Power conversion apparatus and control method therefor

Abstract

The invention provides power conversion equipment and a control method thereof. The power conversion equipment comprises a three-phase power conversion bridge, a controller, a positive bus capacitor and a negative bus capacitor, wherein the positive bus capacitor and the negative bus capacitor are connected in series to the midpoint of a direct-current bus; the first end of the three-phase power conversion bridge is connected with the anode of the DC bus, the second end is connected with the cathode of the DC bus, and the third end is connected with the midpoint of the DC bus; the three-phase power conversion bridge comprises a first bridge arm, a second bridge arm and a third bridge arm; the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the second bridge arm and greater than the absolute value of the output current of the third bridge arm; and under the condition that the voltage of the initial modulation signal of the first bridge arm is larger than that of the initial modulation signal of the second bridge arm and smaller than that of the initial modulation signal of the third bridge arm, the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the direct-current bus. The power conversion equipment can effectively reduce the switching loss in the operation process, and is high in applicability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, in particular to a power conversion device and a control method thereof. BACKGROUND

[0002] Three-phase three-level inverters have the advantages of large output capacity, high output voltage and small current harmonic content, and are therefore widely used in photovoltaic, wind power and energy storage systems. A three-phase three-level inverter contains a large number of switching tubes, and in order to achieve three-level output, the switching frequency of each switching tube is significantly increased, resulting in large switching loss of the three-phase three-level inverter. Therefore, how to reduce the switching loss of the three-phase three-level inverter is a technical problem that needs to be solved by those skilled in the art. SUMMARY

[0003] The present application provides a power conversion device and a control method thereof, which can effectively reduce switching loss during operation, and has high reliability and strong applicability.

[0004] In a first aspect, the present application provides a power conversion device, which is used to connect a direct current source through a positive pole of a direct current bus and a negative pole of the direct current bus, and an alternating current end of the power conversion device is used to connect a power grid or a load; the power conversion device comprises a three-phase power conversion bridge, a controller, and a positive bus capacitor and a negative bus capacitor connected in series at a midpoint of the direct current bus; a first end of the three-phase power conversion bridge and the positive bus capacitor are connected to the positive pole of the direct current bus, a second end of the three-phase power conversion bridge and the negative bus capacitor are connected to the negative pole of the direct current bus, and a third end of the three-phase power conversion bridge is connected to the midpoint of the direct current bus; the controller is used to control the three-phase power conversion bridge to convert direct current provided by the direct current source into alternating current or convert alternating current input from the alternating current end into direct current based on an initial modulation signal of the three-phase power conversion bridge, and the frequency and phase of the initial modulation signal of the three-phase power conversion bridge are the same as the frequency and phase of the alternating current at the alternating current end; wherein the three-phase power conversion bridge comprises a first bridge arm, a second bridge arm and a third bridge arm; the controller is further used to: in a case where an absolute value of a weighted output current of the first bridge arm is greater than an absolute value of an output current of the second bridge arm, and the absolute value of the weighted output current of the first bridge arm is greater than an absolute value of an output current of the third bridge arm, and a voltage size of the initial modulation signal of the first bridge arm is greater than a voltage size of the initial modulation signal of the second bridge arm, and the voltage size of the initial modulation signal of the first bridge arm is less than a voltage size of the initial modulation signal of the third bridge arm, adjust the output voltage size of the first bridge arm to the voltage at the midpoint of the direct current bus.

[0005] In the embodiment, the power conversion device can output three-phase alternating current through the three-phase power conversion bridge, and thus the power conversion device is a three-phase power conversion device. Meanwhile, since the three-phase power conversion bridge is connected to the positive pole of the DC bus, the negative pole of the DC bus and the midpoint of the DC bus respectively, the power conversion device can make the voltage output by each bridge arm equal to the voltage of the positive pole of the DC bus (i.e. the voltage of the positive pole of the DC bus), the voltage of the negative pole of the DC bus (i.e. the voltage of the negative pole of the DC bus) or the voltage of the midpoint of the DC bus (i.e. the voltage of the midpoint of the DC bus) by controlling the switching tubes in each bridge arm of the three-phase power conversion bridge. Therefore, the voltage output by each bridge arm during the operation of the power conversion device includes three levels, and thus the power conversion device is a three-phase three-level power conversion device. Further, the weighted output current absolute value of the first bridge arm refers to the current value obtained by the controller after weighting the output current absolute value of the first bridge arm, the output current absolute value of the second bridge arm refers to the current value obtained by the controller after taking the absolute value of the output current of the second bridge arm, and the output current absolute value of the third bridge arm refers to the current value obtained by the controller after taking the absolute value of the output current of the third bridge arm. During the operation of the power conversion device, if the controller detects that the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, and the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm and less than the voltage of the initial modulation signal of the third bridge arm, it means that the current absolute value flowing through the switching tube of the first bridge arm is the largest among the three bridge arms, i.e. the switching loss generated by the switching state of the switching tube of the first bridge arm is the largest. At this time, the power conversion device adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus through the controller. It can be understood that the output voltage of the first bridge arm remains the voltage of the midpoint of the DC bus, and the switching state of the first bridge arm remains unchanged, thereby reducing the switching loss of the power conversion device and improving the applicability.

[0006] In a possible implementation, the weighted output current absolute value is obtained by the output current absolute value of the first bridge arm and a weighted current coefficient. The weighted output current absolute value is proportional to the output current absolute value of the first bridge arm and the weighted current coefficient, and the weighted current coefficient is greater than 0 and less than or equal to 1.

[0007] In the embodiment, the controller can take the absolute value of the output current of the first bridge arm to obtain an output current absolute value of the first bridge arm after detecting the output current of the first bridge arm, and calculate a weighted output current absolute value based on the output current absolute value of the first bridge arm and a weighted current coefficient. For example, the weighted output current absolute value is equal to the product of the output current absolute value of the first bridge arm and the weighted current coefficient. The weighted current coefficient is greater than 0 and less than or equal to 1. It can be understood that, in the case that the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm, the output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm. For example, assuming that the weighted current coefficient is equal to 0.8, the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm only when the output current absolute value of the first bridge arm is greater than 1.25 times the output current absolute value of the second bridge arm or the third bridge arm. In the embodiment, the controller obtains the weighted output current of the first bridge arm in a simple and easy-to-implement manner.

[0008] In a possible implementation, the controller is further configured to: adjust the weighted current coefficient to decrease when the voltage at the DC bus midpoint exceeds the target voltage range.

[0009] In the embodiment, when the controller detects that the voltage at the DC bus midpoint exceeds the target voltage range, it means that the voltage at the DC bus midpoint fluctuates greatly. At this time, the controller adjusts the weighted current coefficient to decrease, aiming to reduce the length of time for adjusting the output voltage of the first bridge arm to the voltage at the DC bus midpoint, and thus reduce the influence of the first bridge arm on the voltage at the DC bus midpoint. The smaller the weighted current coefficient of the first bridge arm, the smaller the weighted output current absolute value obtained by multiplying the output current absolute value of the first bridge arm by the weighted current coefficient, and the smaller the length of time for which the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm, thereby making the length of time for which the controller adjusts the output voltage of the first bridge arm to the voltage at the DC bus midpoint smaller. Therefore, the controller can reduce the influence of the first bridge arm on the DC bus midpoint and improve the voltage stability of the DC bus midpoint by adjusting the weighted current coefficient to decrease.

[0010] In a possible implementation, the controller adjusts the output voltage of the first bridge arm to the voltage at the DC bus midpoint, specifically by: injecting a first harmonic signal into an initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, the voltage of the first harmonic signal being negatively correlated with the voltage of the initial modulation signal of the first bridge arm, the voltage of the first modulation voltage signal being equal to a first modulation threshold, and the first modulation threshold being a reference threshold; and controlling the first bridge arm to work based on the first modulation voltage signal to adjust the output voltage of the first bridge arm to the voltage at the DC bus midpoint.

[0011] In the embodiment, the controller injects a first harmonic signal into the initial modulation signal of the first bridge arm, aiming to generate a first modulation voltage signal with a voltage magnitude equal to a first modulation threshold. Therefore, the voltage magnitude of the first harmonic signal can be equal to the first modulation threshold minus the voltage magnitude of the initial modulation signal of the first bridge arm. The first modulation threshold is a reference threshold pre-set in the controller, used to represent the voltage magnitude of the first modulation voltage signal when the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint. For example, the first modulation threshold can be equal to 0 volt. The controller controls the first bridge arm according to the first modulation voltage signal, which can make the output voltage of the first bridge arm equal to the voltage of the DC bus midpoint. For example, the controller controls the output end of the first bridge arm to be conductively connected to the DC bus midpoint according to the first modulation voltage signal, so that the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint. In the embodiment, the controller adjusts the output voltage of the first bridge arm, which has a simple implementation principle and strong reliability.

[0012] In a possible implementation, the controller is further configured to: in a case where the voltage magnitude of the initial modulation signal of the second bridge arm is less than a first voltage threshold or the voltage magnitude of the initial modulation signal of the third bridge arm is greater than a second voltage threshold, and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, adjust the output voltage of the second bridge arm to the negative electrode voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the first modulation threshold and the voltage magnitude of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

[0013] In the embodiment, the first voltage threshold refers to the lower limit value of the modulation voltage signal of the second bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus, and the second voltage threshold refers to the upper limit value of the modulation voltage signal of the third bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus. When the controller injects the first harmonic signal into the first bridge arm, the controller can synchronously inject the first harmonic signal into the initial modulation signal of the second bridge arm and the third bridge arm to offset the influence of the first harmonic signal. At this time, if the initial modulation signal of the second bridge arm is less than the first voltage threshold, the second bridge arm will have the problem of over-modulation. Wherein, the over-modulation of the second bridge arm refers to that the voltage of the initial modulation signal of the second bridge arm is too small after superimposing the first harmonic signal, resulting in distortion of the alternating current waveform output by the second bridge arm. Similarly, if the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the third bridge arm will also have the problem of over-modulation after the controller injects the first harmonic signal into the initial modulation signal of the third bridge arm. Further, if the controller also detects that the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, indicating that the switching loss of the second bridge arm is greater than the switching loss of the third bridge arm. At this time, the controller controls the output voltage of the second bridge arm to the negative voltage of the DC bus, and the second bridge arm keeps the switching state unchanged, which can reduce the switching loss of the power conversion device and avoid the problem of over-modulation at the same time, and the application scenarios are rich and the reliability is strong.

[0014] In a possible implementation, the controller adjusts the output voltage of the second bridge arm to the negative voltage of the DC bus, specifically comprising:

[0015] injecting a second harmonic signal into the initial modulation signal of the second bridge arm to obtain a second modulation voltage signal, the voltage of the second harmonic signal being negatively correlated with the voltage of the initial modulation signal of the second bridge arm; and controlling the second bridge arm to work based on the second modulation voltage signal to adjust the output voltage of the second bridge arm to the negative voltage of the DC bus.

[0016] In the embodiment, the controller injects a second harmonic signal into the initial modulation signal of the second bridge arm to generate a second modulation voltage signal, and adjusts the output voltage of the second bridge arm to the negative voltage of the DC bus according to the second modulation voltage signal. For example, the controller can control the output end of the second bridge arm to be conductively connected to the negative electrode of the DC bus according to the second modulation voltage signal, so that the output voltage of the second bridge arm is equal to the negative voltage of the DC bus. In addition, the voltage of the second harmonic signal is negatively correlated with the voltage of the initial modulation signal of the second bridge arm. For example, the voltage of the second harmonic signal can be equal to the second modulation threshold minus the voltage of the initial modulation signal of the second bridge arm. The second modulation threshold is a reference threshold preset in the controller, and is used to represent the voltage of the second modulation voltage signal when the output voltage of the second bridge arm is equal to the negative voltage of the DC bus. For example, the second modulation threshold can be equal to -1 volt. In the embodiment, the controller can adjust the output voltage of the second bridge arm with simple implementation principle and high reliability.

[0017] In a possible implementation, the controller is further configured to: in a case where the voltage of the initial modulation signal of the second bridge arm is less than a first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than a second voltage threshold, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, adjust the output voltage of the third bridge arm to the positive voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the voltage of the initial modulation signal of the first bridge arm, and the first modulation threshold, and the first modulation threshold is a reference threshold.

[0018] In the embodiment, the first voltage threshold refers to the lower limit value of the modulation voltage signal of the second bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus, and the second voltage threshold refers to the upper limit value of the modulation voltage signal of the third bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus. When the controller injects the first harmonic signal into the first bridge arm, the controller can synchronously inject the first harmonic signal into the initial modulation signal of the second bridge arm and the third bridge arm to offset the influence of the first harmonic signal. At this time, if the initial modulation signal of the second bridge arm is less than the first voltage threshold, the second bridge arm will have the problem of over-modulation. Wherein, the over-modulation of the second bridge arm refers to that the voltage of the initial modulation signal of the second bridge arm is too small after superimposing the first harmonic signal, resulting in distortion of the alternating current waveform output by the second bridge arm. Similarly, if the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the third bridge arm will also have the problem of over-modulation after the controller injects the first harmonic signal into the initial modulation signal of the third bridge arm. Further, if the controller also detects that the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, indicating that the switching loss of the second bridge arm is less than the switching loss of the third bridge arm. At this time, the controller controls the output voltage of the third bridge arm to the positive voltage of the DC bus, and the third bridge arm keeps the switching state unchanged, which can reduce the switching loss of the power conversion device and avoid the problem of over-modulation at the same time, and the application scenarios are rich and the reliability is strong.

[0019] In a possible implementation, the controller adjusts the output voltage of the third bridge arm to the positive voltage of the DC bus, specifically comprising: injecting a third harmonic signal into the initial modulation signal of the third bridge arm to obtain a third modulation voltage signal, the voltage size of the third harmonic signal being negatively correlated with the voltage size of the initial modulation signal of the third bridge arm; and controlling the third bridge arm to work based on the third modulation voltage signal to adjust the output voltage size of the third bridge arm to the positive voltage of the DC bus.

[0020] In the embodiment, the controller injects a third harmonic signal into the initial modulation signal of the third bridge arm to generate a third modulation voltage signal, and adjusts the output voltage of the third bridge arm to the positive voltage of the DC bus according to the third modulation voltage signal. For example, the controller can control the output end of the third bridge arm to be conductively connected to the positive of the DC bus according to the third modulation voltage signal, so that the output voltage of the third bridge arm is equal to the positive voltage of the DC bus. In addition, the voltage of the third harmonic signal is negatively correlated with the voltage of the initial modulation signal of the third bridge arm. For example, the voltage of the third harmonic signal can be equal to the third modulation threshold minus the voltage of the initial modulation signal of the third bridge arm. The third modulation threshold is a reference threshold pre-set in the controller, and is used to represent the voltage of the third modulation voltage signal when the output voltage of the third bridge arm is equal to the positive voltage of the DC bus. For example, the third modulation threshold can be equal to 1 volt. In the embodiment, the controller adjusts the output voltage of the third bridge arm, and the implementation principle is simple and the reliability is high.

[0021] In a possible implementation, the first voltage threshold satisfies:

[0022] V1 = V min -(V0-V a )

[0023] The second voltage threshold satisfies:

[0024] V2 = V max -(V0-V a )

[0025] wherein V min is a lower limit value of a working voltage range of the modulation voltage signal, V max is an upper limit value of the working voltage range of the modulation voltage signal, V1 is the first modulation threshold, V a is the voltage of the initial modulation signal of the first bridge arm; the modulation voltage signal is the second modulation voltage signal or the third modulation voltage signal, the second modulation voltage signal is obtained by injecting a second harmonic signal into the initial modulation signal of the second bridge arm by the controller, and the third modulation voltage signal is obtained by injecting a third harmonic signal into the initial modulation signal of the third bridge arm by the controller, and the first modulation threshold is a reference threshold.

[0026] In the embodiment, the controller can calculate the first voltage threshold and the second voltage threshold according to the lower limit value and the upper limit value of the working voltage range of the modulation voltage signal, and the first modulation threshold and the voltage of the initial modulation signal of the first bridge arm. The calculation manner is simple, easy to implement, and has high applicability.

[0027] In a second aspect, the application also provides a control method of a power conversion device, the power conversion device being configured to connect a DC source through a positive pole of a DC bus and a negative pole of the DC bus, and an AC end of the power conversion device being configured to connect to a power grid or a load; the power conversion device comprising a three-phase power conversion bridge and a positive bus capacitor and a negative bus capacitor connected in series to a midpoint of the DC bus; a first end of the three-phase power conversion bridge being connected to the positive pole of the DC bus through the positive bus capacitor, a second end of the three-phase power conversion bridge being connected to the negative pole of the DC bus through the negative bus capacitor, and a third end of the three-phase power conversion bridge being connected to the midpoint of the DC bus; a frequency and a phase of an initial modulation signal of the three-phase power conversion bridge being the same as a frequency and a phase of an AC power of the AC end;

[0028] The three-phase power conversion bridge comprises a first bridge arm, a second bridge arm and a third bridge arm; and the method comprises:

[0029] obtaining a weighted output current absolute value of the first bridge arm, an output current absolute value of the second bridge arm, an output current absolute value of the third bridge arm, an initial modulation signal of the first bridge arm, an initial modulation signal of the second bridge arm and an initial modulation signal of the third bridge arm;

[0030] in a case where the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm, the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the third bridge arm, a voltage size of the initial modulation signal of the first bridge arm is greater than a voltage size of the initial modulation signal of the second bridge arm, and the voltage size of the initial modulation signal of the first bridge arm is less than a voltage size of the initial modulation signal of the third bridge arm, adjusting an output voltage size of the first bridge arm to a voltage of the midpoint of the DC bus.

[0031] In a possible implementation, the weighted output current absolute value is obtained from the output current absolute value of the first bridge arm and a weighted current coefficient, the weighted output current absolute value being directly proportional to the output current absolute value of the first bridge arm and the weighted current coefficient, the weighted current coefficient being greater than 0 and less than or equal to 1.

[0032] In a possible implementation, the method further comprises: in a case where the voltage size of the midpoint of the DC bus exceeds a target voltage range, adjusting the weighted current coefficient to decrease.

[0033] In a possible implementation, adjusting the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus specifically comprises: injecting a first harmonic signal into the initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, the voltage size of the first harmonic signal being negatively correlated to the voltage size of the initial modulation signal of the first bridge arm, the voltage size of the first modulation voltage signal being equal to a first modulation threshold value, and the first modulation threshold value being a reference threshold value; and controlling the first bridge arm to work based on the first modulation voltage signal to adjust the output voltage size of the first bridge arm to the voltage of the midpoint of the DC bus.

[0034] In a possible implementation, the method further includes: in a case where the voltage size of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage size of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, adjusting the output voltage size of the second bridge arm to the negative electrode voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the first modulation threshold, and the voltage size of the initial modulation signal of the first bridge arm, and the first modulation threshold is the reference threshold.

[0035] In a possible implementation, the adjusting of the output voltage of the second bridge arm to the negative electrode voltage of the DC bus specifically includes:

[0036] injecting a second harmonic signal into the initial modulation signal of the second bridge arm to obtain a second modulation voltage signal, the voltage size of the second harmonic signal being negatively correlated with the voltage size of the initial modulation signal of the second bridge arm; and controlling the second bridge arm to work based on the second modulation voltage signal to adjust the output voltage size of the second bridge arm to the negative electrode voltage of the DC bus.

[0037] In a possible implementation, the method further includes: in a case where the voltage size of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage size of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, adjusting the output voltage size of the third bridge arm to the positive electrode voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the first modulation threshold, and the voltage size of the initial modulation signal of the first bridge arm, and the first modulation threshold is the reference threshold.

[0038] In a possible implementation, the adjusting of the output voltage of the third bridge arm to the positive electrode voltage of the DC bus specifically includes: injecting a third harmonic signal into the initial modulation signal of the third bridge arm to obtain a third modulation voltage signal, the voltage size of the third harmonic signal being negatively correlated with the voltage size of the initial modulation signal of the third bridge arm; and controlling the third bridge arm to work based on the third modulation voltage signal to adjust the output voltage size of the third bridge arm to the positive electrode voltage of the DC bus.

[0039] In a possible implementation, the first voltage threshold satisfies:

[0040] V1=V min -(V0-V a )

[0041] The second voltage threshold satisfies:

[0042] V2=V max -(V0-Va

[0043] V1 min V1 max V1 a V1

[0044] The scheme provided by the second aspect can be used to realize or cooperate to realize the power conversion device provided by the first aspect, and thus can achieve the same or corresponding beneficial effects as the corresponding power conversion device in the first aspect, which will not be described here. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 FIG. 1 is a schematic diagram of an application scenario of a grid-connected system provided by the present application;

[0046] Figure 2 FIG. 2 is a structural schematic diagram of a power conversion device provided by an embodiment of the present application;

[0047] Figure 3 FIG. 3 is another structural schematic diagram of a power conversion device provided by an embodiment of the present application;

[0048] Figure 4 FIG. 4 is a structural schematic diagram of a three-phase power conversion bridge provided by an embodiment of the present application;

[0049] Figure 5 FIG. 5 is a structural schematic diagram of a bridge arm provided by an embodiment of the present application;

[0050] Figure 6 FIG. 6 is another structural schematic diagram of a bridge arm provided by an embodiment of the present application;

[0051] Figure 7 FIG. 7 is a signal waveform schematic diagram of a three-phase power conversion bridge provided by an embodiment of the present application;

[0052] Figure 8 FIG. 8 is a signal waveform schematic diagram of a bridge arm provided by an embodiment of the present application;

[0053] Figure 9 FIG. 9 is another signal waveform schematic diagram of a bridge arm provided by an embodiment of the present application;

[0054] Figure 10 FIG. 10 is yet another signal waveform schematic diagram of a bridge arm provided by an embodiment of the present application;​

[0055] Figure 11 A flowchart of a control method of a power conversion device provided in an embodiment of the present application. DETAILED DESCRIPTION

[0056] The power conversion device provided in the present application is applied in a grid-connected system, and can be applied in different application scenarios, such as a light storage power supply application scenario, a wind storage power supply application scenario, a pure storage power supply application scenario, or other application scenarios. The light storage power supply application scenario is taken as an example for description.

[0057] Please refer to Figure 1 , Figure 1 An application scenario diagram of a grid-connected system provided in the present application. In Figure 1 In the light storage power supply scenario shown in FIG. 1, the grid-connected system includes a storage converter and a photovoltaic inverter. The direct current end of the storage converter is connected to a storage battery, the alternating current end of the storage converter is connected to an alternating current bus of the grid-connected system, and the alternating current bus is further connected to a load and a power grid. The storage converter converts direct current from the storage battery into alternating current and outputs the alternating current to the alternating current bus to supply power to the load and the power grid. Alternatively, the storage converter converts alternating current from the power grid into direct current to charge the direct current source. The direct current end of the photovoltaic inverter is connected to a photovoltaic module, the alternating current end of the photovoltaic inverter is connected to the alternating current bus, and the photovoltaic inverter converts direct current from the photovoltaic module into alternating current and outputs the alternating current to the alternating current bus to supply power to the load and the power grid. The storage converter and the photovoltaic inverter are both used to provide power conversion and other functions in the grid-connected system, and the storage converter and the photovoltaic inverter can be collectively referred to as a power conversion device. The storage battery and the photovoltaic module are both used to provide direct current to the power conversion device, and the storage battery and the photovoltaic module can be collectively referred to as a direct current source.

[0058] In actual applications, the grid-connected system can also be provided with a box-type transformer, which is used to adjust and distribute the input alternating current and then output the alternating current to adapt to the voltage requirements of different power transmission nodes. For example, the alternating current transmitted by the alternating current bus usually has a relatively high voltage amplitude, and therefore the power conversion device can transmit the alternating current to the box-type transformer for voltage boosting and then provide the alternating current to the alternating current bus. In addition, the alternating current transmitted by the alternating current bus can be stepped down by the box-type transformer and then provided to the load, or if the rated voltage amplitude of the load is adapted to the voltage amplitude of the alternating current transmitted by the alternating current bus, the alternating current bus can also directly supply power to the load. When the grid-connected switch is closed and the grid-connected system is in grid-connected operation, the alternating current transmitted by the alternating current bus can be stepped up by one or more box-type transformers and then output to the power grid. The specific setting position and number of the box-type transformer can be flexibly adjusted according to the actual application scenario, and the embodiments of the present application do not limit this.

[0059] It should be noted that, according to the number of power supply phases, the power grid can be divided into a single-phase power grid, a three-phase power grid or other multi-phase power grid. The three-phase power grid has the advantages of high transmission efficiency, large power support and flexible voltage. In actual application, the power grid to which the grid-connected system is connected is usually a three-phase power grid. In order to adapt to the three-phase power grid, the power conversion device of the grid-connected system is a three-phase power conversion device. The three-phase power conversion device includes a three-phase power conversion bridge. The three-phase power conversion bridge can convert the direct current provided by the direct current source into stepped wave alternating current. The stepped wave alternating current refers to alternating current with a stepped voltage waveform. Further, the three-phase power conversion device can filter the stepped wave alternating current output by the three-phase power conversion bridge to obtain sinusoidal wave alternating current, thereby realizing the provision of three-phase sinusoidal wave alternating current to the power grid and the load.

[0060] It should be noted that, according to the level of the stepped wave alternating current output by the three-phase power conversion bridge, the power conversion device can be divided into a two-level power conversion device, a three-level power conversion device and other multi-level power conversion devices. The two-level power conversion device refers to a three-phase power conversion bridge outputting stepped wave alternating current including only two levels, while the three-level power conversion device refers to a three-phase power conversion bridge outputting stepped wave alternating current including three levels. The three-level power conversion device can make the waveform of the sinusoidal wave alternating current after filtering smoother by increasing the number of levels of the stepped wave alternating current, while reducing the voltage jump amplitude, thereby reducing the total harmonic distortion. Therefore, in actual application, the power conversion device in the grid-connected system is usually a three-phase three-level power conversion device, such as a three-phase three-level inverter. Therefore, the present application mainly introduces the three-phase three-level power conversion device.

[0061] It should be noted that the three-phase three-level power conversion device contains a large number of switching tubes, and in order to realize three-level output, the switching frequency of each switching tube will increase significantly, resulting in large switching loss of the three-phase three-level power conversion device. Therefore, how to reduce the switching loss of the three-phase three-level power conversion device is a technical problem that needs to be solved by those skilled in the art. For the sake of convenience, the three-phase three-level power conversion device will be referred to as a power conversion device in the following content.

[0062] Therefore, the present application provides a power conversion device which can effectively reduce switching loss during operation, and has high reliability and strong applicability.

[0063] The above is only an example of the application scenario of the power conversion device provided by the present application, and is not exhaustive. The application scenario is not limited by the present application.

[0064] It should be noted that the power conversion device provided by the embodiments of the present application can inversely convert direct current or rectify alternating current. For the sake of convenience, the following content is combined with the power conversion device provided by the present application. Figures 2 to 10The working principle of the power conversion device in the process of inverting conversion is described. The working principle of the power conversion device in the process of rectifying conversion provided by the embodiment of the application can be referred to the following content, and the embodiment of the application does not make redundant description.

[0065] Please refer to Figure 2 , Figure 2 A structural schematic diagram of the power conversion device provided by the embodiment of the application is shown. As shown in Figure 2 , the direct current end of the power conversion device is connected with a direct current source, and the alternating current end of the power conversion device is connected with a power grid or a load. The power conversion device comprises a controller, a three-phase power conversion bridge, a positive bus capacitor C1 and a negative bus capacitor C2. The first end i11 of the three-phase power conversion bridge is connected with the positive pole of the direct current source through the positive pole BUS+ of the direct current bus, and the second end i12 of the three-phase power conversion bridge is connected with the negative pole of the direct current source through the negative pole BUS- of the direct current bus. One end of the positive bus capacitor C1 is connected with the positive pole BUS+ of the direct current bus, and the other end of the positive bus capacitor C1 is connected with the negative pole BUS- of the direct current bus through the negative bus capacitor C2. The connection point of the positive bus capacitor C1 and the negative bus capacitor C2 is the direct current bus midpoint N. The third end i13 of the three-phase power conversion bridge is connected with the direct current bus midpoint N. In addition, the three-phase power conversion bridge is connected with the alternating current end of the power conversion device through three live lines.

[0066] In the process of the power conversion device supplying power to the power grid or the load, the power conversion device controls the three-phase power conversion bridge to convert the direct current provided by the direct current source into stepped wave alternating current through the controller. Since the three-phase power conversion bridge is connected with the positive pole BUS+ of the direct current bus, the negative pole BUS- of the direct current bus and the direct current bus midpoint N respectively, the voltage of the stepped wave alternating current output by the three-phase power conversion bridge can be equal to the positive pole voltage of the direct current bus, the negative pole voltage of the direct current bus or the voltage of the direct current bus midpoint. The size of the positive pole voltage of the direct current bus is equal to the voltage size of the positive pole BUS+ of the direct current bus, the size of the negative pole voltage of the direct current bus is equal to the voltage size of the negative pole BUS- of the direct current bus, and the size of the voltage of the direct current bus midpoint is equal to the voltage size of the direct current bus midpoint N. Therefore, it can be seen that the stepped wave alternating current output by the three-phase power conversion bridge in the process of inverting conversion comprises three levels.

[0067] In some possible embodiments, in order to provide the power grid or the load with sinusoidal wave alternating current, the power conversion device is provided with a filtering module which can filter the stepped wave alternating current output by the three-phase power conversion bridge to obtain the sinusoidal wave alternating current. Specifically, please refer to Figure 3 , Figure 3 Another structural schematic diagram of the power conversion device provided by the embodiment of the application is shown. As shown in Figure 3As shown, the filter module is arranged between the three-phase power conversion bridge and the AC end of the power conversion device, and is composed of a plurality of inductors and a plurality of capacitors. The filter module can filter out high-frequency harmonic components in the stepped wave AC output by the three-phase power conversion bridge to obtain a sinusoidal wave AC, and output the sinusoidal wave AC to the power grid or the load through the AC end of the power conversion device. It can be understood that the specific circuit structure of the filter module can be flexibly adjusted according to the actual scene, Figure 3 The filter module shown is only an example and does not constitute a limitation on the embodiments of the present application.

[0068] Please refer to Figure 2 and Figure 3 , the three-phase power conversion bridge includes three bridge arms: bridge arm a, bridge arm b and bridge arm c. The bridge arm a, the bridge arm b and the bridge arm c are connected in parallel between the first end i11 and the second end i12 of the three-phase power conversion bridge, and the bridge arm a, the bridge arm b and the bridge arm c are also respectively connected with the third end i13 of the three-phase power conversion bridge. At the same time, the output end i21 of the bridge arm a is connected with the first AC end i31 of the power conversion device through a corresponding live line, the output end i22 of the bridge arm b is connected with the second AC end i32 of the power conversion device through a corresponding live line, and the output end i23 of the bridge arm c is connected with the third AC end i33 of the power conversion device through a corresponding live line. It should be noted that, Figure 2 and Figure 3 The positions of the bridge arm a, the bridge arm b and the bridge arm c shown can be exchanged, and the embodiments of the present application do not limit this.

[0069] In some possible implementations, please refer to Figure 4 , Figure 4 for a structural schematic diagram of the three-phase power conversion bridge provided by the embodiments of the present application. As shown in Figure 4 , Figure 4 , Figure 4The parallel connection in the bridge arm a, the bridge arm b and the bridge arm c is between the first end i11 of the three-phase power conversion bridge and the second end i12 of the three-phase power conversion bridge. The bridge arm a includes four switch tubes connected in series: the switch tube Q11, the switch tube Q12, the switch tube Q13 and the switch tube Q14. The connection point of the switch tube Q11 and the switch tube Q12 is connected to the connection point of the switch tube Q13 and the switch tube Q14 through the diode D11 and the diode D12 connected in series. The connection point of the diode D11 and the diode D12 is connected to the third end i13 of the three-phase power conversion bridge. The connection point of the switch tube Q12 and the switch tube Q13 is connected to the output end i21 of the bridge arm a. The bridge arm b includes the switch tube Q21, the switch tube Q22, the switch tube Q23, the switch tube Q24, the diode D21 and the diode D22. The bridge arm c includes the switch tube Q31, the switch tube Q32, the switch tube Q33, the switch tube Q34, the diode D31 and the diode D32. It can be understood that the circuit structures of the bridge arm b and the bridge arm c are the same as those of the bridge arm a, which will not be described here.

[0070] In some application scenarios, please refer to Figure 5 and Figure 6 , Figure 5 the structural schematic diagram of the bridge arm provided by the embodiment of the application, Figure 6 the other structural schematic diagram of the bridge arm provided by the embodiment of the application. Figure 4 The bridge arm a, the bridge arm b and the bridge arm c shown in FIG. 1 can be deformed into Figure 5 shown in FIG. 2. Alternatively, Figure 4 The bridge arm a, the bridge arm b and the bridge arm c shown in FIG. 1 can be deformed into Figure 6 shown in FIG. 3. It can be understood that the circuit structures of the bridge arm a, the bridge arm b and the bridge arm c in the three-phase power conversion bridge can be deformed according to actual needs. Figure 4 、 Figure 5 and Figure 6 shown are only examples and do not constitute a limitation on the embodiments of the application.

[0071] In the embodiment of the application, in the process of the power conversion device supplying power to the power grid or the load, the power conversion device controls the bridge arm a, the bridge arm b and the bridge arm c to work through the controller, so as to convert the direct current provided by the direct current source into stepped alternating current.

[0072] Specifically, the controller first acquires the initial modulation signal of the bridge arm a. The initial modulation signal of the bridge arm a can be understood as the AC reference signal of the first AC terminal i31 connected to the bridge arm a. The controller obtains the pulse width modulation signal of the bridge arm a according to the initial modulation signal of the bridge arm a, and controls the switching tube in the bridge arm a to act, so that the bridge arm a outputs corresponding staircase AC. Further, the staircase AC output by the bridge arm a is output to the first AC terminal i31 after filtering. At this time, the AC frequency of the first AC terminal i31 is the same as the frequency of the initial modulation signal of the bridge arm a, and the phase of the AC of the first AC terminal i31 is the same as the phase of the initial modulation signal of the bridge arm a. Similarly, the controller acquires the initial modulation signal of the bridge arm b. The initial modulation signal of the bridge arm b can be understood as the AC reference signal of the second AC terminal i32 connected to the bridge arm b. The controller obtains the pulse width modulation signal of the bridge arm b according to the initial modulation signal of the bridge arm b, and controls the switching tube in the bridge arm b to act, so that the bridge arm b outputs corresponding staircase AC. The staircase AC output by the bridge arm b is output to the second AC terminal i32 after filtering. At this time, the AC frequency of the second AC terminal i32 is the same as the frequency of the initial modulation signal of the bridge arm a, and the phase of the AC of the first AC terminal i31 is the same as the phase of the initial modulation signal of the bridge arm a. Similarly, the controller acquires the initial modulation signal of the bridge arm c. The initial modulation signal of the bridge arm c can be understood as the AC reference signal of the third AC terminal i33 connected to the bridge arm c. The controller obtains the pulse width modulation signal of the bridge arm c according to the initial modulation signal of the bridge arm c, and controls the switching tube in the bridge arm c to act, so that the bridge arm c outputs corresponding staircase AC. The staircase AC output by the bridge arm c is output to the third AC terminal i33 after filtering. At this time, the AC frequency of the third AC terminal i33 is the same as the frequency of the initial modulation signal of the bridge arm c, and the phase of the AC of the third AC terminal i33 is the same as the phase of the initial modulation signal of the bridge arm a. Therefore, the frequency and phase of the initial modulation signal of each bridge arm (bridge arm a, bridge arm b or bridge arm c) are the same as the frequency and phase of the AC of the corresponding AC terminal (first AC terminal i31, second AC terminal i32 and third AC terminal i33).

[0073] For ease of understanding, the following content is exemplified by taking the controller controlling the bridge arm a to work according to the initial modulation signal of the bridge arm a as an example.

[0074] Please refer to Figure 4 and Figure 7 , Figure 7 The signal waveform diagram of the three-phase power conversion bridge provided in the embodiment of the present application is shown. Taking the controller controlling the bridge arm a to work according to the initial modulation signal of the bridge arm a as an example, Figure 4As shown in the bridge arm a is taken as an example, the controller can generate the pulse width modulation signal of the switch tube of the bridge arm a based on the initial modulation signal and the carrier signal of the bridge arm a, so as to control the switch tube to act, so that the bridge arm a converts the direct current from the direct power supply into the staircase wave alternating current. Specifically, as shown in the figure, Figure 7 As shown in the figure, when the initial modulation signal of the bridge arm a is located in the positive half cycle, the voltage of the initial modulation signal is greater than 0, and when the initial modulation signal of the bridge arm a is located in the negative half cycle, the voltage of the initial modulation signal of the bridge arm a is less than 0. The controller compares the voltage of the initial modulation signal of the bridge arm a with the voltage of the carrier signal to obtain the corresponding pulse width modulation signal of the switch tube Q11, the switch tube Q12, the switch tube Q13 and the switch tube Q14. When the corresponding pulse width modulation signal of each switch tube is 0, the controller controls the switch tube to be off, and when the corresponding pulse width modulation signal is 1, the controller controls the switch tube to be on. As can be seen, in the positive half cycle of the initial modulation signal of the bridge arm a, the controller controls the switch tube Q11 and the switch tube Q13 to be alternately turned on, and controls the switch tube Q12 to be always turned on and the switch tube Q14 to be always turned off. In the negative half cycle of the initial modulation signal of the bridge arm a, the controller controls the switch tube Q12 and the switch tube Q14 to be alternately turned on, and controls the switch tube Q13 to be always turned on and the switch tube Q11 to be always turned off. Thus, the bridge arm a can convert the direct current into the staircase wave alternating current. It can be understood that the specific implementation of generating the pulse width modulation signal by the bridge arm b and the bridge arm c can refer to the description of the bridge arm a, which is not described herein in detail. Figure 4 and Figure 7 As shown in the figure, the embodiment of the present application will not be described herein.

[0075] Further, after the controller controls each bridge arm to convert the direct current into the staircase wave alternating current, the filter module filters the staircase wave alternating current output by each bridge arm to obtain the sinusoidal wave alternating current, and outputs the sinusoidal wave alternating current to each alternating current terminal, so as to realize the provision of the three-phase sinusoidal wave alternating current to the power grid or the load. The sinusoidal wave alternating current output by the first alternating current terminal i31, the second alternating current terminal i32 and the third alternating current terminal i33 has the same frequency and a phase difference of 120 degrees.

[0076] In the embodiment of the present application, during the process that the power conversion device supplies power to the power grid or the load, the controller detects the current of each alternating current terminal. In the case that the absolute value of the weighted output current of the bridge arm a is greater than the absolute value of the output current of the bridge arm b, and the absolute value of the weighted output current of the bridge arm a is greater than the absolute value of the output current of the bridge arm c, the controller takes the bridge arm a as the first bridge arm, takes the bridge arm b as the second bridge arm, takes the bridge arm c as the third bridge arm, and controls the first bridge arm (i.e. the bridge arm a) to keep the switch state unchanged.

[0077] The weighted output current absolute value of the bridge arm a refers to a current value obtained by the controller after weighting calculation of the output current absolute value of the bridge arm a. Specifically, the controller can obtain the output current absolute value of the bridge arm a by taking the absolute value of the output current size of the bridge arm a after detection, and obtain the weighted output current absolute value by multiplying the output current absolute value of the bridge arm a by a weighted current coefficient. The weighted current coefficient is greater than 0 and less than or equal to 1. In addition, the output current absolute value of the bridge arm b refers to a current value obtained by the controller after taking the absolute value of the output current size of the bridge arm b, and the output current absolute value of the bridge arm c refers to a current value obtained by the controller after taking the absolute value of the output current size of the bridge arm c.

[0078] In some possible embodiments, the controller can be connected with the output end i21 of the bridge arm a, and obtain the output current size of the bridge arm a by collecting the current size of the output end i21 of the bridge arm a. Alternatively, the controller can obtain the output current size of the bridge arm a through an external sampling module. The sampling module is electrically connected with the output end i21 of the bridge arm a, and can sample the current size of the output end i21 of the bridge arm a. The sampling module is communicatively connected with the controller, and can send the sampled current size of the output end i21 of the bridge arm a to the controller.

[0079] In some possible embodiments, since the output end i21 of the bridge arm a is connected with the first alternating end i31 through the filtering module, the current size of the first alternating end i31 can be equivalent to the output current size of the bridge arm a, and the current size flowing through the filtering module can also be equivalent to the output current size of the bridge arm a. Therefore, the controller can take the current size of the first alternating end i31 or the current size flowing through the filtering module as the output current size of the bridge arm a. It can be understood that the above is only an example, and the embodiments of the present application do not limit the implementation of the current size of the output end i21 of the bridge arm a of the controller.

[0080] Similarly, the specific implementation of the controller obtaining the output current size of the bridge arm b or the bridge arm c can refer to the above content, and the embodiments of the present application will not be repeated here.

[0081] It can be understood that when the weighted current coefficient is equal to 1, the weighted output current absolute value of the bridge arm a is equal to the output current absolute value of the bridge arm a. When the weighted current coefficient is less than 1, the weighted output current absolute value of the bridge arm a is less than the output current absolute value of the bridge arm a. Therefore, in the case that the weighted output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arm b and the bridge arm c, the output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arm b and the bridge arm c.

[0082] It should be noted that in the process of switching of the switch tube of the bridge arm a (switching from on to off, or switching from off to on), the voltage and current across the switch tube overlap to generate energy loss, which in turn causes switching loss of the bridge arm a. The greater the absolute value of the weighted output current of the bridge arm a, the greater the absolute value of the current flowing through the switch tube in the bridge arm a, and the greater the switching loss generated by the switching of the switch tube in the bridge arm a. Therefore, when the controller detects that the absolute value of the weighted output current of the bridge arm a is greater than the absolute value of the output current of the bridge arm b and greater than the absolute value of the output current of the bridge arm c, it means that among the three bridge arms, the absolute value of the current flowing through the switch tube of the bridge arm a is the largest, that is, the switching loss generated by the switching of the switch tube of the bridge arm a is the largest. At this time, the power conversion device takes the bridge arm a as the first bridge arm, and controls the first bridge arm to keep the switching state unchanged through the controller, so that the switch tube of the first bridge arm does not act, which can minimize the switching loss of the power conversion device.

[0083] It should be noted that from the above, it can be seen that the bridge arm a is connected with the positive electrode BUS+ of the DC bus, the negative electrode BUS- of the DC bus and the midpoint N of the DC bus respectively. In the process of working of the bridge arm a, the controller adjusts the switching state of the bridge arm a so that when the output end i21 of the bridge arm a is turned on with the midpoint N of the DC bus, the voltage at the output end of the bridge arm a is equal to the voltage at the midpoint of the DC bus. At this time, if the controller controls the switching state of the bridge arm a to remain unchanged, the voltage at the output end of the bridge arm a can remain equal to the voltage at the midpoint of the DC bus. Similarly, the controller adjusts the switching state of the bridge arm a so that when the output end i21 of the bridge arm a is turned on with the positive electrode BUS+ of the DC bus, the voltage at the output end of the bridge arm a is equal to the positive electrode voltage of the DC bus. At this time, if the controller controls the switching state of the bridge arm a to remain unchanged, the voltage at the output end of the bridge arm a can remain equal to the positive electrode voltage of the DC bus. Similarly, the controller adjusts the switching state of the bridge arm a so that when the output end i21 of the bridge arm a is turned on with the negative electrode BUS- of the DC bus, the voltage at the output end of the bridge arm a is equal to the negative electrode voltage of the DC bus. At this time, if the controller controls the switching state of the bridge arm a to remain unchanged, the voltage at the output end of the bridge arm a can remain equal to the negative electrode voltage of the DC bus. As can be seen, in the process of working of the bridge arm a, the output voltage of the bridge arm a can be equal to the positive electrode voltage of the DC bus, the negative electrode voltage of the DC bus or the voltage at the midpoint of the DC bus. When the controller controls the output voltage of the bridge arm a to remain equal to the voltage at the midpoint of the DC bus, the positive electrode voltage of the DC bus or the negative electrode voltage of the DC bus, the bridge arm a keeps the switching state unchanged.

[0084] In the embodiments of the present application, when the controller detects that the weighted output current absolute value of the bridge arm a is greater than the output current absolute value of the bridge arm b and greater than the output current absolute value of the bridge arm c, the bridge arm a is the bridge arm with the greatest switching loss among the three bridge arms. Further, if the controller simultaneously detects that the voltage of the initial modulation signal of the bridge arm a is greater than the voltage of the initial modulation signal of the bridge arm b and the voltage of the initial modulation signal of the bridge arm c, the controller takes the bridge arm a as the first bridge arm and adjusts the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint. At this time, the first bridge arm keeps the switching state unchanged, and the switching loss of the power conversion device can be reduced.

[0085] For example, in the case where the voltage of the initial modulation signal of the bridge arm a is greater than the voltage of the initial modulation signal of the bridge arm b and less than the voltage of the initial modulation signal of the bridge arm c, assuming that the weighted current coefficient of the bridge arm a is equal to 1, the weighted output current absolute value of the bridge arm a is equal to the output current absolute value of the bridge arm a. At this time, as long as the output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arms b and c, the weighted output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arms b and c. Therefore, the controller takes the bridge arm a as the first bridge arm and controls the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint when the output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arms b and c.

[0086] For example, in the case where the voltage of the initial modulation signal of the bridge arm a is greater than the voltage of the initial modulation signal of the bridge arm b and less than the voltage of the initial modulation signal of the bridge arm c, assuming that the weighted current coefficient of the bridge arm a is equal to 0.8, the output current absolute value of the bridge arm a multiplied by 0.8 is equal to the weighted output current absolute value of the bridge arm a. It can be understood that the weighted output current absolute value of the bridge arm a is greater than the output current absolute values of the bridge arms b and c only when the output current absolute value of the bridge arm a is greater than 1.25 times the output current absolute values of the bridge arms b and c. Therefore, the controller takes the bridge arm a as the first bridge arm and controls the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint when the output current absolute value of the bridge arm a is greater than 1.25 times the output current absolute values of the bridge arms b and c.

[0087] For example, in the case that the voltage size of the initial modulation signal of the bridge arm a is greater than the voltage size of the initial modulation signal of the bridge arm b and less than the voltage size of the initial modulation signal of the bridge arm c, assuming that the weighting current coefficient of the bridge arm a is equal to 0.5, the output current absolute value of the bridge arm a multiplied by 0.5 is equal to the weighted output current absolute value of the bridge arm a. It can be understood that when the output current absolute value of the bridge arm a is greater than 2 times the output current absolute value of the bridge arm b and the bridge arm c, the weighted output current absolute value of the bridge arm a is greater than the output current absolute value of the bridge arm b and the bridge arm c. Therefore, when the output current absolute value of the bridge arm a is greater than 2 times the output current absolute value of the bridge arm b and greater than 2 times the output current absolute value of the bridge arm c, the controller controls the bridge arm a as the first bridge arm, and controls the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint.

[0088] It can be understood that in other application scenarios, the controller can also control the bridge arm b or the bridge arm c as the first bridge arm. For ease of description, the following content is described by taking the bridge arm a as the first bridge arm, the bridge arm b as the second bridge arm, and the bridge arm c as the third bridge arm.

[0089] In some possible embodiments, the controller can inject a first harmonic signal into the initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, and adjust the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint according to the first modulation voltage signal. The voltage size of the first harmonic signal is negatively correlated with the voltage size of the initial modulation signal of the first bridge arm. Specifically, the voltage size of the first harmonic signal is equal to the first modulation threshold minus the voltage size of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold pre-set in the controller and can represent the voltage size of the first modulation voltage signal when the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint. For example, the first modulation threshold is equal to 0 volt. After the controller injects the first harmonic signal into the initial modulation signal of the first bridge arm, the first harmonic signal with the voltage size equal to the first modulation threshold can be obtained. The controller generates the pulse width modulation signal of the first bridge arm according to the first modulation voltage signal with the voltage size equal to the first modulation threshold, and controls the switch tube in the first bridge arm to conduct when the output end of the first bridge arm is connected to the DC bus midpoint N, and the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint.

[0090] For example, it is assumed that the voltage size of the initial modulation signal of the first bridge arm is equal to 0.1 volt, and the first modulation threshold value is equal to 0 volt. In order to make the first modulation voltage signal of the first bridge arm equal to the first modulation threshold value 0 volt, the controller injects a first harmonic signal with a voltage size equal to -0.1 volt into the initial modulation signal of the first bridge arm. After superimposing the first harmonic signal and the initial modulation signal of the first bridge arm, the first modulation voltage signal with a voltage size equal to 0 volt is obtained. Further, the controller generates the pulse width modulation signal of the first bridge arm according to the first modulation voltage signal with a voltage size equal to 0 volt, and controls the switching tube of the first bridge arm to act, so that the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint. It can be understood that the specific implementation of the controller generating the pulse width modulation signal of the first bridge arm according to the first modulation voltage signal can refer to the specific embodiments shown in the above Figure 4 and Figure 7 The embodiments of the present application will not be described here.

[0091] In some possible embodiments, at the same time when the controller injects the first harmonic signal into the initial modulation signal of the first bridge arm, the controller also injects the first harmonic signal into the initial modulation signals of the other two bridge arms. At this time, the controller injects the initial modulation signals of the three bridge arms with a common mode signal. Since the sine wave alternating current supplied by the three bridge arms to the power grid or the load is a line voltage (i.e. the difference between the output voltages of different bridge arms). Therefore, the first harmonic signals injected by the controller into the initial modulation signals of the three bridge arms can be offset, so as not to affect the sine wave alternating current waveform provided to the power grid or the load.

[0092] As can be seen from the above, the controller controls the output voltage of the first bridge arm to remain equal to the voltage of the DC bus midpoint, so that the first bridge arm remains unchanged in the switching state, thereby reducing the switching loss of the power conversion device. It should be noted that the controller does not control the output voltage of the first bridge arm to remain equal to the positive voltage of the DC bus or the negative voltage of the DC bus to make the first bridge arm remain unchanged in the switching state, and the purpose is to avoid over-modulation of the controller to the second bridge arm or the third bridge arm.

[0093] Among them, the over-modulation of the controller to the second bridge arm or the third bridge arm means that the voltage size of the initial modulation signal of the second bridge arm or the third bridge arm after superimposing the first harmonic signal exceeds the voltage size of the carrier signal. As can be seen from the above, the controller controls the output voltage of the second bridge arm or the third bridge arm to remain equal to the voltage of the DC bus midpoint, so that the second bridge arm or the third bridge arm remains unchanged in the switching state, thereby reducing the switching loss of the power conversion device. Figure 7As shown, after the initial modulation signal of each bridge arm is superimposed with the first harmonic signal to obtain the modulation voltage signal, the controller can compare the voltage of the modulation voltage signal with the voltage of the carrier signal to generate the pulse width modulation signal of each bridge arm. When the voltage amplitude of the modulation voltage signal of the second bridge arm or the third bridge arm exceeds the voltage amplitude of the carrier signal, the pulse width modulation signal of the second bridge arm or the third bridge arm remains constant for a period of time regardless of the change of the voltage of the carrier signal, resulting in the distortion of the alternating current waveform output by the second bridge arm and the third bridge arm.

[0094] For example, it is assumed that the voltage of the initial modulation signal of the first bridge arm is equal to 0.1 volts, the voltage of the initial modulation signal of the second bridge arm is equal to -0.6 volts, the voltage of the initial modulation signal of the third bridge arm is equal to 0.5 volts, the third modulation threshold is equal to 1 volt, and the voltage of the carrier signal is greater than -1 volt and less than 1 volt. The third modulation threshold is a reference threshold preset by the controller. At this time, if the controller injects the first harmonic signal with a voltage equal to 0.9 volts into the initial modulation signal of the first bridge arm. After the first harmonic signal is superimposed with the initial modulation signal of the first bridge arm, the first modulation voltage signal with a voltage equal to the third modulation threshold of 1 volt is obtained. The controller generates the pulse width modulation signal of the first bridge arm according to the first modulation voltage signal to control the operation of the switch in the first bridge arm, so that the output end of the first bridge arm is conducted with the positive pole BUS+ of the DC bus, and the output voltage of the first bridge arm is equal to the positive pole voltage of the DC bus. At the same time, the controller injects the first harmonic signal with a voltage equal to 0.9 volts into the initial modulation signal of the second bridge arm. After the first harmonic signal is superimposed with the initial modulation signal of the second bridge arm, the modulation voltage signal with a voltage equal to 0.3 volts is obtained. At the same time, the controller injects the first harmonic signal with a voltage equal to 0.9 volts into the initial modulation signal of the third bridge arm. After the first harmonic signal is superimposed with the initial modulation signal of the third bridge arm, the modulation voltage signal with a voltage equal to 1.4 volts is obtained. At this time, the voltage of the modulation voltage signal of the third bridge arm is greater than 1 volt, i.e., greater than the voltage of the carrier signal, which can cause over-modulation when the controller controls the operation of the third bridge arm. Similarly, when the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm and less than the voltage of the initial modulation signal of the third bridge arm, the controller adjusts the output voltage of the first bridge arm to be equal to the negative pole voltage of the DC bus, which can also cause over-modulation of the second bridge arm or the third bridge arm by the controller. Therefore, the controller adjusts the output voltage of the first bridge arm to be equal to the voltage of the midpoint of the DC bus in the above-mentioned case in the embodiment of the present application, which can avoid over-modulation of the second bridge arm and the third bridge arm.

[0095] It should be noted that, in the case that the controller adjusts the output voltage of the first bridge arm to keep equal to the voltage of the DC bus midpoint, the output end of the first bridge arm is conductive with the DC bus midpoint N, and the current of the first bridge arm directly affects the voltage of the DC bus midpoint. At the same time, since the DC bus midpoint N is the connection point of the positive bus capacitor C1 and the negative bus capacitor C2, when the voltage of the DC bus midpoint fluctuates, the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 also fluctuate, thereby causing the positive electrode BUS+ of the DC bus and the negative electrode BUS- of the DC bus to produce obvious fluctuations. Therefore, in the embodiment of the present application, when the controller detects that the voltage of the DC bus midpoint N significantly fluctuates, the conductive time of the output end of the first bridge arm with the DC bus midpoint N can be reduced, thereby reducing the influence on the voltage of the DC bus midpoint and the voltage of the DC bus.

[0096] In some possible embodiments, the controller can adjust the weighted current coefficient of the first bridge arm to decrease when detecting that the voltage of the DC bus midpoint N exceeds the target voltage range, thereby reducing the conductive time of the output end of the first bridge arm with the DC bus midpoint N. The target voltage range can be understood as the voltage range of the DC bus midpoint N when the voltages of the positive electrode BUS+ of the DC bus and the negative electrode BUS- of the DC bus remain stable. The specific value of the bus voltage range can be set according to the actual application scenario, and the embodiment of the present application does not limit this.

[0097] It should be noted that, the greater the weighted current coefficient of the first bridge arm is, the greater the weighted output current absolute value obtained by multiplying the output current absolute value of the first bridge arm by the weighted current coefficient is, and the longer the time that the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm is, thereby causing the controller to control the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint for a longer time, that is, the output end of the first bridge arm is conductive with the DC bus midpoint N for a longer time. Conversely, the smaller the weighted current coefficient of the first bridge arm is, the smaller the weighted output current absolute value obtained by multiplying the output current absolute value of the first bridge arm by the weighted current coefficient is, and the shorter the time that the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm is, thereby causing the controller to control the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint for a shorter time, that is, the output end of the first bridge arm is conductive with the DC bus midpoint N for a shorter time. Therefore, when the voltage of the DC bus midpoint N exceeds the target voltage range, the controller can reduce the length of time that the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm by adjusting the weighted current coefficient of the first bridge arm to decrease, thereby reducing the conductive time of the output end of the first bridge arm with the DC bus midpoint N, and further reducing the voltage fluctuation of the DC bus midpoint N.

[0098] For example, in the case that the voltage size of the initial modulation signal of the first bridge arm is greater than the voltage size of the initial modulation signal of the second bridge arm and less than the voltage size of the initial modulation signal of the third bridge arm, assuming that the weighted current coefficient of the first bridge arm is equal to 1, the weighted output current absolute value of the first bridge arm is equal to the output current absolute value of the first bridge arm. At this time, as long as the output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm, the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm. Therefore, the controller controls the output voltage of the third bridge arm to be equal to the voltage of the DC bus midpoint when the output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm.

[0099] Further, if the controller detects that the voltage of the DC bus midpoint N exceeds the target voltage range, the controller adjusts the weighted current coefficient of the first bridge arm from 1 to 0.8. It can be understood that the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm or the third bridge arm only when the output current absolute value of the first bridge arm is greater than 1.25 times the output current absolute value of the second bridge arm or the third bridge arm. Therefore, the length of time during which the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm is reduced, and the conduction time of the output end of the first bridge arm to the DC bus midpoint N is reduced. Further, if the controller still detects that the voltage of the DC bus midpoint N exceeds the target voltage range, the controller adjusts the weighted current coefficient of the first bridge arm from 0.8 to 0.5. At this time, the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm or the third bridge arm only when the output current absolute value of the first bridge arm is greater than 2 times the output current absolute value of the second bridge arm or the third bridge arm. Therefore, the length of time during which the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm is further reduced, and the conduction time of the output end of the first bridge arm to the DC bus midpoint N is further reduced, thereby effectively reducing the voltage fluctuation of the DC bus midpoint N.

[0100] For the convenience of understanding, the following content takes the bridge arm a as an example, and is described in combination with Figure 8 and Figure 9 . Figure 8 A signal waveform diagram of the bridge arm provided by the embodiment of the present application, Figure 9 Another signal waveform diagram of the bridge arm provided by the embodiment of the present application.

[0101] Assuming that the output current of the bridge arm a leads the output voltage of the bridge arm a, and the cosine value of the phase difference between the output current of the bridge arm a and the output voltage of the bridge arm a is equal to 0, that is, the power factor of the power conversion device is equal to 0. At the same time, the output current of the bridge arm a also leads the initial modulation signal of the bridge arm a, and the cosine value of the phase difference between the output current of the bridge arm a and the initial modulation signal of the bridge arm a is equal to 0. At this time, assuming that the weighted current coefficient of the bridge arm a is equal to 1, the signal waveforms of the bridge arm a can be referred to as Figure 8 . As shown in Figure 8 , in the a1 stage and the b1 stage, the absolute value of the output current of the bridge arm a is large, the controller regards the bridge arm a as the first bridge arm, and adjusts the modulation voltage signal of the bridge arm a (that is, the first modulation voltage signal of the first bridge arm) to be equal to the first modulation threshold value 0V, so as to control the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint, and the bridge arm a keeps the switching state unchanged, thereby reducing the switching loss of the power conversion device.

[0102] When the controller detects that the voltage of the DC bus midpoint N is out of the target voltage range, assuming that the controller reduces the weighted current coefficient of the bridge arm a from 1 to 0.6, the signal waveforms of the bridge arm a can be referred to as Figure 9 . As shown in Figure 9 , in the a2 stage and the b2 stage, the absolute value of the output current of the bridge arm a is large, the controller regards the bridge arm a as the first bridge arm, and adjusts the modulation voltage signal of the bridge arm a (that is, the first modulation voltage signal of the first bridge arm) to be equal to the first modulation threshold value 0V, so as to control the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint, and the bridge arm a keeps the switching state unchanged, thereby reducing the switching loss of the power conversion device. Obviously, Figure 9 , the time length of the a2 stage and the b2 stage shown in Figure 8 is obviously smaller than the time length of the a1 stage and the b1 stage shown in, that is, the time length of the controller controlling the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint is reduced. Therefore, the controller can reduce the conduction time of the output end of the first bridge arm and the DC bus midpoint N by adjusting the weighted current coefficient to reduce, and further effectively reduce the voltage fluctuation of the DC bus midpoint N.

[0103] In some feasible embodiments, during the process of the power conversion device supplying power to the power grid or load, if the controller detects that the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the second bridge arm and greater than the absolute value of the output current of the third bridge arm, and the voltage magnitude of the initial modulation signal of the first bridge arm is greater than the voltage magnitude of the initial modulation signal of the second bridge arm and less than the voltage magnitude of the initial modulation signal of the third bridge arm, and at the same time, it is also detected that the voltage magnitude of the initial modulation signal of the second bridge arm is less than the first voltage threshold and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, then the controller adjusts the output voltage of the second bridge arm to be equal to the negative pole voltage of the DC bus.

[0104] Wherein, the first voltage threshold refers to the lower limit value of the modulation voltage signal (such as the second modulation voltage signal) of the second bridge arm when the controller adjusts the output voltage of the first bridge arm to be equal to the voltage at the midpoint of the DC bus. Specifically, when the controller synchronously injects the first harmonic signal into the initial modulation signals of the first bridge arm, the second bridge arm, and the third bridge arm to adjust the output voltage of the first bridge arm to be equal to the voltage at the midpoint of the DC bus, if the initial modulation signal of the second bridge arm is less than the first voltage threshold, the voltage of the initial modulation signal of the second bridge arm after superposition with the first harmonic signal will be less than the voltage of the carrier signal, resulting in overmodulation of the second bridge arm. Therefore, when it is detected that the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold, the controller cannot control the output voltage of the first bridge arm to be equal to the voltage at the midpoint of the DC bus.

[0105] In some feasible implementations, the calculation method of the first voltage threshold may refer to satisfying the following formula (1):

[0106] V1=V min -(V0-V a )Formula (1)

[0107] Among them, V min is the lower limit of the working voltage range of the modulation voltage signal, V0 is the first modulation threshold, V a The lower limit of the operating voltage range of the modulated voltage signal can represent the lower limit of the voltage of the carrier signal. Optionally, the lower limit of the modulated voltage signal is equal to the lower limit of the voltage of the carrier signal - 1 volt.

[0108] For example, assume that the voltage magnitude of the initial modulation signal of the first bridge arm is equal to 0.4 volts, the voltage magnitude of the initial modulation signal of the second bridge arm is equal to -0.7 volts, the voltage magnitude of the initial modulation signal of the third bridge arm is equal to 0.6 volts, the first modulation threshold is equal to 0 volts, and the lower limit value of the modulation voltage signal is -1 volt. At this time, if the controller needs to control the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint, the controller injects a first harmonic signal with a voltage magnitude equal to -0.4 volts into the initial modulation signal of the first bridge arm. After superimposing the first harmonic signal and the initial modulation signal of the first bridge arm, a first modulation voltage signal with a voltage magnitude equal to the first modulation threshold of 0 volts is obtained. At the same time, the controller injects a first harmonic signal with a voltage magnitude equal to -0.4 volts into the initial modulation signal of the second bridge arm. After superimposing the first harmonic signal and the initial modulation signal of the second bridge arm, a modulation voltage signal with a voltage magnitude equal to -1.1 volts is obtained. At this time, the modulation voltage signal of the second bridge arm exceeds the lower limit value of the modulation voltage signal of -1 volt, which will cause over-modulation of the second bridge arm, thereby affecting the AC waveform supplied by the second bridge arm to the power grid or load.

[0109] In some possible embodiments, during the process in which the power conversion device supplies power to the power grid or load, if the controller detects that the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, the voltage magnitude of the initial modulation signal of the first bridge arm is greater than the voltage magnitude of the initial modulation signal of the second bridge arm and less than the voltage magnitude of the initial modulation signal of the third bridge arm, and further detects that the voltage magnitude of the initial modulation signal of the third bridge arm is greater than the second voltage threshold and the output current absolute value of the second bridge arm is greater than the output current absolute value of the third bridge arm, the controller adjusts the output voltage of the second bridge arm to be equal to the negative voltage of the DC bus.

[0110] In some possible embodiments, the second voltage threshold refers to the upper limit value of the modulation voltage signal of the third bridge arm when the controller adjusts the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint. Specifically, when the controller synchronously injects a first harmonic signal into the initial modulation signals of the first bridge arm, the second bridge arm, and the third bridge arm to adjust the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint, if the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the voltage magnitude of the initial modulation signal of the third bridge arm after superimposing the first harmonic signal will exceed the voltage magnitude of the carrier signal, which will cause over-modulation of the third bridge arm. Therefore, when it is detected that the voltage magnitude of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the controller cannot control the output voltage magnitude of the first bridge arm to be equal to the voltage of the DC bus midpoint.

[0111] In some possible embodiments, the second voltage threshold can be calculated by satisfying the following formula (2):

[0112] V2 = V max - (V0- V a ) Formula (2)

[0113] wherein, V max is the upper limit value of the operating voltage range of the modulation voltage signal, V0is the first modulation threshold value, V a is the voltage magnitude of the initial modulation voltage signal of the first bridge arm. The upper limit value of the operating voltage range of the modulation voltage signal can represent the voltage upper limit value of the carrier signal. Optionally, the upper limit value of the modulation voltage signal is equal to the voltage upper limit value 1 volt of the carrier signal.

[0114] In general, in the process of power conversion device supplying power to the grid or load, if the controller detects that the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, the voltage magnitude of the initial modulation signal of the first bridge arm is greater than the voltage magnitude of the initial modulation signal of the second bridge arm and less than the voltage magnitude of the initial modulation signal of the third bridge arm, and at the same time, it is also detected that the voltage magnitude of the initial modulation signal of the second bridge arm is less than the first voltage threshold value or the voltage magnitude of the initial modulation signal of the third bridge arm is greater than the second voltage threshold value, the output current absolute value of the second bridge arm is greater than the output current absolute value of the third bridge arm, the controller adjusts the output voltage of the second bridge arm to be equal to the negative voltage of the DC bus.

[0115] wherein, in the case of the output current absolute value of the second bridge arm being greater than the output current absolute value of the third bridge arm, the current absolute value flowing through the switch tube of the second bridge arm is relatively large, and the switching loss generated by the second bridge arm is greater than the switching loss generated by the third bridge arm. Therefore, the controller adjusts the output voltage of the second bridge arm to be equal to the negative voltage of the DC bus, so that the switching state of the second bridge arm remains unchanged, and the switching loss of the second bridge arm can be reduced.

[0116] In some possible embodiments, the controller can obtain a second modulation voltage signal by injecting a second harmonic signal into the initial modulation signal of the second bridge arm, and adjust the output voltage of the second bridge arm to be equal to the negative voltage of the DC bus according to the second modulation voltage signal. Wherein, the voltage magnitude of the second harmonic signal is negatively correlated with the voltage magnitude of the initial modulation signal of the second bridge arm. When the controller generates the pulse width modulation signal of the second bridge arm according to the second modulation voltage signal, and controls the switch tube in the second bridge arm to act according to the pulse width modulation signal, the output end of the second bridge arm is conducted with the negative pole BUS- of the DC bus, and the output voltage of the second bridge arm is equal to the negative voltage of the DC bus.

[0117] In some possible embodiments, the voltage size of the second harmonic signal is equal to the second modulation threshold minus the voltage size of the initial modulation signal of the second bridge arm. The second modulation threshold is a reference threshold value preset in the controller. For example, the second modulation threshold can be equal to -1 volt. After the controller injects the second harmonic signal into the initial modulation signal of the second bridge arm, the second harmonic signal with the voltage size equal to the second modulation threshold can be obtained. Further, the controller generates the pulse width modulation signal of the second bridge arm according to the second harmonic signal with the voltage size equal to the second modulation threshold, so that the output end of the second bridge arm is conductive to the negative pole BUS- of the DC bus when the switch tube in the first bridge arm is actuated, and the output voltage of the second bridge arm is equal to the negative pole voltage of the DC bus.

[0118] It can be understood that, while the controller injects the second harmonic signal into the initial modulation signal of the second bridge arm, the controller synchronously injects the first harmonic signal into the initial modulation signals of the first bridge arm and the third bridge arm, so that the first harmonic signals can be offset to each other and thus do not affect the sine wave alternating current waveform provided to the power grid or the load. In addition, the specific implementation of the controller injecting the second harmonic signal into the initial modulation signal of the second bridge arm can refer to the specific implementation of the controller injecting the first harmonic signal into the initial modulation signal of the first bridge arm, which will not be described herein again.

[0119] In some possible embodiments, during the power conversion device supplies power to the power grid or the load, if the controller detects that the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the second bridge arm and greater than the absolute value of the output current of the third bridge arm, the voltage size of the initial modulation signal of the first bridge arm is greater than the voltage size of the initial modulation signal of the second bridge arm and less than the voltage size of the initial modulation signal of the third bridge arm, and the controller further detects that the voltage size of the initial modulation signal of the second bridge arm is less than the first voltage threshold value or the voltage size of the initial modulation signal of the third bridge arm is greater than the second voltage threshold value, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, the controller adjusts the output voltage of the third bridge arm to be equal to the positive pole voltage of the DC bus.

[0120] As can be seen from the above, the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, which means that if the controller adjusts the output voltage of the first bridge arm to be equal to the voltage of the midpoint of the DC bus, over-modulation will occur in the second bridge arm or the third bridge arm. Meanwhile, in the case that the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, the absolute value of the current flowing through the switch tube of the third bridge arm is relatively large, and thus the switching loss generated by the second bridge arm is less than the switching loss generated by the third bridge arm. Therefore, the controller adjusts the output voltage of the third bridge arm to be equal to the positive electrode voltage of the DC bus, so that the switching state of the third bridge arm remains unchanged, and the switching loss of the third bridge arm can be reduced.

[0121] In some possible implementation manners, the controller can inject a third harmonic signal into the initial modulation signal of the third bridge arm to obtain a third modulation voltage signal, and adjust the output voltage of the third bridge arm to be equal to the positive electrode voltage of the DC bus according to the third modulation voltage signal. The voltage of the third harmonic signal is negatively correlated with the voltage of the initial modulation signal of the third bridge arm. When the controller generates a pulse width modulation signal of the third bridge arm according to the third modulation voltage signal, and controls the switch tube in the third bridge arm to act according to the pulse width modulation signal, the output end of the third bridge arm is connected to the negative electrode BUS- of the DC bus, and the output voltage of the third bridge arm is equal to the positive electrode voltage of the DC bus.

[0122] In some possible implementation manners, the voltage of the third harmonic signal is equal to the third modulation threshold minus the voltage of the initial modulation signal of the third bridge arm. The third modulation threshold is a reference threshold preset in the controller. For example, the third modulation threshold can be equal to 1 volt. When the controller injects the third harmonic signal into the initial modulation signal of the third bridge arm, the third harmonic signal with the voltage equal to the third modulation threshold can be obtained. Further, the controller generates a pulse width modulation signal of the third bridge arm according to the third modulation voltage signal with the voltage equal to the third modulation threshold, so as to control the switch tube in the third bridge arm to act, and in this way, the output end of the third bridge arm is connected to the positive electrode BUS+ of the DC bus, and the output voltage of the third bridge arm is equal to the positive electrode voltage of the DC bus.

[0123] It can be understood that, while the controller injects the third harmonic signal into the initial modulation signal of the third bridge arm, the controller synchronously injects the third harmonic signal into the initial modulation signals of the first bridge arm and the second bridge arm, so that the third harmonic signals can offset each other, and thus will not affect the sine wave alternating current waveform provided to the power grid or the load. In addition, the specific implementation of the controller injecting the third harmonic signal into the initial modulation signal of the third bridge arm can refer to the specific implementation of the controller injecting the first harmonic signal into the initial modulation signal of the first bridge arm, which is not described herein again.

[0124] In some possible implementation manners, when the controller detects that the absolute value of the output current of the bridge arm a is greater than the absolute value of the output current of the bridge arm b and greater than the absolute value of the output current of the bridge arm c, if the voltage size of the initial modulation signal of the bridge arm a is greater than the voltage size of the initial modulation signal of the bridge arm b and the voltage size of the initial modulation signal of the bridge arm a is greater than the voltage size of the initial modulation signal of the bridge arm c, the controller adjusts the output voltage of the bridge arm a to remain equal to the positive electrode voltage of the DC bus. In the case where the output voltage of the bridge arm a remains equal to the positive electrode voltage of the DC bus, the bridge arm a keeps the switching state unchanged, and thus the switching loss of the bridge arm a can be reduced. Specifically, the specific implementation manner in which the controller adjusts the output voltage of the bridge arm a to remain equal to the positive electrode voltage of the DC bus can be referred to the foregoing content. In addition, the controller can control the output voltage of the bridge arm b or the bridge arm c to remain equal to the positive electrode voltage of the DC bus based on the foregoing content, and the embodiments of the present application will not be described herein again.

[0125] In some possible implementation manners, when the controller detects that the absolute value of the output current of the bridge arm a is greater than the absolute value of the output current of the bridge arm b and greater than the absolute value of the output current of the bridge arm c, if the voltage size of the initial modulation signal of the bridge arm a is less than the voltage size of the initial modulation signal of the bridge arm b and the voltage size of the initial modulation signal of the bridge arm a is less than the voltage size of the initial modulation signal of the bridge arm c. The controller adjusts the output voltage of the bridge arm a to remain equal to the negative electrode voltage of the DC bus. In the case where the output voltage of the bridge arm a remains equal to the negative electrode voltage of the DC bus, the bridge arm a keeps the switching state unchanged, and thus the switching loss of the bridge arm a can be reduced. Specifically, the specific implementation manner in which the controller adjusts the output voltage of the bridge arm a to remain equal to the negative electrode voltage of the DC bus can be referred to the foregoing content. In addition, the controller can control the output voltage of the bridge arm b or the bridge arm c to remain equal to the negative electrode voltage of the DC bus based on the foregoing content, and the embodiments of the present application will not be described herein again.

[0126] It should be noted that, in the embodiments of the present application, the power conversion device controls the bridge arm a, the bridge arm b or the bridge arm c to keep the switching state unchanged to reduce the switching loss through the foregoing specific implementation manners, and when the power factor of the power conversion device changes, the effect of reducing the switching loss can still be maintained well. For ease of understanding, the following content takes the bridge arm a as an example and is illustrated in combination with Figures 8 to 10 the foregoing content. Figure 10 Another signal waveform diagram of the bridge arm provided by the embodiments of the present application.

[0127] It can be known from the foregoing content that, assuming that the cosine value of the phase difference between the output current of the bridge arm a and the output voltage of the bridge arm a is equal to 0 and the weighted current coefficient of the bridge arm a is equal to 1, the various signal waveforms of the bridge arm a can be as shown in Figure 8 the foregoing content. Specifically, in the case where the cosine value of the phase difference between the output current of the bridge arm a and the output voltage of the bridge arm a is equal to 0 and the weighted current coefficient of the bridge arm a is equal to 1, the specific implementation manner in which the controller adjusts the output voltage of the bridge arm a to remain equal to the positive electrode voltage of the DC bus can be referred to the foregoing content. Figure 8In the a1 and b1 phases shown, the absolute value of the output current of arm a is larger. The controller uses arm a as the first arm and adjusts the modulation voltage signal of arm a to be equal to the first modulation threshold of 0 volts to control the output voltage of arm a to be equal to the voltage at the midpoint of the DC bus. Arm a maintains the switching state unchanged. Figure 8 In the c1 and d1 phases shown, the absolute value of the output current of arm a is larger. The controller uses arm a as the third arm and adjusts the modulation voltage signal of arm a to be equal to the third modulation threshold of 1 volt to control the output voltage of arm a to be equal to the positive voltage of the DC bus. Arm a maintains the switching state unchanged. Figure 8 In the stage e1 shown, the absolute value of the output current of bridge arm a is large. The controller uses bridge arm a as the second bridge arm and adjusts the first modulation voltage signal to be equal to the second modulation threshold -1 volt to control the output voltage of bridge arm a to be equal to the negative voltage of the DC bus. Bridge arm a maintains the switching state unchanged, thereby reducing switching losses.

[0128] Similarly, assuming that the cosine value of the phase difference between the output current of bridge arm a and the output voltage of bridge arm a is equal to 0, and the weighted current coefficient of bridge arm a is equal to 0.6, the signal waveforms of bridge arm a can be as follows: Figure 9 Specifically, Figure 9 In the a2 and b2 phases shown, the absolute value of the output current of arm a is larger. The controller uses arm a as the first arm and adjusts the modulation voltage signal of arm a to be equal to the first modulation threshold of 0 volts to control the output voltage of arm a to be equal to the voltage at the midpoint of the DC bus. Arm a maintains the switching state unchanged. Figure 9 In the c2 and d2 phases shown, the absolute value of the output current of arm a is larger. The controller uses arm a as the third arm and adjusts the modulation voltage signal of arm a to be equal to the third modulation threshold of 1 volt to control the output voltage of arm a to be equal to the positive voltage of the DC bus. Arm a maintains the switching state unchanged. Figure 9 In the e2 stage shown, the absolute value of the output current of bridge arm a is large. The controller uses bridge arm a as the third bridge arm and adjusts the modulation voltage signal of bridge arm a to be equal to the second modulation threshold -1 volt to control the output voltage of bridge arm a to be equal to the negative pole voltage of the DC bus. Bridge arm a maintains the switching state unchanged, thereby reducing switching losses.

[0129] Assume that the cosine value of the phase difference between the output current of bridge arm a and the output voltage of bridge arm a is equal to 0.5, that is, the power factor of the power conversion device is equal to -0.5. At the same time, since the phase of the initial modulation signal of bridge arm a is the same as the phase of the AC power at the first AC terminal i31, if the weighted current coefficient of bridge arm a is equal to 1, the waveforms of the various signals of bridge arm a can be as follows: Figure 10 Specifically, Figure 10In the a3 stage and the b3 stage, the output current of the bridge arm a has a large absolute value, the controller regards the bridge arm a as the first bridge arm, and adjusts the modulation voltage signal of the bridge arm a to be equal to the first modulation threshold value 0V, so as to control the output voltage of the bridge arm a to be equal to the voltage of the DC bus midpoint, and the bridge arm a keeps the switching state unchanged. Figure 10 In the c3 stage and the d3 stage, the output current of the bridge arm a has a large absolute value, the controller regards the bridge arm a as the third bridge arm, and adjusts the modulation voltage signal of the bridge arm a to be equal to the third modulation threshold value 1V, so as to control the output voltage of the bridge arm a to be equal to the positive voltage of the DC bus, and the bridge arm a keeps the switching state unchanged. Figure 10 In the e3 stage, the output current of the bridge arm a has a large absolute value, the controller regards the bridge arm a as the second bridge arm, and adjusts the modulation voltage signal of the bridge arm a to be equal to the second modulation threshold value -1V, so as to control the output voltage of the bridge arm a to be equal to the negative voltage of the DC bus, and the bridge arm a keeps the switching state unchanged, thereby reducing the switching loss.

[0130] It can be known that, under different power factors, the power conversion device provided by the embodiment of the present application can keep the switching state of the bridge arm a unchanged when the output current of the bridge arm a has a large absolute value, thereby reducing the switching loss of the power conversion device, and the effect of reducing the switching loss can be optimal under different power factors. Figures 8 to 10

[0131] In summary, the power conversion device provided by the embodiment of the present application is a three-phase three-level power conversion device. In the process of supplying power to the power grid or the load by the power conversion device, the controller detects the current output by each bridge arm, and in the case that the weighted output current of the first bridge arm is greater than the output current of the second bridge arm and greater than the output current of the third bridge arm, and the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm and less than the voltage of the initial modulation signal of the third bridge arm, the controller adjusts the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint. Among the three bridge arms, the current flowing through the switching tube of the first bridge arm has the largest absolute value, that is, the switching loss generated by the switching tube of the first bridge arm is the largest. At this time, the output voltage of the first bridge arm keeps being equal to the voltage of the DC bus midpoint, and the switching state of the first bridge arm keeps unchanged, thereby reducing the switching loss of the power conversion device, and the applicability is strong. In addition, when the power conversion device detects that the voltage of the DC bus midpoint exceeds the target voltage range, the controller adjusts the weighted current coefficient to be reduced, so as to reduce the length of time for adjusting the output voltage of the first bridge arm to be equal to the voltage of the DC bus midpoint, and further reduce the influence of the current of the first bridge arm on the voltage of the DC bus midpoint, and improve the stability of the voltage of the DC bus midpoint.

[0132] Please refer to Figure 11 , Figure 11 ​A flowchart of a control method of a power conversion device is provided in the embodiments of the present application. The control method of the power conversion device provided in the embodiments of the present application is applicable to Figures 2 to 10 the controller in the corresponding detailed description. Specifically, the control method of the power conversion device can include the following steps:

[0133] S101, obtaining the weighted output current absolute value of the first bridge arm, the output current absolute value of the second bridge arm, the output current absolute value of the third bridge arm, the initial modulation signal of the first bridge arm, the initial modulation signal of the second bridge arm, and the initial modulation signal of the third bridge arm.

[0134] It can be understood that the power conversion device provided in the embodiments of the present application can realize three-phase alternating current output through a three-phase power conversion bridge, and therefore the power conversion device is a three-phase power conversion device. Meanwhile, since the three-phase power conversion bridge is connected with the positive pole of the DC bus, the negative pole of the DC bus, and the midpoint of the DC bus respectively, the power conversion device can control the switching tubes in each bridge arm of the three-phase power conversion bridge, so that the voltage output by each bridge arm is equal to the voltage of the positive pole of the DC bus (i.e. the voltage of the positive pole of the DC bus), the voltage of the negative pole of the DC bus (i.e. the voltage of the negative pole of the DC bus), or the voltage of the midpoint of the DC bus (i.e. the voltage of the midpoint of the DC bus). As can be seen, the voltage output by each bridge arm during the operation of the power conversion device includes three levels, and therefore the power conversion device is a three-phase three-level power conversion device. Further, the weighted output current absolute value of the first bridge arm refers to the current value obtained by the controller after weighting calculation on the output current absolute value of the first bridge arm, the output current absolute value of the second bridge arm refers to the current value obtained by the controller after taking the absolute value of the output current of the second bridge arm, and the output current absolute value of the third bridge arm refers to the current value obtained by the controller after taking the absolute value of the output current of the third bridge arm.

[0135] The specific implementation of S101 can refer to the implementation of the controller in the above Figures 2 to 10 The embodiments of the present application will not be described here again.

[0136] S102, in the case that the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm, the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the third bridge arm, the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm, and the voltage of the initial modulation signal of the first bridge arm is less than the voltage of the initial modulation signal of the third bridge arm, the output voltage of the first bridge arm is adjusted to the voltage of the midpoint of the DC bus.

[0137] It can be understood that, during the operation of the power conversion device, if the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, it means that, among the three bridge arms, the current absolute value flowing through the switch tube of the first bridge arm is the largest, that is, the switching loss generated by the action of the switch tube of the first bridge arm is the largest. At this time, the power conversion device keeps the switching state of the first bridge arm unchanged by control, so that the switch tube of the first bridge arm does not act, which can minimize the switching loss of the power conversion device. Further, in the embodiment of the present application, the power conversion device can adjust the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus when the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm and less than the voltage of the initial modulation signal of the third bridge arm. At this time, the output voltage of the first bridge arm is kept as the voltage of the midpoint of the DC bus, and the switching state of the first bridge arm is unchanged, thereby reducing the switching loss of the power conversion device and having strong applicability.

[0138] The specific implementation of S102 can refer to the implementation of the controller in the above Figures 2 to 10 The specific implementation of S102 can refer to the implementation of the controller in the above

[0139] In an optional embodiment, the weighted output current absolute value is obtained by multiplying the output current absolute value of the first bridge arm by a weighted current coefficient. The weighted output current absolute value is directly proportional to the output current absolute value of the first bridge arm and the weighted current coefficient. The weighted current coefficient is greater than 0 and less than or equal to 1.

[0140] It can be understood that, after detecting the output current size of the first bridge arm, the controller can take the absolute value of the output current size of the first bridge arm to obtain the output current absolute value of the first bridge arm, and multiply the output current absolute value of the first bridge arm by a weighted current coefficient to obtain the weighted output current absolute value. The weighted current coefficient is greater than 0 and less than or equal to 1. It can be understood that, in the case where the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, the output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and the output current absolute value of the third bridge arm. For example, assuming that the weighted current coefficient is equal to 0.8, the weighted output current absolute value of the first bridge arm will be greater than the output current absolute value of the second bridge arm and the output current absolute value of the third bridge arm only when the output current absolute value of the first bridge arm is greater than 1.25 times the output current absolute value of the second bridge arm or the output current absolute value of the third bridge arm. As can be seen, when the weighted output current absolute value of the first bridge arm is greater than the output current absolute value of the second bridge arm and greater than the output current absolute value of the third bridge arm, the switching loss generated by the first bridge arm is the largest, and the controller controls the first bridge arm to keep the switching state unchanged, which can effectively reduce the switching loss and has high accuracy. In addition, the controller obtains the weighted output current of the first bridge arm in a simple and easy-to-implement manner.

[0141] In an optional embodiment, the method further comprises: in the case that the voltage of the DC bus midpoint exceeds the target voltage range, adjusting the weighted current coefficient to decrease.

[0142] It can be understood that, when the controller detects that the voltage of the DC bus midpoint exceeds the target voltage range, it means that the voltage of the DC bus midpoint fluctuates greatly. At this time, the controller adjusts the weighted current coefficient to decrease, which can reduce the length of time for adjusting the output voltage of the first bridge arm to the voltage of the DC bus midpoint, and thus reduce the influence of the current of the first bridge arm on the voltage of the DC bus midpoint. It should be noted that, the smaller the weighted current coefficient of the first bridge arm is, the smaller the weighted output current absolute value obtained by multiplying the output current absolute value of the first bridge arm by the weighted current coefficient is, and the smaller the length of time for which the weighted output current absolute value of the first bridge arm is greater than the output current absolute values of the second bridge arm and the third bridge arm is. Therefore, the smaller the length of time for which the controller adjusts the output voltage of the first bridge arm to the voltage of the DC bus midpoint is, and the lower the influence of the current of the first bridge arm on the voltage of the DC bus midpoint is. Therefore, in the case that the voltage of the DC bus midpoint exceeds the target voltage range, the controller can effectively reduce the voltage fluctuation of the DC bus midpoint by adjusting the weighted current coefficient to decrease, which has strong applicability and is easy to implement.

[0143] In an optional embodiment, adjusting the output voltage of the first bridge arm to the voltage of the DC bus midpoint specifically comprises: injecting a first harmonic signal into the initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, the voltage of the first harmonic signal being negatively correlated with the voltage of the initial modulation signal of the first bridge arm, the voltage of the first modulation voltage signal being equal to a first modulation threshold, and the first modulation threshold being a reference threshold; and controlling the first bridge arm to work based on the first modulation voltage signal to adjust the output voltage of the first bridge arm to the voltage of the DC bus midpoint.

[0144] It can be understood that, the voltage of the first harmonic signal is equal to the first modulation threshold minus the voltage of the initial modulation signal of the first bridge arm. The first modulation threshold is a reference threshold pre-set in the controller, and is used to represent the voltage of the first modulation voltage signal when the output voltage of the first bridge arm is equal to the voltage of the DC bus midpoint. For example, the first modulation threshold can be equal to 0 volt. After the controller injects the first harmonic signal into the initial modulation signal of the first bridge arm to generate the first modulation voltage signal, the controller controls the switching tube of the first bridge arm to act according to the first modulation voltage signal, so that the output end of the first bridge arm is connected to the DC bus midpoint, thereby making the output voltage of the first bridge arm equal to the voltage of the DC bus midpoint. In the embodiments of the present application, the controller has a simple implementation principle for adjusting the output voltage of the first bridge arm, and has strong reliability.

[0145] In an optional embodiment, the method further comprises: in the case that the voltage size of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage size of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, adjusting the output voltage size of the second bridge arm to the negative electrode voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the first modulation threshold and the voltage size of the initial modulation signal of the first bridge arm, and the first modulation threshold is the reference threshold.

[0146] It can be understood that the first voltage threshold refers to the lower limit value of the modulation voltage signal of the second bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus, and the second voltage threshold refers to the upper limit value of the modulation voltage signal of the third bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus. When the controller injects the first harmonic signal into the first bridge arm, the controller can synchronously inject the first harmonic signal into the initial modulation signals of the second bridge arm and the third bridge arm to offset the influence of the first harmonic signal. At this time, if the initial modulation signal of the second bridge arm is less than the first voltage threshold, the second bridge arm will have the problem of over-modulation. Wherein, the over-modulation of the second bridge arm refers to that the voltage of the initial modulation signal of the second bridge arm is too small after superimposing the first harmonic signal, which will cause the distortion of the alternating current waveform output by the second bridge arm. Similarly, if the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the initial modulation signal of the third bridge arm will also have the problem of over-modulation after injecting the first harmonic signal. Further, the controller also detects that the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, that is, the absolute value of the current flowing through the switch tube of the second bridge arm is larger, and the switching loss of the second bridge arm is greater than that of the third bridge arm. At this time, the controller controls the output voltage of the second bridge arm to the negative electrode voltage of the DC bus, and the second bridge arm keeps the switching state unchanged, which can reduce the switching loss of the power conversion device and avoid the problem of over-modulation, and the application scenarios are rich and the reliability is strong.

[0147] In an optional embodiment, adjusting the output voltage of the second bridge arm to the negative electrode voltage of the DC bus specifically comprises:

[0148] injecting a second harmonic signal into the initial modulation signal of the second bridge arm to obtain a second modulation voltage signal, the voltage size of the second harmonic signal being negatively correlated with the voltage size of the initial modulation signal of the second bridge arm; and controlling the second bridge arm to work based on the second modulation voltage signal to adjust the output voltage size of the second bridge arm to the negative electrode voltage of the DC bus.

[0149] It can be understood that the voltage size of the second harmonic signal is equal to the second modulation threshold minus the voltage size of the initial modulation signal of the second bridge arm. The second modulation threshold is a reference threshold preset in the controller, and is used to represent the voltage size of the second modulation voltage signal when the output voltage of the second bridge arm is equal to the negative voltage of the DC bus. For example, the second modulation threshold can be equal to -1 volt. After the controller injects the above-mentioned second harmonic signal into the initial modulation signal of the second bridge arm to generate the second modulation voltage signal, the switching tube of the second bridge arm is controlled according to the second modulation voltage signal, so that the output end of the second bridge arm is connected with the negative electrode of the DC bus, thereby making the output voltage of the second bridge arm equal to the negative voltage of the DC bus. In the embodiment of the application, the controller adjusts the output voltage of the second bridge arm, and the implementation principle is simple and the reliability is strong.

[0150] In an optional embodiment, the method further comprises: in the case that the voltage size of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage size of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, adjusting the output voltage of the third bridge arm to the positive voltage of the DC bus; wherein the first voltage threshold and the second voltage threshold are obtained from the working voltage range of the modulation voltage signal, the first modulation threshold and the voltage size of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

[0151] It can be understood that the first voltage threshold refers to the lower limit value of the modulation voltage signal of the second bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus, and the second voltage threshold refers to the upper limit value of the modulation voltage signal of the third bridge arm when the controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus. When the controller injects the first harmonic signal into the first bridge arm, the controller can synchronously inject the first harmonic signal into the initial modulation signal of the second bridge arm and the third bridge arm to offset the influence of the first harmonic signal. At this time, if the initial modulation signal of the second bridge arm is less than the first voltage threshold, the second bridge arm will have the problem of over-modulation. Wherein, the over-modulation of the second bridge arm refers to that the voltage of the initial modulation signal of the second bridge arm is too small after superimposing the first harmonic signal, which will cause the distortion of the alternating current waveform output by the second bridge arm. Similarly, if the initial modulation signal of the third bridge arm is greater than the second voltage threshold, the initial modulation signal of the third bridge arm will have the problem of over-modulation after injecting the first harmonic signal. Further, the controller also detects that the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, that is, the absolute value of the current flowing through the switch tube of the third bridge arm is larger, and the switching loss of the third bridge arm is greater than the switching loss of the second bridge arm. At this time, the controller controls the output voltage of the third bridge arm to the positive electrode voltage of the DC bus, and the second bridge arm keeps the switch state unchanged, which can reduce the switching loss of the power conversion device, avoid the problem of over-modulation, and has rich application scenarios and strong reliability.

[0152] In an optional embodiment, adjusting the output voltage of the third bridge arm to the positive electrode voltage of the DC bus specifically includes: injecting a third harmonic signal into the initial modulation signal of the third bridge arm to obtain a third modulation voltage signal, the voltage size of the third harmonic signal being negatively correlated with the voltage size of the initial modulation signal of the third bridge arm; and controlling the third bridge arm to work based on the third modulation voltage signal to adjust the output voltage size of the third bridge arm to the positive electrode voltage of the DC bus.

[0153] It can be understood that the voltage size of the third harmonic signal is equal to the third modulation threshold minus the voltage size of the initial modulation signal of the third bridge arm. The third modulation threshold is a reference threshold pre-set in the controller and used to represent the voltage size of the third modulation voltage signal when the output voltage of the third bridge arm is equal to the positive electrode voltage of the DC bus. For example, the third modulation threshold can be equal to 1 volt. After the controller injects the third harmonic signal into the initial modulation signal of the third bridge arm to generate the third modulation voltage signal, the switch tube of the third bridge arm is controlled to act according to the third modulation voltage signal, so that the output end of the third bridge arm is connected with the positive electrode of the DC bus, thereby making the output voltage of the third bridge arm equal to the positive electrode voltage of the DC bus. In the embodiment of the application, the controller has simple implementation principle and strong reliability in adjusting the output voltage of the third bridge arm.

[0154] In an optional embodiment, the first voltage threshold satisfies:

[0155] V1 = V min -(V0-V a )

[0156] The second voltage threshold satisfies:

[0157] V2 = V max -(V0-V a )

[0158] wherein V min is a lower limit value of a working voltage range of the modulation voltage signal, V max is an upper limit value of the working voltage range of the modulation voltage signal, V1 is the first modulation threshold, V a is a voltage size of the initial modulation signal of the first bridge arm; the modulation voltage signal is the second modulation voltage signal or the third modulation voltage signal, the second modulation voltage signal is obtained by injecting a second harmonic signal into the initial modulation signal of the second bridge arm by the controller, the third modulation voltage signal is obtained by injecting a third harmonic signal into the initial modulation signal of the third bridge arm by the controller, and the first modulation threshold is the reference threshold.

[0159] It can be understood that the controller can calculate the first voltage threshold and the second voltage threshold according to the lower limit value and the upper limit value of the working voltage range of the modulation voltage signal and the voltage size of the initial modulation signal of the first bridge arm and the first modulation threshold, and the calculation manner is simple, easy to implement and strong in applicability.

[0160] The above merely describes specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the above claims.

Claims

1. A power conversion device, characterized in that: The power conversion device is used to connect a DC source through the positive pole of the DC bus and the negative pole of the DC bus, and the AC end of the power conversion device is used to connect to a power grid or a load; the power conversion device includes a three-phase power conversion bridge, a controller, and a positive bus capacitor and a negative bus capacitor connected in series at the midpoint of the DC bus; the first end of the three-phase power conversion bridge and the positive bus capacitor are connected to the positive pole of the DC bus, the second end of the three-phase power conversion bridge and the negative bus capacitor are connected to the negative pole of the DC bus, and the third end of the three-phase power conversion bridge is connected to the midpoint of the DC bus; the controller is used to control the three-phase power conversion bridge based on the initial modulation signal of the three-phase power conversion bridge to convert the DC power provided by the DC source into AC power or convert the AC power input from the AC end into DC power, and the frequency and phase of the initial modulation signal of the three-phase power conversion bridge are the same as the frequency and phase of the AC power at the AC end; The three-phase power conversion bridge includes a first bridge arm, a second bridge arm and a third bridge arm; the controller is further configured to: When the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the second bridge arm, and the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the third bridge arm, and the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm, and the voltage of the initial modulation signal of the first bridge arm is less than the voltage of the initial modulation signal of the third bridge arm, the output voltage of the first bridge arm is adjusted to the voltage of the midpoint of the DC bus.

2. The power conversion device according to claim 1, characterized in that The weighted output current absolute value is obtained by the output current absolute value of the first bridge arm and the weighted current coefficient. The weighted output current absolute value is proportional to the output current absolute value of the first bridge arm and the weighted current coefficient; the weighted current coefficient is greater than 0 and less than or equal to 1.

3. The power conversion device according to claim 2, characterized in that: The controller is also used for: When the voltage at the midpoint of the DC bus exceeds the target voltage range, the weighted current coefficient is adjusted to decrease.

4. The power conversion device according to any one of claims 1 to 3, characterized in that: The controller adjusts the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus, specifically including: injecting a first harmonic signal into the initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, wherein the voltage of the first harmonic signal is negatively correlated with the voltage of the initial modulation signal of the first bridge arm; the voltage of the first modulation voltage signal is equal to a first modulation threshold, which is a reference threshold; The first bridge arm is controlled to operate based on the first modulation voltage signal to adjust the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus.

5. The power conversion device according to claim 1, characterized in that: The controller is also used for: When the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, adjusting the output voltage of the second bridge arm to the negative pole voltage of the DC bus; The first voltage threshold and the second voltage threshold are obtained by the operating voltage range of the modulation voltage signal, the first modulation threshold and the voltage magnitude of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

6. The power conversion device according to claim 5, characterized in that: The controller adjusts the output voltage of the second bridge arm to the negative voltage of the DC bus, specifically including: Injecting a second harmonic signal into the initial modulation signal of the second bridge arm to obtain a second modulation voltage signal, wherein the voltage of the second harmonic signal is negatively correlated with the voltage of the initial modulation signal of the second bridge arm; The second bridge arm is controlled to operate based on the second modulation voltage signal to adjust the output voltage of the second bridge arm to the negative pole voltage of the DC bus.

7. The power conversion device according to claim 1, characterized in that: The controller is also used for: When the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, adjusting the output voltage of the third bridge arm to the positive voltage of the DC bus; The first voltage threshold and the second voltage threshold are obtained by the operating voltage range of the modulation voltage signal, the first modulation threshold and the voltage magnitude of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

8. The power conversion device according to claim 7, characterized in that: The controller adjusts the output voltage of the third bridge arm to the positive voltage of the DC bus, specifically including: injecting a third harmonic signal into the initial modulation signal of the third bridge arm to obtain a third modulation voltage signal, wherein the voltage of the third harmonic signal is negatively correlated with the voltage of the initial modulation signal of the third bridge arm; The third bridge arm is controlled to operate based on the third modulation voltage signal to adjust the output voltage of the third bridge arm to the positive voltage of the DC bus.

9. The power conversion device according to any one of claims 6 to 8, characterized in that: The first voltage threshold satisfies: V1=V min -(V0-V a ) The second voltage threshold satisfies: V2=V max -(V0-V a ) Among them, V min is the lower limit of the working voltage range of the modulation voltage signal, V max is the upper limit of the working voltage range of the modulation voltage signal, V1 is the first modulation threshold, V a is the voltage magnitude of the initial modulation signal of the first bridge arm; the modulation voltage signal is a second modulation voltage signal or a third modulation voltage signal, the second modulation voltage signal is obtained by the controller injecting a second harmonic signal into the initial modulation signal of the second bridge arm, the third modulation voltage signal is obtained by the controller injecting a third harmonic signal into the initial modulation signal of the third bridge arm, and the first modulation threshold is a reference threshold.

10. A control method for a power conversion device, the power conversion device being configured to be connected to a DC source via a positive DC bus and a negative DC bus, and the AC end of the power conversion device being configured to be connected to a power grid or a load; the power conversion device comprising a three-phase power conversion bridge and a positive bus capacitor and a negative bus capacitor connected in series at a midpoint of the DC bus; a first end of the three-phase power conversion bridge and the positive bus capacitor being connected to the positive DC bus, a second end of the three-phase power conversion bridge and the negative bus capacitor being connected to the negative DC bus, and a third end of the three-phase power conversion bridge being connected to the midpoint of the DC bus; the frequency and phase of an initial modulation signal of the three-phase power conversion bridge being the same as the frequency and phase of the AC power at the AC end; in, The three-phase power conversion bridge includes a first bridge arm, a second bridge arm and a third bridge arm; characterized in that the method includes: Obtaining the absolute value of the weighted output current of the first bridge arm, the absolute value of the output current of the second bridge arm, the absolute value of the output current of the third bridge arm, the voltage of the initial modulation signal of the first bridge arm, the voltage of the initial modulation signal of the second bridge arm, and the voltage of the initial modulation signal of the third bridge arm; When the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the second bridge arm, and the absolute value of the weighted output current of the first bridge arm is greater than the absolute value of the output current of the third bridge arm, and the voltage of the initial modulation signal of the first bridge arm is greater than the voltage of the initial modulation signal of the second bridge arm, and the voltage of the initial modulation signal of the first bridge arm is less than the voltage of the initial modulation signal of the third bridge arm, the output voltage of the first bridge arm is adjusted to the voltage of the midpoint of the DC bus.

11. The method according to claim 10, characterized in that The weighted output current absolute value is obtained by the output current absolute value of the first bridge arm and the weighted current coefficient. The weighted output current absolute value is proportional to the output current absolute value of the first bridge arm and the weighted current coefficient; the weighted current coefficient is greater than 0 and less than or equal to 1.

12. The method according to claim 11, characterized in that The method further comprises: When the voltage at the midpoint of the DC bus exceeds the target voltage range, the weighted current coefficient is adjusted to decrease.

13. The method according to any one of claims 10 to 12, characterized in that The adjusting the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus specifically includes: injecting a first harmonic signal into the initial modulation signal of the first bridge arm to obtain a first modulation voltage signal, wherein the voltage of the first harmonic signal is negatively correlated with the voltage of the initial modulation signal of the first bridge arm; the voltage of the first modulation voltage signal is equal to a first modulation threshold, which is a reference threshold; The first bridge arm is controlled to operate based on the first modulation voltage signal to adjust the output voltage of the first bridge arm to the voltage of the midpoint of the DC bus.

14. The method according to claim 10, characterized in that The method further comprises: When the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is greater than the absolute value of the output current of the third bridge arm, adjusting the output voltage of the second bridge arm to the negative pole voltage of the DC bus; The first voltage threshold and the second voltage threshold are obtained by the operating voltage range of the modulation voltage signal, the first modulation threshold and the voltage magnitude of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

15. The method according to claim 10, characterized in that The method further comprises: When the voltage of the initial modulation signal of the second bridge arm is less than the first voltage threshold or the voltage of the initial modulation signal of the third bridge arm is greater than the second voltage threshold, and the absolute value of the output current of the second bridge arm is less than the absolute value of the output current of the third bridge arm, adjusting the output voltage of the third bridge arm to the positive voltage of the DC bus; The first voltage threshold and the second voltage threshold are obtained by the operating voltage range of the modulation voltage signal, the first modulation threshold and the voltage magnitude of the initial modulation signal of the first bridge arm, and the first modulation threshold is a reference threshold.

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