Power conversion equipment, control method thereof and micro-grid system

Abstract

The invention provides a power conversion device, a control method thereof and a micro-grid system, the power conversion device is used for accessing the micro-grid system, the micro-grid system comprises at least one reference power conversion device, and the power conversion device comprises an inverter circuit and a controller. The controller is used for controlling the inverter circuit to convert the direct current into alternating current and outputting the alternating current, and is also used for adjusting the ratio of active power to reactive power output by the inverter circuit under the condition that the voltage amplitude of the alternating current output by the inverter circuit is smaller than a first threshold value, and the difference between the ratio of the active power to the reactive power output by all the reference power conversion devices is smaller than a second threshold value. According to the power conversion equipment, when the micro-grid system enters the low-voltage ride-through stage, the output power is adjusted, so that the total output power of the multiple power conversion equipment is increased, the effect of maintaining the voltage stability of the alternating-current bus is improved, and the applicability is high.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and particularly to a power conversion device, a control method thereof, and a microgrid system. Background Art

[0002] A microgrid system is a small-scale localized power system integrating new energy power generation, energy storage systems, load management, and control technologies. The microgrid system has two operating modes: grid-connected and off-grid. The grid-connected mode means that under normal circumstances, the microgrid system is connected to the external power grid for operation, supplying excess electrical energy to the external power grid or supplying power to local loads from the external power grid. The off-grid mode means that when it is detected that the external power grid fails or the power quality does not meet the requirements, the microgrid system disconnects from the external power grid to form an island state, and multiple power conversion devices within the microgrid system convert direct current into alternating current to supply power to local loads.

[0003] When a short-circuit fault occurs in the microgrid system, causing the voltage of the AC bus to drop, the microgrid system enters the low-voltage ride-through stage. At this time, if the microgrid system is operating in grid-connected mode, the voltage of the AC bus is determined by the external power grid, and the voltage of the AC bus can be stabilized by the external power grid when the voltage drops. If the microgrid operates independently in off-grid mode, due to the lack of voltage provided by the external power grid, only multiple local power conversion devices can actively adjust the output power. For example, active power and reactive power are provided to the AC bus to maintain the stability of the AC bus voltage. However, the total output power of multiple local power conversion devices is usually relatively small, and the effect of maintaining the stability of the AC bus voltage is poor. Summary of the Invention

[0004] This application provides a power conversion device, a control method thereof, and a microgrid system. The power conversion device is applied in the microgrid system. The microgrid system includes multiple power conversion devices. When the microgrid system enters the low-voltage ride-through stage, each power conversion device can adjust the output power to increase the total output power of multiple power conversion devices, improving the effect of maintaining the stability of the AC bus voltage, and having strong applicability.

[0005] In a first aspect, this application provides a power conversion device for accessing a microgrid system. The microgrid system further includes at least one reference power conversion device. The power conversion device includes an inverter circuit and a controller. The controller is configured to control the inverter circuit to convert direct current from a DC source into alternating current and output it. The controller is further configured to: when the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjust the ratio of the active power to the reactive power output by the inverter circuit so that the difference between the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold.

[0006] In this embodiment, the microgrid system includes a plurality of power conversion devices, and the AC terminals of each power conversion device are electrically connected to the AC bus. When the voltage drop of the AC bus causes the microgrid system to enter the low-voltage ride-through stage, each power conversion device can operate in the grid-forming mode and actively adjust the output power to help maintain the voltage stability of the AC bus. For example, each power conversion device can provide active power and reactive power to the AC bus. It can be understood that since the AC terminal of the power conversion device is electrically connected to the AC bus, the voltage amplitude of the alternating current output by the power conversion device through this AC terminal is the same as the voltage amplitude of the AC bus. Therefore, the power conversion device can determine the voltage amplitude of the AC bus by detecting the voltage amplitude of the alternating current output by the inverter circuit through the controller. When the power conversion device detects through the controller that the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, it indicates that the voltage amplitude of the AC bus is less than this threshold. The first threshold refers to the maximum value of the voltage amplitude of the AC bus during the low-voltage ride-through stage, and this first threshold can be specifically obtained according to the rated voltage amplitude of the AC bus. Therefore, the voltage amplitude of the alternating current output by the inverter circuit being less than the first threshold indicates that the microgrid system enters the low-voltage ride-through stage. At this time, each power conversion device in the microgrid system can adjust its own output power to increase the total output power of the plurality of power conversion devices and improve the effect of maintaining the voltage stability of the AC bus. Further, the magnitude of the total output power of the plurality of power conversion devices is related to the ratio of the active power and reactive power output by each power conversion device. Specifically, compared with the inconsistent ratio of the active power and reactive power output by each power conversion device, when the ratio of the active power and reactive power output by each power conversion device is consistent, the total output power of the plurality of power conversion devices is greater. For this reason, each power conversion device can control the ratio of the active power and reactive power output by the inverter circuit to be consistent through the controller, thereby increasing the total output power. Exemplarily, the power conversion device and the reference power conversion device in the microgrid system both operate in the grid-forming mode, and the power conversion device is one in the microgrid system, and the reference power conversion device is other power conversion devices except this power conversion device. When the power conversion device detects through the controller that the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, the power conversion device adjusts the ratio of the active power and reactive power output by the inverter circuit through the controller, and the difference from the ratio of the active power and reactive power output by all reference power conversion devices is less than the second threshold. At this time, the difference between the ratio of the active power and reactive power output by the power conversion device and all reference power conversion devices can be ignored, that is, they are approximately the same.As can be seen from the above, when the ratio of the active power to the reactive power output by the power conversion device and all reference power conversion devices is the same, the total output power of the power conversion device and all reference power conversion devices is relatively large. Therefore, more active power and reactive power can be provided to the AC bus, thereby enhancing the effect of maintaining the stability of the AC bus voltage, and having strong applicability.

[0007] In a possible implementation, the ratio of the active power to the reactive power output by the inverter circuit is the first ratio, and the ratio of the active power to the reactive power output by the reference power conversion device is the second ratio; the controller adjusts the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices to be less than a second threshold, which specifically includes: adjusting the AC angular frequency output by the inverter circuit so that the AC angular frequency output by the inverter circuit is positively correlated with the first ratio and negatively correlated with the average power ratio until the difference between the first ratio and the second ratio is less than a first threshold; where the average power ratio is the average of the first ratio and the second ratios of all reference power conversion devices. In this embodiment, the magnitude of the first ratio is related to the AC angular frequency output by the inverter circuit in the power conversion device. Therefore, the power conversion device can adjust the AC angular frequency output by the inverter circuit through the controller to adjust the magnitude of the first ratio. At the same time, in order to make the first ratio and the second ratios of all reference power conversion devices approximately the same, the power conversion device can obtain the average of the first ratio and the second ratios of all reference power conversion devices as the average power ratio, and use this average power ratio as the target value of the first ratio when adjusting the AC angular frequency of the inverter circuit, so that the adjusted first ratio can be close to the average power ratio. Similarly, the reference power conversion device can also adjust the second ratio to be close to the average power ratio based on the same implementation manner, so that the difference between the first ratio and the second ratio is less than the first threshold, that is, approximately the same. The implementation principle of this embodiment is simple, and using the average power ratio for adjustment can make the difference between the first ratio and the second ratio decrease faster, that is, the adjustment speed is fast, and the applicability is strong.

[0008] In a possible implementation, the AC frequency output by the inverter circuit satisfies:

[0009]

[0010] where w0 is the rated angular frequency of the inverter circuit, is the average power ratio, is the first ratio, K iis the frequency droop coefficient of the inverter circuit. In this embodiment, the controller can calculate the reference value of the AC output frequency of the inverter circuit by obtaining the first ratio, the average power ratio of the power conversion device and all reference power conversion devices, and combining the rated angular frequency and the frequency droop coefficient of the inverter circuit. Based on the calculated reference value of the AC output frequency of the inverter circuit, the controller adjusts the AC angular frequency output by the inverter circuit, and can adjust the difference between the first ratio and the second ratio to be less than the first threshold. The calculation process is simple and easy to implement.

[0011] In a possible implementation manner, the controller is further configured to: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, and the current amplitude threshold of the inverter circuit is greater than the rated current amplitude of the inverter circuit, adjust the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit. In this embodiment, the magnitude of the power output by the power conversion device is positively correlated with the current amplitude output by the inverter circuit. Therefore, the power conversion device can increase the current amplitude output by the inverter circuit through the controller to increase the magnitude of the output power and further improve the effect of maintaining the AC bus voltage. The magnitude of the current amplitude output by the inverter circuit is limited by the rated current amplitude and the current amplitude threshold of the inverter circuit. Among them, the rated current amplitude refers to the upper limit of the current that the inverter circuit can withstand during operation, and the current amplitude threshold represents the maximum value of the current amplitude that the inverter circuit can output. It can be understood that if the current amplitude threshold of the inverter circuit is greater than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is greater than the rated current amplitude. At this time, the power conversion device controls the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude through the controller, which can avoid overcurrent damage of the inverter circuit, has high safety, and the output power of the power conversion device increases significantly.

[0012] In a possible implementation, the controller is further configured to: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, and the current amplitude threshold of the inverter circuit is less than the rated current amplitude of the inverter circuit, adjust the current amplitude output by the inverter circuit to be less than or equal to the current amplitude threshold of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit. In this embodiment, the magnitude of the power output by the power conversion device is positively correlated with the current amplitude output by the inverter circuit. Therefore, the power conversion device can increase the current amplitude output by the inverter circuit through the controller to increase the magnitude of the output power and further improve the effect of maintaining the AC bus voltage. The magnitude of the current amplitude output by the inverter circuit is limited by the rated current amplitude and the current amplitude threshold of the inverter circuit. Among them, the rated current amplitude refers to the upper limit of the current that the inverter circuit can withstand during operation, and the current amplitude threshold represents the maximum value of the current amplitude that the inverter circuit can output. It can be understood that if the current amplitude threshold of the inverter circuit is smaller than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is less than the rated current amplitude. At this time, the power conversion device controls the current amplitude output by the inverter circuit to be less than or equal to the current amplitude threshold through the controller, which can make the current amplitude output by the inverter circuit reach the maximum value. While the output power of the power conversion device increases significantly, it can avoid overcurrent damage to the power conversion device and has high safety.

[0013] In a possible implementation, the controller is further configured to: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, adjust the difference between the voltage phase angle output by the inverter circuit and the voltage phase angle output by all reference power conversion devices to be less than a third threshold. In this embodiment, when the difference between the voltage phase angles output by the power conversion device and all reference power conversion devices is less than a third threshold, it indicates that the voltage phase angles output by the power conversion device and all reference power conversion devices are approximately the same. At this time, there will be no voltage difference or the voltage difference is very small between the power conversion device and the reference power conversion device, which can reduce the generation of circulating current, thereby reducing the loss of the output power of the power conversion device and ensuring the safety of the device at the same time.

[0014] In a possible implementation, the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices adjusted by the controller is less than a third threshold, which specifically includes: when the difference between the angular frequency of the alternating current output by the inverter circuit and the angular frequencies of the alternating currents output by all reference power conversion devices is less than a fourth threshold, adjusting the voltage vector component in the synchronous rotating coordinate system to be equal to 0, so that the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices is less than the third threshold; wherein, the voltage vector component in the synchronous rotating coordinate system is the vector magnitude of the voltage signal output by the inverter circuit on the q-axis of the synchronous rotating coordinate system. In this embodiment, whether the voltage phase angles output by the power conversion device and the reference power conversion device are consistent can be represented by the directions of different voltage vectors in the synchronous rotating coordinate system. For example, the voltage signal output by the power conversion device can be converted into a first voltage vector in the synchronous rotating coordinate system, and the voltage signal output by the reference power conversion device can be converted into a second voltage vector in the synchronous rotating coordinate system. If the voltage phase angles output by the power conversion device and the reference power conversion device are inconsistent, then in the synchronous rotating coordinate system, the directions of the first voltage vector and the second voltage vector are different. On the contrary, the directions of the first voltage vector and the second voltage vector are the same. Therefore, the power conversion device can adjust the first voltage vector in the synchronous rotating coordinate system to coincide with the d-axis through the controller, and the reference power conversion device also adjusts the second voltage vector in the synchronous rotating coordinate system to coincide with the d-axis, so that the directions of the first voltage vector and the second voltage vector both coincide with the d-axis, and the voltage phase angles output by the power conversion device and the reference power conversion device are kept consistent. In addition, since the voltage vector component of the first voltage vector on the q-axis of the synchronous rotating coordinate system is equal to 0 when the first voltage vector coincides with the d-axis. Therefore, the power conversion device can detect whether the voltage vector component of the first voltage vector on the q-axis of the synchronous rotating coordinate system is equal to 0 through the controller, and then determine whether the first voltage vector coincides with the d-axis.

[0015] In a possible implementation, the magnitude of the voltage vector component in the synchronous rotating coordinate system is negatively correlated with the magnitude of the current vector angle. The current vector angle is the angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis. The controller adjusts the voltage vector component in the synchronous rotating coordinate system to be equal to 0, which specifically includes: adjusting the current vector angle to increase, so that the voltage vector component in the synchronous rotating coordinate system decreases until the voltage vector component in the synchronous rotating coordinate system is equal to 0. In this embodiment, when the voltage signal output by the power conversion device is represented as the first voltage vector in the synchronous rotating coordinate system and the current signal output by the power conversion device is represented as the first current vector in the synchronous rotating coordinate system, the direction of the first voltage vector is related to the direction of the first current vector. Specifically, when the angle between the first current vector and the d-axis (i.e., the current vector angle) increases, the angle between the first voltage vector and the d-axis will decrease, thereby causing the voltage vector component of the first voltage vector on the q-axis to decrease. Therefore, the power conversion device can adjust the current vector angle in the synchronous rotating coordinate system through the controller, so that the voltage vector component decreases until the voltage vector component is equal to 0 and the first voltage vector coincides with the d-axis.

[0016] In a possible implementation, the current vector angle satisfies:

[0017]

[0018] where δ i is the current vector angle, δ0 is the initial angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis, E q is the voltage vector component in the synchronous rotating coordinate system, and w Δi is the rotational angular frequency reference coefficient. In this embodiment, when the voltage signal output by the power conversion device is represented as the first voltage vector in the synchronous rotating coordinate system and the current signal output by the power conversion device is represented as the first current vector in the synchronous rotating coordinate system, the relationship between the magnitude of the angle between the first current vector and the d-axis (i.e., the current vector angle) and the voltage vector component of the first voltage vector on the q-axis satisfies the above formula. Therefore, the controller can adjust the voltage vector component of the first voltage vector on the q-axis by adjusting the magnitude of the current vector angle, and the implementation principle is simple and the applicability is strong.

[0019] In a second aspect, the present application also provides a microgrid system. The microgrid system includes a power conversion device and at least one reference power conversion device. The power conversion device includes an inverter circuit and a controller. The controller is configured to control the inverter circuit to convert direct current from a DC source into alternating current and output it. The controller is further configured to: when the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjust the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices to be less than a second threshold.

[0020] In a third aspect, the present application also provides a control method for a power conversion device. The power conversion device is configured to be connected to a microgrid system, and the microgrid system further includes at least one reference power conversion device. The power conversion device includes an inverter circuit. The controller is configured to control the inverter circuit to convert direct current from a DC source into alternating current and output it. The method includes: obtaining the voltage amplitude of the alternating current output by the inverter circuit; when the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjusting the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices to be less than a second threshold.

[0021] In a possible implementation, the ratio of the active power to the reactive power output by the inverter circuit is a first ratio, and the ratio of the active power to the reactive power output by the reference power conversion device is a second ratio; adjusting the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices to be less than a second threshold specifically includes: adjusting the angular frequency of the alternating current output by the inverter circuit such that the angular frequency of the alternating current output by the inverter circuit is positively correlated with the first ratio and negatively correlated with the average power ratio until the difference between the first ratio and the second ratio is less than a first threshold; wherein, the average power ratio is the average of the first ratio and the second ratios of all reference power conversion devices.

[0022] In a possible implementation, the alternating current frequency output by the inverter circuit satisfies:

[0023]

[0024] wherein, w0 is the rated angular frequency of the inverter circuit, is the average power ratio, is the first ratio, K i is the frequency droop coefficient of the inverter circuit.

[0025] In a possible implementation, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, and the current amplitude threshold of the inverter circuit is greater than the rated current amplitude of the inverter circuit, adjusting the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit.

[0026] In a possible implementation, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, and the current amplitude threshold of the inverter circuit is less than the rated current amplitude of the inverter circuit, adjusting the current amplitude output by the inverter circuit to be less than or equal to the current amplitude threshold of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit.

[0027] In a possible implementation, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, adjusting the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices to be less than a third threshold.

[0028] In a possible implementation, adjusting the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices to be less than a third threshold specifically includes: when the difference between the AC angular frequency output by the inverter circuit and the AC angular frequencies output by all reference power conversion devices is less than a fourth threshold, adjusting the voltage vector component in the synchronous rotating coordinate system to be equal to 0, so that the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices is less than a third threshold; wherein, the voltage vector component in the synchronous rotating coordinate system is the vector magnitude of the voltage signal output by the inverter circuit on the q-axis of the synchronous rotating coordinate system.

[0029] In a possible implementation, the magnitude of the voltage vector component in the synchronous rotating coordinate system is negatively correlated with the magnitude of the current vector angle, and the current vector angle is the angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis; adjusting the voltage vector component in the synchronous rotating coordinate system to be equal to 0 specifically includes: adjusting the current vector angle to increase, so that the voltage vector component in the synchronous rotating coordinate system decreases until the voltage vector component in the synchronous rotating coordinate system is equal to 0.

[0030] In a possible implementation, the current vector angle satisfies:

[0031]

[0032] where δ i is the current vector angle, δ0 is the initial angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis, E q is the voltage vector component in the synchronous rotating coordinate system, and w Δi is the rotational angular frequency reference coefficient.

[0033] The solutions provided in the second and third aspects above are used to implement or cooperate with the implementation of the power conversion device provided in the corresponding first aspect. Therefore, the same or corresponding beneficial effects can be achieved as those of the corresponding power conversion device in the first aspect, and will not be elaborated here. BRIEF DESCRIPTION OF THE DRAWINGS[[ID=I9]]

[0034] Figure 1 is a schematic diagram of an application scenario of the microgrid system provided in this application;

[0035] Figure 2 is a schematic diagram of an application scenario of the power conversion device provided in an embodiment of this application;

[0036] Figure 3a is a schematic diagram of power synthesis provided in an embodiment of this application;

[0037] Figure 3b is another schematic diagram of power synthesis provided in an embodiment of this application;

[0038] Figure 3c is yet another schematic diagram of power synthesis provided in an embodiment of this application;

[0039] Figure 4 is a schematic diagram of angular frequency droop provided in an embodiment of this application;

[0040] [[ID=The]] Figure 5 is a schematic diagram of an application scenario of the controller provided in an embodiment of this application;

[0041] Figure 6a is a schematic diagram of a power ratio curve provided in an embodiment of this application;

[0042] Figure 6b is a schematic diagram of a difference curve provided in an embodiment of this application;

[0043] Figure 6c is a schematic diagram of an alternating current angular frequency curve provided in an embodiment of this application;

[0044] Figure 7Another power combination schematic diagram provided by the embodiment of the present application;

[0045] Figure 8a A schematic diagram of a synchronous rotating coordinate system provided by the embodiment of the present application;

[0046] Figure 8b Another schematic diagram of a synchronous rotating coordinate system provided by the embodiment of the present application;

[0047] Figure 9 Another application scenario schematic diagram of the controller provided by the embodiment of the present application;

[0048] Figure 10 A flowchart schematic diagram of a control method for a power conversion device provided by the embodiment of the present application. Detailed implementation manners

[0049] The microgrid system provided by the present application can be applied to different application scenarios, such as photovoltaic-storage power supply application scenarios, wind-storage power supply application scenarios, pure energy storage power supply application scenarios, or other application scenarios, etc. Hereinafter, the microgrid system will be described by taking the photovoltaic-storage power supply application scenario as an example.

[0050] As a small power system, the microgrid system can rely on internal distributed power sources and energy storage devices to supply power to local loads independently when the external power grid fails. Exemplarily, in remote areas or on islands, the microgrid system uses photovoltaic devices and energy storage devices to supply power to local loads, thereby realizing off-grid power supply and solving the problem of difficult grid extension. In addition, the microgrid system can also operate in parallel with the external power grid. Specifically, please refer to Figure 1 , Figure 1 An application scenario schematic diagram of the microgrid system provided by the present application. As Figure 1 shown, the microgrid system includes a grid connection switch, and the microgrid system is connected to the external power grid through this grid connection switch. When the grid connection switch is closed, the microgrid system operates synchronously with the external power grid, and can transmit electric energy to the external power grid or receive electric energy from the external power grid for utilization. For example, the microgrid system can transmit the surplus electric energy generated by photovoltaic power generation during the day with strong sunlight to the external power grid to improve the utilization efficiency of electric energy, and obtain electric energy from the external power grid when the photovoltaic power generation is insufficient at night to meet the power consumption needs of local loads. When the external power grid fails or disconnects from the grid according to a preset time, the grid connection switch is disconnected, and the microgrid system operates independently off-grid.

[0051] In Figure 1 the application scenario shown, the microgrid system can supply power to the load or the external power grid jointly through an energy storage inverter and a photovoltaic inverter. As Figure 1As shown in the figure, the microgrid system includes an energy storage converter and a photovoltaic inverter. Among them, the DC side of the energy storage converter is connected to the energy storage battery, and the AC side of the energy storage converter is connected to the AC bus of the microgrid system. The energy storage converter converts the DC power from the energy storage battery into AC power and outputs it to the AC bus to supply power to the load or the external power grid. At the same time, when the energy storage converter operates in the grid-forming mode, it can adjust the amplitude, frequency, and waveform of the output voltage to stabilize the voltage of the AC bus. The DC side of the photovoltaic inverter is connected to the photovoltaic module, and the AC side of the photovoltaic inverter is connected to the AC bus. The photovoltaic inverter converts the DC power from the photovoltaic module into AC power and outputs it to the AC bus to supply power to the load or the external power grid. At the same time, the photovoltaic inverter can also operate in the grid-forming mode to help stabilize the voltage of the AC bus. The energy storage converter and the inverter are both used to provide functions such as power conversion in the microgrid system, and both can operate in the grid-forming mode. The energy storage converter and the photovoltaic inverter can be collectively referred to as power conversion devices.

[0052] In practical applications, a box-type transformer is also set in the microgrid system. The box-type transformer is used to adjust and distribute the input AC voltage and then output it to meet the voltage requirements of different power transmission nodes. Exemplarily, the amplitude of the AC voltage transmitted by the AC bus is usually relatively high. Therefore, the power conversion device can transmit the AC power to the box-type transformer for boosting and then supply it to the AC bus. In addition, the AC power transmitted on the AC bus can be stepped down by the box-type transformer and supplied to the load. Or, if the rated voltage amplitude of the load is suitable for the AC voltage amplitude transmitted on the AC bus, the AC bus can also directly supply power to the load. When the grid connection switch is closed and the microgrid system operates in parallel, the AC power transmitted on the AC bus can be boosted by one or more box-type transformers and then output to the external power grid. The specific installation location and quantity 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.

[0053] It should be noted that in Figure 1In the application scenario shown, when a short - circuit fault occurs in the micro - grid system, the AC bus of the micro - grid system is equivalent to being connected to the ground wire through a resistor with a relatively small resistance, resulting in a voltage drop of the AC bus. Generally, when the voltage amplitude of the AC bus drops below 90% of the rated voltage amplitude, the micro - grid system enters the low - voltage ride - through stage. At this time, if the micro - grid system is operating in parallel, the voltage of the AC bus is determined by the external power grid, and the voltage of the AC bus can be stabilized by the external power grid when the voltage drops. However, when the bus voltage of the external power grid is severely distorted, the voltage of the AC bus cannot be stabilized by the external power grid, and only multiple local power conversion devices can actively adjust the output power. For example, provide active power and reactive power to the AC bus to maintain the bus voltage stability and prevent large - scale disconnection from the grid. Or, when the micro - grid system operates independently off - grid, due to the lack of voltage provided by the external power grid, only multiple local power conversion devices can adjust the output power to maintain the bus voltage stability. However, the total output power of multiple local power conversion devices is usually relatively small, resulting in poor voltage stability of the AC bus.

[0054] For this reason, the present application provides a power conversion device. This power conversion device is applied to the micro - grid system. When the voltage of the AC bus of the micro - grid system drops and the micro - grid system enters the low - voltage ride - through stage, each power conversion device in the micro - grid system can adjust the output power, thereby increasing the total output power of multiple power conversion devices and further improving the voltage stability of the AC bus.

[0055] The above is only an example of the application scenario of the power conversion device provided by the present application, not an exhaustive list, and the present application does not limit the application scenario.

[0056] In the present application, the micro - grid system includes multiple power conversion devices. When the micro - grid system enters the low - voltage ride - through stage, the specific implementation methods of each power conversion device for adjusting the output power can be the same. For the convenience of understanding, the following content will illustrate the implementation method of a power conversion device in the micro - grid system for adjusting the output power during the low - voltage ride - through stage, and use the other power conversion devices except this power conversion device as reference power conversion devices. It can be understood that the implementation method of the reference power conversion device for adjusting the output power during the low - voltage ride - through stage can refer to the specific implementation method of the power conversion device, and the present application will not elaborate on this.

[0057] Please refer to Figure 2 , Figure 2 which is a schematic diagram of an application scenario of the power conversion device provided by the embodiment of the present application. As Figure 2As shown, the power conversion device is connected in parallel with the reference power conversion device to the common connection point PCC, and the AC terminal of the power conversion device is electrically connected to the AC bus of the microgrid system. Among them, the power conversion device includes a controller and an inverter circuit, and the inverter circuit is used to convert the DC power input at the DC terminal into AC power and output it through the AC terminal. Among them, the number of reference power conversion devices can be one or more, Figure 2 The illustration is only an example.

[0058] From the above, it can be seen that when the voltage amplitude of the AC bus drops, causing the microgrid system to enter the low voltage ride-through stage, the total output power of the power conversion device and the reference power conversion device is small, resulting in a poor effect of maintaining the voltage stability of the AC bus. Therefore, in order to timely control the voltage stability of the AC bus, the power conversion device can detect the voltage amplitude of the AC bus through the controller to determine whether the microgrid system enters the low voltage ride-through stage.

[0059] Specifically, the power conversion device can determine that the microgrid system enters the low voltage ride-through stage when it detects through the controller that the voltage amplitude of the AC bus drops below a first threshold. Among them, the first threshold can be obtained from the rated voltage amplitude of the AC bus. Exemplarily, assuming that the rated voltage amplitude of the AC bus is 220 volts, when the voltage amplitude of the AC bus drops below 90% of the rated voltage amplitude, the microgrid system enters the low voltage ride-through stage. Therefore, the controller can set the above first threshold to 198 volts (220 volts × 90%), and determine that the microgrid system enters the low voltage ride-through stage when it detects that the voltage amplitude of the AC bus is less than 198 volts. Among them, the specific value of the first threshold can be flexibly adjusted according to the requirements of the actual application scenario, and the embodiments of the present application do not limit this.

[0060] It should be noted that since the AC terminal of the power conversion device is electrically connected to the AC bus, the voltage amplitude of the AC bus can be equivalent to the voltage amplitude of this AC terminal, and the voltage amplitude of the AC terminal of the power conversion device is equivalent to the voltage amplitude of the AC power output by the inverter circuit in the power conversion device. Then, the controller can use the voltage amplitude of the AC power output by the inverter circuit or the voltage amplitude of the AC terminal as the voltage amplitude of the AC bus to determine whether the microgrid system enters the low voltage ride-through stage. For the convenience of description, in the following content, the voltage amplitude of the AC bus is equivalently expressed as the voltage amplitude of the AC power output by the inverter circuit. The controller can determine that the microgrid system enters the low voltage ride-through stage when it detects that the voltage amplitude of the AC power output by the inverter circuit is less than the above first threshold.

[0061] In some feasible embodiments, the controller can be electrically connected to the AC side, and obtain the voltage amplitude of the alternating current output by the inverter circuit by sampling the electrical signal of the AC side. Alternatively, the controller can obtain the voltage amplitude of the alternating current output by the inverter circuit through an external sampling module. The sampling module is electrically connected to the AC side, can sample the electrical signal of the AC side, and thus obtain the voltage amplitude of the alternating current output by the inverter circuit. At the same time, the sampling module is communicatively connected to the controller and can send the sampled voltage amplitude of the alternating current output by the inverter circuit to the controller. It can be understood that the above are only examples, and the embodiments of the present application do not limit the implementation manner of the controller to obtain the voltage amplitude of the alternating current output by the inverter circuit.

[0062] It can be understood that the controller can continuously detect the voltage amplitude of the alternating current output by the inverter circuit. When the controller detects that the voltage amplitude of the alternating current output by the inverter circuit drops below the first threshold, it indicates that the microgrid system enters the low-voltage ride-through stage. At this time, in order to help the voltage of the AC bus recover, the power conversion device can output reactive current to the AC bus. This reactive current can produce the effect of voltage increase under the action of the line impedance, thereby compensating for the voltage amplitude drop of the AC bus. The more the voltage amplitude of the AC bus drops, the greater the reactive power that the power conversion device needs to output to compensate the voltage of the AC bus. At the same time, the power conversion device can also output active power to the AC bus to maintain the frequency stability of the AC bus voltage. The greater the active power and reactive power output by the power conversion device, the better the effect of maintaining the stability of the AC bus voltage. At the same time, the magnitudes of the active power and reactive power output by the power conversion device are related to the apparent power output by the power conversion device, and the apparent power is equal to where U is the effective value of the voltage output by the power conversion device, I is the effective value of the current output by the power conversion device, P is the active power output by the power conversion device, and Q is the reactive power output by the reference power conversion device. Thus, it can be seen that the magnitudes of the active power and reactive power output by the power conversion device are positively correlated with the apparent power output by the power conversion device. If the apparent power output by the first power conversion is greater, then the active power and reactive power output by the power conversion device are greater, and the better the effect of maintaining the stability of the AC bus voltage by the power conversion device. Similarly, the greater the apparent power output by the reference power conversion device, the better the effect of maintaining the stability of the AC bus voltage.

[0063] Therefore, when the microgrid enters the low-voltage ride-through stage, the microgrid system can increase the total apparent power output by adjusting the power conversion device and all reference power conversion devices, thereby improving the effect of maintaining the stability of the AC bus voltage and further enhancing the stability of the AC bus voltage.

[0064] In this application, the total apparent power output by the power conversion device and all reference power conversion devices is related to the ratio of the active power and reactive power output by the power conversion device and each reference power conversion device. Specifically, please refer to Figure 2 , Figure 3a , Figure 3b and Figure 3c , Figure 3a which is a power synthesis schematic diagram provided by an embodiment of this application, Figure 3b which is another power synthesis schematic diagram provided by an embodiment of this application, Figure 3c which is yet another power synthesis schematic diagram provided by an embodiment of this application. Among them, Figure 3a , Figure 3b and Figure 3c the abscissa P of which represents the magnitude of the active power, Figure 3a , Figure 3b and Figure 3c the ordinate Q of which represents the magnitude of the reactive power.

[0065] It should be noted that in Figure 3b and Figure 3c , S1 represents the apparent power output by the power conversion device, and this S1 is synthesized by the active power and reactive power output by the power conversion device. Then, the cotangent value of the angle between S1 and the abscissa (i.e., or ) is equal to the ratio of the active power and reactive power output by the power conversion device. Similarly, S2 represents the apparent power output by a reference power conversion device, and this S2 is synthesized by the active power and reactive power output by the reference power conversion device. Then, the cotangent value of the angle between S2 and the abscissa (i.e., or ) is equal to the ratio of the active power and reactive power output by the reference power conversion device. Similarly, S3 represents the apparent power output by another reference power conversion device, and the cotangent value of the angle between this S3 and the abscissa (i.e., or ) is equal to the ratio of the active power and reactive power output by this reference power conversion device.

[0066] It can be understood that when the ratios of the active power and reactive power output by the power conversion device and each reference power conversion device are not equal, for example, as shown in Figure 2 and Figure 3a , the ratio of the active power and reactive power output by the power conversion device is the ratio of the active power and reactive power output by a reference power conversion device is the ratio of the active power and reactive power output by another reference power conversion device is and the above and are not equal to each other. At this time, the total apparent power output by the power conversion device and all reference power conversion devices can be characterized by Figure 3b shown as St1. Meanwhile, the total reactive power of the power conversion device and all reference power conversion devices is equal to Q1, and the total active power is equal to P1.

[0067] When the ratios of the active power to the reactive power output by the power conversion device and all reference power conversion devices are equal, for example Figure 3c shown as, the ratios of the active power to the reactive power output by the power conversion device and all reference power conversion devices are all equal to At this time, the total apparent power output by the power conversion device and all reference power conversion devices can be characterized by St2. Meanwhile, the total reactive power output by the power conversion device and all reference power conversion devices is equal to Q2, and the total active power is equal to P2. By comparing Figure 3b and Figure 3c it can be known that the magnitude of the total apparent power output by the power conversion device and all reference power conversion devices is related to the difference between the ratios of the active power to the reactive power output by the power conversion device and each reference power conversion device. Obviously, when the ratios of the active power to the reactive power output by the power conversion device and all reference power conversion devices are the same, the total apparent power St2 output by the power conversion device and all reference power conversion devices is significantly greater than Figure 3b shown as St1.

[0068] Therefore, by adjusting the ratio of the output active power to the reactive power of the power conversion device provided in this application to be the same as the ratio of the output active power to the reactive power of all reference power conversion devices, the total apparent power output by the power conversion device and all reference power conversion devices can be increased. When the total apparent power of the power conversion device and all reference power conversion devices increases, it means that the total output power of the power conversion device and all reference power conversion devices increases, thereby improving the effect of maintaining the stability of the AC bus voltage and enhancing the stability of the AC bus voltage.

[0069] Specifically, after the microgrid system enters the low-voltage ride-through stage, the power conversion device provided in this application can control the difference between the ratio of the active power output to the reactive power output and the ratio of the active power output to the reactive power output of the reference power conversion device to be less than a second threshold, so as to increase the total output power of the power conversion device and all reference power conversion devices. Among them, the difference between the ratio of the active power output to the reactive power output of the power conversion device and the ratio of the active power output to the reactive power output of the reference power conversion device being less than the second threshold indicates that the ratios of the active power output to the reactive power output of the power conversion device and the reference power conversion device are approximately the same or the difference can be ignored. The specific value of the second threshold can be flexibly adjusted according to the actual application, and this application does not limit it.

[0070] Generally speaking, when the microgrid system enters the low-voltage ride-through stage, the power conversion devices in the microgrid system operate in the grid-forming mode and can increase the output power to improve the maintenance effect of the AC bus voltage stability. In this application, when the microgrid system includes multiple power conversion devices, the total output power of the multiple power conversion devices is related to the ratio of the active power output to the reactive power output of each power conversion device. When the ratios of the active power output to the reactive power output of each power conversion device are the same, the total output power of the multiple power conversion devices is relatively large. Specifically, the power conversion device in the microgrid includes a controller and an inverter circuit. The power conversion device detects the voltage amplitude of the alternating current output by the inverter circuit through the controller, and determines that the microgrid system enters the low-voltage ride-through stage when it detects that the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold. To ensure the stability of the AC bus voltage during the low-voltage ride-through stage, the power conversion device adjusts the ratio of the active power output to the reactive power output of the inverter circuit through the controller, and the ratio of the active power output to the reactive power output is less than a second threshold compared with that of the reference power conversion device. Among them, the reference power conversion device is other power conversion devices in the microgrid system except the above-mentioned power conversion device. At this time, the ratios of the active power output to the reactive power output of the power conversion device and all reference power conversion devices are approximately the same or the difference can be ignored. Therefore, when the microgrid system enters the low-voltage ride-through stage, the total output power of the power conversion device and all reference power conversion devices can be increased, thereby improving the maintenance effect of the AC bus voltage stability, enhancing the stability of the AC bus voltage, and having strong applicability.

[0071] For ease of explanation, in the following content, the ratio of the active power to the reactive power output by the inverter circuit of the power conversion device is briefly referred to as the first ratio, and the ratio of the active power to the reactive power output by the reference power conversion device is briefly referred to as the second ratio. The difference between the first ratio and the second ratio being less than the second threshold is expressed as the first ratio being approximately consistent with the second ratio. The power conversion device can adjust the magnitude of the first ratio through a controller, making the first ratio approximately consistent with the second ratio, thereby increasing the total output power of the power conversion device and all reference power conversion devices and enhancing the voltage stability of the AC bus during the low-voltage ride-through stage.

[0072] In some feasible embodiments, the magnitude of the first ratio in the power conversion device is related to the AC angular frequency output by the inverter circuit. Therefore, the power conversion device can adjust the AC angular frequency output by the inverter circuit through a controller to adjust the magnitude of the first ratio. Exemplarily, please refer to Figure 4 , Figure 4 which is a schematic diagram of angular frequency droop provided by an embodiment of the present application. Figure 4 The abscissa shown represents the first ratio, Figure 4 and the ordinate ω shown i represents the AC frequency output by the inverter circuit in the power conversion device. As Figure 4 shown, when the first ratio is , the AC frequency output by the inverter circuit is ω1. At this time, if the power conversion device sets as the target value of the first ratio, that is, the power conversion device needs to adjust the first ratio from to then the power conversion device can adjust the AC frequency ω1 output by the inverter circuit to the AC frequency ω2 through the controller, so that the first ratio changes from to

[0073] It should be noted that from the above content, it can be seen that the microgrid system includes multiple power conversion devices. In order to make the first ratio approximately consistent with the second ratio of each reference power conversion device, the controller can, before adjusting the first ratio, obtain the average of the first ratio and the second ratios of all reference power conversion devices as the average power ratio, and use this average power ratio as the target value of the first ratio.

[0074] In some feasible embodiments, the above average power ratio can be calculated by a central controller included in the microgrid system, which is communicatively connected to the power conversion device and the reference power conversion devices. When the microgrid system enters the low voltage ride-through stage, the central controller acquires the AC power parameters (such as voltage amplitude and current amplitude, etc.) output by the power conversion device to calculate the active power and reactive power output by the power conversion device, and then the magnitude of the above first ratio can be obtained. At the same time, the central controller also acquires the AC power parameters output by each reference power conversion device to calculate the active power and reactive power output by each reference power conversion device, and then the magnitude of the second ratio of each reference power conversion device can be obtained. The central controller calculates the average of the first ratio and the second ratios of all reference power conversion devices as the above average power ratio.

[0075] In some feasible embodiments, the central controller can also directly sample Figure 2 the AC power parameters of the shown point of common coupling (PCC) to obtain the total active power and total reactive power of the power conversion device and all reference power conversion devices, and then the above average power ratio can be obtained according to the ratio of the total active power to the total reactive power.

[0076] In some feasible embodiments, the above average power ratio can also be calculated by a controller in the power conversion device, which can be communicatively connected to each reference power conversion device. When the microgrid system enters the low voltage ride-through stage, the controller can sample the AC power parameters output by the inverter circuit to calculate the active power and reactive power output by the power conversion device, and then the magnitude of the above first ratio can be obtained. At the same time, the reference power conversion device can also send the calculated second ratio to the controller of the power conversion device. The controller calculates the average of the first ratio and the second ratios of all reference power conversion devices as the above average power ratio. Similarly, the reference power conversion device can also obtain the second ratios sent by other reference power conversion devices and the first ratio sent by the power conversion device while calculating the second ratio, so as to calculate the above average power ratio.

[0077] It can be understood that the above methods for the power conversion device to obtain the average power ratio are only examples and do not constitute a limitation to the embodiments of the present application.

[0078] Further, the central controller sends the calculated average power ratio to the power conversion device, and the power conversion device adjusts the first ratio based on the average power ratio. Before the first ratio is approximately the same as the second ratios of all reference power conversion devices, the central controller can obtain the adjusted first ratio and the second ratios of all reference power conversion devices based on the above to continuously update and iterate the average power ratio. Similarly, the central controller can send the average power ratio to each reference power conversion device, and the reference power conversion device can use the average power ratio as the target value of the second ratio to synchronously adjust the magnitude of the second ratio.

[0079] It can be understood that from the above, the angular frequency of the alternating current output by the inverter circuit in the power conversion device is related to the first ratio. Therefore, after receiving the above average power ratio, the power conversion device can use the average power ratio as the target value of the first ratio and adjust the angular frequency of the alternating current output by the inverter circuit through the controller to adjust the first ratio so that the first ratio approaches the above average power ratio. Specifically, the controller can calculate the angular frequency of the alternating current of the inverter circuit based on the difference between the average power ratio and the first ratio.

[0080] In some feasible embodiments, the controller can calculate the angular frequency of the alternating current of the inverter circuit based on the following formula (1), and the specific expression of the formula (1) is as follows:

[0081]

[0082] where, w i is the angular frequency of the alternating current of the inverter circuit, w0 is the rated angular frequency of the inverter circuit, is the average power ratio, is the first ratio, K i is the frequency droop coefficient of the inverter circuit. Among them, the rated angular frequency w0 of the inverter circuit usually equals 50 Hz or 60 Hz, and this application does not limit this. It can be seen that the alternating current frequency w i output by the inverter circuit is positively correlated with the first ratio of the inverter circuit and

[0083] It can be seen that since the magnitude of the AC angular frequency is linearly related to the magnitude of the first ratio, the controller can make the first ratio equal to the average power ratio by adjusting the AC angular frequency of the inverter circuit. In addition, before the first ratio is approximately the same as the second ratio of each reference power conversion device, the controller can update and adjust the AC angular frequency output by the inverter circuit according to the updated average power ratio until the first ratio is approximately the same as the second ratio of each other reference power conversion device, and the adjustment of the first ratio is completed. Similarly, each reference power conversion device completes the adjustment of the second ratio.

[0084] In some feasible embodiments, for the convenience of understanding the specific implementation process of the power conversion device adjusting the AC frequency output by the inverter circuit in the embodiments of the present application, the following content is combined with Figure 5 for illustration. Figure 5 FIG. is a schematic diagram of an application scenario of the controller provided in the embodiments of the present application. As Figure 5 shown, first, the controller can obtain the active power Pi and reactive power Qi output by the inverter circuit, and calculate the ratio of the active power Pi and reactive power Qi output by the inverter circuit as That is, the first ratio is At the same time, the controller obtains the total active power Pt and total reactive power Qt of the power conversion device and all reference power conversion devices, and calculates the ratio of the total active power Pt and total reactive power Qt output as That is, the average power ratio is Further, the controller calculates the average power ratio and the first ratio The difference between them is multiplied by the frequency droop coefficient Ki of the inverter circuit and then added to the rated angular frequency w0 of the inverter circuit to obtain the AC angular frequency w of the inverter circuit i . Further, in order to adjust the AC frequency output by the inverter circuit, the controller can perform time integration on the above calculated AC angular frequency w i to obtain the synchronous phase angle θ ref_i . The controller can generate a corresponding modulation voltage signal based on the synchronous phase angle θ ref_i The modulation voltage signal is used to indicate the operation of the switching tubes of each arm in the inverter circuit, so as to adjust the corresponding AC frequency output by the inverter circuit.

[0085] In some feasible embodiments, please refer to Figure 5 again. In order to avoid reciprocally adjusting the AC angular frequency output by the inverter circuit near the zero crossing point of the controller, that is, the AC angular frequency output by the inverter circuit fluctuates greatly back and forth near the target value, the controller can Figure 5In the specific implementation process shown, a dead zone link is set. Further, after the controller sends a modulation voltage signal to the inverter circuit, it waits for the inverter circuit to complete the adjustment of the AC angular frequency, and then the controller executes the next adjustment process to improve the stability of the AC angular frequency output by the inverter circuit.

[0086] In some feasible implementation manners, during the process of the power conversion device adjusting the first ratio, for the convenience of understanding the variation characteristics of parameters such as the first ratio, the AC frequency output by the inverter circuit, and the average power ratio, the following content is described in conjunction with Figures 6a to 6c for illustrative purposes. Figure 6a FIG. is a schematic diagram of a power ratio curve provided by an embodiment of the present application, Figure 6b FIG. is a schematic diagram of a difference curve provided by an embodiment of the present application, Figure 6c FIG. is a schematic diagram of an AC angular frequency curve provided by an embodiment of the present application.

[0087] Among them, Figure 6a The shown cot represents the variation curve of the first ratio, Figure 6a The shown cot represents the variation curve of the average power ratio. It can be understood that during the process of the power conversion device adjusting the first ratio cot , the first ratio cot gradually approaches the above-mentioned average power ratio cot . When the first ratio cot is close to the above-mentioned average power ratio cot , it means that the first ratio cot of the power conversion device is approximately the same as the second ratio of all reference power conversion devices. At this time, the total output power of the power conversion device and all reference power conversion devices in the microgrid system increases, and the power conversion device completes the adjustment of the output power.

[0088] Figure 6b The shown cot represents the variation curve of the difference between the average power ratio cot and the first ratio cot , Figure 6c The shown w i represents the variation curve of the AC angular frequency output by the inverter circuit. It can be understood that during the process of the power conversion device adjusting the first ratio cot , since the controller adjusts the AC angular frequency output by the inverter circuit based on the difference between the first ratio cot and the average power ratio cot , therefore, the difference variation curve cot The same as the variation trend of the alternating current angular frequency variation curve ω i is the same. As Figure 6c shown, when the difference variation curve cot decreases, the alternating current angular frequency variation curve ω i also decreases, and when the difference variation curve cot increases, the alternating current angular frequency variation curve ω i also increases.

[0089] It can be understood that Figures 6a to 6c the parameter variation curves shown are only examples. According to different actual application scenarios, the variation curves of the above-mentioned first ratio, the alternating current frequency output by the inverter circuit, and the average power ratio and other parameters can have different forms, and the embodiments of the present application will not list them one by one here.

[0090] In some feasible embodiments, the total apparent power output by the power conversion device and all reference power conversion devices is related not only to the above-mentioned first ratio and second ratio, but also to the magnitude of the apparent power output by the power conversion device and the magnitudes of the apparent powers output by each reference power conversion device. Specifically, please refer to Figure 7 , Figure 7 which is another power synthesis schematic diagram provided by the embodiment of the present application. Among them, Figure 7 the abscissa P of Figure 7 represents the magnitude of the active power, and Figure 7 the ordinate Q of represents the magnitude of the reactive power. The cotangent value of the included angle between S1, S2, and S3 and the abscissa (i.e., Figure 7 ) is equal to the ratio of the active power and the reactive power output by the power conversion device or the reference power conversion device. At this time, the total apparent power output by the power conversion device and all reference power conversion devices can be represented by

[0091] It should be noted that Figure 7 the apparent power S1 output by the power conversion device shown in Figure 3c is larger than the apparent power S1 output by the power conversion device shown in Figure 7 , that is, Figure 3c the S1 of Figure 3c is longer than the S1 length of

[0092] It can be understood that when the ratio of the active power and reactive power output by the power conversion device and all reference power conversion devices is approximately the same, by increasing the apparent power output by the power conversion device, the total apparent power output by the power conversion device and all reference power conversion devices can be increased, thereby further improving the effect of maintaining the stability of the AC bus voltage and effectively enhancing the stability of the AC bus voltage. Similarly, the reference power conversion device can also increase the apparent power output, so that the total apparent power output by the power conversion device and all reference power conversion devices increases.

[0093] As can be seen from the above, the apparent power output by the power conversion device is equal to UI, where U is the effective value of the voltage output by the power conversion device, and I is the effective value of the current output by the power conversion device. At the same time, the effective value of the current output by the power conversion device is positively correlated with the magnitude of the current amplitude output by the inverter circuit in the power conversion device. Thus, it can be seen that the power conversion device can control the inverter circuit to increase the current amplitude output to increase the apparent power output.

[0094] It should be noted that when the power conversion device increases the current amplitude output by the inverter circuit through the controller, it is necessary to avoid the current amplitude output by the inverter circuit exceeding the rated current amplitude. This rated current amplitude refers to the upper limit of the current that the inverter circuit can withstand during operation. When the current amplitude output by the inverter circuit exceeds the rated current amplitude, it will cause the power conversion device to be damaged by overcurrent. Therefore, by controlling the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude through the controller, the power conversion device can make the current amplitude output by the inverter circuit reach the maximum value. Furthermore, while the apparent power output by the power conversion device increases significantly, the power conversion device can be prevented from being damaged by overcurrent, and the safety is high.

[0095] In addition, the magnitude of the current amplitude output by the inverter circuit in the power conversion device is not only limited by the rated current amplitude of the inverter circuit but also related to the current amplitude threshold of the inverter circuit. Specifically, this current amplitude threshold represents the maximum value of the current amplitude that the inverter circuit can output, and the magnitude of this current amplitude threshold is equal to the ratio of 2S n to E DG , where S n is the rated apparent power of the inverter circuit, and E DG is the magnitude of the voltage amplitude output by the inverter circuit. Thus, it can be seen that when the voltage amplitude output by the inverter circuit is different, the current amplitude threshold that the inverter circuit can output is different.

[0096] It can be understood that when the current amplitude threshold of the inverter circuit is less than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output currently is less than the rated current amplitude. Then, the power conversion device controls the current amplitude output by the inverter circuit to be less than or equal to the current amplitude threshold through the controller, so that the current amplitude output by the inverter circuit can reach the maximum value, and it is ensured that the power conversion device will not be damaged due to overcurrent, with high safety. Or, when the current amplitude threshold of the inverter circuit is greater than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output currently is greater than the rated current amplitude. Then, to avoid damage to the power conversion device due to overcurrent, the power conversion device controls the current amplitude output by the inverter circuit to be less than or equal to the rated circuit amplitude through the controller, thereby ensuring safety while increasing the apparent power output by the inverter circuit.

[0097] In some feasible embodiments, the power conversion device provided by the present application can also adjust the voltage phase angle output by the inverter circuit through the controller to be consistent with the voltage phase angles output by all reference power conversion devices, so as to reduce the loss of the output power of the power conversion device.

[0098] It should be noted that when the voltage phase angles output by the power conversion device and the reference power conversion device are not synchronized, that is, when the voltage phase angles output by the power conversion device and the reference power conversion device do not remain consistent, there will be a voltage difference between the power conversion device and the reference power conversion device, which will cause current to flow between the power conversion device and the reference power conversion device, that is, generate circulating current. The generation of circulating current will cause the current to dissipate in the form of heat under the action of the wire, and then the output power of the power conversion device will be lost. At the same time, the circulating current may also cause overheating, affecting the safety of the device. Therefore, by adjusting the voltage phase angle output by the inverter circuit through the controller to be consistent with the voltage phase angles output by all reference power conversion devices, the power conversion device can reduce the loss of the output power of the power conversion device and ensure the safety of the device at the same time.

[0099] Specifically, after the microgrid system enters the low-voltage ride-through stage, the power conversion device provided by the present application can adjust the voltage phase angle output by the inverter circuit through the controller, and the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices is less than a third threshold. At this time, the voltage phase angle output by the power conversion device and the voltage phase angles output by the reference power conversion devices are approximately the same or the difference can be ignored. The specific value of the third threshold can be flexibly adjusted according to the specific actual application, and the present application does not limit this.

[0100] In some feasible embodiments, whether the voltage phase angles output by the power conversion device and the reference power conversion device are consistent can be represented by the directions of different voltage vectors in the synchronous rotating coordinate system. Specifically, the voltage signal (three-phase AC voltage) output by the power conversion device is converted into a first voltage vector in the synchronous rotating coordinate system, and the voltage signal output by the reference power conversion device is converted into a second voltage vector in the synchronous rotating coordinate system. In the synchronous rotating coordinate system, the first voltage vector and the second voltage vector will rotate at a certain angular frequency, and the rotation angular frequency of the first voltage vector is equal to the AC angular frequency output by the power conversion device, and the rotation angular frequency of the second voltage vector is equal to the AC angular frequency output by the reference power conversion device. At the same time, from the above content and Figure 6c As shown, after the power conversion device completes the adjustment of the first ratio and the second power change device completes the adjustment of the second ratio, the AC angular frequencies output by the power conversion device and the reference power conversion device are both equal to the rated angular frequency. Therefore, in the synchronous rotating coordinate system, the first voltage vector and the second voltage vector will rotate at the same angular frequency. Thus, the relative positions of the first voltage vector and the second voltage vector in the synchronous rotating coordinate system will remain unchanged.

[0101] At this time, if the voltage phase angles output by the power conversion device and the reference power conversion device are inconsistent, then in the synchronous rotating coordinate system, the first voltage vector and the second voltage vector do not coincide, that is, the directions of the first voltage vector and the second voltage vector are different. On the contrary, if the voltage phase angles output by the power conversion device and the reference power conversion device are consistent, then in the synchronous rotating coordinate system, the first voltage vector and the second voltage vector coincide, that is, the directions of the first voltage vector and the second voltage vector are the same.

[0102] Exemplarily, please refer to Figure 8a , Figure 8a which is a schematic diagram of the synchronous rotating coordinate system provided by an embodiment of the present application. Among them, Figure 8a the abscissa shown is the d-axis of the synchronous rotating coordinate system, Figure 8a and the ordinate shown is the q-axis of the synchronous rotating coordinate system. Figure 8a The E in DG1 represents the above-mentioned first voltage vector, Figure 8a The E in DG2 represents the above-mentioned second voltage vector. From the above content, it can be seen that the first voltage vector E DG1 and the second voltage vector E DG2 rotate synchronously at the same angular frequency, so the relative positions of the first voltage vector E DG1 and the second voltage vector E DG2 remain unchanged at different times. Further, in the case where the voltage phase angles output by the power conversion device and the reference power conversion device are inconsistent, ifFigure 8a As shown, the first voltage vector E DG1 and the second voltage vector E DG2 have an included angle therebetween, that is, the directions of the first voltage vector E DG1 and the second voltage vector E DG2 are different.

[0103] In some feasible embodiments, the power conversion device can adjust the first voltage vector to coincide with the d-axis in the synchronous rotating coordinate system through a controller, and the reference power conversion device also adjusts the second voltage vector to coincide with the d-axis in the synchronous rotating coordinate system, so that the directions of the first voltage vector and the second voltage vector both coincide with the d-axis, thereby enabling the voltage phase angles output by the power conversion device and the reference power conversion device to be consistent. In addition, since the voltage vector component of the first voltage vector on the q-axis of the synchronous rotating coordinate system is equal to 0 when the first voltage vector coincides with the d-axis. Therefore, the power conversion device can detect whether the voltage vector component of the first voltage vector on the q-axis of the synchronous rotating coordinate system is equal to 0 through the controller, and further determine whether the first voltage vector coincides with the d-axis.

[0104] In some feasible embodiments, the controller can adjust the direction of the first current vector in the synchronous rotating coordinate system to adjust the direction of the second voltage vector, where the first current vector is the current vector obtained by converting the current signal (three-phase alternating current) output by the power conversion device into the synchronous rotating coordinate system.

[0105] It should be noted that in the synchronous rotating coordinate system, the first current vector rotates at a certain angular frequency, and the angular frequency at which the first current vector rotates is equal to the angular frequency of the alternating current output by the power conversion device. Further, the included angle between the first current vector and the first voltage vector in the synchronous rotating coordinate system is equal to the ratio of the active power and the reactive power output by the power conversion device, that is, equal to the first ratio. At the same time, from the above content and [[ID=BigImage]]Figure 6a as shown, after the power conversion device completes the adjustment of the first ratio, the first ratio output by the power conversion device remains unchanged. Therefore, in the synchronous rotating coordinate system, the included angle between the first voltage vector and the first current vector remains unchanged at different times. Thus, when the controller adjusts the direction of the first current vector in the synchronous rotating coordinate system, the direction of the first voltage vector changes synchronously.

[0106] In some feasible embodiments, the direction of the first current vector in the synchronous rotating coordinate system is related to the included angle of the first current vector, and the included angle of the first current vector is the magnitude of the included angle between the first current vector and the d-axis. Therefore, by adjusting the included angle of the first current vector, the controller can adjust the direction of the first current vector, and further adjust the direction of the first voltage vector. When the controller detects that the voltage vector component of the first voltage vector on the q-axis in the synchronous rotating coordinate system is equal to 0, it can be determined that the first voltage vector coincides with the d-axis.

[0107] Exemplarily, please refer to Figure 8b , Figure 8b which is another schematic diagram of the synchronous rotating coordinate system provided by the embodiment of the present application. Among them, Figure 8a the abscissa shown is the d-axis of the synchronous rotating coordinate system, Figure 8a and the ordinate shown is the q-axis of the synchronous rotating coordinate system. Figure 8a E in DG1 represents the first voltage vector, I DG1 represents the first current vector, and δ i is the magnitude of the included angle between the first current vector I DG1 and the d-axis.

[0108] It can be understood that when the controller adjusts the included angle δ i of the first current vector to increase, the first current vector I DG1 rotates clockwise and drives the first voltage vector E DG1 to rotate clockwise. At this time, the controller can detect that the voltage vector component of the first voltage vector E DG1 on the q-axis gradually decreases. It can be understood that when the voltage vector component of the first voltage vector E DG1 on the q-axis decreases to equal 0, the first voltage vector coincides with the d-axis, and the controller completes the adjustment of the output voltage phase angle.

[0109] In some feasible embodiments, when the controller adjusts the magnitude of the included angle between the first current vector and the d-axis, the variation relationship between the magnitude of the included angle of the current vector and the magnitude of the voltage vector component of the first voltage vector on the q-axis can be expressed by the following formula (2), and the specific formula (2) is:

[0110]

[0111] where δ i is the above-mentioned included angle of the current vector, δ0 is the initial included angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis, E q is the voltage vector component of the above-mentioned first voltage vector in the synchronous rotating coordinate system, and w Δi is the rotational angular frequency reference coefficient. Characterize the frequency-domain integration.

[0112] In this application, the power conversion device can adjust the direction of the first current vector by adjusting the magnitude of the included angle of the current vector in the synchronous rotating coordinate system through the controller, and then can adjust the direction of the first voltage vector. When the power conversion device detects through the controller that the voltage vector component of the first voltage vector on the q-axis is equal to 0, it indicates that the first voltage vector coincides with the d-axis. Similarly, for the implementation method of the reference power conversion device to adjust the voltage phase angle, reference can be made to the specific implementation method of the power conversion device to adjust the voltage phase angle above, which will not be elaborated in this embodiment of the application. When both the power conversion device and the reference power conversion device adjust the corresponding voltage vectors in the synchronous rotating coordinate system to coincide with the d-axis, the voltage phase angles output by the power conversion device and the reference power conversion device are kept consistent, thereby reducing the generation of circulating current and reducing the power loss output by the power conversion device and the reference power conversion device.

[0113] For the convenience of understanding the specific control process of the controller in the power conversion device in this application, the following content is combined with Figure 9 for illustrative examples. Figure 9 This is another schematic diagram of the application scenario of the controller provided by the embodiment of the application. As Figure 9 shown, first, the controller can obtain the rated current amplitude Ilimit and the current amplitude threshold Imax of the inverter circuit, and perform a minimum operation on the rated current amplitude Ilimit and the current amplitude threshold Imax, and output the smaller absolute value of the rated current amplitude Ilimit and the current amplitude threshold Imax as Iref_i, so that the controller can control the current amplitude output by the inverter circuit to be less than or equal to Iref_i. At the same time, the controller also obtains the voltage vector component Eq of the voltage signal output by the inverter circuit on the q-axis of the synchronous rotating coordinate system. The controller calculates the difference between the voltage vector component Eq and 0, multiplies the difference by the rotation angular frequency reference coefficient w of the inverter circuit DG _q. The controller calculates the difference between the voltage vector component Eq DG _q and 0, and multiplies the difference by the rotation angular frequency reference coefficient w of the inverter circuit Δi and then performs time integration, and adds it to the initial included angle δ0 between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis to obtain the current vector included angle δ i . Further, in order to adjust the voltage phase angle output by the inverter circuit, the controller can calculate the reactive current reference value Iref_q of the inverter circuit to be equal to Iref_i×sinδ i , and at the same time calculate the active current reference value Iref_d of the inverter circuit to be equal to Iref_i×cosδ i . Further, the controller can sample the reactive current I DG _ q and the active current I output by the inverter circuit currentlyDG _ d , and calculate the difference between the active current reference value Iref_d and the active current I DG _ d , and the difference between the reactive current reference value Iref_q and the reactive current I DG _ q . Furthermore, the corresponding active modulation voltage signal Uref_d and reactive modulation voltage signal Uref_q can be obtained through the PI proportional-integral module. In addition, the controller also obtains the synchronous phase angle θ Figure 5 based on the control flow shown above ref_i . The controller can convert the active modulation voltage signal Uref_d and the reactive modulation voltage signal Uref_q from the synchronous rotating coordinate system to the control three-phase stationary coordinate system according to the synchronous phase angle θ ref_i to control the switching tubes of the three-phase bridge arms in the inverter circuit

[0114] It can be understood that Figure 9 the example shown is only for illustration. In the present application, the controller can also adjust the magnitude of the current output by the inverter circuit, the AC angular frequency, the ratio of the active power to the reactive power (the first ratio), and the voltage phase angle based on other control flows. Examples are not given one by one in the embodiments of the present application

[0115] Generally speaking, the first ratio in this application is related to the angular frequency of the alternating current output by the inverter circuit. Therefore, the power conversion device can adjust the angular frequency of the alternating current output by the inverter circuit through the controller to adjust the magnitude of the first ratio. At the same time, in order to make the first ratio approximately the same as the second ratios of all reference power conversion devices, the power conversion device can obtain the average value of the first ratio and the second ratios of all reference power conversion devices as the average power ratio, and use this average power ratio as the target value of the first ratio when adjusting the angular frequency of the alternating current of the inverter circuit, so that the adjusted first ratio can be close to the average power ratio. In addition, the magnitude of the output power of the power conversion device is positively correlated with the amplitude of the current output by the inverter circuit. Therefore, the power conversion device can increase the amplitude of the current output by the inverter circuit through the controller to increase the magnitude of the output power, and further improve the effect of maintaining the AC bus voltage. Among them, the magnitude of the current amplitude output by the inverter circuit is limited by the rated current amplitude and the current amplitude threshold of the inverter circuit. If the current amplitude threshold of the inverter circuit is greater than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is greater than the rated current amplitude. At this time, the power conversion device controls the amplitude of the current output by the inverter circuit to be less than or equal to the rated current amplitude through the controller, which can avoid overcurrent damage to the inverter circuit, has high safety, and the output power of the power conversion device increases significantly. If the current amplitude threshold of the inverter circuit is less than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is less than the rated current amplitude. At this time, the power conversion device controls the amplitude of the current output by the inverter circuit to be less than or equal to the current amplitude threshold through the controller, which can make the amplitude of the current output by the inverter circuit reach the maximum value. While the output power of the power conversion device increases significantly, overcurrent damage to the power conversion device is avoided, and the safety is high. In addition, when the voltage phase angles output by the power conversion device and all reference power conversion devices are approximately the same, there will be no voltage difference or the voltage difference is very small between the power conversion device and the reference power conversion device, which can reduce the generation of circulating current, thereby reducing the loss of the output power of the power conversion device and ensuring the safety of the device at the same time. It can be seen that when the power conversion device provided in this application is applied to the microgrid system, the total output power of multiple power conversion devices can be relatively large, thereby improving the effect of maintaining the stability of the AC bus voltage, and has strong applicability.

[0116] Please refer to Figure 10 , Figure 10 which is a schematic flowchart of a control method for a power conversion device provided by an embodiment of this application. The control method for the power conversion device provided by the embodiment of this application is applicable to Figures 2 to 9 the controller in the corresponding specific implementation manner. Specifically, the control method for the power conversion device may include the steps:

[0117] S101. Obtain the voltage amplitude of the alternating current output by the inverter circuit.

[0118] It can be understood that the microgrid system includes multiple power conversion devices, and the AC terminals of each power conversion device are electrically connected to the AC bus. When the voltage drop of the AC bus causes the microgrid system to enter the low-voltage ride-through stage, each power conversion device can operate in the grid-forming mode and actively adjust the output power to help maintain the voltage stability of the AC bus. For example, each power conversion device can provide active power and reactive power to the AC bus. It can be understood that since the AC terminals of the power conversion devices are electrically connected to the AC bus, the voltage amplitude of the alternating current output by the inverter circuit through this AC terminal is the same as the voltage amplitude of the AC bus. Therefore, the power conversion device can determine the voltage amplitude of the AC bus by detecting the voltage amplitude of the alternating current output by the inverter circuit through the controller. When the power conversion device detects through the controller that the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, it indicates that the voltage amplitude of the AC bus is less than this threshold. Among them, the first threshold refers to the maximum value of the voltage amplitude of the AC bus in the low-voltage ride-through stage, and this first threshold can be specifically obtained according to the rated voltage amplitude of the AC bus. Therefore, when the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, it indicates that the microgrid system enters the low-voltage ride-through stage.

[0119] For the specific implementation manner of the above S101, reference can be made to the Figures 2 to 9 implementation manner executed by the controller in the above, which will not be elaborated in this embodiment of the present application.

[0120] S102. When the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, adjust the ratio of the active power to the reactive power output by the inverter circuit so that the difference between the ratio of the active power to the reactive power output by all reference power conversion devices is less than the second threshold.

[0121] It can be understood that when the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, it indicates that the microgrid system enters the low-voltage ride-through stage. At this time, each power conversion device in the microgrid system can adjust its own output power to increase the total output power of multiple power conversion devices and improve the maintenance effect of the stability of the AC bus voltage. Further, the magnitude of the total output power of multiple power conversion devices is related to the ratio of the active power and reactive power output by each power conversion device. Specifically, compared with the inconsistent ratios of the active power and reactive power output by each power conversion device, when the ratios of the active power and reactive power output by each power conversion device are consistent, the total output power of multiple power conversion devices is greater. Therefore, each power conversion device can control the ratio of the active power and reactive power output by the inverter circuit to be consistent through the controller, thereby increasing the total output power. Exemplarily, it is assumed that the microgrid system includes a power conversion device and at least one reference power conversion device, and both the power conversion device and the reference power conversion device operate in the grid-forming mode. When the power conversion device detects through the controller that the voltage amplitude of the alternating current output by the inverter circuit is less than the first threshold, the power conversion device adjusts, through the controller, the ratio of the active power and reactive power output by the inverter circuit so that the difference between the ratio of the active power and reactive power output by the power conversion device and the ratio of the active power and reactive power output by the reference power conversion device is less than the second threshold. At this time, the difference between the ratios of the active power and reactive power output by the power conversion device and all reference power conversion devices can be ignored, that is, they are approximately the same. As can be seen from the above, when the ratios of the active power and reactive power output by the power conversion device and all reference power conversion devices are the same, the total output power of the power conversion device and all reference power conversion devices is relatively large, so that more active power and reactive power can be provided to the AC bus, thereby improving the maintenance effect of the stability of the AC bus voltage, and it has strong applicability.

[0122] For the specific implementation manner of the above S102, reference can be made to the implementation manner executed by the controller in the above Figures 2 to 9 and details are not described herein again in the embodiments of the present application.

[0123] In an optional implementation manner, the ratio of the active power and reactive power output by the inverter circuit is the first ratio, and the ratio of the active power and reactive power output by the reference power conversion device is the second ratio; adjusting the ratio of the active power and reactive power output by the inverter circuit so that the difference between the ratio of the active power and reactive power output by the inverter circuit and the ratio of the active power and reactive power output by all reference power conversion devices is less than the second threshold specifically includes: adjusting the angular frequency of the alternating current output by the inverter circuit so that the angular frequency of the alternating current output by the inverter circuit is positively correlated with the first ratio and negatively correlated with the average power ratio until the difference between the first ratio and the second ratio is less than the first threshold; where the average power ratio is the average of the first ratio and the second ratios of all reference power conversion devices.

[0124] It can be understood that the magnitude of the first ratio is related to the angular frequency of the alternating current output by the inverter circuit in the power conversion device. Therefore, the power conversion device can adjust the angular frequency of the alternating current output by the inverter circuit to adjust the magnitude of the first ratio. At the same time, in order to make the first ratio approximately the same as the second ratio of all reference power conversion devices, the power conversion device can obtain the average power ratio of the first ratio and the second ratio of all reference power conversion devices as the average power ratio, and use this average power ratio as the target value of the first ratio when adjusting the angular frequency of the alternating current of the inverter circuit, so that the adjusted first ratio can be close to the average power ratio. Similarly, the reference power conversion device can also adjust the second ratio to be close to the average power ratio based on the same implementation method, so that the difference between the first ratio and the second ratio is less than the first threshold, that is, approximately the same. The implementation principle of this embodiment is simple, and using the average power ratio for adjustment can make the difference between the first ratio and the second ratio decrease faster, that is, the adjustment speed is fast and the applicability is strong.

[0125] In an alternative embodiment, the alternating current frequency output by the inverter circuit satisfies:

[0126]

[0127] wherein, w0 is the rated angular frequency of the inverter circuit, is the average power ratio, is the first ratio, K i is the frequency droop coefficient of the inverter circuit.

[0128] It can be understood that the power conversion device can calculate the reference value of the alternating current frequency output by the inverter circuit by obtaining the first ratio, the average power ratio of the power conversion device and all reference power conversion devices, and combining the rated angular frequency and the frequency droop coefficient of the inverter circuit. Based on the calculated reference value of the alternating current frequency output by the inverter circuit, the controller adjusts the angular frequency of the alternating current output by the inverter circuit, and can adjust the difference between the first ratio and the second ratio to be less than the first threshold. The calculation process is simple and easy to implement.

[0129] In an alternative embodiment, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than the second threshold, and the current amplitude threshold of the inverter circuit is greater than the rated current amplitude of the inverter circuit, adjusting the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit.

[0130] It can be understood that the magnitude of the output power of the power conversion device is positively correlated with the amplitude of the current output by the inverter circuit. Therefore, the power conversion device can adjust the amplitude of the current output by the inverter circuit to increase, so as to increase the magnitude of the output power and further improve the effect of maintaining the AC bus voltage. The magnitude of the current amplitude output by the inverter circuit is limited by the rated current amplitude and the current amplitude threshold of the inverter circuit. Among them, the rated current amplitude refers to the upper limit of the current that the inverter circuit can withstand during operation, and the current amplitude threshold represents the maximum value of the current amplitude that the inverter circuit can output. It can be understood that if the current amplitude threshold of the inverter circuit is greater than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is greater than the rated current amplitude. At this time, the power conversion device controls the amplitude of the current output by the inverter circuit to be less than or equal to the rated current amplitude, which can avoid overcurrent damage to the inverter circuit, has high safety, and the output power of the power conversion device increases significantly.

[0131] In an optional embodiment, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than the second threshold, and the current amplitude threshold of the inverter circuit is less than the rated current amplitude of the inverter circuit, adjusting the amplitude of the current output by the inverter circuit to be less than or equal to the current amplitude threshold of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the amplitude of the voltage output by the inverter circuit.

[0132] It can be understood that the magnitude of the output power of the power conversion device is positively correlated with the amplitude of the current output by the inverter circuit. Therefore, the power conversion device can adjust the amplitude of the current output by the inverter circuit to increase, so as to increase the magnitude of the output power and further improve the effect of maintaining the AC bus voltage. The magnitude of the current amplitude output by the inverter circuit is limited by the rated current amplitude and the current amplitude threshold of the inverter circuit. Among them, the rated current amplitude refers to the upper limit of the current that the inverter circuit can withstand during operation, and the current amplitude threshold represents the maximum value of the current amplitude that the inverter circuit can output. It can be understood that if the current amplitude threshold of the inverter circuit is less than the rated current amplitude, it means that the maximum current amplitude that the inverter circuit can output at the current moment is less than the rated current amplitude. At this time, the power conversion device controls the amplitude of the current output by the inverter circuit to be less than or equal to the current amplitude threshold, which can make the amplitude of the current output by the inverter circuit reach the maximum value. While the output power of the power conversion device increases significantly, it can avoid overcurrent damage to the power conversion device and has high safety.

[0133] In an alternative embodiment, the method further includes: when the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all reference power conversion devices is less than a second threshold, the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices is less than a third threshold.

[0134] It can be understood that when the difference between the voltage phase angles output by the power conversion device and all reference power conversion devices is less than the third threshold, it indicates that the voltage phase angles output by the power conversion device and all reference power conversion devices are approximately the same. At this time, there will be no voltage difference or a very small voltage difference between the power conversion device and the reference power conversion device, which can reduce the generation of circulating current, thereby reducing the loss of the output power of the power conversion device and ensuring the safety of the device at the same time.

[0135] In an alternative embodiment, the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices being less than a third threshold specifically includes: when the difference between the AC angular frequency output by the inverter circuit and the AC angular frequencies output by all reference power conversion devices is less than a fourth threshold, adjusting the voltage vector component in the synchronous rotating coordinate system to be equal to 0, so that the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all reference power conversion devices is less than the third threshold; wherein, the voltage vector component in the synchronous rotating coordinate system is the vector magnitude of the voltage signal output by the inverter circuit on the q-axis of the synchronous rotating coordinate system.

[0136] It can be understood that whether the voltage phase angles output by the power conversion device and the reference power conversion device are consistent can be represented by the directions of different voltage vectors in the synchronous rotating coordinate system. For example, the voltage signal output by the power conversion device can be converted into a first voltage vector in the synchronous rotating coordinate system, and the voltage signal output by the reference power conversion device can be converted into a second voltage vector in the synchronous rotating coordinate system. If the voltage phase angles output by the power conversion device and the reference power conversion device are inconsistent, then in the synchronous rotating coordinate system, the directions of the first voltage vector and the second voltage vector are different. On the contrary, the directions of the first voltage vector and the second voltage vector are the same. Therefore, the power conversion device adjusts the first voltage vector in the synchronous rotating coordinate system to coincide with the d-axis, and the reference power conversion device also adjusts the second voltage vector in the synchronous rotating coordinate system to coincide with the d-axis, so that the directions of the first voltage vector and the second voltage vector both coincide with the d-axis, and the voltage phase angles output by the power conversion device and the reference power conversion device are kept consistent. In addition, since when the first voltage vector coincides with the d-axis, the voltage vector component of the first voltage vector on the q-axis in the synchronous rotating coordinate system is equal to 0. Therefore, the power conversion device can detect whether the voltage vector component of the first voltage vector on the q-axis in the synchronous rotating coordinate system is equal to 0 through the controller, and then determine whether the first voltage vector coincides with the d-axis.

[0137] In an optional implementation manner, the magnitude of the voltage vector component in the synchronous rotating coordinate system is negatively correlated with the magnitude of the current vector angle, and the current vector angle is the angle between the current vector of the current signal output by the inverter circuit and the d-axis in the synchronous rotating coordinate system; adjusting the voltage vector component in the synchronous rotating coordinate system to be equal to 0 specifically includes: adjusting the current vector angle to increase so that the voltage vector component in the synchronous rotating coordinate system decreases until the voltage vector component in the synchronous rotating coordinate system is equal to 0.

[0138] It can be understood that when the voltage signal output by the power conversion device is characterized as the first voltage vector in the synchronous rotating coordinate system and the current signal output by the power conversion device is characterized as the first current vector in the synchronous rotating coordinate system, the direction of the first voltage vector is related to the direction of the first current vector. Specifically, when the angle between the first current vector and the d-axis (i.e., the current vector angle) increases, the angle between the first voltage vector and the d-axis will decrease, thereby causing the voltage vector component of the first voltage vector on the q-axis to decrease. Therefore, the power conversion device can adjust the current vector angle to increase in the synchronous rotating coordinate system, so that the voltage vector component decreases until the voltage vector component is equal to 0, and the first voltage vector coincides with the d-axis.

[0139] In an optional implementation manner, the current vector angle satisfies:

[0140]

[0141] wherein, δ i is the current vector angle, δ0 is the initial angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis, E q is the voltage vector component in the synchronous rotating coordinate system, w Δi is the rotational angular frequency reference coefficient.

[0142] It can be understood that when the voltage signal output by the power conversion device is characterized as the first voltage vector in the synchronous rotating coordinate system and the current signal output by the power conversion device is characterized as the first current vector in the synchronous rotating coordinate system, the magnitude of the angle between the first current vector and the d-axis (i.e., the current vector angle) and the relationship between the voltage vector component of the first voltage vector on the q-axis satisfy the above formula. Therefore, by adjusting the magnitude of the current vector angle, the power conversion device can adjust the voltage vector component of the first voltage vector on the q-axis, with a simple implementation principle and strong applicability.

[0143] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the above-mentioned claims.

Claims

1. A power conversion device, characterized in that, The power conversion device is used to connect to a microgrid system, and the microgrid system further includes at least one reference power conversion device. The power conversion device includes an inverter circuit and a controller. The controller is used to control the inverter circuit to convert direct current from a DC source into alternating current and output it. The controller is further used for: When the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjusting the ratio of the active power to the reactive power output by the inverter circuit so that the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices is less than a second threshold.

2. The power conversion device according to claim 1, characterized in that, The ratio of the active power to the reactive power output by the inverter circuit is a first ratio, and the ratio of the active power to the reactive power output by the reference power conversion device is a second ratio. The controller adjusts the ratio of the active power to the reactive power output by the inverter circuit so that the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices is less than a second threshold, which specifically includes: Adjusting the angular frequency of the alternating current output by the inverter circuit so that the angular frequency of the alternating current output by the inverter circuit is positively correlated with the first ratio and negatively correlated with the average power ratio until the difference between the first ratio and the second ratio is less than the first threshold; wherein, the average power ratio is the average of the first ratio and the second ratios of each reference power conversion device.

3. The power conversion device according to claim 2, wherein, The alternating current frequency output by the inverter circuit satisfies: where, w0 is the rated angular frequency of the inverter circuit, is the average power ratio, is the first ratio, K i is the frequency droop coefficient of the inverter circuit.

4. The power conversion device according to any one of claims 1 to 3, characterized in that, The controller is further used for: When the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices is less than a second threshold and the current amplitude threshold of the inverter circuit is greater than the rated current amplitude of the inverter circuit, adjusting the current amplitude output by the inverter circuit to be less than or equal to the rated current amplitude of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit.

5. The power conversion device according to any one of claims 1 to 3, characterized in that, The controller is further used for: When the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices is less than a second threshold and the current amplitude threshold of the inverter circuit is less than the rated current amplitude of the inverter circuit, adjusting the current amplitude output by the inverter circuit to be less than or equal to the current amplitude threshold of the inverter circuit; wherein, the current amplitude threshold of the inverter circuit is positively correlated with the rated apparent power of the inverter circuit and negatively correlated with the voltage amplitude output by the inverter circuit.

6. The power conversion device according to claim 2 or 3, characterized in that, The controller is further used for: When the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices is less than a second threshold, adjusting the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all the reference power conversion devices to be less than a third threshold.

7. The power conversion device according to claim 6, wherein, The difference between the voltage phase angle output by the inverter circuit regulated by the controller and the voltage phase angles output by all the reference power conversion devices is less than a third threshold, specifically including: When the difference between the angular frequency of the alternating current output by the inverter circuit and the angular frequencies of the alternating currents output by all the reference power conversion devices is less than a fourth threshold, adjust the voltage vector component in the synchronous rotating coordinate system to be equal to 0, so that the difference between the voltage phase angle output by the inverter circuit and the voltage phase angles output by all the reference power conversion devices is less than the third threshold; wherein, the voltage vector component in the synchronous rotating coordinate system is the vector magnitude of the voltage signal output by the inverter circuit on the q-axis of the synchronous rotating coordinate system.

8. The power conversion device according to claim 7, characterized in that, The magnitude of the voltage vector component in the synchronous rotating coordinate system is negatively correlated with the magnitude of the current vector angle, and the current vector angle is the angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis. The controller adjusts the voltage vector component in the synchronous rotating coordinate system to be equal to 0, specifically including: Adjust the current vector angle to increase, so that the voltage vector component in the synchronous rotating coordinate system decreases until the voltage vector component in the synchronous rotating coordinate system is equal to 0.

9. The power conversion device according to claim 8, wherein, The current vector angle satisfies: where, δ i is the current vector angle, δ0 is the initial angle between the current vector of the current signal output by the inverter circuit in the synchronous rotating coordinate system and the d-axis, E q is the voltage vector component in the synchronous rotating coordinate system, w Δi is the rotational angular frequency reference coefficient.

10. A microgrid system, characterized in that, The microgrid system includes a power conversion device and at least one reference power conversion device. The power conversion device includes an inverter circuit and a controller. The controller is used to control the inverter circuit to convert the direct current from the DC source into alternating current and output it. The controller is further used for: When the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjust the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices to be less than a second threshold.

11. A control method for a power conversion device, characterized in that, The power conversion device is used to access the microgrid system, and the microgrid system further includes at least one reference power conversion device. The power conversion device includes an inverter circuit. The controller is used to control the inverter circuit to convert the direct current from the DC source into alternating current and output it. The method includes: Detect the voltage amplitude of the alternating current output by the inverter circuit; When the voltage amplitude of the alternating current output by the inverter circuit is less than a first threshold, adjust the difference between the ratio of the active power to the reactive power output by the inverter circuit and the ratio of the active power to the reactive power output by all the reference power conversion devices to be less than a second threshold.

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