Power conversion device and method for controlling same

Abstract

The invention provides a power conversion device and a control method thereof. The power conversion device comprises a direct current end, at least two power conversion bridge arms, a controller and an alternating current end. The direct-current end of the power conversion device is used for connecting a photovoltaic module or an energy storage battery, and the alternating-current end of the power conversion device is used for connecting a power grid or a load. And the at least two power conversion bridge arms are used for converting direct current output by the photovoltaic module or the energy storage battery into alternating current and outputting the alternating current to a power grid or a load. The controller is used for controlling the modulation wave of each power conversion bridge arm in the at least two power conversion bridge arms to be periodically and sequentially switched between at least two different discontinuous pulse width modulation waves under the condition that the current value of the alternating current end of the power conversion device is larger than a current threshold value. According to the invention, the loss of the bottleneck switch tube can be balanced, so that the loss of the bottleneck switch tube is effectively reduced.

Description

Technical Field

[0001] The present application relates to the field of power supply technology, and in particular to a power conversion device and a control method thereof. Background Art

[0002] The heat dissipation of the inverter is the main factor limiting the power increase. From the source, the main heat source of the high-power inverter is the loss generated when the semiconductor device is conducting current. In addition, the junction temperature of the semiconductor device usually cannot exceed the upper threshold of its operation, otherwise there is a risk of thermal failure. Based on this, it can be seen that the semiconductor device with the highest loss and junction temperature (i.e., the bottleneck switching device) limits the output power increase of the inverter. Therefore, it is particularly important to reduce the loss of the bottleneck switching device. Summary of the invention

[0003] The present application provides a power conversion device and a control method thereof, which can balance the loss of the bottleneck switch tube, thereby effectively reducing the loss of the bottleneck switch tube.

[0004] In a first aspect, the present application provides a power conversion device, which includes a DC end, at least two power conversion bridge arms, a controller and an AC end. The DC end of the power conversion device is used to connect to a DC source, and the AC end of the power conversion device is used to connect to a power grid or a load. At least two power conversion bridge arms are used to convert the DC power output by a photovoltaic module or an energy storage battery into AC power and output it to a power grid or a load. The controller is used to control the modulation wave of each power conversion bridge arm in the at least two power conversion bridge arms to periodically switch between at least two different discontinuous pulse width modulation waves in turn when the current value at the AC end of the power conversion device is greater than a current threshold, indicating that there is a switch tube with a higher junction temperature in at least two power conversion bridge arms.

[0005] In this embodiment, the power conversion device controls the modulation wave of each power conversion bridge arm to periodically switch between at least two different discontinuous pulse width modulation waves in turn, so that the bottleneck switch tube in the power conversion device periodically switches back and forth between at least two bottleneck switch tubes corresponding to at least two DPWM modulation modes, thereby shortening the working time of each bottleneck switch tube in the at least two bottleneck switch tubes corresponding to the at least two DPWM modulation modes, so as to reduce the junction temperature of each bottleneck switch tube in the at least two bottleneck switch tubes, thereby reducing the loss of each bottleneck switch tube in the at least two bottleneck switch tubes, so as to achieve loss balance of the bottleneck switch tube, thereby facilitating the improvement of the maximum output power of the power conversion device. Among them, the at least two DPWM modulation modes correspond to the at least two different discontinuous pulse width modulation waves one by one.

[0006] In combination with the first aspect, in a first possible implementation, there are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence, and the time interval between any two adjacent switching moments in the multiple switching moments is N times the power frequency period, where N is a positive integer.

[0007] In this embodiment, the power conversion device controls the modulation wave of each power conversion bridge arm to periodically switch between at least two different discontinuous pulse width modulation waves in sequence, and controls the time interval between any two adjacent switching moments to be N times the power frequency cycle of the power grid, so that the junction temperature of the bottleneck switch tube in the power conversion device is reduced to the average value of the junction temperatures of at least two bottleneck switch tubes corresponding to the above-mentioned at least two DPWM modulation modes, thereby reducing the loss of the bottleneck switch tube in the power conversion device to the average value of the loss of at least two bottleneck switch tubes corresponding to the above-mentioned at least two DPWM modulation modes, thereby balancing the loss of the bottleneck switch tube, making the loss of the bottleneck switch tube lower, and further facilitating the maximum output power of the power conversion device. In addition, since the difficulty of calculating the junction temperature or loss of the switch tube in an integer multiple of the power frequency cycle is lower than the difficulty of calculating the junction temperature or loss of the switch tube in a non-integer multiple of the power frequency cycle, therefore, the time interval between any two adjacent switching moments is N times the power frequency cycle, which can also reduce the control difficulty of the power conversion device to achieve switch tube loss balancing, which is conducive to improving the efficiency of the power conversion device.

[0008] In combination with the first aspect or the first possible implementation manner of the first aspect, in a second possible implementation manner, there are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence. The controller is also used to control the amplitudes of two discontinuous pulse width modulation waves before and after any switching moment in the multiple switching moments to be the same at any switching moment in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence.

[0009] In this embodiment, when the power conversion device controls the modulation wave of each power conversion bridge arm to switch between at least two different discontinuous pulse width modulation waves, by controlling the amplitudes of the two discontinuous pulse width modulation waves before and after the same switching moment to be the same at the same switching moment, it can be ensured that the two discontinuous pulse width modulation waves before and after the switching are continuous, so that the power conversion device can achieve smooth switching when switching between at least two DPWM modulation modes, thereby improving the stability of the power conversion device.

[0010] In combination with any one of the first aspect to the second possible implementation of the first aspect, in a third possible implementation, the power conversion device further includes a positive DC bus, a negative DC bus, a positive bus capacitor and a negative bus capacitor, the AC end of the power conversion device includes a three-phase AC end, and at least two power conversion bridge arms include three power conversion bridge arms, each power conversion bridge arm includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube and two clamping diodes. Among them, the positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus, and the positive DC bus and the negative DC bus are connected between the DC end of the power conversion device and the three power conversion bridge arms. The first switch tube, the second switch tube, the third switch tube and the fourth switch tube of each power conversion bridge arm are connected in series between the positive DC bus and the negative DC bus in sequence, and the connection between the second switch tube and the third switch tube of the three power conversion bridge arms is respectively connected to the three-phase AC end. The two clamping diodes of each power conversion bridge arm are connected in series between the connection between the first switch tube and the second switch tube of each power conversion bridge arm and the connection between the third switch tube and the fourth switch tube of each power conversion bridge arm, and the connection between the two clamping diodes of each power conversion bridge arm is connected to the connection between the positive bus capacitor and the negative bus capacitor.

[0011] In this embodiment, the power conversion device is a three-phase three-wire ANPC three-level power conversion device.

[0012] In combination with any one of the first aspect to the second possible implementation of the first aspect, in a fourth possible implementation, the power conversion device further includes a positive DC bus, a negative DC bus, a positive bus capacitor and a negative bus capacitor, the AC end of the power conversion device includes a three-phase AC end, at least two power conversion bridge arms include three power conversion bridge arms, and each power conversion bridge arm includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube. Among them, the positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus, and the positive DC bus and the negative DC bus are connected between the DC end of the power conversion device and the three power conversion bridge arms. The first switch tube and the second switch tube of each power conversion bridge arm are connected in series between the positive DC bus and the negative DC bus, and the connection between the first switch tube and the second switch tube in the three power conversion bridge arms is respectively connected to the three-phase AC end. The third switch tube and the fourth switch tube of each power conversion bridge arm are reversely connected in series between the connection between the first switch tube and the second switch tube of each power conversion bridge arm and the connection between the positive bus capacitor and the negative bus capacitor.

[0013] In this embodiment, the power conversion device is a three-phase three-wire T-type three-level power conversion device.

[0014] In the second aspect, the present application provides a control method for a power conversion device, which is applied to a power conversion device, wherein the power conversion device includes a DC end, at least two power conversion bridge arms and an AC end, wherein the DC end of the power conversion device is used to connect a photovoltaic component or an energy storage battery, and the AC end of the power conversion device is used to connect a power grid or a load; and at least two power conversion bridge arms are used to convert the DC power output by the photovoltaic component or the energy storage battery into AC power and output it to the power grid or the load. The method includes: when the current value at the AC end of the power conversion device is greater than a current threshold, controlling the modulation wave of each of the at least two power conversion bridge arms to periodically switch between at least two different discontinuous pulse width modulation waves in turn.

[0015] In combination with the second aspect, in a first possible implementation, there are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence, and the time interval between any two adjacent switching moments in the multiple switching moments is N times the power frequency period, where N is a positive integer.

[0016] In combination with the second aspect or the first possible implementation manner of the second aspect, in a second possible implementation manner, there are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence. The power conversion device also controls the amplitudes of two discontinuous pulse width modulation waves before and after any switching moment in the multiple switching moments to be the same at any switching moment in the process of controlling the modulation wave of each power conversion bridge arm to periodically switch between at least two different discontinuous pulse width modulation waves in sequence.

[0017] It should be understood that the implementation and beneficial effects of the above-mentioned aspects of the present application can be referenced to each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of an application scenario of the power conversion device provided in this application;

[0019] Figure 2 It is a structural schematic diagram of a power conversion device provided by the present application;

[0020] Figure 3 is another structural schematic diagram of the power conversion device provided by the present application;

[0021] Figure 4 It is a waveform diagram of a modulation wave of a power conversion bridge arm provided by the present application;

[0022] Figure 5a is another structural schematic diagram of the power conversion device provided by the present application;

[0023] Figure 5bis another structural schematic diagram of the power conversion device provided by the present application;

[0024] Figure 5c is another structural schematic diagram of the power conversion device provided by the present application;

[0025] Figure 6 It is a flow chart of the control method of the power conversion device provided in the present application. DETAILED DESCRIPTION

[0026] The power conversion device provided in this application can be applied to various application fields such as photovoltaic power generation, energy storage power generation, new energy intelligent microgrid, power transmission and distribution, etc. The power conversion device provided in this application can be an inverter, a power storage converter (Power Convers ion System, PCS), an uninterruptible power supply (Uninterrupted Power Supply, UPS), etc., which are applicable to different application scenarios, such as photovoltaic power supply scenarios, energy storage power supply scenarios, photovoltaic storage hybrid power supply scenarios, UPS power supply scenarios, etc. The photovoltaic power supply scenario is used as an example for explanation below.

[0027] See also Figure 1 , Figure 1 Schematic diagram of the application scenario of the power conversion device provided by this application. In the photovoltaic power supply scenario, the power conversion device provided by this application is Figure 1 The inverter shown in the figure includes a DC terminal, a first-phase bridge arm, a second-phase bridge arm, a third-phase bridge arm, a controller and a three-phase AC terminal. The DC terminal of the inverter is connected to the photovoltaic string, and the three-phase AC terminals of the inverter are connected to the AC power grid through a box-type substation and a booster station in sequence. The first end and the second end of each phase bridge arm in the first-phase bridge arm, the second-phase bridge arm and the third-phase bridge arm are respectively connected to the two DC terminals of the inverter, and the third end of the first-phase bridge arm, the third end of the second-phase bridge arm and the third end of the third-phase bridge arm are respectively connected to the three-phase AC terminal of the inverter.

[0028] After the inverter starts running, the controller controls the modulation wave of each phase bridge arm in the three-phase bridge arm to be a DPWM wave, inverts the DC power output by the photovoltaic string into AC power, and outputs it to the box-type substation connected to it. The box-type substation boosts the AC power input to its input end and outputs it to the booster station. The booster station boosts the AC power input to its input end to obtain AC power that meets the requirements of the AC power grid, thereby realizing power supply to the AC power grid.

[0029] In the process of the inverter supplying stable power to the AC grid, when the controller detects that the current at the AC end of the inverter is greater than the current threshold, it indicates that there is a switch tube with a higher junction temperature in the above three-phase bridge arm, and the controller controls the modulation wave of each phase bridge arm to periodically switch between at least two different DPWM waves in sequence. There is a phase difference of 120 degrees between the modulation waves of any two phase bridge arms in the above three-phase bridge arm.

[0030] It can be understood that, when there are switch tubes with high junction temperature in the three-phase bridge arm of the inverter, the modulation wave of each phase bridge arm is controlled to switch periodically between at least two different DPWM waves in turn, so that the bottleneck switch tube in the inverter is periodically switched back and forth between at least two bottleneck switch tubes corresponding to at least two DPWM modulation modes, thereby shortening the working time of each bottleneck switch tube in the at least two bottleneck switch tubes corresponding to the at least two DPWM modulation modes, so as to reduce the junction temperature of each bottleneck switch tube in the at least two bottleneck switch tubes, thereby reducing the loss of each bottleneck switch tube in the at least two bottleneck switch tubes, so as to achieve loss balance of the bottleneck switch tube, and thus help to improve the maximum output power of the inverter. Wherein, the number of at least two different DPWM waves is k, specifically including DPWM1 wave, ..., DPWMk wave, then k DPWM waves correspond to k DPWM modulation modes, and the i-th DPWM modulation mode among the k DPWM modulation modes is a modulation mode in which the modulation wave of each phase bridge arm is controlled to be DPWMk wave.

[0031] The above is only an example of the application scenarios of the power conversion device provided in this application, and is not an exhaustive list. This application does not limit the application scenarios.

[0032] Combine the following Figure 2 to Figure 5c The working principle of the power conversion device provided in this application is illustrated by way of example.

[0033] See also Figure 2 , Figure 2 Schematic diagram of a power conversion device provided by the present application. Figure 2As shown, the power conversion device 1 includes a DC terminal (including a DC terminal i11 and a DC terminal i12), a power conversion bridge arm 111, ..., a power conversion bridge arm 11n, a controller 12 and an AC terminal (including an AC terminal o11, ..., an AC terminal o1n), where n is an integer greater than 1. Among them, the DC terminal i11 and the DC terminal i12 of the power conversion device 1 are used to connect a DC source (including a photovoltaic module, an energy storage battery, a capacitor, etc.), and the AC terminal o11, ..., and the AC terminal o1n of the power conversion device 1 are used to connect a power grid or a load. The first end i111+, ..., of the power conversion bridge arm 111, and the first end i11n+ of the power conversion bridge arm 11n are all connected to the positive DC terminal i11 of the power conversion device 1, and the second end i111-, ..., of the power conversion bridge arm 11n, and the second end i11n- of the power conversion bridge arm 11n are all connected to the negative DC terminal i12 of the power conversion device 1. The third end o111 of the power conversion bridge arm 111 is connected to the AC end o11 of the power conversion device 1, ..., and the third end o11n of the power conversion bridge arm 11n is connected to the AC end o1n of the power conversion device 1. Here, Figure 2 The DC terminal i 11 and the DC terminal i 12 shown are only simple illustrations of the positive DC terminal and the negative DC terminal of the power conversion device 1 , and do not represent the actual number of the positive DC terminal and the negative DC terminal of the power conversion device 1 .

[0034] In one embodiment, during the process of the power conversion device 1 supplying power to the grid or the load, the controller 12 detects the AC end current of the power conversion device 1. When the AC end current of the power conversion device 1 is greater than the current threshold, the controller 12 controls the modulation wave of each of the n power conversion bridge arms to periodically switch between at least two different DPWM waves in sequence.

[0035] Among them, the modulation wave of each power conversion bridge arm is at least two different DPWM waves, and the duration of each DPWM wave can be the same or different, and the present application does not impose any restrictions on this. The at least two different DPWM waves corresponding to each power conversion bridge arm are obtained by superimposing at least two different common-mode signals on the initial sinusoidal modulation wave of each power conversion bridge arm, and there is a phase difference between the initial sinusoidal modulation waves of any two power conversion bridge arms in the above n power conversion bridge arms, or there is a voltage difference between the output voltages of any two power conversion bridge arms. Exemplarily, when n=2, the two power conversion bridge arms in the power conversion device 1 are respectively the L-phase bridge arm and the N-phase bridge arm, the AC end of the L-phase bridge arm is used to connect the live wire, and the AC end of the N-phase bridge arm is used to connect the neutral wire, and the power conversion device 1 is a single-phase power conversion device; when n=3, the three power conversion bridge arms in the power conversion device 1 are respectively the A-phase bridge arm, the B-phase bridge arm and the C-phase bridge arm, and the power conversion device 1 is a three-phase power conversion device, and there is a phase difference of 120 degrees between the initial sinusoidal modulation waves of any two power conversion bridge arms; when n=4, the four power conversion bridge arms in the power conversion device 1 are respectively the A-phase bridge arm, the B-phase bridge arm, the C-phase bridge arm and the N-phase bridge arm, and the power conversion device 1 is a three-phase four-bridge arm power conversion device.

[0036] It can be understood that the purpose of controlling the above-mentioned n power conversion bridge arms in a DPWM modulation mode is to ensure that there is always one power conversion bridge arm without switching action within a switching cycle, thereby reducing the overall switching loss of the above-mentioned n power conversion bridge arms. In order to achieve this purpose, the controller 12 injects a common-mode signal into the initial sinusoidal modulation wave of each power conversion bridge arm to obtain the modulation wave of each power conversion bridge arm. In this way, by designing a common-mode signal, the modulation signal of any power conversion bridge arm can be clamped at +1 or -1 within at least one switching cycle. Therefore, when the modulation wave of the power conversion bridge arm is converted into a PWM wave, the switch tube can be controlled to maintain a state unchanged for a period of time. Since the common-mode signal injected into each power conversion bridge arm is the same, and the line voltage (i.e., the difference between the bridge arm phase voltages output by the two power conversion bridge arms) is used to supply power to the power grid or load, the effect of the common-mode signal can be offset, so that the phase voltage of the sine wave waveform can be restored at the AC end of the power conversion device 1, thereby not affecting the power supply to the power grid or load. In addition, the power conversion device 1 controls the modulation wave of each power conversion bridge arm to periodically switch between at least two different DPWM waves in turn, so that the bottleneck switch tube in the power conversion device 1 periodically switches back and forth between at least two bottleneck switch tubes corresponding to at least two DPWM modulation modes, thereby shortening the working time of each bottleneck switch tube in the at least two bottleneck switch tubes corresponding to the at least two DPWM modulation modes, so as to reduce the junction temperature of each bottleneck switch tube in the at least two bottleneck switch tubes, thereby reducing the loss of each bottleneck switch tube in the at least two bottleneck switch tubes, so as to achieve loss balance of the bottleneck switch tube, thereby facilitating the improvement of the maximum output power of the power conversion device 1. Furthermore, compared with the method of achieving loss balance of the inner tube and the outer tube by using different zero-level current channels in two adjacent switching cycles, the switch loss balance control method provided by the power conversion device 1 does not need to rely on the redundant zero-level state brought by a specific topology, and has strong applicability.

[0037] For the sake of convenience, the following takes n=3 as an example. Figure 3 , Figure 3 is another structural schematic diagram of the power conversion device provided by the present application. Figure 3 As shown, Figure 2 Compared with the power conversion device 1 shown, Figure 3The power conversion device 1 shown also includes a positive DC bus BUS+, a negative DC bus BUS-, a positive bus capacitor C1, a negative bus capacitor C2, a filter inductor L1, a filter inductor L2, a filter inductor L3, a filter capacitor C31, a filter capacitor C32 and a filter capacitor C33. Among them, one end of the positive bus capacitor C1 is connected to the positive DC bus BUS+, and the other end of the positive bus capacitor C1 is connected to the negative DC bus BUS- through the negative bus capacitor C2, and the connection between the positive bus capacitor C1 and the negative bus capacitor C2 is the bus midpoint N. The first end i111+ of the power conversion bridge arm 111, the first end i112+ of the power conversion bridge arm 112, and the first end i113+ of the power conversion bridge arm 113 are connected to the DC terminal i11 of the power conversion device 1 through the positive DC bus BUS+, and the second end i111- of the power conversion bridge arm 111, the second end i112- of the power conversion bridge arm 112, and the second end i113- of the power conversion bridge arm 113 are connected to the DC terminal i12 of the power conversion device 1 through the negative DC bus BUS-. The third end o111 of the power conversion bridge arm 111 is connected to the AC terminal o11 of the power conversion device 1 through the filter inductor L1, the third end o112 of the power conversion bridge arm 112 is connected to the AC terminal o12 of the power conversion device 1 through the filter inductor L2, and the third end o113 of the power conversion bridge arm 113 is connected to the AC terminal o13 of the power conversion device 1 through the filter inductor L3. One end of the filter capacitor C31 is connected to the bus midpoint N, and the other end of the filter capacitor C31 is connected to the third end o111 of the power conversion bridge arm 111 through the filter inductor L1; one end of the filter capacitor C32 is connected to the bus midpoint N, and the other end of the filter capacitor C32 is connected to the third end o112 of the power conversion bridge arm 112 through the filter inductor L2; one end of the filter capacitor C33 is connected to the bus midpoint N, and the other end of the filter capacitor C33 is connected to the third end o113 of the power conversion bridge arm 113 through the filter inductor L3.

[0038] In one embodiment, during the process of the power conversion device 1 supplying power to the grid or the load, the controller 12 detects the current at the AC end of the power conversion device 1. When the current value at the AC end of the power conversion device 1 is greater than the current threshold, the controller 12 controls the modulation wave of each power conversion bridge arm in the power conversion bridge arms 111 to 113 to periodically switch between k different DPWM waves in sequence, wherein there is a 120-degree phase difference between the modulation waves of any two power conversion bridge arms in the power conversion bridge arms 111 to 113, and k is a positive integer greater than 1.

[0039] Specifically, when the current value of the AC end of the power conversion device 1 is greater than the current threshold, it indicates that there are switch tubes with higher junction temperatures in the power conversion bridge arms 111 to 113. Then, the controller 12 determines the k preset operating conditions closest to the current operating conditions of the power conversion device 1 from the mapping table between the preset operating conditions of the power conversion device 1 and the optimal DPWM wave for junction temperature based on the current AC end voltage value and the AC end current value of the power conversion device 1, thereby determining the k optimal DPWM waves for junction temperature (such as DPWM1 wave, DPWM2 wave, ..., DPWMk wave) corresponding to the above k preset operating conditions as the target modulation wave of each power conversion bridge arm. Among them, the number of preset operating conditions of the power conversion device 1 in the above mapping table is multiple, and the multiple preset operating conditions are divided into multiple operating condition areas according to the AC end current and AC end voltage of the power conversion device 1. The number of the optimal DPWM waves for junction temperature in the above mapping table is multiple, and the multiple optimal DPWM waves for junction temperature are obtained by simulation or calculation based on the loss model of the circuit and multiple preset operating conditions to obtain the optimal DPWM waves for junction temperature corresponding to each preset operating condition. The optimal DPWM wave for junction temperature refers to the DPWM wave corresponding to the lowest junction temperature of the bottleneck switch tube when the power conversion device 1 adopts the DPWM modulation method. Exemplarily, when k=2, the controller 12 determines the implementation method of the two preset operating conditions closest to the current operating condition of the power conversion device 1 as follows: calculate the absolute value of the voltage difference between the current AC terminal voltage value of the power conversion device 1 and the AC terminal voltage value in each preset operating condition, and calculate the absolute value of the current difference between the current AC terminal current value of the power conversion device 1 and the AC terminal current value in each preset operating condition, and determine the two preset operating conditions with the smallest and second smallest absolute values ​​of the voltage difference and the current difference as the two preset operating conditions closest to the current operating condition of the power conversion device 1.

[0040] After determining the target modulation wave of each power conversion bridge arm, the controller 12 controls the modulation wave of each power conversion bridge arm to periodically switch between DPWM1 wave, DPWM2 wave, ..., DPWMk wave in sequence, wherein there are multiple switching moments in the process of periodically switching the modulation wave of each power conversion bridge arm between DPWM1 wave, DPWM2 wave, ..., DPWMk wave in sequence, and the time interval between any two adjacent switching moments among the multiple switching moments is N times the power frequency period, and N is a positive integer. Specifically, the controller 12 controls the modulation wave of each power conversion bridge arm to be DPWM1 wave, and when the duration of the modulation wave of each power conversion bridge arm being DPWM1 wave is N times the power frequency cycle, the modulation wave of each power conversion bridge arm is controlled to switch from DPWM1 wave to DPWM2 wave; ...; and when the duration of the modulation wave of each power conversion bridge arm being DPWM(k-1) wave is N times the power frequency cycle, the modulation wave of each power conversion bridge arm is controlled to switch from DPWM(k-1) wave to DPWMk wave, thereby forming a cycle of switching between DPWM1 wave, DPWM2 wave, ..., DPWMk wave in sequence, and performing a periodic cycle.

[0041] For example, when k=2, the modulation wave of the power conversion bridge arm 111 is as follows: Figure 4 As shown, the controller 12 controls the modulation wave of the power conversion bridge arm 111 to be DPWM1 wave in the time period of 0 to 0.1s. At 0.1s, the controller 12 controls the modulation wave of the power conversion bridge arm 111 to switch from DPWM1 wave to DPWM2 wave, and controls the modulation wave of the power conversion bridge arm 111 to continue to be DPWM2 wave in the time period of 0.1s to 0.2s. Figure 4 The modulation wave of the power conversion bridge arm 111 is switched only once in 0.1s. In actual operation, the modulation wave of the power conversion bridge arm 111 is switched between the DPWM1 wave and the DPWM2 wave every 5 times the power frequency period. It can be understood that the modulation wave of the power conversion bridge arm 112 and the modulation wave of the power conversion bridge arm 113 are similar to the modulation wave of the power conversion bridge arm 111, and the only difference is that the phase difference is 120 degrees. It should be noted that the specific shapes of the DPWM1 wave and the DPWM2 wave in this application are not limited to Figure 4 The shape of the DPWM wave shown in the figure can be considered as a type of DPWM wave as long as the value is equal to +1, 0, or -1 within a period of time. Figure 4 It can be seen that the two DPWM waves before and after any switching moment (such as the moment of 0.1s) are continuous, so smooth switching can be achieved between multiple DPWM waves, thereby improving the stability of the power conversion device 1.

[0042] It can be understood that when there are switch tubes with higher junction temperatures in the power conversion bridge arms 111 to 113 of the power conversion device 1, the modulation wave of each power conversion bridge arm is controlled to periodically switch between DPWM1 wave, DPWM2 wave, ..., DPWMk wave in sequence, and the time interval between any two adjacent switching moments is controlled to be N times the power frequency cycle of the power grid, so that the junction temperature of the bottleneck switch tube in the power conversion device 1 is reduced to the average value of the junction temperatures of k bottleneck switch tubes corresponding to k DPWM modulation modes, thereby reducing the loss of the bottleneck switch tube in the power conversion device 1 to the average value of the loss of k bottleneck switch tubes corresponding to k DPWM modulation modes, thereby balancing the loss of the bottleneck switch tube, making the loss of the bottleneck switch tube lower, which is beneficial to further improve the maximum output power of the power conversion device 1. In addition, since the preset operating conditions corresponding to the DPWM1 wave, DPWM2 wave, ..., and DPWMk wave are closest to the current operating conditions of the power conversion device 1, the power conversion device 1 will not change the operating conditions of the power conversion device 1 during the process of balancing the switching tube losses, which is beneficial to improving the stability of the power conversion device 1.

[0043] Furthermore, when the power conversion device 1 detects whether there is a switch tube with a high junction temperature in the power conversion bridge arms 111 to 113, it can not only consider the AC end current of the power conversion device 1, but also consider the AC end voltage of the power conversion device 1 at the same time to improve the detection accuracy. Specifically, when the AC end power value of the power conversion device 1 is greater than the power threshold, it means that there is a switch tube with a high junction temperature in the power conversion bridge arms 111 to 113, and the controller 12 controls the modulation wave of each power conversion bridge arm in the power conversion bridge arms 111 to 113 to periodically switch between DPWM1 wave, DPWM2 wave, ..., DPWMk wave in turn. Exemplarily, the power threshold is 80% of the rated power of the power conversion device 1.

[0044] It should be noted that even if the DPWM1 wave, DPWM2 wave, ..., DPWMk wave are not the DPWM waves with the optimal junction temperature, the switch loss balancing control method of the power conversion device 1 provided in the present application can still reduce the loss of the bottleneck switch tube in the power conversion device 1, that is, the loss of the bottleneck switch tube can still be balanced. In addition, in the above embodiment, the switching method of the modulation wave of each power conversion bridge arm in the k DPWM waves is switched in a fixed order. In fact, it is only necessary to ensure that the modulation wave of each power conversion bridge arm periodically traverses the k DPWM waves. Regardless of whether the switching method of the modulation wave of each power conversion bridge arm in the k DPWM waves is switched in a fixed order, the loss of the bottleneck switch tube can still be balanced. Exemplarily, when k=3, the modulation waves of each power conversion bridge arm can be DPWM1 wave, DPWM2 wave, DPWM3 wave, DPWM1 wave, DPWM3 wave, DPWM2 wave, DPWM3 wave, DPWM2 wave, DPWM1 wave, ..., DPWM2 wave, DPWM1 wave, DPWM3 wave. Among them, DPWM1 wave, DPWM2 wave, and DPWM3 wave constitute the first traversal from DPWM1 wave to DPWM3 wave, DPWM1 wave, DPWM3 wave, and DPWM2 wave constitute the second traversal from DPWM1 wave to DPWM3 wave, ..., DPWM2 wave, DPWM1 wave, and DPWM3 wave constitute the jth traversal from DPWM1 wave to DPWM3 wave.

[0045] In the present application, when there are switch tubes with high junction temperature in the power conversion bridge arms 111 to 113, the power conversion device 1 controls the modulation wave of each power conversion bridge arm to periodically traverse the DPWM1 wave, DPWM2 wave, ..., DPWMk wave, and controls the time interval between any two adjacent switching moments to be N times the power frequency cycle of the power grid, so that the junction temperature of the bottleneck switch tube in the power conversion device 1 is reduced to the average value of the junction temperatures of the k bottleneck switch tubes corresponding to the k DPWM modulation modes, thereby reducing the loss of the bottleneck switch tube in the power conversion device 1 to the average value of the loss of the k bottleneck switch tubes corresponding to the k DPWM modulation modes, thereby balancing the loss of the bottleneck switch tube, making the loss of the bottleneck switch tube lower, and further facilitating the maximum output power of the power conversion device 1. In addition, the switch loss balancing control method provided by the power conversion device 1 does not need to rely on the redundant zero-level state brought by a specific topology, and has strong applicability.

[0046] For example, Figure 3 The power conversion bridge arms 111 to 113 shown can be used Figure 5a The topological structure shown in FIG. 1 is a three-phase three-wire T-type three-level power conversion device. Figure 5aAs shown, the power conversion bridge arm 111 includes a first switch tube Q11, a second switch tube Q12, a third switch tube Q13 and a fourth switch tube Q14, the power conversion bridge arm 112 includes a first switch tube Q21, a second switch tube Q22, a third switch tube Q23 and a fourth switch tube Q24, and the power conversion bridge arm 113 includes a first switch tube Q31, a second switch tube Q32, a third switch tube Q33 and a fourth switch tube Q34. The switch tube in each power conversion bridge arm includes a metal oxide semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT) or a gallium nitride (GaN) transistor, etc. The following is an introduction taking the switch tube as an IGBT as an example: the collector of the first switch tube Q11 is connected to the first end i111+ of the power conversion bridge arm 111, the emitter of the first switch tube Q11 is connected to the second end i111- of the power conversion bridge arm 111 through the second switch tube Q12, the emitter of the first switch tube Q11 is connected to the third end o111 of the power conversion bridge arm 111 and the emitter of the fourth switch tube Q14, the collector of the fourth switch tube Q14 is connected to the collector of the third switch tube Q13, and the emitter of the third switch tube Q13 is connected to the fourth end i1110 of the power conversion bridge arm 111. The collector of the first switch tube Q21 is connected to the first end i112+ of the power conversion bridge arm 112, the emitter of the first switch tube Q21 is connected to the second end i112- of the power conversion bridge arm 112 through the second switch tube Q22, the emitter of the first switch tube Q21 is connected to the third end o112 of the power conversion bridge arm 112 and the emitter of the fourth switch tube Q24, the collector of the fourth switch tube Q24 is connected to the collector of the third switch tube Q23, and the emitter of the third switch tube Q23 is connected to the fourth end i1120 of the power conversion bridge arm 112. The collector of the first switch tube Q31 is connected to the first end i113+ of the power conversion bridge arm 113, the emitter of the first switch tube Q31 is connected to the second end i113- of the power conversion bridge arm 113 through the second switch tube Q32, the emitter of the first switch tube Q31 is connected to the third end o113 of the power conversion bridge arm 113 and the emitter of the fourth switch tube Q34, the collector of the fourth switch tube Q34 is connected to the collector of the third switch tube Q33, and the emitter of the third switch tube Q33 is connected to the fourth end i1130 of the power conversion bridge arm 113. In addition, the fourth end i1110 of the power conversion bridge arm 111, the fourth end i1120 of the power conversion bridge arm 112, and the fourth end i1130 of the power conversion bridge arm 113 are all connected to the bus midpoint N.

[0047] For example, Figure 3 The power conversion bridge arms 111 to 113 shown can be used Figure 5b The topology shown in FIG. 1 is a three-phase three-wire ANPC three-level power conversion device. Figure 5bAs shown, the power conversion bridge arm 111 includes a first switch tube Q11, a second switch tube Q12, a third switch tube Q13, a fourth switch tube Q14, a clamping diode D11 and a clamping diode D12, the power conversion bridge arm 112 includes a first switch tube Q21, a second switch tube Q22, a third switch tube Q23, a fourth switch tube Q24, a clamping diode D21 and a clamping diode D22, and the power conversion bridge arm 113 includes a first switch tube Q31, a second switch tube Q32, a third switch tube Q33, a fourth switch tube Q34, a clamping diode D31 and a clamping diode D32. The switch tube in each power conversion bridge arm includes a MOSFET, an IGBT or a GaN transistor, etc. The following is an introduction using the IGBT as an example: the collector of the first switch tube Q11 is connected to the first end i111+ of the power conversion bridge arm 111, the emitter of the first switch tube Q11 is connected to the collector of the fourth switch tube Q14 after passing through the second switch tube Q12 and the third switch tube Q13 in sequence, and the emitter of the fourth switch tube Q14 is connected to the second end i111- of the power conversion bridge arm 111. The connection between the second switch tube Q12 and the third switch tube Q13 is connected to the third end o111 of the power conversion bridge arm 111. The cathode of the clamping diode D11 is connected to the collector of the second switch tube Q12, the anode of the clamping diode D11 is connected to the cathode of the clamping diode D12 and the fourth end i1110 of the power conversion bridge arm 111, and the anode of the clamping diode D12 is connected to the emitter of the third switch tube Q13; the collector of the first switch tube Q21 is connected to the first end i112+ of the power conversion bridge arm 112, the emitter of the first switch tube Q21 is connected to the collector of the fourth switch tube Q24 after passing through the second switch tube Q22 and the third switch tube Q23 in sequence, and the emitter of the fourth switch tube Q24 is connected to the second end i112- of the power conversion bridge arm 112. The connection between the third switch tube Q23 and the fourth switch tube Q24 is connected to the third end o112 of the power conversion bridge arm 112. The cathode of the clamping diode D21 is connected to the collector of the second switch tube Q22, the anode of the clamping diode D21 is connected to the cathode of the clamping diode D22 and the fourth end i1120 of the power conversion bridge arm 112, and the anode of the clamping diode D22 is connected to the emitter of the third switch tube Q23; the collector of the first switch tube Q31 is connected to the first end i113+ of the power conversion bridge arm 113, the emitter of the first switch tube Q31 is connected to the collector of the fourth switch tube Q34 after passing through the second switch tube Q32 and the third switch tube Q33 in sequence, and the emitter of the fourth switch tube Q34 is connected to the second end i113- of the power conversion bridge arm 113. The connection between the third switch tube Q33 and the fourth switch tube Q34 is connected to the third end o113 of the power conversion bridge arm 113.The cathode of the clamping diode D31 is connected to the collector of the second switch tube Q32, the anode of the clamping diode D31 is connected to the cathode of the clamping diode D32 and the fourth end i1130 of the power conversion bridge arm 113, and the anode of the clamping diode D32 is connected to the emitter of the third switch tube Q33. In addition, the fourth end i1110 of the power conversion bridge arm 111, the fourth end i1120 of the power conversion bridge arm 112, and the fourth end i1130 of the power conversion bridge arm 113 are all connected to the bus midpoint N.

[0048] For example, Figure 3 The power conversion bridge arms 111 to 113 shown can be used Figure 5c The topological structure shown in FIG. 1 is a three-phase three-wire two-level power conversion device. Figure 5c As shown, the power conversion bridge arm 111 includes a first switch tube Q11 and a second switch tube Q12, the power conversion bridge arm 112 includes a first switch tube Q21 and a second switch tube Q22, and the power conversion bridge arm 113 includes a first switch tube Q31 and a second switch tube Q32. The switch tube in each power conversion bridge arm includes a MOSFET, an IGBT or a GaN transistor, etc. The following is an introduction using the switch tube as an IGBT as an example: the collector of the first switch tube Q11 is connected to the first end i111+ of the power conversion bridge arm 111, the emitter of the first switch tube Q11 is connected to the collector of the second switch tube Q12 and the third end o111 of the power conversion bridge arm 111, and the emitter of the second switch tube Q12 is connected to the second end i111- of the power conversion bridge arm 111. The collector of the first switch tube Q21 is connected to the first end i112+ of the power conversion bridge arm 112, the emitter of the first switch tube Q21 is connected to the collector of the second switch tube Q22 and the third end o112 of the power conversion bridge arm 112, and the emitter of the second switch tube Q22 is connected to the second end i112- of the power conversion bridge arm 112. The collector of the first switch tube Q31 is connected to the first end i113+ of the power conversion bridge arm 113, the emitter of the first switch tube Q31 is connected to the collector of the second switch tube Q32 and the third end o113 of the power conversion bridge arm 113, and the emitter of the second switch tube Q32 is connected to the second end i113- of the power conversion bridge arm 113.

[0049] See also Figure 6 , Figure 6 The control method of the power conversion device provided in the embodiment of the present application is applicable to Figure 2 to Figure 5c The power conversion device 1 shown. The control method of the power conversion device may include the steps of:

[0050] S101, controlling at least two power conversion bridge arms so that a power conversion device supplies power to a grid or a load.

[0051] In one embodiment, the power conversion device controls the modulation wave of each power conversion bridge arm in at least two power conversion bridge arms to be a DPWM wave, so as to convert the DC power at the DC end of the power conversion device into AC power and output it to the AC end of the power conversion device, thereby realizing power supply to the power grid or the load.

[0052] S102, when the current value at the AC end of the power conversion device is greater than a current threshold, controlling the modulation wave of each power conversion bridge arm to periodically switch between at least two different DPWM waves in sequence.

[0053] There are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in turn, and the time interval between any two adjacent switching moments in the multiple switching moments is N times the power frequency period, where N is a positive integer.

[0054] In one embodiment, the power conversion device controls the amplitudes of two DPWM waves before and after any switching moment to be the same at any switching moment while controlling the modulation wave of each power conversion bridge arm to periodically switch between at least two different DPWM waves in sequence.

[0055] In a specific implementation, more operations performed by the power conversion device in the control method of the power conversion device provided in this application can be found in Figure 2 to Figure 5c The implementation method performed by the power conversion device 1 shown will not be repeated here.

[0056] In the present application, the power conversion device controls the modulation wave of each power conversion bridge arm to periodically traverse at least two different DPWM waves, so that the bottleneck switch tube in the power conversion device periodically switches back and forth between at least two bottleneck switch tubes corresponding to at least two DPWM modulation modes, thereby shortening the working time of each bottleneck switch tube in the at least two bottleneck switch tubes corresponding to the at least two DPWM modulation modes, so as to reduce the junction temperature of each bottleneck switch tube in the at least two bottleneck switch tubes, thereby reducing the loss of each bottleneck switch tube in the at least two bottleneck switch tubes, so as to achieve loss balance of the bottleneck switch tube, thereby facilitating the improvement of the maximum output power of the power conversion device. In addition, the switch loss balance control method provided by the power conversion device does not need to rely on the redundant zero-level state brought by a specific topology, and has strong applicability.

[0057] The above are only specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A power conversion device, It is characterized in that The power conversion device comprises a DC end, at least two power conversion bridge arms, a controller and an AC end, wherein: The DC end of the power conversion device is used to connect to a photovoltaic module or an energy storage battery, and the AC end of the power conversion device is used to connect to a power grid or a load; The at least two power conversion bridge arms are used to convert the direct current output by the photovoltaic module or the energy storage battery into alternating current and output it to the power grid or the load; The controller is used to control the modulation wave of each power conversion bridge arm in the at least two power conversion bridge arms to periodically switch between at least two different discontinuous pulse width modulation waves in sequence when the AC end current value of the power conversion device is greater than the current threshold.

2. The power conversion device according to claim 1, It is characterized in that There are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence, and the time interval between any two adjacent switching moments in the multiple switching moments is N times the power frequency period, where N is a positive integer.

3. The power conversion device according to claim 1 or 2, It is characterized in that There are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence; The controller is also used to control the amplitudes of the two discontinuous pulse width modulation waves before and after any switching moment among the multiple switching moments to be the same at any switching moment in the process of controlling the modulation wave of each power conversion bridge arm to periodically switch between the at least two different discontinuous pulse width modulation waves in sequence.

4. The power conversion device according to any one of claims 1 to 3, It is characterized in that The power conversion device further includes a positive DC bus, a negative DC bus, a positive bus capacitor and a negative bus capacitor, the AC end of the power conversion device includes a three-phase AC end, the at least two power conversion bridge arms include three power conversion bridge arms, each of the power conversion bridge arms includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube and two clamping diodes, wherein: The positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus, and the positive DC bus and the negative DC bus are connected between the DC end of the power conversion device and the three power conversion bridge arms; The first switch tube, the second switch tube, the third switch tube and the fourth switch tube of each power conversion bridge arm are connected in series between the positive DC bus and the negative DC bus in sequence, and the connection between the second switch tube and the third switch tube of the three power conversion bridge arms is respectively connected to the three-phase AC end; The two clamping diodes of each power conversion bridge arm are connected in series between the connection between the first switch tube and the second switch tube of each power conversion bridge arm and the connection between the third switch tube and the fourth switch tube of each power conversion bridge arm, and the connection between the two clamping diodes of each power conversion bridge arm is connected to the connection between the positive bus capacitor and the negative bus capacitor.

5. The power conversion device according to any one of claims 1 to 3, It is characterized in that The power conversion device further includes a positive DC bus, a negative DC bus, a positive bus capacitor and a negative bus capacitor. The AC end of the power conversion device includes a three-phase AC end. The at least two power conversion bridge arms include three power conversion bridge arms. Each of the power conversion bridge arms includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube, wherein: The positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus, and the positive DC bus and the negative DC bus are connected between the DC end of the power conversion device and the three power conversion bridge arms; The first switch tube and the second switch tube of each power conversion bridge arm are connected in series between the positive DC bus and the negative DC bus, and the connection points of the first switch tubes and the second switch tubes in the three power conversion bridge arms are respectively connected to the three-phase AC ends; The third switch tube and the fourth switch tube of each power conversion bridge arm are connected in reverse series between the connection point of the first switch tube and the second switch tube of each power conversion bridge arm and the connection point of the positive bus capacitor and the negative bus capacitor.

6. A control method for a power conversion device, It is characterized in that The power conversion device comprises a DC end, at least two power conversion bridge arms and an AC end, wherein the DC end of the power conversion device is used to connect to a photovoltaic module or an energy storage battery, and the AC end of the power conversion device is used to connect to a power grid or a load; the at least two power conversion bridge arms are used to convert the DC power output by the photovoltaic module or the energy storage battery into AC power and output it to the power grid or the load; The method comprises: When the current value at the AC end of the power conversion device is greater than a current threshold, the modulation wave of each of the at least two power conversion bridge arms is controlled to periodically switch between at least two different discontinuous pulse width modulation waves in sequence.

7. The method according to claim 6, It is characterized in that There are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence, and the time interval between any two adjacent switching moments in the multiple switching moments is N times the power frequency period, where N is a positive integer.

8. The method according to claim 6 or 7, It is characterized in that There are multiple switching moments in the process in which the modulation wave of each power conversion bridge arm periodically switches between at least two different discontinuous pulse width modulation waves in sequence; The method further comprises: In the process of controlling the modulation wave of each power conversion bridge arm to periodically switch between the at least two different discontinuous pulse width modulation waves in sequence, the amplitudes of the two discontinuous pulse width modulation waves before and after any switching moment among the multiple switching moments are controlled to be the same at any switching moment.

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