Power conversion device

Abstract

A power conversion device comprises a three-phase power conversion bridge arm, a balance bridge arm, a three-phase grid-connected switch, an N-phase grid-connected switch, a controller, a three-phase alternating current end and an N-phase alternating current end. Three alternating-current ends of the three-phase power conversion bridge arm are respectively connected with a three-phase alternating-current end through a three-phase grid-connected switch, and an output end of the balance bridge arm is connected with an N-phase alternating-current end through an N-phase grid-connected switch. The controller controls the balance bridge arm to enable the absolute value of the direct-current component to be smaller than or equal to a first voltage threshold value under the condition that the absolute value of the direct-current component of the voltage difference between the two ends of any phase of grid-connected switch in the three-phase grid-connected switch and the N-phase grid-connected switch is larger than the first voltage threshold value; under the condition that the absolute value of the direct current component is smaller than or equal to a first voltage threshold value, an N-phase grid-connected switch is controlled to be closed; and after the N-phase grid-connected switch is closed, the three-phase grid-connected switch is controlled to be closed synchronously. The influence of direct-current common-mode voltage on each phase of grid-connected switch can be eliminated, and small-voltage-difference closing of each phase of grid-connected switch is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power supply, in particular to a power conversion device. BACKGROUND

[0002] In the field of photovoltaic and energy storage power supply, a power converter is needed to convert the direct current of a photovoltaic string or a battery cluster into alternating current to supply the alternating current load. In the process of grid connection, the grid connection switch needs to be closed when the voltage difference across the grid connection switch is less than the voltage difference threshold to complete the grid connection. If the voltage difference across the grid connection switch is too large when the grid connection switch is closed, there is a risk of contact sticking, resulting in failure of the grid connection switch. When the impedance to ground of the positive pole of the direct current bus and the negative pole of the direct current bus is inconsistent, or when the positive bus capacitor and the negative bus capacitor connected between the positive pole of the direct current bus and the negative pole of the direct current bus in the power converter are inconsistent, there will be a direct current common mode voltage on the power converter side of the grid connection switch, resulting in a large voltage difference across the grid connection switch when the grid connection switch is closed. Therefore, before the grid connection switch is closed, it is particularly important to eliminate the influence of the direct current common mode voltage on the grid connection switch to achieve small voltage difference attraction of the grid connection switch. SUMMARY

[0003] The present application provides a power conversion device, which can eliminate the influence of the direct current common mode voltage and / or the alternating current common mode voltage on each phase grid connection switch and achieve small voltage difference attraction of the grid connection switch.

[0004] In a first aspect, the application provides a power conversion device, comprising a positive DC bus pole, a negative DC bus pole, a positive bus capacitor, a negative bus capacitor, a three-phase four-leg power conversion circuit, a three-phase grid-connected switch, an N-phase grid-connected switch, a controller, a three-phase AC terminal and an N-phase AC terminal. The positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus pole and the negative DC bus pole, and the connection point of the positive bus capacitor and the negative bus capacitor is a bus midpoint. The three-phase four-leg power conversion circuit comprises a three-phase power conversion leg and a balancing leg. The DC terminals of each phase power conversion leg in the three-phase power conversion leg are respectively connected to the positive DC bus pole, the negative DC bus pole and the bus midpoint, and the three AC terminals of the three-phase power conversion leg are respectively connected to the three-phase AC terminal through the three-phase grid-connected switch. The input terminals of the balancing leg are respectively connected to the positive DC bus pole, the negative DC bus pole and the bus midpoint, and the output terminal of the balancing leg is connected to the N-phase AC terminal through the N-phase grid-connected switch. The N-phase AC terminal and the three-phase AC terminal are used to connect to an AC grid. The controller is configured to, when the absolute value of the DC component of the voltage difference between any one of the three-phase grid-connected switch and the N-phase grid-connected switch (i.e., the DC common-mode voltage) is greater than a first voltage threshold, control the balancing leg to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor, so that the absolute value of the DC component of the voltage difference between any one of the three-phase grid-connected switch and the N-phase grid-connected switch is less than or equal to the first voltage threshold; when the absolute value of the DC component of the voltage difference between any one of the three-phase grid-connected switch and the N-phase grid-connected switch is less than or equal to the first voltage threshold, i.e., when there is no DC common-mode voltage in the voltage difference between any one of the three-phase grid-connected switch and the N-phase grid-connected switch, control the N-phase grid-connected switch to be closed, thereby realizing small-voltage-difference closing of the N-phase grid-connected switch. After the N-phase grid-connected switch is closed, the AC terminal voltage of the three-phase power conversion leg follows the voltage of the AC grid; when the voltage difference between the three-phase grid-connected switch is less than or equal to a second voltage threshold, the frequency difference between the voltages of the three-phase grid-connected switch is less than or equal to a frequency threshold, and the phase difference between the voltages of the three-phase grid-connected switch is less than or equal to a phase threshold, control the three-phase grid-connected switch to be closed.

[0005] It can be understood that, in the case that the power conversion device exists the DC common-mode voltage in the voltage difference between the any-phase grid-connected switch, the DC common-mode voltage is reduced to zero by controlling the balance bridge arm to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor, so as to eliminate the influence of the DC common-mode voltage on the any-phase grid-connected switch. The control mode of the power conversion device using the balance bridge arm to eliminate the influence of the DC common-mode voltage does not need to additionally increase the hardware bias circuit, and has fast adjustment speed and wide adjustment range. Since the N-phase grid-connected switch is closed in the case that the DC common-mode voltage does not exist in the voltage difference between the any-phase grid-connected switch, the small-voltage-difference closing of the N-phase grid-connected switch can be realized. After the N-phase grid-connected switch is closed, the DC common-mode voltage and the AC common-mode voltage in the voltage difference between the any-phase grid-connected switch in the three-phase grid-connected switch are eliminated, so that the voltage difference between the three-phase grid-connected switch is no longer affected by the DC common-mode voltage and the AC common-mode voltage. At this time, the three-phase grid-connected switch can be controlled to be closed synchronously, and the small-voltage-difference closing of the three-phase grid-connected switch can be realized.

[0006] In the first possible implementation, the controller is specifically configured to control the N-phase grid-connected switch to be closed in the case that the absolute value of the DC component of the voltage difference between the any-phase grid-connected switch is less than or equal to the first voltage threshold, and the effective value of the AC component of the voltage difference between the any-phase grid-connected switch is less than or equal to the third voltage threshold, that is, in the case that the DC common-mode voltage and the AC common-mode voltage do not exist in the voltage difference between the any-phase grid-connected switch.

[0007] In the present embodiment, it is considered that, when the power conversion device is connected to the AC power grid with a heavy unbalanced load, the voltage difference between the any-phase grid-connected switch can also be affected by the AC common-mode voltage. Based on this, the N-phase grid-connected switch is controlled to be closed in the case that the N-phase grid-connected switch is not affected by the DC common-mode voltage and the AC common-mode voltage in the voltage difference between the N-phase grid-connected switch, so as to realize the small-voltage-difference (such as zero-voltage-difference) closing of the N-phase grid-connected switch.

[0008] In the second possible implementation, the controller is specifically configured to control the N-phase grid-connected switch to be closed at the voltage zero-crossing point of the connection between the N-phase grid-connected switch and the N-phase AC terminal in the case that the effective value of the AC component of the voltage difference between the any-phase grid-connected switch is greater than the third voltage threshold, and the absolute value of the DC component of the voltage difference between the any-phase grid-connected switch is less than or equal to the first voltage threshold, that is, in the case that the AC common-mode voltage exists and the DC common-mode voltage does not exist in the voltage difference between the any-phase grid-connected switch. Here, the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal can be understood as the AC common-mode voltage.

[0009] In the embodiment, it is considered that when the AC common-mode voltage reaches the voltage zero-crossing point, the DC common-mode voltage can not have been controlled to a small value, and closing the N-phase grid-connection switch can cause a large voltage difference across the N-phase grid-connection switch. Based on this, the power conversion device can control the N-phase grid-connection switch to be closed at the voltage zero-crossing point of the AC common-mode voltage when the AC common-mode voltage exists in the voltage difference across any phase grid-connection switch and the DC common-mode voltage in the voltage difference across any phase grid-connection switch has been eliminated. Thus, it can be ensured that the DC common-mode voltage has been eliminated before the power conversion device controls the N-phase grid-connection switch to be closed at the voltage zero-crossing point of the AC common-mode voltage, and the N-phase grid-connection switch can be closed with a smaller voltage difference.

[0010] In a third possible implementation, the controller is specifically configured to send a first driving signal to the N-phase grid-connection switch to cause the N-phase grid-connection switch to be closed at the voltage zero-crossing point of the voltage at the connection between the N-phase grid-connection switch and the N-phase AC terminal when the phase-locked angle of the voltage at the connection between the N-phase grid-connection switch and the N-phase AC terminal is located in a leading phase angle interval, where the leading phase angle interval is determined by the phase angle converted by the switching closing time of the N-phase grid-connection switch and a phase angle error threshold.

[0011] In the embodiment, the power conversion device sends a driving signal to the N-phase grid-connection switch when the phase-locked angle of the voltage at the connection between the N-phase grid-connection switch and the N-phase AC terminal reaches the phase angle converted by the switching closing time of the N-phase grid-connection switch at 0 degrees, 180 degrees or 360 degrees, to avoid a time interval between the time when the driving signal is sent and the actual closing time of the N-phase grid-connection switch, which can cause the N-phase grid-connection switch to be greatly affected by the AC common-mode voltage when it is closed. Thus, the N-phase grid-connection switch can be closed when the AC common-mode voltage is zero, and the N-phase grid-connection switch can be closed with a smaller voltage difference. In short, the power conversion device can further reduce the AC common-mode voltage difference when the N-phase grid-connection switch is attracted by using the phase selection closing strategy, and the N-phase grid-connection switch can be closed with a smaller voltage difference.

[0012] In a fourth possible implementation, the controller is further configured to, before sending the first driving signal, control the balanced bridge arm to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor, so that the absolute value of the DC component of the voltage difference across any phase grid-connection switch is a first voltage, where the first voltage is not equal to zero; and send a second driving signal to the N-phase grid-connection switch to obtain the switching closing time of the N-phase grid-connection switch when the absolute value of the DC component of the voltage difference across any phase grid-connection switch is the first voltage, where the switching closing time of the N-phase grid-connection switch is determined by the time interval between the time when the second driving signal is sent by the controller and the time when the current on the N-phase grid-connection switch reaches a current threshold.

[0013] In the embodiment, when the power conversion device detects that there is an AC common-mode voltage in the voltage difference between the two ends of any phase grid-connected switch, the current actual switching closing time of the N-phase grid-connected switch is obtained by sending a driving signal to the N-phase grid-connected switch, and the switching closing time of the N-phase grid-connected switch is calibrated based on the current actual switching closing time of the N-phase grid-connected switch, so that the phase selection closing precision can be improved, and the N-phase grid-connected switch can be closed with a smaller voltage difference.

[0014] In a fifth possible implementation, the controller is further configured to, after the N-phase grid-connected switch is closed, and before controlling the AC end voltage of the three-phase power conversion bridge arm to follow the voltage of the AC power grid, control the balancing bridge arm to make the absolute value of the voltage difference between the positive bus capacitor and the negative bus capacitor less than or equal to a fourth voltage threshold.

[0015] In the embodiment, before controlling the AC end voltage of the three-phase power conversion bridge arm to follow the voltage of the AC power grid, the balancing bridge arm is controlled to make the voltage of the positive bus capacitor equal to the voltage of the negative bus capacitor, so that the AC voltage obtained after the AC voltage output by the three-phase power conversion bridge arm passes through the filter inductor and the filter capacitor is not asymmetric, and the N-phase grid-connected switch can be closed with a smaller voltage difference.

[0016] In a sixth possible implementation, the balancing bridge arm includes a first switch tube, a second switch tube, and an inductor, and the input end of the balancing bridge arm includes a first input end, a second input end, and a third input end. The first input end of the balancing bridge arm is connected to the positive pole of the DC bus, the second input end of the balancing bridge arm is connected to the negative pole of the DC bus, and the third input end of the balancing bridge arm is connected to the bus midpoint. The first switch tube and the second switch tube are connected in series between the first input end and the second input end of the balancing bridge arm, and the connection point of the first switch tube and the second switch tube is connected to the third input end and the output end of the balancing bridge arm through the inductor. The controller is configured to control the second switch tube to be turned on for a first time duration and then turned off, and after the second switch tube is turned off, control the first switch tube to be turned on for a second time duration and then turned off, so that the voltage difference between the positive bus capacitor and the negative bus capacitor is a second voltage. When the voltage difference is the second voltage, the absolute value of the DC component of the voltage difference between the two ends of any phase grid-connected switch is less than or equal to a first voltage threshold.

[0017] In a seventh possible implementation, the balancing bridge arm further includes a third switch tube and a fourth switch tube, and the third switch tube and the fourth switch tube are connected in anti-parallel between the connection point of the first switch tube and the second switch tube and the third input end of the balancing bridge arm.

[0018] In an eighth possible implementation, the balancing bridge arm includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, an inductor, and a capacitor, and the input end of the balancing bridge arm includes a first input end, a second input end, and a third input end. The first input end of the balancing bridge arm is connected to the positive pole of the DC bus, the second input end of the balancing bridge arm is connected to the negative pole of the DC bus, and the third input end of the balancing bridge arm is connected to the bus midpoint. The first switch tube, the second switch tube, the third switch tube, and the fourth switch tube are connected in series between the first input end and the second input end of the balancing bridge arm. One end of the capacitor is connected to the connection point of the first switch tube and the second switch tube, and the other end of the capacitor is connected to the connection point of the third switch tube and the fourth switch tube. The connection point of the second switch tube and the third switch tube is connected to the third input end and the output end of the balancing bridge arm through the inductor. The controller controls the second switch tube and the fourth switch tube to be turned on for a first time duration and then turned off, and after the second switch tube and the fourth switch tube are turned off, controls the first switch tube and the third switch tube to be turned on for a second time duration and then turned off, so that the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is a second voltage, and when the difference is the second voltage, the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to a first voltage threshold.

[0019] In a second aspect, the application provides a power conversion device, which includes a positive pole of a DC bus, a negative pole of the DC bus, a positive bus capacitor, a negative bus capacitor, a three-phase four-bridge-arm power conversion circuit, a three-phase grid-connected switch, an N-phase grid-connected switch, a controller, a three-phase AC end, and an N-phase AC end. The positive bus capacitor and the negative bus capacitor are connected in series between the positive pole of the DC bus and the negative pole of the DC bus, and the connection point of the positive bus capacitor and the negative bus capacitor is a bus midpoint. The three-phase four-bridge-arm power conversion circuit includes a three-phase power conversion bridge arm and a balancing bridge arm. The DC end of each phase power conversion bridge arm in the three-phase power conversion bridge arm is respectively connected to the positive pole of the DC bus, the negative pole of the DC bus, and the bus midpoint, and the three AC ends of the three-phase power conversion bridge arm are respectively connected to the three-phase AC end through the three-phase grid-connected switch. The input end of the balancing bridge arm is respectively connected to the positive pole of the DC bus, the negative pole of the DC bus, and the bus midpoint, and the output end of the balancing bridge arm is connected to the N-phase AC end through the N-phase grid-connected switch. The controller controls the N-phase grid-connected switch to be closed at the voltage zero-crossing point of the connection point of the N-phase grid-connected switch and the N-phase AC end when the effective value of the AC component of the voltage difference across any phase grid-connected switch in the three-phase grid-connected switch and the N-phase grid-connected switch is greater than a third voltage threshold, indicating that there is an AC common-mode voltage in the voltage difference across any phase grid-connected switch. After the N-phase grid-connected switch is closed, the AC end voltage of the three-phase power conversion bridge arm follows the voltage of the AC power grid. When the voltage difference across the three-phase grid-connected switch is less than or equal to a second voltage threshold, the frequency difference of the voltage across the three-phase grid-connected switch is less than or equal to a frequency threshold, and the phase difference of the voltage across the three-phase grid-connected switch is less than or equal to a phase threshold, the three-phase grid-connected switch is controlled to be closed.

[0020] It can be understood that, when the AC power grid connected with the power conversion device has a heavy unbalanced load, the voltage difference across any phase grid-connected switch can be affected by the AC common-mode voltage. Based on this, when the power conversion device has the AC common-mode voltage in the voltage difference across any phase grid-connected switch, the N-phase grid-connected switch is controlled to be closed at the voltage zero-crossing point of the AC common-mode voltage, so as to avoid the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch, and to realize small voltage difference closing of the N-phase grid-connected switch. After the N-phase grid-connected switch is closed, the DC common-mode voltage and the AC common-mode voltage in the voltage difference across each phase grid-connected switch in the three-phase grid-connected switch are eliminated, so that the voltage difference across the three-phase grid-connected switch is no longer affected by the DC common-mode voltage and the AC common-mode voltage. At this time, the three-phase grid-connected switch is controlled to be closed at the same time, and small voltage difference closing of the three-phase grid-connected switch can be realized.

[0021] In the first possible implementation, the controller is specifically configured to control the N-phase grid-connected switch to be closed at the voltage zero-crossing point of the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal, when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, and the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

[0022] In the present embodiment, it is considered that the voltage difference across any phase grid-connected switch can also be affected by the DC common-mode voltage. Based on this, when the power conversion device has no DC common-mode voltage and has the AC common-mode voltage in the voltage difference across any phase grid-connected switch, the N-phase grid-connected switch is controlled to be closed at the voltage zero-crossing point of the AC common-mode voltage. In short, when the voltage difference across the N-phase grid-connected switch is not affected by the DC common-mode voltage and the AC common-mode voltage, the N-phase grid-connected switch is controlled to be closed, so as to realize smaller voltage difference (such as zero voltage difference) closing of the N-phase grid-connected switch.

[0023] In the second possible implementation, the controller is specifically configured to send a first driving signal to the N-phase grid-connected switch to make the N-phase grid-connected switch be closed at the voltage zero-crossing point of the voltage at the second end of the N-phase grid-connected switch, when the phase-locked angle of the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal is located in a leading phase angle interval, wherein the leading phase angle interval is determined by the phase angle converted by the switch closing time of the N-phase grid-connected switch and a phase angle error threshold. In short, the power conversion device further reduces the AC common-mode voltage difference when the N-phase grid-connected switch is attracted by using the phase selection closing strategy, so as to realize smaller voltage difference closing of the N-phase grid-connected switch.

[0024] In a third possible implementation, the controller is further configured to, before sending the first drive signal, control the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, wherein the first voltage is not equal to zero; when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, send a second drive signal to the N-phase grid-connected switch to obtain the switching closing time of the N-phase grid-connected switch, wherein the switching closing time of the N-phase grid-connected switch is determined by the duration between the moment when the controller sends the second drive signal and the moment when the current on the N-phase grid-connected switch reaches a current threshold.

[0025] In this embodiment, when the power conversion device detects an AC common-mode voltage in the voltage difference across any phase grid-connected switch, it sends a drive signal to the N-phase grid-connected switch to obtain the current actual closing time of the N-phase grid-connected switch. Based on the current actual closing time of the N-phase grid-connected switch, it calibrates the closing time of the N-phase grid-connected switch, thereby improving the phase selection closing accuracy and further realizing smaller voltage difference closing of the N-phase grid-connected switch.

[0026] In a fourth possible implementation, the controller is further configured to control the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than a first voltage threshold, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

[0027] In this embodiment, when a DC common-mode voltage exists in the voltage difference across any phase grid-connected switch, the power conversion device reduces the DC common-mode voltage to zero by controlling the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor, thereby eliminating the influence of the DC common-mode voltage on any phase grid-connected switch.

[0028] In a fifth possible implementation, the controller is further configured to control the balancing arm after the N-phase grid-connected switch is closed and before controlling the AC terminal voltage of the three-phase power conversion arm to follow the AC grid voltage, such that the absolute value of the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a fourth voltage threshold.

[0029] In this embodiment, before controlling the AC terminal voltage of the three-phase power conversion bridge arm to follow the AC grid voltage, the power conversion device controls the balance bridge arm to make the voltage of the positive bus capacitor equal to the voltage of the negative bus capacitor. This avoids the situation where the AC voltage output by the three-phase power conversion bridge arm is asymmetrical after passing through the filter inductor and filter capacitor, thereby better realizing the small voltage difference energization of the three-phase grid-connected switch. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the application scenario of the power conversion device provided in this application;

[0031] Figure 2 This is a schematic diagram of the power conversion device provided in this application;

[0032] Figure 3 This is another structural schematic diagram of the power conversion device provided in this application;

[0033] Figure 4 This is a waveform diagram of the voltage difference and current across an N-phase grid-connected switch provided by existing technology;

[0034] Figure 5 This is a schematic diagram of the control method for the power conversion device 1 provided in this application;

[0035] Figure 6 This is a waveform diagram of the voltage difference, current, positive bus capacitor voltage, and negative bus capacitor voltage across the N-phase grid-connected switch provided in this application.

[0036] Figure 7 This is another structural schematic diagram of the power conversion device provided in this application;

[0037] Figure 8 This is another structural schematic diagram of the power conversion device provided in this application. Detailed Implementation

[0038] The power conversion device provided in this application is applicable to various fields such as photovoltaic power generation, energy storage power generation, new energy smart microgrids, and power transmission and distribution. The power conversion device provided in this application can be an inverter, a power conversion system (PCS), an uninterruptible power supply (UPS), etc., suitable for different application scenarios, such as photovoltaic power supply scenarios, energy storage power supply scenarios, photovoltaic-energy storage hybrid power supply scenarios, and UPS power supply scenarios. The following explanation uses an energy storage power supply scenario as an example.

[0039] See Figure 1 , Figure 1 This is a schematic diagram illustrating an application scenario of the power conversion device provided in this application. In a photovoltaic-storage hybrid power supply scenario, the power conversion device provided in this application is... Figure 1The diagram shows a photovoltaic inverter or energy storage converter. A photovoltaic inverter is used to convert direct current (DC) from photovoltaic modules to alternating current (AC) after grid connection is completed via a grid-connection switch, and then transmits the AC power to the grid or load. An energy storage converter is used to convert DC from energy storage battery clusters to AC after grid connection is completed via a grid-connection switch, and then transmit the AC power to the grid or load; alternatively, it can convert AC from the grid to DC and transmit the DC power to the energy storage battery clusters to charge them.

[0040] Because the ground impedance of the positive and negative DC bus terminals of any device in a photovoltaic inverter or energy storage converter is inconsistent, or the positive and negative bus capacitances of any device are inconsistent, a DC common-mode voltage will exist on the device side of the grid-connected switch in any device. This will result in an excessively large voltage difference across the grid-connected switch when it is closed. Therefore, eliminating the influence of the DC common-mode voltage on the grid-connected switch before it is closed is particularly important. Based on this, this application provides a power conversion device that can eliminate the influence of the DC common-mode voltage on the grid-connected switch before it is closed, thereby achieving small voltage difference closing of the grid-connected switch. The following is in conjunction with... Figures 2 to 5 The working principle of the power conversion device provided in this application is illustrated by example.

[0041] See Figure 2 , Figure 2 This is a schematic diagram of the power conversion device provided in this application. Figure 2As shown, the power conversion device 1 includes a DC terminal (e.g., i11 and i12), a DC bus positive terminal BUS+, a DC bus negative terminal BUS-, a positive bus capacitor C1, a negative bus capacitor C2, a three-phase four-bridge power conversion circuit 11, a three-phase grid-connected switch (e.g., the first phase grid-connected switch K1, the second phase grid-connected switch K2, and the third phase grid-connected switch K3), an N-phase grid-connected switch K4, a controller 12, three-phase AC terminals (e.g., the first phase AC terminal o1A, the second phase AC terminal o1B, and the third phase AC terminal o1C), and an N-phase AC terminal o1N. The DC terminal of the power conversion device 1 is used to connect to a DC source (e.g., a photovoltaic string or a battery storage cluster), and the three-phase AC terminals and the N-phase AC terminals of the power conversion device 1 are used to connect to an AC power grid (including a large power grid or a microgrid). The positive terminal BUS+ and the negative terminal BUS- of the DC bus are connected to the DC terminals i11 and i12 of the power conversion device 1, respectively. The positive bus capacitor C1 and the negative bus capacitor C2 are connected in series between the positive terminal BUS+ and the negative terminal BUS- of the DC bus. The connection point of the positive bus capacitor C1 and the negative bus capacitor C2 is the bus midpoint N. The three-phase four-arm power conversion circuit 11 includes three-phase power conversion arms (such as the first phase arm 111, the second phase arm 112, and the third phase arm 113) and a balancing arm 114. The DC terminals i111, i112, and i113 of the first phase bridge arm 111 are connected to the positive terminal BUS+, the negative terminal BUS-, and the midpoint N of the DC bus, respectively. The DC terminals i121, i122, and i123 of the second phase bridge arm 112 are connected to the positive terminal BUS+, the negative terminal BUS-, and the midpoint N of the DC bus, respectively. The DC terminals i131, i132, and i133 of the third phase bridge arm 113 are connected to the positive terminal BUS+, the negative terminal BUS-, and the midpoint N of the DC bus, respectively. The AC terminal o11 of the first phase bridge arm 111 is connected to the first phase AC terminal o1A through the first phase grid-connected switch K1. The AC terminal o12 of the second phase bridge arm 112 is connected to the second phase AC terminal o1B through the second phase grid-connected switch K2. The AC terminal o13 of the third phase bridge arm 113 is connected to the third phase AC terminal o1C through the third phase grid-connected switch K3. The input terminals of the balancing bridge arm 114 (such as i141, i142, and i143) are connected to the positive terminal BUS+, the negative terminal BUS-, and the bus midpoint N, respectively. The output terminal o14 of the balancing bridge arm 114 is connected to the N-phase AC terminal o1N through the N-phase grid-connected switch K4. It should be noted that this explanation uses one grid-connected switch for each phase as an example. In practical applications, multiple grid-connected switches can be connected in series for each phase, so that if one switch becomes stuck, the line can be effectively disconnected by controlling the other switches connected in series with it.

[0042] Specifically, the first terminal of the first-phase grid-connected switch K1 is connected to the AC terminal o11 of the first-phase bridge arm 111, and the second terminal of the first-phase grid-connected switch K1 is connected to the first-phase AC terminal o1A. The first terminal of the second-phase grid-connected switch K2 is connected to the AC terminal o12 of the second-phase bridge arm 112, and the second terminal of the second-phase grid-connected switch K2 is connected to the second-phase AC terminal o1B. The first terminal of the third-phase grid-connected switch K3 is connected to the AC terminal o13 of the third-phase bridge arm 113, and the second terminal of the third-phase grid-connected switch K3 is connected to the third-phase AC terminal o1C. The first terminal of the Nth-phase grid-connected switch K4 is connected to the output terminal o14 of the balancing bridge arm 114, and the second terminal of the Nth-phase grid-connected switch K4 is connected to the Nth-phase AC terminal o1N.

[0043] To facilitate understanding, we will first introduce the voltage difference across grid-connected switches K1 to K4. For the first-phase grid-connected switch K1, the voltage difference ΔU across K1 is... K1 =U iAPE -U gAPE =(U iAN +U iNPE )-(U gAN +U gNPE ). Among them, U iAPE U is the voltage at the first terminal of the first phase grid-connected switch K1 relative to the reference ground PE. gAPE U is the voltage at the second terminal of the first phase grid-connected switch K1 relative to the reference ground PE. iAN U is the voltage from the first terminal of the first phase grid-connected switch K1 to the first terminal of the N-phase grid-connected switch K4. gAN U is the voltage from the second terminal of the first-phase grid-connected switch K1 to the second terminal of the N-phase grid-connected switch K4. iNPE U is the voltage at the first terminal of the N-phase grid-connected switch K4 relative to the reference ground PE, or simply the voltage at the first terminal of the N-phase grid-connected switch K4 relative to ground; gNPE This refers to the voltage at the second terminal of the N-phase grid-connected switch K4 relative to the reference ground PE, or simply the voltage at the second terminal of the N-phase grid-connected switch K4 relative to ground. In actual control, this voltage can be controlled by adjusting the first phase bridge arm 111 to... iAN Follow U gAN Even if U iAN =U gAN Therefore, the voltage difference ΔU across the first phase grid-connected switch K1 is... K1 It can be simplified to U iNPE -U gNPE Similarly, it can be seen that the voltage difference across each grid-connected switch in grid-connected switches K2 to K4 is U. iNPE -U gNPE Here, U iNPEThis is due to the inconsistent ground impedance of the positive DC bus (BUS+) and negative DC bus (BUS-), or the inconsistent capacitance of the positive bus (C1) and negative bus (C2), resulting in a DC common-mode voltage at the first terminal of the N-phase grid-connected switch K4; U gNPE This is due to the presence of an AC common-mode voltage at the second terminal of the N-phase grid-connected switch K4 when there is a heavy unbalanced load in the AC power grid. In summary, the voltage difference across each phase of the grid-connected switch from K1 to K4 is affected by the DC common-mode voltage and / or AC common-mode voltage on the N-phase grid-connected switch. The power conversion device 1 provided in this application can achieve small voltage difference closing of each phase grid-connected switch in three scenarios: the presence of DC common-mode voltage and / or AC common-mode voltage on each phase grid-connected switch.

[0044] For example, each phase power conversion arm adopts a T-type three-level topology, and the balanced arm 114 adopts a two-level arm topology. See [link to relevant documentation]. Figure 3 , Figure 3 This is another structural schematic diagram of the power conversion device provided in this application. For example... Figure 3 As shown, the first phase bridge arm 111 includes switching transistors Q11, Q12, Q13, and Q14. Switching transistors Q11 and Q12 are connected in series between the DC terminals i111 and i112 of the first phase bridge arm 111. The connection between switching transistors Q11 and Q12 is connected to the AC terminal o11 of the first phase bridge arm 111. Switching transistors Q13 and Q14 are connected in reverse series between the DC terminal i113 and the AC terminal o11 of the first phase bridge arm 111. The second phase bridge arm 112 includes switching transistors Q21, Q22, Q23, and Q24. Switching transistors Q21 and Q22 are connected in series between the DC terminals i121 and i122 of the second phase bridge arm 112. In this configuration, the connection between switch Q21 and switch Q22 is connected to the AC terminal o12 of the second phase bridge arm 112. Switch Q23 and switch Q24 are connected in reverse series between the DC terminal i123 and the AC terminal o12 of the second phase bridge arm 112. The third phase bridge arm 113 includes switch Q31, switch Q32, switch Q33 and switch Q34. Switch Q31 and switch Q32 are connected in series between the DC terminals i131 and i132 of the third phase bridge arm 113. The connection between switch Q31 and switch Q32 is connected to the AC terminal o13 of the third phase bridge arm 113. Switch Q33 and switch Q34 are connected in reverse series between the DC terminal i133 and the AC terminal o13 of the third phase bridge arm 113.

[0045] The balanced bridge arm 114 includes a first switch Q41, a second switch Q42, and an inductor L41. The input terminals of the balanced bridge arm 114 include a first input terminal i141, a second input terminal i142, and a third input terminal i143. The first switch Q41 and the second switch Q42 are connected in series between the first input terminal i141 and the second input terminal i142. The connection between the first switch Q41 and the second switch Q42 is connected to the third input terminal i143 and the output terminal i14 through the inductor L41.

[0046] In addition, the power conversion device 1 also includes filter inductors L11, L12, L21, L22, L31, L32, and L42, and filter capacitors C31, C32, and C33. The AC terminal o11 of the first phase bridge arm 111 is connected to the first terminal of the first phase grid-connected switch K1 in sequence through filter inductors L11 and L12. The AC terminal o12 of the second phase bridge arm 112 is connected to the first terminal of the second phase grid-connected switch K2 in sequence through filter inductors L21 and L22. The AC terminal o13 of the third phase bridge arm 113 is connected to the first terminal of the third phase grid-connected switch K3 in sequence through filter inductors L31 and L32. The output terminal o14 of the balanced bridge arm 114 is connected to the first terminal of the N-phase grid-connected switch K4 through filter inductor L42. Filter capacitor C31 is connected between the connection point of filter inductors L11 and L12 and the midpoint N of the busbar; filter capacitor C32 is connected between the connection point of filter inductors L21 and L22 and the midpoint N of the busbar; and filter capacitor C33 is connected between the connection point of filter inductors L31 and L32 and the midpoint N of the busbar.

[0047] Before the three-phase grid-connected switches K1 to K3 and the N-phase grid-connected switch K4 are closed, the controller 12 acquires the voltage difference across any one of the three-phase grid-connected switches K1 to K3 and the N-phase grid-connected switch K4, and determines whether there is a DC common-mode voltage and an AC common-mode voltage in the voltage difference across any one of the phase grid-connected switches based on the DC and AC components in the voltage difference across any one of the phase grid-connected switches.

[0048] In one implementation scenario, a DC common-mode voltage exists in the voltage difference across any phase grid-connected switch. The controller 12 can achieve small voltage difference closure of each phase grid-connected switch in the following manner:

[0049] When the absolute value of the DC component of the voltage difference across any phase grid-connected switch (i.e., the DC common-mode voltage) is greater than a first voltage threshold, controller 12 controls the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold. The first voltage threshold is determined by the DC closing capability of the grid-connected switch. For example, the first voltage threshold is zero. Considering voltage fluctuations in actual control, the first voltage threshold can also be other values ​​that fluctuate slightly around zero. Here, the DC common-mode voltage is equivalent to either the voltage to ground at the bus midpoint N or the voltage to ground at the first terminal of the N-phase grid-connected switch K4.

[0050] Specifically, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than a first voltage threshold, controller 12 controls the second switch Q42 to conduct for a first duration and then turn it off, so that the energy of the negative bus capacitor C2 is transferred to the inductor L41. After the second switch Q42 is turned off, controller 12 controls the first switch Q41 to conduct for a second duration and then turn it off, so that the energy stored in the inductor L41 is transferred to the positive bus capacitor C1, thereby making the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 a second voltage. Wherein, when the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 is the second voltage, the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

[0051] It is understandable that when there is a DC common-mode voltage in the voltage difference across any phase grid-connected switch, the power conversion device 1 reduces the DC common-mode voltage to zero by controlling the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, thereby eliminating the influence of the DC common-mode voltage on any phase grid-connected switch.

[0052] Subsequently, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to a first voltage threshold, that is, when there is no DC common-mode voltage in the voltage difference across the N-phase grid-connected switch, controller 12 controls the N-phase grid-connected switch K4 to close, thereby achieving small voltage difference closure of the N-phase grid-connected switch. Preferably, the voltage difference across any phase grid-connected switch may also be affected by AC common-mode voltage. Based on this, controller 12 controls the N-phase grid-connected switch K4 to close when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to a first voltage threshold, and the effective value of the AC component of the voltage difference across any phase grid-connected switch is less than or equal to a third voltage threshold, that is, when there is no DC common-mode voltage and AC common-mode voltage in the voltage difference across the N-phase grid-connected switch, thereby achieving even smaller voltage difference (e.g., zero voltage difference) closure of the N-phase grid-connected switch. Since after the N-phase grid-connected switch K4 is closed, UiNPE =U gNPE Therefore, the voltage difference between the two ends of each phase grid-connected switch from the first phase grid-connected switch K1 to the third phase grid-connected switch K3 is no longer affected by the DC common-mode voltage and the AC common-mode voltage. The third voltage threshold is determined by the AC closing capability of the grid-connected switch.

[0053] After the N-phase grid-connected switch K4 is closed, the controller 12 controls the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage. That is, by controlling the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113, the controller ensures that the AC voltage after passing through the filter circuit (i.e., the first terminal voltage of the three-phase grid-connected switch) follows the AC grid voltage. When the voltage difference across the three-phase grid-connected switch is less than or equal to a second voltage threshold, the frequency difference across the three-phase grid-connected switch is less than or equal to a frequency threshold, and the phase difference across the three-phase grid-connected switch is less than or equal to a phase threshold, the controller 12 controls the three-phase grid-connected switch to close, thereby controlling the simultaneous closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3, thus achieving small voltage difference closure of the first phase grid-connected switch K1 to the third phase grid-connected switch K3. For example, the second voltage threshold is 10% of the AC grid voltage, the frequency threshold is 0.5Hz, and the phase threshold is 10 degrees.

[0054] When the voltage of the positive bus capacitor C1 deviates from that of the negative bus capacitor C2, the AC voltage output from the three-phase power conversion bridge arm will be asymmetrical after passing through the filter inductor and filter capacitor. This will cause the first-terminal voltage of any of the first-phase grid-connected switches K1 to K3 to fail to follow the AC grid voltage, resulting in an excessively large voltage difference when any of the first-phase grid-connected switches K1 to K3 is closed.

[0055] Based on this, after the N-phase grid-connected switch K4 is closed, and before controlling the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage, the controller 12 controls the balancing bridge arm 114 to ensure that the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to the fourth voltage threshold. The fourth voltage threshold is zero. Considering voltage fluctuations in actual control, the fourth voltage threshold can also take other values ​​that fluctuate slightly around zero.

[0056] Understandably, before the power conversion device 1 controls the first terminal voltage of any one of the first phase grid-connected switches K1 to the third phase grid-connected switch K3 to follow the voltage of the AC grid, it controls the balance bridge arm 114 to make the voltage of the positive bus capacitor C1 equal to the voltage of the negative bus capacitor C2. This is to avoid the situation where the AC voltage output by the three-phase power conversion bridge arm is asymmetrical after passing through the filter inductor and filter capacitor, thereby better realizing the small voltage difference pull-in of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0057] In this implementation scenario, when a DC common-mode voltage exists in the voltage difference across any phase grid-connected switch, and no AC common-mode voltage exists, the power conversion device 1 reduces the DC common-mode voltage to zero by controlling the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 through the balance bridge arm 114, thereby eliminating the influence of the DC common-mode voltage on each phase grid-connected switch. Since the DC common-mode voltage (i.e., the voltage at the first terminal of the N-phase grid-connected switch) has been eliminated and no AC common-mode voltage (i.e., the voltage at the second terminal of the N-phase grid-connected switch) exists, the voltage difference across the N-phase grid-connected switch is close to zero. Subsequently, the power conversion device 1 controls the N-phase grid-connected switch to close, achieving small voltage difference closure of the N-phase grid-connected switch, and controls the first phase grid-connected switch K1 to the third phase grid-connected switch K3 to close simultaneously, achieving small voltage difference closure of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0058] In another implementation scenario, there are both DC common-mode voltage and AC common-mode voltage in the voltage difference across any phase grid-connected switch. The controller 12 can achieve small voltage difference closure of each phase grid-connected switch in the following way:

[0059] When the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than the first voltage threshold, the controller 12 controls the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold, thereby eliminating the influence of DC common-mode voltage on any phase grid-connected switch.

[0060] If the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, it indicates the presence of AC common-mode voltage in the voltage difference across any phase grid-connected switch. Therefore, controller 12 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the voltage at the connection point between the N-phase grid-connected switch K4 and the N-phase AC terminal o1N (the voltage at the second terminal of the N-phase grid-connected switch K4). In other words, controller 12 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the AC common-mode voltage. By controlling the N-phase grid-connected switch K4 to close when the AC common-mode voltage is zero, the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch K4 is avoided, thus achieving small voltage difference closure of the N-phase grid-connected switch K4.

[0061] Specifically, when the effective value of the AC component of the voltage difference across any phase-connected switch is greater than the third voltage threshold, controller 12 performs phase-locked loop processing on the second-terminal voltage of N-phase grid-connected switch K4 to obtain the phase-locked angle of the second-terminal voltage of N-phase grid-connected switch K4. When the phase-locked angle of the second-terminal voltage of N-phase grid-connected switch K4 is 0 degrees, 180 degrees, or 360 degrees, it indicates that the second-terminal voltage of N-phase grid-connected switch K4 is zero. Therefore, controller 12 sends a drive signal to N-phase grid-connected switch K4 to close it at the zero-crossing point of the second-terminal voltage of N-phase grid-connected switch K4.

[0062] Because there is a time interval (e.g., about 20ms) between the time when the controller 12 sends the drive signal to the N-phase grid-connected switch K4 and the time when the N-phase grid-connected switch K4 actually closes, this time interval will cause the controller 14 to send the drive signal to the N-phase grid-connected switch K4 when the voltage at the second terminal of the N-phase grid-connected switch K4 is zero. This will still cause a deviation between the actual closing time of the N-phase grid-connected switch K4 and the time when the voltage at the second terminal of the N-phase grid-connected switch K4 is zero. As a result, the N-phase grid-connected switch K4 is greatly affected by the AC common-mode voltage when it is closed.

[0063] Based on this, when the effective value of the AC component of the voltage difference across any phase-connected switch is greater than the third voltage threshold, controller 12 performs phase-locked loop processing on the second-terminal voltage of N-phase grid-connected switch K4 to obtain the phase-locked angle of the second-terminal voltage of N-phase grid-connected switch K4. Then, based on the frequency of the AC power grid, the closing time of N-phase grid-connected switch K4 is converted into a phase angle θ1. Finally, based on phase angle θ1 and the phase angle error threshold Δθ, the leading phase angle interval [θ1-Δθ, θ1+Δθ] is obtained. The range of the phase angle error threshold Δθ is determined by the phase angle converted from the standard deviation of the closing time distribution curve of the grid-connected switch. For example, assuming the closing time of the N-phase grid-connected switch K4 is 22ms and Δθ = 20 degrees, and since the frequency of the AC power grid is 50Hz, the period of the AC power grid is 20ms. 20ms corresponds to a phase angle of 360 degrees. Therefore, the phase angle θ1 converted by the closing time of the N-phase grid-connected switch K4 is θ1 = (22 / 20-1)*360 degrees = 36 degrees, thus obtaining the leading phase angle range [16 degrees, 56 degrees]. Then, when the phase-locked angle of the second terminal voltage of the N-phase grid-connected switch K4 is within the leading phase angle range, the controller 12 sends a first drive signal to the N-phase grid-connected switch K4 to close it at the zero-crossing point of the second terminal voltage. Here, the closing time of the N-phase grid-connected switch K4 refers to the duration between the moment the controller 12 sends the drive signal to the N-phase grid-connected switch K4 and the actual closing time of the N-phase grid-connected switch K4. The actual closing time of the N-phase grid-connected switch K4 is the moment when the current on the N-phase grid-connected switch K4 reaches the current threshold. The current threshold is determined by the ratio of the first voltage to the loop impedance and the current sampling error. The loop impedance is the impedance of the loop formed between the balance bridge arm 114 and the branch containing the N-phase grid-connected switch K4 and the reference ground when the N-phase grid-connected switch K4 is closed. Here, the closing time of the N-phase grid-connected switch K4 can be preset or it can be the initial closing time of the N-phase grid-connected switch K4. This initial closing time is the duration between the moment when the controller 12 sends a drive signal to the N-phase grid-connected switch K4 when the power conversion device 1 is first turned on and the actual closing time of the N-phase grid-connected switch K4.

[0064] Understandably, when the phase-locked angle of the second terminal voltage of the N-phase grid-connected switch K4 reaches a phase angle that leads the switching closing time of the N-phase grid-connected switch K4 by 0, 180, or 360 degrees, the power conversion device 1 sends a drive signal to the N-phase grid-connected switch K4. This avoids the time interval between the time of sending the drive signal and the actual closing time of the N-phase grid-connected switch K4, which could lead to a significant influence of the AC common-mode voltage when the N-phase grid-connected switch K4 closes. This allows the N-phase grid-connected switch K4 to close when the AC common-mode voltage is zero, thereby achieving a smaller voltage difference closing of the N-phase grid-connected switch K4. In simple terms, the power conversion device 1 further reduces the AC common-mode voltage difference when the N-phase grid-connected switch K4 is energized through a phase-selective closing strategy, achieving a smaller voltage difference closing of the N-phase grid-connected switch K4.

[0065] Considering the wear and tear of the grid-connected switch in actual operation, there will be a deviation between the actual closing time of the N-phase grid-connected switch K4 and the initial closing time or the manually preset closing time of the N-phase grid-connected switch K4. If the initial closing time or the manually preset closing time is still used as the closing time of the N-phase grid-connected switch K4 to control the closing of the N-phase grid-connected switch K4, there will also be a small deviation between the actual closing time of the N-phase grid-connected switch K4 and the time when the voltage at the second terminal of the N-phase grid-connected switch K4 is zero. As a result, the N-phase grid-connected switch K4 will still be affected by a small AC common-mode voltage when it is closed.

[0066] Based on this, when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, and before sending the first drive signal to the N-phase grid-connected switch K4, the controller 12 controls the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, wherein the first voltage is not equal to zero. For the specific implementation of the controller 12 controlling the balance bridge arm 114 to make the absolute value of the DC component of the voltage difference across any phase grid-connected switch the first voltage, please refer to the specific implementation of the controller 12 controlling the balance bridge arm 114 to make the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 the second voltage in the above embodiment, which will not be repeated here. Subsequently, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, controller 12 sends a second drive signal to N-phase grid-connected switch K4 to obtain the closing time of N-phase grid-connected switch K4. The closing time of N-phase grid-connected switch K4 is determined by the duration between the moment controller 12 sends the second drive signal and the actual closing time of N-phase grid-connected switch K4. Here, both the first and second drive signals are drive signals used to control the closing of N-phase grid-connected switch K4; the only difference between them is the timing of their transmission.

[0067] For example, if there is only one second drive signal, the closing time of the N-phase grid-connected switch K4 is the duration between the moment the controller 12 sends the second drive signal and the moment when the current on the N-phase grid-connected switch K4 reaches the current threshold. Alternatively, if there are multiple second drive signals, namely second drive signal a1, second drive signal a2, ..., second drive signal ak, then the controller 12 sends the second drive signal a1 to the N-phase grid-connected switch K4 when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, and records the first duration between the moment the controller 12 sends the second drive signal a1 and the actual closing time of the N-phase grid-connected switch K4. Afterwards, the controller 12 controls the N-phase grid-connected switch K4 to open, and after the N-phase grid-connected switch K4 is open, and when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, it sends the second drive signal a2 to the N-phase grid-connected switch K4, and records the second duration between the moment the controller 12 sends the second drive signal a2 and the actual closing time of the N-phase grid-connected switch K4. Based on this, the controller 12 can obtain a total of k durations, namely the first duration, the second duration, ..., the kth duration, and determine the average or mode of the above k durations as the closing time of the N-phase grid-connected switch K4.

[0068] It is understandable that when the power conversion device 1 detects the presence of AC common-mode voltage in the voltage difference across any phase grid-connected switch, it sends a drive signal to the N-phase grid-connected switch K4 to obtain the current actual closing time of the N-phase grid-connected switch K4, and calibrates the closing time of the N-phase grid-connected switch K4 based on the current actual closing time of the N-phase grid-connected switch K4, thereby improving the phase selection closing accuracy and further realizing the smaller voltage difference closing of the N-phase grid-connected switch K4.

[0069] It should be noted that this application does not restrict the execution order of the steps of controlling the absolute value of the DC component of the voltage difference across any phase grid-connected switch to be less than or equal to the first voltage threshold and controlling the N-phase grid-connected switch K4 to close at the voltage zero-crossing point of the second terminal voltage of the N-phase grid-connected switch K4. Considering that when the AC common-mode voltage reaches the voltage zero-crossing point, the DC common-mode voltage may not yet be controlled to a smaller value, closing the N-phase grid-connected switch K4 at this time would result in a larger voltage difference across the N-phase grid-connected switch K4 when it is closed. Based on this, the controller 12 can control the N-phase grid-connected switch K4 to close at the voltage zero-crossing point of the second terminal voltage of the N-phase grid-connected switch K4 when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold and the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold. This ensures that the DC common-mode voltage is eliminated before the power conversion device 1 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the AC common-mode voltage, thereby enabling the N-phase grid-connected switch K4 to close with a smaller voltage difference.

[0070] Because after the N-phase grid-connected switch K4 is closed, U iNPE =U gNPE This allows the voltage difference between each phase of the grid-connected switch from the first phase K1 to the third phase K3 to no longer be affected by the DC common-mode voltage and the AC common-mode voltage. Therefore, after controlling the N-phase grid-connected switch K4 to close, the controller 12 controls the AC terminal voltage of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage. When the voltage difference between the three phase grid-connected switches is less than or equal to a second voltage threshold, the frequency difference between the three phase grid-connected switches is less than or equal to a frequency threshold, and the phase difference between the three phase grid-connected switches is less than or equal to a phase threshold, the controller 12 controls the three phase grid-connected switches to close, thereby controlling the synchronous closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3, thus achieving small voltage difference closure of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0071] Preferably, after the N-phase grid-connected switch K4 is closed, and before controlling the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage, the controller 12 controls the balancing bridge arm 114 so that the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to the fourth voltage threshold. This avoids the situation where the AC voltage output from the three-phase power conversion bridge arm is asymmetrical after passing through the filter inductor and filter capacitor, and thus better achieves the small voltage difference activation of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0072] In this implementation scenario, when there is both DC common-mode voltage and AC common-mode voltage in the voltage difference across any phase grid-connected switch, the power conversion device 1 reduces the DC common-mode voltage to zero by controlling the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, thereby eliminating the influence of the DC common-mode voltage on each phase grid-connected switch. Furthermore, by controlling the N-phase grid-connected switch K4 to close when the AC common-mode voltage is zero, the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch K4 is avoided, thus achieving small voltage difference closure of the N-phase grid-connected switch. After the N-phase grid-connected switch K4 is closed, the DC common-mode voltage and AC common-mode voltage in the voltage difference between each phase grid-connected switch from the first phase grid-connected switch K1 to the third phase grid-connected switch K3 are eliminated. As a result, the voltage difference between each phase grid-connected switch from the first phase grid-connected switch K1 to the third phase grid-connected switch K3 is no longer affected by the DC common-mode voltage and AC common-mode voltage. At this time, controlling the simultaneous closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3 can achieve the small voltage difference closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0073] To better understand this implementation scenario, the following example demonstrates the control effect by first eliminating the influence of DC common-mode voltage and then eliminating the influence of AC common-mode voltage. The waveform diagram of the voltage difference between the two ends of the N-phase grid-connected switch K4 when it is closed, provided by the prior art, and the waveform diagram of the voltage difference between the two ends of the N-phase grid-connected switch K4 when it is closed, provided by this application.

[0074] See Figure 4 , Figure 4 This is a waveform diagram of the voltage difference and current across an N-phase grid-connected switch provided by existing technology. (Example:) Figure 4 As shown, the voltage difference ΔU across the N-phase grid-connected switch K4 is... K4 This includes both DC common-mode voltage (approximately 30V) and AC common-mode voltage. The N-phase grid-connected switch K4 has a voltage difference ΔU across its terminals. K4 The circuit closes at its maximum value, and before the N-phase grid-connected switch K4 closes, the voltage U of the bus capacitor C1 is not aligned. C1 The voltage U of the negative bus capacitor C2 C2 Adjustments need to be made. Clearly, in the existing technology, the N-phase grid-connected switch K4 is based on the voltage difference ΔU across its terminals. K4 The circuit breaker closes when the DC common-mode voltage is about 30V and the AC common-mode voltage is at its maximum. It does not take into account the influence of the DC common-mode voltage and AC common-mode voltage on the closing of each phase grid-connected switch, which will result in an excessive voltage difference between the two ends when each phase grid-connected switch is closed.

[0075] Based on this, this application provides Figure 5The schematic diagram of the control method for the power conversion device 1 shown illustrates the specific implementation method of first eliminating the DC common-mode voltage and then eliminating the AC common-mode voltage to achieve small-differential voltage drop engagement of the grid-connected switches for each phase. For example... Figure 5 As shown, the control method for the power conversion device 1 includes the following steps:

[0076] S101, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than the first voltage threshold, control the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

[0077] S102, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold, and the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, control the N-phase grid-connected switch K4 to close at the voltage zero-crossing point of the second terminal voltage of the N-phase grid-connected switch K4.

[0078] S103 controls the balance bridge arm 114 so that the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to the fourth voltage threshold.

[0079] After the N-phase grid-connected switch K4 is closed, the controller 12 controls the balance bridge arm 114 so that the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to the fourth voltage threshold.

[0080] S104 controls the AC terminal voltage of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage, and controls the three-phase grid-connected switches K1 to K3 to close when the synchronous closing conditions are met.

[0081] When the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to a fourth voltage threshold, controller 12 controls the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage. Furthermore, when the three-phase grid-connected switches K1 to K3 meet the synchronous closing conditions, controller 12 controls the three-phase grid-connected switches K1 to K3 to close. The synchronous closing conditions for the three-phase grid-connected switches K1 to K3 include: the voltage difference across the three-phase grid-connected switches being less than or equal to a second voltage threshold; the voltage frequency difference across the three-phase grid-connected switches being less than or equal to a frequency threshold; and the voltage phase difference across the three-phase grid-connected switches being less than or equal to a phase threshold.

[0082] For details on the specific implementation of steps S101 to S104, please refer to the description of the corresponding parts in the above embodiments, which will not be repeated here.

[0083] use Figure 5 The control method shown enables small differential voltage engagement of each phase grid-connected switch; see details below. Figure 6 .like Figure 6 As shown, controller 12 detects the voltage difference ΔU across the N-phase grid-connected switch K4. K4 When a DC common-mode voltage (30V) is present, at 0.1s, the balance bridge arm 114 is controlled to adjust the voltage U of the positive bus capacitor C1. C1 Voltage U with negative bus capacitor C2 C2 The difference is used to reduce the DC common-mode voltage. At 0.5s, the DC common-mode voltage decreases to zero. At this time, the voltage difference ΔU across the N-phase grid-connected switch K4 is... K4 The DC common-mode voltage has been eliminated, and only the AC common-mode voltage is included. In other words, the voltage difference ΔU across the N-phase grid-connected switch K4 is now... K4 This refers to the AC common-mode voltage. Subsequently, controller 12 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the AC common-mode voltage to avoid the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch K4, thereby achieving small voltage difference closure of the N-phase grid-connected switch. After the N-phase grid-connected switch K4 closes, both the DC common-mode voltage and the AC common-mode voltage in the voltage difference between the first-phase grid-connected switch K1 and the third-phase grid-connected switch K3 are eliminated, thus ensuring that the voltage difference between the first-phase grid-connected switch K1 and the third-phase grid-connected switch K3 is no longer affected by the DC common-mode voltage and the AC common-mode voltage.

[0084] In another implementation scenario, an AC common-mode voltage exists in the voltage difference across any phase grid-connected switch. Controller 12 can achieve small voltage difference closure of each phase grid-connected switch in the following manner:

[0085] If the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, it indicates the presence of AC common-mode voltage in the voltage difference across that phase grid-connected switch. Therefore, controller 12 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the voltage at its second terminal, i.e., controlling the N-phase grid-connected switch K4 to close at the zero-crossing point of the AC common-mode voltage. By controlling the N-phase grid-connected switch K4 to close when the AC common-mode voltage is zero, the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch K4 is avoided, thus achieving small voltage difference closure of the N-phase grid-connected switch K4. Preferably, the voltage difference across any phase grid-connected switch may also be affected by DC common-mode voltage. Based on this, the controller 12 controls the N-phase grid-connected switch K4 to close at the zero-crossing point of the AC common-mode voltage when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to a first voltage threshold, and the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than a third voltage threshold. That is, when there is no DC common-mode voltage but there is AC common-mode voltage in the voltage difference across any phase grid-connected switch, the controller 12 controls the N-phase grid-connected switch K4 to close. In simple terms, when the voltage difference across the N-phase grid-connected switch is not affected by DC common-mode voltage and AC common-mode voltage, the controller controls the N-phase grid-connected switch K4 to close, thereby achieving a smaller voltage difference (such as zero voltage difference) closure of the N-phase grid-connected switch.

[0086] Specifically, when the effective value of the AC component of the voltage difference across any phase-connected switch is greater than the third voltage threshold, controller 12 performs phase-locked loop processing on the second-terminal voltage of N-phase grid-connected switch K4 to obtain the phase-locked angle of the second-terminal voltage of N-phase grid-connected switch K4. When the phase-locked angle of the second-terminal voltage of N-phase grid-connected switch K4 is 0 degrees, 180 degrees, or 360 degrees, controller 12 sends a drive signal to N-phase grid-connected switch K4 to close it at the zero-crossing point of the second-terminal voltage of N-phase grid-connected switch K4.

[0087] Preferably, when the effective value of the AC component of the voltage difference across any phase-connected switch is greater than a third voltage threshold, the controller 12 performs phase-locked loop processing on the second-terminal voltage of the N-phase grid-connected switch K4 to obtain the phase-locked angle of the second-terminal voltage of the N-phase grid-connected switch K4. Based on the frequency of the AC grid, the closing time of the N-phase grid-connected switch K4 is converted into a phase angle θ1. Then, based on the phase angle θ1 and the phase angle error threshold Δθ, the leading phase angle interval [θ1-Δθ, θ1+Δθ] is obtained. Afterwards, when the phase-locked angle of the second-terminal voltage of the N-phase grid-connected switch K4 is within the leading phase angle interval, the controller 12 sends a first drive signal to the N-phase grid-connected switch K4 to close it at the zero-crossing point of the second-terminal voltage of the N-phase grid-connected switch K4. It can be understood that the power conversion device 1 further reduces the AC common-mode voltage difference when the N-phase grid-connected switch K4 is energized through a phase-selective closing strategy, achieving a smaller voltage difference closing of the N-phase grid-connected switch K4.

[0088] More preferably, when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than a third voltage threshold, and before sending the first drive signal to the N-phase grid-connected switch K4, the controller 12 controls the balance bridge arm 114 to adjust the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, wherein the first voltage is not equal to zero. Then, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, the controller 12 sends a second drive signal to the N-phase grid-connected switch K4 to obtain the closing time of the N-phase grid-connected switch K4, wherein the closing time of the N-phase grid-connected switch K4 is determined by the duration between the moment the controller 12 sends the second drive signal and the moment when the current on the N-phase grid-connected switch K4 reaches a current threshold.

[0089] It is understandable that when the power conversion device 1 detects the presence of AC common-mode voltage in the voltage difference across any phase grid-connected switch, it sends a drive signal to the N-phase grid-connected switch K4 to obtain the current actual closing time of the N-phase grid-connected switch K4, and calibrates the closing time of the N-phase grid-connected switch K4 based on the current actual closing time of the N-phase grid-connected switch K4, thereby improving the phase selection closing accuracy and further realizing the smaller voltage difference closing of the N-phase grid-connected switch K4.

[0090] Because after the N-phase grid-connected switch K4 is closed, U iNPE =U gNPEThis allows the voltage difference between each phase of the grid-connected switch from the first phase K1 to the third phase K3 to no longer be affected by the DC common-mode voltage and the AC common-mode voltage. Therefore, after controlling the N-phase grid-connected switch K4 to close, the controller 12 controls the AC terminal voltage of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage. When the voltage difference between the three phase grid-connected switches is less than or equal to a third voltage threshold, the frequency difference between the three phase grid-connected switches is less than or equal to a frequency threshold, and the phase difference between the three phase grid-connected switches is less than or equal to a phase threshold, the controller 12 controls the three phase grid-connected switches to close, thereby controlling the synchronous closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3, thus achieving small voltage difference closure of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0091] Preferably, after the N-phase grid-connected switch K4 is closed, and before controlling the AC terminal voltages of the first phase bridge arm 111 to the third phase bridge arm 113 to follow the AC grid voltage, the controller 12 controls the balancing bridge arm 114 so that the absolute value of the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is less than or equal to the fourth voltage threshold. This avoids the situation where the AC voltage output from the three-phase power conversion bridge arm is asymmetrical after passing through the filter inductor and filter capacitor, and thus better achieves the small voltage difference activation of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0092] In this implementation scenario, when an AC common-mode voltage exists in the voltage difference across any phase grid-connected switch, the power conversion device 1 avoids the influence of the AC common-mode voltage on the closing of the N-phase grid-connected switch K4 by controlling it to close when the AC common-mode voltage is zero, thereby achieving small voltage difference closure of the N-phase grid-connected switch. After the N-phase grid-connected switch K4 closes, both the DC and AC common-mode voltages in the voltage difference across each phase grid-connected switch from the first phase grid-connected switch K1 to the third phase grid-connected switch K3 are eliminated. Therefore, the voltage difference across each phase grid-connected switch from the first phase grid-connected switch K1 to the third phase grid-connected switch K3 is no longer affected by the DC and AC common-mode voltages. At this time, controlling the simultaneous closing of the first phase grid-connected switch K1 to the third phase grid-connected switch K3 achieves small voltage difference closure of the first phase grid-connected switch K1 to the third phase grid-connected switch K3.

[0093] Additionally, optionally, the balanced bridge arm 114 also includes two reverse-connected switches to form a freewheeling circuit for inductor L41 after one of the first switch Q41 and the second switch Q42 is turned off and before the other switch is turned on. See details in [link to relevant documentation]. Figure 7 .like Figure 7As shown, the balanced bridge arm 114 also includes a third switch Q43 and a fourth switch Q44, which are connected in reverse series between the connection point of the first switch Q41 and the second switch Q42 and the third input terminal i143 of the balanced bridge arm 114. Figure 7 The specific implementation method of the balanced bridge arm 114 controlling the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 is shown below. Figure 3 The balance bridge arm 114 shown is the same.

[0094] For example, each phase power conversion arm and balancing arm 114 employs a Type I three-level topology. See [link to relevant documentation]. Figure 8 , Figure 8 This is another structural schematic diagram of the power conversion device provided in this application. For example... Figure 8As shown, the first phase bridge arm 111 includes switching transistors Q11, Q12, Q13, and Q14, diodes D11 and D12. Switches Q11, Q12, Q13, and Q14 are connected in series between the DC terminals i111 and i112 of the first phase bridge arm 111. The connection between switching transistors Q12 and Q13 is connected to the AC terminal o11 of the first phase bridge arm 111. Diodes D11 and D12 are connected in series between switching transistors Q11 and Q12. The connection between the junction of diodes Q11 and Q12 and the connection between diodes Q13 and Q14 is connected to the DC terminal i113 of the first phase bridge arm 111; the second phase bridge arm 112 includes diodes Q21, Q22, Q23, Q24, diodes D21 and D22, and diodes Q21, Q22, Q23, and Q24 are connected in series between the DC terminals i121 and i122 of the second phase bridge arm 112, and diodes Q22 and D24 are connected in series. The connection of switch Q23 is connected to the AC terminal o12 of the second phase bridge arm 112. Diodes D21 and D22 are connected in series between the connection of switch Q21 and Q22 and the connection of switch Q23 and Q24. The connection of diodes D21 and D22 is connected to the DC terminal o123 of the second phase bridge arm 112. The third phase bridge arm 113 includes switch Q31, switch Q32, switch Q33, switch Q34, diodes D31 and D32, and switch Q3... 1. Switches Q32, Q33, and Q34 are connected in series between the DC terminals i131 and i132 of the third phase bridge arm 113. The connection between switches Q32 and Q33 is connected to the AC terminal o13 of the third phase bridge arm 113. Diodes D31 and D32 are connected in series between the connection between switches Q31 and Q32 and the connection between switches Q33 and Q34. The connection between diodes D31 and D32 is connected to the DC terminal i133 of the third phase bridge arm 113.

[0095] The balanced bridge arm 114 includes a first switch Q41, a second switch Q42, a third switch Q43, a fourth switch Q44, an inductor L41, and a capacitor C4. The input terminals of the balanced bridge arm 114 include a first input terminal i141, a second input terminal i142, and a third input terminal i143. The first switch Q41, the second switch Q42, the third switch Q43, and the fourth switch Q44 are connected in series between the first input terminal i141 and the second input terminal i142. One end of the capacitor C4 is connected to the connection point of the first switch Q41 and the second switch Q42, and the other end of the capacitor C4 is connected to the connection point of the third switch Q43 and the fourth switch Q44. The connection point of the second switch Q42 and the third switch Q43 is connected to the third input terminal i143 and the output terminal i14 through the inductor L41.

[0096] When the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than a first voltage threshold, controller 12 controls the second switch Q42 and the fourth switch Q44 to conduct for a first duration and then turn them off, so that the energy of the negative bus capacitor C2 is transferred to the inductor L41 and the capacitor C4. After the second switch Q42 and the fourth switch Q44 are turned off, controller 12 controls the first switch Q41 and the third switch Q43 to conduct for a second duration and then turn them off, so that the energy stored in the inductor L41 and the capacitor C4 is transferred to the positive bus capacitor C1, thereby making the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 a second voltage. Wherein, when the voltage difference between the positive bus capacitor C1 and the negative bus capacitor C2 is the second voltage, the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

[0097] It should be noted that, Figure 3 The power conversion device 1 shown illustrates a specific implementation method for achieving small-difference closing of each phase grid-connected switch (hereinafter referred to as small-difference closing of each phase grid-connected switch) under three conditions: DC common-mode voltage and / or AC common-mode voltage on each phase grid-connected switch. This method is also applicable to... Figure 8 The power conversion device 1 shown. Figure 8 The power conversion device 1 shown provides a small differential pressure control method for engaging the grid-connected switches of each phase, which is similar to... Figure 3 Compared to the small differential voltage pull-in phase grid-connected switch control method provided by the power conversion device 1 shown, the two are identical only in the control method of the balancing bridge arm 114. Furthermore, the circuit structure of the three-phase power conversion bridge arm is unrelated to the circuit structure of the balancing bridge arm 114. In short, Figure 8 The balance bridge arm 114 shown is Figure 3 The balance bridge arms 114 shown can be interchanged.

[0098] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A power conversion device, characterized in that, The power conversion device includes a DC bus positive terminal, a DC bus negative terminal, a positive bus capacitor, a negative bus capacitor, a three-phase four-arm power conversion circuit, a three-phase grid-connected switch, an N-phase grid-connected switch, a controller, three-phase AC terminals, and an N-phase AC terminal, wherein: The positive bus capacitor and the negative bus capacitor are connected in series between the positive terminal and the negative terminal of the DC bus, and the connection point of the positive bus capacitor and the negative bus capacitor is the midpoint of the bus. The three-phase four-bridge-arm power conversion circuit includes three-phase power conversion bridge arms and a balancing bridge arm; The DC terminal of each phase of the three-phase power conversion bridge arm is connected to the positive terminal of the DC bus, the negative terminal of the DC bus, and the midpoint of the bus, respectively. The three AC terminals of the three-phase power conversion bridge arm are connected to the three-phase AC terminals through the three-phase grid-connected switch. The input terminals of the balancing bridge arm are respectively connected to the positive terminal of the DC bus, the negative terminal of the DC bus, and the midpoint of the bus. The output terminal of the balancing bridge arm is connected to the N-phase AC terminal through the N-phase grid-connected switch. The N-phase AC terminal and the three-phase AC terminal are used to connect to the AC power grid. The controller is used for: If the absolute value of the DC component of the voltage difference across any phase of the three-phase grid-connected switch and the N-phase grid-connected switch is greater than the first voltage threshold, the balance bridge arm is controlled to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor so that the absolute value of the DC component of the voltage difference across any phase of the grid-connected switch is less than or equal to the first voltage threshold. If the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold, the N-phase grid-connected switch is controlled to close. After the N-phase grid-connected switch is closed, the AC terminal voltage of the three-phase power conversion bridge arm is controlled to follow the voltage of the AC grid. When the voltage difference across the three-phase grid-connected switch is less than or equal to a second voltage threshold, the voltage frequency difference across the three-phase grid-connected switch is less than or equal to a frequency threshold, and the voltage phase difference across the three-phase grid-connected switch is less than or equal to a phase threshold, the three-phase grid-connected switch is controlled to close.

2. The power conversion device according to claim 1, characterized in that, The controller is specifically configured to control the N-phase grid-connected switch to close when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to a first voltage threshold, and the effective value of the AC component of the voltage difference across any phase grid-connected switch is less than or equal to a third voltage threshold.

3. The power conversion device according to claim 1, characterized in that, The controller is specifically configured to, when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than a third voltage threshold, and the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold, control the N-phase grid-connected switch to close at the zero-crossing point of the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal.

4. The power conversion device according to claim 3, characterized in that, The controller is specifically configured to send a first drive signal to the N-phase grid-connected switch when the phase-locked angle of the voltage at the connection point between the N-phase grid-connected switch and the N-phase AC terminal is in the leading phase angle range, so that the N-phase grid-connected switch closes at the zero-crossing point of the voltage at the connection point between the N-phase grid-connected switch and the N-phase AC terminal. The leading phase angle range is determined by the phase angle converted from the switching closing time of the N-phase grid-connected switch and the phase angle error threshold.

5. The power conversion device according to claim 4, characterized in that, The controller is further configured to, before sending the first drive signal, control the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, wherein the first voltage is not equal to zero; when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, send a second drive signal to the N-phase grid-connected switch to obtain the closing time of the N-phase grid-connected switch, wherein the closing time of the N-phase grid-connected switch is determined by the duration between the moment when the controller sends the second drive signal and the moment when the current on the N-phase grid-connected switch reaches a current threshold.

6. The power conversion device according to any one of claims 1-5, characterized in that, The controller is further configured to control the balancing arm after the N-phase grid-connected switch is closed and before controlling the AC terminal voltage of the three-phase power conversion arm to follow the voltage of the AC grid, so that the absolute value of the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a fourth voltage threshold.

7. The power conversion device according to any one of claims 1-6, characterized in that, The balanced bridge arm includes a first switching transistor, a second switching transistor, and an inductor. The input terminals of the balanced bridge arm include a first input terminal, a second input terminal, and a third input terminal, wherein: The first input terminal of the balance bridge arm is connected to the positive terminal of the DC bus, the second input terminal of the balance bridge arm is connected to the negative terminal of the DC bus, and the third input terminal of the balance bridge arm is connected to the midpoint of the bus. The first switch and the second switch are connected in series between the first input terminal and the second input terminal of the balanced bridge arm, and the connection between the first switch and the second switch is connected to the third input terminal and the output terminal of the balanced bridge arm through the inductor. The controller is configured to control the second switch to turn off after a first period of conduction, and after the second switch is turned off, to control the first switch to turn off after a second period of conduction, so that the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is a second voltage, wherein when the difference is the second voltage, the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

8. The power conversion device according to claim 7, characterized in that, The balance bridge arm also includes a third switch and a fourth switch, which are connected in reverse series between the connection point of the first switch and the second switch and the third input terminal of the balance bridge arm.

9. The power conversion device according to any one of claims 1-6, characterized in that, The balanced bridge arm includes a first switch, a second switch, a third switch, a fourth switch, an inductor, and a capacitor. The input terminals of the balanced bridge arm include a first input terminal, a second input terminal, and a third input terminal, wherein: The first input terminal of the balance bridge arm is connected to the positive terminal of the DC bus, the second input terminal of the balance bridge arm is connected to the negative terminal of the DC bus, and the third input terminal of the balance bridge arm is connected to the midpoint of the bus. The first switch, the second switch, the third switch, and the fourth switch are connected in series between the first input terminal and the second input terminal of the balanced bridge arm; One end of the capacitor is connected to the connection point of the first switch and the second switch, and the other end of the capacitor is connected to the connection point of the third switch and the fourth switch. The connection between the second switch and the third switch is connected to the third input and output terminals of the balanced bridge arm through the inductor; The controller is configured to control the second and fourth switches to be turned on for a first duration and then turned off, and after the second and fourth switches are turned off, to control the first and third switches to be turned on for a second duration and then turned off, so that the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is a second voltage, wherein when the difference is the second voltage, the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

10. A power conversion device, characterized in that, The power conversion device includes a DC bus positive terminal, a DC bus negative terminal, a positive bus capacitor, a negative bus capacitor, a three-phase four-arm power conversion circuit, a three-phase grid-connected switch, an N-phase grid-connected switch, a controller, three-phase AC terminals, and an N-phase AC terminal, wherein: The positive bus capacitor and the negative bus capacitor are connected in series between the positive terminal and the negative terminal of the DC bus, and the connection point of the positive bus capacitor and the negative bus capacitor is the midpoint of the bus. The three-phase four-bridge-arm power conversion circuit includes three-phase power conversion bridge arms and a balancing bridge arm; The DC terminal of each phase of the three-phase power conversion bridge arm is connected to the positive terminal of the DC bus, the negative terminal of the DC bus, and the midpoint of the bus, respectively. The three AC terminals of the three-phase power conversion bridge arm are connected to the three-phase AC terminals through the three-phase grid-connected switch. The input terminals of the balancing bridge arm are respectively connected to the positive terminal of the DC bus, the negative terminal of the DC bus, and the midpoint of the bus. The output terminal of the balancing bridge arm is connected to the N-phase AC terminal through the N-phase grid-connected switch. The N-phase AC terminal and the three-phase AC terminal are used to connect to the AC power grid. The controller is used for: If the effective value of the AC component of the voltage difference across any one of the three-phase grid-connected switches and the N-phase grid-connected switches is greater than the third voltage threshold, the N-phase grid-connected switch is controlled to close at the zero-crossing point of the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal. After the N-phase grid-connected switch is closed, the AC terminal voltage of the three-phase power conversion bridge arm is controlled to follow the voltage of the AC grid. When the voltage difference across the three-phase grid-connected switch is less than or equal to a second voltage threshold, the voltage frequency difference across the three-phase grid-connected switch is less than or equal to a frequency threshold, and the voltage phase difference across the three-phase grid-connected switch is less than or equal to a phase threshold, the three-phase grid-connected switch is controlled to close.

11. The power conversion device according to claim 10, characterized in that, The controller is specifically configured to, when the effective value of the AC component of the voltage difference across any phase grid-connected switch is greater than the third voltage threshold, and the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold, control the N-phase grid-connected switch to close at the zero-crossing point of the voltage at the connection between the N-phase grid-connected switch and the N-phase AC terminal.

12. The power conversion device according to claim 11, characterized in that, The controller is specifically configured to send a first drive signal to the N-phase grid-connected switch when the phase-locked angle of the voltage at the connection point between the N-phase grid-connected switch and the N-phase AC terminal is in the leading phase angle range, so that the N-phase grid-connected switch closes at the zero-crossing point of the voltage at the connection point between the N-phase grid-connected switch and the N-phase AC terminal. The leading phase angle range is determined by the phase angle converted from the switching closing time of the N-phase grid-connected switch and the phase angle error threshold.

13. The power conversion device according to claim 12, characterized in that, The controller is further configured to, before sending the first drive signal, control the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is a first voltage, wherein the first voltage is not equal to zero; when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is the first voltage, send a second drive signal to the N-phase grid-connected switch to obtain the closing time of the N-phase grid-connected switch, wherein the closing time of the N-phase grid-connected switch is determined by the duration between the moment when the controller sends the second drive signal and the moment when the current on the N-phase grid-connected switch reaches a current threshold.

14. The power conversion device according to any one of claims 10-13, characterized in that, The controller is further configured to, when the absolute value of the DC component of the voltage difference across any phase grid-connected switch is greater than a first voltage threshold, control the balance bridge arm to adjust the voltage difference between the positive bus capacitor and the negative bus capacitor, so that the absolute value of the DC component of the voltage difference across any phase grid-connected switch is less than or equal to the first voltage threshold.

15. The power conversion device according to any one of claims 10-14, characterized in that, The controller is further configured to control the balancing arm after the N-phase grid-connected switch is closed and before controlling the AC terminal voltage of the three-phase power conversion arm to follow the voltage of the AC grid, so that the absolute value of the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a fourth voltage threshold.

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