Power conversion device

Abstract

The power conversion device provided by the invention comprises a direct current end, a positive direct current bus, a negative direct current bus, a positive bus capacitor, a negative bus capacitor, at least three inverter bridge arms, a switch unit, a controller, filter inductors corresponding to the at least three inverter bridge arms one by one, a filter capacitor and an alternating current end. And the third end of each inverter bridge arm sequentially passes through the filter inductor, the filter capacitor and the switch unit corresponding to each inverter bridge arm and then is connected with the midpoint of the bus. The controller controls the switch unit to be switched off or the impedance of the switch unit to be increased when the voltage of the power grid is smaller than or equal to a first voltage threshold value; and after the switch unit is switched off or the impedance of the switch unit is increased, the modulation wave of each inverter bridge arm is controlled to be the modulation wave after the first common-mode modulation signal is superposed on the initial modulation wave of each inverter bridge arm, so that the amplitude of the modulation wave of each inverter bridge arm is increased. Furthermore, not only can the loss of the switching tube in each inverter bridge arm be balanced, but also the problem of common-mode oscillation caused by modulation wave change can be improved.

Description

Technical Field

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

[0002] When a low voltage ride-through fault occurs in the power grid, the amplitude of the three-phase grid-connected voltage output by the power storage converter (PCS) decreases, resulting in a decrease in the modulation index (i.e., the ratio of the grid voltage to the semi-DC bus voltage). Since the modulation index is positively correlated with the modulation wave amplitude of the inverter bridge arm in the PCS, as the modulation index decreases, the modulation wave amplitude of the inverter bridge arm in the PCS also decreases, which reduces the time that the inverter bridge arm outputs a positive or negative level, thereby increasing the time that the inverter bridge arm outputs a zero level. Figure 1 Taking the PCS using the Neutra lPoint Clamped (NPC) three-level inverter topology as an example, for the inverter bridge arm composed of switch tubes Q11 to Q14 and diodes D15 and D16, when the grid voltage is in the positive half cycle, the time when the inverter bridge arm outputs the positive level decreases, corresponding to the decrease in the conduction time of the switch tube Q11, and the time when the inverter bridge arm outputs the zero level increases, corresponding to the increase in the conduction time of the diode D15 and the switch tube Q12 on the zero level loop; when the grid voltage is in the negative half cycle, the time when the inverter bridge arm outputs the negative level decreases, corresponding to the decrease in the conduction time of the switch tube Q14, and the time when the inverter bridge arm outputs the zero level increases, corresponding to the increase in the conduction time of the diode D16 and the switch tube Q13 on the zero level loop. Based on this, it can be seen that when a low voltage ride-through fault occurs in the grid, the loss of the switch devices located in the zero level loop in each inverter bridge arm will increase, resulting in an unbalanced loss of the switch devices in each inverter bridge arm. Among them, the zero level loop refers to the current loop corresponding to the inverter bridge arm outputting the zero level. Summary of the invention

[0003] The present application provides a power conversion device, which can not only balance the loss of the switch tube in each inverter bridge arm, but also improve the common mode oscillation problem caused by the change of the modulation wave.

[0004] In the first aspect, the present application provides a power conversion device, which includes a DC terminal, a positive DC bus, a negative DC bus, a positive bus capacitor, a negative bus capacitor, at least three inverter bridge arms, a switch unit, a controller, and filter inductors, filter capacitors, and AC terminals corresponding to at least three inverter bridge arms. Among them, the DC terminal is used to connect a photovoltaic module or an energy storage battery, and the AC terminals corresponding to at least three inverter bridge arms are used to connect to the power grid. The positive DC bus and the negative DC bus are connected to the DC terminal, and the positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus in sequence. The first end and the second end of each inverter bridge arm in the at least three inverter bridge arms are respectively connected to the positive DC bus and the negative DC bus, and the third end of each inverter bridge arm is connected to the bus midpoint after passing through the filter inductor, filter capacitor, and switch unit corresponding to each inverter bridge arm in sequence, and the bus midpoint is the connection point of the positive bus capacitor and the negative bus capacitor. The series connection of the filter inductor and filter capacitor corresponding to at least three inverter bridge arms is respectively used to connect the AC terminals corresponding to at least three inverter bridge arms. The controller is used to control the switch unit to disconnect or increase the impedance of the switch unit when the voltage of the power grid is less than or equal to a first voltage threshold, indicating that the power grid has a low voltage ride-through fault; and after the switch unit is disconnected or the impedance of the switch unit is increased, control the modulation wave of each inverter bridge arm to be a modulation wave after superimposing a first common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to increase the amplitude of the modulation wave of each inverter bridge arm, wherein the initial modulation wave is the modulation wave of each inverter bridge arm when the power conversion device is working normally.

[0005] In this embodiment, the power conversion device adds a switch unit to the common-mode branch (a branch located between the third end of each inverter bridge arm and the midpoint of the bus and including the filter inductor and filter capacitor corresponding to each inverter bridge arm). When a low voltage ride-through fault occurs in the power grid, the switch unit is first controlled to be disconnected or the impedance of the switch unit is controlled to be increased to cut off the common-mode branch or increase the impedance of the common-mode branch, thereby directly avoiding the generation of common-mode oscillation or slowing down the common-mode oscillation phenomenon. After the switch unit is disconnected or the impedance of the switch unit is increased, the amplitude of the modulation wave of each inverter bridge arm is increased by superimposing a common-mode modulation signal on the initial modulation wave of each inverter bridge arm. Here, the modulation wave of the inverter bridge arm refers to the modulation wave of the reference switch tube in the inverter bridge arm, and the reference switch tube is a switch tube directly connected to the positive DC bus or the negative DC bus in the inverter bridge arm. Specifically, when the grid voltage is in the positive half cycle, the reference switch is the switch directly connected to the positive DC bus in the inverter bridge arm; when the grid voltage is in the negative half cycle, the reference switch is the switch directly connected to the negative DC bus in the inverter bridge arm. The modulation wave amplitude of the reference switch determines the duration of the inverter bridge arm outputting a positive level or a negative level. Specifically, the larger the modulation wave amplitude of the reference switch, the larger the duty cycle of the reference switch, and the longer the conduction time of the reference switch, that is, the longer the duration of the inverter bridge arm outputting a positive level or a negative level; the smaller the modulation wave amplitude of the reference switch, the smaller the duty cycle of the reference switch, and the shorter the conduction time of the reference switch, that is, the shorter the duration of the inverter bridge arm outputting a positive level or a negative level. For a three-level power conversion device, when the grid voltage is in the positive half cycle, the inverter bridge arm either outputs a positive level or a zero level; when the grid voltage is in the negative half cycle, the inverter bridge arm either outputs a zero level or a negative level. In other words, the duration of the inverter bridge arm outputting zero level is negatively correlated with the duration of the outputting positive level or negative level. Therefore, the power conversion device can increase the duration of each inverter bridge arm outputting positive level or negative level and reduce the duration of each inverter bridge arm outputting zero level by increasing the modulation wave amplitude of each inverter bridge arm. In other words, the conduction time of the reference switch tube in each inverter bridge arm can be increased, and the conduction time of the switch tube on the zero-level loop in each inverter bridge arm can be reduced, thereby achieving loss balance of the switch tube in each inverter bridge arm. In summary, when a low voltage ride-through fault occurs in the power grid, the power conversion device can not only balance the loss of the switch tube in each inverter bridge arm, but also improve the common-mode oscillation problem caused by the change of the modulation wave, which is beneficial to the stable operation of the power conversion device.

[0006] In a possible implementation, the controller is also used to, when the voltage of the power grid is greater than or equal to a second voltage threshold, indicate that a high voltage ride-through fault has occurred in the power grid, and then control the switch unit to be disconnected or the impedance of the switch unit to be increased; and after the switch unit is disconnected or the impedance of the switch unit is increased, control the modulation wave of each inverter bridge arm to be a modulation wave after superimposing a second common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to reduce the amplitude of the modulation wave of each inverter bridge arm, wherein the second voltage threshold is greater than the first voltage threshold.

[0007] In this embodiment, the power conversion device can also control the switch unit to be disconnected or the impedance of the switch unit to be increased when a high voltage ride-through fault occurs in the power grid, so as to cut off the common mode branch or increase the impedance of the common mode branch, thereby directly avoiding the generation of common mode oscillation or slowing down the common mode oscillation phenomenon. After the switch unit is disconnected or the impedance of the switch unit is increased, the amplitude of the modulation wave of each inverter bridge arm is reduced by superimposing the common mode modulation signal on the initial modulation wave of each inverter bridge arm, thereby reducing the duration of each inverter bridge arm outputting a positive level or a negative level, and increasing the duration of each inverter bridge arm outputting a zero level. In other words, the conduction time of the reference switch tube in each inverter bridge arm can be reduced, and the conduction time of the switch tube on the zero level loop in each inverter bridge arm can be increased, thereby achieving loss balance of the switch tube in each inverter bridge arm. Therefore, when a high voltage ride-through fault occurs in the power grid, the power conversion device can not only balance the loss of the switch tube in each inverter bridge arm, but also improve the common mode oscillation problem caused by the change of the modulation wave, which is conducive to the stable operation of the power conversion device.

[0008] In a possible implementation, the switch unit includes a mechanical switch and a semiconductor switch connected in parallel. The controller is used to control the mechanical switch to be disconnected first and then control the semiconductor switch to be turned off to disconnect the switch unit when the voltage of the power grid is less than or equal to a first voltage threshold or greater than or equal to a second voltage threshold.

[0009] In this embodiment, when a voltage ride-through fault occurs in the power grid, the power conversion device uses a control method in which the mechanical switch is disconnected earlier than the semiconductor switch, so that the switch unit is disconnected, thereby putting the common-mode branch in an open-circuit state. Therefore, not only can the common-mode oscillation problem be completely solved, but the conduction loss of the semiconductor switch can also be effectively reduced, thereby reducing the loss of the power conversion device.

[0010] In a possible implementation, the switch unit includes a switch device and an impedance element connected in parallel, the switch device includes a mechanical switch or a semiconductor switch, and the impedance element includes an inductor, a capacitor, or a resistor. The controller is used to control the switch device to disconnect when the voltage of the power grid is less than or equal to a first voltage threshold or greater than or equal to a second voltage threshold, so as to increase the impedance of the switch unit.

[0011] In this embodiment, the switch unit adopts a structure in which a switch device and an impedance element are connected in parallel. When a voltage crossing fault occurs in the power grid, the power conversion device can increase the impedance of the switch unit only by controlling the switch device to disconnect, thereby putting the common-mode branch in a high-impedance state, thereby improving the common-mode oscillation problem. The control method is simple and easy to implement.

[0012] In a possible implementation, the controller is also used to control the switch unit to close when the voltage of the power grid is greater than a first voltage threshold and less than a second voltage threshold, indicating that the power grid voltage has returned to normal; and after the switch unit is closed, control the modulation wave of each inverter bridge arm to be the initial modulation wave of each inverter bridge arm.

[0013] In this embodiment, after the grid voltage returns to normal, the problem of increased losses in some switching tubes in each inverter bridge arm disappears. There is no need to superimpose a common-mode modulation signal on the initial modulation wave of each inverter bridge arm, and restore the common-mode branch to a pass state or a low-impedance state, thereby restoring the power conversion device to the working state when the grid voltage is normal.

[0014] In a possible implementation, the switch unit includes a mechanical switch and a semiconductor switch connected in parallel. The controller is used to control the semiconductor switch to conduct first and then control the mechanical switch to close, so that the switch unit is closed when the voltage of the power grid is greater than a first voltage threshold and less than a second voltage threshold.

[0015] In this embodiment, after the grid voltage returns to normal, the power conversion device adopts a control method in which the mechanical switch closes later than the semiconductor switch, so that the switch unit is closed, which not only restores the common-mode branch to a conductive state, but also effectively reduces the conduction loss of the semiconductor switch, thereby reducing the loss of the power conversion device.

[0016] In a possible implementation, the switch unit includes a switch device and an impedance element connected in parallel, the switch device includes a mechanical switch or a semiconductor switch, and the impedance element includes an inductor, a capacitor, or a resistor. The controller is used to control the switch device to close when the voltage of the power grid is greater than a first voltage threshold and less than a second voltage threshold, so as to close the switch unit.

[0017] In this embodiment, after the grid voltage returns to normal, the power conversion device can close the switch unit only by controlling the switch device to close, thereby restoring the common-mode branch to a low-impedance state. The control method is simple and easy to implement.

[0018] In a possible implementation, at least three inverter bridge arms in the power conversion device adopt an active neutral point clamped (ANPC) three-level inverter topology. Specifically, the fourth end of each inverter bridge arm is connected to the bus midpoint, and each inverter bridge arm includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube and a sixth switch tube. Among them, the first switch tube, the second switch tube, the third switch tube and the fourth switch tube of each inverter bridge arm are sequentially connected in series between the first end and the second end of each inverter bridge arm, and the connection between the second switch tube and the third switch tube of each inverter bridge arm is connected to the third end of each inverter bridge arm. The fifth switch tube and the sixth switch tube of each inverter bridge arm are connected in series between the connection between the first switch tube and the second switch tube of each inverter bridge arm and the connection between the third switch tube and the fourth switch tube of each inverter bridge arm, and the connection between the fifth switch tube and the sixth switch tube of each inverter bridge arm is connected to the fourth end of each inverter bridge arm.

[0019] In a possible implementation, at least three inverter bridge arms in the power conversion device adopt a T-type three-level inverter topology. Specifically, the fourth end of each inverter bridge arm is connected to the bus midpoint, and each inverter bridge arm includes a first switch tube, a second switch tube, a third switch tube, and a fourth switch tube. Among them, the first switch tube and the second switch tube of each inverter bridge arm are connected in series between the first end and the second end of each inverter bridge arm, and the connection between the first switch tube and the second switch tube in each inverter bridge arm is connected to the third end of each inverter bridge arm. The third switch tube and the fourth switch tube of each inverter bridge arm are reversely connected in series between the third end and the fourth end of each inverter bridge arm.

[0020] In a possible implementation, the power conversion device further includes an energy storage element, a seventh switch tube and an eighth switch tube, wherein the seventh switch tube and the eighth switch tube are sequentially connected in series between the positive DC bus and the negative DC bus, and the energy storage element is connected between the bus midpoint and the connection point of the seventh switch tube and the eighth switch tube. The controller is used to control the seventh switch tube to turn on for a first preset time and then turn off when the voltage of the positive bus capacitor is greater than the voltage of the negative bus capacitor; after the seventh switch tube is turned off, control the eighth switch tube to turn on for a second preset time and then turn off, so as to transfer the energy stored in the positive bus capacitor to the negative bus capacitor; or, the controller is used to control the eighth switch tube to turn on for a third preset time and then turn off when the voltage of the positive bus capacitor is less than the voltage of the negative bus capacitor; after the eighth switch tube is turned off, control the seventh switch tube to turn on for a fourth preset time and then turn off, so as to transfer the energy stored in the negative bus capacitor to the positive bus capacitor.

[0021] In this embodiment, the circuit composed of the energy storage element, the seventh switch tube and the eighth switch tube can be understood as a bus voltage balancing circuit. The power conversion device can control the conduction time of the two switch tubes in the bus voltage balancing circuit to balance the voltage of the positive bus capacitor and the voltage of the negative bus capacitor, thereby selecting a switch tube with lower voltage resistance, thereby reducing the cost of the power conversion device. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of an energy storage converter provided by the prior art;

[0023] Figure 2 It is a schematic diagram of an application scenario of the power conversion device provided in this application;

[0024] Figure 3 It is a structural schematic diagram of a power conversion device provided by the present application;

[0025] Figure 4a is another structural schematic diagram of the power conversion device provided by the present application;

[0026] Figure 4b is another structural schematic diagram of the power conversion device provided by the present application;

[0027] Figure 5 is another structural schematic diagram of the power conversion device provided by the present application;

[0028] Figure 6 This is another structural schematic diagram of the power conversion device provided by the present application. DETAILED DESCRIPTION

[0029] The power conversion device provided in this application can be applied to various application fields such as photovoltaic power generation, energy storage power generation, new energy smart microgrid, power transmission and distribution, etc. The power conversion device provided in this application can be an inverter, PCS, uninterruptible power supply (UPS), etc., which is applicable to different application scenarios, such as photovoltaic power supply scenario, energy storage power supply scenario, photovoltaic storage hybrid power supply scenario, UPS power supply scenario, etc. The following is an example of energy storage power supply scenario.

[0030] See also Figure 2 , Figure 2 Schematic diagram of the application scenario of the power conversion device provided by this application. In the energy storage power supply scenario, the power conversion device provided by this application is Figure 2The PCS shown includes a DC terminal, a positive DC bus BUS+, a negative DC bus BUS-, a positive bus capacitor C1, a negative bus capacitor C2, a first phase bridge arm, a second phase bridge arm, a third phase bridge arm, a switch unit, a controller, filter inductors L1 to L3 corresponding to the first phase bridge arm to the third phase bridge arm, filter capacitors C31 to C33, and a three-phase AC terminal. The DC terminal of the PCS is connected to the energy storage battery cluster, and the three-phase AC terminals of the PCS are connected to the AC power grid through a box-type substation and a booster station in turn. The positive DC bus BUS+ and the negative DC bus BUS- are connected to the DC terminal of the PCS, and the positive bus capacitor C1 and the negative bus capacitor C2 are connected in series between the positive DC bus BUS+ and the negative DC bus BUS- in turn. The first end and the second end of each phase bridge arm in the first phase bridge arm, the second phase bridge arm and the third phase bridge arm are connected to the positive DC bus BUS+ and the negative DC bus BUS- respectively. The third end of the first phase bridge arm is connected to the bus midpoint N after passing through the filter inductor L1, filter capacitor C31 and switch unit corresponding to the first phase bridge arm in sequence. The third end of the second phase bridge arm is connected to the bus midpoint N after passing through the filter inductor L2, filter capacitor C32 and switch unit corresponding to the second phase bridge arm in sequence. The third end of the third phase bridge arm is connected to the bus midpoint N after passing through the filter inductor L3, filter capacitor C33 and switch unit corresponding to the third phase bridge arm in sequence. The bus midpoint N is the connection point of the positive bus capacitor C1 and the negative bus capacitor C2. The first phase bridge arm to the third phase bridge arm are all inverter bridge arms. The series connection point of the filter inductor L1 and the filter capacitor C31, the series connection point of the filter inductor L2 and the filter capacitor C32, and the series connection point of the filter inductor L3 and the filter capacitor C33 are respectively connected to the three-phase AC end of the PCS.

[0031] After the PCS starts running, the switch unit is in the closed state, and the controller controls the first-phase bridge arm to the third-phase bridge arm to invert the DC power output by the energy storage battery cluster into AC power, and output it to the box-type substation connected to it. The box-type substation boosts the AC power input to its input end and outputs it to the booster station. The booster station boosts the AC power input to its input end to obtain AC power that meets the requirements of the AC power grid, thereby realizing power supply to the AC power grid.

[0032] In the process of the PCS supplying stable power to the AC power grid, when the voltage of the AC power grid is less than or equal to the first voltage threshold, the controller indicates that a low voltage ride-through fault occurs in the AC power grid, and then controls the switch unit to be disconnected or the impedance of the switch unit to be increased. After the switch unit is disconnected or the impedance of the switch unit is increased, the modulation wave of each phase bridge arm is controlled to be a modulation wave after the first common-mode modulation signal is superimposed on the initial modulation wave of each phase bridge arm, so as to increase the amplitude of the modulation wave of each phase bridge arm. Among them, the initial modulation wave of each phase bridge arm is the modulation wave of each phase bridge arm when the PCS is working normally.

[0033] It is understandable that when a low voltage ride-through fault occurs in the power grid, the PCS can increase the amplitude of the modulation wave of each phase bridge arm by superimposing a common-mode modulation signal on the basis of the initial modulation wave of each phase bridge arm, thereby increasing the duration of the output of the positive level or negative level of each phase bridge arm and reducing the duration of the output of the zero level of each phase bridge arm. In other words, the conduction time of the reference switch tube in each phase bridge arm can be increased, and the conduction time of the switch tube on the zero-level loop in each phase bridge arm can be reduced, thereby achieving loss balance of the switch tube in each phase bridge arm. Among them, the reference switch tube is a switch tube directly connected to the positive DC bus BUS+ or the negative DC bus BUS- in each phase bridge arm. However, if the above-mentioned control method for balancing the loss of the switch tube is directly applied to the existing PCS, the superimposed common-mode modulation signal is equivalent to being directly added to the two ends of the series branch of the filter inductor and the filter capacitor corresponding to each phase bridge arm, thereby causing common-mode oscillation. Based on this, the PCS in the present application adds a switch unit to the common-mode branch (a branch located between the third end of each phase bridge arm and the bus midpoint N and including the filter inductor and filter capacitor corresponding to each phase bridge arm) compared to the existing PCS. When a low voltage ride-through fault occurs in the power grid, the switch unit is first controlled to be disconnected or the impedance of the switch unit is controlled to increase, so as to cut off the common-mode branch or increase the impedance of the common-mode branch, thereby directly avoiding the generation of common-mode oscillation or slowing down the common-mode oscillation phenomenon. After the switch unit is disconnected or the impedance of the switch unit is increased, the loss balance of the switch tube in each phase bridge arm is achieved by superimposing the common-mode modulation signal on the initial modulation wave of each phase bridge arm. Therefore, when a low voltage ride-through fault occurs in the power grid, the PCS can not only balance the loss of the switch tube in each phase bridge arm, but also improve the common-mode oscillation problem caused by the change of the modulation wave, which is beneficial to the stable operation of the PCS.

[0034] The above is only an example of the application scenarios of the power conversion device provided in this application, and is not an exhaustive list. This application does not limit the application scenarios.

[0035] The present application mainly optimizes the control method of the power conversion device in the scenario of a power grid crossing fault. Specifically, the present application divides the power grid crossing fault scenarios into two categories. One category is the low-voltage crossing fault scenario of the power grid, and the modulation index is reduced, which increases the loss of the switch tube located on the zero-level loop in each inverter bridge arm. The other category is the high-voltage crossing fault scenario of the power grid, and the modulation index is increased, which increases the loss of the reference switch tube in each inverter bridge arm. The power conversion device provided by the present application can solve the problem of increased loss of some switch tubes in the above two scenarios, so as to enable the power conversion device to operate stably.

[0036] Combine the following Figures 3 to 6 The working principle of the power conversion device provided in this application is illustrated by way of example.

[0037] See also Figure 3, Figure 3 Schematic diagram of a power conversion device provided by the present application. Figure 3 As shown, the power conversion device 1 includes a DC terminal (including a DC terminal i11 and a DC terminal i12), a positive DC bus BUS+, a negative DC bus BUS-, a positive bus capacitor C1, a negative bus capacitor C2, an inverter bridge arm 111, an inverter bridge arm 112, ..., an inverter bridge arm 11n, a switch unit 12, a controller 13, a filter inductor L1, a filter inductor L2, ..., a filter inductor Ln, a filter capacitor C1, a filter capacitor C2, ..., a filter capacitor Cn and an AC terminal (including an AC terminal o11, an AC terminal o12, ..., and an AC terminal o13). The inverter bridge arm 111 corresponds to the filter inductor L1, the filter capacitor C31, and the AC terminal o11; the inverter bridge arm 112 corresponds to the filter inductor L2, the filter capacitor C32, and the AC terminal o12; ...; the inverter bridge arm 11n corresponds to the filter inductor Ln, the filter capacitor C3n, and the AC terminal o1n. Here, Figure 3 The DC terminal i11 and the DC terminal i12 shown are only simple illustrations of the positive DC terminal and the negative DC terminal of the power conversion device 1, and do not represent the actual number of the positive DC terminal and the negative DC terminal of the power conversion device 1. In practical applications, the power conversion device 1 has multiple groups of DC terminals, and one group of DC terminals includes a positive DC terminal and a negative DC terminal.

[0038] Among them, the DC terminal i11 and the DC terminal i12 of the power conversion device 1 are used to connect to a DC source (including a photovoltaic module, an energy storage battery, a capacitor, etc.), and the AC terminals o11 to o1n of the power conversion device 1 are used to connect to the power grid. The positive DC bus BUS+ is connected to the DC terminal i11 of the power conversion device 1, and the negative DC bus BUS- is connected to the DC terminal i12 of the power conversion device 1. The positive bus capacitor C1 and the negative bus capacitor C2 are connected in series between the positive DC bus BUS+ and the negative DC bus BUS-. The first end i111+ of the inverter bridge arm 111 is connected to the positive DC bus BUS+, the second end i111- of the inverter bridge arm 111 is connected to the negative DC bus BUS-, and the third end o111 of the inverter bridge arm 111 is connected to the bus midpoint N after passing through the filter inductor L1, the filter capacitor C31 and the switch unit 12 in sequence; the first end i112+ of the inverter bridge arm 112 is connected to the positive DC bus BUS+, the second end i112- of the inverter bridge arm 112 is connected to the negative DC bus BUS-, and the third end o112 of the inverter bridge arm 112 is connected to the bus midpoint N after passing through the filter inductor L2, the filter capacitor C32 and the switch unit 12 in sequence; ...; the first end i11n+ of the inverter bridge arm 11n is connected to the positive DC bus BUS+, and the second end i11n of the inverter bridge arm 11n is connected to the negative DC bus BUS-. 11n- is connected to the negative DC bus BUS-, and the third end o11n of the inverter bridge arm 11n is connected to the bus midpoint N after passing through the filter inductor Ln, the filter capacitor C3n and the switch unit 12 in sequence. The series connection of the filter inductor L1 and the filter capacitor C31 is used to connect the AC end o11 of the power conversion device 1; the series connection of the filter inductor L2 and the filter capacitor C32 is used to connect the AC end o12 of the power conversion device 1; ...; the series connection of the filter inductor Ln and the filter capacitor C3n is used for the AC end o1n of the power conversion device 1. Optionally, the filter circuit composed of the filter inductor and the filter capacitor corresponding to each inverter bridge arm can also be replaced by an LCL filter network. A DC / DC converter can also be connected between the power conversion device 1 and the DC source. At this time, the bus midpoint N in the power conversion device 1 is connected to the N line of the DC / DC converter.

[0039] Here, n is an integer greater than or equal to 3, and usually, n is mostly 3 or 4. When n=3, the three inverter bridge arms in the power conversion device 1 are respectively the A-phase bridge arm, the B-phase bridge arm and the C-phase bridge arm, and the power conversion device 1 is a three-phase three-wire power conversion device; when n=4, the four inverter bridge arms in the power conversion device 1 are respectively the A-phase bridge arm, the B-phase bridge arm, the C-phase bridge arm and the N-phase bridge arm, and the power conversion device 1 is a three-phase four-wire power conversion device. In addition, the inverter bridge arms 111 to 11n can adopt a multi-level inverter topology, including but not limited to a two-level inverter topology, an NPC three-level inverter topology, an ANPC three-level inverter topology and a T-type three-level inverter topology.

[0040] In one implementation scenario, during the process of the power conversion device 1 supplying power to the power grid, when the voltage of the power grid is less than or equal to the first voltage threshold, the controller 13 indicates that a low voltage ride-through fault occurs in the power grid, and then controls the switch unit 12 to be disconnected or the impedance of the switch unit 12 to be increased. After the switch unit 12 is disconnected or the impedance of the switch unit 12 is increased, the controller 13 controls the modulation wave of each inverter bridge arm in the inverter bridge arms 111 to 11n to be a modulation wave after superimposing the first common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to increase the amplitude of the modulation wave of each inverter bridge arm. Among them, the initial modulation wave of each inverter bridge arm is the modulation wave of each inverter bridge arm when the power conversion device 1 is working normally. Here, the normal operation of the power conversion device 1 refers to that the AC terminal voltage of the power conversion device 1 is basically consistent with the grid voltage when the grid is operating normally. In other words, it refers to that the AC terminal voltage of the power conversion device 1 is within the normal voltage range of the grid, and the normal voltage range of the grid can be greater than the first voltage threshold and less than the second voltage threshold. The first voltage threshold is less than the rated voltage of the grid, and the second voltage threshold is greater than the rated voltage of the grid. The specific settings of the first voltage threshold and the second voltage threshold can be adjusted based on actual operation requirements. Exemplarily, the first voltage threshold is 0.9 times the rated voltage of the grid, and the second voltage threshold is 1.1 times the rated voltage of the grid. In addition, the initial modulation wave of each inverter bridge arm includes a discontinuous pulse width modulation (DPWM) wave, a sinusoidal pulse width modulation (SPWM) wave, or a space vector pulse width modulation (SVPWM) wave.

[0041] It can be understood that when a low voltage ride-through fault occurs in the power grid, the power conversion device 1 cuts off the common mode branch or increases the impedance of the common mode branch by controlling the switch unit 12, thereby directly avoiding the generation of common mode oscillation or slowing down the common mode oscillation phenomenon. Afterwards, the power conversion device 1 increases the amplitude of the modulation wave of each inverter bridge arm by superimposing the first common mode modulation signal on the initial modulation wave of each inverter bridge arm, that is, increases the modulation wave amplitude of the reference switch tube in each inverter bridge arm. Since the modulation wave amplitude of the reference switch tube determines the duration of the inverter bridge arm outputting a positive level or a negative level, specifically, the larger the modulation wave amplitude of the reference switch tube, the larger the duty cycle of the reference switch tube, the longer the conduction time of the reference switch tube, that is, the longer the duration of the inverter bridge arm outputting a positive level or a negative level; the smaller the modulation wave amplitude of the reference switch tube, the smaller the duty cycle of the reference switch tube, the shorter the conduction time of the reference switch tube, that is, the shorter the duration of the inverter bridge arm outputting a positive level or a negative level. Since the duration of the inverter bridge arm outputting zero level is negatively correlated with the duration of the outputting positive level or negative level, the power conversion device 1 can increase the duration of each inverter bridge arm outputting positive level or negative level and reduce the duration of each inverter bridge arm outputting zero level by increasing the modulation wave amplitude of each inverter bridge arm. In other words, the conduction time of the reference switch tube in each inverter bridge arm can be increased, and the conduction time of the switch tube on the zero level loop in each inverter bridge arm can be reduced, thereby achieving loss balance of the switch tube in each inverter bridge arm.

[0042] In another implementation scenario, during the process of the power conversion device 1 supplying power to the power grid, when the voltage of the power grid is greater than or equal to the second voltage threshold, the controller 13 indicates that a high voltage ride-through fault occurs in the power grid, and then controls the switch unit 12 to be disconnected or controls the impedance of the switch unit 12 to increase. After the switch unit 12 is disconnected or the impedance of the switch unit 12 is increased, the controller 13 controls the modulation wave of each inverter bridge arm in the inverter bridge arms 111 to 11n to be a modulation wave after superimposing the second common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to reduce the amplitude of the modulation wave of each inverter bridge arm. Among them, the initial modulation wave of each inverter bridge arm is the modulation wave of each inverter bridge arm when the power conversion device 1 works normally. Both the first common-mode modulation signal and the second common-mode modulation signal can be understood as common-mode voltages, and the shapes and amplitudes of the second common-mode modulation signal and the first common-mode modulation signal are different.

[0043] It can be understood that when a high voltage ride-through fault occurs in the power grid, the power conversion device 1 cuts off the common mode branch or increases the impedance of the common mode branch by controlling the switch unit 12, thereby directly avoiding the generation of common mode oscillation or slowing down the common mode oscillation phenomenon. Afterwards, the power conversion device 1 reduces the amplitude of the modulation wave of each inverter bridge arm by superimposing the second common mode modulation signal on the initial modulation wave of each inverter bridge arm, thereby reducing the duration of each inverter bridge arm outputting a positive level or a negative level, and increasing the duration of each inverter bridge arm outputting a zero level. In other words, the conduction time of the switch tube directly connected to the DC bus in each inverter bridge arm can be reduced, and the conduction time of the switch tube on the zero level loop in each inverter bridge arm can be increased, thereby achieving loss balance of the switch tube in each inverter bridge arm.

[0044] In the present application, when a through fault occurs in the power grid, the power conversion device 1 can control the switch unit 12 to disconnect the common-mode branch or increase the impedance of the common-mode branch, thereby improving the problem of common-mode oscillation, and then superimposing the common-mode modulation signal on the initial modulation wave of each inverter bridge arm to change the amplitude of the modulation wave of each inverter bridge arm, so as to achieve loss balance of the switch tube in each inverter bridge arm. In addition, since the common-mode modulation signal injected into each inverter bridge arm is the same, and the line voltage (i.e., the difference between the bridge arm phase voltages output by the two inverter bridge arms) is used to supply power to the grid, the effect of the common-mode modulation signal can be offset, thereby not affecting the power supply to the grid.

[0045] Since the working principle of the power conversion device 1 is the same regardless of any value of n greater than or equal to 3, for the convenience of description, n=3 is taken as an example for illustration below.

[0046] For example, see Figure 4a , Figure 4a is another structural schematic diagram of the power conversion device provided by the present application. Figure 4aAs shown, the inverter bridge arms 111 to 113 adopt an NPC three-level inverter topology. Specifically, the inverter bridge arm 111 includes a first switch tube Q11, a second switch tube Q12, a third switch tube Q13, a fourth switch tube Q14, a clamping diode D15 and a clamping diode D16, the inverter bridge arm 112 includes a first switch tube Q21, a second switch tube Q22, a third switch tube Q23, a fourth switch tube Q24, a clamping diode D25 and a clamping diode D26, and the inverter bridge arm 113 includes a first switch tube Q31, a second switch tube Q32, a third switch tube Q33, a fourth switch tube Q34, a clamping diode D35 and a clamping diode D36. The switch tube in each inverter bridge arm can be understood as a semiconductor switch, including a metal oxide semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT) or a gallium nitride (GaN) transistor, etc. The following is an introduction using the switch tube as an IGBT as an example: the first switch tube Q11, the second switch tube Q12, the third switch tube Q13 and the fourth switch tube Q14 are sequentially connected in series between the first end i111+ and the second end i111- of the inverter bridge arm 111. Specifically, the collector of the first switch tube Q11 is connected to the first end i111+ of the inverter bridge arm 111, the emitter of the first switch tube Q11 is connected to the collector of the fourth switch tube Q14 after passing through the second switch tube Q12 and the third switch tube Q13 in sequence, and the emitter of the fourth switch tube Q14 is connected to the second end i111- of the inverter bridge arm 111. The connection point between the second switch tube Q12 and the third switch tube Q13 is connected to the third end o111 of the inverter bridge arm 111 . The cathode of the clamping diode D15 is connected to the collector of the second switching tube Q12, the anode of the clamping diode D15 is connected to the cathode of the clamping diode D16 and the fourth end i1110 of the inverter bridge arm 111, and the anode of the clamping diode D16 is connected to the emitter of the third switching tube Q13; the first switching tube Q21, the second switching tube Q22, the third switching tube Q23 and the fourth switching tube Q24 are sequentially connected in series between the first end i112+ and the second end i112- of the inverter bridge arm 112. Specifically, the collector of the first switching tube Q21 is connected to the first end i112+ of the inverter bridge arm 112, the emitter of the first switching tube Q21 is connected to the collector of the fourth switching tube Q24 after passing through the second switching tube Q22 and the third switching tube Q23 in sequence, and the emitter of the fourth switching tube Q24 is connected to the second end i112- of the inverter bridge arm 112. The connection point between the third switch tube Q23 and the fourth switch tube Q24 is connected to the third end o112 of the inverter bridge arm 112 .The cathode of the clamping diode D25 is connected to the collector of the second switching tube Q22, the anode of the clamping diode D25 is connected to the cathode of the clamping diode D26 and the fourth end i1120 of the inverter bridge arm 112, and the anode of the clamping diode D26 is connected to the emitter of the third switching tube Q23; the first switching tube Q31, the second switching tube Q32, the third switching tube Q33 and the fourth switching tube Q34 are sequentially connected in series between the first end i113+ and the second end i113- of the inverter bridge arm 113. Specifically, the collector of the first switching tube Q31 is connected to the first end i113+ of the inverter bridge arm 113, the emitter of the first switching tube Q31 is connected to the collector of the fourth switching tube Q34 after passing through the second switching tube Q32 and the third switching tube Q33 in sequence, and the emitter of the fourth switching tube Q34 is connected to the second end i113- of the inverter bridge arm 113. The connection point between the third switch tube Q33 and the fourth switch tube Q34 is connected to the third end o113 of the inverter bridge arm 113. The cathode of the clamping diode D35 is connected to the collector of the second switch tube Q32, the anode of the clamping diode D35 is connected to the cathode of the clamping diode D36 and the fourth end i1130 of the inverter bridge arm 113, and the anode of the clamping diode D36 is connected to the emitter of the third switch tube Q33. In addition, the fourth end i1110 of the inverter bridge arm 111, the fourth end i1120 of the inverter bridge arm 112, and the fourth end i1130 of the inverter bridge arm 113 are all connected to the bus midpoint N. In addition, the switch unit 12 includes a mechanical switch K1 and a semiconductor switch Q4 in parallel, and the mechanical switch here includes but is not limited to a relay, a contactor, etc. Here,. Figure 4a For the specific connection relationship of the other parts of the power conversion device 1 except the inverter bridge arms 111 to 113 and the switch unit 12, please refer to Figure 3 The description of the corresponding parts in the power conversion device 1 shown is not repeated here.

[0047] Specifically, after the power conversion device 1 is connected to the grid, the switch unit 12 is closed, specifically, the mechanical switch K1 is in a closed state, and the semiconductor switch Q4 is in an off state. The controller 13 obtains the voltage of the grid in real time after the power conversion device 1 is connected to the grid, and controls the semiconductor switch Q4 to turn on when the voltage of the grid is less than or equal to the first voltage threshold or greater than or equal to the second voltage threshold. After the semiconductor switch Q4 is turned on, the controller 13 first controls the mechanical switch K1 to turn off, and then controls the semiconductor switch Q4 to turn off, so as to control the switch unit 12 to turn off, thereby cutting off the common-mode branch, and thus completely solving the common-mode oscillation problem caused by the superposition of the common-mode modulation signal. In addition, for the specific operation of the controller 13 after the switch unit 12 is disconnected, please refer to Figure 3It is understandable that, since the power conversion device 1 uses the implementation mode that the mechanical switch K1 is turned off before the semiconductor switch Q4 when the switch unit 12 is turned off, the conduction loss of the semiconductor switch Q4 can be effectively reduced.

[0048] In addition, when the voltage of the power grid is greater than the first voltage threshold and less than the second voltage threshold, indicating that the power grid voltage has returned to normal, the controller 13 controls the switch unit 12 to close, specifically, first controls the semiconductor switch Q4 to turn on, and then controls the mechanical switch K1 to close, so that the common-mode branch is restored to the on-state. After the switch unit 12 is closed, that is, after the mechanical switch K1 is closed, the controller 13 controls the modulation wave of each inverter bridge arm in the inverter bridge arms 111 to 113 to be the initial modulation wave of each inverter bridge arm, and controls the semiconductor switch Q4 to turn off. It can be understood that after the power grid voltage returns to normal, the problem of increased loss of some switch tubes in each inverter bridge arm disappears, so there is no need to superimpose the common-mode modulation signal on the initial modulation wave of each inverter bridge arm, and restore the on-state of the common-mode branch, so that the power conversion device 1 can restore the working state when the power grid voltage is normal. In addition, because the power conversion device 1 adopts the implementation method of closing the mechanical switch K1 later than the semiconductor switch Q4 when controlling the switch unit 12 to close, the on-state loss of the semiconductor switch Q4 can be effectively reduced.

[0049] Optionally, the power conversion device 1 may further include a bus voltage balancing circuit, which is used to transfer the electric energy stored in the bus capacitor with a higher voltage in the positive bus capacitor C1 and the negative bus capacitor C2 to the bus capacitor with a lower voltage when there is a deviation between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2. Exemplarily, the bus voltage balancing circuit 14 includes a seventh switch tube Q5, an eighth switch tube Q6 and an inductor L4, wherein the seventh switch tube Q5 and the eighth switch tube Q6 are connected in series between the positive DC bus BUS+ and the negative DC bus BUS-, and the inductor L4 is connected between the bus midpoint N and the connection between the seventh switch tube Q5 and the eighth switch tube Q6. Optionally, the inductor L4 can also be replaced by other energy storage elements, such as a capacitor or a combination of an inductor and a capacitor.

[0050] In one embodiment, when the voltage of the positive bus capacitor C1 is greater than the voltage of the negative bus capacitor C2, the controller 13 controls the seventh switch tube Q5 to be turned on for a first preset time and then turned off, so as to transfer the energy stored in the positive bus capacitor C1 to the inductor L4. After the seventh switch tube Q5 is turned off, the controller 13 controls the eighth switch tube Q6 to be turned on for a second preset time and then turned off, so as to transfer the energy stored in the inductor L4 to the negative bus capacitor C2, thereby realizing the transfer of the energy stored in the positive bus capacitor C1 to the negative bus capacitor C2.

[0051] In another embodiment, when the voltage of the positive bus capacitor C1 is less than the voltage of the negative bus capacitor C2, the controller 13 controls the eighth switch tube Q6 to be turned on for a third preset time and then turned off, so as to transfer the energy stored in the negative bus capacitor C2 to the inductor L4. After the eighth switch tube Q6 is turned off, the controller 13 controls the seventh switch tube Q5 to be turned on for a fourth preset time and then turned off, so as to transfer the energy stored in the inductor L4 to the positive bus capacitor C1, thereby realizing the transfer of the energy stored in the negative bus capacitor C2 to the positive bus capacitor C1.

[0052] It can be understood that the power conversion device 1 can balance the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 by controlling the conduction time of the two switching tubes in the bus voltage balancing circuit, so that a switching tube with lower voltage resistance can be selected, thereby reducing the cost of the power conversion device 1.

[0053] In the embodiment of the present application, when a through fault occurs in the power grid, the power conversion device 1 can disconnect the common-mode branch by controlling the switch unit 12 to disconnect, thereby completely solving the common-mode oscillation problem caused by the superposition of the common-mode modulation signal, and then changing the amplitude of the modulation wave of each inverter bridge arm by superimposing the common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to achieve the loss balance of the switch tube in each inverter bridge arm. In addition, since the switch unit 12 adopts a structure in which a mechanical switch is connected in parallel with a semiconductor switch, and the power conversion device 1 adopts a control method in which the mechanical switch is disconnected before the semiconductor switch and closed later than the semiconductor switch when controlling the switch unit 12 to operate, the conduction loss of the semiconductor switch can be effectively reduced, thereby reducing the loss of the power conversion device 1.

[0054] For example, see Figure 4b , Figure 4b is another structural schematic diagram of the power conversion device provided by the present application. Figure 4b As shown, Figure 4a Compared with the power conversion device 1 shown in FIG. 1 , the two devices differ only in the switch unit 12. Specifically, Figure 4b The switch unit 12 shown includes a switch device S1 and an impedance element Z1 connected in parallel, wherein the switch device S1 includes a mechanical switch or a semiconductor switch, and the impedance element Z1 includes an inductor, a capacitor, or a resistor.

[0055] Specifically, after the power conversion device 1 is connected to the grid, the switch unit 12 is closed, specifically, the switch device S1 is in a closed state. The controller 13 obtains the voltage of the power grid in real time after the power conversion device 1 is connected to the grid, and when the voltage of the power grid is less than or equal to the first voltage threshold or greater than or equal to the second voltage threshold, controls the switch device S1 to be disconnected, so as to control the impedance of the switch unit 12 to increase, thereby increasing the impedance of the common-mode branch, so that the common-mode branch is in a high-impedance state, and then improves the common-mode oscillation problem caused by the superposition of the common-mode modulation signal. In practical applications, the improvement effect of the common-mode oscillation problem is determined by the impedance value of the impedance element Z1. In order to achieve the best improvement effect of the common-mode oscillation problem, the impedance element Z1 usually selects a resistor with a large resistance value, an inductor with a large inductance value, or a capacitor with a small capacitance value, so that the common-mode branch reaches a state close to a disconnection.

[0056] In addition, when the voltage of the power grid is greater than the first voltage threshold and less than the second voltage threshold, indicating that the power grid voltage has returned to normal, the controller 13 controls the switch unit 12 to close, specifically controls the switch device S1 to close, so that the common-mode branch returns to a low-impedance state. After the switch unit 12 is closed, that is, after the switch device S1 is closed, the controller 13 controls the modulation wave of each inverter bridge arm in the inverter bridge arms 111 to 113 to be the initial modulation wave of each inverter bridge arm. It can be understood that after the power grid voltage returns to normal, the problem of increased loss of some switch tubes in each inverter bridge arm disappears, so there is no need to superimpose the common-mode modulation signal on the initial modulation wave of each inverter bridge arm, and restore the common-mode branch to a low-impedance state, so that the power conversion device 1 returns to the working state when the power grid voltage is normal.

[0057] Here, the specific operation of the controller 13 after the switch unit 12 is disconnected and the specific operation of achieving bus voltage balancing are shown in FIG. Figure 3 and Figure 4a The description of the corresponding parts in the illustrated embodiment will not be repeated here.

[0058] In the embodiment of the present application, when a through fault occurs in the power grid, the power conversion device 1 can increase the impedance of the common mode branch by connecting the impedance element Z1 in the switch unit 12 to the common mode branch, thereby improving the common mode oscillation problem caused by the subsequent superposition of the common mode modulation signal, and then by superimposing the common mode modulation signal on the initial modulation wave of each inverter bridge arm, the amplitude of the modulation wave of each inverter bridge arm is changed to achieve loss balance of the switch tube in each inverter bridge arm. In addition, the switch unit 12 adopts a structure in which the switch device and the impedance element are connected in parallel. The power conversion device 1 can improve the common mode oscillation problem only by controlling the switch device, and the control method is simple and easy to implement.

[0059] It should be noted that the switch unit 12 in the present application is any circuit for realizing on-off control of the common mode branch, or realizing switching control between the high impedance state and the low impedance state of the common mode branch, except for the Figure 4a and Figure 4b In addition to the specific structure shown, a switch-to-fixed potential point clamping circuit may also be included.

[0060] also, Figure 4a and Figure 4b The structure of the switch unit for realizing switch loss balance in the power conversion device 1 shown in the figure, the structure for realizing bus voltage balance and the control method of the power conversion device 1 are also applicable to Figure 5 and Figure 6 The power conversion device 1 is shown.

[0061] like Figure 5 As shown, the inverter bridge arms 111 to 113 adopt an ANPC three-level inverter topology. Figure 5 The inverter bridge arms shown are equivalent to Figure 4a The bridge arm after the two clamping diodes in each inverter bridge arm are replaced by the fifth switch tube and the sixth switch tube. Specifically, the fifth switch tube Q15 and the sixth switch tube Q16 in the inverter bridge arm 111 are connected in series between the connection point of the first switch tube Q11 and the second switch tube Q12 and the connection point of the third switch tube Q13 and the fourth switch tube Q14, and the connection point of the fifth switch tube Q15 and the sixth switch tube Q16 is connected to the fourth end i 1110 of the inverter bridge arm 111; the fifth switch tube Q25 and the sixth switch tube Q26 in the inverter bridge arm 112 are connected in series between the connection point of the first switch tube Q21 and the second switch tube Q22 and the connection point of the third switch tube Q23 and the fourth switch tube Q24, and the connection point of the fifth switch tube Q25 and the sixth switch tube Q26 is connected to the fourth end i 1110 of the inverter bridge arm 112. 1120; the fifth switch tube Q35 and the sixth switch tube Q36 in the inverter bridge arm 113 are connected in series between the connection point of the first switch tube Q31 and the second switch tube Q32 and the connection point of the third switch tube Q33 and the fourth switch tube Q34, and the connection point of the fifth switch tube Q35 and the sixth switch tube Q36 is connected to the fourth end i 1130 of the inverter bridge arm 113. Here, Figure 5 For a detailed description of the connection relationship and control method of other parts of the power conversion device 1 except the inverter bridge arms, please refer to Figure 4a and Figure 4b The description of the corresponding parts in the power conversion device 1 shown is not repeated here.

[0062] like Figure 6As shown, the inverter bridge arms 111 to 113 adopt a T-type three-level inverter topology. Specifically, the inverter bridge arm 111 includes a first switch tube Q11, a second switch tube Q12, a third switch tube Q13 and a fourth switch tube Q14, the inverter bridge arm 112 includes a first switch tube Q21, a second switch tube Q22, a third switch tube Q23 and a fourth switch tube Q24, and the inverter bridge arm 113 includes a first switch tube Q31, a second switch tube Q32, a third switch tube Q33 and a fourth switch tube Q34. The switch tube in each inverter bridge arm can be understood as a semiconductor switch, including a MOSFET, an IGBT or a GaN transistor, etc. The following is an introduction taking the switch tube as an IGBT as an example: the collector of the first switch tube Q11 is connected to the first end i111+ of the inverter bridge arm 111, the emitter of the first switch tube Q11 is connected to the second end i111- of the inverter bridge arm 111 through the second switch tube Q12, and the emitter of the first switch tube Q11 is connected to the third end o111 of the inverter bridge arm 111. The third switch tube Q13 and the fourth switch tube Q14 are reversely connected in series between the third end o111 and the fourth end i1110 of the inverter bridge arm 111. Specifically, the emitter of the fourth switch tube Q14 is connected to the third end o111 of the inverter bridge arm 111, the collector of the fourth switch tube Q14 is connected to the collector of the third switch tube Q13, and the emitter of the third switch tube Q13 is connected to the fourth end i1110 of the inverter bridge arm 111; the collector of the first switch tube Q21 is connected to the first end i112+ of the inverter bridge arm 112, the emitter of the first switch tube Q21 is connected to the second end i112- of the inverter bridge arm 112 through the second switch tube Q22, and the emitter of the first switch tube Q21 is connected to the third end o112 of the inverter bridge arm 112. The third switch tube Q23 and the fourth switch tube Q24 are reversely connected in series between the third end o112 and the fourth end i1120 of the inverter bridge arm 112. Specifically, the emitter of the fourth switch tube Q24 is connected to the third end o112 of the inverter bridge arm 112, the collector of the fourth switch tube Q24 is connected to the collector of the third switch tube Q23, and the emitter of the third switch tube Q23 is connected to the fourth end i1120 of the inverter bridge arm 112; the collector of the first switch tube Q31 is connected to the first end i113+ of the inverter bridge arm 113, the emitter of the first switch tube Q31 is connected to the second end i113- of the inverter bridge arm 113 through the second switch tube Q32, and the emitter of the first switch tube Q31 is connected to the third end o113 of the inverter bridge arm 113. The third switch tube Q33 and the fourth switch tube Q34 are reversely connected in series between the third end o113 and the fourth end i1130 of the inverter bridge arm 113. Specifically, the emitter of the fourth switch tube Q34 is connected to the third end o113 of the inverter bridge arm 113, the collector of the fourth switch tube Q34 is connected to the collector of the third switch tube Q33, and the emitter of the third switch tube Q33 is connected to the fourth end i1130 of the inverter bridge arm 113.In addition, the fourth end i1110 of the inverter bridge arm 111, the fourth end i1120 of the inverter bridge arm 112, and the fourth end i1130 of the inverter bridge arm 113 are all connected to the bus midpoint N. Here, . Figure 6 For a detailed description of the connection relationship and control method of other parts of the power conversion device 1 except the inverter bridge arms, please refer to Figure 4a and Figure 4b The description of the corresponding parts in the power conversion device 1 shown is not repeated here.

[0063] The above are only specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A power conversion device, characterized in that: The power conversion device includes a DC terminal, a positive DC bus, a negative DC bus, a positive bus capacitor, a negative bus capacitor, at least three inverter bridge arms, a switch unit, a controller, filter inductors, filter capacitors and AC terminals corresponding to the at least three inverter bridge arms, wherein: The DC end is used to connect to a photovoltaic module or an energy storage battery, and the AC ends corresponding to the at least three inverter bridge arms are used to connect to a power grid; The positive DC bus and the negative DC bus are connected to the DC terminal, and the positive bus capacitor and the negative bus capacitor are sequentially connected in series between the positive DC bus and the negative DC bus; The first end and the second end of each of the at least three inverter bridge arms are connected to the positive DC bus and the negative DC bus respectively, and the third end of each inverter bridge arm is connected to the bus midpoint after sequentially passing through the filter inductor, filter capacitor and the switch unit corresponding to each inverter bridge arm, and the bus midpoint is the connection point of the positive bus capacitor and the negative bus capacitor, and the series connection points of the filter inductors and filter capacitors corresponding to the at least three inverter bridge arms are respectively used to connect the AC ends corresponding to the at least three inverter bridge arms; The controller is used to control the switch unit to be disconnected or the impedance of the switch unit to be increased when the voltage of the power grid is less than or equal to a first voltage threshold; after the switch unit is disconnected or the impedance of the switch unit is increased, control the modulation wave of each inverter bridge arm to be a modulation wave after superimposing a first common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to increase the amplitude of the modulation wave of each inverter bridge arm, wherein the initial modulation wave is the modulation wave of each inverter bridge arm when the power conversion device is working normally.

2. The power conversion device according to claim 1, characterized in that: The controller is also used to control the switch unit to be disconnected or the impedance of the switch unit to increase when the voltage of the power grid is greater than or equal to a second voltage threshold; after the switch unit is disconnected or the impedance of the switch unit increases, control the modulation wave of each inverter bridge arm to be a modulation wave after superimposing a second common-mode modulation signal on the initial modulation wave of each inverter bridge arm, so as to reduce the amplitude of the modulation wave of each inverter bridge arm, wherein the second voltage threshold is greater than the first voltage threshold.

3. The power conversion device according to claim 1 or 2, characterized in that: The switch unit includes a mechanical switch and a semiconductor switch connected in parallel; The controller is used for controlling the mechanical switch to open first and then controlling the semiconductor switch to turn off when the voltage of the power grid is less than or equal to the first voltage threshold or greater than or equal to the second voltage threshold.

4. The power conversion device according to claim 1 or 2, characterized in that: The switch unit includes a switch device and an impedance element connected in parallel, the switch device includes a mechanical switch or a semiconductor switch, and the impedance element includes an inductor, a capacitor or a resistor; The controller is used for controlling the switch device to be disconnected when the voltage of the power grid is less than or equal to the first voltage threshold or greater than or equal to the second voltage threshold.

5. The power conversion device according to any one of claims 2 to 4, characterized in that: The controller is also used to control the switching unit to close when the voltage of the power grid is greater than the first voltage threshold and less than the second voltage threshold; after the switching unit is closed, the modulation wave of each inverter bridge arm is controlled to be the initial modulation wave of each inverter bridge arm.

6. The power conversion device according to claim 5, characterized in that: The switch unit includes a mechanical switch and a semiconductor switch connected in parallel; The controller is used for controlling the semiconductor switch to be turned on first and then controlling the mechanical switch to be closed when the voltage of the power grid is greater than the first voltage threshold and less than the second voltage threshold.

7. The power conversion device according to claim 5, characterized in that: The switch unit includes a switch device and an impedance element connected in parallel, the switch device includes a mechanical switch or a semiconductor switch, and the impedance element includes an inductor, a capacitor or a resistor; The controller is used for controlling the switching device to close when the voltage of the power grid is greater than the first voltage threshold and less than the second voltage threshold.

8. The power conversion device according to any one of claims 1 to 7, characterized in that: The fourth end of each inverter bridge arm is connected to the busbar midpoint, and each inverter bridge arm includes a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube and a sixth switch tube, wherein: The first switch tube, the second switch tube, the third switch tube and the fourth switch tube of each inverter bridge arm are sequentially connected in series between the first end and the second end of each inverter bridge arm, and the connection between the second switch tube and the third switch tube of each inverter bridge arm is connected to the third end of each inverter bridge arm; The fifth switch tube and the sixth switch tube of each inverter bridge arm are connected in series between the connection point of the first switch tube and the second switch tube of each inverter bridge arm and the connection point of the third switch tube and the fourth switch tube of each inverter bridge arm, and the connection point of the fifth switch tube and the sixth switch tube of each inverter bridge arm is connected to the fourth end of each inverter bridge arm.

9. The power conversion device according to any one of claims 1 to 7, characterized in that: The fourth end of each inverter bridge arm is connected to the busbar midpoint, and each inverter bridge arm includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube, wherein: The first switch tube and the second switch tube of each inverter bridge arm are connected in series between the first end and the second end of each inverter bridge arm, and the connection between the first switch tube and the second switch tube in each inverter bridge arm is connected to the third end of each inverter bridge arm; The third switch tube and the fourth switch tube of each inverter bridge arm are reversely connected in series between the third end and the fourth end of each inverter bridge arm.

10. The power conversion device according to any one of claims 1 to 9, characterized in that: The power conversion device further includes an energy storage element, a seventh switch tube and an eighth switch tube, wherein the seventh switch tube and the eighth switch tube are sequentially connected in series between the positive DC bus and the negative DC bus, and the energy storage element is connected between the midpoint of the bus and the connection between the seventh switch tube and the eighth switch tube; The controller is used to control the seventh switch tube to be turned on for a first preset time and then turned off when the voltage of the positive bus capacitor is greater than the voltage of the negative bus capacitor; after the seventh switch tube is turned off, control the eighth switch tube to be turned on for a second preset time and then turned off, so as to transfer the energy stored in the positive bus capacitor to the negative bus capacitor; or, The controller is used to control the eighth switch tube to be turned on for a third preset time period and then turned off when the voltage of the positive bus capacitor is less than the voltage of the negative bus capacitor; after the eighth switch tube is turned off, control the seventh switch tube to be turned on for a fourth preset time period and then turned off, so as to transfer the energy stored in the negative bus capacitor to the positive bus capacitor.

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