Power conversion device and slow start control method of power conversion device

Abstract

The invention provides a power conversion device and a slow start control method of the power conversion device, the power conversion device comprises a plurality of bus capacitors connected in parallel, a positive direct current bus, a negative direct current bus and an inverter circuit, and the plurality of bus capacitors are connected in parallel and then connected between the positive direct current bus and the negative direct current bus. The positive DC bus and the negative DC bus are used for connecting a DC power supply. The inverter circuit comprises at least one phase of bridge arm connected in parallel between the positive direct current bus and the negative direct current bus, the bridge arm comprises an upper bridge arm connected with the positive direct current bus and a lower bridge arm connected with the negative direct current bus, and a bridge arm midpoint of the bridge arm is connected to the positive direct current bus or the negative direct current bus through a filter inductor and a filter capacitor which are connected in series; the series connection point of the filter capacitor and the filter inductor is connected with a power grid. The power conversion device further comprises a voltage dividing capacitor which is connected between the filter capacitor and the positive direct current bus or the negative direct current bus, the number of thin film capacitors can be reduced, and the size of the power conversion device can be effectively reduced.

Description

Technical Field

[0001] This application relates to the field of electronic power technology, and in particular to a power conversion device and a method for slow-start control of the power conversion device. Background Technology

[0002] Power conversion devices (PCDs) are widely used in photovoltaic power generation, energy storage, wind power generation, and motor drives because they can convert direct current (DC) to alternating current (AC). The DC voltage is converted into a three-phase high-frequency pulse voltage by the bridge circuit of the PCD, and then fed into the power grid through filter inductors and capacitors. Currently, the mainstream topology in bridge circuits is the three-level topology, such as the T-type and I-type three-level topologies. For example... Figure 2 Taking the T-type three-level topology as an example, multiple sets of series-connected thin-film capacitors are connected in parallel between the positive and negative DC buses. In the three-phase bridge arms, the midpoint of each phase bridge arm is connected to the midpoint of the DC-side thin-film capacitor through a series-connected filter inductor and filter capacitor. The midpoint of the thin-film capacitor provides a zero-level output, enabling the power conversion device to output high, zero, and low levels. However, due to the low power density of the thin-film capacitor, the power conversion device is relatively large. Summary of the Invention

[0003] This application provides a power conversion device and a soft-start control method for the power conversion device, which can reduce the number of thin-film capacitors and avoid the large size of the power conversion device due to the low power density of the thin-film capacitors, thus effectively reducing the size of the power conversion device.

[0004] In a first aspect, this application provides a power conversion device, which includes multiple parallel bus capacitors, a positive DC bus, a negative DC bus, and an inverter circuit. Multiple parallel-connected thin-film capacitors are connected between the positive and negative DC buses, which are used to connect to a DC power supply. The inverter circuit includes at least one phase bridge arm connected in parallel between the positive and negative DC buses. The bridge arm includes an upper bridge arm connected to the positive DC bus and a lower bridge arm connected to the negative DC bus. The midpoint of the bridge arm is connected to either the positive or negative DC bus via a series-connected filter inductor and filter capacitor. The series connection point of the filter capacitor and filter inductor is used to connect to the power grid. The power conversion device also includes a voltage-dividing capacitor connected between the filter capacitor and either the positive or negative DC bus. It should be understood that when the inverter circuit samples a T-type or I-type three-level topology, the midpoint of each phase arm needs to be connected to the series connection terminal of a set of two series-connected thin-film capacitors. This results in multiple sets of series-connected thin-film capacitors on the DC side of the power conversion device being required to connect between the positive and negative DC buses in the case of multi-phase arms. Since the power density of thin-film capacitors is relatively low, the size of the power conversion device is relatively large. Therefore, the power conversion device provided in this application, which samples multiple thin-film capacitors in parallel on the DC side, can reduce the number of thin-film capacitors, thereby reducing the size of the power conversion device to a certain extent, and also reducing the heat dissipation burden of the power conversion device.

[0005] In one possible implementation, the power conversion device further includes a controller. When the power conversion device is started, the controller can be used to control the target switch in any phase arm to conduct, thereby charging the voltage divider capacitor. Specifically, when the midpoint of each phase arm is connected to the positive DC bus via a series-connected filter inductor and filter capacitor, the target switch is the switch in the lower arm of each phase arm. When the midpoint of each phase arm is connected to the negative DC bus via a series-connected filter inductor and filter capacitor, the target switch is the switch in the upper arm of each phase arm. Furthermore, the controller can also be used to control the switches in the upper and lower arms of each phase arm to alternately conduct when the capacitor voltage of the voltage divider capacitor reaches the target voltage, so that the inverter circuit converts the DC power from the DC power supply into AC power and outputs the AC power to the power grid. It should be understood that when the midpoint of any phase arm is connected to the positive DC bus BUS+ or the negative DC bus BUS- through a series filter inductor and filter capacitor, the filter capacitor may have a DC bias voltage resulting from voltage division of the positive or negative bus voltage. For example, when the output voltage of the DC power supply is Vdc, the DC bias voltage can be approximately equal to half of Vdc. When the power conversion device operates in a high-voltage scenario, i.e., when the output voltage of the DC power supply is large, the filter capacitor may withstand a high capacitor voltage. Therefore, when the power conversion device starts up, a large instantaneous current may flow through the filter capacitor, potentially damaging it. Based on this, a voltage divider capacitor can be added between the filter capacitor and the positive DC bus BUS+ or the negative DC bus BUS-. When the power conversion device starts up, the target switch in any phase arm can be controlled to conduct to pre-charge the voltage divider capacitor. In this way, after the power conversion device is connected to a DC power source or to the power grid, the change in the voltage of the filter capacitor is reduced to a certain extent, thereby reducing the rate of change of the voltage of the filter capacitor. This avoids a sudden surge in the voltage of the filter capacitor, which would cause a large instantaneous current to flow through the filter capacitor. This can improve the safety of the power conversion device and also improve the power generation stability of the power conversion device.

[0006] In one possible implementation, the voltage divider unit may further include a varistor connected in parallel across the voltage divider capacitor. In the event of a voltage surge across the voltage divider capacitor, the varistor can be used for current-limiting protection. It should be understood that when a lightning surge occurs on the DC side of the power conversion device, the surge current can flow through the positive or negative DC bus, through the voltage divider capacitor and filter capacitor, and then into the power grid. When the surge current flows through the voltage divider capacitor, the capacitor may experience a large capacitor voltage, potentially leading to overvoltage damage. A varistor is a resistor with nonlinear volt-ampere characteristics, capable of voltage clamping and absorbing excess current to protect sensitive components when the circuit experiences overvoltage. In summary, by connecting a varistor in parallel across the voltage divider capacitor, the varistor can absorb excess current during surges, protecting the voltage divider from damage.

[0007] In one possible implementation, the voltage divider capacitor can be a film capacitor, which is a capacitor with a polymer film as the dielectric. Compared with other types of capacitors, film capacitors have low parasitic effects, low loss, long life and bidirectional withstand voltage, and strong ability to withstand high ripple current. They are more suitable for power storage and low-frequency filtering scenarios, and can improve the reliability and safety of power conversion devices to a certain extent.

[0008] Secondly, this application provides a soft-start control method for a power conversion device. The power conversion device includes a controller, multiple parallel-connected thin-film capacitors, a positive DC bus, a negative DC bus, and an inverter circuit. The multiple parallel-connected thin-film capacitors are connected between the positive DC bus and the negative DC bus, which are used to connect to a DC power supply. The inverter circuit includes at least one phase bridge arm connected in parallel between the positive DC bus and the negative DC bus. Each phase bridge arm includes an upper bridge arm and a lower bridge arm connected in series. The midpoint of each phase bridge arm is connected to the positive DC bus or the negative DC bus through a series-connected filter inductor and filter capacitor. The series connection point of the filter capacitor and filter inductor is used to connect to the power grid. The power conversion device also includes a voltage divider capacitor connected between the filter capacitor and the positive DC bus or the negative DC bus. The method includes:

[0009] When the power conversion device is started, the target switch in any phase arm is turned on to charge the voltage divider capacitor. When the midpoint of each phase arm is connected to the positive DC bus through a series filter inductor and filter capacitor, the target switch is the switch in the lower arm of each phase arm. When the midpoint of each phase arm is connected to the negative DC bus through a series filter inductor and filter capacitor, the target switch is the switch in the upper arm of each phase arm.

[0010] When the voltage of the voltage divider capacitor reaches the target voltage, the switching transistors in the upper and lower arms of each phase bridge arm are alternately turned on to enable the inverter circuit to convert the DC power from the DC power supply into AC power and output the AC power to the power grid, thereby realizing the slow start of the power conversion device.

[0011] It should be understood that, compared to the method where the midpoint of each phase bridge arm is connected to the series connection point of two film capacitors connected in series, the DC side of the power conversion device provided in this application only requires multiple film capacitors connected in parallel between the positive and negative DC buses. The number of film capacitors is reduced by at least half, thus reducing the size of the power conversion device to a certain extent. When the power conversion device starts up, the switch in the upper bridge arm of any phase bridge arm can be turned on to pre-charge the voltage divider capacitor, ensuring that the voltage of the voltage divider capacitor Cd reaches the target voltage. This reduces the change in the voltage of the filter capacitors connected to each phase bridge arm during startup, thereby reducing the rate of change of the filter capacitor voltage. This prevents a sudden surge in the voltage of the filter capacitor, which could lead to a large instantaneous current flowing through it, improving the safety and power generation stability of the power conversion device.

[0012] In one possible implementation, the voltage divider unit may further include a varistor connected in parallel across the voltage divider capacitor. In the event of a voltage surge across the voltage divider capacitor, the varistor can provide current-limiting protection. It should be understood that a varistor is a resistor with non-linear volt-ampere characteristics, capable of voltage clamping and absorbing excess current to protect sensitive components when the circuit experiences overvoltage. In summary, by connecting a varistor in parallel across the voltage divider capacitor, excess current can be absorbed during surges, protecting the voltage divider from damage.

[0013] In one possible implementation, the voltage divider capacitor can be a film capacitor, which is a capacitor with a polymer film as the dielectric. Compared with other types of capacitors, film capacitors have low parasitic effects, low loss, long life and bidirectional withstand voltage, and strong ability to withstand high ripple current. They are more suitable for power storage and low-frequency filtering scenarios, and can improve the reliability and safety of power conversion devices to a certain extent.

[0014] Thirdly, this application provides a photovoltaic system including a DC power supply and a photovoltaic power conversion device as described in the first aspect, the power conversion device being used to convert DC power from the DC power supply into AC power and output the AC power to the power grid or a load. Attached Figure Description

[0015] Figure 1 This is a system architecture diagram of the photovoltaic power generation system provided in the embodiments of this application;

[0016] Figure 2 This is a schematic diagram of the power conversion device provided in an embodiment of this application;

[0017] Figure 3 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0018] Figure 4 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0019] Figure 5 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0020] Figure 6 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0021] Figure 7 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0022] Figure 8 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application;

[0023] Figure 9 A schematic flowchart of the soft-start control method for the power conversion device provided in this application. Detailed Implementation

[0024] The power conversion device provided in this application can be applied to different application scenarios, such as photovoltaic power generation systems, energy storage systems, wind power generation systems, motor drives, or other application scenarios. The following explanation uses a photovoltaic power generation application scenario as an example.

[0025] A photovoltaic (PV) power generation system can be a power supply system based on solar photovoltaic power generation. Solar PV power generation has the characteristics of no moving parts, no noise, no pollution, and high reliability, and has excellent application prospects in communication power supply systems in remote areas. The PV power generation system provided in this application can be used to power various types of electrical equipment, such as base station equipment, battery power, or household appliances (such as refrigerators, air conditioners, etc.), depending on the actual application scenario, and is not limited here. The power conversion device provided in this application can be used in the above-mentioned PV power generation system. It can be used to convert the variable DC voltage generated by DC power sources such as photovoltaic solar panels into AC power at the mains frequency, and output the AC power to the mains grid for use by the mains grid. Specifically, it can be used by base station equipment, batteries, or household appliances in the mains grid. The PV system provided in this application can be adapted to different application scenarios, such as solar power supply scenarios and solar hybrid power supply scenarios, depending on the actual application scenario, and is not limited here. This application uses a solar power supply scenario as an example for illustration.

[0026] See Figure 1 , Figure 1 This is a schematic diagram of a photovoltaic power generation system. (Refer to...) Figure 1 In a photovoltaic (PV) power generation system, PV strings 1 utilize the photovoltaic effect to directly convert solar energy into electrical energy. PV strings 1 are typically a series of strings, each containing multiple PV modules connected in series. Each PV module may contain multiple solar cells connected in series or parallel. A power conversion device 2 converts the direct current (DC) from PV strings 1 into alternating current (AC), and then sends the AC to a corresponding prefabricated substation 3 for voltage transformation. The prefabricated substation 3 converts the low-voltage AC output from the power conversion device 2 into medium-voltage AC, which is then transmitted to a step-up substation 4 and finally to the power grid 5 or other loads. In a PV power generation system, the power conversion device typically employs a three-phase three-level topology. This topology includes phase A, phase B, and phase C arms. The midpoint of each phase arm can be coupled to the series connection point of two series-connected thin-film capacitors, which are connected between the positive DC bus BUS+ and the negative DC bus BUS-. The three-level topology can be either a T-type or an I-type three-level topology. For ease of description, a T-type three-level topology will be used as an example. See [link / reference] Figure 2 , Figure 2 This is a schematic diagram of the power conversion device provided in an embodiment of this application. Figure 2As shown, multiple sets of series-connected thin-film capacitors are connected between the positive DC bus BUS+ and the negative DC bus BUS-. For example, series-connected thin-film capacitors C11 and C12 are connected between the positive DC bus BUS+ and the negative DC bus BUS-. Taking phase A as an example, phase A may include a first switch Q11, a second switch Q12, a third switch Q13, and a fourth switch Q14. The midpoint N1 of phase A is connected to the series connection point of thin-film capacitors C11 and C12, where the series connection point of thin-film capacitors C11 and C12 can be the midpoint O of the DC bus (hereinafter referred to as bus midpoint O). Here, the upper arm of phase A includes the first switch Q11, the lower arm includes the fourth switch Q14, and the midpoint N1 of phase A is connected to the bus midpoint O through the second switch Q12 and the third switch Q13. In a three-level topology, each phase arm can generate three voltage levels: high, zero, and low. Therefore, each phase arm requires two film capacitors connected in series to control the midpoint balance and provide a zero-level output. Thus, on the DC side of the power converter, the midpoint of each phase arm needs to be connected to the series connection point of two film capacitors, which are connected between the positive DC bus BUS+ and the negative DC bus BUS-. Consequently, when film capacitors are used for the bus voltage, a three-level topology requires a large number of film capacitors on the DC side. Furthermore, due to the low power density of film capacitors, this results in a larger size for the power converter.

[0027] Therefore, this application provides a power conversion device that can reduce the number of DC-side thin-film capacitors, reduce the size of the power conversion device, and has low cost and wide applicability.

[0028] The specific implementation principle of the photovoltaic power generation system provided in the embodiments of this application will be described below with reference to the accompanying drawings.

[0029] See Figure 3 , Figure 3 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application. For example... Figure 3As shown, the power conversion device 2 includes a positive DC bus BUS+, a negative DC bus BUS-, and an inverter circuit 21. The positive DC bus BUS+ and negative DC bus BUS- are used to connect to a DC power source. Multiple thin-film capacitors are connected in parallel between the positive and negative DC buses. Here, the DC power source can be a photovoltaic string, and the power conversion device 2 can convert the DC power from the photovoltaic string into AC power. Optionally, the DC power source may include a photovoltaic string and a DC-DC converter circuit. The output terminal of the photovoltaic string can be connected to the input terminal of the DC-DC converter circuit, the positive output terminal of the DC-DC converter circuit can be connected to the positive DC bus BUS+, and the negative output terminal of the DC-DC converter circuit can be connected to the negative DC bus BUS-. The DC-DC converter circuit can perform voltage conversion (e.g., boost) on the DC power from the photovoltaic string and output the voltage-converted DC power to the power conversion device 2. The power conversion device 2 sequentially inverts the input DC power to obtain AC power that meets the requirements of the power grid or load, thereby enabling power supply to various types of electrical equipment, including the power grid. Optionally, the DC power supply may also include an energy storage battery, which can be connected to a DC-DC converter circuit. The DC-DC converter circuit can convert the DC power from the energy storage battery into voltage and output the voltage-converted DC power to the power conversion device 2. It should be understood that the DC power supply can be determined according to the actual application scenario and is not limited thereto.

[0030] The inverter circuit 21 may include at least one phase bridge arm connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. The bridge arm may include an upper bridge arm and a lower bridge arm connected in series. The midpoint of each phase bridge arm is connected to the positive DC bus BUS+ or the negative DC bus BUS- through a series-connected filter inductor and filter capacitor. The series connection point of the filter capacitor and filter inductor is used to connect to the power grid. The inverter circuit 21 may also include a voltage divider capacitor connected between the filter capacitor and the positive DC bus BUS+ or the negative DC bus BUS-. Here, when the inverter circuit includes multiple phase bridge arms, the midpoint of each phase bridge arm can be connected to the first end of the filter inductor, and the second end of the filter inductor can be connected to the first end of the filter capacitor. The second end of the filter capacitor connected to the midpoint of each bridge arm is then connected to the positive DC bus BUS+ or the negative DC bus BUS- through the voltage divider capacitor. When the inverter circuit includes a single-phase bridge arm, the midpoint of the bridge arm is connected to the positive DC bus BUS+ or the negative DC bus BUS- through a series-connected filter inductor, filter capacitor, and voltage divider capacitor. It should be understood that this application does not limit the number of bridge arms included in the inverter circuit; for ease of description, a single-phase bridge arm will be used as an example for detailed description below. When the midpoint of the bridge arm is connected to the negative DC bus BUS- through a series-connected filter inductor and filter capacitor, the structure of the power conversion device 2 can be as follows: Figure 3As shown, taking phase A as an example, the upper arm of phase A may include a first switch Q11, and the lower arm may include a second switch Q12. The connection point of the first switch Q11 and the second switch Q12 can be the midpoint N1 of phase A. The midpoint N1 of phase A can be connected to the negative DC bus BUS- through a series-connected filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd. When the inverter circuit includes one phase arm, the number of film capacitors connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS- can be one. When the midpoint of the arm is connected to the positive DC bus BUS+ through a series-connected filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd, the structure of the power conversion device 2 can be found in [reference needed]. Figure 4 , Figure 4 This is another structural schematic diagram of the power conversion device 2 provided in the embodiments of this application. For example... Figure 4 As shown, taking phase A bridge arm as an example, the upper bridge arm of phase A bridge arm may include a first switch Q11, and the lower bridge arm of phase A bridge arm may include a second switch Q12. The connection point of the first switch Q11 and the second switch Q12 can be the midpoint N1 of the phase A bridge arm. The midpoint N1 of the phase A bridge arm can be connected to the positive DC bus BUS+ through a series connection of a filter inductor La, a filter capacitor Ca, and a voltage divider capacitor Cd. It can be seen that... Figure 3 , Figure 4 and Figure 2 In comparison, taking phase A bridge arm as an example, Figure 3 , Figure 4 In this design, only one thin-film capacitor needs to be connected between the positive DC bus BUS+ and the negative DC bus BUS- on the DC side, which reduces the number of thin-film capacitors on the DC side and can reduce the size of the power conversion device to some extent.

[0031] Since the midpoint of any phase arm is connected to the positive DC bus BUS+ or the negative DC bus BUS- through a series filter inductor and filter capacitor, the filter capacitor may be subjected to a large DC bias voltage. Therefore, a voltage divider capacitor can be added between the filter capacitor and the positive DC bus BUS+ or the negative DC bus BUS- to avoid damage to the filter capacitor due to overvoltage.

[0032] See also Figure 3 and Figure 4Because the power conversion device 2 can convert DC to AC, there is an AC voltage similar to a sine wave on the filter capacitor Ca. Since the second terminal of the filter capacitor Ca is connected to either the positive DC bus BUS+ or the negative DC bus BUS-, there may also be a DC bias voltage on the filter capacitor Ca obtained by dividing the positive or negative bus voltage. For example, when the output voltage of the DC power supply is Vdc, the DC bias voltage can be approximately equal to half of Vdc. That is, the capacitor voltage of the filter capacitor Ca can be superimposed with Vdc / 2 on top of the sine wave. Since photovoltaic power generation systems typically operate under high voltage conditions, the capacitor voltage of the filter capacitor Ca may be relatively high. When the power conversion device 2 starts up, there may be a momentary large current flowing through the filter capacitor Ca. The rate of change of the current and voltage through the filter capacitor Ca with time is related to the current, as shown in formula (1).

[0033]

[0034] Where I is the current flowing through the filter capacitor, and C is the capacitance value of the filter capacitor Ca. Let be the rate of change of the voltage across the filter capacitor Ca over time.

[0035] As shown in formula (1), at the instant the power conversion device 2 starts up, the voltage across the filter capacitor Ca may increase rapidly in a short time. Therefore, a large instantaneous current flows through the filter capacitor Ca, which may cause the filter capacitor Ca to burn out. To solve the above problem, the voltage divider capacitor Cd can be pre-charged so that its voltage reaches the target voltage (e.g., Vdc / 2). This can avoid a large DC bias voltage on the filter capacitor Ca and prevent a large instantaneous current from flowing through it due to the rapid increase in voltage.

[0036] In some feasible embodiments, the power conversion device 2 may further include a controller 22. When the power conversion device 2 is started, the controller 22 can control the target switch in any phase arm to conduct in order to charge the voltage divider capacitor Cd. Specifically, when the midpoint of the bridge arm is connected to the positive DC bus BUS+ through a series-connected filter inductor and filter capacitor, the target switch is the switch in the lower bridge arm of each phase arm. When the midpoint of the bridge arm is connected to the negative DC bus BUS- through a series-connected filter inductor and filter capacitor, the target switch is the switch in the upper bridge arm of each phase arm.

[0037] Specifically, with Figure 3Taking phase A of the bridge arm as an example, the midpoint N1 of phase A is connected to the negative DC bus BUS- through a series-connected filter inductor La and filter capacitor Ca. The target switch can be the first switch Q11. The controller 22 can adjust the duty cycle of the first switch Q11 and the second switch Q12 to turn on the first switch Q11 and turn off the second switch Q12. At this time, the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd can form a BUCK circuit. Current flows from the positive DC bus BUS+ through the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd. In this way, the voltage divider capacitor Cd and the filter capacitor Ca can divide the bus voltage between the positive DC bus BUS+ and the negative DC bus BUS-, preventing the filter capacitor Ca from being damaged by overvoltage due to DC bias.

[0038] Similarly, with Figure 4 Taking the example where the midpoint of the bridge arm is connected to the positive DC bus BUS+ via a series-connected filter inductor La and filter capacitor Ca, the target switch is the second switch Q12. The controller 22 can adjust the duty cycle of the first switch Q11 and the second switch Q12 to turn on the second switch Q12 and turn off the first switch Q11. At this time, the second switch Q12, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd form a BUCK circuit. Current flows from the negative DC bus BUS- through the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the positive DC bus BUS+, thus pre-charging the voltage divider capacitor Cd. In this way, the voltage divider capacitor Cd and the filter capacitor Ca can divide the bus voltage between the positive DC bus BUS+ and the negative DC bus BUS-, preventing the filter capacitor Ca from being damaged by overvoltage due to DC bias.

[0039] Furthermore, when the voltage across the voltage divider capacitor Cd reaches the target voltage, the controller 22 can also control the switching transistors in the upper and lower arms of each phase bridge arm to alternately conduct, causing the inverter circuit 21 to convert the DC power from the DC power supply into AC power and output the AC power to the grid, thereby achieving a soft start-up of the power conversion device 2. For example, in Figure 3 or Figure 4 When the voltage of the voltage divider capacitor Cd connected to the midpoint of the A-phase bridge arm reaches the target voltage, the power conversion device 2 can enter normal operation, and the switching transistors in the power conversion device 2 can be controlled to operate so that the power conversion device 2 outputs power.

[0040] In some feasible implementations, the voltage divider capacitor Cd can be a film capacitor. A film capacitor is a capacitor with a polymer film as the dielectric. Compared with other types of capacitors, film capacitors have low parasitic effects, low loss, long life and bidirectional withstand voltage characteristics, and strong ability to withstand high ripple current. They are more suitable for power storage and low-frequency filtering scenarios, and can improve the reliability and safety of power conversion devices to a certain extent.

[0041] The power conversion device should understand that when the power conversion device 2 starts up, the controller 22 can pre-charge the voltage divider capacitor Cd by controlling the switching transistor in the upper or lower bridge arm of any phase bridge arm, so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. In this way, when the power conversion device 2 starts up, the change in the capacitor voltage of the filter capacitors connected to each phase bridge arm can be reduced to a certain extent, thereby reducing the rate of change of the filter capacitor voltage and preventing a sudden surge in the capacitor voltage of the filter capacitor that would cause a large instantaneous current to flow through the filter capacitor. Therefore, by pre-charging the voltage divider capacitor Cd, the power conversion device 2 can achieve a soft start-up, avoiding large current surges in the power devices of the power conversion device 2, improving the safety of the power conversion device 2, and also improving the power generation stability of the power conversion device 2.

[0042] In some feasible implementations, the power conversion device 2 further includes a varistor connected in parallel across the voltage divider capacitor Cd. In the event of a voltage surge on the voltage divider capacitor Cd, the varistor can be used to provide current-limiting protection for Cd. See also Figure 5 and Figure 6 , Figure 5 and Figure 6 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application. For example... Figure 5 and Figure 6As shown, a varistor R11 can be connected in parallel across the voltage divider capacitor Cd. It should be understood that photovoltaic power generation systems are widely distributed outdoors and are easily threatened by lightning strikes. In particular, the equipment on the DC side and the power conversion device side is very sensitive to surges, and any damage will lead to system shutdown or a significant reduction in power generation efficiency. When a lightning surge occurs on the DC side of the power conversion device 2, the surge current can flow through the positive DC bus BUS+ or the negative DC bus BUS- through the voltage divider capacitor Cd and the filter capacitor Ca, and then into the power grid. Among them, when the surge current flows through the voltage divider capacitor Cd, according to formula (1), the voltage divider capacitor Cd may bear a large capacitor voltage, which may cause the voltage divider capacitor Cd to be damaged due to overvoltage. The varistor R11 is a resistor device with nonlinear volt-ampere characteristics, which can perform voltage clamping when the circuit is subjected to overvoltage, and absorb excess current to protect sensitive devices. In summary, by connecting a varistor R11 in parallel across the voltage divider capacitor Cd, the varistor R11 can absorb excess current when there is a surge current, thus protecting the voltage divider capacitor Cd from damage.

[0043] Understandably, the above Figures 3 to 6 The power conversion device 2 shown is a single-arm device. The power conversion device 2 provided in this application can also be a multi-arm device. A detailed description will follow using a three-phase arm device as an example. The structure of the power conversion device 2 can be found in [reference needed]. Figure 7 and Figure 8 , Figure 7 and Figure 8 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application. For example... Figure 7 and Figure 8 As shown, the power conversion device 2 may include a controller 22, a positive DC bus BUS+, a negative DC bus BUS-, and an inverter circuit 21. The positive DC bus BUS+ and negative DC bus BUS- are used to connect to a DC power supply. The inverter circuit 21 may include an A-phase bridge arm, a B-phase bridge arm, and a C-phase bridge arm connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-, and these A-phase, B-phase, and C-phase bridge arms correspond to the A-phase, B-phase, and C-phase outputs of the power conversion device 2, respectively. Multiple thin-film capacitors are connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. For example, as shown... Figure 7 and Figure 8As shown, when the inverter circuit 21 includes phase A, phase B, and phase C bridge arms, the power conversion device may also include film capacitors Cf1, Cf2, and Cf3. These film capacitors are connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. It should be understood that this application does not limit the number of film capacitors connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. The phase A bridge arm may include a first switch Q11 and a second switch Q12; the phase B bridge arm may include a first switch Q21 and a second switch Q22; and the phase C bridge arm may include a first switch Q31 and a second switch Q32. It is understood that... Figure 7 In the power conversion device 2 shown, the circuit structures of phase A bridge arm, phase B bridge arm, and phase C bridge arm are the same as those described above. Figure 3 The bridge arms in the power conversion device 2 shown are the same, and will not be described again here. The midpoint N1 of the A-phase bridge arm, the midpoint N2 of the B-phase bridge arm, and the midpoint N3 of the C-phase bridge arm can all be connected to the negative DC bus BUS- through a series of filter inductors and filter capacitors. When the inverter circuit includes multi-phase bridge arms, the common point after the connection of each filter capacitor connected to each phase bridge arm can be recorded as the capacitor midpoint O'. Specifically, the midpoint N1 of the A-phase bridge arm is connected to the first end of the filter inductor La, and the second end of the filter inductor La is connected to the first end of the filter capacitor Ca. The midpoint N2 of the B-phase bridge arm is connected to the first end of the filter inductor Lb, and the second end of the filter inductor Lb is connected to the first end of the filter capacitor Cb. The midpoint N3 of the C-phase bridge arm, where the second end of the filter capacitor Ca is located, is connected to the first end of the filter inductor Lc. The second end of the filter inductor Lc is connected to the first end of the filter capacitor Cc. The connection of the second end of the filter capacitor Cc and the second end of the filter capacitor Cb can be recorded as the capacitor midpoint O'. The power conversion device 2 may further include a voltage-dividing capacitor Cd and a varistor R11. The voltage-dividing capacitor Cd is connected between the midpoint O' of the filter capacitor and the negative DC bus BUS-. The series connection points of the filter inductor La and the filter capacitor Ca, the filter inductor Lb and the filter capacitor Cb, and the filter inductor Lc and the filter capacitor Cc can be used to connect to the power grid. Similarly, Figure 8 In the power conversion device 2 shown, the circuit structures of phase A bridge arm, phase B bridge arm, and phase C bridge arm are the same as those described above. Figure 4 The bridge arms in the power conversion device 2 shown are the same, and the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the positive DC bus BUS+.

[0044] In specific implementation, when the power conversion device 2 has multiple bridge arms, when the power conversion device 2 is started, the controller 22 can control the target switch in any phase bridge arm to conduct in order to charge the voltage divider capacitor Cd. For example... Figure 7As shown, the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the negative DC bus BUS-. The target switching transistors are the switching transistors in the lower arm of each phase bridge arm, such as the first switching transistor Q11 in phase A, the first switching transistor Q21 in phase B, and the first switching transistor Q31 in phase C. When the power conversion device 2 starts up, the controller 22 can control the first switching transistor Q11 in phase A to turn on and the second switching transistor Q12 to turn off. At this time, the first switching transistor Q11, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the positive DC bus BUS+ through the first switching transistor Q11, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. Alternatively, the controller 22 can control the first switching transistor Q21 in phase B to turn on and the second switching transistor Q22 to turn off. At this time, the first switch Q21, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the positive DC bus BUS+ through the first switch Q21, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd so that its capacitor voltage reaches the target voltage. Alternatively, the controller 22 can control the first switch Q31 in the C-phase bridge arm to turn on and the second switch Q32 to turn off. At this time, the first switch Q31, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the positive DC bus BUS+ through the first switch Q31, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd so that its capacitor voltage reaches the target voltage.

[0045] Similarly, as Figure 8 As shown, the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the positive DC bus BUS+. The target switching transistors are the switching transistors in the lower arm of each phase bridge arm, such as the second switching transistor Q12 in phase A, the second switching transistor Q22 in phase B, and the second switching transistor Q32 in phase C. When the power conversion device 2 starts up, the controller 22 can control the target switching transistor in any phase bridge arm to conduct, so as to charge the voltage divider capacitor Cd. Figure 8As shown, when the power conversion device 2 is started, the controller 22 can control the first switch Q11 in the A-phase bridge arm to turn off and the second switch Q12 to turn on. At this time, the first switch Q11, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the negative DC bus BUS- through the first switch Q11, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the positive DC bus BUS+ to pre-charge the voltage divider capacitor Cd so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. Alternatively, the controller 22 can control the first switch Q21 in the B-phase bridge arm to turn off and the second switch Q22 to turn on. At this time, the first switch Q21, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd can form a BUCK circuit. Current flows from the negative DC bus BUS- through the first switch Q21, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the positive DC bus BUS+ to pre-charge the voltage divider capacitor Cd so that its capacitor voltage reaches the target voltage. Alternatively, the controller 22 can control the first switch Q31 in the C-phase bridge arm to turn off and the second switch Q32 to turn on. At this time, the first switch Q31, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd can form a BUCK circuit. Current flows from the negative DC bus BUS- through the first switch Q31, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the positive DC bus BUS+ to pre-charge the voltage divider capacitor Cd so that its capacitor voltage reaches the target voltage.

[0046] Furthermore, when the voltage of the voltage divider capacitor Cd reaches the target voltage, the controller 22 can control the switching transistors in the upper and lower arms of each phase bridge arm to alternately conduct, so that the inverter circuit 21 converts the DC power from the DC power supply into AC power and outputs the AC power to the power grid, thereby realizing the slow start of the power conversion device 2.

[0047] It should be understood that when the power conversion device 2 has multiple bridge arms, the filter capacitors connected to the midpoints of each phase bridge arm can be connected to the positive DC bus BUS+ or the negative DC bus BUS- through voltage divider capacitors Cd. When the filter capacitors connected to the midpoints of each phase bridge arm are connected to the positive DC bus BUS+ through voltage divider capacitors Cd, the controller 22 can pre-charge the voltage divider capacitor Cd by controlling the switching transistor in the lower bridge arm of any phase bridge arm when the power conversion device 2 starts up, so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. When the filter capacitors connected to the midpoints of each phase bridge arm are connected to the negative DC bus BUS- through voltage divider capacitors Cd, the controller 22 can pre-charge the voltage divider capacitor Cd by controlling the switching transistor in the upper bridge arm of any phase bridge arm when the power conversion device 2 starts up, so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. In this way, when the power conversion device 2 starts up, the change in the capacitor voltage of the filter capacitors connected to each phase bridge arm can be reduced to a certain extent, thereby reducing the rate of change of the capacitor voltage and preventing a sudden surge in the capacitor voltage that would cause a large instantaneous current to flow through the filter capacitor. It is evident that by pre-charging the voltage divider capacitor Cd, the power conversion device 2 can achieve a soft start-up, avoiding a large current impact on the power devices in the power conversion device 2, thus improving the safety and power generation stability of the power conversion device 2.

[0048] Please see Figure 9 , Figure 9 This is a flowchart illustrating the soft-start control method for the power conversion device provided in this application. The soft-start control method for the power conversion device provided in this application is applicable to… Figures 3 to 8 The controller in any of the specific embodiments shown in the figure. The power conversion device 2 includes a controller 22, a positive DC bus BUS+, a negative DC bus BUS-, and an inverter circuit 21. The positive DC bus BUS+ and the negative DC bus BUS- are used to connect to a DC power source. The inverter circuit 21 may include at least one phase bridge arm connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. The bridge arm may include an upper bridge arm and a lower bridge arm connected in series. The midpoint of each phase bridge arm is connected to the positive DC bus BUS+ or the negative DC bus BUS- through a series-connected filter inductor and filter capacitor. The series connection point of the filter capacitor and the filter inductor is used to connect to the power grid. The inverter circuit 21 may also include a voltage-dividing capacitor connected between the filter capacitor and the positive DC bus BUS+ or the negative DC bus BUS-. When the inverter circuit includes one phase bridge arm, and the midpoint of the bridge arm is connected to the negative DC bus BUS- through a series filter inductor and filter capacitor, the structure of the power conversion device can be as follows: Figure 3As shown, taking phase A as an example, the upper arm of phase A may include a first switch Q11, and the lower arm may include a second switch Q12. The connection point of the first switch Q11 and the second switch Q12 can be the midpoint N1 of phase A. The midpoint N1 of phase A can be connected to the negative DC bus BUS- through a series-connected filter inductor La and filter capacitor Ca. The series connection point of the filter inductor La and filter capacitor Ca is used to connect to the power grid. The voltage divider capacitor Cd can be connected between the filter capacitor Ca and the negative DC bus BUS-. When the inverter circuit includes one phase arm, the number of film capacitors connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS- can be 1. When the inverter circuit includes one phase arm, and the midpoint of the arm is connected to the positive DC bus BUS+ through a series-connected filter inductor and filter capacitor, the structure of the power conversion device can be as follows: Figure 4 As shown, the structure of phase A bridge arm can be found in [reference needed]. Figure 3 The structure of phase A bridge arm shown will not be described in detail here. Figure 4 and Figure 3 The difference lies in the fact that the voltage divider capacitor Cd can be connected between the filter capacitor Ca and the positive DC bus BUS+. When the inverter circuit includes multi-phase bridge arms, and the midpoint of each bridge arm is connected to the negative DC bus BUS- through a series filter inductor and filter capacitor, the structure of the power conversion device can be as follows: Figure 7 As shown, multiple thin-film capacitors are connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. For example, as shown... Figure 7 and Figure 8 As shown, when the inverter circuit 21 includes phase A, phase B, and phase C bridge arms, the power conversion device may also include film capacitors Cf1, Cf2, and Cf3. These film capacitors are connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. The circuit structures of phase A, phase B, and phase C bridge arms are the same as described above. Figure 3 The bridge arms in the power conversion device 2 shown are the same, and will not be described again here. It is worth mentioning that the filter capacitor Cc connected to phase A, the filter capacitor Cb connected to phase B, and the filter capacitor Cc connected to phase C form the midpoint O' of the filter capacitor when connected together. The voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the negative DC bus BUS-. When the inverter circuit includes multi-phase bridge arms, and the midpoint of each bridge arm is connected to the positive DC bus BUS+ through a series filter inductor and filter capacitor, the structure of the power conversion device can be as follows: Figure 8 As shown, the circuit structures of phase A, phase B, and phase C bridge arms are the same as those described above. Figure 4The bridge arms in the power conversion device 2 shown are the same, and will not be described again here. It is worth mentioning that the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the positive DC bus BUS+. Regarding... Figures 3 to 8 In any power conversion device corresponding to the illustration, the soft-start control method for the power conversion device provided in this application embodiment may include the following steps:

[0049] S101. When the power conversion device is started, the target switch in any phase bridge arm is controlled to be turned on to charge the voltage divider capacitor. Wherein, when the midpoint of each phase bridge arm is connected to the positive DC bus through a series filter inductor and filter capacitor, the target switch is the switch in the lower bridge arm of each phase bridge arm; when the midpoint of each phase bridge arm is connected to the negative DC bus through a series filter inductor and filter capacitor, the target switch is the switch in the upper bridge arm of each phase bridge arm.

[0050] In specific implementation, Figure 3 Taking phase A bridge arm as an example, the midpoint N1 of phase A bridge arm is connected to the negative DC bus BUS- through a series-connected filter inductor La and filter capacitor Ca. The target switch can be the first switch Q11. The controller 22 adjusts the duty cycle of the first switch Q11 and the second switch Q12 to turn on the first switch Q11 and turn off the second switch Q12. At this time, the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd form a BUCK circuit. Current flows from the positive DC bus BUS+ through the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd. In this way, the voltage divider capacitor Cd and the filter capacitor Ca can divide the bus voltage between the positive DC bus BUS+ and the negative DC bus BUS-, avoiding a large DC bias voltage on the filter capacitor Ca. This also prevents the filter capacitor Ca from being damaged by a large current when the power conversion device is started.

[0051] Similarly, with Figure 4Taking the example where the midpoint of the bridge arm is connected to the positive DC bus BUS+ via a series-connected filter inductor La and filter capacitor Ca, the target switch is the second switch Q12. The controller 22 adjusts the duty cycle of the first switch Q11 and the second switch Q12 to turn on the second switch Q12 and turn off the first switch Q11. At this time, the second switch Q12, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd form a BUCK circuit. Current flows from the negative DC bus BUS- through the first switch Q11, filter inductor La, filter capacitor Ca, and voltage divider capacitor Cd into the positive DC bus BUS+, thus pre-charging the voltage divider capacitor Cd. In this way, the voltage divider capacitor Cd and the filter capacitor Ca can divide the bus voltage between the positive DC bus BUS+ and the negative DC bus BUS-, avoiding a large DC bias voltage on the filter capacitor Ca. This also prevents the filter capacitor Ca from being damaged by a large current when the power conversion device is started.

[0052] In some feasible implementations, the power conversion device 2 provided in this application can also be a multi-arm bridge. Taking a three-phase bridge as an example, the structure of the power conversion device 2 can be found in [reference needed]. Figure 7 and Figure 8 .like Figure 7 and Figure 8 As shown, when the power conversion device 2 is started, the controller 22 can control the target switch in any phase bridge arm to conduct in order to charge the voltage divider capacitor Cd. Figure 7 As shown, the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the negative DC bus BUS-. The target switching transistors are the switching transistors in the lower arm of each phase bridge arm, such as the first switching transistor Q11 in phase A, the first switching transistor Q21 in phase B, and the first switching transistor Q31 in phase C. When the power conversion device 2 starts up, the controller 22 can control any one of the first switching transistors Q11 in phase A, Q21 in phase B, and Q31 in phase C to turn on. At this time, the target switching transistor, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the positive DC bus BUS+ through the target switching transistor, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the negative DC bus BUS- to pre-charge the voltage divider capacitor Cd so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage.

[0053] Similarly, as Figure 8As shown, the voltage divider capacitor Cd is connected between the midpoint O' of the filter capacitor and the positive DC bus BUS+. The target switching transistors are the switching transistors in the lower arm of each phase bridge arm, such as the second switching transistor Q12 in phase A, the second switching transistor Q22 in phase B, and the second switching transistor Q32 in phase C. When the power conversion device 2 starts up, the controller 22 can control the target switching transistor in any phase bridge arm to conduct, so as to charge the voltage divider capacitor Cd. Figure 8 As shown, when the power conversion device 2 is started, the controller 22 can control any one of the following switches to turn on: the second switch in phase A, the second switch Q22 in phase B, and the second switch Q32 in phase C. At this time, the target switch, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd can form a BUCK circuit. Current flows from the negative DC bus BUS- through the target switch, the filter inductor La, the filter capacitor Ca, and the voltage divider capacitor Cd into the positive DC bus BUS+ to pre-charge the voltage divider capacitor Cd so that its voltage reaches the target voltage.

[0054] In some feasible implementations, the voltage divider capacitor Cd can be a film capacitor. A film capacitor is a capacitor with a polymer film as the dielectric. Compared with other types of capacitors, film capacitors have low parasitic effects, low loss, long life and bidirectional withstand voltage characteristics, and strong ability to withstand high ripple current. They are more suitable for power storage and low-frequency filtering scenarios, and can improve the reliability and safety of power conversion devices to a certain extent.

[0055] S102. When the voltage of the voltage divider capacitor reaches the target voltage, the switching transistors in the upper and lower arms of each phase bridge arm are controlled to conduct alternately, so that the inverter circuit converts the DC power from the DC power supply into AC power and outputs the AC power to the power grid, thereby realizing the slow start of the power conversion device.

[0056] It should be understood that, compared to the method where the midpoint of each phase bridge arm is connected to the series connection point of two film capacitors connected in series, the DC side of the power conversion device provided in this application only requires multiple film capacitors connected in parallel between the positive DC bus BUS+ and the negative DC bus BUS-. The number of film capacitors is reduced by at least half, thus reducing the size of the power conversion device to a certain extent. When the filter capacitor connected to the midpoint of each phase bridge arm is connected to the positive DC bus BUS+ through a voltage divider capacitor Cd, when the power conversion device 2 is started, the switch in the lower bridge arm of any phase bridge arm can be turned on to pre-charge the voltage divider capacitor Cd, so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. When the filter capacitor connected to the midpoint of each phase bridge arm is connected to the negative DC bus BUS- through a voltage divider capacitor Cd, when the power conversion device 2 is started, the switch in the upper bridge arm of any phase bridge arm can be turned on to pre-charge the voltage divider capacitor Cd, so that the capacitor voltage of the voltage divider capacitor Cd reaches the target voltage. In this way, when the power converter 2 starts up, the change in the capacitor voltage of the filter capacitors connected to each phase bridge arm can be reduced to a certain extent, thereby reducing the rate of change of the filter capacitor voltage and preventing a sudden surge in the filter capacitor voltage that would cause a large instantaneous current to flow through the filter capacitor. It is evident that by pre-charging the voltage divider capacitors, the power converter 2 can achieve a slow start-up, avoiding large current surges in the power devices within the power converter 2, thus improving the safety and power generation stability of the power converter 2.

[0057] In some feasible implementations, the power conversion device 2 further includes a varistor connected in parallel across the voltage divider capacitor Cd. In the event of a voltage surge on the voltage divider capacitor Cd, the varistor can be used to provide current-limiting protection for Cd. See also Figure 5 and Figure 6 , Figure 5 and Figure 6 This is another structural schematic diagram of the power conversion device provided in the embodiments of this application. For example... Figure 5 and Figure 6As shown, a varistor R11 can be connected in parallel across the voltage divider capacitor Cd. It should be understood that photovoltaic power generation systems are widely distributed outdoors and are easily threatened by lightning strikes. In particular, the equipment on the DC side and the power conversion device side is very sensitive to surges, and any damage will lead to system shutdown or a significant reduction in power generation efficiency. When a lightning surge occurs on the DC side of the power conversion device 2, the surge current can flow through the positive DC bus BUS+ or the negative DC bus BUS- through the voltage divider capacitor Cd and the filter capacitor Ca, and then into the power grid. Among them, when the surge current flows through the voltage divider capacitor Cd, according to formula (1), the voltage divider capacitor Cd may bear a large capacitor voltage, which may cause the voltage divider capacitor Cd to be damaged due to overvoltage. The varistor R11 is a resistor device with nonlinear volt-ampere characteristics, which can perform voltage clamping when the circuit is subjected to overvoltage, and absorb excess current to protect sensitive devices. In summary, by connecting a varistor R11 in parallel across the voltage divider capacitor Cd, the varistor R11 can absorb excess current when there is a surge current, thus protecting the voltage divider capacitor Cd from damage.

[0058] In summary, compared to the previous method where the midpoint of each phase arm is connected to the series connection point of two film capacitors, the power conversion device provided in this application only requires multiple film capacitors connected in parallel between the positive and negative DC buses on the DC side. This reduces the number of film capacitors by at least half, thus reducing the size of the power conversion device to some extent. Furthermore, by setting a voltage divider capacitor between the filter capacitor and either the positive or negative DC bus, the target switch in any phase arm can be turned on during power conversion to charge the voltage divider capacitor, thereby raising the voltage of the filter capacitor to the target voltage. Specifically, when the midpoint of each phase arm is connected to the positive DC bus via a series filter inductor and filter capacitor, the target switch is the switch in the lower phase arm of each phase arm; when the midpoint of each phase arm is connected to the negative DC bus via a series filter inductor and filter capacitor, the target switch is the switch in the upper phase arm of each phase arm. In this way, when the power converter starts up, the change in the capacitor voltage of the filter capacitors connected to each phase bridge arm can be reduced to a certain extent, thereby reducing the rate of change of the filter capacitor voltage and preventing a sudden surge in the filter capacitor voltage that would cause a large instantaneous current to flow through the filter capacitor. It is evident that pre-charging the voltage divider capacitors can achieve a slow start-up of the power converter, avoiding large current surges to the power devices within the power converter, thus improving the safety and power generation stability of the power converter.

Claims

1. A power conversion device, the power conversion device comprising a plurality of parallel bus capacitors, a positive DC bus, a negative DC bus and an inverter circuit, wherein the plurality of parallel thin film capacitors are connected between the positive DC bus and the negative DC bus, and the positive DC bus and the negative DC bus are used to connect to a DC power supply; in, The inverter circuit includes at least one phase bridge arm connected in parallel between the positive DC bus and the negative DC bus. The bridge arm includes an upper bridge arm connected to the positive DC bus and a lower bridge arm connected to the negative DC bus. The midpoint of the bridge arm is connected to the positive DC bus or the negative DC bus through a series-connected filter inductor and filter capacitor. The series connection point of the filter capacitor and the filter inductor is used to connect to the power grid. The power conversion device further includes a voltage divider capacitor, which is connected between the filter capacitor and the positive DC bus or the negative DC bus.

2. The power conversion device according to claim 1, characterized in that, The power conversion device also includes a controller; When the power conversion device is started, the controller is used to control the target switch in any phase arm of the bridge to conduct in order to charge the voltage divider capacitor; when the midpoint of each phase arm is connected to the positive DC bus through the series-connected filter inductor and filter capacitor, the target switch is the switch in the lower arm of each phase arm; when the midpoint of each phase arm is connected to the negative DC bus through the series-connected filter inductor and filter capacitor, the target switch is the switch in the upper arm of each phase arm. The controller is further configured to, when the voltage of the voltage divider capacitor reaches the target voltage, control the switching transistors in the upper and lower bridge arms of each phase of the bridge arm to alternately conduct, so that the inverter circuit converts the DC power from the DC power supply into AC power and outputs the AC power to the power grid.

3. The power conversion device according to claim 2, characterized in that, The power conversion device also includes a varistor, which is connected in parallel across the voltage divider capacitor; In the event of a voltage surge in the voltage divider capacitor, the varistor is used to provide current-limiting protection for the voltage divider capacitor.

4. The power conversion device according to claim 2, characterized in that, The voltage divider capacitor is a thin-film capacitor.

5. A soft-start control method for a power conversion device, characterized in that, The power conversion device includes a controller, multiple parallel-connected thin-film capacitors, a positive DC bus, a negative DC bus, and an inverter circuit. The multiple parallel-connected thin-film capacitors are connected between the positive DC bus and the negative DC bus, which are used to connect to a DC power supply. The inverter circuit includes at least one phase bridge arm connected in parallel between the positive DC bus and the negative DC bus. The bridge arm includes an upper bridge arm connected to the positive DC bus and a lower bridge arm connected to the negative DC bus. The midpoint of the bridge arm is connected to the positive DC bus or the negative DC bus through a series-connected filter inductor and filter capacitor. The power conversion device also includes a voltage-dividing capacitor connected between the filter capacitor and the positive DC bus or the negative DC bus. The series connection point of the filter capacitor and the filter inductor is used to connect to the power grid. The method includes: When the power conversion device is started, the target switch in any phase arm is turned on to charge the voltage divider capacitor. Specifically, when the midpoint of each phase arm is connected to the positive DC bus via a series-connected filter inductor and filter capacitor, the target switch is the switch in the lower arm of each phase arm; when the midpoint of each phase arm is connected to the negative DC bus via a series-connected filter inductor and filter capacitor, the target switch is the switch in the upper arm of each phase arm. When the voltage of the voltage divider capacitor reaches the target voltage, the switching transistors in the upper and lower arms of each phase of the bridge arm are controlled to alternately conduct, so that the inverter circuit converts the DC power from the DC power supply into AC power and outputs the AC power to the power grid, thereby realizing the slow start of the power conversion device.

6. The method according to claim 5, characterized in that, The power conversion device also includes a varistor, which is connected in parallel across the voltage divider capacitor; In the event of a voltage surge in the voltage divider capacitor, the varistor is used to provide current-limiting protection for the voltage divider capacitor.

7. The method according to claim 5, characterized in that, The voltage divider capacitor is a thin-film capacitor.

8. A photovoltaic system, characterized in that, The photovoltaic system includes a DC power supply and a power conversion device as described in any one of claims 1 to 4; The power conversion device is used to convert the DC power from the DC power source into AC power and output the AC power to the power grid or load.

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