Power conversion device and control method thereof

Abstract

The embodiment of the application discloses a kind of power conversion device and its control method, it is related to power electronics field, it is solved how to inhibit the problem of high-frequency common-mode leakage current of Heric inversion circuit.Power conversion device includes first inductance, second inductance, bus capacitor connected between positive DC bus and negative DC bus, first bridge arm, second bridge arm and capacitor branch, and third bridge arm connected between the midpoint of first bridge arm and the midpoint of second bridge arm.First bridge arm includes first switch tube and second switch tube connected in series, second bridge arm includes third switch tube and fourth switch tube connected in series, and third bridge arm includes fifth switch tube and sixth switch tube.Capacitor branch includes first capacitor, second capacitor and third capacitor connected in series, first inductance is connected between the first end of third bridge arm and the first end of second capacitor, and second inductance is connected between the second end of third bridge arm and the second end of second capacitor.

Description

Technical Field

[0001] This application relates to the field of power electronics, and more particularly to a power conversion device and its control method. Background Technology

[0002] Compared to isolated inverter circuits with power frequency transformers, non-isolated inverter circuits offer higher conversion efficiency and lower cost. Therefore, traditional residential low-power single-phase inverters in photovoltaic power generation systems typically employ non-isolated inverter circuits. Among non-isolated inverter circuits, the highly efficient and reliable inverter concept (Heric) inverter circuit boasts lower component costs, higher conversion efficiency, and superior performance in terms of system leakage current. Consequently, traditional residential low-power single-phase inverters often utilize this Heric inverter circuit.

[0003] However, during the freewheeling phase, the Heric inverter circuit forms an LC common-mode resonant circuit, generating high-frequency common-mode leakage current. This high-frequency common-mode leakage current increases the total harmonic current distortion (THDi) of the grid current, leading to increased electromagnetic interference (EMI), increased losses in traditional low-power single-phase inverters for residential applications, and reduced inverter reliability. Therefore, suppressing the high-frequency common-mode leakage current in the Heric inverter circuit has become an urgent problem to be solved. Summary of the Invention

[0004] This application provides a power conversion device and its control method, which solves the problem of how to suppress the high-frequency common-mode leakage current of Heric inverter circuit.

[0005] To achieve the above objectives, the embodiments of this application adopt the following technical solutions:

[0006] A first aspect of this application provides a power conversion device for converting direct current (DC) output from a photovoltaic array or energy storage battery into alternating current (AC). The power conversion device includes a first inductor, a second inductor, a positive DC bus, a negative DC bus, a bus capacitor connected between the positive and negative DC buses, a first bridge arm, a second bridge arm, and a capacitor branch, as well as a third bridge arm connected between the midpoints of the first and second bridge arms. The first bridge arm includes a first and a second switch connected in series. The second bridge arm includes a third and a fourth switch connected in series. The third bridge arm includes a fifth and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first capacitor, a second capacitor, and a third capacitor connected in series. The first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. The second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used to connect to the power grid or load.

[0007] Based on this scheme, the resonant frequency of the LC common-mode resonant circuit formed in the freewheeling stage is related to the sum of the equivalent capacitance values ​​of the first to fourth switching transistors, as well as the sum of the equivalent capacitance values ​​of the first and third capacitors. By selecting the first or third capacitor with different capacitance values, the sum of the equivalent capacitance values ​​of the first and third capacitors can be increased, which can reduce the resonant frequency of the LC common-mode resonant circuit and increase the impedance of the LC common-mode resonant circuit. This can reduce the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit, reduce the losses of THDi, EMI and power conversion devices, and improve the reliability of power conversion devices.

[0008] In conjunction with the first aspect, in one embodiment, the power conversion device further includes a first resistor and a second resistor, one end of the first resistor being connected to a first end of a second capacitor, one end of the second resistor being connected to a second end of the second capacitor, and the other ends of both the first resistor and the second resistor being used to connect to a negative DC bus and a reference ground.

[0009] Based on this scheme, by setting a first resistor and a second resistor, the reference ground and the negative DC bus can be made to be at the same potential, which can simplify the hardware design of the power conversion device and reduce the cost.

[0010] In conjunction with the first aspect, in one embodiment, the power conversion device further includes a controller for controlling the synchronously conducting switching transistors and the third bridge arm to alternately conduct. After controlling the synchronously conducting switching transistors and the third bridge arm to alternately conduct, when the voltage across the second capacitor is equal, the power conversion device is controlled to start normal operation. The conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold. The first and fourth switching transistors are synchronously conducting switching transistors, or the second and third switching transistors are synchronously conducting switching transistors.

[0011] Based on this scheme, at the instant the power conversion device starts working, the voltage at the midpoint of the first bridge arm and the midpoint of the second bridge arm may differ significantly, and the stress on the switching transistors may be uneven. First, the controller controls the synchronously conducting switching transistors and the third bridge arm to conduct alternately, and the conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold, thereby avoiding damage to the switching transistors due to excessive stress. Second, when the voltage across the second capacitor is equal, the controller controls the power conversion device to start normal operation. At the instant the power conversion device starts working, the stress and losses of the switching transistors are equal, thereby improving the reliability and efficiency of the power conversion device.

[0012] In conjunction with the first aspect, in one embodiment, the sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values, the sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor, the sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch and the fourth switch, and N is greater than 1.

[0013] Based on this scheme, the larger the sum of the first capacitance values, the lower the resonant frequency of the LC common-mode resonant circuit, the larger the impedance of the LC common-mode resonant circuit, and the smaller the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit. This can reduce the losses of THDi, EMI and power conversion devices, and improve the reliability of power conversion devices.

[0014] In conjunction with the first aspect, in one implementation, N equals 10 or 20.

[0015] Based on this scheme, when the sum of the first capacitance values ​​is much greater than the sum of the second capacitance values, the resonant frequency of the LC common-mode resonant circuit is lower, the impedance of the LC common-mode resonant circuit is greater, and the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit is lower. This can significantly reduce the losses of THDi, EMI and power conversion devices, and improve the reliability of power conversion devices.

[0016] In conjunction with the first aspect, in one embodiment, the power conversion device further includes a magnetic ring through which a cable between the first inductor and a first terminal of the second capacitor passes, and a cable between the second inductor and a second terminal of the second capacitor also passes through the magnetic ring.

[0017] In one embodiment, the cable between the first inductor and the first end of the second capacitor can be wound around a magnetic ring, or the cable between the second inductor and the second end of the second capacitor can be wound around a magnetic ring. The specific number of turns of the cable wound around the magnetic ring is not limited in this application embodiment.

[0018] Based on this scheme, by setting a magnetic ring, the oscillation amplitude of the voltage at the midpoint of the first bridge arm and the voltage at the midpoint of the second bridge arm can be further reduced, and the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit can be further reduced, thereby further reducing the losses of THDi, EMI and power conversion device, and improving the reliability of power conversion device.

[0019] In conjunction with the first aspect, in one embodiment, the magnetic ring is a magnetic ring made of an amorphous alloy material.

[0020] Based on this scheme, since amorphous alloy materials have the advantages of high resistivity, low hysteresis loss and good high-frequency characteristics, the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit can be further reduced, and the losses of THDi, EMI and power conversion devices can be further reduced, thereby improving the reliability of the power conversion devices.

[0021] A second aspect of this application provides a control method for a power conversion device, applied to the power conversion device. The method includes: controlling a synchronously conducting switch and a third bridge arm to alternately conduct; after the synchronously conducting switch and the third bridge arm alternately conduct, when the voltage across a second capacitor is equal, controlling the power conversion device to start normal operation; the conduction time of the synchronously conducting switch is less than or equal to a preset time threshold; a first switch and a fourth switch are synchronously conducting switches, or a second switch and a third switch are synchronously conducting switches. The power conversion device is used to convert direct current (DC) output from a photovoltaic array or energy storage battery into alternating current (AC). The power conversion device includes a first inductor, a second inductor, a positive DC bus, a negative DC bus, a bus capacitor connected between the positive and negative DC buses, a first bridge arm, a second bridge arm, and a capacitor branch, and a third bridge arm connected between the midpoint of the first bridge arm and the midpoint of the second bridge arm. The first bridge arm includes a first and a second switch connected in series. The second bridge arm includes a third and a fourth switch connected in series. The third bridge arm includes a fifth and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first, a second, and a third capacitor connected in series. A first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. A second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used for connection to the power grid or load.

[0022] In conjunction with the second aspect, in one embodiment, the power conversion device further includes a first resistor and a second resistor, one end of the first resistor being connected to a first end of a second capacitor, one end of the second resistor being connected to a second end of the second capacitor, and the other ends of both the first resistor and the second resistor being used to connect to the negative DC bus and the reference ground.

[0023] In conjunction with the second aspect, in one embodiment, the sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values, the sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor, the sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch and the fourth switch, and N is greater than 1.

[0024] In conjunction with the second aspect, in one implementation, N equals 10 or 20.

[0025] A third aspect of this application provides a photovoltaic energy storage system, which includes a power conversion device and an energy storage battery. The positive terminal of the energy storage battery is connected to a positive DC bus, and the negative terminal of the energy storage battery is connected to a negative DC bus. The power conversion device is used to convert the DC power output from the photovoltaic array or the energy storage battery into AC power. The power conversion device includes a first inductor, a second inductor, a positive DC bus, a negative DC bus, a bus capacitor connected between the positive DC bus and the negative DC bus, a first bridge arm, a second bridge arm, and a capacitor branch, and a third bridge arm connected between the midpoint of the first bridge arm and the midpoint of the second bridge arm. The first bridge arm includes a first switch and a second switch connected in series. The second bridge arm includes a third switch and a fourth switch connected in series. The third bridge arm includes a fifth switch and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first capacitor, a second capacitor, and a third capacitor connected in series. The first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. The second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used to connect to the power grid or load.

[0026] In conjunction with the third aspect, in one embodiment, the power conversion device further includes a first resistor and a second resistor, one end of the first resistor being connected to a first end of a second capacitor, one end of the second resistor being connected to a second end of the second capacitor, and the other ends of both the first resistor and the second resistor being used to connect to a negative DC bus and a reference ground.

[0027] In conjunction with the third aspect, in one embodiment, the power conversion device further includes a controller for controlling the synchronously conducting switching transistors and the third bridge arm to alternately conduct. After controlling the synchronously conducting switching transistors and the third bridge arm to alternately conduct, when the voltage across the second capacitor is equal, the power conversion device is controlled to start normal operation. The conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold. The first and fourth switching transistors are synchronously conducting switching transistors, or the second and third switching transistors are synchronously conducting switching transistors.

[0028] In conjunction with the third aspect, in one implementation, the sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values, the sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor, the sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch and the fourth switch, and N is greater than 1.

[0029] In conjunction with the third aspect, in one implementation, N equals 10 or 20.

[0030] The descriptions of the second and third aspects in this application can be referenced to the detailed description of the first aspect; and the beneficial effects of the second and third aspects can be referenced to the analysis of the beneficial effects of the first aspect, which will not be repeated here. Attached Figure Description

[0031] Figure 1 A circuit topology diagram of a photovoltaic energy storage system provided in this application embodiment;

[0032] Figure 2 This is a schematic diagram of the circuit topology of a power conversion device;

[0033] Figure 3 This is a waveform diagram of a voltage.

[0034] Figure 4 This is a schematic diagram of the circuit topology of an LC common-mode resonant circuit;

[0035] Figure 5 This is a circuit topology diagram of a power conversion device provided in an embodiment of this application;

[0036] Figure 6 A circuit topology diagram of an LC common-mode resonant circuit provided in an embodiment of this application;

[0037] Figure 7 A waveform diagram of a voltage provided for an embodiment of this application;

[0038] Figure 8 A circuit topology diagram of another power conversion device provided in the embodiments of this application;

[0039] Figure 9 A waveform diagram of another voltage provided in an embodiment of this application;

[0040] Figure 10 A circuit topology diagram of another power conversion device provided in the embodiments of this application;

[0041] Figure 11 This is a flowchart illustrating a control method for a power conversion device provided in an embodiment of this application. Detailed Implementation

[0042] The following sections will discuss the fabrication and use of various embodiments in detail. However, it should be understood that many applicable inventive concepts provided in this application can be implemented in a variety of specific environments. The specific embodiments discussed are merely illustrative of specific ways of implementing and using this description and technology, and do not limit the scope of this application.

[0043] Unless otherwise defined, all technical terms used herein have the same meaning as commonly known to one of ordinary skill in the art.

[0044] Each circuit or other component may be described or referred to as "for" performing one or more tasks. In this context, "for" is used to imply a structure by indicating that the circuit / component includes a structure (e.g., a circuit system) that performs one or more tasks during operation. Therefore, even when the specified circuit / component is currently inoperable (e.g., not turned on), it can still be referred to as "for performing that task." Circuits / components used with the term "for" include hardware, such as circuits that perform operations.

[0045] The technical solutions of the embodiments of this application will now be described with reference to the accompanying drawings. In this application, the terms "first," "second," etc., do not limit the quantity or order of the embodiments.

[0046] In this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or illustration. Any embodiment or design described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0047] Before introducing the embodiments of this application, the background technology involved in this application will be introduced first.

[0048] like Figure 1 The diagram shows a circuit topology of a photovoltaic energy storage system 100 according to an embodiment of this application. The photovoltaic energy storage system 100 includes a photovoltaic inverter 110 and a first energy storage battery 120. A first DC port of the photovoltaic inverter 110 is connected to a photovoltaic array 200, a second DC port of the photovoltaic inverter 110 is connected to the electrodes of the first energy storage battery 120, and an AC port of the photovoltaic inverter 110 is connected to the power grid or a load 300. The photovoltaic array 200 converts solar energy into DC power, and the photovoltaic inverter 110 converts this DC power into AC power and transmits it to the power grid or the load 300. Alternatively, the photovoltaic inverter 110 converts the DC power output from the first energy storage battery 120 into AC power and transmits it to the power grid or the load 300. This photovoltaic energy storage system 100 can also be called a photovoltaic power generation system, and the photovoltaic inverter 110 can also be called a photovoltaic energy storage inverter.

[0049] In one implementation, reference Figure 1The photovoltaic energy storage system 100 may further include a second energy storage battery 130 and a power conversion system (PCS) 140. The electrodes of the second energy storage battery 130 are connected to the DC port of the power conversion system 140, and the AC port of the power conversion system 140 is connected to the AC port of the photovoltaic inverter 110. The power conversion system 140 is used to convert the AC power output from the photovoltaic inverter 110 into DC power, or to convert the AC power supplied by the grid or load 300 into DC power to charge the second energy storage battery 130. The power conversion system 140 is also used to convert the DC power output from the second energy storage battery 130 into AC power to supply power to the grid or load 300.

[0050] Both the photovoltaic inverter 110 and the energy storage converter 140 mentioned above can be referred to as power conversion devices. Since the Heric inverter circuit has advantages such as low device cost, high conversion efficiency, and superior performance in terms of system leakage current, this power conversion device can include the Heric inverter circuit, thus possessing the advantages of the Heric inverter circuit. For example... Figure 2 The diagram shows a circuit topology of a power conversion device 400. The power conversion device 400 includes a first inductor L1, a second inductor L2, a capacitor C, a positive DC bus BUS+, a negative DC bus BUS-, a bus capacitor Cbus connected between the positive DC bus BUS+ and the negative DC bus BUS-, a first bridge arm, a second bridge arm, and a third bridge arm connected between the midpoint A of the first bridge arm and the midpoint B of the second bridge arm. The first bridge arm includes a first switch Q1 and a second switch Q2 connected in series. The second bridge arm includes a third switch Q3 and a fourth switch Q4 connected in series. The third bridge arm includes a fifth switch Q5 and a sixth switch Q6. The drain or collector of the fifth switch Q5 is connected to the drain or collector of the sixth switch Q6, or the source or emitter of the fifth switch Q5 is connected to the source or emitter of the sixth switch Q6. The first inductor L1 is connected between the first end of the third bridge arm and the first end of the capacitor C, and the second inductor L2 is connected between the second end of the third bridge arm and the second end of the capacitor C. The two ends of the capacitor C are used to connect to the power grid or the load 300.

[0051] The switching transistor in this embodiment can be a metal-oxide-semiconductor field-effect transistor (MOSFET), which can also be simply referred to as a MOS transistor. Each MOS transistor includes a reverse-biased body diode. Alternatively, the switching transistor in this embodiment can be an insulated-gate bipolar transistor (IGBT), for example, referring to... Figure 2 The first switching transistor Q1 to the sixth switching transistor Q6 are IGBTs, and the power conversion device 400 also includes diodes D1 to D6 that are connected in reverse parallel with the first switching transistor Q1 to the sixth switching transistor Q6 respectively.

[0052] However, due to the parasitic capacitance Cpv between the photovoltaic array 200 and the ground (also known as protective earthing, PE) and the parasitic parameters inherent in the Heric inverter circuit itself, when the Heric inverter circuit is operating in the freewheeling phase, the freewheeling loop in the Heric inverter circuit, as shown... Figure 2 In the case shown by the broken line Z, as Figure 3 The diagram shows the voltage waveforms at midpoint A of the first bridge arm and midpoint B of the second bridge arm. Without fixed voltage clamping, these voltages will oscillate significantly, resulting in a large high-frequency common-mode leakage current. Specifically, as shown... Figure 4 As shown in (a), this is a schematic diagram of the circuit topology of the LC common-mode resonant circuit formed by the parasitic parameters of the photovoltaic energy storage system 100, the bridge arm in Heric, the first inductor L1, the second inductor L2, the capacitor C, and the grid impedance Zg. Cpv is the parasitic capacitance between the photovoltaic array 200 and the ground, and Vg is the equivalent AC source of the grid or load 300. Figure 4 Figure (b) shows the equivalent circuit topology of the LC common-mode resonant circuit, where Udcm is the DC bus common-mode voltage, which is the average of the voltages of the positive DC bus BUS+ and the negative DC bus BUS-. (Refer to...) Figures 2 to 4 When the Heric inverter circuit is operating in the freewheeling phase, due to the large voltage oscillations at the midpoint A of the first bridge arm and the midpoint B of the second bridge arm, the aforementioned LC common-mode resonant circuit will generate a large high-frequency common-mode leakage current. This high-frequency common-mode leakage current will cause THDi to increase, leading to increased EMI, increased losses in the power conversion device 400 (also known as a single-phase inverter), and reduced reliability of the power conversion device 400.

[0053] Based on this, this application provides a power conversion device that, by adding a capacitor, changes the equivalent circuit topology of the LC common-mode resonant circuit in the freewheeling stage of the Heric inverter circuit, thereby reducing the resonant frequency of the changed LC common-mode resonant circuit and reducing the high-frequency common-mode leakage current.

[0054] The power conversion device provided in this application embodiment can be a photovoltaic inverter, which can replace the photovoltaic inverter 110 in the aforementioned photovoltaic energy storage system 100. The power conversion device provided in this application embodiment can also be an energy storage converter, which can replace the energy storage converter 140 in the aforementioned photovoltaic energy storage system 100. The detailed descriptions of the photovoltaic inverter 110 and the energy storage converter 140 described above can all be applied to this power conversion device, and will not be repeated here.

[0055] In one implementation, such as Figure 5 The diagram shown is a circuit topology schematic of a power conversion device 500 provided in an embodiment of this application. (The last sentence appears to be incomplete and possibly refers to a different application.) Figure 2 The difference between the power conversion device 400 and the power conversion device 500 shown is that the power conversion device 500 does not include capacitor C. The power conversion device 500 also includes a capacitor branch connected between the positive DC bus BUS+ and the negative DC bus BUS-. The capacitor branch includes a first capacitor C1, a second capacitor C2 and a third capacitor C3 connected in series. A first inductor L1 is connected between the first end of the third bridge arm and the first end of the second capacitor C2. A second inductor L2 is connected between the second end of the third bridge arm and the second end of the second capacitor C2. The first capacitor C1 is connected between the positive DC bus BUS+ and the first end of the second capacitor C2. The third capacitor C3 is connected between the second end of the second capacitor C2 and the negative DC bus BUS-. The two ends of the second capacitor C2 are used to connect to the power grid or the load 300.

[0056] Due to the parasitic capacitance Cpv between the photovoltaic array 200 and the ground, and the parasitic parameters inherent in the power conversion device 500 itself, when the power conversion device 500 operates in the freewheeling phase, the freewheeling loop in the power conversion device 500 is as follows: Figure 5 The broken lines X1, X2, and X3 are shown in the diagram, where the freewheeling loops X1 and X3 freewheel through the parasitic capacitance of the switching transistor. Figure 6 The diagram shows the equivalent circuit topology of the LC common-mode resonant circuit formed by the parasitic parameters of the photovoltaic energy storage system 100, the first inductor L1, the second inductor L2, the first capacitor C1, the second capacitor C2, the third capacitor C3, and the grid impedance Zg. In this circuit, I is the equivalent current source of the photovoltaic array 200, capacitor Ct is the equivalent capacitance of the first switch Q1 to the fourth switch Q4 in the power conversion device 500, capacitor Cw is the equivalent capacitance of the first capacitor C1 and the third capacitor C3, and inductor Ld is the equivalent inductance of the first inductor L1 and the second inductor L2.

[0057] Reference Figure 6 At this time, the resonant frequency of the LC common-mode resonant circuit satisfies the following formula:

[0058]

[0059] Where f is the resonant frequency of the LC common-mode resonant circuit at this time, π is pi, Ld is the sum of the equivalent inductance values ​​of the first inductor L1 and the second inductor L2, Ct is the sum of the equivalent capacitance values ​​of the first switch Q1 to the fourth switch Q4, and Cw is the sum of the equivalent capacitance values ​​of the first capacitor C1 and the third capacitor C3. Cw can be called the first capacitance sum, and Ct can be called the second capacitance sum. It can be understood that the resonant frequency f of the LC common-mode resonant circuit at this time is related to the sum of the equivalent capacitance values ​​Ct of the first switch Q1 to the fourth switch Q4, and also to the sum of the equivalent capacitance values ​​Cw of the first capacitor C1 and the third capacitor C3. By choosing different capacitance values ​​for the first capacitor C1 or the third capacitor C3, the sum of the equivalent capacitance values ​​Cw of the first capacitor C1 and the third capacitor C3 can be increased, thereby reducing the resonant frequency of the LC common-mode resonant circuit. Furthermore, the reactance of the capacitor (Ct or Cw) and inductor (Ld) is related to the resonant frequency. The lower the resonant frequency, the larger the capacitive reactance of the capacitor and the smaller the inductive reactance of the inductor. As the resonant frequency of the LC common-mode resonant circuit decreases, the capacitive reactance of the equivalent capacitors Ct and Cw will increase, and the inductive reactance of the equivalent inductor Ld will decrease. This increases the impedance of the LC common-mode resonant circuit, thereby reducing the high-frequency common-mode leakage current generated by the circuit. For example... Figure 7 The diagram shows the voltage waveforms at the midpoint A of the first bridge arm and the midpoint B of the second bridge arm when the power conversion device 500 is operating in the freewheeling phase. This is consistent with the above... Figure 3 In comparison, the oscillation amplitude of the voltage at the midpoint A of the first bridge arm and the voltage at the midpoint B of the second bridge arm is significantly reduced, resulting in a smaller high-frequency common-mode leakage current.

[0060] In one embodiment, the sum of the first capacitance values ​​(Cw) is greater than or equal to N times the sum of the second capacitance values ​​(Ct), where N is greater than 1. This application does not limit the specific value of N. Referring to the above formula, the larger the sum of the first capacitance values ​​(Cw), the lower the resonant frequency of the LC common-mode resonant circuit, the greater the impedance of the LC common-mode resonant circuit, and the smaller the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit. This reduces THDi, EMI, and the losses of the power conversion device 500, thereby improving the reliability of the power conversion device 500.

[0061] In one implementation, N equals 10 or 20, in which case the resonant frequency of the LC common-mode resonant circuit satisfies the following formula:

[0062]

[0063] Understandably, when the sum of the first capacitance values ​​(Cw) is much greater than the sum of the second capacitance values ​​(Ct), the resonant frequency of the LC common-mode resonant circuit is lower, the impedance of the LC common-mode resonant circuit is greater, and the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit is lower. This can significantly reduce the losses of THDi, EMI and power conversion device 500, and improve the reliability of power conversion device 500.

[0064] The resonant frequency of the LC common-mode resonant circuit formed during the freewheeling stage of the power conversion device 500 provided in this application embodiment is related to the sum of the equivalent capacitance values ​​Ct of the first switch Q1 to the fourth switch Q4, and also to the sum of the equivalent capacitance values ​​Cw of the first capacitor C1 and the third capacitor C3. By selecting different capacitance values ​​for the first capacitor C1 or the third capacitor C3, the sum of the equivalent capacitance values ​​Cw of the first capacitor C1 and the third capacitor C3 can be increased, which can reduce the resonant frequency of the LC common-mode resonant circuit and increase the impedance of the LC common-mode resonant circuit. This can reduce the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit, reduce THDi, EMI and the losses of the power conversion device 500, and improve the reliability of the power conversion device 500.

[0065] In one implementation, reference Figure 5 In the power conversion device 500, due to the influence of circuit parasitic parameters, inconsistent drive signals, etc., at the moment the power conversion device 500 starts working, the voltage at the midpoint A of the first bridge arm and the voltage at the midpoint B of the second bridge arm may differ greatly. The stress on the switching transistor may be uneven, the switching transistor may be damaged, the loss of the switching transistor may be uneven, and the efficiency may be reduced, which will lead to low reliability and low efficiency of the power conversion device 500.

[0066] In one implementation, in the power conversion device 500, to simplify hardware design and reduce costs, the reference ground and the negative DC bus BUS- are designed to be at the same potential. Specifically, such as... Figure 8 The diagram shown is a circuit topology diagram of another power conversion device 500 provided in an embodiment of this application. The power conversion device 500 also includes a first resistor R1 and a second resistor R2. One end of the first resistor R1 is connected to the first end of the second capacitor C2, and one end of the second resistor R2 is connected to the second end of the second capacitor C2. The other ends of the first resistor R1 and the other ends of the second resistor R2 are both used to connect to the negative DC bus BUS- and the reference ground, which can be analog ground (AGND).

[0067] However, by designing the reference ground and the negative DC bus BUS- to be at the same potential, before the power conversion device 500 operates, the voltage at the midpoint A of the first bridge arm and the voltage at the midpoint B of the second bridge arm will both be much less than half of the bus voltage. The specific voltage is related to the resistance values ​​of the first resistor R1 and the second resistor R2. This bus voltage is the voltage between the positive DC bus BUS+ and the negative DC bus BUS-. When the power conversion device 500 starts operating, as... Figure 9 The diagram shows the voltage waveforms at the midpoint A of the first bridge arm and the midpoint B of the second bridge arm. It can be understood that at the instant the power conversion device 500 starts working, the voltage at the midpoint A of the first bridge arm and the midpoint B of the second bridge arm differ significantly. This results in uneven stress on the switching transistors, which may damage them. Uneven losses in the switching transistors will reduce efficiency, leading to lower reliability and efficiency of the power conversion device 500.

[0068] Based on the above, in one implementation, with Figure 8 For example, refer to Figure 8 The power conversion device 500 provided in this application embodiment also includes a controller 510. The controller 510 controls the synchronously conducting switching transistors and the third bridge arm to alternately conduct, thereby discharging the first capacitor C1 and charging the third capacitor C3. After controlling the synchronously conducting switching transistors and the third bridge arm to alternately conduct, when the voltage across the second capacitor C2 is equal (the voltage at the midpoint A of the first bridge arm and the voltage at the midpoint B of the second bridge arm are equal), and the stress and losses during the operation of the switching transistors are equal, the power conversion device 500 is controlled to start normal operation, thereby improving the reliability and efficiency of the power conversion device 500. The conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold. The first switching transistor Q1 and the fourth switching transistor Q4 are synchronously conducting switching transistors, or the second switching transistor Q2 and the third switching transistor Q3 are synchronously conducting switching transistors.

[0069] For example, taking the first switch Q1 and the fourth switch Q4 as synchronously conducting switches, refer to... Figure 8 During the process where the controller 510 controls the synchronously conducting switching transistor and the third bridge arm to alternately conduct in order to discharge the first capacitor C1 and charge the third capacitor C3, such as Figure 8 As shown in (a), with the synchronously conducting switching transistors on, the current loop in the power conversion device 500 is as shown by the broken line T, as follows: Figure 8 As shown in (b) above, when the synchronously conducting switch is turned off, the current loop in the power conversion device 500 is shown as broken lines U1, U2 and U3.

[0070] In one embodiment, the specific value of the preset time threshold can be determined based on the stress that the switching transistor can withstand. In this application embodiment, the specific value of the preset time threshold is not limited.

[0071] In the power conversion device 500 provided in this application embodiment, at the instant the power conversion device 500 starts working, the voltage at the midpoint of the first bridge arm and the midpoint of the second bridge arm may differ significantly, and the stress on the switching transistors may be uneven. First, the controller 510 controls the synchronously conducting switching transistors and the third bridge arm to alternately conduct, and the conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold, thereby avoiding damage to the switching transistors due to excessive stress. Second, when the voltage across the second capacitor C2 is equal, the controller 510 controls the power conversion device 500 to start normal operation. At the instant the power conversion device 500 starts working, the stress and losses of the switching transistors are equal, thereby improving the reliability and efficiency of the power conversion device 500.

[0072] In one implementation, such as Figure 10 The diagram shows a circuit topology of another power conversion device 500 provided in an embodiment of this application. The power conversion device 500 also includes a magnetic ring 520, through which the cable between the first inductor L1 and the first end of the second capacitor C2 passes, and the cable between the second inductor L2 and the second end of the second capacitor C2 also passes through the magnetic ring 520.

[0073] In one embodiment, the cable between the first inductor L1 and the first end of the second capacitor C2 can be wound around the magnetic ring 520, or the cable between the second inductor L2 and the second end of the second capacitor C2 can be wound around the magnetic ring 520. The specific number of turns of the cable wound around the magnetic ring 520 is not limited in this embodiment.

[0074] In one embodiment, the magnetic ring 520 is an amorphous alloy material. Since the amorphous alloy material has the advantages of high resistivity, low hysteresis loss and good high-frequency characteristics, the high-frequency common-mode leakage current generated by the above-mentioned LC common-mode resonant circuit can be further reduced.

[0075] The power conversion device 500 provided in this application embodiment can further reduce the oscillation amplitude of the voltage at the midpoint A of the first bridge arm and the voltage at the midpoint B of the second bridge arm by setting a magnetic ring 520, and can further reduce the high-frequency common-mode leakage current generated by the LC common-mode resonant circuit, thereby further reducing THDi, EMI and the loss of the power conversion device 500, and improving the reliability of the power conversion device 500.

[0076] like Figure 11The diagram shown is a flowchart illustrating a control method for a power conversion device 500 according to an embodiment of this application. This method is applied to the power conversion device 500 and includes steps S1101-S1102.

[0077] S1101, controller 510 controls the synchronously conducting switching transistors and the third bridge arm to conduct alternately, the conduction time of the synchronously conducting switching transistors being less than or equal to a preset time threshold. Specifically, the first switching transistor Q1 and the fourth switching transistor Q4 are synchronously conducting switching transistors, or the second switching transistor Q2 and the third switching transistor Q3 are synchronously conducting switching transistors.

[0078] After the synchronously conducting switching transistors and the third bridge arm are alternately turned on, the controller 510 controls the power conversion device 500 to start normal operation when the voltage across the second capacitor C2 is equal.

[0079] In one embodiment, after the controller 510 controls the synchronously conducting switch and the third bridge arm to conduct alternately, if the voltages across the second capacitor C2 are not equal, the controller 510 can continue to control the synchronously conducting switch and the third bridge arm to conduct alternately.

[0080] The control method for the power conversion device 500 provided in this application embodiment addresses the situation where, at the instant the power conversion device 500 starts working, the voltage at the midpoint of the first bridge arm and the midpoint of the second bridge arm may differ significantly, and the stress on the switching transistors may be uneven. Firstly, the controller 510 controls the synchronously conducting switching transistors and the third bridge arm to alternately conduct, and the conduction time of the synchronously conducting switching transistors is less than or equal to a preset time threshold. This avoids damage to the switching transistors due to excessive stress. Secondly, when the voltage across the second capacitor C2 is equal, the controller 510 controls the power conversion device 500 to start normal operation. At the instant the power conversion device 500 starts working, the stress and losses of the switching transistors are equal, thereby improving the reliability and efficiency of the power conversion device 500.

[0081] Based on this, such as Figure 1 As shown in the figure, this application embodiment also provides a photovoltaic energy storage system 100, which includes a photovoltaic inverter 110 and a first energy storage battery 120. The first DC port of the photovoltaic inverter 110 is used to connect to a photovoltaic array 200, the second DC port of the photovoltaic inverter 110 is connected to the electrode of the first energy storage battery 120, and the AC port of the photovoltaic inverter 110 is used to connect to the power grid or a load 300. The circuit topology of the photovoltaic inverter 110 is as described above. Figure 5 , Figure 8 or Figure 10 The circuit topology of the power conversion device 500 shown in any of the attached figures.

[0082] In one implementation, reference Figure 1 The photovoltaic energy storage system 100 may further include a second energy storage battery 130 and an energy storage inverter 140. The electrodes of the second energy storage battery 130 are connected to the DC port of the energy storage inverter 140, and the AC port of the energy storage inverter 140 is connected to the AC port of the photovoltaic inverter 110. The circuit topology of the energy storage inverter 140 is as described above. Figure 5 , Figure 8 or Figure 10 The circuit topology of the power conversion device 500 shown in any of the attached figures.

[0083] The above detailed description of the power conversion device 500, as well as the analysis of its beneficial effects, can be directly applied to the control method of the power conversion device and... Figure 1 The photovoltaic energy storage system 100 shown in this application will not be described in detail here.

[0084] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A power conversion device, characterized in that, The power conversion device is used to convert the direct current output by the photovoltaic array or energy storage battery into alternating current. The power conversion device includes a first inductor, a second inductor, a positive DC bus, a negative DC bus, a bus capacitor connected between the positive DC bus and the negative DC bus, a first bridge arm, a second bridge arm and a capacitor branch, and a third bridge arm connected between the midpoint of the first bridge arm and the midpoint of the second bridge arm. The first bridge arm includes a first switch and a second switch connected in series. The second bridge arm includes a third switch and a fourth switch connected in series. The third bridge arm includes a fifth switch and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first capacitor, a second capacitor, and a third capacitor connected in series. The first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. The second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used to connect to the power grid or load.

2. The power conversion device according to claim 1, characterized in that, The power conversion device further includes a first resistor and a second resistor. One end of the first resistor is connected to the first end of the second capacitor, and one end of the second resistor is connected to the second end of the second capacitor. The other ends of the first resistor and the second resistor are both used to connect to the negative DC bus and the reference ground.

3. The power conversion device according to claim 1 or 2, characterized in that, The power conversion device also includes a controller; The controller is used to control the synchronously conducting switching transistor and the third bridge arm to conduct alternately. After controlling the synchronously conducting switching transistor and the third bridge arm to conduct alternately, when the voltage across the second capacitor is equal, the controller controls the power conversion device to start normal operation. The conduction time of the synchronously conducting switching transistor is less than or equal to a preset time threshold. The first switching transistor and the fourth switching transistor are the synchronously conducting switching transistors, or the second switching transistor and the third switching transistor are the synchronously conducting switching transistors.

4. The power conversion device according to any one of claims 1-3, characterized in that, The sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values. The sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor. The sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch, and the fourth switch. N is greater than 1.

5. The power conversion device according to claim 4, characterized in that, N equals 10 or 20.

6. The power conversion device according to any one of claims 1-5, characterized in that, The power conversion device further includes a magnetic ring through which a cable between the first inductor and the first end of the second capacitor passes, and a cable between the second inductor and the second end of the second capacitor also passes through the magnetic ring.

7. The power conversion device according to claim 6, characterized in that, The magnetic ring is made of amorphous alloy material.

8. A control method for a power conversion device, characterized in that, Applied to a power conversion device, the method includes: The synchronously conducting switching transistor and the third bridge arm are alternately turned on. After the synchronously conducting switching transistor and the third bridge arm are alternately turned on, when the voltage across the second capacitor is equal, the power conversion device is controlled to start normal operation. The conduction time of the synchronously conducting switching transistor is less than or equal to a preset time threshold. The first and fourth switching transistors are the synchronously conducting switching transistors, or the second and third switching transistors are the synchronously conducting switching transistors. The power conversion device is used to convert the direct current output by the photovoltaic array or energy storage battery into alternating current. The power conversion device includes a first inductor, a second inductor, a positive DC bus, a negative DC bus, a bus capacitor connected between the positive DC bus and the negative DC bus, a first bridge arm, a second bridge arm and a capacitor branch, and a third bridge arm connected between the midpoint of the first bridge arm and the midpoint of the second bridge arm. The first bridge arm includes a first switch and a second switch connected in series. The second bridge arm includes a third switch and a fourth switch connected in series. The third bridge arm includes a fifth switch and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first capacitor, a second capacitor, and a third capacitor connected in series. The first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. The second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used to connect to the power grid or load.

9. The method according to claim 8, characterized in that, The power conversion device further includes a first resistor and a second resistor. One end of the first resistor is connected to the first end of the second capacitor, and one end of the second resistor is connected to the second end of the second capacitor. The other ends of the first resistor and the second resistor are both used to connect to the negative DC bus and the reference ground.

10. The method according to claim 8 or 9, characterized in that, The sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values. The sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor. The sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch, and the fourth switch. N is greater than 1.

11. The method according to claim 10, characterized in that, N equals 10 or 20.

12. A photovoltaic energy storage system, characterized in that, The photovoltaic energy storage system includes a power conversion device and an energy storage battery. The positive terminal of the energy storage battery is connected to a positive DC bus, and the negative terminal of the energy storage battery is connected to a negative DC bus. The power conversion device is used to convert the DC power output by the photovoltaic array or the energy storage battery into AC power. The power conversion device includes a first inductor, a second inductor, the positive DC bus, the negative DC bus, a bus capacitor connected between the positive DC bus and the negative DC bus, a first bridge arm, a second bridge arm, and a capacitor branch, as well as a third bridge arm connected between the midpoint of the first bridge arm and the midpoint of the second bridge arm. The first bridge arm includes a first switch and a second switch connected in series. The second bridge arm includes a third switch and a fourth switch connected in series. The third bridge arm includes a fifth switch and a sixth switch. The drain or collector of the fifth switch is connected to the drain or collector of the sixth switch, or the source or emitter of the fifth switch is connected to the source or emitter of the sixth switch. The capacitor branch includes a first capacitor, a second capacitor, and a third capacitor connected in series. The first inductor is connected between the first end of the third bridge arm and the first end of the second capacitor. The second inductor is connected between the second end of the third bridge arm and the second end of the second capacitor. The first capacitor is connected between the positive DC bus and the first end of the second capacitor. The third capacitor is connected between the second end of the second capacitor and the negative DC bus. The two ends of the second capacitor are used to connect to the power grid or load.

13. The system according to claim 12, characterized in that, The power conversion device further includes a first resistor and a second resistor. One end of the first resistor is connected to the first end of the second capacitor, and one end of the second resistor is connected to the second end of the second capacitor. The other ends of the first resistor and the second resistor are both used to connect to the negative DC bus and the reference ground.

14. The system according to claim 12 or 13, characterized in that, The power conversion device also includes a controller; The controller is used to control the synchronously conducting switching transistor and the third bridge arm to conduct alternately. After controlling the synchronously conducting switching transistor and the third bridge arm to conduct alternately, when the voltage across the second capacitor is equal, the controller controls the power conversion device to start normal operation. The conduction time of the synchronously conducting switching transistor is less than or equal to a preset time threshold. The first switching transistor and the fourth switching transistor are the synchronously conducting switching transistors, or the second switching transistor and the third switching transistor are the synchronously conducting switching transistors.

15. The system according to any one of claims 12-14, characterized in that, The sum of the first capacitance values ​​is greater than or equal to N times the sum of the second capacitance values. The sum of the first capacitance values ​​is the sum of the equivalent capacitance values ​​of the first capacitor and the third capacitor. The sum of the second capacitance values ​​is the sum of the equivalent capacitance values ​​of the first switch, the second switch, the third switch, and the fourth switch. N is greater than 1.

16. The system according to claim 15, characterized in that, N equals 10 or 20.

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