Power converter and method of overvoltage protection of operation of a power converter

Abstract

This application provides a power converter and an overvoltage protection method for the power converter, relating to the field of power electronics technology. The power converter includes a power conversion circuit and a controller. The power conversion circuit includes multiple switching transistors, which are used to control the on / off state of these transistors to convert direct current (DC) to alternating current (AC), which is then supplied to the power grid and load through the first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the power conversion circuit. The controller, based on the first instantaneous value of each phase voltage and the first instantaneous value of the zero-sequence voltage output from the three-phase voltages of the power conversion circuit, determines that at least one phase terminal of the power conversion circuit is disconnected from the power grid. In this case, the controller controls the switching transistors in the power conversion circuit to turn off, preventing further voltage increases on the power converter and load, thus ensuring the safety of the power converter and load.

Description

Technical Field

[0001] This application relates to the field of power electronics, and in particular to a power converter and a method for protecting the operation overvoltage of the power converter. Background Technology

[0002] A photovoltaic (PV) and energy storage system is a comprehensive energy system that combines photovoltaic power generation with energy storage to achieve the production, storage, distribution, and stable supply of electricity. A PV-energy storage system typically includes a power converter, such as a PV grid-connected inverter or a power conversion system (PCS), or one or more of these. The power converter is connected to the power grid. When the connection between the power converter and the grid is normal, the phase voltage output by the power converter is controlled by the grid voltage, outputting alternating current with the same frequency and phase as the grid voltage, thus supplying power to loads (such as televisions, washing machines, machine tools, and other electrical equipment) together with the grid. When the connection between the power converter and the grid is abnormal, the phase voltage output by the power converter is no longer controlled by the grid voltage, resulting in a momentary high voltage output. The power converter and the load may be damaged under the impact of this momentary high voltage. Summary of the Invention

[0003] This application provides a power converter and an overvoltage protection method for the power converter, which can protect the power converter and load in a timely manner when an abnormality occurs in the connection between the power converter and the power grid, so as to avoid damage to the power converter and load due to the impact of instantaneous high voltage.

[0004] In a first aspect, this application provides a power converter. The power converter includes a power conversion circuit and a controller, wherein the power conversion circuit includes a plurality of switching transistors. The power conversion circuit controls the on / off switching of the plurality of internal switching transistors to convert direct current (DC) to alternating current (AC), and supplies it to the power grid and load through a first phase terminal, a second phase terminal, a third phase terminal, and a neutral terminal of the power conversion circuit. The controller controls all switching transistors in the power conversion circuit to turn off when it detects that at least one phase terminal of the power conversion circuit is disconnected from the power grid.

[0005] As can be seen, the power converter provided in this application can detect whether the connection between the power conversion circuit and the power grid is abnormal. These abnormalities include three situations: single-phase disconnection, two-phase disconnection, and three-phase disconnection. A single-phase disconnection means that the connection between one phase of the power converter and the power grid 200 is broken, while the other two phases remain connected. A two-phase disconnection means that the connection between two phases of the power converter and the power grid is broken, while the other phase remains connected. A three-phase disconnection means that the connection between all three phases of the power converter and the power grid is broken. When any of the above situations is detected, the controller can control the switching transistors in the power conversion circuit to turn off. After all the switching transistors in the power conversion circuit are turned off, the power conversion circuit will no longer generate AC power, preventing further voltage increases on the power converter and load, thus protecting the power converter and load from damage due to excessive voltage surges.

[0006] In some feasible implementations, the controller is used to control all switches in the power conversion circuit to turn off when it detects that the absolute value of the first instantaneous value of at least one phase voltage in the three-phase voltages output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold. The three-phase voltages output by the power conversion circuit include a first phase voltage, a second phase voltage, and a third phase voltage. The first phase voltage is the voltage between the first phase line terminal and the neutral line terminal of the power conversion circuit; the second phase voltage is the voltage between the second phase line terminal and the neutral line terminal of the power conversion circuit; the third phase voltage is the voltage between the third phase line terminal and the neutral line terminal of the power conversion circuit; and the zero-sequence voltage is the vector sum of the first phase voltage, the second phase voltage, and the third phase voltage.

[0007] It is understood that when the connection between the power conversion circuit and the power grid is normal, the voltage of each phase output by the power conversion circuit is controlled by the grid voltage, ensuring that the instantaneous values ​​of the first phase voltage, second phase voltage, third phase voltage, and zero-sequence voltage are stably maintained within a certain range. When the instantaneous value of at least one phase voltage and the instantaneous value of the zero-sequence voltage exceed the above range, it indicates that the connection between the power conversion circuit and the power grid is likely abnormal. Based on this, the above implementation method can accurately detect whether the connection between the power conversion circuit and the power grid is abnormal, thereby ensuring timely protection of the power conversion circuit and the load.

[0008] In some feasible implementations, the first instantaneous value of the first phase voltage includes multiple instantaneous values ​​of the first phase voltage collected within a first time period; the first instantaneous value of the second phase voltage includes multiple instantaneous values ​​of the second phase voltage collected within the first time period; the first instantaneous value of the third phase voltage includes multiple instantaneous values ​​of the third phase voltage collected within the first time period; and the first instantaneous value of the zero-sequence voltage includes multiple instantaneous values ​​of the zero-sequence voltage within the first time period. Then, the controller is configured to turn off all switches in the power conversion circuit when it detects that the absolute values ​​of multiple instantaneous values ​​of at least one phase voltage within the first time period are all greater than a first threshold, and the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage within the first time period are all greater than a second threshold.

[0009] It can be understood that the instantaneous voltage value refers to the specific value of the voltage at a certain moment, which changes dynamically over time. When the absolute values ​​of multiple instantaneous phase voltage values ​​over a period of time are all greater than a first threshold, and the absolute values ​​of multiple instantaneous zero-sequence voltage values ​​are all greater than a second threshold, it indicates that the increase in phase voltage and zero-sequence voltage during that period is not random, but is caused by an abnormal connection between the power conversion circuit and the power grid. Based on this, the above implementation method can accurately detect whether the connection between the power conversion circuit and the power grid is abnormal, thereby ensuring timely protection of the power conversion circuit and the load.

[0010] In some feasible implementations, the controller is also used to control the power converter to operate in grid-connected mode when the absolute value of the second instantaneous value of each phase voltage is less than a third threshold, the effective value of each phase voltage is less than a fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than a fifth threshold. The second instantaneous value of each phase voltage, the effective value of each phase voltage, and the second instantaneous value of the zero-sequence voltage are all acquired after all switches in the power conversion circuit have been turned off.

[0011] It can be understood that the effective value of phase voltage refers to the root mean square value of the instantaneous phase voltage over a complete cycle. When the connection between the power conversion circuit and the grid is normal, the effective value of each phase voltage of the power conversion circuit remains constant. Changes in the effective value of the phase voltage more accurately reflect the connection status between the power conversion circuit and the grid. Therefore, when the absolute value of the second instantaneous value of each phase voltage output by the power conversion circuit is less than the third threshold, the effective value of each phase voltage is less than the fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than the fifth threshold, it indicates that the instantaneous values ​​of each phase voltage and the zero-sequence voltage have stabilized back within the normal range. This change is not random but is caused by the restoration of normal connection between the power conversion circuit and the grid. In other words, when the absolute value of the second instantaneous value of each phase voltage is less than the third threshold, the effective value of each phase voltage is less than the fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than the fifth threshold, it indicates that the connection between the power conversion circuit and the grid has returned to normal. When the connection between the power conversion circuit and the power grid is restored to normal, the controller controls the power converter to operate in parallel with the grid, so that the power conversion circuit continues to convert DC power into AC power with the same frequency and phase as the power grid, and then supplies power to the load together with the power grid.

[0012] In some feasible implementations, the controller is also used to control the power converter to shut down when the difference between a first frequency and a second frequency of the power conversion circuit output voltage is detected to be greater than a sixth threshold. The first frequency is obtained by sampling the frequency of the power conversion circuit output voltage after all switches in the power conversion circuit have been turned off. The second frequency is obtained by sampling the frequency of the power conversion circuit output voltage during grid-connected operation of the power converter.

[0013] It is understandable that while turning off the switching transistor stops the power conversion circuit from outputting AC power, it cannot cut off the energy input from the DC side (such as photovoltaic strings or energy storage batteries) to the power conversion circuit. Continuous energy input from the DC side will cause the power input to the DC side to exceed the power consumed by the load. This unconsumed power will be converted into reactive power, causing the voltage across the power converter and load to continuously rise, potentially damaging them. In the above embodiment, the frequency change of the output voltage of the power conversion circuit (i.e., whether the difference between the first and second frequencies is greater than a sixth threshold) is detected to determine whether the load has consumed all the power input from the DC side. When the difference between the first and second frequencies is greater than the sixth threshold, it indicates that the load has not consumed all the power input from the DC side, and there is a risk of damage to the power converter and load. In this case, the controller shuts down the power converter, cutting off the energy input from the DC side to the power conversion circuit, preventing damage to the power converter and load due to DC side energy input, and ensuring the safety of the power converter and load.

[0014] Secondly, this application provides an overvoltage protection method for a power converter. This method is applied to a controller in the power converter. In addition to the controller, the power converter also includes a power conversion circuit, which comprises multiple switching transistors. The power conversion circuit controls the on / off switching of the multiple internal switching transistors to convert direct current (DC) to alternating current (AC), and supplies it to the power grid and load through a first phase terminal, a second phase terminal, a third phase terminal, and a neutral terminal of the power conversion circuit. When the controller detects that at least one phase terminal of the power conversion circuit is disconnected from the power grid, it controls all switching transistors in the power conversion circuit to turn off.

[0015] In some feasible implementations, when the controller detects that the connection between at least one phase terminal of the power conversion circuit and the power grid is broken, it controls all switches in the power conversion circuit to turn off. This includes: the controller detecting that the absolute value of the first instantaneous value of at least one phase voltage of the three-phase voltage output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold, and then controlling all switches in the power conversion circuit to turn off. The three-phase voltage output by the power conversion circuit includes a first phase voltage, a second phase voltage, and a third phase voltage. The first phase voltage is the voltage between the first phase terminal and the neutral terminal of the power conversion circuit; the second phase voltage is the voltage between the second phase terminal and the neutral terminal of the power conversion circuit; the third phase voltage is the voltage between the third phase terminal and the neutral terminal of the power conversion circuit; and the zero-sequence voltage is the vector sum of the first, second, and third phase voltages.

[0016] In some feasible implementations, the first instantaneous value of the first phase voltage includes multiple instantaneous values ​​of the first phase voltage collected within a first time period; the first instantaneous value of the second phase voltage includes multiple instantaneous values ​​of the second phase voltage collected within the first time period; the first instantaneous value of the third phase voltage includes multiple instantaneous values ​​of the third phase voltage collected within the first time period; and the first instantaneous value of the zero-sequence voltage includes multiple instantaneous values ​​of the zero-sequence voltage within the first time period. Then, if the controller detects that the absolute value of the first instantaneous value of at least one phase voltage output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold, this includes: if the controller detects that the absolute values ​​of multiple instantaneous values ​​of at least one phase voltage within the first time period are all greater than the first threshold, and the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage within the first time period are all greater than the second threshold, then the controller controls all switches in the power conversion circuit to turn off.

[0017] In some feasible implementations, after the controller turns off all the switches in the power conversion circuit, it further detects that the absolute value of the second instantaneous value of each phase voltage is less than a third threshold, the effective value of each phase voltage is less than a fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than a fifth threshold. Then, the power converter is controlled to operate in grid-connected mode. The second instantaneous value of each phase voltage, the effective value of each phase voltage, and the second instantaneous value of the zero-sequence voltage are all acquired after all the switches in the power conversion circuit have been turned off.

[0018] In some feasible implementations, after the controller turns off all the switches in the power conversion circuit, and further detects that the difference between the first frequency and the second frequency of the power conversion circuit output voltage is greater than a sixth threshold, the power converter is then controlled to shut down. The first frequency is obtained by sampling the frequency of the power conversion circuit output voltage after all the switches in the power conversion circuit have been turned off. The second frequency is obtained by sampling the frequency of the power conversion circuit output voltage during grid-connected operation of the power converter.

[0019] It is understood that the implementation and beneficial effects of the above-mentioned aspects of this application can be referenced from each other. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of a photovoltaic energy storage system provided in this application; Figure 2 This is a schematic diagram of the structure of a power converter provided in this application; Figure 3 This is a schematic diagram of the structure of a power conversion circuit provided in this application; Figure 4 This is a schematic diagram of another power conversion circuit provided in this application; Figure 5 This is a schematic diagram illustrating the working principle of a controller provided in this application; Figure 6 This is a schematic diagram of the structure of an experimental test circuit provided in this application; Figure 7 This is a flowchart illustrating an overvoltage protection method for a power converter provided in this application. Detailed Implementation

[0021] See Figure 1 , Figure 1 This is a schematic diagram of the structure of a photovoltaic energy storage system provided in this application. Figure 1 As shown, the photovoltaic-storage system 100 includes a photovoltaic string 110, a photovoltaic grid-connected inverter 120, an energy storage battery 130, and an energy storage converter 140.

[0022] The photovoltaic string 110 is a circuit unit composed of multiple photovoltaic modules (also known as "solar panels") connected in series. These photovoltaic modules can include one or more types such as monocrystalline silicon photovoltaic modules, polycrystalline silicon photovoltaic modules, thin-film photovoltaic modules, and concentrated photovoltaic modules. The photovoltaic string 110 is used to convert solar energy into direct current using the photovoltaic effect and output it to the photovoltaic grid-connected inverter 120.

[0023] The photovoltaic grid-connected inverter 120 is used to convert the direct current generated by the photovoltaic string 110 into alternating current and supply it to the grid 200 and / or load 300. The grid 200 here is a low-voltage grid, which is used to safely and reliably distribute electrical energy from substations or distribution transformers to every electrical device, often referred to as the "last mile" of power supply. The load 300 is the electrical device, which may include one or more of the following: residential loads (such as refrigerators, air conditioners, and televisions), commercial loads (such as servers and printers), industrial loads (such as industrial furnaces, machine tools, and welding equipment), and public loads (such as traffic lights and streetlights).

[0024] In this embodiment, the photovoltaic grid-connected inverter 120 adopts a three-phase four-wire system. Specifically, the output terminals of the photovoltaic grid-connected inverter 120 include a first phase terminal, a second phase terminal, a third phase terminal, and a neutral terminal. The first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the photovoltaic grid-connected inverter 120 are respectively connected to the first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the power grid 200. At least one of the first phase terminal, second phase terminal, and third phase terminal of the photovoltaic grid-connected inverter 120 is connected to the neutral terminal via a load 300.

[0025] The energy storage battery 130 includes one or more of the following: lead-acid battery, lithium-ion battery, sodium-sulfur battery, supercapacitor, and superconducting energy storage battery. The energy storage battery 130 stores electrical energy. In situations where the power grid 200 is insufficient (e.g., during peak electricity consumption or due to weather conditions causing insufficient power generation from the photovoltaic string 110) or when grid electricity prices are high, the energy storage battery 130 releases electrical energy and outputs it to the energy storage converter 140. The energy storage converter 140 converts the direct current (DC) from the energy storage battery 130 into alternating current (AC) and supplies it to the grid 200 and / or the load 300. In situations where the photovoltaic string 110 generates excessive electrical energy or when grid electricity prices are low, the energy storage converter 140 converts the excess AC to DC and outputs it to the energy storage battery 130 to charge it.

[0026] Similar to the photovoltaic grid-connected inverter 120, the energy storage converter 140 also uses a three-phase four-wire system to connect to the grid 200 and supply power to the load 300. For simplicity, the connection relationship between the energy storage converter 140, the grid 200, and the load 300 will not be described here. Please refer to the above description of the connection relationship between the photovoltaic grid-connected inverter 120, the grid 200, and the load 300.

[0027] It is worth noting that when the photovoltaic grid-connected inverter 120 is normally connected to the grid 200 (i.e., the connection between the three phase terminals of the photovoltaic grid-connected inverter 120 and the three phase terminals of the grid 200 is normal), the photovoltaic grid-connected inverter 120 is in grid-connected operation. In this state, the three-phase voltage output by the photovoltaic grid-connected inverter 120 is controlled by the voltage of the grid 200. The three-phase voltage output by the photovoltaic grid-connected inverter 120 includes the first phase voltage, the second phase voltage, and the third phase voltage. The first phase voltage is the voltage between the first phase terminal and the neutral terminal of the photovoltaic grid-connected inverter 120; the second phase voltage is the voltage between the second phase terminal and the neutral terminal of the photovoltaic grid-connected inverter 120; and the third phase voltage is the voltage between the third phase terminal and the neutral terminal of the photovoltaic grid-connected inverter 120. The three-phase voltage output by the photovoltaic grid-connected inverter 120 being controlled by the grid voltage 200 means that the first-phase voltage, second-phase voltage, and third-phase voltage output by the photovoltaic grid-connected inverter 120 always follow the first-phase voltage, second-phase voltage, and third-phase voltage of the grid 200, and will not fluctuate significantly due to changes in the photovoltaic grid-connected inverter 120 or the load 300, so that the photovoltaic grid-connected inverter 120 and the load 300 can operate within a safe voltage range.

[0028] In situations such as grid 200 failure, grid 200 maintenance, or cable detachment between the photovoltaic grid-connected inverter 120 and grid 200, the connection between the photovoltaic grid-connected inverter 120 and grid 200 may become abnormal. This abnormality can be a single-phase disconnection, a two-phase disconnection, or a three-phase disconnection. A single-phase disconnection means that the connection between one phase of the photovoltaic grid-connected inverter 120 and grid 200 is broken, while the other two phases remain connected. A two-phase disconnection means that the connection between two phases of the photovoltaic grid-connected inverter 120 and grid 200 is broken, while the other phase remains connected. A three-phase disconnection means that the connection between all three phases of the photovoltaic grid-connected inverter 120 and grid 200 is broken. When a single-phase, two-phase, or three-phase disconnection occurs between the photovoltaic grid-connected inverter 120 and grid 200, the phase voltage of the disconnected phase will no longer be controlled by the grid 200 voltage, resulting in a momentary high voltage phenomenon. For ease of description, the disconnected connection between the photovoltaic grid-connected inverter 120 and the grid 200 is referred to as the "disconnected phase." For example, if the connection between the first phase of the photovoltaic grid-connected inverter 120 and the grid 200 is broken, while the connections between the other two phases and the grid 200 are normal, then the disconnected phase is the connection between the first phase of the photovoltaic grid-connected inverter 120 and the grid 200, and the phase voltage of the disconnected phase is the first phase voltage. If the load 300 connected to the disconnected phase is small, or if no load 300 is connected to the disconnected phase, then the electrical energy output by the photovoltaic grid-connected inverter 120 cannot be fully consumed on the disconnected phase. In this case, the excess electrical energy will be converted into reactive power, and over time, both the photovoltaic grid-connected inverter 120 and the load 300 will be damaged. Similarly, the connection between the energy storage converter 140 and the power grid 200 may also experience the above-mentioned abnormalities. If the connection between the energy storage converter 140 and the power grid 200 experiences the above-mentioned abnormalities, the energy storage converter 140 and the load 300 will also be damaged.

[0029] To address the damage caused by abnormal connections between the photovoltaic grid-connected inverter 120 (or energy storage converter 140) and the power grid 200, this application provides a power converter, which can be either the aforementioned photovoltaic grid-connected inverter 120 or the aforementioned energy storage converter 140. The power converter can detect whether its connection with the power grid 200 is abnormal. If so, it will perform operations such as voltage blocking and shutdown to protect the power converter and its load, preventing damage from instantaneous high-voltage surges.

[0030] The following is combined with Figures 2 to 5 This paper introduces the power converter provided in this application.

[0031] See Figure 2 , Figure 2This is a schematic diagram of a power converter provided in this application. Figure 2 As shown, the power converter 400 includes a power conversion circuit 410 and a controller 420.

[0032] (1) Power conversion circuit 410 The power conversion circuit 410 includes multiple switching transistors, such as IGBTs and MOSFETs. The power conversion circuit 410 controls the on / off switching of these transistors to convert direct current (DC) to alternating current (AC). Specifically, when the power converter 400 is a photovoltaic grid-connected inverter 120, the DC power comes from the photovoltaic string 110. When the power converter 400 is an energy storage converter 140, the DC power comes from the energy storage battery 130.

[0033] The power conversion circuit 410 is also used to supply the generated AC power to the power grid 200 and / or the load 300. Specifically, the output terminals of the power conversion circuit 410 include a first phase terminal (shown as "A" in the figure), a second phase terminal (shown as "B" in the figure), a third phase terminal (shown as "C" in the figure), and a neutral terminal (shown as "N" in the figure). The first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the power conversion circuit 410 are respectively connected to the first phase terminal (shown as "B" in the figure) of the power grid 200. "), the second phase line end (shown as " in the figure) The third phase line terminal (shown as " in the diagram") ") and the neutral line end (shown as "" in the figure) The power conversion circuit 410 has one or more of its first phase line terminal, second phase line terminal, and third phase line terminal connected to a load 300 between the first phase line terminal, the second phase line terminal, and the third phase line terminal and the neutral line terminal. For ease of description, Figure 2 The following embodiments are all described using the example of a load 300 connected between the first phase line terminal, the second phase line terminal, and the third phase line terminal of the power conversion circuit 410 and the neutral line terminal. The power conversion circuit 410 can provide the generated AC power to the power grid 200 and / or the load 300 through the above connections.

[0034] To facilitate understanding, the following will combine... Figure 3 and Figure 4 The power conversion circuit 410 will be further described.

[0035] See Figure 3 , Figure 3 A possible power conversion circuit 410 is shown. For example... Figure 3As shown, the power conversion circuit 410 includes a DC-DC converter circuit and an inverter circuit. The DC-DC converter circuit performs DC-DC conversion (e.g., filtering, boosting) on ​​the DC power from the photovoltaic string 110 or the energy storage battery 130, and outputs the converted DC power to the inverter circuit via a DC bus. The inverter circuit includes multi-phase bridge arms, each phase of which includes multiple switching transistors (e.g., IGBTs, MOSFETs). Each multi-phase bridge arm includes at least a first phase bridge arm, a second phase bridge arm, and a third phase bridge arm. The first, second, and third phase bridge arms are used to convert the DC power into first-phase AC power, second-phase AC power, and third-phase AC power, respectively, and output them through the first phase terminal (shown as "A" in the figure), the second phase terminal (shown as "B" in the figure), the third phase terminal (shown as "C" in the figure), and the neutral terminal (shown as "N" in the figure) of the inverter circuit to supply power to the grid 200 and / or the load 300.

[0036] See Figure 4 , Figure 4 Another possible power conversion circuit 410 is shown. Compared to Figure 3 The power conversion circuit 410 shown is... Figure 4 The power conversion circuit 410 shown has been augmented with a filter circuit. This filter circuit filters the first-phase AC power generated by the first phase bridge arm, the second-phase AC power generated by the second phase bridge arm, and the third-phase AC power generated by the third phase bridge arm in the inverter circuit. The filtered first-phase AC power, second-phase AC power, and third-phase AC power are then output through the first-phase line terminal (shown as "A" in the diagram), the second-phase line terminal (shown as "B" in the diagram), the third-phase line terminal (shown as "C" in the diagram), and the neutral line terminal (shown as "N" in the diagram) of the filter circuit, respectively, to supply the grid 200 and / or the load 300.

[0037] Understandable. Figure 3 and Figure 4 Two possible power conversion circuits 410 are only exemplarily described. In practical applications, the power conversion circuit 410 may have other forms, which are not limited in this application.

[0038] (2) Controller 420 The controller 420 can be implemented using one or more of the following: application-specific integrated circuit (ASIC), digital signal processor (DSP), microcontroller unit (MCU), system-on-chip (SoC), central processing unit (CPU), and programmable logic device (PLD). The aforementioned PLD can be a complex programmable logical device (CPLD), field-programmable gate array (FPGA), generic array logic (GAL), or any combination thereof.

[0039] The controller 420 is used to detect whether the connection between the power conversion circuit 410 and the power grid 200 is abnormal. An abnormality here refers to a disconnection between at least one phase terminal of the power conversion circuit 410 and the power grid 200. Specifically, the controller 420 can determine whether the connection between the power conversion circuit 410 and the power grid 200 is abnormal based on the instantaneous value of each phase voltage and / or the instantaneous value of the zero-sequence voltage in the three-phase voltages output by the power conversion circuit 410. The three-phase voltages output by the power conversion circuit 410 include a first-phase voltage, a second-phase voltage, and a third-phase voltage. The first-phase voltage is the voltage between the first phase terminal and the neutral terminal of the power conversion circuit 410; the second-phase voltage is the voltage between the second phase terminal and the neutral terminal of the power conversion circuit 410; the third-phase voltage is the voltage between the third phase terminal and the neutral terminal of the power conversion circuit 410; and the zero-sequence voltage is the vector sum of the first-phase voltage, the second-phase voltage, and the third-phase voltage.

[0040] The controller 420 is also used to control the switching transistor in the power conversion circuit 410 to turn off when an abnormal connection between the power conversion circuit 410 and the power grid 200 is detected. After the switching transistor in the power conversion circuit 410 is turned off, the power conversion circuit 410 will no longer generate AC power, preventing the voltage across the power converter 400 and the load 300 from further increasing, thus preventing damage to the power converter 400 and the load 300 due to excessive voltage surges, achieving the purpose of protecting the power converter 400 and the load 300.

[0041] In some feasible implementations, the controller 420 is also used to detect whether the frequency change of the output voltage of the power conversion circuit 410 is abnormal after the switching transistor in the power conversion circuit 410 is turned off. If so, the controller controls the power converter 400 to shut down. An abnormal frequency change in the output voltage of the power conversion circuit 410 refers to a difference between the first frequency and the second frequency of the output voltage of the power conversion circuit 410 exceeding a threshold value of 4. The first frequency is obtained by sampling the frequency of the output voltage of the power conversion circuit 410 after the switching transistor in the power conversion circuit 410 is turned off. The second frequency is obtained by sampling the frequency of the output voltage of the power conversion circuit 410 during grid-connected operation of the power converter 400.

[0042] It is understandable that while turning off the switching transistor in the power conversion circuit 410 stops the power conversion circuit 410 from outputting AC power, it cannot cut off the energy input to the power conversion circuit 410 from the DC side (such as the photovoltaic string 110 or the energy storage battery 130). Continuous energy input from the DC side will cause the power input to the DC side to exceed the power consumed by the load 300. This unconsumed power will be converted into reactive power, causing the voltage across the power converter 400 and the load 300 to continuously increase, potentially damaging the power converter 400 and the load 300. In the above embodiment, when the difference between the first frequency and the second frequency of the output voltage of the power conversion circuit 410 is detected to be greater than the threshold 4, it indicates that the frequency of the output voltage of the power conversion circuit 410 has increased, which means that the power input from the DC side is greater than the power consumed by the load 300. The power converter 400 and the load 300 are at risk of being damaged. In this situation, the controller 420 controls the power converter 400 to shut down, which can cut off the DC side energy input to the power conversion circuit 410, preventing the power converter 400 and the load 300 from being damaged by the DC side energy input, and ensuring the safety of the power converter 400 and the load 300.

[0043] In some feasible implementations, the controller 420 is also used to detect whether the connection between the power conversion circuit 410 and the power grid 200 has been restored to normal. Here, "normal" means that the first phase line terminal, the second phase line terminal, the third phase line terminal, and the neutral line terminal of the power conversion circuit 410 are all connected to the power grid 200. Specifically, the controller 420 can determine whether the connection between the power conversion circuit 410 and the power grid 200 has been restored to normal based on the instantaneous value of each phase voltage output by the power conversion circuit 410, the effective value of each phase voltage, and the instantaneous value of the zero-sequence voltage.

[0044] The controller 420 is also used to control the power conversion circuit 410 to reconnect to the grid when it detects that the connection between the power conversion circuit 410 and the grid 200 has been restored to normal. After the power conversion circuit 410 reconnects to the grid, it will convert the DC power provided by the DC side (such as the photovoltaic string 110 or the energy storage battery 130) into AC power with the same frequency and phase as the grid 200, and then supply power to the load 300 together with the grid 200.

[0045] To facilitate understanding, the following will combine... Figure 5 The working principle of controller 420 will be further explained.

[0046] ① The controller 420 detects that the power conversion circuit 410 meets the first condition.

[0047] If the power conversion circuit 410 does not meet the first condition, the controller 420 can determine that the connection between the power conversion circuit 410 and the power grid 200 is abnormal. The first condition includes condition 1 and / or condition 2. Condition 1 means that the absolute value of the first instantaneous value of at least one phase voltage output by the power conversion circuit 410 is greater than threshold 1, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than threshold 2. Condition 2 means that the absolute value of the third instantaneous value of at least one phase voltage output by the power conversion circuit 410 is greater than threshold 3.

[0048] Condition 1 and Condition 2 will be described below.

[0049] (1) Condition 1 The first instantaneous value of the first phase voltage includes one or more instantaneous values ​​of the first phase voltage acquired within time T1. The first instantaneous value of the second phase voltage includes one or more instantaneous values ​​of the second phase voltage acquired within time T1. The first instantaneous value of the third phase voltage includes one or more instantaneous values ​​of the third phase voltage acquired within time T1. The first instantaneous value of the zero-sequence voltage includes one or more instantaneous values ​​of the zero-sequence voltage within time T1.

[0050] When the first instantaneous values ​​of the first phase voltage, the second phase voltage, the third phase voltage, and the zero-sequence voltage respectively include multiple instantaneous values ​​of the first phase voltage, the second phase voltage, the third phase voltage, and the zero-sequence voltage within time T1, condition 1 means that within time T1, the absolute values ​​of multiple instantaneous values ​​of at least one phase voltage are all greater than threshold 1, and the multiple instantaneous values ​​of the zero-sequence voltage are all greater than threshold 2.

[0051] It is worth noting that the instantaneous voltage value refers to the specific value of the voltage at a certain moment, which changes dynamically over time. When, within time T1, the absolute values ​​of multiple instantaneous values ​​of at least one phase voltage are all greater than the first threshold, and the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage are all greater than the second threshold, it indicates that the increase in phase voltage and zero-sequence voltage during this period is not random, but rather caused by an abnormal connection between the power conversion circuit and the power grid. In other words, based on condition 1, it is possible to accurately detect whether the connection between the power conversion circuit and the power grid is abnormal.

[0052] To make it easier to understand, we will further explain condition 1 with an example below.

[0053] If only one instantaneous value of the first-phase voltage, the second-phase voltage, and the third-phase voltage is collected within time T1, denoted as Ua1, Ub1, and Uc1 respectively, and Ua1, Ub1, and Uc1 are collected at the same instant, then the first instantaneous value of the first-phase voltage is Ua1, the first instantaneous value of the second-phase voltage is Ub1, the first instantaneous value of the third-phase voltage is Uc1, and the first instantaneous value of the zero-sequence voltage is Uz1, where Uz1 = Ua1 + Ub1 + Uc1. In this case, condition 1 means that at least one of the absolute values ​​of Ua1, Ub1, and Uc1 is greater than threshold 1, and the absolute value of Uz1 is greater than threshold 2.

[0054] If multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage are collected within time T1, for simplicity, the following explanation will use the collection of two instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage as an example. Assume that Ua11, Ub11, and Uc11 are collected at time T11, and Ua12, Ub12, and Uc12 are collected at time T12, where both time T11 and time T12 are within time T1. Then, the first instantaneous value of the first-phase voltage includes Ua11 and Ua12, the first instantaneous value of the second-phase voltage includes Ub11 and Ub12, the first instantaneous value of the third-phase voltage includes Uc11 and Uc12, and the first instantaneous value of the zero-sequence voltage includes Uz11 and Uz12, where Uz11 = Ua11 + Ub11 + Uc11, and Uz12 = Ua12 + Ub12 + Uc12. In this case, condition 1 means that at least one of the following is true: the absolute values ​​of Ua11 and Ua12 are both greater than threshold 1, the absolute values ​​of Ub11 and Ub12 are both greater than threshold 1, and the absolute values ​​of Uc11 and Uc12 are both greater than threshold 1, and the absolute values ​​of Uz11 and Uz12 are both greater than threshold 2.

[0055] The thresholds 1, 2, and time T1 mentioned above can be configured based on anomalies in the connection between the power conversion circuit 410 and the power grid 200, as well as the load 300. Specifically, the disconnected phase between the power conversion circuit 410 and the power grid 200, and the load 300 connected to the disconnected phase are taken as independent variables, while the withstand time of the load 300 connected to the disconnected phase, the instantaneous phase voltage of the disconnected phase, and the instantaneous zero-sequence voltage are taken as dependent variables. Multiple experimental tests are conducted to obtain multiple instantaneous phase voltage values, multiple instantaneous zero-sequence voltage values, and multiple load withstand times. Among them, the withstand time of the load 300 refers to the longest time that the load 300 can maintain its function without being broken down or damaged when subjected to instantaneous high voltage. Threshold 1 can be set to the minimum absolute value among the multiple instantaneous phase voltage values, threshold 2 can be set to the minimum absolute value among the multiple instantaneous zero-sequence voltage values, and time T1 can be set to the minimum among the multiple load withstand times.

[0056] The above multiple experimental tests can be based on Figure 6 The circuit shown is implemented as follows. Figure 6 As shown, the first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the power converter 400 (specifically the power conversion circuit 410) are connected to the first phase terminal, second phase terminal, third phase terminal, and neutral terminal of the power grid 200 via the first phase line, second phase line, third phase line, and neutral line, respectively. A switch device K1 is provided on the first phase line, which is used to control the connection between the first phase terminal of the power conversion circuit 410 and the first phase terminal of the power grid 200. When K1 is open, the connection between the first phase terminal of the power conversion circuit 410 and the first phase terminal of the power grid 200 is disconnected; when K1 is closed, the connection between the first phase terminal of the power conversion circuit 410 and the first phase terminal of the power grid 200 is maintained. Similarly, a switch device K2 is provided on the second phase line, which is used to control the connection between the second phase terminal of the power conversion circuit 410 and the second phase terminal of the power grid 200. A switch K3 is installed on the third phase line. K3 is used to control the connection between the third phase line terminal of the power conversion circuit 410 and the third phase line terminal of the power grid 200. When K1, K2, and K3 are all closed, the power converter 400 is in grid-connected operation. In this state, the power conversion circuit 410 converts DC power into AC power with the same frequency and phase as the power grid 200, and together with the power grid 200, supplies power to the first load Ra, the second load Rb, and the third load Rc. Here, Ra, Rb, and Rc are respectively the loads 300 connected between the first phase line terminal and the neutral line terminal of the power conversion circuit 410, the loads 300 connected between the second phase line terminal and the neutral line terminal of the power conversion circuit 410, and the loads 300 connected between the third phase line terminal and the neutral line terminal of the power conversion circuit 410.

[0057] The control device changes the disconnected phase between the power conversion circuit 410 and the power grid 200 by instructing K1, K2, and K3 to open and close; and changes the impedance value of the load on the disconnected phase by instructing Ra, Rb, and Rc to adjust the impedance values. Multiple experimental tests can be performed by repeatedly changing the disconnected phase between the power conversion circuit 410 and the power grid 200, and by repeatedly changing the impedance value of the load on the disconnected phase. In each experimental test, the control device can instruct the controller 420 in the power converter 400 to sample the instantaneous phase voltage output of the power conversion circuit 410, and instruct Ra, Rb, and Rc to detect the withstand time of the local load. Thus, multiple instantaneous phase voltage values, multiple instantaneous zero-sequence voltage values, and multiple load withstand times can be obtained, and threshold 1, threshold 2, and time T1 can be configured.

[0058] To facilitate understanding, the following six experimental tests will be used as examples to introduce the configuration process of threshold 1, threshold 2, and time T1.

[0059] Experiment 1: After the power converter 400 has been running in grid-connected operation for a period of time, the control device instructs K1 to open, while K2 and K3 remain closed. Then, the control device instructs the controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t1. The control device obtains time t1 from Ra, and the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t1 from the controller 420. Based on these instantaneous values, it calculates the instantaneous value of the zero-sequence voltage within time t1. Finally, the control device records time t1, the instantaneous value u1 of the first-phase voltage within time t1, and the instantaneous value u2 of the zero-sequence voltage within time t1.

[0060] Experiment 2: After the power converter 400 has been running in grid-connected operation for a period of time, the control device instructs K1 and K2 to open simultaneously, while K3 remains closed. Then, the control device instructs controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t2, and instructs Rb to detect the load's withstand time t3. The control device obtains time t2 from Ra, time t3 from Rb, and the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t4 from controller 420. Time t4 is the maximum value between time t2 and time t3. Based on the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t4, the instantaneous value of the zero-sequence voltage within time t4 is calculated. Afterwards, the control device records the instantaneous values ​​u3 of the first-phase voltage within time t2, time t3, and time t4, u4 of the second-phase voltage within time t4, and u5 of the zero-sequence voltage within time t4.

[0061] Experiment 3: After the power converter 400 has been running in grid-connected operation for a period of time, the control device instructs K1, K2, and K3 to open simultaneously. Then, the control device instructs controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t5, Rb to detect the load's withstand time t6, and Rc to detect the load's withstand time t7. The control device obtains time t5 from Ra, time t6 from Rb, and time t7 from Rc. It also obtains the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t8 from controller 420. Time t8 is the maximum value among time t5, time t6, and time t7. Based on the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t8, the instantaneous value of the zero-sequence voltage within time t8 is calculated. Subsequently, the control device records the instantaneous values ​​of the first phase voltage u6, the second phase voltage u7, the third phase voltage u8, and the zero-sequence voltage u9 within time t5, time t6, time t7, and time t8.

[0062] Experiment 4: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in grid-connected mode for a period of time under the new impedance value, the control device instructs K1 to open, while K2 and K3 remain closed. Subsequently, the control device instructs controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t9. The control device obtains time t9 from Ra, and the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t9 from controller 420. Based on these instantaneous values, it calculates the instantaneous value of the zero-sequence voltage within time t9. Then, the control device records time t9, the instantaneous value u10 of the first-phase voltage within time t9, and the instantaneous value u11 of the zero-sequence voltage within time t9.

[0063] Experiment 5: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in grid-connected mode for a period of time under the new impedance value, the control device instructs K1 and K2 to open simultaneously, while K3 remains closed. Subsequently, the control device instructs controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t10, and instructs Rb to detect the load's withstand time t11. The control device obtains time t10 from Ra, time t11 from Rb, and the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t12 from controller 420. Time t12 is the maximum value between time t10 and time t11. Based on the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t12, the instantaneous value of the zero-sequence voltage within time t12 is calculated. Subsequently, the control device records the instantaneous values ​​of the first phase voltage u12, the second phase voltage u13, and the zero-sequence voltage u14 within time t10, time t11, and time t12.

[0064] Experiment 6: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in grid-connected mode for a period of time under the new impedance value, the control device instructs K1, K2, and K3 to open simultaneously. Subsequently, the control device instructs controller 420 to collect multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and instructs Ra to detect the load's withstand time t13, Rb to detect the load's withstand time t14, and Rc to detect the load's withstand time t15. The control device obtains time t13 from Ra, time t14 from Rb, time t15 from Rc, and the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t16 from controller 420. Time t16 is the maximum value among times t13, t14, and t15. Based on the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage within time t16, the instantaneous value of the zero-sequence voltage within time t16 is calculated. Subsequently, the control device records the instantaneous values ​​u15 of the first phase voltage, u16 of the second phase voltage, u17 of the third phase voltage, and u18 of the zero-sequence voltage within time t13, time t14, time t15, and time t16.

[0065] The following results are obtained through the above six experimental tests: instantaneous phase voltage values ​​u1, u3, u4, u6, u7, u8, u10, u12, u13, u15, u16, u17; instantaneous zero-sequence voltage values ​​u2, u5, u9, u11, u14, u18; and load withstand times t1, t2, t3, t5, t6, t7, t9, t10, t11, t13, t14, t15. The control device compares the absolute values ​​of the above multiple instantaneous phase voltage values ​​and configures threshold 1, where threshold 1 is the minimum absolute value among the above multiple instantaneous phase voltage values; compares the absolute values ​​of the above multiple instantaneous zero-sequence voltage values ​​and configures threshold 2, where threshold 2 is the minimum absolute value among the above multiple instantaneous zero-sequence voltage values; and compares the above multiple load withstand times and configures time T1, where time T1 is the minimum among the above multiple load withstand times.

[0066] It is understandable that, in addition to the six experimental tests mentioned above, the control device can also make other changes to the disconnected phase between the power conversion circuit 410 and the power grid 200, as well as the load 300 connected to the disconnected phase, to conduct other experimental tests. For example, it can instruct Ra to change the impedance value again and control K1 to open while K2 and K3 to close. Or, it can instruct Rb to change the impedance value and control K1 and K2 to open simultaneously. Or, it can instruct both Ra and Rb to change the impedance values ​​and control K1 and K2 to open simultaneously. Or, it can instruct Rb to change the impedance value and control K1, K2, and K3 to open simultaneously. Or, it can instruct both Ra and Rb to change the impedance values ​​and control K1, K2, and K3 to open simultaneously. Or, it can instruct Ra, Rb, and Rc to change the impedance values ​​and control K1, K2, and K3 to open simultaneously. The control device can also be configured to obtain threshold 1, threshold 2, and time T1 through other experimental tests. The specific implementation idea is similar to the above content, and for the sake of simplicity, it will not be repeated here.

[0067] (2) Condition 2 The third instantaneous value of the first phase voltage includes one or more instantaneous values ​​of the first phase voltage acquired within time T2. The third instantaneous value of the second phase voltage includes one or more instantaneous values ​​of the second phase voltage acquired within time T2. The third instantaneous value of the third phase voltage includes one or more instantaneous values ​​of the third phase voltage acquired within time T2.

[0068] Condition 2 means that when the third instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage respectively include multiple instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage collected within time T2, the absolute value of at least one of the first phase voltage, the second phase voltage, and the third phase voltage collected within time T2 is greater than the threshold 3.

[0069] To make it easier to understand, the following example will further illustrate condition 2.

[0070] If only one instantaneous value of the first-phase voltage, the second-phase voltage, and the third-phase voltage is collected within time T2, namely Ua2, Ub2, and Uc2 respectively, and Ua2, Ub2, and Uc2 are collected at the same instant, then the third instantaneous value of the first-phase voltage is Ua2, the third instantaneous value of the second-phase voltage is Ub2, and the third instantaneous value of the third-phase voltage is Uc2. In this case, condition ② means that at least one of the absolute values ​​of Ua2, Ub2, and Uc2 is greater than the threshold value 3.

[0071] If multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage are collected within time T2, for simplicity, the following explanation will use the collection of two instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage as an example. Assume that Ua21, Ub21, and Uc21 are collected at time t21, and Ua22, Ub22, and Uc22 are collected at time t22, where both time t21 and time t22 fall within time T2. Then, the third instantaneous value of the first-phase voltage includes Ua21 and Ua22, the third instantaneous value of the second-phase voltage includes Ub21 and Ub22, and the third instantaneous value of the third-phase voltage includes Uc21 and Uc22. In this case, condition 2 means that at least one of the following conditions is met: the absolute values ​​of Ua21 and Ua22 are both greater than threshold 3; the absolute values ​​of Ub21 and Ub22 are both greater than threshold 3; and the absolute values ​​of Uc21 and Uc22 are both greater than threshold 3.

[0072] Similar to the configuration process of threshold 1, threshold 2, and time T1, threshold 3 and time T2 can also be obtained through multiple experimental tests. Specifically, after the power converter 400 has been running in grid-connected operation for a period of time, switches K1, K2, and K3 are simultaneously turned on, and the withstand time of Ra, the withstand time of Rb, the withstand time of Rc, and the instantaneous values ​​of the first-phase voltage, the second-phase voltage, and the third-phase voltage are tested. Then, the withstand time of Ra, the withstand time of Rb, the withstand time of Rc, and the instantaneous values ​​of the first-phase voltage, the second-phase voltage, and the third-phase voltage are recorded. Next, the impedance values ​​of Ra, Rb, and Rc are changed, ensuring that the difference between any two of the new Ra, Rb, and Rc impedance values ​​is less than a preset difference. Following the same approach, the withstand time of the new Ra, the withstand time of the new Rb, the withstand time of the new Rc, and the instantaneous values ​​of the first-phase voltage, the second-phase voltage, and the third-phase voltage under the new Ra, Rb, and Rc are tested again. By analogy, multiple experimental tests are conducted to obtain multiple instantaneous phase voltage values ​​and multiple load withstand times. Therefore, threshold 3 can be set as the minimum absolute value among the multiple instantaneous phase voltage values, and time T2 can be set as the minimum among the multiple load withstand times.

[0073] It is worth noting that threshold 3 is usually greater than threshold 1. The reason is as follows: the instantaneous phase voltage of the disconnected phase is related to at least one of the following: the disconnection status between the power conversion circuit 410 and the power grid 200 (i.e., single-phase disconnection, two-phase disconnection, and three-phase disconnection), and the load 300 connected to the disconnected phase. Specifically, in the case of a three-phase disconnection and balanced three-phase load, the instantaneous phase voltage of the disconnected phase will fluctuate significantly. In the cases of single-phase disconnection, two-phase disconnection, three-phase disconnection, and unbalanced three-phase load, the instantaneous phase voltage of the disconnected phase will not fluctuate significantly, but the instantaneous value of the zero-sequence voltage will increase significantly. Since threshold 1 is the absolute value of the smallest instantaneous phase voltage obtained through experimental testing under single-phase disconnection, two-phase disconnection, and three-phase disconnection conditions, while threshold 3 is the absolute value of the smallest instantaneous phase voltage obtained through experimental testing under the case of three-phase disconnection and balanced three-phase load, threshold 3 is greater than threshold 1.

[0074] The first condition has been described in detail above. The following describes how the controller 420 detects that the power conversion circuit 410 meets the first condition, which can be divided into the following three cases.

[0075] (1) The first condition includes only condition 1. When the first condition includes only condition 1, the controller 420 can detect that the power conversion circuit 410 satisfies the first condition in the following way: Within time T1, controller 420 acquires one or more instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage to obtain the first instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage. Then, based on the first instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, controller 420 calculates the first instantaneous value of the zero-sequence voltage. Finally, based on the first instantaneous values ​​of the first-phase voltage, second-phase voltage, third-phase voltage, and zero-sequence voltage, controller 420 determines whether the power conversion circuit 410 satisfies condition 1. If it does, then the power conversion circuit 410 is determined to satisfy the first condition. Otherwise, it is determined that the power conversion circuit 410 does not satisfy the first condition.

[0076] The specific implementation methods of "the controller 420 determines whether the power conversion circuit 410 meets condition 1 based on the first instantaneous values ​​of the first phase voltage, the second phase voltage, the third phase voltage, and the zero-sequence voltage" mentioned above include one or more of the following: Method 1: Controller 420 compares the absolute values ​​of the first instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage with threshold 1, and compares the absolute value of the first instantaneous value of the zero-sequence voltage with threshold 2. If at least one of the absolute values ​​of the first instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage is greater than threshold 1, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than threshold 2, then the power conversion circuit 410 is determined to satisfy condition 1. Otherwise, the power conversion circuit 410 is determined not to satisfy condition 1.

[0077] Method 2: The controller 420 compares the absolute values ​​of the first instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage to determine the maximum value among the three. Then, it compares the determined maximum value with threshold 1 and the absolute value of the first instantaneous value of the zero-sequence voltage with threshold 2. If the maximum value is greater than threshold 1 and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than threshold 2, then the power conversion circuit 410 is determined to satisfy condition 1. Otherwise, the power conversion circuit 410 is determined not to satisfy condition 1.

[0078] (2) The first condition only includes condition 2. When the first condition includes only condition 2, the controller 420 can detect that the power conversion circuit 410 satisfies the first condition in the following way: Within time T2, controller 420 acquires one or more instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage to obtain a third instantaneous value of the first-phase voltage, second-phase voltage, and third-phase voltage. Then, based on the third instantaneous value of the first-phase voltage, second-phase voltage, and third-phase voltage, controller 420 determines whether power conversion circuit 410 satisfies condition 2. If yes, power conversion circuit 410 is determined to satisfy the first condition. Otherwise, power conversion circuit 410 is determined not to satisfy the first condition.

[0079] The specific implementation methods of "the controller 420 determines whether the power conversion circuit 410 meets condition 2 based on the third instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage" mentioned above include one or more of the following: Method 1: Controller 420 compares the absolute values ​​of the third instantaneous values ​​of the first-phase voltage, the second-phase voltage, and the third-phase voltage with threshold 3. If at least one of these values ​​is greater than threshold 3, then the power conversion circuit 410 satisfies condition 2. Otherwise, it is determined that the power conversion circuit 410 does not satisfy condition 2.

[0080] Method 2: The controller 420 compares the absolute values ​​of the third instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage to determine the maximum value among the three. Then, it compares the determined maximum value with a threshold 3. If the maximum value is greater than the threshold 3, the power conversion circuit 410 is determined to satisfy condition 2. Otherwise, the power conversion circuit 410 is determined not to satisfy condition 2.

[0081] (3) The first condition includes condition 1 and condition 2. When the first condition includes both condition 1 and condition 2, the controller 420 determines that the power conversion circuit 410 satisfies the first condition if it detects that the power conversion circuit 410 satisfies at least one of condition 1 and condition 2. Conversely, if it detects that the power conversion circuit 410 does not satisfy either condition 1 or condition 2, it determines that the first condition is not satisfied. It is worth noting that the specific implementation methods for the controller 420 to determine whether the power conversion circuit 410 satisfies condition 1 and condition 2 can be found above, and will not be repeated here for brevity.

[0082] It is understood that when the connection between the power conversion circuit 410 and the power grid 200 is normal, the phase voltage output by the power conversion circuit 410 is controlled by the voltage of the power grid 200, so that the instantaneous value of each phase voltage output by the power conversion circuit 410 is stably maintained within a fixed range. However, when the connection between the power conversion circuit 410 and the power grid 200 is abnormal, the phase voltage of the disconnected phase is no longer controlled by the voltage of the power grid 200, and the instantaneous value of the phase voltage of the disconnected phase will fluctuate beyond the aforementioned fixed range (i.e., satisfying condition 2). Based on the above reasons, the controller 420 can detect the abnormal connection between the power conversion circuit 410 and the power grid 200 based on condition 2.

[0083] In addition, considering scenarios such as single-phase disconnection, two-phase disconnection, three-phase disconnection and unbalanced three-phase load (i.e., the difference in impedance values ​​of the loads 300 connected to each phase is greater than or equal to the preset difference), the instantaneous change in the phase voltage of the disconnected phase is relatively insignificant, while the change in the zero-sequence voltage is significant. Therefore, the controller 420 can combine the two (i.e., satisfy condition 1) to detect whether there is an abnormality in the connection between the power conversion circuit 410 and the power grid 200.

[0084] In summary, the first condition can accurately detect whether the connection between the power conversion circuit 410 and the power grid 200 is abnormal.

[0085] ② The controller 420 controls the power converter 400 to switch from grid-connected operation to overvoltage protection operation.

[0086] When the power conversion circuit 410 meets the first condition, it indicates an abnormal connection between the power conversion circuit 410 and the power grid 200. At this time, the state of the power converter 400 can be switched from grid-connected operation to overvoltage protection mode. Wherein: The grid-connected operation state refers to the working state of the power converter 400 when the connection between the power conversion circuit 410 and the power grid 200 is normal. In this state, the power conversion circuit 410 converts DC power into AC power with the same frequency and phase as the power grid 200 by orderly controlling the conduction and cutoff of the internal switching transistors, and together with the power grid 200, supplies power to the load 300.

[0087] The overvoltage protection state refers to the operating state of the power converter 400 when the connection between the power conversion circuit 410 and the power grid 200 is abnormal. In order to reduce the damage to the power converter 400 and the load 300 caused by the instantaneous high voltage of the disconnected phase output, the controller 420 can execute step ③ to protect the power converter 400 and the load 300.

[0088] ③ The controller 420 provides overvoltage protection for the power conversion circuit 410.

[0089] The controller 420 can implement overvoltage protection for the power conversion circuit 410 in any of the following ways: Method 1: Controller 420 controls power conversion circuit 410 to block the pulse signal. Blocking the pulse signal means cutting off the drive pulse signal of the internal switching transistor of power conversion circuit 410.

[0090] For example, the controller 420 can block the power conversion circuit 410 by controlling some or all of the switches in the power conversion circuit 410 to turn off. Specifically, when controlling some switches in the power conversion circuit 410 to turn off, the switches in the positive level group and the negative level group can be selectively turned off, while the switches in the zero level group are turned on. This allows the AC power generated by the power conversion circuit 410 to be fed back to the DC bus, thus preventing the phase voltage of the disconnected phase from continuing to rise. The switches in the positive level group are those used to output a positive level in the power conversion circuit 410, the switches in the negative level group are those used to output a negative level, and the switches in the zero level group are those used to output a zero level.

[0091] Method 2: The controller 420 first controls the power conversion circuit 410 to block the waveform, and then determines whether the power conversion circuit 410 meets the second condition. If it does, the controller controls the power converter 400 to shut down; if not, the controller controls the power conversion circuit 410 to continue blocking the waveform.

[0092] The second condition refers to the difference between the first frequency and the second frequency of the output voltage of the power conversion circuit 410 being greater than the threshold 4. The first frequency includes one or more frequencies obtained by sampling the frequency of the output voltage of the power conversion circuit 410 once or multiple times within time T3. The second frequency refers to the frequency of the output voltage of the power conversion circuit 410 during grid-connected operation of the power converter 400. It is worth noting that when the first frequency includes multiple frequencies obtained by sampling the frequency of the output voltage of the power conversion circuit 410 multiple times within time T3, the second frequency refers to the difference between these multiple frequencies and the second frequency being greater than the threshold 4.

[0093] Similar to the configuration of thresholds 1, 2, and time T1 described above, threshold 4 and time T3 can also be configured based on anomalies in the connection between the power conversion circuit 410 and the power grid 200, as well as the load 300. Specifically, the disconnected phase between the power conversion circuit 410 and the power grid 200, and the load 300 connected to the disconnected phase, are used as independent variables. The frequency of the output voltage of the power conversion circuit 410 and the withstand time of the load 300 connected to the disconnected phase are used as dependent variables. Multiple experimental tests are conducted to obtain multiple abnormal frequencies and multiple load withstand times. Threshold 4 can be set to the minimum value among the differences between the above multiple abnormal frequencies and the second frequency, and time T3 can be set to the minimum value among the above multiple load withstand times.

[0094] The above multiple experimental tests can also be based on Figure 6 The circuit implementation is shown below. For ease of understanding, the following seven experimental tests will be used as examples to introduce the configuration process of threshold 4 and time T3.

[0095] Experimental Test 1: During grid-connected operation of the power converter 400, the control device instructs the controller 420 to sample the frequency of the output voltage of the power conversion circuit 410 multiple times. After obtaining the multiple frequencies sampled from the controller 420, the control device filters these frequencies (e.g., calculates the average) and records the filtered frequency F.

[0096] Experiment 2: The control device instructs K1 to open, while K2 and K3 remain closed. Then, the control device instructs controller 410 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times, and instructs Ra to detect the load's withstand time p1. The control device obtains time t1 from Ra and the frequency f1 of the output voltage of the power conversion circuit 410 within time p1 from controller 420. Afterwards, the control device records time p1 and frequency f1.

[0097] Experiment 3: The control device instructs K1 and K2 to open simultaneously, while K3 remains closed. Then, the control device instructs controller 420 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times. It also instructs Ra to detect the load's withstand time p2 and Rb to detect the load's withstand time p3. The control device obtains time p2 from Ra, time p3 from Rb, and the frequency f2 of the power conversion circuit 410's output voltage within time p4 from controller 420, where time p4 is the maximum value of time p2 and time p3. Afterwards, the control device records time p2, time p3, and frequency f2.

[0098] Experiment 4: The control device instructs K1, K2, and K3 to be turned on simultaneously. Then, the control device instructs controller 420 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times. It also instructs Ra to detect the load's withstand time p5, Rb to detect the load's withstand time p6, and Rc to detect the load's withstand time p7. The control device obtains time p5 from Ra, time p6 from Rb, time p7 from Rc, and the frequency f3 of the power conversion circuit 410's output voltage within time p8 from controller 420, where time p8 is the maximum value among times p5, p6, and p7. Afterward, the control device records times p5, p6, p7, and frequency f3.

[0099] Experiment 5: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in parallel with the grid for a period of time at the new impedance value, the control device instructs K1 to open, while K2 and K3 remain closed. Subsequently, the control device instructs controller 420 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times, and instructs Ra to detect the load's withstand time p9. The control device obtains time p9 from Ra and the frequency f4 of the output voltage of the power conversion circuit 410 within time p9 from controller 420. Afterwards, the control device records time p9 and frequency f4.

[0100] Experiment 6: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in parallel with the grid for a period of time at the new impedance value, the control device instructs K1 and K2 to open simultaneously, while K3 remains closed. Then, the control device instructs controller 420 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times. It also instructs Ra to detect the load's withstand time p10 and instructs Rb to detect the load's withstand time p11. The control device obtains time p10 from Ra, time p11 from Rb, and the frequency f5 of the power conversion circuit 410's output voltage within time p12 from controller 420, where time p12 is the maximum value between time p10 and time p11. Afterwards, the control device records time p10, time p11, and frequency f5.

[0101] Experiment 7: The control device instructs Ra to adjust the impedance value. After the power converter 400 operates in parallel with the grid for a period of time at the new impedance value, the control device instructs K1 to open, while K2 and K3 remain closed. Then, the control device instructs controller 420 to turn off all switches in the power conversion circuit 410, and samples the frequency of the output voltage of the power conversion circuit 410 multiple times. It also instructs Ra to detect the load's withstand time p13, Rb to detect the load's withstand time p14, and Rc to detect the load's withstand time p15. The control device obtains time p13 from Ra, time p14 from Rb, time p15 from Rc, and the frequency f6 of the power conversion circuit 410's output voltage within time p16 from controller 420. Time p16 is the maximum value among times p13, p14, and p15. Afterwards, the control device records times p13, p14, p15, and the frequency f6.

[0102] Through the above 7 experimental tests, the following can be obtained: frequency F; abnormal frequencies f1, f2, f3, f4, f5, f6; and load tolerance times p1, p2, p3, p5, p6, p7, p9, p10, p11, p13, p14, p15. The control device calculates the difference between the above 6 abnormal frequencies and frequency F, compares the above 6 differences, and sets a threshold 4, where threshold 4 is the minimum value among the above 6 differences; it also compares the above multiple load tolerance times and sets a time T3, where time T3 is the minimum value among the above multiple load tolerance times.

[0103] It is understandable that, in addition to the seven experimental tests mentioned above, the control device can also make other changes to the disconnected phase between the power conversion circuit 410 and the power grid 200, as well as the load 300 connected to the disconnected phase, to conduct other experimental tests. For example, it can control K2 to open and K1 and K3 to close. Or, it can instruct Rb to change its impedance value and control K1 and K2 to open simultaneously. Or, it can instruct both Rb and Rc to change their impedance values ​​and control K2 and K3 to open simultaneously. Or, it can instruct Rb to change its impedance value and control K1, K2, and K3 to open simultaneously. Or, it can instruct both Ra and Rb to change their impedance values ​​and control K1, K2, and K3 to open simultaneously. Or, it can instruct Ra, Rb, and Rc to change their impedance values ​​and control K1, K2, and K3 to open simultaneously. The control device can also be configured to obtain the threshold 4 and time T3 through other experimental tests. The specific implementation idea is similar to the above content, and for the sake of simplicity, it will not be repeated here.

[0104] The controller 420 can determine whether the power conversion circuit 410 meets the second condition in the following way: The power conversion circuit 410 includes a phase-locked loop (PLL) circuit, which is used to output the frequency of the power conversion circuit 410's output voltage. When the power converter 400 is in grid-connected operation, the controller 420 obtains multiple frequencies of the power conversion circuit 410's output voltage through the PLL circuit, and then filters (e.g., averages) these frequencies to obtain the second frequency of the power conversion circuit 410's output voltage. After the power conversion circuit 410 is blocked, the controller 420 obtains the first frequency of the power conversion circuit 410's output voltage within time T3 through the PLL circuit. Then, the controller 420 calculates the difference between the first and second frequencies and compares the calculated value with a threshold 4. If the calculated value is greater than the threshold 4, the second condition is satisfied. Otherwise, the second condition is not satisfied.

[0105] ④ The controller 420 detects that the power conversion circuit 410 meets the third condition.

[0106] If the power conversion circuit 410 meets the third condition, the controller 420 can determine that the connection between the power conversion circuit 410 and the power grid 200 has been restored to normal. The third condition refers to the following: the absolute value of the second instantaneous value of each phase voltage output by the power conversion circuit 410 is less than threshold 5, the first effective value of each phase voltage output by the power conversion circuit 410 is less than threshold 6, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than threshold 7.

[0107] Next, we will further explain the content of the third condition.

[0108] The phase voltages output by the power conversion circuit 410 include the first phase voltage, the second phase voltage, and the third phase voltage. The second instantaneous value of the first phase voltage includes multiple instantaneous values ​​of the first phase voltage collected after time T3. The second instantaneous value of the second phase voltage includes multiple instantaneous values ​​of the second phase voltage collected after time T3. The second instantaneous value of the third phase voltage includes multiple instantaneous values ​​of the third phase voltage collected after time T3. Therefore, the third condition, "the absolute value of the second instantaneous value of each phase voltage output by the power converter 400 is less than the threshold 5," means that after time T3, the absolute values ​​of multiple instantaneous values ​​of each phase voltage are all less than the threshold 5.

[0109] The first effective value of the first phase voltage includes one or more effective values ​​calculated based on the second instantaneous value of the first phase voltage. The first effective value of the second phase voltage includes one or more effective values ​​calculated based on the second instantaneous value of the second phase voltage. The first effective value of the third phase voltage includes one or more effective values ​​calculated based on the second instantaneous value of the third phase voltage. When the first effective values ​​of the first, second, and third phase voltages respectively include multiple effective values ​​of the first, second, and third phase voltages, the condition "the absolute value of the first effective value of each phase voltage output by the power converter 400 is less than the threshold 6" means that after time T3, multiple effective values ​​of each phase voltage are less than the threshold 6.

[0110] The second instantaneous value of the zero-sequence voltage includes multiple instantaneous values ​​of the zero-sequence voltage calculated based on the second instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage. Therefore, the third condition, "the absolute value of the second instantaneous value of the zero-sequence voltage is less than the threshold 7", means that after time T3, the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage are all less than the threshold 7.

[0111] To make it easier to understand, let's take another example to explain further.

[0112] If, after time T3, the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage are sampled for one cycle, then n instantaneous values ​​of the first-phase voltage (hereinafter referred to as "n Ua3"), n instantaneous values ​​of the second-phase voltage (hereinafter referred to as "n Ub3"), and n instantaneous values ​​of the third-phase voltage (hereinafter referred to as "n Uc3") can be obtained. Then, the second instantaneous value of the first-phase voltage includes n Ua3, the second instantaneous value of the second-phase voltage includes n Ub3, and the second instantaneous value of the third-phase voltage includes n Uc3. The first effective value of the first-phase voltage is Ua4, which is obtained by performing a root mean square (RMS) calculation on the n Ua3. The first effective value of the second-phase voltage is Ub4, which is obtained by performing a RMS calculation on the n Ub3. The first effective value of the third-phase voltage is Uc4, which is obtained by performing a RMS calculation on the n Uc3. The third effective value of the zero-sequence voltage includes n Uz3 values, where each Uz3 is calculated by vector summation of Ua3, Ub3, and Uc3 acquired at the same time. In this case, the third condition is that the absolute values ​​of the n Ua3 values, the n Ub3 values, and the n Uc3 values ​​are all less than threshold 5; and Ua4, Ub4, and Uc4 are all less than threshold 6; and the absolute values ​​of the n Uz3 values ​​are all less than threshold 7.

[0113] If, after time T3, the instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage are sampled for two cycles, then 2n instantaneous values ​​of the first-phase voltage (hereinafter referred to as "2n Ua3"), 2n instantaneous values ​​of the second-phase voltage (hereinafter referred to as "2n Ub3"), and 2n instantaneous values ​​of the third-phase voltage (hereinafter referred to as "2n Uc3") can be obtained. Therefore, the second instantaneous value of the first-phase voltage includes 2n Ua3, the second instantaneous value of the second-phase voltage includes 2n Ub3, and the second instantaneous value of the third-phase voltage includes 2n Uc3. The first effective value of the first-phase voltage includes Ua5 and Ua6, where Ua5 is obtained by performing root mean square calculation on the n Ua3 samples collected in the first cycle, and Ua6 is obtained by performing root mean square calculation on the n Ua3 samples collected in the second cycle. The first effective value of the second phase voltage includes Ub5 and Ub6, where Ub5 is calculated by performing a root mean square (RMS) calculation on n Ub3 values ​​collected in the first cycle, and Ub6 is calculated by performing a RMS calculation on n Ub3 values ​​collected in the second cycle. The first effective value of the third phase voltage includes Uc5 and Uc6, where Uc5 is calculated by performing a RMS calculation on n Uc3 values ​​collected in the first cycle, and Uc6 is calculated by performing a RMS calculation on n Ub3 values ​​collected in the second cycle. The third effective value of the zero-sequence voltage includes 2n Uz3 values, where each Uz3 is calculated by performing a vector sum of Ua3, Ub3, and Uc3 values ​​collected at the same time. In this case, the third condition is: the absolute values ​​of 2n Ua3, 2n Ub3, and 2n Uc3 are all less than the threshold 5; and Ua5, Ub5, Uc5, Ua6, Ub6, and Uc6 are all less than the threshold 6; and the absolute values ​​of 2n Uz3 are all less than the threshold 7.

[0114] The aforementioned thresholds 5, 6, and 7 can be configured based on the instantaneous phase voltage, effective phase voltage, and instantaneous zero-sequence voltage output by the power conversion circuit 410 when the connection between the power conversion circuit 410 and the power grid 200 is normal. Specifically, they can be configured based on... Figure 6 The circuit shown was tested using the following experiments: The control device instructs K1, K2, and K3 to all be closed, and then instructs controller 420 to acquire multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage. After acquiring the acquired instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage from controller 420, the control device performs filtering processing (e.g., averaging) on ​​the multiple instantaneous values ​​of the first-phase voltage. Threshold 5 can be set to the value obtained after the filtering processing. The control device also performs root mean square calculation on the multiple instantaneous values ​​of the first-phase voltage to obtain multiple effective values ​​of the first-phase voltage, and then performs filtering processing (e.g., averaging) on ​​the multiple effective values ​​of the first-phase voltage. Threshold 6 can be set to the value obtained after the filtering processing. The control device also performs vector sum calculation on the multiple instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage to obtain multiple instantaneous values ​​of the zero-sequence voltage, and then performs filtering processing (e.g., averaging) on ​​the multiple instantaneous values ​​of the zero-sequence voltage. Threshold 7 can be set to the value obtained after the filtering processing.

[0115] It is understandable that, in addition to the above experimental tests, the control device can also configure threshold 5 and threshold 6 based on multiple instantaneous values ​​of the second phase voltage or the third phase voltage. The specific implementation idea is similar to the above content, and for the sake of brevity, it will not be repeated here.

[0116] The controller 420 can detect that the power conversion circuit 410 meets the third condition in the following way: After time T3, controller 420 acquires one or more instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage to obtain second instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage. Then, based on the second instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, respectively, controller 420 obtains the first effective values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage. Controller 420 also obtains the second instantaneous value of the zero-sequence voltage based on the second instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage. Subsequently, based on the second instantaneous values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, the first effective values ​​of the first-phase voltage, second-phase voltage, and third-phase voltage, and the second instantaneous value of the zero-sequence voltage, it detects that the power conversion circuit 410 satisfies the third condition.

[0117] The specific implementation of the above-mentioned "controller 420 detects that the power conversion circuit 410 meets the third condition based on the second instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage, the first effective values ​​of the first phase voltage, the second phase voltage, and the third phase voltage, and the second instantaneous value of the zero-sequence voltage" includes one or more of the following: Method 1: Controller 420 compares the absolute values ​​of the second instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage with threshold 5, respectively; compares the first effective values ​​of the first phase voltage, the second phase voltage, and the third phase voltage with threshold 6, respectively; and compares the absolute value of the second instantaneous value of the zero-sequence voltage with threshold 7. If the absolute values ​​of the second instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage are all less than threshold 5, and the first effective values ​​of the first phase voltage, the second phase voltage, and the third phase voltage are all less than threshold 6, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than threshold 7, controller 420 determines that the power conversion circuit 410 satisfies the third condition.

[0118] Method 2: Controller 420 compares the absolute values ​​of the second instantaneous values ​​of the first phase voltage, the second phase voltage, and the third phase voltage to determine the maximum value among the three (hereinafter referred to as the "maximum instantaneous value"), and then compares the determined maximum value with threshold 5. Controller 420 also compares the first effective values ​​of the first phase voltage, the second phase voltage, and the third phase voltage to determine the maximum value among the three (hereinafter referred to as the "maximum effective value"), and then compares the determined maximum value with threshold 6. Controller 420 also compares the absolute value of the second instantaneous value of the zero-sequence voltage with threshold 7. If the maximum instantaneous value is less than threshold 1, the maximum effective value is less than threshold 6, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than threshold 7, controller 420 determines that the power conversion circuit 410 satisfies the third condition.

[0119] It is worth noting that the effective value of phase voltage refers to the root mean square value of the instantaneous phase voltage over a complete cycle. When the connection between the power conversion circuit and the grid is normal, the effective value of each phase voltage of the power conversion circuit remains constant. Compared to the instantaneous value of the phase voltage, the change in the effective value of the phase voltage more accurately reflects the connection status between the power conversion circuit and the grid. When the absolute value of the second instantaneous value of each phase voltage output by the power conversion circuit is less than threshold 5, the effective value of each phase voltage is less than threshold 6, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than threshold 7, it indicates that the instantaneous values ​​of each phase voltage and the zero-sequence voltage have stabilized back within the normal range. This change is not random but is caused by the restoration of normal connection between the power conversion circuit and the grid. In other words, when the third condition is met, it indicates that the connection between the power conversion circuit and the grid has returned to normal.

[0120] ⑤ The controller 420 controls the power converter 400 to switch from the overvoltage protection state to the grid-connected operation state.

[0121] When the power conversion circuit 410 meets the third condition, it indicates that the connection between the power conversion circuit 410 and the power grid 200 has been restored to normal. At this time, the controller 420 can control the power converter 400 to re-enter the grid-connected operation state. After the power converter 400 re-enters the grid-connected operation state, the power conversion circuit 410 will orderly control the conduction and cutoff of the internal switching transistors, thereby converting the DC power from the DC side (such as the photovoltaic string 110 or the energy storage battery 130) into AC power with the same frequency and phase as the power grid 200, and jointly supplying power to the load 300 with the power grid 200.

[0122] The above text combined Figures 2 to 6 This application provides a detailed description of the power converter. This power converter can detect whether there are abnormal conditions such as single-phase disconnection, two-phase disconnection, or three-phase disconnection in its connection with the power grid. Furthermore, in the event of such abnormalities, the power converter can promptly control the turn-off of its internal switching transistors to prevent further voltage increases on the power converter and its load. Moreover, after the switching transistors are turned off, the power converter can also determine whether there is still a risk of damage to the power converter and its load based on the frequency change of the output voltage. If so, it will shut down directly to ensure the safety of the power converter and its load. In addition, the power converter can also detect whether its connection with the power grid has been restored to normal, and when it is restored, it will reconnect to the grid for operation.

[0123] This application also provides a method for overvoltage protection of a power converter. This method is applicable to the above-mentioned... Figures 2 to 4 The power converter shown has overvoltage protection. This method can be implemented by the controller in the power converter. Figure 7 As shown, the method includes the following steps: S101, The controller detected an abnormal connection between the power conversion circuit in the power converter and the power grid.

[0124] The power conversion circuit includes multiple switching transistors. By controlling the on and off states of these transistors, the power conversion circuit can convert direct current (DC) into alternating current (AC), which is then supplied to the power grid and the load through the first phase terminal, the second phase terminal, the third phase terminal, and the neutral terminal of the power conversion circuit. Here, the power conversion circuit can be the power conversion circuit 410 mentioned above, the power grid can be the power grid 200 mentioned above, and the load can be the load 300 mentioned above.

[0125] An abnormal connection between the power conversion circuit and the power grid refers to an abnormal connection between at least one phase of the power conversion circuit and the power grid. Specifically, it can be divided into three situations: single-phase disconnection, two-phase disconnection, and three-phase disconnection. A single-phase disconnection means that the connection between one phase of the power conversion circuit and the power grid is broken, while the other two phases remain connected. A two-phase disconnection means that the connection between two phases of the power conversion circuit and the power grid is broken, while the third phase remains connected. A three-phase disconnection means that the connection between all three phases of the power conversion circuit and the power grid is broken.

[0126] The controller can detect abnormal connections between the power conversion circuit and the power grid in the following ways: The controller determines the connection anomaly between the power conversion circuit and the power grid based on the instantaneous value of each phase voltage and / or the instantaneous value of the zero-sequence voltage in the three-phase voltage output by the power conversion circuit.

[0127] More specifically, the controller determines whether the power conversion circuit meets the first condition based on the instantaneous values ​​of each phase voltage and / or the zero-sequence voltage output by the power converter circuit. If the power conversion circuit meets the first condition, an anomaly is determined between the power conversion circuit and the power grid. The first condition and its specific implementation process can be found above. Figure 5 For the sake of brevity, the relevant introduction of ① will not be repeated here.

[0128] S102, The controller controls the switching transistor in the power conversion circuit to turn off.

[0129] For details on how to implement this step, please refer to the above text. Figure 5 For the sake of brevity, the relevant information regarding ③ will not be repeated here.

[0130] After the switching transistor in the power conversion circuit is turned off, the drive pulse signal of the switching transistor can be quickly blocked, stopping the energy conversion between DC and AC in the power converter and preventing the voltage of the power converter and its load from rising further.

[0131] While turning off the switching transistor in the power conversion circuit stops the power conversion circuit 410 from outputting AC power, it cannot cut off the energy input to the DC side of the power converter (such as the photovoltaic string 110 or the energy storage battery 130). Continuous energy input to the DC side will increase the voltage across the power converter and its load, potentially damaging them. To avoid this problem, the controller can also shut down the power converter. See the above for details on the implementation process. Figure 5 For the sake of brevity, the relevant information regarding ③ will not be repeated here.

[0132] In some feasible implementations, the controller can also periodically determine whether the connection between the power conversion circuit and the power grid has been restored to normal after the switching transistors in the power conversion circuit are turned off or the power converter is shut down, based on the instantaneous value of each phase voltage, the effective value of each phase voltage, and the instantaneous value of the zero-sequence voltage output by the power conversion circuit. If so, the controller controls the power converter to re-enter the grid-connected operation state. In this state, the power conversion circuit will regenerate AC power and supply it to the power grid and load through the first phase line terminal, the second phase line terminal, the third phase line terminal, and the neutral line terminal of the power conversion circuit. For details on the implementation process, please refer to the above. Figure 5 For the sake of brevity, the relevant information in section ④ will not be repeated here.

[0133] The above text combined Figure 7 This application details the overvoltage protection method for power converters provided in this application. This method can detect whether there are abnormal conditions such as single-phase disconnection, two-phase disconnection, or three-phase disconnection in the connection between the power converter and the power grid. Furthermore, in the event of such abnormalities, operations such as turning off the switching transistors and shutting down the power converter are performed to prevent the voltage across the power converter and its load from continuing to rise, thereby protecting the power converter and its load. In addition, this method can also detect whether the connection between the power converter and the power grid has been restored to normal, and when it is restored, the power converter is promptly reconnected to the grid.

[0134] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A power converter, characterized in that, The power converter includes a power conversion circuit and a controller, and the power conversion circuit includes multiple switching transistors; The power conversion circuit is used to control the on and off of the plurality of switching transistors to convert DC power into AC power, and provide it to the power grid and load through the first phase terminal, the second phase terminal, the third phase terminal and the neutral terminal of the power conversion circuit. The controller is configured to turn off all switches in the power conversion circuit when it detects that the connection between at least one phase terminal of the power conversion circuit and the power grid is broken.

2. The power converter according to claim 1, characterized in that, The controller is configured to turn off all switches in the power conversion circuit when the absolute value of the first instantaneous value of at least one phase voltage in the three-phase voltage output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold. The three-phase voltage includes a first-phase voltage, a second-phase voltage, and a third-phase voltage. The first-phase voltage is the voltage between the first-phase line terminal and the neutral line terminal. The second-phase voltage is the voltage between the second-phase line terminal and the neutral line terminal. The third-phase voltage is the voltage between the third-phase line terminal and the neutral line terminal. The zero-sequence voltage is the vector sum of the first-phase voltage, the second-phase voltage, and the third-phase voltage.

3. The power converter according to claim 2, characterized in that, The first instantaneous value of the first phase voltage includes multiple instantaneous values ​​of the first phase voltage collected within the first time period; the first instantaneous value of the second phase voltage includes multiple instantaneous values ​​of the second phase voltage collected within the first time period; the first instantaneous value of the third phase voltage includes multiple instantaneous values ​​of the third phase voltage collected within the first time period; and the first instantaneous value of the zero-sequence voltage includes multiple instantaneous values ​​of the zero-sequence voltage collected within the first time period. The controller is configured to turn off all switches in the power conversion circuit when it is detected that the absolute values ​​of multiple instantaneous values ​​of the at least one phase voltage within the first time period are all greater than a first threshold, and the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage within the first time period are all greater than a second threshold.

4. The power converter according to any one of claims 1 to 3, characterized in that, The controller is further configured to control the power converter to operate in grid-connected mode when the absolute value of the second instantaneous value of each phase voltage is less than a third threshold, the effective value of each phase voltage is less than a fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than a fifth threshold. The second instantaneous value of each phase voltage, the effective value of each phase voltage, and the second instantaneous value of the zero-sequence voltage are all acquired after all the switching transistors in the power conversion circuit are turned off.

5. The power converter according to any one of claims 1 to 4, characterized in that, The controller is also configured to control the power converter to shut down when the difference between the first frequency and the second frequency of the output voltage of the power conversion circuit is detected to be greater than a sixth threshold. The first frequency is obtained by sampling the frequency of the output voltage of the power conversion circuit after all the switching transistors in the power conversion circuit are turned off, and the second frequency is obtained by sampling the frequency of the output voltage of the power conversion circuit during the grid-connected operation of the power converter.

6. A method for overvoltage protection of a power converter, characterized in that, The power converter includes a power conversion circuit, which includes multiple switching transistors. The power conversion circuit controls the on and off of the multiple switching transistors to convert direct current into alternating current, and supplies it to the power grid and load through the first phase line terminal, the second phase line terminal, the third phase line terminal and the neutral line terminal of the power conversion circuit. The method includes: If it is detected that the connection between at least one phase terminal of the power conversion circuit and the power grid is broken, all switches in the power conversion circuit are controlled to turn off.

7. The method according to claim 6, characterized in that, The step of controlling all switches in the power conversion circuit to turn off when at least one phase terminal of the power conversion circuit is detected to be disconnected from the power grid includes: The absolute value of the first instantaneous value of at least one phase voltage in the three-phase voltage output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold. Control all switching transistors in the power conversion circuit to turn off; The three-phase voltage includes a first-phase voltage, a second-phase voltage, and a third-phase voltage. The first-phase voltage is the voltage between the first-phase line terminal and the neutral line terminal. The second-phase voltage is the voltage between the second-phase line terminal and the neutral line terminal. The third-phase voltage is the voltage between the third-phase line terminal and the neutral line terminal. The zero-sequence voltage is the vector sum of the first-phase voltage, the second-phase voltage, and the third-phase voltage.

8. The method according to claim 7, characterized in that, The first instantaneous value of the first phase voltage includes multiple instantaneous values ​​of the first phase voltage collected within the first time period; the first instantaneous value of the second phase voltage includes multiple instantaneous values ​​of the second phase voltage collected within the first time period; the first instantaneous value of the third phase voltage includes multiple instantaneous values ​​of the third phase voltage collected within the first time period; and the first instantaneous value of the zero-sequence voltage includes multiple instantaneous values ​​of the zero-sequence voltage collected within the first time period. The detection that the absolute value of the first instantaneous value of at least one phase voltage in the three-phase voltage output by the power conversion circuit is greater than a first threshold, and the absolute value of the first instantaneous value of the zero-sequence voltage is greater than a second threshold, includes: The absolute values ​​of multiple instantaneous values ​​of the at least one phase voltage within the first time period are all greater than a first threshold, and the absolute values ​​of multiple instantaneous values ​​of the zero-sequence voltage within the first time period are all greater than a second threshold.

9. The method according to any one of claims 6 to 8, characterized in that, After all the switches in the power conversion circuit are turned off, the method further includes: The absolute value of the second instantaneous value of each phase voltage is less than a third threshold, the effective value of each phase voltage is less than a fourth threshold, and the absolute value of the second instantaneous value of the zero-sequence voltage is less than a fifth threshold. Control the power converter to operate in grid-connected mode.

10. The method according to any one of claims 6 to 9, characterized in that, After all the switches in the power converter are turned off, the method further includes: The difference between a first frequency and a second frequency of the output voltage of the power converter is detected to be greater than a sixth threshold. The first frequency is obtained by sampling the frequency of the output voltage of the power converter circuit after all the switching transistors in the power converter circuit are turned off, and the second frequency is obtained by sampling the frequency of the output voltage of the power converter circuit during the grid-connected operation of the power converter. Control the power converter to shut down.

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