Photovoltaic inverter, photovoltaic power generation system and method for controlling photovoltaic power generation system

[0001] The present application claims priority to China Patent Application No. 202410233733.3, titled “PHOTOVOLTAIC INVERTER, PHOTOVOLTAIC POWER GENERATION SYSTEM AND METHOD FOR CONTROLLING PHOTOVOLTAIC POWER GENERATION SYSTEM”, filed on Feb. 29, 2024 with the China National Intellectual Property Administration, which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates to the technical field of power electronics, and in particular to a photovoltaic inverter, a photovoltaic power generation system and a method for controlling the photovoltaic power generation system.

[0003] In the process of photovoltaic power generation, a photovoltaic inverter converts energy outputted by a photovoltaic sub-array connected to the photovoltaic inverter into a sine-wave current with a same frequency and a same phase as a power grid, and feeds the sine-wave current into the power grid. In this process, a potential induced degradation (PID) effect is generated on a photovoltaic panel due to the characteristics of the photovoltaic panel, leading to a gradual deterioration of the operation performance of the photovoltaic panel, eventually resulting in the loss of power generation capacity. Therefore, the PID effect is to be suppressed during operation of the photovoltaic inverter.

[0004] In the conventional technology, an anti-PID circuit is usually provided or a DC side is directly connected to ground, to implement an anti-PID operation, boosting a voltage of a negative electrode of the photovoltaic module in the photovoltaic sub-array relative to the ground, so as to suppress the PID effect of the photovoltaic panel.

[0005] However, in the process of the anti-PID operation of the photovoltaic inverter, an insulation impedance of the system to ground fails to be detected due to the existence of a grounding loop. As a result, insulation detection conflicts with and is incompatible with the anti-PID operation.

[0006] In view of this, a photovoltaic inverter, a photovoltaic power generation system and a method for controlling the photovoltaic power generation system are provided, to achieve the compatibility between insulation detection and an anti-PID operation, and to save costs.

[0007] In order to achieve the foregoing objective, the following technical solutions are provided according to the present disclosure.

[0008] In a first aspect, a method for controlling a photovoltaic power generation system is provided according to the present disclosure. In the photovoltaic power generation system, an anti-potential induced degradation (anti-PID) power supply is arranged between ground and a direct-current (DC) bus of a photovoltaic inverter, at least one detection resistor connected in series with the anti-PID power supply is arranged between at least one side of the photovoltaic inverter and the ground, and the anti-PID power supply and the detection resistor are provided with respective switches. The method includes:

• • [0009] controlling only the anti-PID power supply to operate during an anti-PID operation;
  • [0010] controlling both the anti-PID power supply and a target detection resistor of the detection resistor to operate during insulation detection;
  • [0011] obtaining voltages to the ground of a position of the photovoltaic inverter to which the target detection resistor is connected under different conditions by controlling the anti-PID power supply; and
  • [0012] calculating an insulation impedance to the ground of the photovoltaic inverter based on the obtained voltages, a voltage outputted by the anti-PID power supply and a resistance value of the target detection resistor.

[0013] In a second aspect, a photovoltaic inverter is further provided according to the present disclosure, which includes an inverter circuit, an anti-potential induced degradation (anti-PID) circuit, a voltage detection module, and a controller; where

• • [0014] a direct-current (DC) side of the inverter circuit is connected to a DC bus, and an alternating-current (AC) side of the inverter circuit is configured to connect a power grid and/or a load;
  • [0015] the anti-PID circuit includes an anti-PID power supply and at least one detection resistor connected in series with the anti-PID power supply, and the anti-PID power supply and the detection resistor are provided with respective switches;
  • [0016] the anti-PID power supply is arranged between the DC bus and ground, and the at least one detection resistor is arranged between at least one side of the photovoltaic inverter and the ground;
  • [0017] the inverter circuit and the respective switches are controlled by the controller;
  • [0018] the voltage detection module is configured to detect a voltage to the ground of a position of the photovoltaic inverter to which a target detection resistor of the at least one detection resistor is connected and a voltage outputted by the anti-PID power supply, and output the detected voltages to the controller; and
  • [0019] the controller is configured to perform the method for controlling the photovoltaic power generation system according to any one of the first aspect.

[0020] In a third aspect, a photovoltaic power generation system is provided according to the present disclosure, which includes a photovoltaic array and at least one photovoltaic inverter connected to the photovoltaic array; and the photovoltaic inverter is the photovoltaic inverter according to any one of the second aspect described above.

[0031] The technical solutions according to the embodiments of the present disclosure are described clearly and completely as follows in conjunction with the drawings in the embodiments of the present disclosure. It is apparent that the described embodiments are only some of the embodiments according to the present disclosure, rather than all the embodiments. Any other embodiments obtained by those skilled in the art based on the embodiments of the present disclosure without any creative work fall within the protection scope of the present disclosure.

[0032] The terms “include”, “comprise” or any other variants thereof are intended to be non-exclusive. Therefore, a process, method, article or device including a series of elements includes not only these elements but also other elements that are not clearly enumerated, or further includes elements inherent in the process, method, article or device. Unless expressively limited, the statement “including a . . . ” does not exclude the case that other similar elements may exist in the process, method, article or device including the series of elements.

[0033] A method for controlling a photovoltaic power generation system is provided according to the present disclosure, to achieve the compatibility between insulation detection and the anti-PID operation, and to save costs.

[0034] In the photovoltaic power generation system, an anti-PID power supply is arranged between a DC bus of a photovoltaic inverter and the ground. In practical applications, the anti-PID power supply is arranged between a negative electrode of the DC bus and the ground, or arranged between a positive electrode of the DC bus and the ground, depending on requirements of the application scenario. Hereinafter, the anti-PID power supply is arranged between the negative electrode of the DC bus and the ground.

[0035] Moreover, a detection resistor connected in series with the anti-PID power supply is further arranged between the ground and at least one side of the photovoltaic inverter, that is, at least one of a DC side and an AC side of the photovoltaic inverter. As shown in FIG. 1, the negative electrode of the DC bus is connected to the ground through an anti-PID power supply V_PID and a detection resistor R₀₁ connected in series. In an embodiment, as shown in FIG. 2, the negative electrode of the DC bus is connected to the ground through the anti-PID power supply V_PID, and one phase of the AC side of the photovoltaic inverter is connected to the ground through the anti-PID power supply V_PID and a detection resistor R₀₂ connected in series. In an embodiment, as shown in FIG. 3, the negative electrode of the DC bus is connected to the ground through the anti-PID power supply V_PID and the detection resistor R₀₁ connected in series, and one phase of the AC side of the photovoltaic inverter is connected to the ground through the anti-PID power supply V_PID and the detection resistor R₀₂ connected in series. Moreover, the anti-PID power supply V_PID, the detection resistor R₀₁ and the detection resistor R₀₂ are provided with respective switches. As shown in FIG. 1, the anti-PID power supply V_PID is provided with the switch K₁, and the detection resistor R₀₁ is provided with the switch K₂. Alternatively, as shown in FIG. 2, the respective switches include switches K₁, K₂ and K₄, the anti-PID power supply V_PID operates only when K₁ and K₂ are closed, and both the anti-PID power supply V_PID and the detection resistor R₀₂ operate only when K₁ and K₄ are closed. Alternatively, as shown in FIG. 3, the respective switches include switches K₁ to K₄, the anti-PID power supply V_PID operates only when K₁ and K₂ are closed, both the anti-PID power supply V_PID and the detection resistor R₀₁ operate only when K₁ and K₃ are closed, and the anti-PID power supply V_PID and the detection resistor R₀₂ operate only when K₁ and K₄ are closed.

[0036] As shown in FIG. 4, the method for controlling the photovoltaic power generation system includes S101 during the anti-PID operation, and S102, S103, and S104 during the insulation detection.

[0037] In practical applications, a controller of the photovoltaic inverter may determine whether to perform the anti-PID operation or the insulation detection based on a current requirement, such as a received instruction. In an embodiment, during the anti-PID operation, the controller determines to perform the insulation detection in response to receiving an instruction for insulation detection, and the method proceeds from S101 to S102, and then continues to proceed S103 and S104.

[0038] In S101, only the anti-PID power supply is controlled to operate.

[0039] For the structure shown in FIG. 1, a branch where the anti-PID power supply V_PID is located is connected between the negative electrode of the DC bus and the ground by closing the switch K₁, and the detection resistor R₀₁ is bypassed by closing the switch K₂, thus being disconnected from the branch, such that only the anti-PID power supply V_PID operates.

[0040] For the structure shown in FIG. 2, the branch where the anti-PID power supply V_PID is located is connected between the negative electrode of the DC bus and the ground by closing the switches K₁ and K₂, and the detection resistor R₀₂ is disconnected from the branch by opening the switch K₄, such that only the anti-PID power supply V_PID operates.

[0041] For the structure shown in FIG. 3, the branch where the anti-PID power supply V_PID is located is connected between the negative electrode of the DC bus and the ground by closing the switches K₁ and K₂, the detection resistor R₀₁ is disconnected from the branch by opening the switch K₃, and the detection resistor R₀₂ is disconnected from the branch by opening the switch K₄, such that only the anti-PID power supply V_PID operates.

[0042] When only the anti-PID power supply V_PID operates, the negative voltage of the DC bus in FIG. 1, FIG. 2, or FIG. 3 is boosted, thus suppressing the PID effect.

[0043] In S102, both the anti-PID power supply and a target detection resistor are controlled to operate.

[0044] The target detection resistor may refer to the detection resistor R₀₁ in FIG. 1 or FIG. 3, or the detection resistor R₀₂ in FIG. 2 or FIG. 3, thereby detecting different insulation impedances to the ground of the photovoltaic inverter in the photovoltaic power generation system, which is not limited herein, depending on the requirements of the application process.

[0045] For the structure in FIG. 1, a branch formed by the anti-PID power supply V_PID and the detection resistor R₀₁ is connected between the negative electrode of the DC bus and the ground by closing the switch K₁ and opening the switch K₂. That is, the target detection resistor refers to the detection resistor R₀₁ in FIG. 1.

[0046] For the structure in FIG. 2, a branch formed by the anti-PID power supply V_PID and the detection resistor R₀₂ is connected between a corresponding phase of the AC side and the ground by closing the switches K₁ and K₄, and opening the switch K₂. That is, the target detection resistor refers to the detection resistor R₀₂ in FIG. 2.

[0047] For the structure in FIG. 3, a branch formed by the anti-PID power supply V_PID and the detection resistor R₀₁ is connected between the negative electrode of the DC bus and the ground by closing the switches K₁ and K₃ and opening the switches K₂ and K₄, that is, the target detection resistor refers to the detection resistor R₀₁ in FIG. 3. Alternatively, a branch formed by the anti-PID power supply V_PID and the detection resistor R₀₂ is connected between a corresponding phase of the AC side and the ground by closing the switches K₁ and K₄ and opening the switches K₂ and K₃, that is, the target detection resistor refers to the detection resistor R₀₂ in FIG. 3.

[0048] In S103, voltages to the ground of a position of the photovoltaic inverter to which the target detection resistor is connected under different conditions are obtained by controlling the anti-PID power supply.

[0049] When the target detection resistor is a detection resistor arranged between the DC side of the photovoltaic inverter and the ground, a voltage of the position of the inverter to which the target detection resistor is connected refers to a voltage of the DC bus to the ground. When only the switch K₁ is closed in FIG. 1, or only the switches K₁ and K₃ are closed in FIG. 3, the target detection resistor is a detection resistor arranged between the negative electrode of the DC side of the photovoltaic inverter and the ground, that is, the detection resistor R₀₁ as shown in FIG. 1 and FIG. 3. The position of the inverter to which the target detection resistor is connected may refer to the negative electrode of the DC bus. Therefore, the voltage to the ground of the position of the inverter to which the target detection resistor is connected is a voltage to the ground of the negative electrode of the DC bus of the photovoltaic inverter. In such case, the process of obtaining the voltages to the ground of the position of the inverter to which the target detection resistor is connected under different conditions includes obtaining voltages to the ground of the negative electrode of the DC bus under different conditions. In practical application, in a case that a preceding stage of an inverter circuit of the photovoltaic inverter is provided with at least one DC/DC conversion circuit, and a side of the at least one DC/DC conversion circuit connected to a photovoltaic module and the DC bus are in common negative connection, the target detection resistor is the detection resistor arranged between the negative electrode of the DC side of the photovoltaic inverter and the ground, and the position of the inverter to which the target detection resistor is connected is a negative electrode of the side of one of the at least one DC/DC conversion circuit connected to the photovoltaic module. In such case, voltages of positions in common negative connection are equal to each other, and thus the voltage to the ground of the position of the inverter to which the target detection resistor is connected is still the voltage to the ground of the negative electrode of the DC bus of the photovoltaic inverter.

[0050] When only the switches K₁ and K₄ are closed in FIG. 2 or FIG. 3, the target detection resistor refers to the detection resistor arranged between one phase of the AC side of the photovoltaic inverter and the ground, that is, the detection resistor R₀₂ as shown in FIG. 2 and FIG. 3. The position of the inverter to which the target detection resistor is connected refers to the phase of the AC side of the photovoltaic inverter. Therefore, the voltage to the ground of the position of the inverter to which the target detection resistor is connected is the voltage to the ground of the phase of the AC side of the photovoltaic inverter. In such case, the process of obtaining the voltages to the ground of the position of the inverter to which the target detection resistor is connected under different conditions includes obtaining voltages to the ground of the phase of the AC side of the photovoltaic inverter under different conditions.

[0051] The anti-PID power supply is controlled to obtain different conditions in the following two manners. In an embodiment, the anti-PID power supply V_PID in each figure is controlled to output different voltages, a voltage detection module detects the voltages to the ground of the position of the inverter to which the target detection resistor is connected under different output voltages and transmits detected voltages to the controller, such that the controller can obtain the voltages to the ground of the position of the inverter to which the target detection resistor is connected under different conditions. In another embodiment, the anti-PID power supply V_PID in each figure is controlled to output a voltage, and the anti-PID power supply V_PID is disconnected from the photovoltaic inverter. In other words, in practical application, only the anti-PID power supply is controlled to output different voltages, or both the voltage outputted by the anti-PID power supply and the switch are controlled, which is not limited here, depending on the application environment.

[0052] In S104, an insulation impedance to the ground of the photovoltaic inverter is calculated based on the obtained voltages, a voltage outputted by the anti-PID power supply and a resistance value of the target detection resistor.

[0053] Taking the structure in FIG. 1 as an example, a resistance value of the detection resistor R₀₁ in FIG. 1 is known, therefore, equations are listed based on the voltages to the ground of the negative electrode of the DC bus and the voltages outputted by the anti-PID power supply V_PID under different conditions, an equation set is formed by simultaneous equations under different conditions, and the insulation impedance to the ground of the photovoltaic inverter is calculated by eliminating other unknown parameters. The process of listing the equations and the calculation is referred to the following descriptions.

[0054] With the method according to the embodiments of the present disclosure, during the anti-PID operation, only the anti-PID power supply is controlled to operate, and a voltage of a DC bus is boosted by using the anti-PID power supply, so as to suppress the PID effect. During the insulation detection, both the anti-PID power supply and the target detection resistor are controlled to operate, voltages to the ground of a position of the inverter to which the target detection resistor is connected under different conditions are obtained by controlling the anti-PID power supply, such as the voltage to the ground of the negative electrode of the DC bus of the photovoltaic inverter as shown in FIG. 1, so as to perform the insulation detection by using the anti-PID power supply. Then, the insulation impedance to the ground of the photovoltaic inverter can be calculated based on the obtained voltages, the voltage outputted by the anti-PID power supply and the resistance value of the target detection resistor. In other words, the present disclosure can solve the problem of the conflict between the insulation detection and the anti-PID operation in the conventional technology, so as to achieve the compatibility between the insulation detection and the anti-PID operation.

[0055] Moreover, the anti-PID power supply and the target detection resistor are controlled to operate, thus selectively performing the insulation detection or the anti-PID operation in the embodiments of the present disclosure, so that an insulation detection device in the conventional technology is not arranged, saving the hardware cost.

[0056] On the basis of the previous embodiment, in an embodiment, in the method, S103 may include: controlling the anti-PID power supply to be in two states, and obtaining the voltages to the ground of the position of the inverter to which the target detection resistor is connected in the two states. There may be multiple implementation manners for controlling the anti-PID power supply to be in the two states.

[0057] In an implementation manner, the process of controlling the anti-PID power supply to be in two states includes: controlling the anti-PID power supply to output different test voltages.

[0058] In this case, the S103 includes: controlling the anti-PID power supply to output two test voltages, and obtaining the voltage to the ground of a position of the inverter to which the target detection resistor is connected corresponding to each of the two test voltages. For example, as shown in FIG. 1, the voltage to the ground of the negative electrode of the DC bus corresponding to each test voltage is obtained. Alternatively, as shown in FIG. 2, the voltage to the ground of the corresponding phase of the AC side corresponding to each test voltage is obtained. Alternatively, as shown in FIG. 3, the voltage to the ground of the negative electrode of the DC bus corresponding to each test voltage is obtained, or the voltage to ground of the corresponding phase of the AC side corresponding to each test voltage is obtained.

[0059] In practical application, the test voltages differ in at least one of absolute values and directions. For example, the test voltages are two positive voltages with different absolute values, or two negative voltages with different absolute values, or a positive voltage and a negative voltage. In a case that the test voltages are a positive voltage and a negative voltage, the absolute values of the two voltages are the same or different from each other, depending on the actual application environment. All implementations fall within the protection scope of the present disclosure.

[0060] In an embodiment, a power supply being capable of outputting both a positive voltage and a negative voltage serves as the anti-PID power supply. Alternatively, a power supply being capable of only outputting a positive voltage serves as the anti-PID power supply, and the switches are arranged to provide a positive output branch and a negative output branch for the anti-PID power supply, in order to save cost. Taking the structure in FIG. 1 as an example, referring to FIG. 5, the switches S₁ to S₅ are provided. When the switches S₁ and S₂ are closed, and the switches S₄ and S₅ are opened, the anti-PID power supply V_PID outputs a positive voltage to the negative electrode PV− of the DC bus. When the switches S₁ and S₂ are opened, and the switches S₄ and S₅ are closed, the anti-PID power supply V_PID outputs a negative voltage to the negative electrode PV− of the DC bus.

[0061] As shown in FIG. 5, the switches S₁, S₂, and S₃ are closed during performing the anti-PID operation. In response to receiving the instruction for insulation detection, the switch S₃ is first opened, the anti-PID power supply V_PID is switched to a constant voltage mode and outputs a constant test voltage V₁. After the voltage to the ground of the negative electrode PV− of the DC bus (that is, the voltage to the ground of the negative electrode of the DC bus described above) is stable, the voltage to the ground of the negative electrode of the DC bus is recorded as V_pv1. Then, the anti-PID power supply V_PID is switched to output a test voltage V₂, and the voltage to the ground of the negative electrode of the DC bus is recorded as V_pv2 after the voltage to the ground of the negative electrode PV− of the DC bus is stable again. In such case, an equivalent circuit diagram for detection is shown in FIG. 6.

[0062] In FIG. 6, R_PV− represents an impedance to the ground of the negative electrode PV− of the DC bus of the photovoltaic inverter. R₁ represents an impedance to the ground of the positive electrode PV+ of the DC bus in a case that the photovoltaic inverter is in an off-grid state, while R₁ represents a coupling impedance of the positive electrode PV+ of the DC bus and the AC side in a case that the photovoltaic inverter is in a grid-connected state. Hereinafter, R_S represents an impedance of R₁ and R_PV− connected in parallel, that is, the insulation impedance to the ground of the photovoltaic inverter, which may be referred to as a system impedance. A constant current source I represents an influence of other loops on a detected loop of the negative electrode PV− of the DC bus, which is a dynamic variable that changes with the change of the system impedance R_S, and is a positive value or a negative value.

[0063] When the anti-PID power supply V_PID outputs the test voltage V₁, the following equation is obtained as:

[0064] $V_{\mathrm{pv}1}=\left(I+\frac{V_1-V_{\mathrm{pv}1}}{R_{01}}\right)R_S$

[0065] When the anti-PID power supply V_PID outputs the test voltage V₂, the following equation is obtained as:

[0066] $V_{\mathrm{pv}2}=\left(I+\frac{V_2-V_{\mathrm{pv}2}}{R_{01}}\right)R_S$

[0067] A linear equation in two unknowns is formed by the above two simultaneous equations, and the system impedance R_S is calculated as:

[0068] $R_S=\frac{V_{\mathrm{pv}1}-V_{\mathrm{pv}2}}{\left(V_1-V_{\mathrm{pv}1}\right)-\left(V_2-V_{\mathrm{pv}2}\right)}{R}_{01}$

[0069] In another implementation manner, the process of controlling the anti-PID power supply to be in two states includes: controlling the anti-PID power supply to output different test voltages, such as V₁ and V₂. In an embodiment, in one of the two states, the anti-PID power supply outputs one of the test voltages (such as V₂) that is the same as the voltage to the ground of the position of the inverter to which the target detection resistor is connected, such as the voltage to the ground of the negative electrode of the DC bus in FIG. 1. The method further includes: disconnecting the target detection resistor from the photovoltaic inverter in this state. Taking the structure in FIG. 1 as an example, the target detection resistor is disconnected from the photovoltaic inverter by opening the switch K₁ in FIG. 1 or the switch S₃ in FIG. 5. In the other state, the anti-PID power supply outputs the other test voltage (such as V₁) that is different from the voltage to the ground of the position of the inverter to which the target detection resistor is connected (such as the voltage to the ground of the negative electrode of the DC bus in FIG. 1).

[0070] When the anti-PID power supply V_PID outputs the test voltage V₁, the following equation is obtained as:

[0071] $V_{\mathrm{pv}1}=\left(I+\frac{V_1-V_{\mathrm{pv}1}}{R_{01}}\right)R_S$

[0072] When the anti-PID power supply V_PID outputs the test voltage V₂, a potential of the anti-PID power supply V_PID is equal to a potential of the negative electrode PV− of the DC bus, that is:

[0073] $V_{\mathrm{pv}2}={\mathrm{IR}}_S$

[0074] A linear equation in two unknowns is formed by the above two simultaneous equations, and the system impedance R_S is calculated as:

[0075] $R_S=\frac{V_{\mathrm{pv}1}-V_{\mathrm{pv}2}}{V_1-V_{\mathrm{pv}1}}{R}_{01}$

[0076] In another implementation manner, the process of controlling the anti-PID power supply to be in two states includes: controlling the anti-PID power supply to output a test voltage, and disconnecting the anti-PID power supply from the photovoltaic inverter. That is, the process of outputting the test voltage V₂ in the above two implementation manners is changed to disconnecting the anti-PID power supply from the photovoltaic inverter.

[0077] When the anti-PID power supply V_PID outputs the test voltage V₁, the following equation is obtained as:

[0078] $V_{\mathrm{pv}1}=\left(I+\frac{V_1-V_{\mathrm{pv}1}}{R_{01}}\right)R_S$

[0079] When the anti-PID power supply V_PID is disconnected from the photovoltaic inverter, only the current of the constant current source I flows through the system impedance R_S,

[0080] $V_{\mathrm{pv}2}={\mathrm{IR}}_S$

[0081] A linear equation in two unknowns is formed by the above two simultaneous equations, and the system impedance R_S is calculated as:

[0082] $R_S=\frac{V_{\mathrm{pv}1}-V_{\mathrm{pv}2}}{V_1-V_{\mathrm{pv}1}}{R}_{01}$

[0083] The situations where the target detection resistor is R₀₂ in the structures shown in FIG. 2 and FIG. 3 may be deduced in the same way, which are not repeated herein.

[0084] All the above implementation manners are examples for S103, but not limited to this. Other manners can be used in practical applications, as long as the above system impedance R_S can be solved, the manners fall within the protection scope of the present disclosure.

[0085] It is worth noting that in the conventional technology, for the photovoltaic power generation system in FIG. 7, the DC side of each photovoltaic inverter is provided with the corresponding anti-PID power supply V_PID for the anti-PID operation, and the AC side of the photovoltaic inverter is further provided with an insulation monitoring device (IMD) to achieve the insulation impedance detection of the AC side. Moreover, according to the requirements of an IEC standard, AC insulation detection is performed on the system for 24 hours. However, the anti-PID operation results in an existence of a grounding loop, and the AC insulation detection on the system fails to be performed all day due to the grounding loop.

[0086] In embodiments of the present disclosure, the equivalent resistance R₁ in FIG. 6 represents the impedance to the ground of the positive electrode PV+ of the DC bus in a case that the photovoltaic inverter is in the off-grid state, while R₁ represents the coupling impedance of the positive electrode PV+ of the DC bus and the AC side in a case that the photovoltaic inverter is in the grid-connected state. Therefore, in a case that the target detection resistor is the detection resistor R₀₁ shown in each figure, the system impedance R_S calculated in the above implementation manners represents the insulation impedance to the ground of the DC side of the photovoltaic inverter in the off-grid state, and R_S represents an insulation impedance to the ground of the DC side and the AC side of the photovoltaic inverter in the grid-connected state. In other words, with the method, the insulation impedance to the ground of the DC side during an off-grid operation may be detected, and the insulation impedance to the ground of the DC side and the AC side during a grid-connected operation may be detected. Moreover, regardless of the off-grid operation or the grid-connected operation, the photovoltaic inverter can perform the anti-PID operation as required. Therefore, for the above photovoltaic power generation system, each photovoltaic inverter can achieve the compatibility of the anti-PID operation and the AC insulation detection during the grid-connected operation, thereby solving the problem of failing to perform insulation detection all day in the conventional technology. Moreover, in the embodiments of the present disclosure, only the anti-PID power supply is changed without changing the existing photovoltaic inverter, that is, the detection resistors and the respective switches are arranged to achieve the compatibility of the insulation detection and the anti-PID operation without arranging the IMD for the AC insulation detection described above, and a Y-type resistor bridge for DC insulation detection in the conventional technology is not arranged. Therefore, according to the embodiments of the present disclosure, the cost is low, implementation is convenient, and the method is suitable for all inverters without distinguishing between models.

[0087] In addition, in a case that the target detection resistor is the detection resistor R₀₂ shown in the corresponding figure, the detailed process of S103 and S104 in the method is the same as above process, except that the voltage to the ground of the position of the inverter to which the target detection resistor is connected obtained in S103 is the voltage to the ground of the corresponding phase of the AC side of the photovoltaic inverter, instead of the voltage to the ground of the DC bus (such as the voltage to the ground of the negative electrode mentioned above). In this case, the insulation impedance to the ground calculated in S104 is the insulation impedance to the ground of the AC side of the photovoltaic inverter. The solution can also achieve the compatibility of the anti-PID operation and AC insulation detection during the grid-connected operation, and has the advantages of a low cost, a convenient implementation and is suitable for all inverters without distinguishing between models.

[0088] It should also be noted that during the process of S103, an off-grid/grid-connected state of the photovoltaic inverter if the anti-PID power supply is in one of the two states is different from an off-grid/grid-connected state of the photovoltaic inverter if the anti-PID power supply is in the other state, the system impedance R_S under the two states changes, which is reflected in a change of the dynamic variable, i.e., the constant current source I involved in the above multiple implementation manners. Therefore, in order to ensure the accuracy of the detection process, the constant current source I in the two equations are always equal to each other in various implementation manners to ensure that the two equations can be solved simultaneously.

[0089] Therefore, the process of controlling the anti-PID power supply to be in two states, and obtaining the voltages to the ground of the position of the photovoltaic inverter to which the target detection resistor is connected in the two states includes: switching the anti-PID power supply between the two states at least twice, and obtaining the voltage to the ground of the position of the inverter to which the target detection resistor is connected after each switching until a difference between voltages obtained in one of the two states during successive two times of switching meets a predetermined difference requirement. The predetermined difference requirement indicates that the corresponding two voltages are the same or close, that is, a difference between the two voltages is within a range. The voltage to the ground of the position of the inverter to which the target detection resistor is connected may be the voltage to the ground of the negative electrode of the DC bus when the detection resistor R₀₁ is controlled to operate in FIG. 1 or FIG. 2, or the voltage to the ground of the corresponding phase of the AC side when the detection resistor R₀₂ is controlled to operate in FIG. 2 or FIG. 3.

[0090] Taking the controlling the detection resistor R₀₁ to operate in FIG. 1 or FIG. 2 in conjunction with the above implementation manner of S103 as an example, the anti-PID power supply is switched between outputting the test voltage V₁ and outputting the test voltage V₂ at least twice. For example, the anti-PID power supply is first controlled to output the test voltage V₁, then the anti-PID power supply is controlled to output the test voltage V₂, and next the anti-PID power supply is controlled to output the test voltage V₁. If the voltage V_pv1 to the ground of the negative electrode of the DC bus corresponding to the second test voltage V₁ is different from the voltage V_pv1 to the ground of the negative electrode of the DC bus corresponding to the first test voltage V₁, it indicates that the constant current source I changes when the anti-PID power supply is switched between outputting the two test voltages, that is, switching between the off-grid state and the grid-connected state occurs. In this case, the two equations cannot be solved simultaneously to calculate the system impedance R_S. While if the voltage V_pv1 to the ground of the negative electrode of the DC bus corresponding to the second test voltage V₁ is equal to the voltage V_pv1 to the ground of the negative electrode of the DC bus corresponding to the first test voltage V₁, it indicates that the constant current source I does not change when the anti-PID power supply is switched between outputting the two test voltages, that is, no switching between the off-grid state and the grid-connected state occurs. In this case, the two equations are solved simultaneously to calculate the system impedance R_S.

[0091] The above process can ensure that the voltages are detected in two adjacent times on condition of constant system impedance R_S, which can improve the detection accuracy. In an embodiment, only the anti-PID power supply is switched from one state to another state, that is, state switching once, and the voltages to the ground of the position of the inverter to which the target detection resistor is connected are obtained, which also fall within the protection scope of the present disclosure, depending on the actual application.

[0092] A photovoltaic inverter is further provided according to another embodiment of the present disclosure. As shown in FIG. 1 to FIG. 3, the photovoltaic inverter includes an inverter circuit 10, an anti-PID circuit 20, a voltage detection module (not shown), and a controller (not shown).

[0093] An AC side of the inverter circuit 10 is configured to connect a power grid and/or a load. In a grid-connected state, the AC side of the inverter circuit 10 is connected to the power grid, and is connected to or not connected to the load. In an off-grid state, the AC side of the inverter circuit 10 is disconnected from the power grid and is only connected to the load.

[0094] A DC side of the inverter circuit 10 is connected to the DC bus. In practical application, the DC bus may be directly connected to a corresponding photovoltaic module, such as at least one photovoltaic string (as shown in FIG. 1 to FIG. 3), or may be connected to the corresponding photovoltaic module through at least one DC/DC conversion circuit, such as, each DC/DC conversion circuit is connected to at least one photovoltaic string. In this case, one side of each DC/DC conversion circuit is connected to the corresponding photovoltaic string, and the other side of each DC/DC conversion circuit is connected to the DC bus.

[0095] A resistor is connected by a dotted line between the negative electrode of the DC bus and the ground, which represents the insulation resistance to the ground of the negative electrode at the DC side of the photovoltaic inverter, that is, R_PV− in FIG. 6. A resistor is connected by a dotted line between the positive electrode of the DC bus and the ground, which represents the insulation resistance to the ground of the positive electrode at the DC side of the photovoltaic inverter, that is, R₁ in the off-grid state in FIG. 6. A resistor is connected by a dotted line between the AC side of the inverter circuit 10 and the ground, which represents the insulation resistance to the ground of the AC side of the photovoltaic inverter. In the grid-connected state, the insulation resistance to the ground of the AC side and the insulation resistance to the ground of the positive electrode of the DC side together form R₁ in FIG. 6.

[0096] The anti-PID circuit 20 includes an anti-PID power supply V_PID and at least one detection resistor (R₀₁ and/or R₀₂). The anti-PID power supply V_PID is arranged between the DC bus and the ground. In practical applications, the anti-PID power supply V_PID may be arranged between the negative electrode of the DC bus and the ground, or between the positive electrode of the DC bus and the ground, depending on the requirements of the application scenario. The anti-PID power supply V_PID arranged between the negative electrode of the DC bus and the ground is described hereinafter as an example.

[0097] A corresponding detection resistor (R₀₁ and/or R₀₂) is arranged between at least one side of the photovoltaic inverter and the ground. As shown in FIG. 1, the anti-PID circuit 20 includes the anti-PID power supply V_PID and the detection resistor R₀₁ connected in series. Alternatively, as shown in FIG. 2, the anti-PID circuit 20 includes the anti-PID power supply V_PID and the detection resistor R₀₂ connected in series. Alternatively, as shown in FIG. 3, the anti-PID circuit 20 includes the anti-PID power supply V_PID, and the detection resistors R₀₁ and R₀₂, where a branch formed by the anti-PID power supply V_PID connected in series with the detection resistors R₀₁ is different from a branch formed by the anti-PID power supply V_PID connected in series with the detection resistors R₀₂. The branch formed by the anti-PID power supply V_PID connected in series with the detection resistor R₀₁ is connected between the DC bus and the ground, and each figure exemplarily illustrates that the branch is connected between the negative electrode of the DC bus and the ground. The branch formed by the anti-PID power supply V_PID connected in series with the detection resistor R₀₂ is connected between one phase at the AC side of the photovoltaic inverter and the ground. FIG. 1 to FIG. 3 exemplarily illustrate position relationships between the anti-PID power supply V_PID connected in series with the detection resistor, and the position relationship is not limited. The detection resistor R₀₁ and the detection resistor R₀₂ share one anti-PID power supply V_PID in FIG. 3. In another embodiment, the two detection resistors may be connected in series with different anti-PID power supplies, depending on the application environment. All implementations fall within the protection scope of the present disclosure.

[0098] Moreover, the anti-PID power supply V_PID and the detection resistor R₀₁ and R₀₂ are provided with respective switches. As shown in FIG. 1, the anti-PID power supply V_PID is provided with a switch K₁, and the detection resistor R₀₁ is provided with a switch K₂. When the switch K₁ is closed, a branch where the anti-PID power supply V_PID is located is connected between the negative electrode of the DC bus and the ground. When the switch K₁ is opened, the branch where the anti-PID power supply V_PID is located is disconnected from the negative electrode of the DC bus and the ground. When the switch K₂ is closed, the detection resistor R₀₁ is bypassed, thus being disconnected from the negative electrode of the DC bus and the ground. When the switch K₁ is closed and the switch K₂ is opened, the anti-PID power supply V_PID and the detection resistor R₀₁ are controlled to operate. For the structure in FIG. 2, the switches include switches K₁, K₂ and K₄. When only K₁ and K₂ are closed, the anti-PID power supply V_PID is controlled to operate. When only K₁ and K₄ are closed, the anti-PID power supply V_PID and the detection resistor R₀₂ are controlled to operate. Moreover, the switches K₂ and K₄ are mutually exclusive, that is, only one of them is closed at a time. For the structure in FIG. 3, the switches include switches K₁ to K₄, where the anti-PID power supply V_PID is controlled to operate when only K₁ and K₂ are closed, the anti-PID power supply V_PID and the detection resistor R₀₁ are controlled to operate when only K₁ and K₃ are closed, and the anti-PID power supply V_PID and the detection resistor R₀₂ are controlled to operate when only K₁ and K₄ are closed. Moreover, the switches K₂, K₃ and K₄ are mutually exclusive, that is, only one of them is closed at a time.

[0099] In an embodiment, a power supply being capable of outputting both a positive voltage and a negative voltage serves as the anti-PID power supply. Alternatively, a power supply being capable of only outputting a positive voltage serves as the anti-PID power supply, depending on the application environment. All implementations fall within the protection scope of the present disclosure.

[0100] In a case that the anti-PID power supply V_PID is capable of only outputting a positive voltage, the anti-PID power supply V_PID may output positive voltages with different absolute values to the negative electrode PV− of the DC bus. The structure of the anti-PID circuit 20 is further improved to output a positive voltage or a negative voltage to the negative electrode PV− of the DC bus. For example, for the structure in FIG. 1, that is, the number of the detection resistor is 1 and the detection resistor is arranged between the DC bus, such as the negative electrode of the DC bus, and the ground, the structure of the anti-PID circuit 20 may be shown in FIG. 5. The switches include a first switch S₁, a second switch S₂, a third switch S₃, a fourth switch S₄, and a fifth switch S₅. The negative electrode of the DC bus is connected to the positive electrode of the anti-PID power supply V_PID through the detection resistor R₀₁ and the first switch S₁ sequentially. The negative electrode of the anti-PID power supply V_PID is connected to the ground through the second switch S₂. The third switch S₃ is connected in parallel with the detection resistor R₀₁. A branch formed by the first switch S₁ connected in series with the anti-PID power supply V_PID is connected in parallel with the fourth switch S₄. A branch formed by the second switch S₂ connected in series with the anti-PID power supply V_PID is connected in parallel with the fifth switch S₅.

[0101] As shown in FIG. 5, when the switches S₁ and S₂ are closed, and the switches S₄ and S₅ are opened, the anti-PID power supply V_PID outputs a positive voltage to the negative electrode PV− of the DC bus. When the switches S₁ and S₂ are opened, and the switches S₄ and S₅ are closed, the anti-PID power supply V_PID outputs a negative voltage to the negative electrode PV− of the DC bus. Therefore, in a case that a power supply being capable of only outputting a positive voltage serves as the anti-PID power supply V_PID, the anti-PID power supply V_PID can output the positive voltage and negative voltage to the negative electrode PV− of the DC bus, thereby reducing a power supply cost.

[0102] In another embodiment, for the structure in FIG. 2, that is, the number of the detection resistor is 1, and the detection resistor is arranged between one phase of the AC side of the photovoltaic inverter and the ground, the structure of the anti-PID circuit 20 may be shown in FIG. 8. The switches includes a first switch S₁, a second switch S₂, a third switch S₃, a fourth switch S₄, a fifth switch S₅, and a seventh switch S₇. The negative electrode of the DC bus is connected to a first terminal of the first switch S₁ through the third switch S₃. A second terminal of the first switch S₁ is connected to the positive electrode of the anti-PID power supply V_PID. The negative electrode of the anti-PID power supply V_PID is connected to the ground through the second switch S₂. The seventh switch S₇ is connected in series with the detection resistor R₀₂ to form a branch, and the branch is connected between the phase of the AC side of the inverter circuit and the first terminal the first switch S₁. A branch formed by the first switch S₁ connected in series with the anti-PID power supply V_PID is connected in parallel with the fourth switch S₄. A branch formed by the second switch S₂ connected in series with the anti-PID power supply V_PID is connected in parallel with the fifth switch S₅. Moreover, the third switch S₃ and the seventh switch S₇ are mutually exclusive, that is, only one of them is closed at a time.

[0103] For the structure in FIG. 3, that is, the number of the detection resistors is 2. The detection resistors include a first detection resistor R₀₁ and a second detection resistor R₀₂. The structure of the anti-PID circuit 20 may be shown in FIG. 9, and the switches include a first switch S₁, a second switch S₂, a third switch S₃, a fourth switch S₄, a fifth switch S₅, a sixth switch S₆, and a seventh switch S₇. The negative electrode of the DC bus is connected to the first terminal of the first switch S₁ through the sixth switch S₆ and the first detection resistor R₀₁ connected in series. The seventh switch S₇ is connected in series with the second detection resistor R₀₂ to form a branch, and the branch is connected between the phase of the AC side of the inverter circuit and the first terminal of the first switch S₁. The second terminal of the first switch S₁ is connected to the positive electrode of the anti-PID power supply V_PID. The negative electrode of the anti-PID power supply V_PID is connected to the ground through the second switch S₂. The third switch S₃ is connected in parallel with a branch formed by the first detection resistor R₀₁ connected in series with the sixth switch S₆. A branch formed by the first switch S₁ connected in series with the anti-PID power supply V_PID is connected in parallel with the fourth switch S₄. A branch formed by the second switch S₂ connected in series with the anti-PID power supply V_PID is connected in parallel with the fifth switch S₅. Moreover, the third switch S₃, the sixth switch S₆, and the seventh switch S₇ are mutually exclusive, that is, only one of them can be closed at a time.

[0104] In an embodiment, in order to ensure that a direction of the current flow follows the design and avoid current flowing back into the anti-PID power supply V_PID, a first diode D₁ and a second diode D₂ are arranged in the anti-PID circuit 20. The positive electrode of the anti-PID power supply V_PID is connected to the first switch S₁ through the first diode D₁. The positive electrode of the anti-PID power supply V_PID is connected to the fifth switch S₅ through the second diode D₂.

[0105] The inverter circuit 10 and the respective switches are controlled by the controller. The voltage detection module is configured to detect a voltage to the ground of a position of the inverter to which the target detection resistor is connected and a voltage outputted by the anti-PID power supply V_PID, and output the detected voltages to the controller. In a case that the target detection resistor is R₀₁, the voltage to the ground of the position of the inverter to which the target detection resistor is connected is a voltage to the ground of the negative electrode of the DC bus of the photovoltaic inverter. In a case that the target detection resistor is R₀₂, the voltage to the ground of the position of the inverter to which the target detection resistor is connected is a voltage to the ground of the corresponding phase of the AC side of the photovoltaic inverter.

[0106] The controller is configured to perform the method for controlling the photovoltaic power generation system according to any one of the above embodiments. The detailed process and principle of the method may be referred to the above embodiments, which are not repeated herein.

[0107] With the method, the photovoltaic inverter can achieve both the insulation detection and the anti-PID operation by means of the anti-PID circuit 20, depending on actual needs. In an embodiment, the target detection resistor is R₀₁, the insulation detection can be performed on the DC side during the off-grid operation, and the insulation detection can be performed on the AC side and the DC side during the grid-connected operation. For the structure in FIG. 3 and FIG. 8, the insulation detection can be performed on the DC side and AC side of the photovoltaic inverter, respectively. In addition, the structure of the anti-PID circuit 20 is simple and easy to be implemented, which has a low cost and a convenient implementation in a real time manner, and is suitable for various types of inverters.

[0108] A photovoltaic power generation system is further provided according to another embodiment of the present disclosure, which includes a photovoltaic array and at least one photovoltaic inverter connected to the photovoltaic array. The photovoltaic array includes at least one photovoltaic string. The photovoltaic inverter is the photovoltaic inverter described in any one of the above embodiments, and the structure and principle of the photovoltaic inverter may be referred to the above embodiments, which are not repeated herein.

[0109] It is worth noting that the photovoltaic inverter may be a central inverter, or a string inverter. In a case that the photovoltaic inverter is the central inverter, at least one maximum power point tracking (MPPT) branch is arranged between the photovoltaic inverter and the photovoltaic array, which is not described in detail, depending on the application environment.

[0110] In addition, in the photovoltaic power generation system, the number of the photovoltaic inverter may be one or more, depending on the application environment.

[0111] Regardless of the structure of the photovoltaic power generation system, as long as the insulation detection can be compatible with the anti-PID operation for the photovoltaic inverter by the method described in any one of the above embodiments, the structure is fall within the protection scope of the present disclosure. Other structures of the photovoltaic power generation system may be referred to the conventional technology, which are not repeated herein.

[0112] The same or similar parts among the embodiments in this specification may be referred to each other, and each of the embodiments emphasizes differences from other embodiments. In particular, the system or system embodiment is basically similar to the method embodiment, and therefore is described relatively briefly. For relevant details, reference can be made to the corresponding description of the method embodiment. The system and system embodiments described above are only illustrative, in which the units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, may be located in one place, or distributed over multiple network elements. Some or all of the modules may be selected as needed to achieve the object of the solution of the embodiments. Those skilled in the art can understand and implement the solution without any creative effort.

[0113] Those skilled in the art may further realize that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, computer software, or a combination of both. In order to clearly illustrate the interchangeability of hardware and software, compositions and steps of each example have been described in general terms of functionality in the above description. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art may implement the described functionality in various manners for each specific application, while such implementations should not be considered to be beyond the scope of the present disclosure.

[0114] Based on the above description of the disclosed embodiments, the features described in the different embodiments in this specification may be replaced or combined with each other, so that those skilled in the art can implement or use the present disclosure. Various modifications to these embodiments are apparent by those skilled in the art, and the generic principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Hence, the present disclosure is not limited to the embodiments disclosed herein, but shall conform to the widest scope in accordance with the principle and novel features disclosed herein.