Photovoltaic grid-connected inverter system and insulation impedance on-line detection method and device thereof

Abstract

The application discloses a photovoltaic grid-connected inverter system and an insulation impedance online detection method and device thereof. The photovoltaic grid-connected inverter system comprises a plurality of photovoltaic arrays and an inverter. The plurality of photovoltaic arrays are connected in parallel with a direct-current bus of the inverter. Each photovoltaic array comprises a photovoltaic power station, a direct-current leakage current sensor and a direct-current relay. A photovoltaic branch of each photovoltaic array is connected in series with the photovoltaic power station, the direct-current relay and the direct-current leakage current sensor. The positive and negative polar insulation impedance of each photovoltaic branch is connected in parallel with a pair of unbalanced resistors controlled by a first auxiliary switching switch. The positive and negative polar insulation impedance of the direct-current bus of the inverter is connected in parallel with another pair of unbalanced resistors controlled by a second auxiliary switching switch. The application can facilitate insulation impedance detection and accurately locate the photovoltaic array with insulation impedance fault, thereby facilitating automatic removal of the fault photovoltaic array and maintaining the continuous operation of the normal photovoltaic array.

Description

Technical Field

[0001] This invention relates to the field of inverter technology, and in particular to a photovoltaic grid-connected inverter system and its online insulation impedance detection method and device. Background Technology

[0002] Testing the insulation impedance of photovoltaic (PV) arrays is a mandatory standard and requirement for inverters. Centralized PV power plants mostly use non-isolated inverters. According to grid-connected inverter safety requirements, the insulation impedance at the PV array input is measured before the inverter system starts. If the insulation impedance is less than Umax,pv / 30mA (Umax,pv is the maximum output voltage of the PV array), the inverter will display a fault and fail to start. When the inverter is already connected to the grid, residual current monitoring of the PV array is continuously performed. When continuous residual current or a sudden change in residual current exceeds a threshold, the inverter disconnects and issues a fault report. Low insulation impedance at the input of some PV arrays can cause the inverter to immediately report a fault, leading to the shutdown of all connected, normally functioning arrays and wasting resources.

[0003] Therefore, the relevant DC insulation online monitoring method cannot perform insulation impedance detection when the inverter is running in grid connection. It can only continuously detect the residual current of the photovoltaic array. When an insulation fault occurs at the array input, the inverter will immediately disconnect and issue a low insulation impedance fault message. It cannot determine which array input insulation impedances have decreased and automatically disconnects the faulty array to maintain continuous operation. Summary of the Invention

[0004] The present invention aims to at least partially solve the technical problems in related technologies. Therefore, the first objective of the present invention is to provide a photovoltaic grid-connected inverter system that facilitates insulation impedance detection and accurately locates photovoltaic arrays with insulation impedance faults, thereby facilitating the automatic disconnection of faulty arrays to maintain the continuous operation of normal photovoltaic arrays.

[0005] The second objective of this invention is to provide an online method for detecting insulation impedance.

[0006] The third objective of this invention is to provide an online insulation resistance detection device.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0008] A photovoltaic grid-connected inverter system includes multiple photovoltaic arrays and an inverter. The photovoltaic arrays are connected in parallel with the inverter's DC bus. Each photovoltaic array includes a photovoltaic power station, a DC leakage current sensor, and a DC relay. The photovoltaic branch containing each photovoltaic array is connected in series with the photovoltaic power station, the DC relay, and the DC leakage current sensor. The positive and negative insulation impedances of each photovoltaic branch are connected in parallel with a pair of unbalanced resistors controlled by a first auxiliary switching switch. The positive and negative insulation impedances of the inverter's DC bus are connected in parallel with another pair of unbalanced resistors controlled by a second auxiliary switching switch.

[0009] Preferably, the positive and negative insulation impedances of the inverter DC bus are also connected in parallel with another pair of unbalanced resistors controlled by the third auxiliary switching switch.

[0010] To achieve the above objectives, a second aspect of the present invention provides an online insulation impedance detection method, applied to the photovoltaic grid-connected inverter system as described in claim 1 or 2, the method comprising:

[0011] When the inverter is running in grid-connected mode, the DC bus voltage to ground is sampled in real time, and the residual current or residual current abrupt change is sampled for each photovoltaic array through a DC leakage current sensor.

[0012] For each photovoltaic array, determine whether the residual current and / or the residual current abrupt change is greater than the corresponding current threshold.

[0013] When the residual current and / or the sudden change in the residual current is greater than the corresponding current threshold, the corresponding photovoltaic array is designated as the target photovoltaic array.

[0014] The real-time insulation impedance of the target photovoltaic array is calculated based on the DC bus-to-ground voltage, and when the real-time insulation impedance is greater than a preset impedance threshold, it is determined that the target photovoltaic array has an insulation fault.

[0015] The DC relay controlling the target photovoltaic array is disconnected to isolate the photovoltaic array circuit with insulation impedance fault and to send an alarm to the backend.

[0016] Preferably, the residual current is the sum of the positive and negative currents input to the corresponding photovoltaic array.

[0017] Preferably, the method further includes:

[0018] Before the inverter is started, the initial insulation impedance of the inverter DC bus and the initial insulation impedance and initial residual current of each photovoltaic array are calculated. When the initial insulation impedance and initial residual current are both lower than the corresponding threshold, the first to third auxiliary switching switches are disconnected and the inverter is started.

[0019] Preferably, when the residual current and / or the sudden change in the residual current exceeds the corresponding current threshold, the DC relay of the corresponding photovoltaic array is controlled to disconnect to isolate the photovoltaic array circuit with insulation impedance fault and to issue an alarm to the background.

[0020] Preferably, before the inverter is started, the first to third auxiliary switching switches are switched to calculate the initial insulation impedance of the inverter DC bus and the initial insulation impedance of each photovoltaic array input.

[0021] Preferably, the real-time insulation impedance calculation of the target photovoltaic array is achieved by switching the unbalanced resistor of the target photovoltaic array.

[0022] To achieve the above objectives, a third aspect of the present invention provides an online insulation impedance detection device, comprising:

[0023] The detection module is used to detect the DC bus voltage to ground, the residual current input to each photovoltaic array, or the abrupt change in residual current when the inverter is running in grid-connected mode.

[0024] The first calculation module is used to calculate and determine whether the residual current and / or the residual current mutation amount is greater than the corresponding current threshold, and when the residual current and / or the residual current mutation amount is greater than the corresponding current threshold, to mark the corresponding photovoltaic array as the target photovoltaic array, and to control the first action module or the second action module to act.

[0025] The first action module is used to control the unbalanced resistance of the target photovoltaic array when the residual current and / or the sudden change in the residual current is greater than the corresponding current threshold.

[0026] The second calculation module is used to calculate the real-time insulation impedance of the target photovoltaic array input based on the DC bus-to-ground voltage after the first action module is activated, and to control the second action module to activate when the real-time insulation impedance is greater than a preset impedance threshold.

[0027] The second action module is used to control the DC relay of the target photovoltaic array to disconnect after receiving a control command from the first calculation module or the second calculation module that the target photovoltaic array has an insulation fault, so as to isolate the photovoltaic array circuit with insulation impedance fault and send an alarm to the background.

[0028] Preferably, the residual current detected by the detection module is the sum of the positive and negative currents input to the corresponding photovoltaic array.

[0029] This invention has at least the following technical effects:

[0030] This invention, during inverter grid-connected operation, can sample and acquire the DC bus-to-ground voltage and the residual current or residual current fluctuation of each photovoltaic array in real time. Then, for each photovoltaic array, it determines whether the residual current and / or residual current fluctuation exceeds a corresponding current threshold. If so, the corresponding photovoltaic array is designated as the target photovoltaic array. The real-time insulation impedance of the target photovoltaic array is calculated based on the DC bus-to-ground voltage. When the real-time insulation impedance exceeds a preset impedance threshold, an insulation fault is identified in the target photovoltaic array. The DC relay of the target photovoltaic array is then disconnected, thereby isolating the photovoltaic array circuit with the insulation impedance fault. Thus, this invention can calculate the real-time insulation impedance of the photovoltaic array input, and combined with the residual current input, determine whether the photovoltaic array insulation impedance is abnormal. Abnormal arrays can be promptly disconnected via DC relays, thereby ensuring the safety of the grid-connected inverter system and improving system operating efficiency.

[0031] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of a photovoltaic grid-connected inverter system according to an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the structure of the online insulation impedance detection device according to an embodiment of the present invention.

[0034] Figure 3 This is a schematic flowchart of the online insulation impedance detection method according to an embodiment of the present invention. Detailed Implementation

[0035] The following describes this embodiment in detail. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0036] According to the technical safety requirements of grid-connected inverters, centralized photovoltaic power plants currently use multi-photovoltaic array input non-isolated inverters. Insulation impedance is only detected before the inverter system is started and connected to the grid. When the inverter is running in grid-connected mode, insulation impedance cannot be detected. Only the residual current of the photovoltaic array is continuously detected. When an insulation impedance fault occurs at the input of the photovoltaic array, the inverter will immediately disconnect and issue a low insulation impedance fault. It is impossible to determine which photovoltaic arrays have reduced input insulation impedance and automatically disconnects the faulty photovoltaic array to maintain continuous operation.

[0037] This embodiment provides a photovoltaic grid-connected inverter system based on a DC system and its online insulation impedance detection method and device. This scheme can measure the residual current input to each photovoltaic array of the inverter and the DC bus voltage to ground in real time. When the residual current or its sudden change in the residual current of some photovoltaic arrays exceeds the set threshold, the unbalanced resistor is switched on and off in real time. By measuring the positive and negative bus voltage to ground and the residual current under different conditions, the insulation impedance of the abnormal photovoltaic array can be calculated, and the real-time insulation impedance of the abnormal photovoltaic array can be obtained. This allows it to be determined whether there is an insulation impedance fault in the abnormal photovoltaic array, and further isolation measures can be taken.

[0038] This embodiment calculates the real-time insulation impedance of the photovoltaic array and combines it with the residual current to determine whether the insulation impedance of the photovoltaic array is abnormal. Abnormal photovoltaic arrays can be promptly disconnected through DC relays, which can ensure the safety of the grid-connected inverter system and improve the system operating efficiency.

[0039] The photovoltaic grid-connected inverter system and its online insulation impedance detection method and apparatus of this embodiment are described below with reference to the accompanying drawings.

[0040] Figure 1 This is a schematic diagram of the structure of a photovoltaic grid-connected inverter system according to an embodiment of the present invention. Figure 1 As shown, the photovoltaic grid-connected inverter system includes multiple photovoltaic arrays and inverters. The photovoltaic arrays are connected in parallel with the inverter's DC bus. Each photovoltaic array includes a photovoltaic power station (PV1, ..., PVn), a DC leakage current sensor (T1, ..., Tn), and a DC relay (Km1, ..., Kmn). Each photovoltaic branch is connected in series with the photovoltaic power station, DC relay, and DC leakage current sensor. The positive and negative insulation resistances of each photovoltaic branch are respectively parallel to a pair of terminals controlled by the first auxiliary switching switch. The unbalanced resistors are connected in parallel. For example, the positive and negative insulation impedances Rn± of the nth photovoltaic branch are connected in parallel with a pair of unbalanced resistors Rn1 and Rn2 controlled by the first auxiliary switching switch Kn on the nth photovoltaic branch, respectively. The positive and negative insulation impedances of the inverter DC bus are connected in parallel with another pair of unbalanced resistors controlled by the second auxiliary switching switch. For example, the positive and negative insulation impedances R± of the inverter DC bus are connected in parallel with unbalanced resistors RA1 and RA2 switched by the second auxiliary switching switch KA, respectively. The inverter side is connected to the grid. It should be noted that the positive and negative insulation impedances R± of the inverter DC bus are also connected in parallel with unbalanced resistors RB1 and RB2 switched by the third auxiliary switching switch KB, respectively. In this embodiment, the resistance values ​​of unbalanced resistors R11 to Rn1 are all 100K, the resistance values ​​of unbalanced resistors R12 to Rn2 are all 110K, the resistance values ​​of unbalanced resistors Rm1 and Rm2 are 224K and 226K respectively, the resistance values ​​of unbalanced resistors RA1 and RB2 are 300K respectively, and the resistance values ​​of unbalanced resistors RA2 and RB1 are 400K respectively.

[0041] It should be noted that the DC leakage current sensor in this embodiment is installed on the positive and negative input terminals of each photovoltaic array, mainly for accurately monitoring the residual current of each photovoltaic array. In this embodiment, a set of unbalanced resistors, controlled by a first auxiliary switching switch, is installed between the positive and negative terminals and the grounding terminal of each photovoltaic array for residual current calibration and insulation impedance calculation of each photovoltaic array. In this embodiment, the DC relay connected in series on each photovoltaic array between the photovoltaic array interface and its circuit breaker is used to accurately, quickly, and controllably execute the action module commands, segmenting the insulation impedance fault input to the photovoltaic array to ensure that the inverter does not carry the residual current of the faulty photovoltaic array. The DC relay uses a normally open contact and only engages after the inverter's startup self-test is normal. When the insulation impedance fault is calculated and detected, the corresponding DC relay of the photovoltaic array is disconnected. At the same time, the DC relay will also be automatically disconnected when the system fails to power down, ensuring the safety of the inverter.

[0042] To address the technical issues of not being able to detect the DC-side insulation impedance in real time during grid-connected inverter operation, and the inability to isolate the input branch of insulation faults, this embodiment's online insulation impedance detection method relies on the attached... Figure 2 The device shown, Figure 2 This is a schematic diagram of the online insulation resistance detection device according to an embodiment of the present invention. Figure 2 As shown, the online insulation impedance detection device includes a detection module, a first calculation module, a first action module, a second calculation module, and a second action module.

[0043] This embodiment provides a method for online detection of insulation impedance. Figure 3 This is a schematic flowchart of the online insulation impedance detection method according to an embodiment of the present invention. Figure 3 As shown, the method includes:

[0044] Step S101: When the inverter is running in grid-connected mode, the DC bus voltage to ground is sampled in real time, and the residual current or residual current abrupt change is sampled and obtained for each photovoltaic array through a DC leakage current sensor.

[0045] The residual current is the sum of the positive and negative currents input to the corresponding photovoltaic array.

[0046] Step S102: For each photovoltaic array, determine whether the residual current and / or residual current mutation is greater than the corresponding current threshold.

[0047] Step S103: When the residual current and / or the sudden change in residual current is greater than the corresponding current threshold, the corresponding photovoltaic array is designated as the target photovoltaic array.

[0048] It should be noted that when the residual current and / or the sudden change in residual current is significantly greater than the corresponding current threshold, the DC relay of the corresponding photovoltaic array can also be directly controlled to disconnect, so as to isolate the photovoltaic array circuit with insulation impedance fault and send an alarm to the background.

[0049] Alternatively, the residual current and / or residual current abrupt change can be compared with the corresponding initial residual current measured before the inverter is started to determine whether the corresponding photovoltaic array has an insulation impedance fault.

[0050] Step S104: Calculate the real-time insulation impedance of the target photovoltaic array based on the DC bus-to-ground voltage, and determine that the target photovoltaic array has an insulation fault when the real-time insulation impedance is greater than the preset impedance threshold.

[0051] In this embodiment, the real-time insulation impedance calculation of the target photovoltaic array input can be achieved by switching the unbalanced resistance of the target photovoltaic array, i.e., switching the first auxiliary switching switch.

[0052] The real-time insulation impedance can also be compared with the corresponding initial insulation impedance measured before the inverter is started. If it deviates from the initial insulation impedance, it is determined that the target photovoltaic array has an insulation impedance fault.

[0053] Step S105: Control the DC relay of the target photovoltaic array to disconnect, so as to isolate the photovoltaic array circuit with insulation impedance fault, and send an alarm to the background.

[0054] In one embodiment of the present invention, the method further includes calculating the initial insulation impedance of the inverter DC bus and the initial insulation impedance and initial residual current of each photovoltaic array input before the inverter is started, and controlling the first to third auxiliary switching switches to disconnect and control the inverter to start when both the initial insulation impedance and the initial residual current are lower than the corresponding thresholds. Specifically, the initial insulation impedance of the inverter DC bus and the initial insulation impedance of each photovoltaic array input are calculated by switching the first to third auxiliary switching switches before the inverter is started.

[0055] Specifically, this online insulation impedance detection method consists of the following steps:

[0056] Step 1: After the inverter meets the start-up conditions, the residual current input to each photovoltaic array of the inverter and the DC bus voltage to ground are obtained in real time and filtered.

[0057] In this embodiment, a detection module can be used to acquire the residual current and DC bus-to-ground voltage of each photovoltaic array in the inverter in real time. The specific process of extracting the residual current and DC bus-to-ground voltage is as follows:

[0058] (1) Sampling of DC bus voltage to ground involves sampling the positive voltage of PV to ground and the negative voltage of PV to ground of the photovoltaic power station.

[0059] (2) Sampling of residual current is generally achieved by sampling the sum of the positive and negative input currents of each photovoltaic array using a DC leakage current sensor. For example, the DC leakage current sensor Tn samples the sum of the positive and negative input currents ΔIn of the photovoltaic array in the nth photovoltaic branch.

[0060] Step 2: When the residual current input to the photovoltaic array and its sudden change exceed the corresponding current threshold, it is inferred that the photovoltaic array has an insulation abnormality.

[0061] In this embodiment, the first calculation module can be used to calculate and compare in real time whether the residual current and its sudden change in the input of each photovoltaic array exceed the corresponding current threshold. If they exceed the threshold, the photovoltaic array is calibrated and designated as the target photovoltaic array.

[0062] Optionally, in this embodiment of the invention, the current thresholds can be modified by comparing the recommended values ​​(300mA / 30mA) and historical measurement values ​​with the actual on-site operating environment.

[0063] It should be noted that when the residual current and its sudden change in the first calculation module clearly determine that the insulation of the relevant photovoltaic array is abnormal, the second action module can be used to automatically control the DC relay of the photovoltaic array to disconnect, so as to isolate the photovoltaic array circuit with insulation impedance fault and send an alarm to the background.

[0064] When the first calculation module detects an insulation anomaly in the photovoltaic array and identifies it as the target photovoltaic array, it can calculate the real-time insulation impedance of the target photovoltaic array. Specifically, the first action module can be used to control the unbalanced resistor controlled by the first auxiliary switching switch, so that the measured positive and negative bus voltages and residual currents are different. Then, the second calculation module is used to calculate the real-time insulation impedance of the target photovoltaic array at this time.

[0065] For example, when the first calculation module identifies the photovoltaic array of the first photovoltaic branch as the target photovoltaic array, the first action module can be used to control the first auxiliary switching switch K1 on the first photovoltaic branch to switch the corresponding unbalanced resistors R11 and R12 into the photovoltaic array circuit. Then, the second calculation module is used to calculate the real-time insulation impedance of the first photovoltaic array.

[0066] When the real-time insulation impedance calculated by the target photovoltaic array exceeds the preset impedance threshold, it can be determined that the target photovoltaic array has an insulation fault. Then, the second action module is used to automatically control the DC relay of the target photovoltaic array to disconnect, so as to isolate the photovoltaic array circuit with insulation impedance fault and send an alarm to the background.

[0067] Optionally, this embodiment can directly calculate the initial insulation impedance of the inverter bus DC and the input of each photovoltaic array to determine whether to start the inverter. R+ and R- are the insulation impedances of the positive and negative DC buses to ground, i.e., the initial insulation impedances. Rm1 and Rm2 are the unbalanced resistances of the positive and negative buses to ground, respectively. Under normal circumstances, these two resistors balance the DC bus voltage to ground, preventing the DC bus voltage from becoming unstable due to excessive insulation resistance to ground. The two sets of resistors switched by the second auxiliary switching switch KA and the third auxiliary switching switch KB cause the measured positive and negative DC bus voltages to be different under different conditions, so as to calculate the initial insulation impedance of the DC bus to ground.

[0068] With all switches open, the voltages to ground U+ and U- of the positive and negative DC buses are measured respectively, and the following equations about R+ and R- are obtained:

[0069]

[0070] Among them, the 224k resistor is Rm1, and the 226k resistor is Rm2.

[0071] Close the second auxiliary switching switch KA and measure the voltage to ground U' of the positive and negative DC buses respectively. + and U' - We obtain the following equations for R+ and R-:

[0072]

[0073] The 300k resistor is RA1, and the 400k resistor is RA2.

[0074] Optionally, Equations 1 and 2 form a system of two equations concerning R+ and R-, which can be solved to calculate the initial insulation impedance R+ and R- of the DC bus to ground. In practical applications, considering factors such as the equal decrease in insulation impedance between the positive and negative buses, a third auxiliary switching switch KB is set to calculate the DC bus to ground insulation impedance under different operating conditions.

[0075] It is understandable that by closing the first auxiliary switching switch Kn on the nth photovoltaic branch, the initial insulation impedance Rn+ and Rn- of each photovoltaic array can be calculated using the same principle.

[0076] The initial insulation impedance calculation is completed before the inverter starts up and performs a self-test. When the calculated initial residual current and initial insulation impedance are within the corresponding thresholds, the auxiliary switching switch is opened and the inverter starts up.

[0077] Furthermore, the present invention also provides an online insulation impedance detection device, comprising a detection module, a first calculation module, a first action module, a second calculation module, and a second action module connected in sequence, wherein the first calculation module is also connected to the second action module.

[0078] In this embodiment, the detection module is used to detect the DC bus-to-ground voltage, the residual current input to each photovoltaic array, or the residual current fluctuation when the inverter is running in grid connection. The first calculation module is used to calculate and determine whether the residual current and / or residual current fluctuation is greater than the corresponding current threshold, and when the residual current and / or residual current fluctuation is greater than the corresponding current threshold, to mark the corresponding photovoltaic array as the target photovoltaic array and control the first action module or the second action module to act. The first action module is used to control the switching of the unbalanced resistance of the target photovoltaic array when the residual current and / or residual current fluctuation is greater than the corresponding current threshold. The second calculation module is used to calculate the real-time insulation impedance input to the target photovoltaic array based on the DC bus-to-ground voltage after the first action module acts, and to control the second action module to act when the real-time insulation impedance is greater than the preset impedance threshold. The second action module is used to control the DC relay of the target photovoltaic array to disconnect after receiving the control command from the first calculation module or the second calculation module that the target photovoltaic array has an insulation fault, so as to isolate the photovoltaic array circuit with insulation impedance fault and issue an alarm to the background.

[0079] The residual current detected by the detection module is the sum of the positive and negative currents input to the corresponding photovoltaic array.

[0080] It should be noted that the specific implementation of the online insulation impedance detection device in this embodiment can be found in the specific implementation of the online insulation impedance detection method described above, and will not be repeated here to avoid redundancy.

[0081] In summary, this invention can perform real-time online insulation monitoring of all photovoltaic arrays in the inverter, and accurately isolate the faulty photovoltaic array through a DC relay after locating it, thereby ensuring that the remaining photovoltaic arrays with normal insulation impedance continue to generate electricity and ensuring that the inverter operates without interruption.

[0082] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0083] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A photovoltaic grid-connected inverter system, characterized in that, It includes multiple photovoltaic arrays and inverters. The photovoltaic arrays are connected in parallel with the DC bus of the inverters. Each photovoltaic array includes a photovoltaic power station, a DC leakage current sensor, and a DC relay. The photovoltaic branch where each photovoltaic array is located is connected in series with the photovoltaic power station, the DC relay, and the DC leakage current sensor. The positive and negative insulation impedances of each photovoltaic branch are connected in parallel with a pair of unbalanced resistors controlled by a first auxiliary switching switch. The positive and negative insulation impedances of the inverter DC bus are connected in parallel with another pair of unbalanced resistors controlled by a second auxiliary switching switch. The positive and negative insulation resistances of the inverter's DC bus are also connected in parallel with another pair of unbalanced resistors controlled by the third auxiliary switching switch; R+ and R- are the initial insulation impedances of the positive and negative DC buses to ground, respectively; Rm1 and Rm2 are the unbalanced resistances of the positive and negative DC buses to ground, respectively; RA1 and RA2 are a pair of unbalanced resistors controlled by the second auxiliary switching switch; the two pairs of unbalanced resistors controlled by the second and third auxiliary switching switches cause the measured positive and negative DC bus voltages to be different under different conditions, so as to calculate the initial insulation impedances R+ and R- of the positive and negative DC buses to ground; With all switches open, the voltages to ground U+ and U- of the positive and negative DC buses are measured respectively, yielding the following equations regarding the initial insulation impedances R+ and R-: （1） Close the second auxiliary switching switch and measure the voltage to ground of the positive and negative DC buses respectively. and The following equations are obtained regarding the initial insulation resistances R+ and R-: （2） By combining equations (1) and (2), the initial insulation resistances R+ and R- of the positive and negative DC bus to ground can be solved.

2. A method for online detection of insulation impedance, characterized in that, Applied to the photovoltaic grid-connected inverter system as described in claim 1, the method includes: When the inverter is running in grid-connected mode, the DC bus voltage to ground is sampled in real time, and the residual current or residual current abrupt change is sampled for each photovoltaic array through a DC leakage current sensor. For each photovoltaic array, determine whether the residual current and / or the residual current abrupt change is greater than the corresponding current threshold. When the residual current and / or the sudden change in the residual current is greater than the corresponding current threshold, the corresponding photovoltaic array is designated as the target photovoltaic array. The real-time insulation impedance of the target photovoltaic array is calculated based on the DC bus-to-ground voltage, and when the real-time insulation impedance is greater than a preset impedance threshold, it is determined that the target photovoltaic array has an insulation fault. The DC relay controlling the target photovoltaic array is disconnected to isolate the photovoltaic array circuit with insulation impedance fault and to send an alarm to the backend.

3. The online insulation impedance detection method as described in claim 2, characterized in that, The residual current is the sum of the positive and negative currents input to the corresponding photovoltaic array.

4. The online insulation impedance detection method as described in claim 2, characterized in that, The method further includes: Before the inverter is started, the initial insulation impedance of the inverter DC bus and the initial insulation impedance and initial residual current of each photovoltaic array are calculated. When the initial insulation impedance and initial residual current are both lower than the corresponding threshold, the first to third auxiliary switching switches are disconnected and the inverter is started.

5. The online insulation impedance detection method as described in claim 2, characterized in that, When the residual current and / or the sudden change in the residual current exceeds the corresponding current threshold, the DC relay of the corresponding photovoltaic array is controlled to disconnect to isolate the photovoltaic array circuit with insulation impedance fault and to send an alarm to the background.

6. The online insulation impedance detection method as described in claim 4, characterized in that, Before the inverter is started, the first to third auxiliary switching switches are switched to calculate the initial insulation impedance of the inverter DC bus and the initial insulation impedance of each photovoltaic array input.

7. The online insulation impedance detection method as described in claim 2, characterized in that, The real-time insulation impedance calculation of the target photovoltaic array input is achieved by switching the unbalanced resistor of the target photovoltaic array.

8. An online insulation resistance testing device, characterized in that, The device, applied to the photovoltaic grid-connected inverter system as described in claim 1, comprises: The detection module is used to detect the DC bus voltage to ground, the residual current input to each photovoltaic array, or the abrupt change in residual current when the inverter is running in grid-connected mode. The first calculation module is used to calculate and determine whether the residual current and / or the residual current mutation amount is greater than the corresponding current threshold, and when the residual current and / or the residual current mutation amount is greater than the corresponding current threshold, to mark the corresponding photovoltaic array as the target photovoltaic array, and to control the first action module or the second action module to act. The first action module is used to control the unbalanced resistance of the target photovoltaic array when the residual current and / or the sudden change in the residual current is greater than the corresponding current threshold. The second calculation module is used to calculate the real-time insulation impedance of the target photovoltaic array input based on the DC bus-to-ground voltage after the first action module is activated, and to control the second action module to activate when the real-time insulation impedance is greater than a preset impedance threshold. The second action module is used to control the DC relay of the target photovoltaic array to disconnect after receiving a control command from the first calculation module or the second calculation module that the target photovoltaic array has an insulation fault, so as to isolate the photovoltaic array circuit with insulation impedance fault and send an alarm to the background.

9. The online insulation resistance detection device as described in claim 8, characterized in that, The residual current detected by the detection module is the sum of the positive and negative currents input to the corresponding photovoltaic array.

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