Insulation resistance detection method of photovoltaic inverter, photovoltaic inverter and photovoltaic system

Abstract

The invention relates to an insulation resistance detection method for a photovoltaic inverter, the photovoltaic inverter and a photovoltaic system, and the method comprises the steps: detecting the bus voltage of a DC bus during the detection of the insulation resistance of the inverter, terminating the detection of the insulation resistance of the inverter when the bus voltage is lower than a first voltage threshold value, and stopping the detection of the insulation resistance of the inverter when the bus voltage is lower than the first voltage threshold value; and controlling the increase of the bus voltage until the bus voltage is increased to a second voltage threshold value, and controlling the inverter to carry out insulation resistance detection, the second voltage threshold value being not less than the first voltage threshold value. Thus, the bus voltage can be maintained within a certain threshold range in the inverter insulation impedance detection process, the problem that the insulation impedance detection precision becomes poor due to bus voltage change is avoided, the accuracy of inverter insulation impedance detection is effectively improved, and the detection accuracy of the inverter insulation impedance is improved. And therefore, the photovoltaic inverter and the photovoltaic system can be ensured to work more safely and stably.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter safety control, and in particular to an insulation impedance detection method of a photovoltaic inverter, a photovoltaic inverter, and a photovoltaic system. Background Art

[0002] As a device that converts DC power into AC power, the inverter plays a vital role in applications such as photovoltaic power stations. In order to protect the safety and stable operation of the inverter, insulation impedance testing (ISO testing) is an essential step before the system is connected to the grid. The main purpose of the inverter insulation impedance test is to measure the insulation resistance between the input terminal of the photovoltaic array and the ground to ensure that its resistance meets safety requirements, thereby avoiding the risk of electric shock caused by excessive residual leakage current.

[0003] The photovoltaic inverters currently on the market generally use the method of calculating the insulation resistance by detecting the PV+ to ground and PV- to ground voltages. However, the existing insulation impedance detection technology still has some shortcomings in practical applications, especially when calculating the impedance value, it is easily affected by the changes in the sampling voltage and the bus voltage, resulting in ISO underreporting. Specifically, the existing insulation impedance detection method of the photovoltaic inverter usually includes sampling the voltage values ​​before and after the relay is energized, and then calculating the ISO calculated value of the inverter based on these voltage values ​​and the external impedance value, and comparing it with the ISO alarm threshold. However, when performing self-test, since the sampling voltage changes with the decrease of the bus voltage, the calculated impedance value is larger than the actual external impedance, which in turn causes the ISO calculated value to be larger than the actual value, resulting in ISO underreporting. Summary of the invention

[0004] A main purpose of the present invention is to overcome at least one of the above-mentioned defects, and to provide an insulation impedance detection method, a photovoltaic inverter, and a photovoltaic system for a photovoltaic inverter, which can realize accurate and reliable detection of the insulation impedance of the inverter to the ground, and avoid the problem of poor insulation impedance detection accuracy caused by bus voltage changes.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: The present invention provides an insulation impedance detection method for a photovoltaic inverter. The DC side of the inverter is connected to a DC power supply through a DC bus, the AC side of the inverter is connected to a power grid, a bus capacitor is arranged between the DC bus, and a detection circuit for performing inverter insulation impedance detection is arranged between the DC bus and a PE terminal. When performing the inverter insulation impedance detection, the bus voltage of the DC bus is detected. When the bus voltage is lower than a first voltage threshold, the inverter insulation impedance detection is terminated, and the bus voltage is controlled to increase until the bus voltage increases to a second voltage threshold, and the inverter is controlled to perform insulation impedance detection, and the second voltage threshold is not less than the first voltage threshold.

[0006] According to one embodiment of the present invention, the detection circuit includes two groups of discharge branches, wherein the first discharge branch is arranged between the negative bus and the PE terminal, and the second discharge branch is arranged between the positive bus and the PE terminal. When performing the insulation impedance detection of the inverter, the first discharge branch or the second discharge branch is connected to the DC bus at intervals, and the bus voltage when different discharge branches are switched and the detection values ​​of the voltage of any pole of the DC bus to the ground are obtained, and the insulation impedance value to the ground of the DC side of the inverter is calculated based on the detection values.

[0007] According to one embodiment of the present invention, when the insulation impedance detection of the inverter is performed, two control signals are output at intervals, each control signal controls the first discharge branch or the second discharge branch to be switched on and off to connect to the DC bus in a time period, and there is a time gap between the two control signals output at intervals, and the bus voltage is controlled to increase to the second voltage threshold within the time gap.

[0008] According to one embodiment of the present invention, before performing the inverter insulation impedance detection, the bus voltage of the DC bus is detected. If the bus voltage is not lower than a third voltage threshold, the inverter insulation impedance detection is started. Otherwise, the bus voltage is controlled to be increased until the bus voltage is increased to a fourth voltage threshold and then the inverter insulation impedance detection is started.

[0009] According to one embodiment of the present invention, the third voltage threshold is not less than the first voltage threshold.

[0010] According to one embodiment of the present invention, in the process of controlling the increase of the bus voltage, The bus capacitor is charged via the DC power supply or the power grid.

[0011] According to one embodiment of the present invention, the DC power source is a photovoltaic cell assembly or an energy storage battery.

[0012] According to one embodiment of the present invention, during the process of charging the bus capacitor with a DC power supply, the output power of the DC power supply is judged. If the output power of the DC power supply is greater than a preset DC power threshold, the DC power supply is used to charge the bus capacitor; otherwise, the power grid is used to charge the bus capacitor.

[0013] According to one embodiment of the present invention, an AC starting circuit is provided in the inverter, the input side of the AC starting circuit is connected to the power grid, and the output side is connected to the bus capacitor, and is used to draw power from the power grid to charge the bus capacitor. When the insulation impedance of the inverter is detected, the AC starting circuit is stopped.

[0014] According to one embodiment of the present invention, the AC starting circuit includes a rectifier circuit and a control switch, the input side of the rectifier circuit is connected to the power grid, and the output side is connected to the bus capacitor. The control switch is connected in series with the rectifier circuit to control the conduction or disconnection between the bus capacitor and the power grid.

[0015] In particular, the present invention also provides a photovoltaic inverter, wherein the DC side of the inverter is connected to a DC power supply through a DC bus, the AC side of the inverter is connected to a power grid, a bus capacitor is arranged between the DC bus, and a detection circuit for detecting the insulation impedance of the inverter is arranged between the DC bus and the PE terminal, and the detection circuit operates according to the insulation impedance detection method as described above.

[0016] In addition, the present invention also provides a photovoltaic system, which includes the photovoltaic inverter as described above.

[0017] Compared with the prior art, the advantages and beneficial effects of the insulation impedance detection method of the photovoltaic inverter, the photovoltaic inverter, and the photovoltaic system of the patent application of the present invention are: The insulation impedance detection method of the photovoltaic inverter of the present application maintains the detection of the bus voltage of the DC bus during the process of performing the insulation impedance detection of the inverter, so that the bus voltage is maintained within a certain threshold range, thereby avoiding the problem of poor insulation impedance detection accuracy caused by bus voltage changes, effectively improving the accuracy of the insulation impedance detection of the inverter, and thus ensuring that the operation of the photovoltaic inverter and the photovoltaic system is safer and more stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings: Figure 1is a schematic structural diagram of an inverter and an insulation impedance detection circuit according to an embodiment of the present invention; Figure 2 is a schematic diagram of an equivalent circuit of an insulation impedance detection circuit according to an embodiment of the present invention; Figure 3 is a schematic diagram of the working process of an inverter insulation impedance detection method according to an embodiment of the present invention; Figure 4 It is a timing diagram of performing insulation impedance detection of an inverter according to an embodiment of the present invention, wherein, it is for smoothly performing insulation impedance detection; Figure 5 It is a timing diagram of inverter insulation impedance detection according to an embodiment of the present invention, wherein it is a re-detection after a failure in the first process of ISO detection; Figure 6 It is a timing diagram of performing inverter insulation impedance detection according to an embodiment of the present invention, wherein it is a re-detection after a failure in the second process of ISO detection; Figure 7 is a schematic diagram of a judgment process for AC / DC charging according to an embodiment of the present invention; Figure 8 is a schematic diagram of a judgment process for AC / DC charging according to an embodiment of the present invention; Fig. 9 The present invention is a flowchart of detecting and charging the bus voltage before detecting the insulation impedance of the inverter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] Embodiment 1: This embodiment describes a method for detecting insulation impedance of a photovoltaic inverter. Figure 1 As shown, the DC side of the inverter is connected to the DC power supply through the DC bus, the AC side of the inverter is connected to the power grid, a bus capacitor is arranged between the DC bus, and a detection circuit for performing inverter insulation impedance detection is arranged between the DC bus and the PE terminal. When performing the inverter insulation impedance detection, the bus voltage of the DC bus is detected, such as Figure 3As shown, when the bus voltage is lower than the first voltage threshold, the insulation impedance detection of the inverter is terminated, and the bus voltage is controlled to increase until the bus voltage increases to the second voltage threshold, and the inverter is controlled to perform insulation impedance detection, and the second voltage threshold is not less than the first voltage threshold.

[0022] The second voltage threshold and the first voltage threshold are preset fixed values ​​or a range of values. By monitoring the bus voltage of the DC bus and controlling charging under necessary conditions, the bus voltage of the DC bus can be maintained above a relatively stable threshold during the insulation impedance detection, avoiding the problem of large sampling voltage value errors caused by bus voltage fluctuations during insulation impedance detection, thereby effectively improving the accuracy and reliability of the final insulation impedance detection.

[0023] In some implementations, the bus voltage may be tested before the inverter insulation impedance test is performed to ensure that the voltage is stable before the insulation impedance test is started. Fig. 9 As shown, before the inverter insulation impedance detection is performed, the bus voltage of the DC bus is first detected. If the bus voltage is not lower than the third voltage threshold, the inverter insulation impedance detection is started. Otherwise, the bus voltage is controlled to increase until the bus voltage increases to the fourth voltage threshold, and the inverter insulation impedance detection is started. The third voltage threshold is not less than the first voltage threshold. As for the value of the fourth voltage threshold, it can be greater than the third voltage threshold, the value of the fourth voltage threshold can be equal to the second voltage threshold, or it can be slightly less than or greater than the second voltage threshold, and it can be set according to specific working conditions.

[0024] Based on the detection and charging control of the bus voltage during the above insulation impedance detection process, the bus voltage of the DC bus can be higher than a certain voltage threshold before the inverter insulation impedance detection is performed, and the bus capacitor can be charged when necessary so that the starting bus voltage before the inverter insulation impedance detection is maintained within a certain threshold range, thereby avoiding the problem of poor insulation impedance detection accuracy caused by inconsistent bus voltage starting voltage, and effectively improving the accuracy of inverter insulation impedance detection.

[0025] When the bus voltage is increased by controlling the above, the bus capacitor can be charged by the DC power supply or the power grid. That is to say, if necessary, the bus capacitor can be charged by the DC power supply or the power grid, and then the bus capacitor is charged when the inverter insulation impedance is detected so that the bus voltage is increased from lower than the first voltage threshold to the second voltage threshold or higher than the second voltage threshold, or the bus capacitor is charged before the inverter insulation impedance is detected so that the bus voltage is increased from lower than the third voltage threshold to the fourth voltage threshold or higher than the fourth voltage threshold.

[0026] When charging the bus capacitor as described above, the DC power source that can be used can be a photovoltaic cell assembly or an energy storage battery.

[0027] Before performing the inverter insulation impedance detection, the output power of the DC power supply can be judged. If the power of the DC power supply is sufficient, it is used to charge the bus capacitor, otherwise the power grid is used to charge the bus capacitor. Therefore, in the process of charging the bus capacitor with the DC power supply, the output power of the DC power supply can be judged. If the output power of the DC power supply is greater than the preset DC power threshold, the DC power supply is used to charge the bus capacitor, otherwise the power grid is used to charge the bus capacitor.

[0028] There are generally two methods for judging the output power of the photovoltaic cell assembly. One is to judge the bus voltage stability on the DC side of the inverter, and the other is to judge the PV voltage, PV current or PV power output by the photovoltaic cell assembly.

[0029] like Figure 7 As shown, the DC boost circuit in the start-up inverter transforms the output voltage of the photovoltaic cell assembly and then monitors the bus voltage of the DC bus. If the bus voltage is greater than a specific threshold voltage state for a certain period of time, it means that the output power of the photovoltaic cell assembly, that is, the PV power, is sufficient, and the bus capacitor can be charged through the photovoltaic cell assembly, otherwise the power grid is used to charge the bus capacitor.

[0030] like Figure 8 As shown, the output of the photovoltaic cell assembly is monitored. If one of the PV voltage, PV current or PV power is greater than a specific threshold, it means that the output power of the photovoltaic cell assembly, that is, the PV power, is sufficient, and the bus capacitor can be charged through the photovoltaic cell assembly, otherwise the power grid is used to charge the bus capacitor.

[0031] An AC starting circuit may be provided in the inverter, and after introducing AC power from one side of the power grid through the AC starting circuit and converting it into DC power, the bus capacitor is charged, thereby increasing the bus voltage to a required voltage value. The input side of the AC starting circuit is connected to the power grid, and the output side is connected to the bus capacitor, and is used to draw power from the power grid to charge the bus capacitor. When the insulation impedance of the inverter is detected, the AC starting circuit is stopped.

[0032] The control switch inside the AC starting circuit is disconnected when the inverter insulation impedance is detected, that is, the bus voltage will not be affected by the voltage fluctuation of the power grid when the inverter insulation impedance is detected, which can further improve the accuracy of the insulation impedance detection.

[0033] In addition, before performing the inverter insulation impedance test, the load mounted on the DC bus, such as fans and energy storage auxiliary sources, can be turned off. In addition, the mounted load can be operated in low power consumption mode, such as USB devices and converter (BST, inverter) drive circuit blocking.

[0034] Specifically, the AC starting circuit can generally include a rectifier circuit and a control switch, the input side of the rectifier circuit is connected to the power grid, and the output side is connected to the bus capacitor. The control switch is connected in series with the rectifier circuit to control the conduction or disconnection between the bus capacitor and the power grid.

[0035] For the circuit that specifically performs inverter insulation impedance detection, the insulation impedance detection method of the photovoltaic inverter of the present application preferably adopts a technical solution of two groups of discharge branches, the first discharge branch is arranged between the negative bus and the PE terminal, and the second discharge branch is arranged between the positive bus and the PE terminal. When performing the inverter insulation impedance detection, two control signals are output at intervals, and the actions of the control switches in each discharge branch in the two groups of discharge branches are controlled in time periods by each control signal to realize the connection and switching of each discharge branch to the DC bus. For different switching conditions of the discharge branch, the detection values ​​of the bus voltage in the inverter and the voltage of any pole of the DC bus to the ground are respectively obtained, and the insulation impedance value of the DC side of the inverter to the ground is calculated according to each of the detection values.

[0036] And when the insulation impedance detection of the inverter is performed, there is a time gap between the two control signals of the control switches in the two groups of discharge branches, and the bus capacitor is controlled to be charged until the bus voltage is increased to the second voltage threshold value during the time gap. In other words, after the first discharge branch is switched on and connected to complete the detection, the second discharge branch is controlled to switch on and connect to the DC bus after a certain time gap, and the bus voltage of the DC bus is increased during the time gap. This control design can ensure that the bus voltage of the two groups of discharge branches is maintained within a stable voltage threshold range when they are switched on, thereby ensuring the accuracy of the insulation impedance detection.

[0037] by Figure 1 Taking the inverter and insulation impedance detection circuit shown as an example, the inverter circuit in the inverter is a three-phase T-type inverter circuit, the photovoltaic cell assembly is connected to the DC side of the inverter circuit via a DC bus, a bus capacitor is arranged between the DC bus, and each phase line of the AC side of the inverter circuit is connected to each phase of the power grid via relays S1 and S2; the AC starting circuit includes a rectifier circuit composed of a diode D1 and a diode D2, the control switch is a relay K, the input side of the rectifier circuit is connected to the power grid, and the output side is connected to the bus capacitor, the relay K is connected in series with the rectifier circuit, and the relay K is used to control the conduction or disconnection between the bus capacitor and the power grid. In addition, current limiting protection resistors R1 and R2 are also connected in series in the AC starting circuit. The AC starting circuit draws power from the B and C phases of the power grid, and the power drawing position is located between the relay S1 and the relay S2. In this way, when drawing power from the power grid, the relay S2 is closed and the relay S1 is disconnected. The bus capacitor can be charged through the AC starting circuit while the connection between the power grid and the AC side of the inverter is disconnected.

[0038] The detection circuit for performing insulation impedance detection in this embodiment includes a first discharge branch and a second discharge branch, wherein the first discharge branch is arranged between the negative bus and the PE terminal, and the second discharge branch is arranged between the positive bus and the PE terminal. Figure 1 As shown, the first discharge branch includes a resistor R3, a resistor R5 and an ISO relay S3, the resistor R3 is connected in series with the ISO relay S3 and connected between the negative bus and the PE terminal, and the resistor R5 is connected in parallel with the ISO relay S3; the second discharge branch includes a resistor R4, a resistor R6 and an ISO relay S4, the resistor R4 is connected in series with the ISO relay S4 and connected between the positive bus and the PE terminal, and the resistor R6 is connected in parallel with the ISO relay S3.

[0039] In order to better explain the calculation process of the insulation impedance detection performed by the detection circuit of this embodiment, this detection circuit can be equivalent to the following: Figure 2 The circuit diagram shown, in which the bus voltage between the positive and negative busbars of the DC bus is equivalent to the voltage source Ubus, and the voltage between the negative busbar BUS- and the PE terminal is equivalent to the series connection of the voltage source U and the insulation resistance Riso, wherein the voltage of the negative busbar BUS- to the PE terminal is called the ISO voltage, recorded as Uiso. When detecting the insulation resistance Riso, firstly energize the ISO relay S3 and disconnect the ISO relay S4, and record the bus voltage Ubus1 and the ISO voltage Uiso1; then disconnect the ISO relay S3 and energize the ISO relay S4, and record the bus voltage Ubus2 and the ISO voltage Uiso2. The insulation resistance Riso can be calculated through the bands Ubus1, Uiso1, Ubus2, and Uiso2. The calculation formula of the insulation resistance Riso is: Among them, Ro=R3+R4, Rc=R3.

[0040] It can be imagined that the present application can maintain Ubus1 and Ubus2 basically within a stable range through voltage detection and control of the bus voltage, so they can be directly regarded as the voltage source Ubus, which can ensure that the insulation impedance result finally calculated is accurate and reliable.

[0041] The working process of the insulation impedance detection method of this embodiment is as follows: Figure 3 As shown, this embodiment is based on Figure 1 The AC starting circuit in the circuit structure shown is connected to increase the bus voltage. The specific working process is as follows: (1) Verify the current insulation impedance detection times to see if they exceed the preset upper limit of monitoring times. If they do, the insulation impedance detection fails. Otherwise, the insulation impedance detection begins.

[0042] (2) Turn off relay S1 and relay S2, then close and turn on ISO relay S3, turn off ISO relay S4, and then sample the bus voltage and ISO voltage. This step is called the first ISO detection process. In the first ISO detection process, it is detected in real time whether the bus voltage is lower than the first voltage threshold; If the bus voltage detected is lower than the first voltage threshold, ISO relay S3 and ISO relay S4 are turned off, and then the two sub-relays S2B and S2C corresponding to phase B and phase C in relay S2 are closed, and relay K is closed. The AC starting circuit draws power from the grid to charge the bus until the bus voltage rises to the second voltage threshold and then returns to step (1); If the bus voltage obtained through detection is not lower than the first voltage threshold, the ISO detection is continued.

[0043] (3) Close and turn on the ISO relay S4, turn off the ISO relay S3, and sample the bus voltage and ISO voltage. This step is called the second ISO detection process. In the first ISO detection process, it is detected in real time whether the bus voltage is lower than the first voltage threshold; If the bus voltage detected is lower than the first voltage threshold, ISO relay S3 and ISO relay S4 are turned off, and then the two sub-relays S2B and S2C corresponding to phase B and phase C in relay S2 are closed, and relay K is closed. The AC starting circuit draws power from the grid to charge the bus until the bus voltage rises to the second voltage threshold and then returns to step (1); If the bus voltage obtained through detection is not lower than the first voltage threshold, the ISO detection is continued.

[0044] (4) Calculate the insulation resistance Riso of the PV inverter based on the bus voltage and ISO voltage obtained in step (2) and step (3).

[0045] If there is no process failure in each step of the above working process, the timing of the insulation impedance test is as follows: Figure 4 As shown, the timing is interpreted as follows: At time t1->t2, the bus voltage of the DC bus is increased and detected. In this embodiment, the third voltage threshold is designed to be equal to the first voltage threshold, and the fourth voltage threshold is designed to be equal to the second voltage threshold. Starting from time t1, ISO relay S3 and ISO relay S4 are turned off, and then the two sub-relays S2B and S2C corresponding to phase B and phase C in relay S2 are closed, and relay K is closed. The AC starting circuit draws power from the power grid to charge the bus until the bus voltage is increased to or above the second voltage threshold; At time t2->t3, the first ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold; The time t3->t4 is the time gap (the time interval between the two control signals sent to the ISO relay S3 and the ISO relay S4). During this time gap, the ISO relay S3 and the ISO relay S4 are turned off, the relay K is closed, and the AC starting circuit takes power from the grid to charge the bus until the bus voltage rises to or above the second voltage threshold. At time t4->t5, the second ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold.

[0046] Finally, the calculation of the inverter insulation impedance is completed according to the bus voltage and ISO voltage obtained in the first ISO detection process and the second ISO detection process.

[0047] If there is a process failure in step (2), after the first ISO detection process fails because the bus voltage obtained during the detection process is lower than the first voltage threshold, the timing of re-performing the first ISO detection process is as follows: Figure 5 As shown, the timing is interpreted as follows: At time t1->t2, the bus voltage of the DC bus is increased and detected, so that the bus voltage is increased to or above a second voltage threshold; At time t2->t3, the first ISO detection process is performed, during which the bus voltage detected is lower than the first voltage threshold; At time t3->t4, ISO relay S3 and ISO relay S4 are turned off, relay K is closed, and the AC starting circuit takes power from the grid to charge the bus until the bus voltage rises to the second voltage threshold, and then the first ISO detection process continues; At time t4->t5, the first ISO detection process is performed again, and the bus voltage detected in this process is not lower than the first voltage threshold; The time interval t5->t6 is the time interval during which the AC starting circuit draws power from the grid to charge the bus until the bus voltage increases to or above the second voltage threshold; At time t6->t7, the second ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold.

[0048] Finally, the calculation of the inverter insulation impedance is completed according to the bus voltage and ISO voltage obtained by the first ISO detection process at time t4->t5 and the second ISO detection process at time t6->t7.

[0049] If there is a process failure in step (3), the ISO detection second process fails because the bus voltage obtained during the detection is lower than the first voltage threshold, the timing of re-performing the ISO detection first process and the ISO detection second process is as follows: Figure 6 As shown, the timing is interpreted as follows: At time t1->t2, the bus voltage of the DC bus is increased and detected, so that the bus voltage is increased to or above a second voltage threshold; At time t2->t3, the first ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold; The time interval t3->t4 is the time interval during which the AC starting circuit draws power from the grid to charge the bus until the bus voltage increases to or above the second voltage threshold; At time t4->t5, the second ISO detection process is performed, during which the bus voltage detected is lower than the first voltage threshold; At t5->t6, the ISO relay S3 and the ISO relay S4 are turned off, and the relay K is closed. The AC starting circuit takes power from the grid to charge the bus until the bus voltage rises to the second voltage threshold, and then prepares to re-perform the first ISO detection process; At time t6->t7, the first ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold; The time interval t7->t8 is the time interval during which the AC starting circuit draws power from the grid to charge the bus until the bus voltage increases to or above the second voltage threshold; At time t8->t9, the second ISO detection process is performed, during which the bus voltage detected is not lower than the first voltage threshold.

[0050] Finally, the calculation of the inverter insulation impedance is completed according to the bus voltage and ISO voltage obtained by the first ISO detection process at time t6->t7 and the second ISO detection process at time t8->t9.

[0051] It can be seen from the above timing diagrams that when performing insulation impedance detection, the inverter is put into low-power operation mode through enabling control, and controllable loads such as fans, energy storage auxiliary sources, USB devices, drive power supplies, etc. are turned off.

[0052] Embodiment 2: This embodiment describes a photovoltaic inverter, wherein the DC side of the inverter is connected to a DC power supply via a DC bus, the AC side of the inverter is connected to a power grid, a bus capacitor is arranged between the DC bus, and a detection circuit for detecting the insulation impedance of the inverter is arranged between the DC bus and the PE terminal, and the detection circuit operates according to the insulation impedance detection method described in Example 1.

[0053] Embodiment 3: This embodiment describes a photovoltaic system, which includes the photovoltaic inverter described in Embodiment 2.

[0054] In summary, the insulation impedance detection method of the photovoltaic inverter of the present application maintains the detection of the bus voltage of the DC bus before and during the insulation impedance detection of the inverter, so that the bus voltage is maintained within a certain threshold range, thereby avoiding the problem of deterioration of the insulation impedance detection accuracy due to changes in the bus voltage, and effectively improving the accuracy of the insulation impedance detection of the inverter.

[0055] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for detecting insulation impedance of a photovoltaic inverter, characterized in that: The DC side of the inverter is connected to a DC power supply through a DC bus, and the AC side of the inverter is connected to a power grid. A bus capacitor is arranged between the DC bus bars, and a detection circuit for performing inverter insulation impedance detection is arranged between the DC bus bar and the PE terminal. When performing the inverter insulation impedance detection, the bus voltage of the DC bus bar is detected. When the bus voltage is lower than a first voltage threshold, the inverter insulation impedance detection is terminated, and the bus voltage is controlled to increase until the bus voltage increases to a second voltage threshold. The inverter is controlled to perform insulation impedance detection, and the second voltage threshold is not less than the first voltage threshold.

2. The method for detecting insulation impedance of a photovoltaic inverter according to claim 1, characterized in that: The detection circuit includes two groups of discharge branches, wherein the first discharge branch is arranged between the negative bus and the PE terminal, and the second discharge branch is arranged between the positive bus and the PE terminal. When performing the insulation impedance detection of the inverter, the first discharge branch or the second discharge branch is switched on and off at intervals to connect the DC bus, and the detection values ​​of the bus voltage and the voltage of any pole of the DC bus to the ground when different discharge branches are switched on are obtained, and the insulation impedance value to the ground of the DC side of the inverter is calculated based on the detection values.

3. The method for detecting insulation impedance of a photovoltaic inverter according to claim 2, characterized in that: When performing the inverter insulation impedance detection, two control signals are output at intervals, each control signal controls the first discharge branch or the second discharge branch to be switched on and off to connect to the DC bus in a time period, and there is a time gap between the two control signals output at intervals, and the bus voltage is controlled to increase to the second voltage threshold within the time gap.

4. The method for detecting insulation impedance of a photovoltaic inverter according to claim 1, characterized in that: Before performing the inverter insulation impedance detection, the bus voltage of the DC bus is detected. If the bus voltage is not lower than the third voltage threshold, the inverter insulation impedance detection is started. Otherwise, the bus voltage is controlled to be increased until the bus voltage is increased to the fourth voltage threshold, and then the inverter insulation impedance detection is started.

5. The method for detecting insulation impedance of a photovoltaic inverter according to claim 4, characterized in that: The third voltage threshold is not less than the first voltage threshold.

6. The method for detecting insulation impedance of a photovoltaic inverter according to any one of claims 1 to 5, characterized in that: In the process of controlling the increase of the bus voltage, The bus capacitor is charged via the DC power supply or the power grid.

7. The method for detecting insulation impedance of a photovoltaic inverter according to claim 6, characterized in that: The DC power source is a photovoltaic cell assembly or an energy storage battery.

8. The method for detecting insulation impedance of a photovoltaic inverter according to claim 6, characterized in that: In the process of charging the bus capacitor with a DC power supply, the output power of the DC power supply is judged. If the output power of the DC power supply is greater than a preset DC power threshold, the DC power supply is used to charge the bus capacitor; otherwise, the power grid is used to charge the bus capacitor.

9. The method for detecting insulation impedance of a photovoltaic inverter according to claim 6, characterized in that: The inverter is provided with an AC starting circuit, the input side of which is connected to the power grid, and the output side of which is connected to the bus capacitor, for charging the bus capacitor by drawing power from the power grid, and stopping the operation of the AC starting circuit when performing insulation impedance detection of the inverter.

10. The method for detecting insulation impedance of a photovoltaic inverter according to claim 9, characterized in that: The AC starting circuit includes a rectifier circuit and a control switch. The input side of the rectifier circuit is connected to the power grid, and the output side is connected to the bus capacitor. The control switch is connected in series with the rectifier circuit to control the conduction or disconnection between the bus capacitor and the power grid.

11. A photovoltaic inverter, characterized in that: The DC side of the inverter is connected to a DC power supply through a DC bus, the AC side of the inverter is connected to a power grid, a bus capacitor is arranged between the DC bus, and a detection circuit for inverter insulation impedance detection is arranged between the DC bus and the PE terminal, and the detection circuit works according to the insulation impedance detection method according to any one of claims 1 to 10.

12. A photovoltaic system, characterized in that: Comprising the photovoltaic inverter as claimed in claim 11.

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