Insulation Impedance Detection Method for Photovoltaic Inverter, Photovoltaic Inverter, and Photovoltaic System

Abstract

The present invention relates to a method for detecting the insulation impedance of a photovoltaic inverter, a photovoltaic inverter, and a photovoltaic system. When detecting the insulation impedance of the inverter, the bus voltage of the DC bus is detected. When the bus voltage is lower than a first voltage threshold, the detection of the insulation impedance of the inverter is terminated, and the increase of the bus voltage is controlled until the bus voltage is increased to a second voltage threshold, and then the inverter is controlled to perform the insulation impedance detection. The second voltage threshold is not less than the first voltage threshold. In this way, during the process of detecting the insulation impedance of the inverter, the bus voltage can be maintained within a certain threshold range, thereby avoiding the problem that the accuracy of the insulation impedance detection deteriorates due to the change of the bus voltage, effectively improving the accuracy of the insulation impedance detection of the inverter, and further ensuring that the operation of the photovoltaic inverter and the photovoltaic system is safer and more stable.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter safety control, and particularly relates to a method for detecting insulation impedance of a photovoltaic inverter, a photovoltaic inverter, and a photovoltaic system. Background Art

[0002] As a device for converting direct current electrical energy into alternating current electrical energy, an inverter plays a crucial role in applications such as photovoltaic power stations. In order to protect the safe and stable operation of the inverter, it is an essential step to perform insulation impedance detection (ISO detection) before the system is connected to the grid. The main purpose of the inverter insulation impedance detection is to measure the insulation resistance between the input end of the photovoltaic array and the ground to ensure that its resistance value meets the safety requirements, thereby avoiding the electric shock risk caused by excessive residual leakage current.

[0003] Currently, photovoltaic inverters on the market generally adopt a method of calculating the insulation resistance by detecting the voltages of PV+ to the ground and PV- to the ground. However, there are still some deficiencies in the existing insulation impedance detection technology 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 the phenomenon of ISO false negative. Specifically, the existing insulation impedance detection method of a photovoltaic inverter usually includes sampling the voltage values before and after the sampling relay is closed, 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, during self-check, since the sampling voltage changes with the decrease of the bus voltage, the calculated impedance value is larger than the actual external impedance, and then the ISO calculated value is larger than the actual value, thus resulting in the phenomenon of ISO false negative. Summary of the Invention

[0004] A main object of the present invention is to overcome at least one of the above-mentioned defects, and to provide a method for detecting insulation impedance of a photovoltaic inverter, a photovoltaic inverter, and a photovoltaic system, which can accurately and reliably detect the insulation impedance between the inverter and the ground, and avoid the problem of poor insulation impedance detection accuracy caused by the change of the bus voltage.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] 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 the power grid, a bus capacitor is arranged between the DC buses, and a detection circuit for detecting the insulation impedance of the inverter is arranged between the DC bus and the PE terminal. When detecting the insulation impedance of the inverter, the bus voltage of the DC bus is detected. When the bus voltage is lower than the first voltage threshold, the insulation impedance detection of the inverter is terminated, and the increase of the bus voltage is controlled until the bus voltage is increased to the second voltage threshold, and then the inverter is controlled to perform the insulation impedance detection. The second voltage threshold is not less than the first voltage threshold.

[0007] According to one embodiment of the present invention, the detection circuit includes two discharge branches. Among them, 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 intermittently switched to connect to the DC bus, and the detection values of the bus voltage and the voltage of any pole of the DC bus to the ground are obtained when different discharge branches are switched. The insulation impedance value of the DC side of the inverter to the ground is calculated according to the detection values.

[0008] According to one embodiment of the present invention, when performing the insulation impedance detection of the inverter, two control signals are intermittently output. Each control signal controls the first discharge branch or the second discharge branch to be switched to connect to the DC bus in different time periods. There is a time gap between the two intermittently output control signals, and during the time gap, the bus voltage is controlled to be increased to the second voltage threshold.

[0009] According to one embodiment of the present invention, before performing the insulation impedance detection of the inverter, the bus voltage of the DC bus is detected. If the bus voltage is not less than the third voltage threshold, the insulation impedance detection of the inverter is started; otherwise, the increase of the bus voltage is controlled until the bus voltage is increased to the fourth voltage threshold and then the insulation impedance detection of the inverter is started.

[0010] According to one embodiment of the present invention, the third voltage threshold is not less than the first voltage threshold.

[0011] According to one embodiment of the present invention, during the process of controlling the increase of the bus voltage,

[0012] the bus capacitor is charged through the DC power supply or the power grid.

[0013] According to one embodiment of the present invention, the DC power supply is a photovoltaic cell module or a storage battery.

[0014] 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.

[0015] 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, and stops the operation of the AC starting circuit when detecting the insulation impedance of the inverter.

[0016] According to one embodiment of the present invention, the AC starting circuit includes a rectifying circuit and a control switch. The input side of the rectifying circuit is connected to the power grid, the output side is connected to the bus capacitor, and the control switch is connected in series with the rectifying circuit and is used to control the conduction or cut-off between the bus capacitor and the power grid.

[0017] In particular, the present invention also provides 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 the power grid, a bus capacitor is provided between the DC buses, and a detection circuit for detecting the insulation impedance of the inverter is provided between the DC bus and the PE terminal. The detection circuit operates according to the insulation impedance detection method described above.

[0018] In addition, the present invention also provides a photovoltaic system, which includes the photovoltaic inverter described above.

[0019] Compared with the prior art, the advantages and beneficial effects of the insulation impedance detection method, the photovoltaic inverter, and the photovoltaic system of the present invention patent application are as follows:

[0020] 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 detecting the insulation impedance of the inverter, so that the bus voltage is maintained within a certain threshold range, thereby avoiding the problem that the detection accuracy of the insulation impedance deteriorates due to the change of the bus voltage, effectively improving the accuracy of the insulation impedance detection of the inverter, and further ensuring that the photovoltaic inverter and the photovoltaic system work more safely and stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Some specific embodiments of the present invention will be described in detail hereinafter with reference to the drawings in an exemplary but not restrictive manner. The same reference numerals in the drawings denote the same or similar components or parts. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings:

[0022] Figure 1 is a schematic structural diagram of an inverter and an insulation impedance detection circuit according to an embodiment of the present invention;

[0023] Figure 2 is a schematic equivalent circuit diagram of an insulation impedance detection circuit according to an embodiment of the present invention;

[0024] Figure 3 is a schematic working flow diagram of an inverter insulation impedance detection method according to an embodiment of the present invention;

[0025] Figure 4 is a schematic timing diagram for performing inverter insulation impedance detection according to an embodiment of the present invention, wherein it is for smoothly performing insulation impedance detection;

[0026] Figure 5 is a schematic timing diagram for performing inverter insulation impedance detection according to an embodiment of the present invention, wherein it is for re-detection after failure in the first process of ISO detection;

[0027] Figure 6 is a schematic timing diagram for performing inverter insulation impedance detection according to an embodiment of the present invention, wherein it is for re-detection after failure in the second process of ISO detection;

[0028] Figure 7 is a schematic diagram of the judgment process for AC-DC charging according to an embodiment of the present invention;

[0029] Figure 8 is a schematic diagram of the judgment process for AC-DC charging according to an embodiment of the present invention;

[0030] Figure 9 is a schematic flow diagram for detecting and charging the bus voltage before performing inverter insulation impedance detection according to an embodiment of the present invention. Detailed implementation manners

[0031] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0032] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] Embodiment 1:

[0034] This embodiment describes an insulation impedance detection method for a photovoltaic inverter, asFigure 1 As shown, 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 the power grid. A bus capacitor is provided between the DC buses. A detection circuit for detecting the insulation impedance of the inverter is provided between the DC bus and the PE terminal. When detecting the insulation impedance of the inverter, the bus voltage of the DC bus is detected. As Figure 3 shown, when the bus voltage is lower than the first voltage threshold, the insulation impedance detection of the inverter is terminated, and the increase of the bus voltage is controlled until the bus voltage is increased to the second voltage threshold, and the inverter is controlled to perform the insulation impedance detection. The second voltage threshold is not less than the first voltage threshold.

[0035] The second voltage threshold and the first voltage threshold are preset fixed values or a value range. By monitoring the bus voltage of the DC bus and controlling the charging under necessary conditions, the bus voltage of the DC bus can be guaranteed to be maintained above a relatively stable threshold during the insulation impedance detection, avoiding the problem of large sampling voltage value errors caused by the fluctuation of the bus voltage during the insulation impedance detection, thereby effectively improving the accuracy and reliability of the final insulation impedance detection.

[0036] In some embodiments, the bus voltage can also be detected before the insulation impedance detection of the inverter to ensure the voltage stability before the start of the insulation impedance detection. As Figure 9 shown, before the insulation impedance detection of the inverter, the bus voltage of the DC bus is first detected. If the bus voltage is not lower than the third voltage threshold, the insulation impedance detection of the inverter is started. Otherwise, the increase of the bus voltage is controlled until the bus voltage is increased to the fourth voltage threshold and the insulation impedance detection of the inverter 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 slightly less than or greater than the second voltage threshold, and can be set according to the specific working conditions.

[0037] Based on the above detection and charging control of the bus voltage during the insulation impedance detection, the bus voltage of the DC bus can be higher than a certain voltage threshold before the insulation impedance detection of the inverter, and the bus capacitor can be charged under necessary conditions so that the starting bus voltage before the insulation impedance detection of the inverter is maintained within a certain threshold range, thereby avoiding the problem of poor insulation impedance detection accuracy caused by inconsistent starting bus voltages, and effectively improving the accuracy of the insulation impedance detection of the inverter.

[0038] When boosting the bus voltage as described above, the bus capacitor can be charged through the DC power supply or the power grid. That is to say, in necessary cases, the bus capacitor can be charged through the DC power supply or the power grid, so as to charge the bus capacitor during the inverter insulation impedance detection to increase the bus voltage from lower than the first voltage threshold to the second voltage threshold or higher than the second voltage threshold, or to charge the bus capacitor before the inverter insulation impedance detection to increase the bus voltage from lower than the third voltage threshold to the fourth voltage threshold or higher than the fourth voltage threshold.

[0039] When charging the bus capacitor as described above, the available DC power supply can be a photovoltaic cell module or a storage battery.

[0040] 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, during 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.

[0041] Generally, there are two methods for judging the output power of the photovoltaic cell module. One is to judge the stability of the bus voltage 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 module.

[0042] As Figure 7 shown, start the DC boost circuit in the inverter to transform the output voltage of the photovoltaic cell module and then monitor the bus voltage of the DC bus. If the bus voltage is greater than the specific threshold voltage for a certain period of time, it means that the output power of the photovoltaic cell module, i.e., the PV power, is sufficient, and the bus capacitor can be charged through the photovoltaic cell module; otherwise, the power grid is used to charge the bus capacitor.

[0043] As Figure 8 shown, monitor the output of the photovoltaic cell module. If any one of the PV voltage, PV current or PV power is greater than the specific threshold, it means that the output power of the photovoltaic cell module, i.e., the PV power, is sufficient, and the bus capacitor can be charged through the photovoltaic cell module; otherwise, the power grid is used to charge the bus capacitor.

[0044] An AC starting circuit can be set in the inverter. After introducing the alternating current on the grid side through the AC starting circuit and converting it into direct current, the bus capacitor is charged, so as to raise the bus voltage to the required voltage value. The input side of the AC starting circuit is connected to the grid, and the output side is connected to the bus capacitor, which is used to draw power from the grid to charge the bus capacitor, and stops the operation of the AC starting circuit when detecting the insulation impedance of the inverter.

[0045] When detecting the insulation impedance of the inverter, the control switch inside the AC starting circuit is disconnected. That is to say, when detecting the insulation impedance of the inverter, the bus voltage will not be affected by the voltage fluctuation of the grid, which can further improve the accuracy of the insulation impedance detection.

[0046] In addition, before detecting the insulation impedance of the inverter, the loads mounted on the DC bus can be turned off, such as fans and energy storage auxiliary sources. In addition, the mounted loads can also be made to work in a low-power mode, such as turning off the driving circuits of USB devices and converters (BST, inverter).

[0047] Specifically, the AC starting circuit generally can include a rectifier circuit and a control switch. The input side of the rectifier circuit is connected to the grid, and the output side is connected to the bus capacitor. The control switch is connected in series with the rectifier circuit and is used to control the conduction or disconnection between the bus capacitor and the grid.

[0048] For the circuit specifically for detecting the insulation impedance of the inverter, the insulation impedance detection method of the photovoltaic inverter of the present application preferably adopts the technical solution of two 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 detecting the insulation impedance of the inverter, two control signals are output at intervals, and the action of the control switch in each discharge branch of the two discharge branches is controlled in different time periods through each control signal, so as to realize the connection and switching of each discharge branch to the DC bus. For different switching situations of the discharge branch, the detection values of the bus voltage and the voltage of any pole of the DC bus to the ground in the inverter are respectively obtained, and according to each of the detection values, the insulation impedance value of the DC side of the inverter to the ground is calculated.

[0049] When performing the inverter insulation impedance detection, there is a time gap between the two control signals for the control switches in the two discharge branches output at intervals. During this time gap, the bus capacitor is controlled to be charged until the bus voltage is increased to the second voltage threshold. That is to say, after the first discharge branch is switched in and the detection is completed, after a certain time gap, the second discharge branch is controlled to be switched and connected to the DC bus. During the time gap, the bus voltage of the DC bus is increased. This control design can ensure that the bus voltage during the switching-in of the two discharge branches is maintained within a stable voltage threshold range, thereby ensuring the accuracy of the insulation impedance detection.

[0050] Taking Figure 1 the inverter and the 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 module is connected to the DC side of the inverter circuit through the DC bus. A bus capacitor is provided between the DC buses. Each phase line on the AC side of the inverter circuit is connected to each phase of the power grid through the relay S1 and the relay S2; the AC startup circuit includes a rectifier circuit composed of the diode D1 and the diode D2. The control switch is the 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. The relay K is used to control the conduction or cut-off 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 startup circuit. The AC startup circuit takes power from the B and C phases of the power grid, and the power-taking position is between the relay S1 and the relay S2. In this way, when taking power from the power grid, the relay S2 is closed and the relay S1 is opened, so that the connection between the power grid and the AC side of the inverter can be disconnected while the bus capacitor is charged through the AC startup circuit.

[0051] The detection circuit for performing the insulation impedance detection in this embodiment includes a first discharge branch and a second discharge branch. Among them, 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. Specifically, as Figure 1 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 then connected between the negative bus and the PE terminal. 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 then connected between the positive bus and the PE terminal. The resistor R6 is connected in parallel with the ISO relay S4.

[0052] To better explain the calculation process of the insulation impedance detection by the detection circuit of this embodiment and to simplify the calculation, the resistance values of resistor R3 and resistor R4 are selected to be equal, and the resistance values of resistor R5 and resistor R6 are selected to be equal. This detection circuit can be equivalent to the circuit diagram as shown in Figure 2 As shown. Among them, the bus voltage between the positive and negative buses of the DC bus is equivalent to the voltage source Ubus, and the voltage between the negative bus BUS- and the PE terminal is equivalent to the series connection of the voltage source U and the insulation impedance Riso. The voltage of the negative bus BUS- to the PE terminal is called the ISO voltage, denoted as Uiso. When detecting the insulation impedance Riso, first close the ISO relay S3 and open the ISO relay S4, and record the bus voltage Ubus1 and the ISO voltage Uiso1; then open the ISO relay S3 and close the ISO relay S4, and record the bus voltage Ubus2 and the ISO voltage Uiso2. The insulation impedance Riso can be calculated through Ubus1, Uiso1, Ubus2, and Uiso2. The calculation formula of the insulation impedance Riso is

[0053]

[0054] where Ro = R3 + R4 and Rc = R3.

[0055] It can be imagined that through the voltage detection and control of the bus voltage in this application, Ubus1 and Ubus2 can be basically maintained within a stable range, so it can be directly regarded as the voltage source Ubus, which can ensure the accuracy and reliability of the finally calculated insulation impedance result.

[0056] The working process of the insulation impedance detection method of this embodiment is as shown in Figure 3 As shown. This embodiment uses the AC start circuit in the circuit structure shown in Figure 1 to access and boost the bus voltage. The specific working process is as follows:

[0057] (1) Verify the number of times of the current insulation impedance detection to check whether it exceeds the preset upper limit of the monitoring times. If it exceeds, the insulation impedance detection fails; otherwise, start the insulation impedance detection.

[0058] (2) Turn off the relay S1 and the relay S2, then close and conduct the ISO relay S3 and turn off the ISO relay S4, and then sample the bus voltage and the ISO voltage. This step is called the first process of ISO detection. During the first process of ISO detection, continuously detect whether the bus voltage is lower than the first voltage threshold;

[0059] If the bus voltage obtained by detection is lower than the first voltage threshold, turn off the ISO relays S3 and S4, then close the two sub-relays S2B and S2C corresponding to the B-phase and C-phase in the relay S2, close the relay K, and the AC starting circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold and then return to step (1);

[0060] If the bus voltage obtained by detection is not lower than the first voltage threshold, continue the ISO detection.

[0061] (3)Close and turn on the ISO relay S4 and turn off the ISO relay S3, sample the bus voltage and the ISO voltage. This step is called the second process of ISO detection. During the first process of ISO detection, it is detected in real time whether the bus voltage is lower than the first voltage threshold;

[0062] If the bus voltage obtained by detection is lower than the first voltage threshold, turn off the ISO relays S3 and S4, then close the two sub-relays S2B and S2C corresponding to the B-phase and C-phase in the relay S2, close the relay K, and the AC starting circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold and then return to step (1);

[0063] If the bus voltage obtained by detection is not lower than the first voltage threshold, continue the ISO detection.

[0064] (4)Calculate the insulation impedance Riso of the photovoltaic inverter according to the bus voltage and the ISO voltage obtained in step (2) and step (3).

[0065] If there is no process failure in each step of the above working process, the timing of the insulation impedance detection is as Figure 4 shown, and the interpretation of this timing is as follows:

[0066] From t1 to t2, the bus voltage of the DC bus is being boosted 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 the t1 moment, turn off the ISO relays S3 and S4, then close the two sub-relays S2B and S2C corresponding to the B-phase and C-phase in the relay S2, close the relay K, and the AC starting circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold or above;

[0067] From t2 to t3, the first process of ISO detection is carried out, and during this process, the bus voltage obtained by detection is not lower than the first voltage threshold;

[0068] The time period from t3 to t4 is the time gap (the time interval between the two control signals sent to ISO relay S3 and ISO relay S4). During this time gap, ISO relay S3 and ISO relay S4 are turned off, and relay K is closed. The AC startup circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold or higher.

[0069] The time period from t4 to t5 is the second ISO detection process. During this process, the detected bus voltage is not lower than the first voltage threshold.

[0070] Finally, based on the bus voltage and ISO voltage obtained from the first ISO detection process and the second ISO detection process, the calculation of the inverter insulation impedance is completed.

[0071] If there is a process failure in step (2), after the first ISO detection process fails because the detected bus voltage during the process is lower than the first voltage threshold, the timing for re - performing the first ISO detection process is as Figure 5 shown, and the interpretation of this timing is as follows:

[0072] The time period from t1 to t2 is for boosting and detecting the bus voltage of the DC bus so that the bus voltage rises to the second voltage threshold or higher.

[0073] The time period from t2 to t3 is the first ISO detection process. During this process, the detected bus voltage is lower than the first voltage threshold.

[0074] The time period from t3 to t4, ISO relay S3 and ISO relay S4 are turned off, and relay K is closed. The AC startup circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold, and then the first ISO detection process continues.

[0075] The time period from t4 to t5 is the first ISO detection process again. During this process, the detected bus voltage is not lower than the first voltage threshold.

[0076] The time period from t5 to t6 is the time gap. During this time gap, the AC startup circuit draws power from the power grid to charge the bus until the bus voltage rises to the second voltage threshold or higher.

[0077] The time period from t6 to t7 is the second ISO detection process. During this process, the detected bus voltage is not lower than the first voltage threshold.

[0078] Finally, based on the bus voltage and ISO voltage obtained from the first ISO detection process at t4 - >t5 and the second ISO detection process at t6 - >t7, the calculation of the inverter insulation impedance is completed.

[0079] If there is a process failure in step (3), after the ISO detection second process fails because the bus voltage obtained during the detection in the process is lower than the first voltage threshold, the timing sequence of re-performing the ISO detection first process and the ISO detection second process is as Figure 6 shown, and the interpretation of this timing sequence is as follows:

[0080] From time t1 to t2, the bus voltage of the DC bus is boosted and detected to boost the bus voltage to the second voltage threshold or above;

[0081] From time t2 to t3, the ISO detection first process is performed, and the bus voltage obtained during this process is not lower than the first voltage threshold;

[0082] From time t3 to t4, it is the time interval. During this time interval, the AC startup circuit draws power from the power grid to charge the bus until the bus voltage is boosted to the second voltage threshold or above;

[0083] From time t4 to t5, the ISO detection second process is performed, and the bus voltage obtained during this process is lower than the first voltage threshold;

[0084] From time t5 to t6, the ISO relay S3 and the ISO relay S4 are turned off, the relay K is closed, and the AC startup circuit draws power from the power grid to charge the bus until the bus voltage is boosted to the second voltage threshold, and then prepares to re-perform the ISO detection first process;

[0085] From time t6 to t7, the ISO detection first process is performed, and the bus voltage obtained during this process is not lower than the first voltage threshold;

[0086] From time t7 to t8, it is the time interval. During this time interval, the AC startup circuit draws power from the power grid to charge the bus until the bus voltage is boosted to the second voltage threshold or above;

[0087] From time t8 to t9, the ISO detection second process is performed, and the bus voltage obtained during this process is not lower than the first voltage threshold.

[0088] Finally, according to the bus voltage and the ISO voltage obtained during the ISO detection first process from time t6 to t7 and the ISO detection second process from time t8 to t9, the calculation of the inverter insulation impedance is completed.

[0089] As can be seen in the above timing diagrams, when performing the insulation impedance detection, through enabling control, the inverter is in a low-power operating mode, and the controllable loads are turned off, such as fans, energy storage auxiliary sources, USB devices, drive power supplies, etc.

[0090] Embodiment 2:

[0091] This embodiment describes a photovoltaic inverter. The DC side of the inverter is connected to a DC power source through a DC bus. The AC side of the inverter is connected to the power grid. A bus capacitor is provided between the DC buses. A detection circuit for detecting the insulation impedance of the inverter is provided between the DC bus and the PE terminal. The detection circuit operates according to the insulation impedance detection method described in Embodiment 1.

[0092] Embodiment 3:

[0093] This embodiment describes a photovoltaic system, which includes the photovoltaic inverter described in Embodiment 2.

[0094] In summary, for the insulation impedance detection method of the photovoltaic inverter of the present application, before and during the detection of the insulation impedance of the inverter, the detection of the bus voltage of the DC bus is maintained, so that the bus voltage is maintained within a certain threshold range, thereby avoiding the problem that the detection accuracy of the insulation impedance deteriorates due to the change of the bus voltage, and effectively improving the accuracy of the insulation impedance detection of the inverter.

[0095] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It should not 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 covered within the protection scope of the present invention.

Claims

1. An insulation impedance detection method for a photovoltaic inverter, characterized in that, The DC side of the inverter is connected to a DC power source through a DC bus. The AC side of the inverter is connected to the power grid. A bus capacitor is provided between the DC buses. A detection circuit for detecting the insulation impedance of the inverter is provided between the DC bus and the PE terminal. When detecting the insulation impedance of the inverter, the bus voltage of the DC bus is detected. When the bus voltage is lower than the first voltage threshold, the detection of the insulation impedance of the inverter is terminated, and the increase of the bus voltage is controlled until the bus voltage is increased to the second voltage threshold, and then the inverter is controlled to perform the insulation impedance detection. The second voltage threshold is not less than the first voltage threshold. The detection circuit includes two discharge branches. Among them, the first discharge branch is provided between the negative bus and the PE terminal, and the second discharge branch is provided between the positive bus and the PE terminal. When detecting the insulation impedance of the inverter, the first discharge branch or the second discharge branch is intermittently switched to connect to 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 are obtained, and the insulation impedance value of the DC side of the inverter to the ground is calculated according to the detection values. When detecting the insulation impedance of the inverter, two control signals are intermittently output. Each control signal controls the first discharge branch or the second discharge branch to be switched to connect to the DC bus in different time periods. There is a time gap between the two intermittently output control signals, and during the time gap, the bus voltage is controlled to be increased to the second voltage threshold.

2. The insulation impedance detection method of the photovoltaic inverter according to claim 1, characterized in that, Before detecting the insulation impedance of the inverter, the bus voltage of the DC bus is detected. If the bus voltage is not less than the third voltage threshold, the detection of the insulation impedance of the inverter is started; otherwise, the increase of the bus voltage is controlled until the bus voltage is increased to the fourth voltage threshold and then the detection of the insulation impedance of the inverter is started.

3. The insulation impedance detection method of the photovoltaic inverter according to claim 2, characterized in that, The third voltage threshold is not less than the first voltage threshold.

4. The insulation impedance detection method of the photovoltaic inverter according to any one of claims 1 to 3, characterized in that, During the process of controlling the increase of the bus voltage, the bus capacitor is charged through the DC power source or the power grid.

5. The insulation impedance detection method of the photovoltaic inverter according to claim 4, characterized in that, The DC power source is a photovoltaic cell module or a storage battery.

6. The insulation impedance detection method of the photovoltaic inverter according to claim 4, characterized in that, During the process of charging the bus capacitor with the DC power source, the output power of the DC power source is judged. If the output power of the DC power source is greater than a preset DC power threshold, the bus capacitor is charged with the DC power source; otherwise, the bus capacitor is charged with the power grid.

7. The insulation impedance detection method of the photovoltaic inverter according to claim 4, wherein, 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. The operation of the AC starting circuit is stopped when detecting the insulation impedance of the inverter.

8. The insulation impedance detection method of the photovoltaic inverter according to claim 7, wherein The AC starting circuit includes a rectifying circuit and a control switch. The input side of the rectifying circuit is connected to the power grid, the output side is connected to the bus capacitor, and the control switch is connected in series with the rectifying circuit and is used to control the conduction or cut-off between the bus capacitor and the power grid.

9. A photovoltaic inverter, characterized in that, The DC side of the inverter is connected to a DC power source through a DC bus. The AC side of the inverter is connected to the power grid. A bus capacitor is provided between the DC buses. A detection circuit for detecting the insulation impedance of the inverter is provided between the DC bus and the PE terminal. The detection circuit operates according to the insulation impedance detection method described in any one of claims 1 to 8.

10. A photovoltaic system, characterized in that, It includes a photovoltaic inverter as described in claim 9.

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