Insulation impedance detection circuit and insulation impedance detection method of photovoltaic inverter system

Abstract

A photovoltaic inverter system insulation impedance detection circuit and detection method. The photovoltaic inverter system includes at least one input circuit and an inverter circuit, each input circuit and the inverter circuit are electrically coupled through a positive bus and a negative bus, and there is a first equivalent resistance R between the positive bus and the protective grounding point. p There is a second equivalent resistance R between the negative busbar and the protective grounding point n The detection circuit includes: a bus voltage sampling circuit, coupled to the positive and negative buses, for sampling different first and second bus voltages; a protective grounding voltage sampling circuit, coupled to the protective grounding point, for sampling first and second protective grounding voltages corresponding to the first and second bus voltages, respectively; a controller, coupled to the bus voltage sampling circuit and the protective grounding voltage sampling circuit, for calculating the equivalent insulation impedance according to the first and second bus voltages, the first and second protective grounding voltages, and when the equivalent insulation impedance is greater than or equal to a threshold, the controller controls the inverter circuit to be connected to a power grid.

Description

Technical Field

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

[0002] In order to ensure personal safety, the insulation level of the system needs to be determined before the non-isolated photovoltaic inverter is connected to the grid. If the insulation impedance is less than the standard requirement, the photovoltaic inverter is not allowed to be connected to the grid. The usual insulation impedance detection circuit is composed of multiple relays, switch tubes, diodes and resistors, which increases the size and cost of the photovoltaic inverter.

[0003] The Chinese patent with publication number CN201610243810 discloses an insulation impedance detection circuit and detection method suitable for multi-input photovoltaic inverters. By switching two sets of switches and detection resistors, the insulation impedance can be accurately detected within the full voltage range, which is suitable for multi-string input. However, this patent requires the use of two sets of switches and corresponding drive circuits, which increases the size and cost of the photovoltaic inverter.

[0004] A Chinese patent with publication number CN201610240480 discloses a method and device for detecting the insulation resistance to ground of a two-way input photovoltaic inverter, which also requires the switching of two sets of switches and detection resistors to detect the insulation resistance. This solution analyzes the impact of the string input voltage on the detection accuracy, and additionally improves the accuracy of insulation resistance detection through two controllable short-circuit branches. However, this patent is only applicable to the case of two-way string input, and the detection method for the case of multi-way component input will be very complicated. At the same time, the detection circuit not only requires two sets of switches and corresponding drive circuits, but also requires two controllable short-circuit branches to improve the insulation resistance detection accuracy, which greatly increases the size and cost of the photovoltaic inverter.

[0005] Therefore, there is an urgent need for a simpler insulation impedance detection circuit to further reduce the size and cost of the system. Summary of the invention

[0006] The purpose of the present invention is to provide an insulation resistance detection circuit and an insulation resistance detection method for a photovoltaic inverter system, which can reduce the volume and cost of the insulation resistance detection circuit while ensuring the safe operation of the photovoltaic inverter system and the detection accuracy of the insulation resistance.

[0007] In order to achieve the above object, the present invention provides an insulation impedance detection circuit of a photovoltaic inverter system, wherein the photovoltaic inverter system includes at least one input circuit and an inverter circuit, each of the input circuits and the inverter circuit is electrically coupled through a positive bus and a negative bus, and there is a first equivalent resistance R between the positive bus and a protective grounding point. pThere is a second equivalent resistance R between the negative bus and the protective grounding point n , which is characterized in that the insulation impedance detection circuit comprises:

[0008] A bus voltage sampling circuit, coupled to the positive bus and the negative bus, for sampling different first bus voltages and second bus voltages;

[0009] a protective grounding voltage sampling circuit, coupled to the protective grounding point, for sampling a first protective grounding voltage and a second protective grounding voltage corresponding to the first bus voltage and the second bus voltage, respectively;

[0010] A controller is coupled to the bus voltage sampling circuit and the protective grounding voltage sampling circuit, and is used to calculate the equivalent insulation impedance of the photovoltaic inverter system according to the first bus voltage, the second bus voltage, the first protective grounding voltage, and the second protective grounding voltage, and when the equivalent insulation impedance is greater than or equal to a threshold, the controller is used to control the inverter circuit to be connected to a power grid.

[0011] In one embodiment of the present invention, the input circuit is a multi-way input circuit, and the multi-way input circuit is coupled in parallel to the positive bus and the negative bus.

[0012] In an embodiment of the present invention, each of the input circuits has a maximum power point tracking circuit.

[0013] In an embodiment of the present invention, the insulation impedance detection circuit further includes: a PWM driving circuit, coupled between the controller and the inverter circuit, and configured to drive and control the inverter circuit through a PWM signal.

[0014] In one embodiment of the present invention, the controller is further used to adjust the PWM drive circuit according to different light intensities, wherein when the light intensity is greater than a light intensity threshold, the controller increases the voltage difference between the first bus voltage and the second bus voltage.

[0015] In one embodiment of the present invention, the inverter circuit is a single-phase inverter circuit or a multi-phase inverter circuit.

[0016] In order to achieve the above object, the present invention further provides an insulation impedance detection method for a photovoltaic inverter system, wherein the photovoltaic inverter system includes at least one input circuit and an inverter circuit, each of the input circuits and the inverter circuit is electrically coupled through a positive bus and a negative bus, and there is a first equivalent resistance R between the positive bus and a protective grounding point. p There is a second equivalent resistance R between the negative bus and the protective grounding point n, which is characterized in that the insulation impedance detection method includes:

[0017] Sampling bus voltages, using a bus voltage sampling circuit coupled to the positive bus and the negative bus to sample a first bus voltage and a second bus voltage;

[0018] Sampling the protective grounding voltage, using a protective grounding voltage sampling circuit coupled to the protective grounding point to sample a first protective grounding voltage and a second protective grounding voltage corresponding to the first bus voltage and the second bus voltage respectively;

[0019] The insulation impedance is detected by coupling a controller with the bus voltage sampling circuit and the protective grounding voltage sampling circuit, and calculating the equivalent insulation impedance of the photovoltaic inverter system according to the first bus voltage, the second bus voltage, the first protective grounding voltage and the second protective grounding voltage, and when the equivalent insulation impedance is greater than or equal to a threshold, the inverter circuit is controlled by the controller to be connected to a power grid.

[0020] In another embodiment of the present invention, the input circuit is a multi-way input circuit, and the multi-way input circuit is coupled in parallel to the positive bus and the negative bus, and the equivalent insulation impedance of the photovoltaic inverter system is the parallel value of the equivalent insulation resistance of each channel of the multi-way input circuit.

[0021] In another embodiment of the present invention, each of the input circuits has a maximum power point tracking circuit.

[0022] In another embodiment of the present invention, a PWM driving circuit is coupled between the controller and the inverter circuit, and the inverter circuit is driven and controlled by a PWM signal.

[0023] In another embodiment of the present invention, the insulation impedance detection method also includes: the controller adjusts the PWM drive circuit according to different light intensities, wherein when the light intensity is greater than a light intensity threshold, the controller increases the voltage difference corresponding to the first bus voltage and the second bus voltage.

[0024] In another embodiment of the present invention, the controller adjusts the PWM driving circuit by adjusting a voltage modulation ratio of the inverter circuit or adjusting a switching frequency of the inverter circuit.

[0025] The present invention has the following advantages:

[0026] (1) No additional switching switch and corresponding drive circuit are required, which reduces the size and cost of the photovoltaic inverter system;

[0027] (2) It is suitable for insulation resistance detection of photovoltaic inverter systems with multi-string input. The detection method, detection time and detection accuracy are not affected by the number of input strings and have good versatility.

[0028] (3) By controlling the working state of the inverter circuit to change, the bus voltage changes, and the insulation resistance is solved by establishing a set of equations using two sets of detection values. This method is applicable to the full input voltage range and has good safety.

[0029] (4) By controlling the voltage modulation ratio of the inverter circuit and the switching frequency, the bus voltage can be adjusted over a wide range to ensure the detection accuracy of the insulation resistance.

[0030] 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings.

[0032] Figure 1 A schematic diagram of the structure of an insulation impedance detection circuit according to a preferred embodiment of the present invention;

[0033] Figure 2A for Figure 1 When the bus voltage is U bus+ Insulation impedance detection equivalent circuit when

[0034] Figure 2B for Figure 1 When the bus voltage is U' bus+ Insulation impedance detection equivalent circuit when

[0035] Figure 3 A schematic diagram of an insulation impedance detection method according to a preferred embodiment of the present invention;

[0036] Figure 4 It is a circuit diagram of an insulation impedance detection circuit and detection method shown in the present invention applied to a single-phase photovoltaic inverter with multi-channel string input;

[0037] Figure 5 The insulation impedance detection circuit and detection method shown in the present invention are applied to a circuit diagram of a three-phase photovoltaic inverter with multi-channel string input.

[0038] Figure 6 It is a schematic diagram of string input power curves under different illumination conditions and a stable operating point of the bus voltage control of the present invention. DETAILED DESCRIPTION

[0039] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in a variety of forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the present invention will be comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar structures, and thus their detailed description will be omitted.

[0040] When introducing the elements / components / etc. described and / or illustrated herein, the terms "a", "an", "the", "said", and "at least one" are used to indicate that there are one or more elements / components / etc. The terms "comprising", "including", and "having" are used to indicate an open-ended inclusive meaning and mean that there may be additional elements / components / etc. in addition to the listed elements / components / etc. In addition, the terms "first", "second", etc. in the claims are used only as labels and are not numerical limitations of their objects.

[0041] Figure 1 FIG. 1 is a schematic diagram of the structure of an insulation impedance detection circuit of a photovoltaic inverter system according to a preferred embodiment of the present invention. Figure 1 As shown, the photovoltaic inverter system 100 includes at least one input circuit 10 and an inverter circuit 20. Each of the input circuits 10 and the inverter circuit 20 is electrically coupled via a positive bus 201 and a negative bus 202, and a first equivalent resistance R is provided between the positive bus 201 and a protective grounding point PE. p There is a second equivalent resistance R between the negative bus 202 and the protective grounding point PE n The output end of the inverter circuit 20 is connected to a power grid 30 .

[0042] The insulation impedance detection circuit 40 of the photovoltaic inverter system 100 mainly includes a bus voltage sampling circuit 41, a protective grounding voltage sampling circuit (i.e., a PE voltage sampling circuit) 42, and a controller 43. The bus voltage sampling circuit 41 is coupled to the positive bus 201 and the negative bus 202, and is used to sample different first bus voltages and second bus voltages. The PE voltage sampling circuit 42 is coupled to the protective grounding point PE, and is used to sample the first protective grounding voltage and the second protective grounding voltage corresponding to the first bus voltage and the second bus voltage, respectively. The controller 43 is coupled to the bus voltage sampling circuit 41 and the PE voltage sampling circuit 42, and is used to calculate the equivalent insulation impedance R of the photovoltaic inverter system 100 according to the first bus voltage, the second bus voltage, the first protective grounding voltage, and the second protective grounding voltage. ISO , R ISOis the parallel value of the equivalent insulation resistance of multiple strings, and when the equivalent insulation resistance R ISO When the voltage is greater than or equal to a threshold, the controller 43 controls the inverter circuit 20 to be connected to the power grid 30 .

[0043] exist Figure 1 In the embodiment shown, the input circuit 10 may be, for example, a multi-channel input circuit, for example, including but not limited to n-channel input circuits 11, 12, ..., 1n, and they are coupled in parallel to the positive bus 201 and the negative bus 202. Each input circuit 11, 12, ..., 1n includes a string formed by connecting a plurality of photovoltaic panels in series, and each string has a positive and negative input terminal, for example, the first string has a positive input terminal U pv1+ and the negative input terminal U pv1- . And, R ISOx (x=1,2,…n) is the equivalent insulation resistance of the nth string to ground, U pvx (x=1,2,…n) is the equivalent insulation resistance R of the nth string ISOx (x=1,2,…n) For the voltage of the protective grounding terminal PE, the minimum is 0V and the maximum is the string input voltage U pvn+ .

[0044] In the present invention, each input circuit 10 also has a maximum power point tracking (MPPT) circuit, the maximum power point tracking circuits MPPT 1, MPPT 2, ..., MPPT n. It can be understood that the circuit topology of the MPPT circuit can be a Boost circuit or other circuits that can realize the MPPT function. The circuit topology of the inverter circuit 20 can be a full-bridge circuit, or other inverter topologies. These are not intended to limit the present invention.

[0045] In the present invention, the insulation impedance detection circuit 40 may further include a PWM driving circuit 44 , which may be coupled between the controller 43 and the inverter circuit 20 , and is used to drive and control the inverter circuit 20 through a PWM signal.

[0046] exist Figure 1 In the embodiment shown, the controller 43 can collect the bus voltage of the inverter circuit when the inverter is not operating and the bus voltage after the inverter is operating, or collect the corresponding bus voltages under two inverter operating modes, so as to calculate the equivalent insulation resistance R of the photovoltaic inverter system according to the protective grounding voltage (i.e., PE voltage) sampled by the PE voltage sampling circuit 42 under different bus voltages. ISO .

[0047] The specific working principle of the present invention is shown in FIG2 , wherein Figure 2A yes Figure 1 When the bus voltage is U bus+ The insulation detection equivalent circuit is: Figure 2B yes Figure 1 When the bus voltage is U b ' us+ As shown above, the two bus voltages can be the maximum string input voltage and the bus voltage after the inverter is working, or the bus voltages under two inverter working modes. Figure 2A The voltage value of the PE point is U PE , Figure 2B The voltage value of the mid-PE point is U' PE , with negative bus voltage U bus- is the reference potential point, according to Figure 2A and Figure 2B The following system of equations can be established:

[0048]

[0049]

[0050] At the same time, the equivalent insulation resistance R of the photovoltaic inverter system is defined ISO is the parallel value of the equivalent insulation resistance of multiple strings, as shown in formula (3),

[0051]

[0052] By combining equations (1), (2) and (3), we can solve the equivalent insulation resistance R of the photovoltaic inverter system: ISO , as shown in formula (4),

[0053]

[0054] Among them, R p and R n are the equivalent resistance values ​​from the positive bus and negative bus of the photovoltaic inverter system to the protective grounding point (i.e., PE point), which are known quantities. Figure 2A and Figure 2B The voltage sampling values ​​of the two equivalent circuits can be used to calculate the equivalent insulation resistance R of the photovoltaic inverter system according to formula (4): ISO .

[0055] like Figure 3 As shown, an insulation impedance detection method 300 of a preferred embodiment of the present invention is shown, which mainly includes:

[0056] Step 302: sampling bus voltages, which is to use a bus voltage sampling circuit 41 coupled to the positive bus 201 and the negative bus 202 to sample different first bus voltages (eg, Ubus+ ) and a second bus voltage (eg U' bus+ );

[0057] Step 304: sampling the protective grounding voltage, which is to use a PE voltage sampling circuit 42 coupled to the protective grounding point PE to sample the voltages corresponding to the first bus voltages (e.g., U bus+ ) and the second bus voltage (eg U' bus+ ) of the first protective ground voltage (e.g. U PE ) and a second protective ground voltage (eg U' PE );

[0058] Step 306: Detect insulation impedance by using a controller 43 coupled to the bus voltage sampling circuit 41 and the PE voltage sampling circuit 42, and detecting the insulation impedance according to the first bus voltage (e.g., U bus+ ), the second bus voltage (eg U' bus+ ), the first protective ground voltage (for example, U PE ) and the second protective ground voltage (eg U' PE ) Calculate the equivalent insulation resistance R of the photovoltaic inverter system ISO (For example, calculated according to formula (4)), and when the equivalent insulation resistance is greater than or equal to a threshold, the controller controls the inverter circuit 20 to be connected to a power grid 30.

[0059] The insulation impedance detection circuit and detection method of the present invention can be applied to a single-phase inverter circuit or a multi-phase inverter circuit, that is, Figure 1 The inverter circuit 20 in the embodiment may be a single-phase inverter circuit or a multi-phase inverter circuit. Figure 4 As shown, it shows a circuit diagram of the insulation impedance detection circuit and detection method of the present invention applied to a single-phase photovoltaic inverter with multi-string input, wherein the power grid 30 is a single-phase power grid. Figure 5 As shown, it shows a circuit diagram of the insulation impedance detection circuit and detection method of the present invention applied to a three-phase photovoltaic inverter with multi-channel string input, wherein the power grid 30 is a three-phase power grid. Figure 4 and Figure 5 In the illustrated embodiment, the circuit topology of the MPPT 1 to MPPT n circuits is a Boost circuit, and the circuit topology of the inverter circuit 20 is a bridge circuit, but it is understood that the present invention is not limited thereto. In other words, the insulation impedance detection circuit and detection method of the present invention are independent of the topology of the inverter circuit of the photovoltaic inverter system, and are applicable to all topological conditions, and can also be applied to photovoltaic inverter systems with any string input, and the equivalent insulation resistance R ISO The calculation formula can be unified into formula (4).

[0060] In other embodiments, the controller 43 may also be used to adjust the PWM drive circuit according to different light intensities, wherein when the light intensity is greater than a light intensity threshold, the controller increases the voltage difference between the first bus voltage and the second bus voltage. Figure 6 It is a schematic diagram of string input power curves under different illumination conditions and a stable operating point of the bus voltage control of the present invention. Figure 6 The PV curves of the strings under four light intensities are given, where light intensity 1 < light intensity 2 < light intensity 3 < light intensity 4. The present invention drives and controls the inverter through a PWM signal, so that the inverter works in two states, where the overall power consumption of the photovoltaic inverter is P 1 and P 2 , bus voltage difference (U bus+ -U' bus+ ) is greater than the bus voltage difference (U bus+ -U' bus+ ). Therefore, when the light intensity is high, the detection accuracy of the insulation impedance will be lower than when the light intensity is low. In order to improve the detection accuracy of the insulation impedance under high light intensity, the PWM control signal of the inverter can be adjusted to adjust the voltage modulation ratio of the inverter circuit, the operating frequency of the inverter circuit, etc., so as to increase the difference in the overall power consumption of the photovoltaic inverter under the two detection conditions, thereby improving the detected bus voltage difference (U bus+ -U' bus+ ) to ensure better insulation resistance detection accuracy.

[0061] The insulation impedance detection circuit and detection method of the present invention have the following advantages:

[0062] 1. No additional switching switch and corresponding drive circuit are required, which reduces the volume and cost of the photovoltaic inverter system;

[0063] 2. It is suitable for insulation impedance detection of photovoltaic inverter systems with multi-string input. The detection method, detection time and detection accuracy are not affected by the number of input strings and have high versatility;

[0064] 3. By controlling the working state of the inverter circuit to change, the bus voltage changes, and the equivalent insulation impedance is solved by establishing a set of equations through two sets of detection values. This method is applicable to the full input voltage range and has good safety.

[0065] 4. By controlling the voltage modulation ratio of the inverter circuit and the switching frequency, the bus voltage can be adjusted in a wide range to ensure the detection accuracy of the insulation impedance.

[0066] The exemplary embodiments of the present invention are specifically shown and described above. It should be understood that the present invention is not limited to the disclosed embodiments, but rather is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. An insulation impedance detection circuit of a photovoltaic inverter system, the photovoltaic inverter system comprising at least an input circuit and an inverter circuit, each of the input circuits and the inverter circuit being electrically coupled via a positive bus and a negative bus, and a first equivalent resistance R is provided between the positive bus and the protective grounding point. p There is a second equivalent resistance R between the negative bus and the protective grounding point n , characterized in that, The insulation impedance detection circuit comprises: A bus voltage sampling circuit, coupled to the positive bus and the negative bus, for sampling different first bus voltages and second bus voltages; a protective grounding voltage sampling circuit, coupled to the protective grounding point, for sampling a first protective grounding voltage and a second protective grounding voltage corresponding to the first bus voltage and the second bus voltage, respectively; A controller is coupled to the bus voltage sampling circuit and the protective grounding voltage sampling circuit, and is used to calculate the equivalent insulation impedance of the photovoltaic inverter system according to the first bus voltage, the second bus voltage, the first protective grounding voltage, and the second protective grounding voltage, and when the equivalent insulation impedance is greater than or equal to a threshold, the controller is used to control the inverter circuit to be connected to the power grid.

2. The insulation impedance detection circuit according to claim 1, characterized in that: The input circuit is a multi-channel input circuit, and the multi-channel input circuit is coupled to the positive bus and the negative bus in parallel.

3. The insulation impedance detection circuit according to claim 1, characterized in that: Also includes: The PWM driving circuit is coupled between the controller and the inverter circuit, and is used for driving and controlling the inverter circuit through a PWM signal.

4. The insulation impedance detection circuit according to claim 3, characterized in that: The controller is also used to adjust the PWM drive circuit according to different light intensities, wherein when the light intensity is greater than a light intensity threshold, the controller increases the voltage difference between the first bus voltage and the second bus voltage.

5. The insulation impedance detection circuit according to claim 1, characterized in that: The inverter circuit is a single-phase inverter circuit or a multi-phase inverter circuit.

6. The insulation impedance detection circuit according to claim 1, characterized in that: Each of the input circuits has a maximum power point tracking circuit.

7. A method for detecting insulation impedance of a photovoltaic inverter system, wherein the photovoltaic inverter system comprises at least an input circuit and an inverter circuit, wherein each of the input circuits and the inverter circuit is electrically coupled via a positive bus and a negative bus, and a first equivalent resistance R is provided between the positive bus and the protective grounding point. p There is a second equivalent resistance R between the negative bus and the protective grounding point n , characterized in that, The insulation impedance detection method comprises: Sampling bus voltages, using a bus voltage sampling circuit coupled to the positive bus and the negative bus to sample different first bus voltages and second bus voltages; Sampling the protective grounding voltage, which is to use a protective grounding voltage sampling circuit coupled to the protective grounding point to sample a first protective grounding voltage and a second protective grounding voltage corresponding to the first bus voltage and the second bus voltage respectively; The insulation impedance is detected by coupling a controller with the bus voltage sampling circuit and the protective grounding voltage sampling circuit, and calculating the equivalent insulation impedance of the photovoltaic inverter system according to the first bus voltage, the second bus voltage, the first protective grounding voltage and the second protective grounding voltage, and when the equivalent insulation impedance is greater than or equal to a threshold, the controller is used to control the inverter circuit to be connected to the power grid.

8. The insulation impedance detection method according to claim 7, characterized in that: The input circuit is a multi-channel input circuit, and the multi-channel input circuit is coupled in parallel to the positive bus and the negative bus, and the equivalent insulation impedance of the photovoltaic inverter system is the parallel value of the equivalent insulation resistance of each channel of the multi-channel input circuit.

9. The insulation impedance detection method according to claim 7, characterized in that: The inverter circuit is driven and controlled by a PWM signal using a PWM driving circuit coupled between the controller and the inverter circuit.

10. The insulation impedance detection method according to claim 9, characterized in that: Also includes: The controller adjusts the PWM driving circuit according to different light intensities, wherein when the light intensity is greater than a light intensity threshold, the controller increases the voltage difference between the first bus voltage and the second bus voltage.

11. The insulation impedance detection method according to claim 9, characterized in that: The controller adjusts the PWM driving circuit by adjusting a voltage modulation ratio of the inverter circuit or adjusting a switching frequency of the inverter circuit.

12. The insulation impedance detection method according to claim 7, characterized in that: Each of the input circuits has a maximum power point tracking circuit.

Images