Insulation impedance detection circuit and method for non-isolated photovoltaic inverter

Abstract

The present invention discloses an insulation impedance detection circuit and method for a non-isolated photovoltaic inverter. An insulation impedance detection circuit for a non-isolated photovoltaic inverter comprises: an insulation impedance function module for detecting the insulation impedance of a photovoltaic module to ground; the insulation impedance detection circuit further comprises: a first switch and a detection resistor, one end of the first switch is used to be electrically connected to the photovoltaic module, the other end of the first switch is electrically connected to one end of the detection resistor, and the other end of the detection resistor is used to be electrically connected to a power grid and grounded through the neutral point of the power grid. The present invention solves the detection blind area that exists when the casing is not grounded.

Description

Technical Field

[0001] The invention belongs to the field of photovoltaic inverters and relates to an insulation impedance detection circuit and method for a non-isolated photovoltaic inverter. Background Art

[0002] In the global context of energy shortage and environmental degradation, the use of new clean energy is particularly important. Solar energy is widely developed and applied due to its renewable and abundant resources. With the popularization of photovoltaic grid-connected power generation applications, the safety of photovoltaic grid-connected power generation systems has received more and more attention. The insulation impedance detection function of the photovoltaic inverter is necessary to ensure the safe and reliable operation of the photovoltaic system and to prevent electric shock. How to achieve a true and reliable detection function is very meaningful to meet safety requirements and reduce the cost of the inverter. In non-isolated grid-connected inverter products, it is necessary to detect the insulation impedance of the photovoltaic inverter before grid connection.

[0003] However, in existing non-isolated photovoltaic grid-connected inverters, the insulation impedance detection circuit is mostly based on the earth equivalent of the casing, and the premise for accurate detection is also good grounding of the casing. When the inverter is not well connected to the ground, the insulation impedance circuit cannot normally detect the parasitic impedance of the photovoltaic module to the ground. At this time, the machine grid connection will have an impact on the machine itself and the human body. Summary of the invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide an insulation impedance detection circuit and method for a non-isolated photovoltaic inverter to solve the detection blind area that exists when the casing is not grounded.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] An insulation impedance detection circuit for a non-isolated photovoltaic inverter, comprising:

[0007] Insulation impedance function module, used to detect the insulation impedance of photovoltaic modules to ground;

[0008] The insulation impedance detection circuit also includes:

[0009] A first switch and a detection resistor, wherein one end of the first switch is used to be electrically connected to the photovoltaic component, the other end of the first switch is electrically connected to one end of the detection resistor, and the other end of the detection resistor is used to be electrically connected to a power grid and grounded through a neutral point of the power grid.

[0010] In one embodiment, the insulation impedance functional module includes a DC converter, the input end of the DC converter is electrically connected to the positive electrode and the negative electrode of the photovoltaic inverter respectively, and the output end of the DC converter includes an output positive electrode and an output negative electrode; the output positive electrode is electrically connected to the normally closed contact of the first relay, a first capacitor and a second capacitor are connected between the output positive electrode and the output negative electrode, and the normally open contact of the first relay is electrically connected between the first capacitor and the second capacitor; the moving contact of the first relay is electrically connected to one end of the first sampling resistor, the other end of the first sampling resistor is electrically connected to one end of the second switch and one end of the second sampling resistor respectively, the other end of the second sampling resistor is electrically connected to one end of the third sampling resistor, and the voltage operation device is electrically connected between the second sampling resistor and the third sampling resistor; the other end of the second switch is grounded.

[0011] Furthermore, the other end of the third sampling resistor is grounded.

[0012] Furthermore, the second switch is a second relay.

[0013] The present invention also adopts the following technical solution:

[0014] An insulation impedance detection method for an insulation impedance detection circuit of a non-isolated photovoltaic inverter as described above, the detection method comprises the following steps:

[0015] A. disconnecting the first switch, and causing the insulation impedance function module to perform detection to obtain a first insulation impedance value Risox1;

[0016] B. closing the first switch, and the insulation impedance function module performs detection to obtain a second insulation impedance value Risox2;

[0017] C. Disconnect the first switch and calculate the absolute value of the difference between the first insulation impedance Risox1 and the second insulation impedance Risox2. When the absolute value of the difference is greater than or equal to a set threshold, it is determined that the casing of the photovoltaic inverter is well connected to the ground, and the insulation impedance of the photovoltaic component to the ground to be detected is equal to the first insulation impedance value Risox1; when the absolute value of the difference is less than the set threshold, it is determined that the casing of the photovoltaic inverter is not well connected to the ground, and the insulation impedance data of the photovoltaic component to the ground detected by the insulation impedance function module is not true.

[0018] Furthermore, in step C, when the absolute value of the difference is less than a set threshold, a warning is issued and re-detection is performed.

[0019] In one embodiment, in step A and step B, the insulation impedance function module performs detection specifically including the following steps:

[0020] a. Perform circuit self-test to confirm that the circuit components are normal;

[0021] b. Closing the second switch of the insulation impedance functional module, detecting the first BUS voltage value of the positive electrode of the output terminal of the DC converter, detecting the first voltage value output by the voltage operation device, and recording the first voltage value when the first BUS voltage value and the first voltage value are stable;

[0022] c. Switching the moving contact of the first relay of the insulation impedance functional module to the normally open end, detecting the second BUS voltage value between the first capacitor and the second capacitor, detecting the second voltage value output by the voltage operation device, and recording the second voltage value when the second BUS voltage value and the second voltage value are stable;

[0023] d. Calculate insulation resistance according to the first BUS voltage value, the second BUS voltage value, the first voltage value, and the second voltage value.

[0024] Furthermore, in step d, the insulation resistance is calculated according to the following formula:

[0025]

[0026] Among them, Risox represents the insulation resistance value to be detected; R1, R2, R3 represent the resistance values ​​of the first sampling resistor, the second sampling resistor and the third sampling resistor respectively; BUS1, BUS2 represent the first BUS voltage value, the second BUS voltage value respectively; Uriso1, Uriso2 represent the first voltage value, the second voltage value respectively.

[0027] Furthermore, the insulation impedance function module performs the detection and further comprises the following steps: after the detection is completed, the second switch is disconnected and the moving contact of the first relay is switched to a normally closed contact.

[0028] The present invention adopts the above solution, which has the following advantages compared with the prior art:

[0029] The present invention adds a first switch and a detection resistor between the PV side and the grid side, utilizes the remote grounding of the grid neutral point, and the characteristic that each phase of the grid voltage is based on the earth as a reference, and involves the resistance value of the detection resistor in the insulation impedance detection, so as to accurately identify the authenticity of the insulation impedance data, solve the detection blind area when the casing is not grounded, and ensure the dead zone-free detection of the insulation impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solution of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0031] Figure 1 is a schematic diagram of an insulation impedance detection circuit according to an embodiment of the present invention;

[0032] Figure 2 for Figure 1 Circuit diagram of the insulation resistance function module in;

[0033] Figure 3 It is a schematic diagram of an insulation impedance detection circuit according to an embodiment of the present invention, wherein the housing is well grounded;

[0034] Figure 4 is a schematic diagram of an insulation impedance detection circuit according to an embodiment of the present invention, wherein the housing is not well grounded;

[0035] Figure 5 Schematic diagram of the equivalent area of ​​the PV side terminal. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] like Figure 1 As shown, the photovoltaic array (PV) input terminal 1 is electrically connected to the inverter DC side through the PV side terminal 3. The photovoltaic array has a parasitic resistance to the earth, which is the insulation impedance of the photovoltaic array to the ground that needs to be detected in this embodiment. Figure 1 The equivalent resistance in is Riso. An insulation impedance function module 6 is provided in the housing 4 of the inverter for detecting the value of Riso. The housing 4 of the inverter can be well connected to the ground in most cases. In some cases, there may be grounding anomalies. This embodiment can also accurately detect the insulation impedance of the photovoltaic module to the ground in the case of grounding anomalies. Figure 1The housing equipotential symbol in the figure represents the housing potential of the inverter. The grounding point of the internal circuit of the inverter is considered to be connected to the housing potential. The AC side terminal 8 of the inverter is electrically connected to the U, V, W phases and the neutral point N of the power grid respectively. The neutral point N at the far end of the power grid is grounded. 2, 5, and 7 represent the PV side insulation impedance to ground, the PV side wiring, and the grid side wiring. Figure 5 The PV side equivalent access point is shown. Rinternal is the equivalent resistance of the parasitic impedance existing on the DC side of the inverter relative to the housing 4. The first switch K and the detection resistor R are located between the PV side and the grid side.

[0038] The insulation impedance detection circuit for a non-isolated photovoltaic inverter of this embodiment includes:

[0039] Insulation impedance function module, used to detect the insulation impedance of photovoltaic modules to ground; and

[0040] A first switch K and a detection resistor R, wherein one end of the first switch K is electrically connected to the photovoltaic component, the other end of the first switch K is electrically connected to one end of the detection resistor R, and the other end of the detection resistor R is electrically connected to a power grid and grounded through a neutral point N of the power grid.

[0041] This embodiment provides a specific insulation impedance function module (such as Figure 2 As shown), in another embodiment, the insulation impedance functional module can also be a photovoltaic inverter insulation impedance detection circuit known in the prior art. Figure 2 As shown, the insulation impedance functional module in this embodiment includes a DC converter DC / DC, the input end of the DC converter DC / DC is electrically connected to the positive pole PV+ and the negative pole PV- of the photovoltaic inverter respectively, and the output end of the DC converter DC / DC includes an output positive pole BUS+ and an output negative pole BUS-; the output positive pole BUS+ is electrically connected to the normally closed contact of the first relay K1, and a first capacitor and a second capacitor are connected between the output positive pole BUS+ and the output negative pole BUS-, and the normally open contact of the first relay K1 is electrically connected between the first capacitor and the second capacitor; the moving contact of the first relay K1 is electrically connected to one end of the first sampling resistor R1, and the other end of the first sampling resistor R1 is electrically connected to one end of the second switch K2 and one end of the second sampling resistor R2 respectively, and the other end of the second sampling resistor R2 is electrically connected to one end of the third sampling resistor R3, and the voltage operation device Uriso is electrically connected between the second sampling resistor R2 and the third sampling resistor R3; the other end of the second switch K2 is grounded GND.

[0042] The other end of the third sampling resistor R3 is grounded. The inverter housing is grounded, and the DC side of the inverter has a parasitic impedance relative to the housing. Figure 2 The equivalent resistance in is Rinternal.

[0043] The second switch K2 is specifically a second relay.

[0044] The insulation impedance detection method of the insulation impedance detection circuit for the non-isolated photovoltaic inverter of this embodiment includes the following steps:

[0045] A. disconnecting the first switch, and causing the insulation impedance function module to perform detection to obtain a first insulation impedance value Risox1;

[0046] B. closing the first switch, and the insulation impedance function module performs detection to obtain a second insulation impedance value Risox2;

[0047] C. Disconnect the first switch and calculate the absolute value of the difference between the first insulation impedance Risox1 and the second insulation impedance Risox2. When the absolute value of the difference is greater than or equal to a set threshold, it is determined that the casing of the photovoltaic inverter is well connected to the ground, the data detected by the insulation impedance functional module is true and reliable, and the insulation impedance of the photovoltaic component to the ground to be detected is equal to the first insulation impedance value Risox1; when the absolute value of the difference is less than the set threshold, it is determined that the casing of the photovoltaic inverter is not well grounded to the ground, and the data detected by the insulation impedance functional module is not true.

[0048] Specifically, in step C, when the absolute value of the difference is greater than or equal to a set threshold, the insulation impedance to be detected is equal to the first insulation impedance value Risox1; and / or, when the difference is less than the set threshold, a warning is issued and re-detection is performed.

[0049] In step A and step B, the insulation impedance function module performs detection specifically including the following steps:

[0050] a. Perform circuit self-test based on BUS voltage and resistor voltage division to confirm that the circuit components are normal;

[0051] b. Closing the second switch of the insulation impedance functional module, detecting the first BUS voltage value of the positive electrode of the output terminal of the DC converter, detecting the first voltage value output by the voltage operation device, and recording the first voltage value when the first BUS voltage value and the first voltage value are stable;

[0052] c. Switching the moving contact of the first relay of the insulation impedance functional module to the normally open end, detecting the second BUS voltage value between the first capacitor and the second capacitor, detecting the second voltage value output by the voltage operation device, and recording the second voltage value when the second BUS voltage value and the second voltage value are stable;

[0053] d. calculating insulation resistance according to the first BUS voltage value, the second BUS voltage value, the first voltage value and the second voltage value; and

[0054] After the detection is completed, the second switch is disconnected and the moving contact of the first relay is switched to the normally closed contact.

[0055] In step d, the insulation resistance is calculated according to the following formula:

[0056]

[0057] Among them, Risox represents the insulation resistance value to be detected; R1, R2, and R3 represent the resistance values ​​of the first sampling resistor, the second sampling resistor, and the third sampling resistor respectively; BUS1 and BUS2 represent the first BUS voltage value and the second BUS voltage value respectively; Uriso1 and Uriso2 represent the first voltage value and the second voltage value respectively. Risox is the equivalent result of Riso and Rinternal in parallel. In general systems, the impedance value of Rinternal is very large, so the value of Risox can accurately represent the value of the insulation resistance Riso.

[0058] Figure 3 The insulation impedance detection circuit is shown in the case of a good grounding of the chassis. Figure 3 As shown, the working mode and results when the chassis is well grounded are as follows:

[0059] The first switch K remains in the disconnected state, and the insulation impedance function module performs a detection action. At this time, the calculated insulation impedance Risox is recorded as the first insulation impedance value Risox1. The first insulation impedance value Risox1 is actually the equivalent result of Riso and Rinternal in parallel, that is, Risox1 = Riso||Rinternal;

[0060] The first switch K is switched to a closed state, and the insulation impedance function module performs a detection action. At this time, the calculated insulation impedance Risox is recorded as the second insulation impedance value Risox2. The second insulation impedance value Risox2 is actually the equivalent result of Riso, Rinternal and R in parallel, that is, Risox2 = Riso||Rinternal||R;

[0061] Disconnect the first switch K and determine the detection data. Riso can be approximately represented by Risox1, Risox2=Risox1||R. When the appropriate R value is matched, there will be an obvious and calculable difference between the first insulation impedance value Risox1 and the second insulation impedance value Risox2 (i.e. the set threshold value mentioned above). This difference can indicate that the system is well grounded and the insulation impedance detection data is true and reliable.

[0062] Figure 4 The insulation impedance detection circuit is shown in the case where the chassis is not properly grounded. Figure 4 As shown, the working mode and results when the chassis is not well grounded are as follows:

[0063] The first switch K remains in the off state, and the insulation impedance function module performs a detection action. At this time, the calculated insulation impedance Risox is recorded as Risox1, and Risox1 is actually the Rinternal detection value, that is, Risox1 = Rinternal;

[0064] The first switch K is switched to a closed state, and the insulation impedance function module performs a detection action. At this time, the calculated insulation impedance Risox is recorded as Risox2, and Risox2 is actually the Rinternal detection value, that is, Risox2 = Rinternal;

[0065] Disconnect the first switch K, and check the test data. If there is no obvious difference between Risox1 and Risox2, it means that the system is not well grounded and the insulation impedance test data is not true. At this time, a warning can be sent out through communication and retesting can be carried out regularly until the result is normal.

[0066] In this embodiment, a first switch K and a detection resistor R are added between the PV side and the grid side, and the neutral point of the grid is remotely grounded. The characteristics that each phase of the grid voltage is based on the earth as a reference are utilized, and the resistance value of the detection resistor R is involved in the insulation impedance detection, which can accurately identify the authenticity of the insulation impedance data, solve the detection blind area when the casing is not grounded, and ensure the dead zone detection of the insulation impedance, thereby realizing the function of insulation impedance detection that is independent of system grounding. It is possible to judge the authenticity of the insulation impedance detection value and improve the reliability of the photovoltaic system in application.

[0067] The above embodiment is only for illustrating the technical concept and features of the present invention, and is a preferred embodiment. Its purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. An insulation impedance detection method for an insulation impedance detection circuit of a non-isolated photovoltaic inverter, characterized in that: The insulation impedance detection circuit comprises: Insulation impedance function module, used to detect the insulation impedance of photovoltaic modules to ground; A first switch and a detection resistor, wherein one end of the first switch is used to be electrically connected to the photovoltaic component, the other end of the first switch is electrically connected to one end of the detection resistor, and the other end of the detection resistor is used to be electrically connected to a power grid and grounded through a neutral point of the power grid; A direct current converter, wherein the input end of the direct current converter is electrically connected to the positive electrode and the negative electrode of the photovoltaic inverter respectively, and the output end of the direct current converter includes an output positive electrode and an output negative electrode; the output positive electrode is electrically connected to the normally closed contact of the first relay, a first capacitor and a second capacitor are connected between the output positive electrode and the output negative electrode, and the normally open contact of the first relay is electrically connected between the first capacitor and the second capacitor; the moving contact of the first relay is electrically connected to one end of the first sampling resistor, the other end of the first sampling resistor is electrically connected to one end of the second switch and one end of the second sampling resistor respectively, the other end of the second sampling resistor is electrically connected to one end of the third sampling resistor, and a voltage operation device is electrically connected between the second sampling resistor and the third sampling resistor; the other end of the second switch is grounded; the other end of the third sampling resistor is grounded; The detection method comprises the following steps: A. disconnecting the first switch, and causing the insulation impedance function module to perform detection to obtain a first insulation impedance value Risox1; B. closing the first switch, and the insulation impedance function module performs detection to obtain a second insulation impedance value Risox2; C. Disconnect the first switch, calculate the absolute value of the difference between the first insulation impedance value Risox1 and the second insulation impedance value Risox2, and when the absolute value of the difference is greater than or equal to a set threshold, it is determined that the housing of the photovoltaic inverter is well connected to the ground, and the insulation impedance of the photovoltaic module to the ground to be detected is equal to the first insulation impedance value Risox1; when the absolute value of the difference is less than the set threshold, it is determined that the housing of the photovoltaic inverter is not well connected to the ground, and the insulation impedance data of the photovoltaic module to the ground detected by the insulation impedance function module is not true; Among them, the insulation resistance is calculated according to the following formula: Among them, Risox represents the insulation resistance value to be detected; R1, R2, and R3 represent the resistance values ​​of the first sampling resistor, the second sampling resistor, and the third sampling resistor, respectively; BUS1 and BUS2 represent the first BUS voltage value and the second BUS voltage value, respectively; Uriso1 is the first voltage value output by the voltage operation device after the second switch is closed, and Uriso2 is the second voltage value output by the voltage operation device after the moving contact of the first relay is cut to the normally open end.

2. The insulation impedance detection method according to claim 1, characterized in that: In step C, when the absolute value of the difference is less than a set threshold, a warning is issued and re-detection is performed.

3. The insulation impedance detection method according to claim 1, characterized in that: In step A and step B, the insulation impedance function module performs detection specifically including the following steps: a. Perform circuit self-test to confirm that the circuit components are normal; b. Closing the second switch of the insulation impedance functional module, detecting the first BUS voltage value of the positive electrode of the output terminal of the DC converter, detecting the first voltage value output by the voltage operation device, and recording the first voltage value when the first BUS voltage value and the first voltage value are stable; c. Switching the moving contact of the first relay of the insulation impedance functional module to the normally open end, detecting the second BUS voltage value between the first capacitor and the second capacitor, detecting the second voltage value output by the voltage operation device, and recording the second voltage value when the second BUS voltage value and the second voltage value are stable; d. Calculate the insulation resistance according to the formula from the first BUS voltage value, the second BUS voltage value, the first voltage value and the second voltage value.

4. The insulation impedance detection method according to claim 3, characterized in that: The insulation impedance function module further includes the following steps when executing the test: after the test is completed, the second switch is disconnected, and the moving contact of the first relay is switched to a normally closed contact.

5. The insulation impedance detection method according to claim 1, characterized in that: The second switch is a second relay.

Images