A grounding detection circuit and method for a non-isolated photovoltaic inverter

Abstract

The present invention discloses a grounding detection circuit and method for a non-isolated photovoltaic inverter. A grounding detection circuit for a non-isolated photovoltaic inverter includes an inversion unit, the inversion unit includes a first resistor and a first capacitor connected in parallel with each other, one end of the first resistor and one end of the first capacitor are electrically connected to the AC side of the inversion unit, and the inversion unit further includes a second resistor and a switch; the other end of the first resistor and the other end of the first capacitor are electrically connected to one end of the second resistor, and the one end of the second resistor is electrically connected to the casing of the photovoltaic inverter; the switch has a first contact, a second contact and a moving contact, the first contact is used for electrically connecting to the positive electrode of the output end of the photovoltaic module, the second contact is used for electrically connecting to the negative electrode of the output end of the photovoltaic module, and the moving contact is electrically connected to the other end of the second resistor. The present invention has no detection dead zone, and the detection result is accurate and reliable.

Description

Technical Field

[0001] The present invention belongs to the field of photovoltaic inverters, and relates to a grounding (PE connection) detection circuit and method for a non-isolated photovoltaic inverter. Background Art

[0002] Under the global background of energy shortage and environmental deterioration, it is particularly important to utilize new clean energy. Solar energy is widely developed and applied due to its renewable and rich resources. With the popularization of photovoltaic grid-connected power generation applications, the safety of photovoltaic grid-connected power generation systems has received increasing attention. The PE connection detection function of a photovoltaic inverter is necessary to ensure the safe and reliable operation of the photovoltaic system and prevent electric shock to people. How to achieve a simple and reliable detection function is of great significance for meeting safety requirements and reducing the cost of the inverter.

[0003] In existing non-isolated three-phase grid-connected inverters, the voltage between the grid N and the ground PE is the most reliable and intuitive method to judge whether the PE wiring is good. However, in many working conditions, even if the machine case is not well grounded, there will be a situation where the voltage between the machine case and the grid neutral point N is basically undetectable. This is because due to the symmetry of devices such as EMC and surge in the inverter, the machine case will be balanced at the neutral point potential through charging and discharging through the three-phase grid. When the inverter is not well connected to the ground, the detection result of the PE connection detection circuit is inaccurate, and at this time, grid connection of the machine will affect the machine itself and people. Summary of the Invention

[0004] In order to solve the above technical problems, the object of the present invention is to provide a grounding detection circuit and method for a non-isolated photovoltaic inverter, which has no detection dead zone and the detection result is accurate and reliable.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A grounding detection circuit for a non-isolated photovoltaic inverter, including an inverter unit, the inverter unit includes a first resistor and a first capacitor connected in parallel, one end of the first resistor and one end of the first capacitor are electrically connected to the AC side of the inverter unit,

[0007] The inverter unit further includes a second resistor and a switch;

[0008] The other end of the first resistor and the other end of the first capacitor are electrically connected to one end of the second resistor, and the one end of the second resistor is electrically connected to the machine case of the photovoltaic inverter;

[0009] The switch has a first contact, a second contact and a moving contact. The first contact is used for electrically connecting to the positive electrode of the output terminal of the photovoltaic module. The second contact is used for electrically connecting to the negative electrode of the output terminal of the photovoltaic module. The moving contact is electrically connected to the other end of the second resistor.

[0010] In one embodiment, the inverter unit further includes a second capacitor connected in parallel across the two ends of the second resistor.

[0011] In one embodiment, the ground detection circuit further includes a first DC voltage sampling unit and a first AC voltage sampling unit electrically connected between the positive electrode of the output terminal of the photovoltaic module and the ground.

[0012] In one embodiment, the ground detection circuit further includes a second DC voltage sampling unit and a second AC voltage sampling unit electrically connected between the negative electrode of the output terminal of the photovoltaic module and the ground.

[0013] In one embodiment, the switch is a relay.

[0014] The present invention also adopts the following technical solutions:

[0015] A ground detection method for the ground detection circuit of a non-isolated photovoltaic inverter as described above. The ground detection method includes the following steps:

[0016] A. Switch the moving contact of the switch to the first contact to connect the second resistor between the positive electrode of the output terminal of the photovoltaic module and the ground, detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid, and determine whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly proceed to step B; otherwise, continuously detect until the pressure difference is not within the error reporting range, and then proceed to step B;

[0017] B. Switch the moving contact of the switch to the second contact to connect the second resistor between the negative electrode of the output terminal of the photovoltaic module and the ground, detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid, and determine whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly proceed to step C; otherwise, continuously detect until the pressure difference is not within the error reporting range, and then proceed to step B;

[0018] C. Keep the switch state in step B, continuously detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid after the photovoltaic inverter is connected to the grid. When it is determined that the pressure difference falls within the error reporting range, report an error message.

[0019] In one embodiment, a second capacitor is connected in parallel across both ends of the second resistor; in step A, the second resistor and the second capacitor are connected between the positive output terminal of the photovoltaic module and the ground; in step B, the second resistor and the second capacitor are connected between the negative output terminal of the photovoltaic module and the ground.

[0020] Preferably, in step A, the voltage difference Ux is as shown in the following formula:

[0021]

[0022] In steps B and C, the voltage difference Ux is as shown in the following formula:

[0023]

[0024] Wherein, R21, R22, R23, and R24 respectively represent the impedances of the neutral point of the power grid, the W phase, the U phase, and the V phase relative to the machine shell, R11 represents the resistance value of the second resistor or the equivalent impedance value of the second resistor and the second capacitor connected in parallel, U, V, and W respectively represent the three-phase voltages of the power grid, and DC1, DC2, AC1, and AC2 are used to characterize the system common-mode voltage of the PV array with respect to the ground.

[0025] Furthermore, in step A, the voltage difference Ux is simplified as shown in the following formula;

[0026]

[0027] In steps B and C, the voltage difference Ux is simplified as shown in the following formula

[0028]

[0029] Preferably, the error reporting ranges in steps A, B, and C are the same. Specifically, the error reporting ranges in steps A, B, and C are 30V respectively.

[0030] The present invention adopts the above scheme and has the following advantages compared with the prior art:

[0031] The grounding detection circuit and grounding detection method of the present invention can eliminate the sampling interference caused by three-phase self-balancing by adding a second resistor, can utilize the existing voltage in the system, has a clear principle, no detection dead zone, and is simple and reliable. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 is a schematic diagram of a grounding detection circuit according to an embodiment of the present invention;

[0034] Figure 2 is a schematic diagram of the grounding detection circuit of the embodiment of the present invention. Specific embodiments

[0035] The following describes in detail the preferred embodiments of the present invention with reference to 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 here that the description of these embodiments is for helping to understand the present invention, but does not constitute a limitation to 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.

[0036] As Figure 1 shown, the input end 1 of the photovoltaic module array (abbreviated as PV) is electrically connected to the DC side of the inverter through the PV-side wiring terminal 3. The housing 4 of the inverter can be well connected to the ground in most cases, and there may be abnormal grounding in some cases. Figure 1 The equipotential symbol of the housing in [[ ]] represents the potential of the inverter housing, and the grounding point of the internal circuit of the inverter is considered to be the potential of the connected housing. The AC-side wiring terminals 8 of the inverter are respectively electrically connected to the U, V, and W phases and the neutral point N of the power grid, and the neutral point N of the remote power grid is grounded. 2, 5, 6, and 7 respectively represent the integrated equivalent of the PV-side insulation impedance to the ground, the integrated equivalent of the PV-side wiring, the integrated equivalent of the AC-side wiring, and the voltage indication between the neutral point N of the power grid and the PE (grounding point). U, V, and W represent the three-phase power grid. Rinternal is the equivalent resistance of the parasitic impedance existing between the DC side and the housing 1 in the inverter. The capacitor C3 and the resistor R3 respectively equivalently represent the parasitic capacitance and parasitic resistance of the photovoltaic module array to the ground.

[0037] Combined with Figure 1 and Figure 2As shown in the figure, a grounding detection circuit for a non-isolated photovoltaic inverter according to this embodiment includes an inverter unit. The inverter unit includes a first resistor R2 and a first capacitor C2. The first resistor R2 and the first capacitor C2 are connected in parallel, and one end of both is electrically connected to the AC side of the inverter unit, specifically connected to the power grid through the AC side wiring terminal of the inverter. The inverter unit further includes a second resistor R1. A connection point of the other ends of the first resistor R2 and the first capacitor C2 is electrically connected to one end of the second resistor R1. Further, a second capacitor C1 may be connected in parallel across both ends of the second resistor R1. The inverter unit further includes a switch K1. The switch K1 has a first contact, a second contact, and a moving contact. The first contact is electrically connected to the positive electrode PV+ of the output terminal of the photovoltaic module, the second contact is electrically connected to the negative electrode PV- of the output terminal of the photovoltaic module, and the moving contact is electrically connected to the other end of the second resistor R1. The said one end of the second resistor R1 is electrically connected to the housing 1 of the photovoltaic inverter. As Figure 1 stated, the said other ends of the first resistor R2 and the first capacitor C2 are also electrically connected to the housing 1 of the photovoltaic inverter. Thus, the first capacitor C2 and the first resistor R2 are the capacitor and resistor connected between the AC side of the photovoltaic inverter and the housing 1 for functions such as EMC. The second capacitor C1 and the second resistor R1 are the additional capacitor and resistor between the PV side wiring terminal of the photovoltaic inverter and the housing 1. By setting the second capacitor C1 and the second resistor R1, the common-mode voltage of the DC side to the ground system is used to break the potential self-balance between the housing 1 and the neutral point N of the three-phase power grid. When the housing 1 is not well grounded, a voltage sufficient to accurately implement the judgment function is generated between the housing 1 and the neutral point N of the power grid, thereby realizing the detection.

[0038] As Figure 2 shown, the grounding detection circuit further includes a first DC voltage sampling unit DC1 and a first AC voltage sampling unit AC1 electrically connected between the positive electrode PV+ of the output terminal of the photovoltaic module and the ground; the grounding detection circuit further includes a second DC voltage sampling unit DC2 and a second AC voltage sampling unit AC2 electrically connected between the negative electrode PV- of the output terminal of the photovoltaic module and the ground. The detection values of the four voltage sampling units DC1, DC2, AC1, and AC2 are used to represent the system common-mode voltage of the photovoltaic module array to the ground.

[0039] In this embodiment, only a resistor (i.e., the second resistor R1) is added to the PV side wiring terminal of the photovoltaic inverter relative to the housing 1. Then Figure 2R11 therein is the equivalent impedance of the second resistor R1; in some other embodiments, a resistor (i.e., the second resistor R1) and a capacitor (i.e., the second capacitor C1) are added to the PV side terminal of the photovoltaic inverter relative to the chassis 1, and then R11 in the figure is the equivalent impedance of the second resistor R1 and the second capacitor C1 connected in parallel. The switch K1 and the impedance R11 can ensure that there is sufficient voltage between PE and N in the case of poor grounding of the inverter. By switching the switch K1, the impedance R11 can be switched between the positive terminal PV+ of the output of the photovoltaic module and the chassis 1, or between the negative terminal PV- of the output of the photovoltaic module and the chassis 1. Figure 2 The resistors R21, R22, R23, and R24 therein are used to characterize the impedances of the neutral point of the power grid, the W phase, the U phase, and the V phase relative to the chassis 1.

[0040] In this embodiment, the switch K1 is specifically a relay, such as a single-pole double-throw relay.

[0041] This embodiment also provides a grounding detection method for the grounding detection circuit of the non-isolated photovoltaic inverter as described above. The grounding detection method includes the following steps:

[0042] A. Switch the moving contact of the switch K1 to the first contact to connect the second resistor R1 between the positive terminal PV+ of the output of the photovoltaic module and the ground, detect the pressure difference between the chassis 1 of the photovoltaic inverter and the neutral point N of the power grid, and determine whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly proceed to step B; otherwise, continuously detect until the pressure difference is not within the error reporting range and then proceed to step B;

[0043] B. Switch the moving contact of the switch K1 to the second contact to connect the second resistor R1 between the negative terminal PV- of the output of the photovoltaic module and the ground, detect the pressure difference between the chassis 1 of the photovoltaic inverter and the neutral point N of the power grid, and determine whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly proceed to step C; otherwise, continuously detect until the pressure difference is not within the error reporting range and then proceed to step B;

[0044] C. Keep the switch state in step B, continuously detect the pressure difference between the chassis 1 of the photovoltaic inverter and the neutral point N of the power grid after the photovoltaic inverter is connected to the grid. When it is determined that the pressure difference falls within the error reporting range, report an error message.

[0045] The error reporting ranges in the above steps A, B, and C are 30V respectively.

[0046] Figure 2 The working state of the circuit in is described as follows:

[0047] Considering the case where the chassis is not well grounded,

[0048] When the switch K1 connects the second resistor R1 between the negative output terminal PV- of the photovoltaic module and the chassis, the pressure difference between the chassis and the neutral point N of the power grid is denoted as Ux, then

[0049] The parameters R22, R21, R23, R24 on the AC side of the inverter are mostly symmetrically set due to performance requirements. Considering the actual application situation, the expression of the pressure difference Ux can be further optimized as follows:

[0050]

[0051] When the switch K1 connects the second resistor R1 between the positive output terminal PV+ of the photovoltaic module and the chassis, the pressure difference Ux can be expressed by the following expression:

[0052]

[0053] Considering the symmetry of the actual settings of R22, R21, R23, R24, the pressure difference Ux can be further simplified to the following expression:

[0054]

[0055] In the formula, (DC1+AC1) and (DC2+AC2) in the case of the inverter being connected to the grid, when simplifying the working condition to when the input voltage of the photovoltaic module array PV is equal to the BUS voltage, their absolute values are always |1 / 2BUS+Ucom|. Ucom is the relative common-mode voltage output by the inverter module, and it will have different values when different modulation algorithms are used, and it is generally around 1 / 6BUS. It can be seen from the above description that in the case of the inverter being connected to the grid, regardless of whether K1 switches R11 to PV- or PV+, the result of Ux is stable.

[0056] In the case of the (DC1+AC1) and (DC2+AC2) in the formula being off-grid, (DC1+AC1)+(DC2+AC2) = Upv can be satisfied, where Upv refers to the PV input voltage of the inverter. However, the voltage division ratio of (DC1+AC1) and (DC2+AC2) changes with the change of the accidental impedance of the positive output terminal PV+ and the negative input terminal PV- of the photovoltaic module to the ground, but the two generally satisfy the total balance of this increase and decrease. It can be seen from the above that before grid connection, the switch K1 needs to connect the second resistor R1 to the ground of the negative output terminal PV- and the positive output terminal PV+ of the photovoltaic module respectively, read the pressure difference Ux values respectively, and take the larger amplitude for the final determination.

[0057] Based on the above analysis, regardless of the grid-connected, off-grid, and accidental impedance working conditions, a more suitable pressure difference Ux for detection can be obtained by matching a suitable impedance value R11 to the chassis on the DC side.

[0058] In this embodiment, by analyzing and summarizing the common-mode voltage of the ground system under various conditions on the PV side (DC side), the characteristics of the common-mode voltage are ingeniously utilized. By additionally adding a first capacitor and a first resistor, the sampling interference caused by three-phase self-balancing is eliminated. The circuit and detection method described in the present invention can utilize the existing voltage in the system, with clear principles, no detection dead zone, and being simple and reliable.

[0059] The practice of connecting the second resistor R1 to the equivalent path before the inverter grid-side relay through the switch K1 to achieve the same function is considered a variant of the method in this patent.

[0060] In some other embodiments, a second capacitor C1 is connected in parallel across the two ends of the second resistor R1; in step A, the second resistor R1 and the second capacitor C1 are connected between the positive pole of the output end of the photovoltaic module and the ground; in step B, the second resistor R1 and the second capacitor C1 are connected between the negative pole of the output end of the photovoltaic module and the ground. In this case, R11 in the above formula is the equivalent impedance value of the second resistor R1 and the second capacitor C1.

[0061] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention. They are a preferred embodiment, and their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly. However, the protection scope of the present invention cannot be limited thereby. Any equivalent transformation or modification made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A grounding detection circuit for a non-isolated photovoltaic inverter, including an inverter unit. The inverter unit includes a first resistor and a first capacitor connected in parallel. One end of the first resistor and one end of the first capacitor are electrically connected to the AC side of the inverter unit. It is characterized in that: The inverter unit further includes a second resistor and a switch; The other end of the first resistor and the other end of the first capacitor are electrically connected to one end of the second resistor, and the one end of the second resistor is electrically connected to the casing of the photovoltaic inverter; The switch has a first contact, a second contact and a moving contact. The first contact is used to be electrically connected to the positive pole of the output end of the photovoltaic module, the second contact is used to be electrically connected to the negative pole of the output end of the photovoltaic module, and the moving contact is electrically connected to the other end of the second resistor; The inverter unit further includes a second capacitor connected in parallel with both ends of the second resistor; The grounding detection circuit further includes a first DC voltage sampling unit and a first AC voltage sampling unit electrically connected between the positive pole of the output end of the photovoltaic module and the ground; The grounding detection circuit further includes a second DC voltage sampling unit and a second AC voltage sampling unit electrically connected between the negative pole of the output end of the photovoltaic module and the ground.

2. The grounding detection circuit according to claim 1, wherein: The switch is a relay.

3. A grounding detection method for the grounding detection circuit of a non-isolated photovoltaic inverter as described in claim 1, characterized in that, The grounding detection method includes the following steps: A. Switch the moving contact of the switch to the first contact to connect the second resistor between the positive pole of the output end of the photovoltaic module and the ground, detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid, and judge whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly enter step B; otherwise, continuously detect until the pressure difference is not within the error reporting range, and then enter step B; B. Switch the moving contact of the switch to the second contact to connect the second resistor between the negative pole of the output end of the photovoltaic module and the ground, detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid, and judge whether the pressure difference falls within the error reporting range. When the pressure difference is not within the error reporting range, directly enter step C; otherwise, continuously detect until the pressure difference is not within the error reporting range, and then enter step C; C. Keep the switch state in step B, continuously detect the pressure difference between the casing of the photovoltaic inverter and the neutral point of the power grid after the photovoltaic inverter is connected to the grid. When it is judged that the pressure difference falls within the error reporting range, report an error message.

4. The grounding detection method according to claim 3, wherein In step A, the second resistor and the second capacitor are connected between the positive pole of the output end of the photovoltaic module and the ground; in step B, the second resistor and the second capacitor are connected between the negative pole of the output end of the photovoltaic module and the ground.

5. The grounding detection method according to claim 3 or 4, characterized in that, In step A, the pressure difference Ux is shown by the following formula: In steps B and C, the pressure difference Ux is shown by the following formula: Among them, R21, R22, R23, and R24 respectively represent the impedances of the neutral point of the power grid, phase W, phase U, and phase V relative to the machine case, R11 represents the resistance value of the second resistor or the equivalent impedance value of the second resistor and the second capacitor connected in parallel, U, V, and W respectively represent the three-phase voltages of the power grid, and DC1, DC2, AC1, and AC2 are used to characterize the system common-mode voltage of the PV array to the ground.

6. The grounding detection method according to claim 5, wherein In the said step A, the pressure difference Ux is simplified as shown in the following formula; In the said steps B and C, the pressure difference Ux is simplified as shown in the following formula 7. The grounding detection method according to claim 3, wherein The error reporting ranges in the said steps A, B, and C are the same.

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