Detection circuit for grounding reliability of three-phase four-wire grid-connected inverter system

Abstract

The invention discloses a detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system, and the detection circuit comprises a three-phase symmetric circuit and a detection circuit, the three-phase symmetric circuit comprises three-phase live wires, grounding resistors and a safety capacitor, the three-phase live wires are electrically connected with a grounding wire through the grounding resistors, and the safety capacitor is electrically connected with the three-phase live wires through the safety capacitor. The safety capacitor is connected with the grounding resistor in parallel, the detection circuit comprises an external resistor and a controllable switch, and the first phase live wire is electrically connected with the grounding wire through the external resistor and the controllable switch. The external resistor is additionally arranged between the live wire and the grounding wire to destroy the symmetry of the system, so that the voltage difference is generated between the grounding wire and the zero wire to judge whether the system is reliably grounded or not.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system. Background Technology

[0002] In substations, inverters are installed outdoors and are greatly affected by the environment. Prolonged exposure to sunlight, rain, and lightning strikes can cause grounding cables, screws, and other components to age and rust, leading to unreliable grounding of the inverter casing and causing safety issues. For single-phase inverters, the reliability of grounding can be determined by detecting the voltage difference between the neutral (N) line and the protective earth (PE) casing. When the voltage difference exceeds a threshold, a grounding fault is identified, grid connection of the inverter is prohibited, and the grounding point is investigated to improve system safety.

[0003] Grounding detection technology for single-phase grid-connected inverters involves adding a hardware sampling circuit between the neutral (N) line and the protective earth (PE) line to detect the voltage difference between them. However, for three-phase four-wire inverters, due to the presence of safety capacitors and the symmetry of the three-phase circuit, the voltage difference between the PE and N lines will be very small even if the system lacks reliable grounding. Summary of the Invention

[0004] Based on this, in order to solve the problem of how to determine whether the system is reliably grounded by detecting the voltage difference between the N line and PE line, the present invention provides a detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system.

[0005] This invention provides a detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system, comprising:

[0006] A three-phase symmetrical circuit and a detection circuit are provided. The three-phase symmetrical circuit includes three-phase live wires, a grounding resistor, and a safety capacitor. The three-phase live wires are electrically connected to the grounding wire through the grounding resistor. The safety capacitor is connected in parallel with the grounding resistor. The detection circuit includes an external resistor and a controllable switch. The first phase live wire is electrically connected to the grounding wire through the external resistor and the controllable switch.

[0007] The grounding resistance includes a first resistor, a second resistor, and a third resistor. The first phase live wire is electrically connected to the grounding wire through the first resistor, the second phase live wire is electrically connected to the grounding wire through the second resistor, and the third phase live wire is electrically connected to the grounding wire through the third resistor.

[0008] The resistance values ​​of the first resistor, the second resistor, and the third resistor are equal.

[0009] The safety capacitor includes a first capacitor, a second capacitor, and a third capacitor. The first capacitor is connected in parallel with the first resistor, the second capacitor is connected in parallel with the second resistor, and the third capacitor is connected in parallel with the third resistor.

[0010] The capacitances of the first capacitor, the second capacitor, and the third capacitor are equal.

[0011] The sixth, seventh, and eighth resistors are three-phase load resistors, which are connected between the live wire and the ground wire.

[0012] The neutral wire and the ground wire are electrically connected through a fourth resistor.

[0013] The fourth capacitor is connected in parallel with the fourth resistor.

[0014] When the controllable switch is open and the external resistor is not connected to the circuit, the potential of the grounding wire is fixed at the three-phase midpoint by the first capacitor, the second capacitor, and the third capacitor, and the potential of the grounding wire is the same as the potential of the neutral wire. When the controllable switch is closed and the external resistor is infinite, the system has three-phase symmetry, and the potential of the grounding wire is the same as the potential of the neutral wire. When the controllable switch is closed and the external resistor is a finite value, the system wiring is normal. When the grounding wire and the neutral wire are short-circuited, the voltage difference between the grounding wire and the neutral wire is detected to determine whether the system is reliably grounded.

[0015] Beneficial effects: This invention adds an external resistor between the live wire and the ground wire PE to disrupt the system symmetry, thereby creating a voltage difference between the ground wire PE and the neutral wire N, which can be used to determine whether the system is reliably grounded.

[0016] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0017] The accompanying drawings are provided for a better understanding of this solution and do not constitute a limitation of the invention. Wherein:

[0018] Figure 1 This is a circuit topology diagram provided according to the present invention. Detailed Implementation

[0019] The following description, in conjunction with the accompanying drawings, illustrates exemplary embodiments of the present invention, including various details to aid understanding. These details should be considered merely exemplary. Therefore, those skilled in the art will recognize that various changes and modifications can be made to the embodiments described herein without departing from the scope and spirit of the invention. Similarly, for clarity and brevity, descriptions of well-known functions and structures are omitted in the following description.

[0020] like Figure 1 As shown, this invention provides a detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system, comprising:

[0021] The three-phase symmetrical circuit and the detection circuit include three-phase live wires, a grounding resistor and a safety capacitor. The three-phase live wires are electrically connected to the grounding wire PE through the grounding resistor. The safety capacitor is connected in parallel with the grounding resistor. The detection circuit includes an external resistor R5 and a controllable switch S1. The first phase live wire L1 is electrically connected to the grounding wire PE through the external resistor R5 and the controllable switch S1.

[0022] The first phase live wire L1, the second phase live wire L2, the third phase live wire L3, and the neutral wire N constitute a three-phase four-wire input.

[0023] The grounding resistance includes a first resistor R1, a second resistor R2, and a third resistor R3. The first phase live wire L1 is electrically connected to the grounding wire PE through the first resistor R1, the second phase live wire L2 is electrically connected to the grounding wire PE through the second resistor R2, and the third phase live wire L3 is electrically connected to the grounding wire PE through the third resistor R3.

[0024] The resistance values ​​of the first resistor R1, the second resistor R2, and the third resistor R3 are equal.

[0025] The safety capacitors include a first capacitor C1, a second capacitor C2, and a third capacitor C3. The first capacitor C1 is connected in parallel with the first resistor R1, the second capacitor C2 is connected in parallel with the second resistor R2, and the third capacitor C3 is connected in parallel with the third resistor R3.

[0026] The capacitances of the first capacitor C1, the second capacitor C2, and the third capacitor C3 are equal.

[0027] The sixth resistor R6, the seventh resistor R7, and the eighth resistor R8 are three-phase load resistors, which are connected between the live wire and the ground wire.

[0028] The first phase live wire L1 is electrically connected to the second phase live wire L2 through the sixth resistor R6 and the seventh resistor R7. The first phase live wire L1 is also grounded through the sixth resistor R6. The first phase live wire L1 is electrically connected to the third phase live wire L3 through the sixth resistor R6 and the eighth resistor R8. The third phase live wire L3 is also grounded through the eighth resistor R8. The second phase live wire L2 is electrically connected to the third phase live wire L3 through the seventh resistor R7 and the eighth resistor R8. The second phase live wire L2 is also grounded through the seventh resistor R7.

[0029] The sixth resistor R6, the seventh resistor R7, and the eighth resistor R8 are the equivalent loads of the first phase live wire L1, the second phase live wire L2, and the third phase live wire L3 to the neutral wire N, respectively.

[0030] The neutral wire N and the ground wire PE are electrically connected through the fourth resistor R4.

[0031] The fourth capacitor C4 is connected in parallel with the fourth resistor R4.

[0032] The first resistor R1, the second resistor R2, the third resistor R3, and the fourth resistor R4 are the resistances of the first phase live wire L1, the second phase live wire L2, the third phase live wire L3, and the neutral wire N to the ground wire PE, respectively. The external resistor R5 is the resistance applied between the first phase live wire L1 and the ground wire PE. The sixth resistor R6, the seventh resistor R7, and the eighth resistor R8 are the equivalent loads of the first phase live wire L1, the second phase live wire L2, and the third phase live wire L3 to the neutral wire N, respectively. The first capacitor C1, the second capacitor... C2, C3, and C4 are the safety capacitors for the first phase live wire L1, the second phase live wire L2, the third phase live wire L3, and the neutral wire N to the ground wire PE, respectively. The capacitances of C1, C2, and C3 are equal. The impedances of C1, C2, C3, and C4 are the impedances of the first phase live wire L1, the second phase live wire L2, the third phase live wire L3, and the neutral wire N to the ground wire PE, respectively. The controllable switch S1 can control whether the external resistor R5 is connected to the circuit.

[0033] When the controllable switch S1 is open and the external resistor R5 is not connected to the circuit, the potential of the grounding wire PE is fixed at the three-phase midpoint by the first capacitor C1, the second capacitor C2, and the third capacitor C3, and the potential of the grounding wire PE is the same as the potential of the neutral wire N. When the controllable switch S1 is closed and the external resistor R5 is infinite, the system has three-phase symmetry, and the potential of the grounding wire PE is the same as the potential of the neutral wire N. When the controllable switch S1 is closed and the external resistor R5 is a finite value, the system wiring is normal. When the grounding wire PE and the neutral wire N are short-circuited, the voltage difference between the grounding wire PE and the neutral wire N is detected to determine whether the system is reliably grounded.

[0034] Theoretically, the resistances of the first resistor R1, the second resistor R2, and the third resistor R3 are equal and infinite. When the controllable switch S1 is not closed, the external resistor R5 is not connected to the circuit. Due to the three-phase symmetry, the potential of the grounding wire PE is fixed at the midpoint of the three phases by the first capacitor C1, the second capacitor C2, and the third capacitor C3. The potential of the grounding wire PE is the same as the potential of the neutral wire N. Therefore, even if a grounding fault occurs in the system, there will be no voltage difference between the neutral wire N and the grounding wire PE.

[0035] Let the voltages between the first phase live wire L1, the second phase live wire L2, and the third phase live wire L3 and the neutral wire N be V1, V2, and V3, respectively.

[0036] ;

[0037] ;

[0038] ;

[0039] Where V represents the peak voltage, W represents the angular velocity, and π represents pi.

[0040] Let I1, I2, I3, and I4 be the branch currents between the first phase live wire L1, the second phase live wire L2, the third phase live wire L3, the neutral wire N, and the ground wire PE, respectively; and let V4 be the voltage between the ground wire PE and the neutral wire N.

[0041] Based on Kirchhoff's laws, we obtain:

[0042] ;

[0043] at the same time, ;

[0044] set up , , ,So,

[0045] ;

[0046] ;

[0047] in, This indicates the impedance of the first capacitor C1. This represents the impedance of the second capacitor C2. This represents the impedance of the third capacitor, C3. This represents the impedance of the fourth capacitor, C4.

[0048] When R9 = R11, that is, R5 is infinite and the system has three-phase symmetry, the potential of the grounding wire PE is equal to the potential of the neutral wire N. Therefore, it is not possible to determine whether the system is reliably grounded by detecting the potential difference between the two.

[0049] When R5 is a finite value, i.e., R9 When R11 is used, the potential of the grounding wire PE is equal to the potential of the neutral wire N only when R10=0, that is, when the resistance of R4 is very small (the system is properly grounded and the grounding wire PE is short-circuited with the neutral wire N).

[0050] In summary, for a three-phase four-wire grid-connected inverter, when the inverter is not connected to the grid, the system symmetry can be disrupted by adding an external resistor R5 between the live wire L and the ground wire PE, and the voltage difference between the ground wire PE and the neutral wire N can be detected to determine whether the system is reliably grounded.

[0051] Taking a 120kW three-phase four-wire inverter as an example, when the grounding is good, the voltage difference between the neutral wire (N) and the grounding wire (PE) is 0; when there is a ground fault (the measured resistance is in the megohm range), the voltage difference between the neutral wire (N) and the grounding wire (PE) is 5.8V; when there is a ground fault and a 6kΩ resistor is connected between the live wire and PE, the voltage between the N wire and PE is 217V, showing a significant voltage difference. Due to the inherent differences in the properties of different systems, the specific resistance and voltage values ​​will vary.

[0052] For a three-phase four-wire grid-connected inverter, due to the presence of safety capacitors and the symmetry of the three-phase circuit, the voltage difference between the grounding wire PE and the neutral wire N will be very small even if the system does not have reliable grounding.

[0053] This invention determines whether the system is reliably grounded by adding an external resistor between the live wire L and the ground wire PE to disrupt the system's symmetry, thereby creating a voltage difference between the ground wire PE and the neutral wire N.

[0054] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions within the technical scope disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system, characterized in that, include: A three-phase symmetrical circuit and a detection circuit are provided. The three-phase symmetrical circuit includes three-phase live wires, a grounding resistor, and a safety capacitor. The three-phase live wires are electrically connected to the grounding wire through the grounding resistor. The safety capacitor is connected in parallel with the grounding resistor. The detection circuit includes an external resistor and a controllable switch. The first phase live wire is electrically connected to the grounding wire through the external resistor and the controllable switch.

2. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 1, characterized in that: The grounding resistance includes a first resistor, a second resistor, and a third resistor. The first phase live wire is electrically connected to the grounding wire through the first resistor, the second phase live wire is electrically connected to the grounding wire through the second resistor, and the third phase live wire is electrically connected to the grounding wire through the third resistor.

3. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 2, characterized in that: The resistance values ​​of the first resistor, the second resistor, and the third resistor are equal.

4. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 3, characterized in that: The safety capacitor includes a first capacitor, a second capacitor, and a third capacitor. The first capacitor is connected in parallel with the first resistor, the second capacitor is connected in parallel with the second resistor, and the third capacitor is connected in parallel with the third resistor.

5. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 4, characterized in that: The capacitances of the first capacitor, the second capacitor, and the third capacitor are equal.

6. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 5, characterized in that: The sixth, seventh, and eighth resistors are three-phase load resistors, which are connected between the live wire and the ground wire.

7. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 6, characterized in that: The neutral wire and the ground wire are electrically connected through a fourth resistor.

8. The detection circuit for grounding reliability of a three-phase four-wire grid-connected inverter system according to claim 7, characterized in that: The fourth capacitor is connected in parallel with the fourth resistor.

9. A detection circuit for the grounding reliability of a three-phase four-wire grid-connected inverter system according to any one of claims 1-8, characterized in that: When the controllable switch is open and the external resistor is not connected to the circuit, the potential of the grounding wire is fixed at the three-phase midpoint by the first capacitor, the second capacitor, and the third capacitor, and the potential of the grounding wire is the same as the potential of the neutral wire. When the controllable switch is closed and the external resistor is infinite, the system has three-phase symmetry, and the potential of the grounding wire is the same as the potential of the neutral wire. When the controllable switch is closed and the external resistor is a finite value, the system wiring is normal. When the grounding wire and the neutral wire are short-circuited, the voltage difference between the grounding wire and the neutral wire is detected to determine whether the system is reliably grounded.

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