A method and system for detecting and correcting voltage of PT secondary circuit ground fault

Abstract

The application provides a PT secondary circuit grounding defect detection and voltage correction method and system, comprising obtaining real-time zero sequence current on both sides of a first grounding point of a voltage transformer (PT) secondary neutral line, judging whether it exceeds a corresponding threshold to determine whether a second grounding point exists on the PT secondary circuit; calculating effective common-mode interference voltage and interference voltage phase caused by the second grounding point on the PT secondary circuit, generating compensation voltage with the same amplitude as the effective common-mode interference voltage and the opposite phase as the interference voltage phase, and putting the compensation device into the PT secondary circuit to offset the negative impact of system voltage distortion caused by the presence of the second grounding point. The application can efficiently locate the approximate position of the second grounding point on the PT secondary circuit, effectively compensate the common-mode interference voltage caused by the grounding defect, make the relay protection device accurately identify the real system voltage, and avoid misoperation and refusal to operate accidents.

Description

Technical Field

[0001] This invention relates to the field of grounding detection and voltage correction compensation technology, specifically to a method and system for detecting grounding defects and correcting voltage in a PT secondary circuit. Background Technology

[0002] According to the "Eighteen Major Power Grid Accident Prevention Measures of State Grid Corporation of China", the secondary circuit of the voltage transformer (PT) in the substation should be grounded at one and only one point on the voltage transfer panel. This is to ensure that the neutral point voltage of the PT is consistent with the grounding grid potential, so that the relay protection device can sample the correct secondary voltage of the power system, thereby meeting the selectivity and reliability requirements of the relay protection device.

[0003] However, in the practical operation and maintenance of substation relay protection, factors such as incorrect secondary circuit connections, damaged cable shielding, and poor circuit debugging accuracy often lead to two or even multiple grounding points in the PT secondary circuit. Since the substation grounding grid is not at a standard zero potential, when two grounding points occur in the PT secondary circuit, a certain potential difference, i.e., common-mode interference voltage, will be generated between the grounding points. The amplitude and phase of the common-mode interference voltage are jointly determined by the current flowing into the grounding grid and the combined impedance between the grounding points. Because the three-phase voltages of the PT are all referenced to the neutral point potential, the presence of common-mode interference voltage will cause a certain degree of deviation and distortion in the amplitude and phase of the PT's three-phase voltages. This may lead to maloperation and failure to operate of relay protection devices that use voltage as the distinguishing electrical quantity (such as distance protection and zero-sequence current directional protection), resulting in cascading trips and expanding the power outage area.

[0004] Therefore, two-point grounding defects in the PT secondary circuit can negatively impact the safe operation of the power grid. During static operation (without system faults), the current flowing between the grounding points of the PT secondary circuit mainly consists of stray system currents and capacitive currents. Due to their small amplitude, the resulting common-mode interference voltage does not significantly affect the PT three-phase voltage. However, when a grounding fault occurs, a large zero-sequence current flows into the grounding grid, generating a significant common-mode interference voltage between the grounding points. This leads to severe distortion of the PT three-phase voltage. In this situation, the relay protection device cannot accurately identify the true system voltage, inevitably causing circuit breaker maloperation and failure to operate. Therefore, accurately identifying two-point grounding defects in the PT secondary circuit and effectively correcting and compensating for the system voltage is of significant engineering importance and practical value for ensuring power grid safety. Summary of the Invention

[0005] In view of this, the problem to be solved by the present invention is to provide a method and system for detecting grounding defects and correcting voltage in the secondary circuit of a PT, which can efficiently locate the approximate position of the second grounding point on the secondary circuit of the PT and effectively compensate for the system voltage anomalies caused by common-mode interference voltage, so that the relay protection device can accurately identify the true system voltage and reduce the probability of false tripping or failure to trip.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for detecting grounding defects and correcting voltage in the secondary circuit of a PT (Pressure Transmission Platform), comprising detecting a second grounding point and acquiring the real-time zero-sequence current on both sides of the first grounding point on the secondary neutral line of the PT. To determine whether a second grounding point exists on the PT secondary circuit, it is necessary to determine whether the corresponding threshold is exceeded. Generate compensation voltage and calculate the effective common-mode interference voltage on the PT secondary circuit caused by the second grounding point. Interference voltage phase Generate amplitude and effective common-mode interference voltage Same phase and interference voltage phase The opposite compensation voltage is applied to the PT secondary circuit for voltage correction compensation.

[0007] Furthermore, the real-time zero-sequence current The first zero-sequence current or the second zero-sequence current on both sides of the first grounding point of the PT; The step of detecting the second grounding point includes: verifying whether the first zero-sequence current and the second zero-sequence current of all PTs connected to the same grounding grid are accurate based on the zero-sequence current verification formula; if yes, continue to execute the step of detecting the second grounding point; if no, reacquire the first zero-sequence current and the second zero-sequence current and continue to verify. The zero-sequence current verification formula is: , Where Q represents the zero-sequence current discrimination threshold. , These represent the first zero-sequence current and the second zero-sequence current of the nth PT, respectively.

[0008] Furthermore, the effective common-mode interference voltage is calculated. Includes: the combined impedance based on the two grounding points Real-time zero-sequence current on the secondary neutral line Calculate the current first common-mode interference voltage ; Based on the PT, the self-generated zero-sequence voltage and the external zero-sequence voltage are obtained from the corresponding circuits of its secondary and tertiary windings, respectively, and the difference is calculated to determine the second common-mode interference voltage. First common-mode interference voltage With the second common-mode interference voltage The effective common-mode interference voltage is calculated using the first weighted summation strategy. .

[0009] Furthermore, the summation formula for the first weighted summation strategy is as follows: , Where ɛ1 and ɛ2 represent the first common-mode interference voltage, respectively. Second common-mode interference voltage Error coefficient These represent the first common-mode interference voltages. Second common-mode interference voltage The weighting coefficients.

[0010] Furthermore, the combined impedance is calculated. The formula is: , in, This represents the first zero-sequence current obtained after the second grounding point occurs on the secondary neutral line of the nth group of PTs during static operation of the power grid. Or the second zero-sequence current , This represents the offset voltage vector during static operation of the power grid. Offset voltage vector The formula is: , in, These represent the accurate three-phase symmetrical voltages collected before the second grounding point appears in the secondary circuit of the nth PT group. These represent the actual three-phase voltages after the offset voltage is superimposed, collected after the second grounding point appears in the secondary circuit of the nth PT.

[0011] Furthermore, the phase of the interference voltage is calculated. Includes: based on real-time zero-sequence current Determine the measurement phase Determining the reference phase based on the fault type of the power grid The measured phase and reference phase The phase of the interference voltage is calculated using the second weighted summation strategy. .

[0012] Furthermore, the summation formula for the second weighted summation strategy is: , Where β1 and β2 represent the measurement phases, respectively. Reference phase The error coefficient, These represent the measured phases. Reference phase The weighting coefficients.

[0013] Furthermore, the step of generating compensation voltage includes a first verification strategy: when the power grid is in static operation, it is determined whether the verification effect of the compensation voltage meets the threshold based on the first verification formula. If not, ɛ1 and ɛ2 are adjusted and the process jumps to the step of generating compensation voltage. If yes, feedback that the compensation is effective is output. The first verification formula is: , in, These represent the zero-sequence components of the monitoring voltages for phases A, B, and C, respectively, with δ being the amplitude discrimination threshold.

[0014] Furthermore, the step of generating compensation voltage includes a second verification strategy: when the power grid is in a fault state, the verification effect of the compensation voltage is judged based on the second verification formula to determine whether it meets the threshold. If not, β1 and β2 are adjusted and the process jumps to the step of generating compensation voltage. If yes, feedback that the compensation is effective is output. The second verification formula is: , Where Z represents a specific phase among phases A, B, and C, β Z2 β Z0 These represent the negative sequence component phase and the zero sequence component phase of a specific phase, respectively, and γ is the phase discrimination threshold.

[0015] A PT secondary circuit grounding defect detection and voltage correction system includes a PT electrically connected to a relay protection device via a voltage transfer panel. The secondary neutral line drawn from the neutral point of the PT is sequentially connected to a first grounding point located on the voltage transfer panel and the relay protection device. A first current monitoring device for collecting a first zero-sequence current is provided on the secondary neutral line between the first grounding point and the PT. A second current monitoring device for collecting a second zero-sequence current is provided on the secondary neutral line between the first grounding point and the protection device. A first three-phase synchronous conduction valve is connected in series between the PT and the voltage transfer panel, and between the voltage transfer panel and the protection device. The first three-phase synchronous conduction valve is connected in parallel with a three-phase voltage correction and compensation device and a second three-phase synchronous conduction valve connected in series.

[0016] The beneficial effects of this invention are: By acquiring the real-time zero-sequence current on both sides of the first grounding point on the secondary neutral line of several PTs respectively. By comparing it with the corresponding threshold, it is possible to effectively determine whether a second grounding point has appeared on the secondary circuit of any PT, through real-time zero-sequence current. and offset voltage vector Calculate the composite impedance Based on comprehensive impedance The approximate location of the second grounding point can be determined.

[0017] Based on real-time zero-sequence current Pre-calculated comprehensive impedance Determine the first common-mode interference voltage The second common-mode interference voltage is obtained by taking advantage of the fact that the PT third winding is not affected by the second grounding point of the neutral line and has good data stability. ; through the first common-mode interference voltage Second common-mode interference voltage Jointly determine the effective common-mode interference voltage Through real-time zero-sequence current Directly acquire the measurement phase The reference phase is obtained based on the characteristic differences of power grid fault types, sequence network mode, and vector characteristics. By measuring the phase Reference phase Jointly determine the phase of the interference voltage , The compensation voltage generated by the above method can effectively offset the interference of the second grounding point on the system voltage, regardless of whether the power grid is in static operation or fault state. This enables the relay protection device to accurately identify the true system voltage and avoid false tripping or failure to trip.

[0018] The first and second verification formulas are set up to verify whether the voltage compensation is accurate during static operation of the power grid and whether the phase compensation is accurate during power grid fault conditions, thereby improving the accuracy of the compensation voltage correction.

[0019] The voltage correction and compensation device can accurately intervene and compensate when a grounding defect occurs in the secondary circuit of the PT. It automatically exits compensation after the grounding defect is eliminated, without having a negative impact on the system operation. It does not make any changes or upgrades to the existing strategies of the relay protection device, is compatible with conventional substations and smart substations, has good versatility, and is easy to promote and apply. Attached Figure Description

[0020] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a circuit diagram of a PT secondary circuit grounding defect detection and voltage correction system with a first current monitoring device and a second current monitoring device installed. Figure 2This is a circuit diagram of a PT secondary circuit grounding defect detection and voltage correction system with a three-phase voltage correction and compensation device installed according to the present invention. Figure 3 This is a flowchart of a method for detecting grounding defects and correcting voltage in a PT secondary circuit according to the present invention; Figure 4 This is a schematic diagram of the setting of measuring points in a PT secondary circuit grounding defect detection and voltage correction method of the present invention; Figure 5 This invention relates to a method for detecting grounding defects in a PT secondary circuit and correcting voltage, which incorporates a comprehensive impedance. Relationship diagram; Among them, 1. Voltage transfer panel; 101. First current monitoring device; 102. Second current monitoring device; 2. Grounding grid; 3. First three-phase synchronous conduction valve; 4. Second three-phase synchronous conduction valve; 5. Three-phase voltage correction and compensation device. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] This invention provides a PT secondary circuit grounding defect detection and voltage correction system, such as... Figure 1 As shown, the system includes a voltage transformer (PT) that converts high-voltage electrical energy into low-voltage electrical energy in the primary power grid system; a voltage transfer panel that distributes, transfers, and transmits low-voltage signals; and a relay protection device that receives low-voltage signals and isolates faulty equipment within the primary power grid system. Several PTs within the substation are connected to their corresponding relay protection devices through the voltage transfer panel.

[0024] The secondary neutral line drawn from the neutral point of the PT is sequentially connected to the first grounding point located on the voltage transfer panel and the corresponding contact of the relay protection device. The first grounding point is connected to the grounding grid, making the voltage of the entire secondary neutral line consistent with the voltage of the first grounding point. A first current monitoring device for collecting the first zero-sequence current is installed on the secondary neutral line between the first grounding point and the PT, and a second current monitoring device for collecting the second zero-sequence current is installed on the secondary neutral line between the first grounding point and the protection device. In one embodiment of this application, both the first current monitoring device and the second current monitoring device are located close to the first grounding point.

[0025] When no second grounding point appears on the secondary neutral line of the PT (multi-point grounding defect), there is no voltage difference on the secondary neutral line, and the current collected by the first and second current monitoring devices is 0. When a second grounding point appears on the secondary neutral line of the PT, since the substation grounding grid is not at a standard zero potential, there is a voltage difference between the two grounding points, and a zero-sequence current composed of stray current and capacitive current of the grounding grid will flow. Based on the position and direction of the second grounding point relative to the first grounding point, the first and second zero-sequence currents with amplitudes are collected. By continuously monitoring whether the first or second zero-sequence current on both sides of the first grounding point of the PT is greater than the corresponding threshold, it is determined whether a second grounding point has appeared on the secondary neutral line of the PT.

[0026] Each PT in the substation has a first current monitoring device and a second current monitoring device installed on both sides of its first grounding point. When the zero-sequence current collected by a set of first and second current monitoring devices is greater than the threshold, the PT group (number) where the second grounding point occurs can be determined based on the correspondence between the PT and the first and second current monitoring devices.

[0027] like Figure 2 As shown, a first three-phase synchronous conduction valve is connected in series between the PT and the voltage transfer panel, and between the voltage transfer panel and the relay protection device, so that the secondary circuit led out from the secondary winding of the PT is connected to the corresponding relay protection device through the first three-phase synchronous conduction valve. A three-phase voltage correction and compensation device and a second three-phase synchronous conduction valve are connected in series with the first three-phase synchronous conduction valve.

[0028] When no second grounding point appears on the secondary neutral line of the PT, the first three-phase synchronous conduction valve is open, and the second three-phase synchronous conduction valve is closed. The three-phase voltage correction and compensation device is not connected to the PT secondary circuit. When a second grounding point appears on the secondary neutral line of the PT, the first three-phase synchronous conduction valve is closed, and the second three-phase synchronous conduction valve is open. The three-phase voltage correction and compensation device is connected to the secondary circuit and outputs compensation voltage to counteract the interference effect of the common-mode interference voltage generated by the second grounding point on the primary system voltage, ensuring that the relay protection device collects accurate primary system voltage and preventing false tripping or failure to trip. The voltage correction and compensation device remains connected to the circuit until the second grounding point is successfully eliminated. Then, the first and second three-phase synchronous conduction valves reset, restoring the normal operation mode of the PT's secondary circuit.

[0029] One embodiment of this application is as follows: the secondary circuit of the PT includes a secondary neutral line and a three-phase voltage circuit, and a voltage monitoring device is connected in parallel to the secondary circuit of the PT for continuous monitoring and acquisition of the three-phase voltage.

[0030] A method for detecting grounding defects and correcting voltage in a PT secondary circuit, such as Figure 3 As shown, this includes detecting the second grounding point and acquiring the real-time zero-sequence current on both sides of the first grounding point on the secondary neutral line of the PT. To determine whether a second grounding point exists on the secondary circuit of PT, it is necessary to determine whether the corresponding threshold is exceeded.

[0031] Real-time zero-sequence current The formula for determining whether a grounding defect has occurred at the second grounding point is: (This is the first or second zero-sequence current on either side of the first grounding point of the PT.) or , in, , Let represent the first zero-sequence current and the second zero-sequence current corresponding to the nth group PT, respectively, and P represent the defect discrimination threshold.

[0032] or When the fault diagnosis formula is met, it indicates that a second grounding point has appeared on the secondary circuit of the nth PT group (grounding defect detection is completed). Then, based on... and The actual flow direction determines whether the second grounding point is located on the PT side or the relay protection device side of the first grounding point. If the grounding defect formula is not met, it indicates that the secondary neutral line of the nth group of PTs is normal and no second grounding point has appeared.

[0033] To avoid abnormal data in the first and second zero-sequence currents due to malfunctions of the first and second current monitoring devices, which could affect the accuracy of grounding defect detection, the accuracy of the first and second zero-sequence currents in the secondary circuits of all PTs connected to the same grounding grid is verified using the zero-sequence current verification formula. If accurate, the second grounding point detection step continues; otherwise, the first and second zero-sequence currents are reacquired and verified.

[0034] A zero-sequence current verification formula is constructed based on Kirchhoff's current law (the vector sum of the currents flowing into and out of a node is zero): , Where Q represents the zero-sequence current discrimination threshold, which is set to account for reasonable measurement and calculation errors. This formula indicates that the sum of the current flowing into and out of the grounding grid is less than the corresponding zero-sequence current discrimination threshold Q. If the sum of all first and second zero-sequence currents corresponding to the voltage transfer panel does not satisfy this current verification formula, it indicates that there is a defect in the first or second current monitoring device or that the collected values ​​are abnormal, requiring equipment repair and re-collection.

[0035] One embodiment of this application is as follows: Q is set to 0. When the sum of the zero-sequence currents of all PTs connected to the same grounding grid is 0, it indicates that the first current monitoring device and the second current monitoring device corresponding to all PTs are normal and the step of detecting the second grounding point can be performed.

[0036] Generate compensation voltage and calculate the effective common-mode interference voltage on the secondary circuit of the PT caused by the second grounding point. Interference voltage phase Generate amplitude and effective common-mode interference voltage Same phase and interference voltage phase The opposite compensation voltage is applied to the PT secondary circuit for voltage correction compensation.

[0037] Determine the effective common-mode interference voltage Includes: the combined impedance based on the two grounding points The current first common-mode interference voltage is calculated from the real-time zero-sequence current on the secondary neutral line. Based on the PT, the self-generated zero-sequence voltage and the external zero-sequence voltage are obtained from the corresponding circuits of its secondary and tertiary windings, respectively, and the difference is calculated to determine the second common-mode interference voltage. First common-mode interference voltage With the second common-mode interference voltage The effective common-mode interference voltage is calculated using the first weighted summation strategy. .

[0038] Calculate the combined impedance between grounding points The formula is: , in, This represents the combined impedance between grounding points. This represents the first zero-sequence current acquired after the second grounding point appears on the secondary neutral line of the nth PT group. Or the second zero-sequence current , This represents the offset voltage vector during static operation of the power grid.

[0039] The offset voltage vector can be solved using the symmetrical component method. Offset voltage vector The formula is: , in, These represent the accurate three-phase symmetrical voltages collected before the second grounding point appears in the secondary circuit of the nth PT group. These represent the actual three-phase voltages, superimposed with the offset voltage, collected after the second grounding point occurs in the secondary circuit of the nth PT. In one embodiment of this application, both the accurate three-phase symmetrical voltage and the actual three-phase voltage can be collected by voltage detection equipment installed on the secondary circuit of the PT.

[0040] First common-mode interference voltage The formula is: , in, This represents the real-time zero-sequence current collected before and after the power grid fault. Since the combined impedance of the grounding grid between the first and second grounding points depends on the impedance of the secondary circuit, soil, and copper busbar between the two points, the combined impedance of the grounding point during the existence of the second grounding point... This remains unchanged so that, in the event of a grid fault (when a ground fault occurs in the grid, causing a large zero-sequence current to flow into the grounding grid), the calculated composite impedance can be used as a basis. and the collected real-time zero-sequence current Follow up to determine the first common-mode interference voltage .

[0041] If the second grounding point defect in the secondary circuit of the PT is not eliminated in time during the static operation of the power grid, a system fault in the power grid will cause the real-time zero-sequence current to be collected to be affected. Changes can easily lead to changes in the calculated first common-mode interference voltage. Reduced accuracy affects the voltage correction effect of the compensation voltage. Specific fault types in the power grid include: (I) Three-phase ground fault: The system voltage remains three-phase symmetrical, and the fault current does not contain a zero-sequence component; (II) Two-phase short circuit: The system voltage is significantly unbalanced and contains a negative sequence component, while the fault current does not contain a zero sequence component; (III) Single-phase grounding: The system voltage is significantly asymmetrical and contains negative sequence and zero sequence components, and the fault current contains a zero sequence component; (IV) Two-phase ground fault: The system voltage is significantly asymmetrical and contains negative sequence and zero sequence components, and the fault current contains a zero sequence component.

[0042] As can be seen from the above, when a single-phase ground fault or a two-phase ground fault occurs in the power grid, the fault current on the secondary winding of the corresponding PT contains a zero-sequence component. This zero-sequence component generated by the fault is crucial for effectively characterizing the ground fault and ensuring the correct operation of the relay protection device. If only the first common-mode interference voltage is used under power grid fault conditions... Generating compensation voltage can easily lead to overcompensation (excessively removing the zero-sequence component generated by the fault itself), thus affecting the correct operation of the relay protection device.

[0043] Since the neutral lines of the secondary and tertiary windings (open delta) of the PT are grounded at a single point through the voltage transfer panel, even if there is a second grounding point on the corresponding circuit of the secondary winding of the PT, it will not affect the corresponding circuit of the tertiary winding of the PT.

[0044] Therefore, after a second grounding point appears on the secondary neutral line of the PT, the three-phase voltage can be collected based on the corresponding circuit of the PT's tertiary winding and the vector sum can be calculated to generate an external zero-sequence voltage; the three-phase voltage can also be collected based on the corresponding circuit of the PT's secondary winding (secondary circuit) and the vector sum can be calculated to generate a self-generated zero-sequence voltage. The second common-mode interference voltage is calculated based on the difference formula. The formula for finding the difference is: , in, This represents the self-generated zero-sequence voltage of the secondary circuit of the nth PT group after the second grounding point appears. This represents the external zero-sequence voltage after the second grounding point appears in the secondary circuit of the nth PT group.

[0045] The summation formula for the first weighted summation strategy is: , Where ɛ1 and ɛ2 represent the first common-mode interference voltage, respectively. Second common-mode interference voltage Error coefficient These represent the first common-mode interference voltages. Second common-mode interference voltage The weighting coefficients are as follows. Specifically, ɛ1 and ɛ2 are jointly determined by factors such as the sampling accuracy of the protection device, the accuracy of the current monitoring device, and calculation errors.

[0046] Obtain the phase of the interference voltage Includes: based on real-time zero-sequence current Determine the measurement phase Determining the reference phase based on the fault type of the power grid The measured phase and reference phase The phase of the interference voltage is calculated using the second weighted summation strategy. .

[0047] Because the impedance angle of the grounding grid is much smaller than that of the power grid system, it is approximately resistive. Therefore, the phase of the common-mode interference voltage is related to the real-time zero-sequence current flowing through the grounding point. With approximately the same phase, the zero-sequence current flowing through the first and second grounding points, obtained by the current monitoring device, can be directly converted. / The phase is defined as the measured phase. .

[0048] When a fault occurs in the power grid (types (III) and (IV)), the reference phase is determined using fault vector analysis based on the specific fault type. Reference phase The method for obtaining it is as follows: For type (III) faults, the zero-sequence current reference phase The zero-sequence current reference phase is the same as the fault phase current; for Class (IV) faults, the zero-sequence current reference phase is... It is 180° out of phase with the healthy phase current.

[0049] The summation formula for the second weighted summation strategy is: , Where β1 and β2 represent the measurement phases, respectively. Reference phase The error coefficient, These represent the measured phases. Reference phase The weighting coefficients.

[0050] Generate compensation voltage The formula is: .

[0051] To improve the accuracy of the verification of the compensation voltage during static operation of the power grid, the first verification strategy is executed after the compensation voltage is injected into the secondary circuit: when the power grid is static, the verification effect of the compensation voltage is judged based on the first verification formula to determine whether it meets the threshold. If not, ɛ1 and ɛ2 are adjusted and the process jumps to the step of generating the compensation voltage. If yes, feedback that the compensation is effective is output. The first verification formula is: , in, These represent the zero-sequence components within the monitoring voltages of phases A, B, and C, respectively. δ The amplitude discrimination threshold should be set to avoid system unbalanced voltage.

[0052] Specifically, during static operation of the power grid, the three-phase voltage on the secondary circuit of the PT is obtained, and the superimposed zero-sequence component is separated using the symmetrical component method. The first verification formula is then used to determine whether it is valid. If it is valid, the compensation effect of the compensation voltage is determined to be ideal. If it is not valid, the values ​​of ɛ1 and ɛ2 are continuously adjusted, and the step of generating the compensation voltage is re-executed until the first verification formula is valid.

[0053] To improve the accuracy of compensation voltage verification during grid fault conditions, a second verification strategy is executed after the compensation voltage is injected into the secondary circuit: when a grid fault occurs, the verification effect of the compensation voltage is judged based on the second verification formula to determine whether it meets the threshold. If not, β1 and β2 are adjusted and the process jumps to the compensation voltage generation step. If yes, feedback that the compensation is effective is output. The second verification formula is: , Where Z represents a specific phase among phases A, B, and C, β Z2 and β Z0 These represent the negative-sequence phase component and the zero-sequence phase component of a specific phase, respectively. γ is the phase discrimination threshold, which should be set considering the differences in soil distribution characteristics of the grounding grid and the transition resistance. The specific phase Z is defined as follows: when a single-phase ground fault occurs in the power grid, Z represents the faulty phase; when a two-phase ground fault occurs in the power grid, Z represents the healthy phase.

[0054] Specifically, when a grid fault occurs, the three-phase voltages on the secondary circuit of the power PT are obtained, and the superimposed zero-sequence component is calculated. Based on a specific phase Z, the phases of the negative-sequence component and the zero-sequence component are determined. The validity of the second verification formula is then checked. If valid, the compensation voltage is deemed to have an ideal compensation effect; if not, the values ​​of β1 and β2 are adjusted, and the compensation voltage generation step is executed until the second verification formula is valid.

[0055] When a second grounding point appears in the PT secondary circuit, the combined impedance between the two grounding points is... The impedance is mainly composed of the secondary circuit impedance of the PT and the impedance of the grounding grid connected in series. Since the resistivity of the soil in the grounding grid is much greater than that of copper, the overall impedance between the grounding points is basically determined by the impedance of the grounding grid. Because the grounding grid is a complex three-dimensional conductor network composed of flat steel, copper busbars, and soil, the current flow path is complex and the soil resistivity distribution is uneven; therefore, the overall impedance between the two grounding points... There is no obvious correlation with distance.

[0056] like Figure 4 As shown, to roughly locate the second grounding point, test sections (sections I+, II+, III+, I-, II-) are divided between the PT terminal box and the relay protection device along the actual route of the PT secondary circuit. Measurement points (S1, S2, S3) are marked at the boundaries of each section, and each measurement point is grounded. A reference secondary voltage is applied, and the first and second zero-sequence currents of the current monitoring device are collected. The offset voltage vector during static operation of the power grid is obtained using the symmetrical component method. Then, the combined impedance between the first grounding point and each measuring point is calculated. .

[0057] like Figure 5 As shown, the location of the test section and the overall impedance are established. The corresponding relationship table. Based on the above method, the comprehensive impedance of the second grounding point between different measuring points in the PT secondary circuit is determined. The mapping relationship between the location and the measurement point. This is so that in subsequent actual monitoring, based on the real-time zero-sequence current... Flow direction and overall impedance The approximate location range of the second grounding point is estimated based on the interval in which it is located.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for detecting grounding defects and correcting voltage in a PT secondary circuit, characterized in that, include: Detect the second grounding point and obtain the real-time zero-sequence current on both sides of the first grounding point on the secondary neutral line of the PT. To determine whether a second grounding point exists on the PT secondary circuit, it is necessary to determine whether the corresponding threshold is exceeded. Generate compensation voltage and calculate the effective common-mode interference voltage on the PT secondary circuit caused by the second grounding point. Interference voltage phase Generate amplitude and effective common-mode interference voltage Same phase and interference voltage phase The opposite compensation voltage is applied, and voltage correction compensation is performed on the PT secondary circuit. Calculate the effective common-mode interference voltage Includes: the combined impedance based on the two grounding points Real-time zero-sequence current on the secondary neutral line Calculate the current first common-mode interference voltage ; Based on the PT, the self-generated zero-sequence voltage and the external zero-sequence voltage are obtained from the corresponding circuits of its secondary and tertiary windings, respectively, and the difference is calculated to determine the second common-mode interference voltage. First common-mode interference voltage With the second common-mode interference voltage The effective common-mode interference voltage is calculated using the first weighted summation strategy. .

2. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 1, characterized in that, The real-time zero-sequence current The first zero-sequence current or the second zero-sequence current on both sides of the first grounding point of the PT; The step of detecting the second grounding point includes: verifying whether the first zero-sequence current and the second zero-sequence current of all PTs connected to the same grounding grid are accurate based on the zero-sequence current verification formula; if yes, continue to execute the step of detecting the second grounding point; if no, reacquire the first zero-sequence current and the second zero-sequence current and continue to verify. The zero-sequence current verification formula is: ， Where Q represents the zero-sequence current discrimination threshold. , These represent the first zero-sequence current and the second zero-sequence current of the nth PT, respectively.

3. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 1, characterized in that, The summation formula for the first weighted summation strategy is as follows: ， Where ɛ1 and ɛ2 represent the first common-mode interference voltage, respectively. Second common-mode interference voltage Error coefficient These represent the first common-mode interference voltages. Second common-mode interference voltage The weighting coefficients.

4. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 1, characterized in that, Calculate the combined impedance The formula is: ， in, This represents the first zero-sequence current obtained after the second grounding point occurs on the secondary neutral line of the nth group of PTs during static operation of the power grid. Or the second zero-sequence current , This represents the offset voltage vector during static operation of the power grid. Offset voltage vector The formula is: ， in, These represent the accurate three-phase symmetrical voltages collected before the second grounding point appears in the secondary circuit of the nth PT group. These represent the actual three-phase voltages after the offset voltage is superimposed, collected after the second grounding point appears in the secondary circuit of the nth PT.

5. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 1, characterized in that, Calculate the phase of the interference voltage Includes: based on real-time zero-sequence current Determine the measurement phase Determining the reference phase based on the fault type of the power grid The measured phase and reference phase The phase of the interference voltage is calculated using the second weighted summation strategy. .

6. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 5, characterized in that, The summation formula for the second weighted summation strategy is: ， Where β1 and β2 represent the measurement phases, respectively. Reference phase The error coefficient, These represent the measurement phases. Reference phase The weighting coefficients.

7. The method and system for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 3, characterized in that, The step of generating compensation voltage includes a first verification strategy: when the power grid is in static operation, it is determined whether the verification effect of the compensation voltage meets the threshold based on the first verification formula. If not, ɛ1 and ɛ2 are adjusted and the process jumps to the step of generating compensation voltage. If yes, feedback that the compensation is effective is output. The first verification formula is: ， in, These represent the zero-sequence components of the monitoring voltages for phases A, B, and C, respectively, with δ being the amplitude discrimination threshold.

8. The method for detecting grounding defects and correcting voltage in a PT secondary circuit according to claim 6, characterized in that, The step of generating compensation voltage includes a second verification strategy: when the power grid is in a fault state, the verification effect of the compensation voltage is judged based on the second verification formula to determine whether it meets the threshold. If not, β1 and β2 are adjusted and the process jumps to the step of generating compensation voltage. If yes, feedback that the compensation is effective is output. The second verification formula is: ， Where Z represents a specific phase among phases A, B, and C, β Z2 β Z0 These represent the negative sequence component phase and the zero sequence component phase of a specific phase, respectively, and γ is the phase discrimination threshold.

9. A detection and correction system for the second grounding point of a PT secondary circuit, comprising a method for detecting grounding defects and correcting voltage in a PT secondary circuit according to any one of claims 1 to 8, characterized in that, The device includes a PT connected to a relay protection device via a voltage transfer panel. The secondary neutral line drawn from the neutral point of the PT is connected in sequence to a first grounding point located on the voltage transfer panel and the relay protection device. A first current monitoring device for collecting the first zero-sequence current is provided on the secondary neutral line between the first grounding point and the PT. A second current monitoring device for collecting the second zero-sequence current is provided on the secondary neutral line between the first grounding point and the protection device. A first three-phase synchronous conduction valve is connected in series between the PT and the voltage transfer panel, and between the voltage transfer panel and the protection device. The first three-phase synchronous conduction valve is connected in parallel with a three-phase voltage correction and compensation device and a second three-phase synchronous conduction valve connected in series.

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