A device and method for measuring impedance of a high-voltage cable grounding system

Abstract

The present invention discloses a device and method for measuring the impedance of a high-voltage cable grounding system under energization. The measuring device of the present invention includes a control unit, a drive circuit, a DC-AC circuit, a coupling transmission circuit, a filter circuit, a current conditioning circuit and a voltage conditioning circuit; the drive circuit is connected to the control unit, and is used to amplify the four complementary drive signals output by the control unit and output them to the DC-AC circuit, and control the on-off of the switch tube of the DC-AC circuit; the DC-AC circuit receives the drive signal of the drive circuit, and realizes the output of AC voltage through SPWM control; the front end of the filter circuit is connected to the secondary side of the transformer, and the rear end of the filter circuit is used to connect the three-phase grounding system to be measured; the impedance value of the three-phase grounding system to be measured is calculated by the control unit. The present invention can perform reliable live measurement of the impedance of the high-voltage cable grounding system in operation.

Description

Technical Field

[0001] The invention belongs to the technical field of impedance measurement of high-voltage cable grounding systems, and in particular relates to an isolated high-voltage cable grounding system impedance live measurement device and method. Background Art

[0002] As an important part of the high-voltage cable line, the high-voltage cable grounding system includes the cable metal sheath, accessory lead seal, grounding cable and the connectors between the parts. If there is a defect in any part (such as cracked lead seal, poor connection of grounding cable, damaged sheath, loose connection, etc.), it is very easy to cause abnormal impedance of the grounding system, which in turn affects the cable sealing and electrical connection performance, and easily causes the joint or terminal breakdown failure. Since these defects are located inside the cable line, they are difficult to find by naked eyes or general detection means. The grounding system impedance caused by electrical oxidation, rust, and improper grounding of the grounding cable is too large, resulting in failure or poor grounding of the cable, thereby causing defects in the cable grounding system, and also increasing the risk of electrical faults such as operating overvoltage and overload in the electrical system or equipment. Therefore, in order to ensure the safety and reliability of the cable system and the performance of the grounding system, it is very necessary to conduct live detection and defect troubleshooting of the high-voltage cable grounding system parameters under the normal operation of the high-voltage cable line for the safe and reliable operation of the entire power grid. The grounding impedance of the grounding system is generally very small, and the measurement accuracy is very high. The cable line distribution is complex, and the line parameters are greatly affected by external factors such as the environment. Therefore, accurate measurement of the measurement loop impedance, such as the resistance and inductance parameters of the grounding system, can predict the state of the grounding system in advance and provide effective data support for the safe operation of the cable. At the same time, since the grounding system fault may cause high voltage when the grounding system is measured under power, the measurement system needs to be electrically isolated to ensure personal safety.

[0003] In order to detect the status of the high-voltage cable grounding system, passive infrared temperature measurement, circulating current detection, eddy current flaw detection and other methods are generally used. At present, these detection methods are all based on circulating current, temperature, and flaw detection methods to indirectly judge the defects of the grounding system, and cannot directly reflect the specific parameters of the grounding impedance. CN108982968A discloses an online measurement system and measurement method for contact resistance, which adopts an improved discrete Fourier algorithm to achieve high-precision measurement of small resistance parameters. However, the algorithm process used in this measurement method is relatively complex. It can achieve accurate measurement of contact resistance, but it is not suitable for live measurement of grounding impedance affected by complex line parameters. Summary of the invention

[0004] In view of the above-mentioned defects in the prior art, the present invention provides a device and method for measuring the impedance of a high-voltage cable grounding system under live conditions, which can reliably measure the loop impedance of a high-voltage cable grounding system under live conditions.

[0005] To this end, the present invention adopts the following technical solution.

[0006] In a first aspect, the present invention provides a device for measuring the impedance of a high-voltage cable grounding system, comprising a control unit, a drive circuit, a DC-AC circuit, a coupling transmission circuit, a filter circuit, a current conditioning circuit, and a voltage conditioning circuit;

[0007] The driving circuit is connected to the control unit, and is used to amplify the four complementary driving signals output by the control unit and output them to the DC-AC circuit, so as to control the on and off of the switch tube of the DC-AC circuit;

[0008] The DC-AC circuit receives the driving signal from the driving circuit and outputs the AC voltage through SPWM control;

[0009] The coupling transmission circuit includes a primary coil of a transformer, a secondary coil of a transformer, a sensor coil, a compensation inductor L c , primary compensation capacitor C 1 and secondary compensation capacitor C 2. The primary compensation capacitor C One end of 1 is connected to the DC-AC circuit, and the primary compensation capacitor C The other end of 1, the primary coil and the compensation inductor L c One end of the primary side is connected in series to compensate for the inductance L c The other end of the primary side is connected to the DC-AC circuit; the secondary side coil and the secondary side compensation capacitor C 2 in series, compensating inductance L c The secondary side and the sensing coil are connected in series, and the sensing coil is coupled with the primary coil and the secondary coil;

[0010] The front end of the filter circuit is connected to the secondary side of the transformer, and the rear end of the filter circuit is used to connect to the three-phase grounding system of the high-voltage cable to be tested;

[0011] Transformer primary current I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1. The sampling signal is amplified and filtered by the current conditioning circuit and the voltage conditioning circuit respectively, and then input into the AD sampling pin of the control unit, and the impedance value of the three-phase grounding system of the high-voltage cable to be tested is calculated by the control unit.

[0012] The present invention utilizes a coupling transmission circuit, controls the designated phase to be measured to be connected to a measuring device through a relay, calculates based on the coupling relationship of each coil in the coupling transmission circuit, according to the voltage sampling values ​​of both ends of the sensor coil and the compensation inductor in the coupling transmission circuit obtained by the conditioning circuit after being connected in series, and the primary current sampling value of the coupling transformer, and then obtains the impedance value of the grounding system after parameter correction. The present invention is a detection method for the loop impedance of the grounding system with high measurement accuracy, strong anti-interference performance, convenient device, and wide applicability.

[0013] Furthermore, the filter circuit is a filter circuit that can achieve series resonance and parallel resonance of inductance and capacitance at the operating frequency.

[0014] Furthermore, the filter circuit includes an inductor L x1 , capacitor C x1 、Inductance L y1 and capacitor C y1 , the inductance L y1 and capacitor C y1 Connect in series with the secondary compensation capacitor C 2 in series, the inductor L x1 With capacitor C x1 After parallel connection with the secondary compensation capacitor C 2 in series; the inductor L y1 With capacitor C y1 To achieve series resonance, the inductor L x1 With capacitor C x1 Achieve parallel resonance.

[0015] Furthermore, in the coupling transmission circuit, the self-inductance coefficient of the primary coil is L 1. The self-inductance coefficient of the secondary coil is L 2. Satisfy:

[0016] ,

[0017] ,

[0018] In the formula, Indicates the resonant center frequency;

[0019] The mutual inductance coefficient between the sensor coil and the primary coil is M 1s , compensation inductance L c The primary-secondary mutual inductance coefficient is M r ,satisfy:

[0020] ,

[0021] It is known that the mutual inductance coefficient between the primary coil and the secondary coil isM 12 , the mutual inductance coefficient between the sensor coil and the secondary coil is M 2s , sensor coil and compensation inductor L c The output voltage after the secondary side is connected in series is V 1. The calculation formula for the loop impedance value of the high-voltage cable three-phase grounding system is:

[0022] .

[0023] Furthermore, the DC-AC circuit is a full-bridge DC-AC circuit, including MOS transistor S1, MOS transistor S2, MOS transistor S3 and MOS transistor S4;

[0024] The MOS transistor S1 and the MOS transistor S2 form a first half bridge, and the MOS transistor S3 and the MOS transistor S4 form a second half bridge. Both half bridges are driven by a set of complementary driving signals including a dead zone output by the driving circuit.

[0025] Furthermore, in the first half bridge, a first connection point is provided between the MOS transistor S1 and the MOS transistor S2; in the second half bridge, a second connection point is provided between the MOS transistor S3 and the MOS transistor S4; the primary compensation capacitor C 1 is connected to the first connection point, the compensation inductor L c The other end of the original side is connected to the second connection point.

[0026] In a second aspect, the present invention provides a method for measuring the impedance of a high-voltage cable grounding system under live conditions, which uses the above-mentioned device for measuring the impedance of a high-voltage cable grounding system under live conditions to perform measurement, and comprises the following steps:

[0027] 1) When the DC power supply is input into the DC-AC circuit, the energy enters the primary side of the transformer in the form of a sine wave and is transmitted to the secondary side through the coupling of the coil;

[0028] 2) Primary current of transformer I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1. The sampling signal is amplified and filtered by the current conditioning circuit and the voltage conditioning circuit respectively, and then input into the control unit. The initial value of the loop impedance of the three-phase grounding system to be measured is calculated according to the voltage and current sampling values, thereby avoiding the direct measurement of the secondary current.

[0029] 3) Before the formal measurement, the loop impedance of the three-phase grounding system to be measured is measured when the load is open circuit, short circuit and 0.2 ohm respectively. According to the measurement results, the initial value of the loop impedance of the three-phase grounding system to be measured is corrected and calculated to obtain the corrected measured value of the loop impedance of the three-phase grounding system to be measured, and the influence of the filter circuit impedance and parasitic parameters in the measuring device on the measurement of the loop impedance of the three-phase grounding system to be measured is eliminated;

[0030] 4) During the measurement process, the relays in the three-phase measurement circuit connected in series with the loads of each phase are controlled so that the designated phase among the three phases A, B, and C is connected to the measurement device to measure the loop impedance of the grounding system of that phase;

[0031] 5) During the measurement process, the host computer sends an information frame to the control unit. After receiving a frame of information, the control unit verifies and processes it and executes the corresponding instructions; the control unit sends an information frame to the host computer, the host computer program verifies and processes it, and displays the measurement results, thereby realizing communication between the host computer and the control unit, achieving the purpose of real-time interaction of controlling the measurement process through the host computer and displaying the measurement results through the host computer interface.

[0032] Furthermore, an information frame is sent to the control unit through the host computer, and the content of the information frame includes a reference current value, an SPWM control signal, a correction instruction and each relay control instruction; the control unit sends an information frame to the host computer, and the information frame includes voltage and current sampling values, and a three-phase grounding system loop impedance value.

[0033] Furthermore, the initial value of the grounding system loop impedance is calculated based on a result obtained by performing DFT operation on the sampled values ​​and the mutual inductance coefficient of the coupling between the coils.

[0034] Furthermore, the correction calculation in step 3) takes into account the impedance of the transformer secondary side rear stage measurement device itself, assuming that Z1 and Z2 are the equivalent series impedance and equivalent parallel impedance of the equivalent circuit of the measurement device, respectively. When the load is open-circuited, the measured impedance value is Z=Z1+Z2; when the load is short-circuited, the measured impedance value is Z1; when the load is connected to impedance Z x When the measured impedance value is Z1+Z2 / / Z x (“ / / ” indicates parallel connection), satisfying:

[0035] ,

[0036] According to the impedance values ​​measured in the three cases of open load, short load and load connected to a given 0.2 ohm resistor, the equivalent series impedance Z1 and equivalent parallel impedance Z2 of the measuring device and the mutual inductance coefficient are calculated, and then the unknown load Z is calculated. x Calculation of

[0037] Through phase correction, the phase deviation between the sampled voltage and the sampled current realized through the current transformer, as well as the phase deviation effect caused by component errors in the voltage conditioning circuit and the current conditioning circuit are eliminated.

[0038] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0039] The present invention can realize automatic switching and automatic detection, and has the performance of eliminating AC power frequency induced voltage and current interference, effectively improving measurement efficiency and reducing the impact on power grid operation. The present invention can effectively measure potential defects that affect the electrical connection performance of the three-phase grounding system, such as cracking of high-voltage cable lead seals, loose grounding cables, and rusted bolts, and realize the live measurement, phase verification and defect precise location functions of parameters such as the three-phase grounding system loop impedance, ground induced voltage, and cable sheath inductance. The measurement method and measurement device of the present invention are suitable for outdoor measurement operations, and the measurement is all realized on the primary side, avoiding direct measurement of the electrical signal on the secondary load side, simplifying the measurement device, making on-site operation simple and convenient, and having high measurement accuracy and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the impedance live measurement device of the high-voltage cable grounding system of the present invention;

[0041] Figure 2 The present invention is a circuit diagram of a device for measuring impedance of a high-voltage cable grounding system. DETAILED DESCRIPTION

[0042] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0043] Example 1

[0044] This embodiment is a device for measuring the impedance of a high-voltage cable grounding system. Figure 1 and Figure 2 As shown, it is composed of a control unit, a driving circuit, a DC-AC circuit 101, a coupling transmission circuit 102, a filtering circuit 103, a current conditioning circuit and a voltage conditioning circuit. The MCU in the control unit is a single-chip microcomputer, using the single-chip microcomputer STM32G431C8T6 of STMicroelectronics, and the driving circuit uses the IR2181 chip of Infineon. The A / D sampling chip in the conditioning circuit uses the MCP6022 chip of Microchip.

[0045] In this embodiment, the operating frequency f s =1 kHz, the DC power supply is input into the DC-AC circuit, the energy enters the primary side of the transformer in the form of a sine wave, and is transmitted to the secondary side through the coupling of the inductor coil, thereby applying excitation to the measuring device, and finally outputs to the load to be measured after passing through the series and parallel resonant filter circuit; at the same time, the primary and secondary coils of the coupling transmission circuit are respectively connected to the sensor coil L s coupling.

[0046] The drive circuit is connected to the control unit, and is used to amplify the four complementary drive signals output by the control unit and output them to the DC-AC circuit to control the on and off of the switch tube of the DC-AC circuit. The DC-AC circuit receives the drive signal of the drive circuit and realizes the output of AC voltage through SPWM control.

[0047] The front end of the filter circuit is connected to the secondary side of the transformer, and the rear end of the filter circuit is connected to the three-phase grounding system of the high-voltage cable to be tested, wherein the measurement output end of each phase is connected in series with a relay, and the control unit outputs a control signal to enable the relay to control the loop impedance access of the phase to be tested, thereby realizing the selective measurement of the loop impedance of the three-phase grounding system of the high-voltage cable to be tested. The filter circuit is a filter circuit that realizes series resonance and parallel resonance of inductance and capacitance at the operating frequency.

[0048] Specifically, the filter circuit includes an inductor L x1 , capacitor C x1 、Inductance L y1 and capacitor C y1 , the inductance L y1 and capacitor C y1 Connect in series with the secondary compensation capacitor C 2 in series, the inductor L x1 With capacitor C x1 After parallel connection with the secondary compensation capacitor C 2 in series; the inductor L y1 With capacitor C y1 To achieve series resonance, the inductor L x1 With capacitor C x1 Parallel resonance is achieved to prevent the live side from introducing interference from other frequency bands from the load end and coupling into the measuring device, thus affecting the measurement of the loop impedance of the three-phase grounding system.

[0049] Specifically, the DC-AC circuit is a full-bridge DC-AC circuit, including MOS transistor S1, MOS transistor S2, MOS transistor S3 and MOS transistor S4; the MOS transistor S1 and MOS transistor S2 form a first half bridge, and a first connection point is provided between MOS transistor S1 and MOS transistor S2; the MOS transistor S3 and MOS transistor S4 form a second half bridge, and a second connection point is provided between MOS transistor S3 and MOS transistor S4; both half bridges are driven by a set of complementary drive signals including a dead zone output by a drive circuit. Specifically, the coupling transmission circuit includes a primary coil of a transformer, a secondary coil of a transformer, a sensor coil, a compensation inductor L c , primary compensation capacitor C 1 and secondary compensation capacitor C 2. The primary compensation capacitor C One end of 1 is connected to the first connection point, and the primary compensation capacitor C The other end of 1, the primary coil and the compensation inductor L c One end of the primary side is connected in series to compensate for the inductance L c The other end of the primary side is connected to the second connection point; the secondary side coil and the secondary side compensation capacitor C 2 in series, compensating inductance L c The secondary side is connected in series with the sensing coil, and the sensing coil is coupled with the primary coil and the secondary coil.

[0050] Transformer primary current I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1. The sampling signal is amplified and filtered by the current conditioning circuit and the voltage conditioning circuit respectively, and then input into the AD sampling pin of the control unit. The control unit calculates the impedance value of the three-phase grounding system to be measured by the DFT algorithm.

[0051] Specifically, in the coupling transmission circuit, the self-inductance coefficient of the primary coil is L 1. The self-inductance coefficient of the secondary coil is L 2. Satisfy:

[0052] ,

[0053] ,

[0054] In the formula, Indicates the resonant center frequency;

[0055] The mutual inductance coefficient between the sensor coil and the primary coil is M 1s, compensation inductance L c The primary-secondary mutual inductance coefficient is M r ,satisfy:

[0056] ,

[0057] Through the sensor coil L s With compensation inductor L c Output voltage after series connection V 1 The secondary current is obtained indirectly by sampling and measuring through the conditioning circuit I 2. Sensing coil L s With compensation inductor L c The open-circuit output voltage of the two ends after series connection V 1Satisfy:

[0058] ,

[0059] In the formula, To compensate for the inductance L c The output voltage, For sensor coil L s Output voltage

[0060] It is known that the mutual inductance coefficient between the primary coil and the secondary coil is M 12 , the mutual inductance coefficient between the sensor coil and the secondary coil is M 2s , sensor coil and compensation inductor L c The output voltage after the secondary side is connected in series is V 1. The calculation formula for the loop impedance value of the high-voltage cable three-phase grounding system is:

[0061] ,

[0062] The capacitor compensates the self-inductance of the primary and secondary coils of the transformer, simplifying the voltage and current ratio of the primary and secondary coils. For the transformer, in order to prevent the excessive magnetic induction intensity in the magnetic core from causing the transformer linearity to decrease and affecting the accuracy of the measuring device, the relative magnetic permeability is selected in this embodiment. u r =6 0 The core has good linearity under the working current.

[0063] Example 2

[0064] This embodiment provides a method for measuring the impedance of a high-voltage cable grounding system under live conditions, which uses the device for measuring the impedance of a high-voltage cable grounding system under live conditions described in Embodiment 1 for measurement. The specific steps are as follows:

[0065] 1) The control unit outputs two pairs of complementary driving signals through the driving circuit to drive the two half bridges in the full-bridge DC-AC circuit 101 respectively, so that S1 and S2, S3 and S4 complement each other and switch, and sends instructions to the host computer to control the SPWM signal to adjust the inverter output. Since the secondary current is large during the measurement process, the number of turns of the inductance components in the coupling coil, filter circuit and parasitic inductance correction circuit is large and the inductance value is large, the component characteristics are nonlinear when the current changes, and the inductance value in the measurement loop will change, affecting the accurate measurement of the loop impedance of the three-phase grounding system to be measured. Therefore, during the measurement, the current closed-loop control is used to control the secondary current to be constant, so that the measurement of the loop impedance of the three-phase grounding system to be measured is carried out under the conditions of constant current on the secondary side and unchanged loop component characteristics. The control unit in this embodiment is a single-chip microcomputer.

[0066] 2) The DC power supply is input into the full-bridge DC-AC circuit. The full-bridge DC-AC circuit 101 outputs an SPWM waveform through a filtering circuit. The energy enters the primary side of the transformer in the form of a sine wave and is transmitted to the secondary side through the coupling of the coil, thereby applying excitation to the measuring device and inducing an electromotive force in the sensing coil.

[0067] 3) Primary current of transformer I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1 is sampled, the sampling signal is amplified and filtered by the current conditioning circuit and the voltage conditioning circuit respectively, and then input into the single chip microcomputer, and the initial value of the loop impedance of the three-phase grounding system to be measured is obtained by performing DFT operation according to the voltage and current sampling values.

[0068] 4) Due to the high-precision measurement requirements of the grounding system loop impedance, the equivalent series resistance of the passive components on the secondary side of the coupling transformer and the contact resistance of the relay in the measuring device have a non-negligible impact on the accuracy of the measurement of the loop impedance of the three-phase grounding system to be measured. Therefore, before the formal measurement, the loop impedance of the three-phase grounding system to be measured is measured when the load is open circuit, short circuit, and 0.2 ohms respectively. The host computer sends instructions to the microcontroller and saves the measurement results as correction parameters. During the formal measurement, the initial value of the loop impedance of the three-phase grounding system to be measured is corrected and calculated to eliminate the influence of the filter circuit impedance, parasitic parameters, etc. in the measurement loop on the loop impedance measurement of the three-phase grounding system to be measured.

[0069] 5) In actual measurement, it is necessary to measure the grounding system loop impedance of each three-phase circuit. During the measurement process, the host computer sends a command for measuring the grounding system loop impedance of the specified phase to the single-chip microcomputer. The single-chip microcomputer outputs a control signal to control the relays in series with the loads of each phase in the three-phase measurement circuit, so that the specified phase among the three phases A, B, and C is connected to the measurement circuit, thereby realizing the measurement of the grounding system loop impedance of that phase.

[0070] The measuring end of the designated phase is clamped to the incoming and outgoing terminals of the three-phase grounding system to be tested through an electric test clamp, which effectively improves the safety and reliability of outdoor testing.

[0071] 6) During the measurement process, the host computer sends an information frame to the single-chip microcomputer, and the single-chip microcomputer verifies and processes it after receiving a frame of information, and executes the corresponding instructions; the single-chip microcomputer sends an information frame to the host computer, and the host computer program verifies and processes it, and displays the measurement results, realizing the communication between the host computer and the single-chip microcomputer, achieving the purpose of controlling the measurement process through the host computer and displaying the measurement results through the host computer interface in real time, and realizing the interactive adjustment and control of the measurement circuit. The host computer sends instructions to the single-chip microcomputer, which can be in the form of serial port, Ethernet, Bluetooth, etc. This embodiment uses the single-chip microcomputer serial port to connect the HC-05 Bluetooth module to the host computer respectively, and realizes communication through the transparent transmission between the two paired Bluetooth modules. Thereby, the measurement method of the present invention can adopt a wireless power transmission method to physically isolate the detection loop from the control loop, so as to greatly improve the electrical insulation withstand voltage of the measurement side and the measured end.

[0072] The host computer sends an information frame to the single-chip microcomputer, and the content of the information frame includes reference current value, SPWM control signal, correction instruction, each relay control instruction and other contents; the single-chip microcomputer sends an information frame to the host computer, and the information frame includes voltage and current sampling values, and the loop impedance value of the three-phase grounding system to be measured.

[0073] The initial value of the loop impedance of the three-phase grounding system to be measured is calculated based on the result obtained by performing DFT operation on the sampled values ​​and the mutual inductance coefficient of the coupling between the coils.

[0074] The correction calculation in step 4) takes into account the impedance of the transformer secondary side rear stage measurement device itself, assuming that Z1 and Z2 are the equivalent series impedance and equivalent parallel impedance of the equivalent circuit of the measurement device, respectively. When the load is open-circuited, the measured impedance value is Z=Z1+Z2; when the load is short-circuited, the measured impedance value is Z1; when the load is connected to impedance Z x When the measured impedance value is Z1+Z2 / / Z x ,satisfy:

[0075] .

[0076] According to the impedance values ​​measured in the three cases of open load, short load and load connected to a given 0.2 ohm resistor, the equivalent series impedance Z1 and equivalent parallel impedance Z2 of the measuring device and the mutual inductance coefficient are calculated, and then the unknown load Z is calculated. x Calculation of

[0077] Through phase correction, the phase deviation between the sampled voltage and the sampled current realized through the current transformer, as well as the phase deviation effect caused by component errors in the voltage conditioning circuit and the current conditioning circuit are eliminated.

[0078] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A device for measuring the impedance of a high-voltage cable grounding system, characterized in that: It includes a control unit, a driving circuit, a DC-AC circuit, a coupling transmission circuit, a filtering circuit, a current conditioning circuit and a voltage conditioning circuit; The driving circuit is connected to the control unit, and is used to amplify the four complementary driving signals output by the control unit and output them to the DC-AC circuit, so as to control the on and off of the switch tube of the DC-AC circuit; The DC-AC circuit receives the driving signal from the driving circuit and outputs the AC voltage through SPWM control; The coupling transmission circuit includes a primary coil of a transformer, a secondary coil of a transformer, a sensor coil, a compensation inductor L c , primary compensation capacitor C 1 and secondary compensation capacitor C 2. The primary compensation capacitor C One end of 1 is connected to the DC-AC circuit, and the primary compensation capacitor C The other end of 1, the primary coil and the compensation inductor L c One end of the primary side is connected in series to compensate for the inductance L c The other end of the primary side is connected to the DC-AC circuit; the secondary side coil and the secondary side compensation capacitor C 2 in series, compensating inductance L c The secondary side and the sensing coil are connected in series, and the sensing coil is coupled with the primary coil and the secondary coil; The front end of the filter circuit is connected to the secondary side of the transformer, and the rear end of the filter circuit is used to connect to the three-phase grounding system of the high-voltage cable to be tested; Transformer primary current I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1. Amplify and filter the sampling signal through the current conditioning circuit and the voltage conditioning circuit respectively, and then input the amplified signal into the AD sampling pin of the control unit, and calculate the impedance value of the three-phase grounding system of the high-voltage cable to be tested through the control unit; The filter circuit is a filter circuit that can realize series resonance and parallel resonance of inductance and capacitance at the operating frequency; The filter circuit includes an inductor L x1 , capacitor C x1 、Inductance L y1 and capacitor C y1 , the inductance L y1 and capacitor C y1 Connect in series with the secondary compensation capacitor C 2 in series, the inductor L x1 With capacitor C x1 After parallel connection with the secondary compensation capacitor C 2 in series; the inductor L y1 With capacitor C y1 To achieve series resonance, the inductor L x1 With capacitor C x1 Achieve parallel resonance; In the coupling transmission circuit, the self-inductance coefficient of the primary coil is L 1. The self-inductance coefficient of the secondary coil is L 2. Satisfy: ， ， In the formula, Indicates the resonant center frequency; The mutual inductance coefficient between the sensor coil and the primary coil is M 1s , compensation inductance L c The primary-secondary mutual inductance coefficient is M r ,satisfy: ， It is known that the mutual inductance coefficient between the primary coil and the secondary coil is M 12 , the mutual inductance coefficient between the sensor coil and the secondary coil is M 2s , sensor coil and compensation inductor L c The output voltage after the secondary side is connected in series is V 1. The calculation formula for the loop impedance value of the high-voltage cable three-phase grounding system is: 。 2. The high-voltage cable grounding system impedance live measurement device according to claim 1, characterized in that: The DC-AC circuit is a full-bridge DC-AC circuit, including MOS transistor S1, MOS transistor S2, MOS transistor S3 and MOS transistor S4; The MOS transistor S1 and the MOS transistor S2 form a first half bridge, and the MOS transistor S3 and the MOS transistor S4 form a second half bridge. Both half bridges are driven by a set of complementary driving signals including a dead zone output by the driving circuit.

3. The high-voltage cable grounding system impedance live measurement device according to claim 2, characterized in that: In the first half bridge, a first connection point is provided between the MOS transistor S1 and the MOS transistor S2; in the second half bridge, a second connection point is provided between the MOS transistor S3 and the MOS transistor S4; the primary compensation capacitor C 1 is connected to the first connection point, the compensation inductor L c The other end of the original side is connected to the second connection point.

4. A method for measuring the impedance of a high-voltage cable grounding system under live conditions, which uses the device for measuring the impedance of a high-voltage cable grounding system under live conditions as described in any one of claims 1 to 3, characterized in that: Includes steps: 1) When the DC power supply is input into the DC-AC circuit, the energy enters the primary side of the transformer in the form of a sine wave and is transmitted to the secondary side through the coupling of the coil; 2) Primary current of transformer I 1. Sensor coil and compensation inductor L c The voltage output at both ends after series connection V 1. Sampling is performed, and the sampling signal is amplified and filtered by the current conditioning circuit and the voltage conditioning circuit respectively, and then input into the control unit, and the initial value of the loop impedance of the three-phase grounding system to be measured is calculated according to the voltage and current sampling values; 3) Before the formal measurement, the loop impedance of the three-phase grounding system is measured when the load is open circuit, short circuit and 0.2 ohm respectively. According to the measurement results, the initial value of the loop impedance of the grounding system to be measured is corrected and calculated to obtain the corrected loop impedance value of the three-phase grounding system to be measured, and the influence of the filter circuit impedance and parasitic parameters in the measuring device on the measurement of the loop impedance of the three-phase grounding system to be measured is eliminated; 4) During the measurement process, the relays in the three-phase measurement circuit connected in series with the loads of each phase are controlled so that the designated phase among the three phases A, B, and C is connected to the measurement device to measure the loop impedance of the grounding system of that phase; 5) During the measurement process, an information frame is sent to the control unit through the host computer. The content of the information frame includes a reference current value, an SPWM control signal and a correction instruction. After receiving a frame of information, the control unit verifies and processes it and executes the corresponding instruction; the control unit sends the information frame to the host computer, and the host computer program verifies and processes it, and displays the measurement result, thereby realizing communication between the host computer and the control unit, and achieving the purpose of real-time interaction in which the measurement process is controlled by the host computer and the measurement result is displayed through the host computer interface.

5. The method for measuring the impedance of a high-voltage cable grounding system according to claim 4 is characterized in that: An information frame is sent to the control unit through the host computer, and the content of the information frame includes a reference current value, an SPWM control signal, a correction instruction and each relay control instruction; the control unit sends an information frame to the host computer, and the information frame includes voltage and current sampling values, and the loop impedance value of the three-phase grounding system to be measured.

6. The measuring method according to claim 4, characterized in that: The initial value of the loop impedance of the three-phase grounding system to be measured is calculated based on the result obtained by performing DFT operation on the sampled values ​​and the mutual inductance coefficient of the coupling between the coils.

7. The method for measuring the impedance of a high-voltage cable grounding system according to claim 4, characterized in that: The correction calculation in step 3) takes into account the impedance of the transformer secondary side rear stage measurement device itself, assuming that Z1 and Z2 are the equivalent series impedance and equivalent parallel impedance of the equivalent circuit of the measurement device, respectively. When the load is open circuit, the measured impedance value is Z=Z1+Z2; When the load is short-circuited, the measured impedance value is Z1; when the load is connected to impedance Z x When the measured impedance value is Z1+Z2 / / Z x ,satisfy: ， According to the impedance values ​​measured in the three cases of open load, short load and load connected to a given 0.2 ohm resistor, the equivalent series impedance Z1 and equivalent parallel impedance Z2 of the measuring device and the mutual inductance coefficient are calculated, and then the unknown load Z is calculated. x Calculation of Through phase correction, the phase deviation between the sampled voltage and the sampled current realized through the current transformer, as well as the phase deviation effect caused by component errors in the voltage conditioning circuit and the current conditioning circuit are eliminated.

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