A 10kV cable-to-ground capacitance current value measuring device and method

Abstract

The application belongs to the technical field of power systems, and specifically discloses a 10kV cable-to-ground capacitance current value measuring device and method, which comprises the following steps: injecting a current signal of a predetermined resonant frequency at a current signal injection end, measuring the voltage value and the current value of the primary side of a voltage transformer, and calculating the system impedance under the predetermined resonant frequency; injecting a current signal of a specified frequency at the current signal injection end, measuring the voltage value and the current value of the primary side of the voltage transformer, and calculating the system impedance under the specified frequency; the specified frequency is determined through the predetermined resonant frequency; and calculating the cable-to-ground capacitance of the cable to be measured according to the predetermined resonant frequency, the system impedance under the predetermined resonant frequency, the specified frequency and the system impedance under the specified frequency. The application can improve the measurement accuracy, and has strong anti-interference ability and adaptability.

Description

Technical Field

[0001] This invention belongs to the field of power system technology, specifically relating to a device and method for measuring the capacitance current of a 10kV cable to ground. Background Technology

[0002] During the long-term operation of 10kV cable power grids, with the continuous expansion of power system scale and the sustained increase in cable laying length, the cable-to-ground capacitance shows a gradual upward trend, leading to a corresponding increase in the frequency of faults. Among these, single-phase grounding faults caused by increased capacitance have become a major cause of cable operation failures, severely affecting the continuity and stability of power supply and potentially causing secondary damage to power grid equipment, increasing operation and maintenance costs and safety risks. Therefore, accurate and efficient monitoring of the capacitance current value of 10kV cable systems is of crucial practical significance for ensuring the safe and stable operation of cable power grids and is one of the key technical problems urgently needing to be solved in the current power operation and maintenance field. Currently, the industry's methods for monitoring the capacitive current value of 10kV cable systems are mainly divided into two categories: indirect measurement methods and direct measurement methods. Among them, indirect measurement methods are relatively mature, forming various technical solutions including the bias capacitor method, resonance method, and impedance triangulation method, while direct measurement methods are represented by the neutral point injection method. Although these measurement techniques have been applied in practice to some extent, they generally have many shortcomings due to limitations in technical principles and field conditions, making it difficult to meet the current power grid's high requirements for the accuracy, stability, and safety of capacitive current monitoring. Indirect measurement methods have weak anti-interference capabilities. These methods require injecting a stable frequency signal into the system during measurement. However, the complex power frequency electromagnetic fields in the field can easily intrude into the measurement circuit through electromagnetic induction or capacitive coupling, forming common-mode or differential-mode interference. This causes the weak capacitance current signal to be overwhelmed by strong interference signals. Because power frequency interference and the frequency characteristics of the measurement signal are intertwined, this interference is difficult to effectively avoid under the existing framework of indirect measurement technology, directly affecting the validity of the measurement data. The measurement accuracy of indirect methods is also low. Some indirect measurement techniques require changing the inductance value of the arc suppression coil to infer the system capacitance current. During this process, differences in cable joint structure, installation position deviations, and dynamic fluctuations in the power grid load can directly interfere with the acquisition and calculation of electrical parameters, leading to significant errors in the measurement results. Furthermore, the raw data obtained by indirect measurement methods often cannot directly reflect the true value of the capacitance current, requiring secondary processing and analysis to extract effective information. This process not only increases data processing costs but may also further amplify errors due to the limitations of the analysis model. Furthermore, traditional indirect measurement methods pose safety hazards to existing operating systems. Taking the resonance method as an example, its technical principle is to use an arc-suppression coil and the system's distributed capacitance to form a resonant circuit, and calculate the capacitor current by analyzing the electrical parameters under resonant conditions. When severe resonance occurs in the system during measurement, the voltage difference at the cable neutral point will increase sharply, potentially exceeding the equipment's insulation limit, posing a secondary threat to the safe operation of the power grid, and bringing significant risks to operation and maintenance work. Traditional neutral point injection methods have poor adaptability to field deployment. For example, the frequency sweep method requires continuous transmission of frequency conversion signals to the power grid system. This not only places extremely high demands on the frequency stability and power output capability of the signal source, increasing equipment investment costs, but also imposes stringent standards on the accuracy of resonant point identification and analysis. The complex technical requirements and high operational threshold greatly limit the scope of application of this method in 10kV cable systems of different operating conditions and scales. In summary, the shortcomings of existing capacitance current measurement technologies in terms of anti-interference capability, measurement accuracy, system safety, and field adaptability make it difficult to meet the operation and maintenance needs of the rapidly developing 10kV cable power grid. Developing a more efficient, accurate, and safe capacitance current monitoring method has become an inevitable trend to promote the upgrading of power system operation and maintenance technology. Summary of the Invention

[0003] The purpose of this invention is to provide a device and method for measuring the capacitance current of a 10kV cable to ground, which can improve measurement accuracy and has strong anti-interference ability and adaptability.

[0004] To achieve the above objectives, the present invention employs the following technical solution: According to one aspect of the present invention, a device for measuring the capacitance current of a 10kV cable to ground is provided, comprising: A voltage transformer is used, with its secondary side connected to the cable under test. A current signal injection terminal is reserved at the neutral point of the secondary side of the voltage transformer. An adjustable three-phase capacitor structure is introduced on the primary side of the voltage transformer. This adjustable three-phase capacitor structure includes three symmetrically connected adjustable inductor-adjustable capacitor series structures. In the adjustable inductor-adjustable capacitor series structure, the end of the adjustable inductor furthest from the adjustable capacitor is grounded, and the end of the adjustable capacitor furthest from the adjustable inductor is grounded. The adjustable inductor is matched with the primary side of the voltage transformer. In the three adjustable inductor-adjustable capacitor series structures, the nodes of the adjustable inductor and the adjustable capacitor are connected to the same common node.

[0005] Furthermore, in the adjustable inductor-adjustable capacitor series structure, the end of the adjustable inductor furthest from the adjustable capacitor is grounded through a lightning protection grounding module, and the end of the adjustable capacitor furthest from the adjustable inductor is grounded through a filter grounding module. The specifications of the adjustable inductor are precisely matched with the rated parameters of the primary side of the voltage transformer to achieve resonant coupling of the 10kV cable to ground capacitance. An impedance matching unit is set at the common node to eliminate node signal reflection and improve measurement stability.

[0006] Furthermore, in the adjustable inductor-adjustable capacitor series structure, a programmable AC current source is also provided between the nodes of the adjustable inductor and the adjustable capacitor and the common node.

[0007] By employing the above technical solution and introducing a voltage transformer and a three-phase capacitor circuit with a specific structure, a stable measurement system was constructed. Using this device to measure the cable-to-ground capacitance current effectively suppresses power frequency interference, improves the signal-to-noise ratio of the measurement signal, and thus enhances the measurement accuracy of the capacitance current to ground. Simultaneously, its structural design simplifies field deployment and avoids the safety risks to the power grid system that traditional resonant methods may pose, thereby enhancing the operational reliability of the device.

[0008] According to a second aspect of the present invention, a method for measuring the capacitance current of a 10kV cable to ground is provided, based on any of the above-described 10kV cable-to-ground capacitance current measuring devices, comprising the following steps: Inject a current signal at the desired resonant frequency at the current signal injection terminal, and measure the voltage and current values ​​on the primary side of the voltage transformer at the desired resonant frequency; calculate the system impedance at the desired resonant frequency based on the voltage and current values ​​on the primary side of the voltage transformer at the desired resonant frequency. A current signal of a specified frequency is injected at the current signal injection terminal, and the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency are measured; the system impedance at the specified frequency is calculated based on the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency; the specified frequency is determined by the proposed resonant frequency, and the specified frequency and the proposed resonant frequency meet a preset difference requirement; Calculate the capacitance to ground of the cable under test based on the proposed resonant frequency, the system impedance at the proposed resonant frequency, the specified frequency, and the system impedance at the specified frequency.

[0009] By employing the above technical solution, different frequency current signals are injected stepwise and the system impedance is calculated, ultimately leading to an accurate calculation of the capacitance to ground. This effectively solves the interference, accuracy, and safety issues inherent in traditional methods. Impedance measurement and calculation at dual frequency points (the intended resonant frequency and the specified frequency) avoids the error accumulation that may occur with single-frequency measurements, reduces dependence on system parameter variations, and improves the robustness and accuracy of the calculation. The specified frequency and the intended resonant frequency meet a preset difference requirement, ensuring the validity and independence of the data points, further enhancing anti-interference capabilities, and helping to reduce errors and improve accuracy during the measurement of the capacitance to ground current, resulting in more reliable measurement results.

[0010] Furthermore, by employing a reasonable current injection method and selecting a multi-frequency injection form, the measurement of ground capacitance systems of different values ​​was achieved. Moreover, the method utilizes an ungrounded system connection, making it applicable to grounded systems without arc suppression coils, thus demonstrating strong adaptability.

[0011] According to one embodiment of the present invention, the determination of the intended resonant frequency includes the following steps: A sweep frequency current signal is injected at the current signal injection terminal, and the voltage signal on the primary side of the voltage transformer is monitored at the same time, and the amplitude change curve of the voltage signal is recorded. The resonant frequency is determined based on the amplitude variation curve of the voltage signal on the primary side of the voltage transformer; the determined resonant frequency is the current frequency corresponding to the sudden change in the voltage signal on the primary side of the voltage transformer.

[0012] Furthermore, the slope threshold method is used to determine the sudden change in the voltage signal on the primary side of the voltage transformer. By setting a preset voltage amplitude change slope threshold, the resonant frequency is automatically identified, avoiding the subjective error of manual judgment and improving the accuracy of resonant frequency determination.

[0013] Furthermore, the amplitude of the swept current signal is stable between 5mA and 20mA, and the amplitude fluctuation does not exceed ±0.5mA, with the frequency continuously increasing from small to large.

[0014] Furthermore, the sweep frequency step size is adaptively adjusted according to the degree of proximity to the resonant frequency. The step size increases when the frequency is far from the resonant frequency and decreases when the frequency is close to the resonant frequency, which improves the sweep frequency efficiency and ensures the accuracy of resonant frequency identification. At the same time, the sweep frequency current signal adopts a weak signal design with different frequencies to avoid interference with the cable relay protection device and voltage transformer itself, without interrupting the normal operation of the cable and realizing uninterrupted measurement.

[0015] This effectively solves the problem of inaccurate resonant point identification in traditional methods. By injecting a swept-frequency current signal, the system is fully excited over a wide frequency range, ensuring the capture of the true resonant frequency and avoiding measurement errors caused by insufficient frequency matching. This method utilizes the dynamic characteristics of the swept-frequency signal to make the system's resonant response more easily distinguishable from background noise at a fixed frequency, reducing the stringent requirements for precise signal source control, simplifying the complexity of resonant point analysis, and enhancing the system's immunity to power frequency interference. Thus, the intended resonant frequency can be determined more accurately, laying the foundation for subsequent precise calculations of system impedance and capacitance to ground.

[0016] According to one embodiment of the present invention, the step of calculating the capacitance to ground based on the proposed resonant frequency, the system impedance at the proposed resonant frequency, the specified frequency, and the system impedance at the specified frequency is calculated according to the following formula: ; In the formula, C Capacitance to ground; k This is a correction factor; n The inductance coefficient of the voltage transformer; w a The proposed resonant frequency; w b For a specified frequency; Z a The system impedance at the proposed resonant frequency; Z b The system impedance at a specified frequency.

[0017] According to one embodiment of the present invention, the difference between the specified frequency and the intended resonant frequency is in the range of 50Hz-200Hz, and the specified frequency does not exceed the maximum value of the carrier communication frequency.

[0018] Furthermore, the specified frequency is greater than the intended resonant frequency, and the specified frequency does not exceed the maximum value of the carrier communication frequency.

[0019] Preferably, the difference between the specified frequency and the intended resonant frequency should be as large as possible.

[0020] Preferably, the specified frequency is the maximum value of the carrier communication frequency.

[0021] According to one embodiment of the present invention, the system impedance at the corresponding frequency is calculated based on the voltage and current values ​​on the primary side of the voltage transformer at a specified frequency or a predetermined resonant frequency, using the following formula: ; In the formula, Z y The system impedance at the corresponding frequency; y = a or b , Z a The system impedance at the proposed resonant frequency, Z b The system impedance at a specified frequency; U y This represents the voltage value on the primary side of the voltage transformer at the corresponding frequency. I y This represents the current value on the primary side of the voltage transformer at the corresponding frequency. n The inductance coefficient of the voltage transformer; R y This represents the total circuit resistance on the primary side of the voltage transformer at the corresponding frequency.

[0022] According to one embodiment of the present invention, the voltage value on the primary side of the voltage transformer is the voltage vector sum of three adjustable inductors and adjustable capacitors connected in parallel.

[0023] Furthermore, the voltage of the adjustable inductor-adjustable capacitor series structure is calculated according to the following formula: ; In the formula, U x The voltage is for an adjustable inductor-adjustable capacitor series structure. x =1, 2, or 3 U 1 represents the voltage in the first adjustable inductor-adjustable capacitor series structure. U 2 represents the voltage in the second adjustable inductor-adjustable capacitor series structure. U 3 represents the voltage in the third adjustable inductor-adjustable capacitor series structure; n The inductance coefficient of the voltage transformer; R x A resistor with an adjustable inductor-adjustable capacitor series structure; w y The frequency of the injected current signal, y = a or b ; L xIt is an inductor with an adjustable inductor-adjustable capacitor series structure; it is adaptively adjusted and then locked according to the parameters of the cable under test. C x It is a capacitor with an adjustable inductor-adjustable capacitor series structure; it is adaptively adjusted and then locked according to the parameters of the cable under test. I x The current is for the series connection of an adjustable inductor and adjustable capacitor; the current is equal for three parallel series connections of an adjustable inductor and adjustable capacitor.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The present invention provides a device and method for measuring the capacitance current of a 10kV cable to ground. By setting up a voltage transformer and a three-phase capacitor structure, it effectively reduces external interference and provides a stable measurement environment, thereby improving anti-interference capability, enhancing measurement accuracy, and reducing safety risks. The impedance measurement and calculation at dual frequency points avoids the error accumulation that may exist in single-frequency measurement, reduces dependence on system parameter changes, and improves the robustness and accuracy of the calculation. Furthermore, the multi-frequency injection method, using an ungrounded system connection, has strong adaptability.

[0025] 2. This invention sets the specified frequency above the intended resonant frequency by pre-defined difference requirements. This avoids the highly nonlinear region near the resonant point during the measurement process and also helps reduce errors caused by the primary excitation impedance, resulting in a more stable and accurate system impedance measured at the specified frequency. This stable impedance measurement result, combined with the system impedance at the intended resonant frequency, significantly improves the accuracy of ground capacitance calculation, thereby enhancing the anti-interference capability and reliability of the entire measurement method. Attached Figure Description

[0026] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is the equivalent circuit of the 10kV cable-to-ground capacitance current measuring device in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the neutral point injection current signal in the method for measuring the capacitance current of a 10kV cable to ground according to Embodiment 1 of the present invention. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0028] The following detailed description is exemplary and intended to provide further detailed explanation of the invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this invention is for describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention.

[0029] Example 1 Existing methods for measuring the capacitance current of 10kV cables to ground, such as indirect measurement and neutral point injection methods, generally suffer from poor resistance to power frequency interference, low measurement accuracy, significant impact on the safety of operating systems, and poor adaptability to field deployment. These methods are susceptible to electromagnetic interference, have large data errors, and may affect power grid safety due to resonance and other factors, thus limiting their application scope and measurement effectiveness. Therefore, this embodiment provides a device for measuring the capacitance current of 10kV cables to ground.

[0030] This device connects the secondary side of a voltage transformer to the cable under test and introduces a three-phase capacitor structure on the primary side of the voltage transformer. This three-phase capacitor structure consists of three parallel adjustable inductor-adjustable capacitor series structures. The end of the adjustable inductor furthest from the adjustable capacitor is grounded through a lightning protection grounding module, and the end of the adjustable capacitor furthest from the adjustable inductor is grounded through a filter grounding module. Furthermore, the inductor is matched to the primary side of the voltage transformer, with the specifications of the adjustable inductor precisely matching the rated parameters of the primary side of the voltage transformer, achieving resonant coupling of the 10kV cable-to-ground capacitance. In the three adjustable inductor-adjustable capacitor series structures, the nodes of the adjustable inductor and adjustable capacitor are connected to the same common node, which can be configured as a multi-port connector. A programmable AC current source is also provided between the nodes of the adjustable inductor and adjustable capacitor and the common node. The programmable AC current source can output swept frequency current signals, resonant frequency current signals, and specified frequency current signals. The signal amplitude and frequency can be adaptively adjusted according to the length and cross-sectional specifications of the cable under test. It also integrates a signal filtering unit to suppress 50Hz power frequency interference and external electromagnetic interference, solving the technical problems of weak anti-interference ability and poor adaptability of traditional measuring devices.

[0031] Combination Figure 1 As shown, in the equivalent circuit of the 10kV cable-to-ground capacitance current measuring device in this embodiment, the primary side of the voltage transformer is equivalent to an excitation inductor, and the three-phase capacitor structure on the primary side is equivalent to three parallel structures consisting of secondary impedances (Z1, Z2, or Z3), secondary inductances (L1, L2, or L3), and capacitances to ground (C1, C2, or C3) connected in series, which fully reflects the coupling relationship between the transformer and the cable system. When a current signal is injected into the reserved current signal injection terminal at the neutral point of the secondary side of the voltage transformer, this equivalent model can be used to analyze the system impedance characteristics at different frequencies.

[0032] A voltage transformer is an electrical device that proportionally converts high voltage to low voltage. Conventional electromagnetic voltage transformers, which transform voltage through electromagnetic induction, can be used, or electronic voltage transformers can be employed, utilizing fiber optics or electronic technology for voltage measurement. In this embodiment, the secondary side of the voltage transformer can be connected to the cable under test via wires, terminal blocks, or intermediate connectors. The primary side is coupled with a three-phase capacitor structure to indirectly measure the capacitance current to ground of the high-voltage cable. The three-phase capacitor structure refers to a circuit composed of three independent adjustable inductor-adjustable capacitor series structures connected in parallel, used to simulate and compensate for capacitive loads in the power grid, and its resonant characteristics aid in measurement.

[0033] In the adjustable inductor-adjustable capacitor series structure of this embodiment, one end of both the adjustable inductor and the adjustable capacitor is grounded, and the other end is connected to a common node, forming a resonant circuit. The adjustable inductor is connected in series or in parallel with the primary side of the voltage transformer.

[0034] Combination Figure 2 As shown, the method for measuring the ground capacitance current value using the aforementioned 10kV cable-to-ground capacitance current measuring device involves injecting a three-phase current signal with the same amplitude and phase at the current signal injection terminal reserved at the neutral point on the secondary side of the voltage transformer. i After amplification, the values ​​are injected into the cable system under test through three-phase adjustable inductors. By adjusting the three-phase adjustable inductors and the capacitances to ground C1, C2, and C3, the three parallel adjustable inductors-adjustable capacitors in the three-phase capacitor structure are kept symmetrical, and the impedance of the common node is calibrated to ensure impedance matching of the measurement circuit.

[0035] During measurement, the intended resonant frequency is first injected. w a The current signal is obtained in real time by the primary side signal acquisition unit of the voltage transformer, which acquires the voltage and three-phase current at that frequency. i 1. i 2. i 3. Calculate the system impedance Z after noise reduction processing. a Subsequently, the resonant frequency was injected. w a A specified frequency that meets the preset difference requirement w b Similarly, the system impedance Z can be calculated from the current signal. b Finally, combining the voltage transformer inductance coefficient n, and using an improved capacitance calculation model, the proposed resonant frequency is determined. w a The proposed resonant frequency w a The system impedance Z under the following conditions a Specified frequencyw b and specified frequency w b The system impedance Z under the following conditions b Accurately calculate the capacitance to ground of the cable under test, effectively eliminate the influence of factors such as loop resistance and grounding resistance, and improve measurement accuracy.

[0036] Specifically, it includes the following steps: S1. Preprocessing: Based on the length and cross-sectional parameters of the cable under test, adjust the parameters of each adjustable inductor and adjustable capacitor C1, C2 and C3 in the three-phase capacitor structure to keep the three-phase structure symmetrical. The impedance of the common node is calibrated through the impedance matching unit to ensure impedance matching of the measurement circuit.

[0037] S2. Inject the intended resonant frequency into the current signal injection terminal reserved at the neutral point on the secondary side of the voltage transformer. w a The current signal is measured in real time through the signal acquisition unit on the primary side of the voltage transformer to determine the intended resonant frequency. w a Voltage and current values ​​on the primary side of the voltage transformer i The proposed resonant frequency w a Voltage and current values ​​on the primary side of the voltage transformer i After noise reduction, the values ​​are substituted into the impedance calculation model to calculate the intended resonant frequency. w a The system impedance Z under the following conditions a .

[0038] S3. Inject a specified frequency signal into the current signal injection terminal reserved at the neutral point on the secondary side of the voltage transformer. w b The current signal is measured in real time at a specified frequency through the signal acquisition unit on the primary side of the voltage transformer. w b The voltage and current values ​​on the primary side of the voltage transformer; the specified frequency. w b After noise reduction processing of the voltage and current values ​​on the primary side of the voltage transformer, they are substituted into the impedance calculation model to calculate the specified frequency. w b The system impedance Z under the following conditions b ;Specified frequency w b By the proposed resonant frequency w a Confirm, specify frequency w b With the intended resonant frequency w a It meets the preset difference requirement.

[0039] S4. Based on the proposed resonant frequency w a The proposed resonant frequency w a The system impedance Z under the following conditions a Specified frequency w b Specified frequency w b The system impedance Z under the following conditions b By combining the voltage transformer's inductance coefficient n, an improved capacitance calculation model is used to accurately calculate the ground capacitance of the cable under test. This improved capacitance calculation model eliminates the influence of loop resistance and grounding resistance on the measurement results, addressing the technical pain point of large errors in traditional measurement methods.

[0040] Combination Figure 1 and Figure 2 By calculating the proposed resonant frequencies respectively w a and specified frequency w b The system impedance Z under the following conditions a and Z b By combining the voltage transformer's inductance coefficient n, the interference of secondary circuit resistance, inductance, and excitation inductance on the measurement results can be eliminated, and the three-phase cable-to-ground capacitance can be accurately calculated, providing theoretical support for the accurate calculation of capacitance current.

[0041] Specifically, in step S2, a predetermined resonant frequency is injected into the neutral point on the secondary side of the voltage transformer. w a The current signal excites the system at the predetermined resonant frequency. w a The response at this point provides a reference point for subsequent calculations of capacitance to ground. The proposed method for determining the resonant frequency is as follows: S2-1. A sweep frequency current signal is injected into the current signal injection terminal reserved at the neutral point on the secondary side of the voltage transformer through a programmable AC current source. The amplitude of the sweep frequency current signal is stable between 5mA and 20mA, and the amplitude fluctuation does not exceed ±0.5mA. The frequency increases continuously from small to large, and the sweep frequency range covers 10Hz-1000Hz. The sweep frequency step size can be adaptively adjusted.

[0042] S2-2. While injecting the sweep frequency signal, monitor the voltage signal on the primary side of the voltage transformer in real time and record the amplitude change curve of the voltage signal; S2-3. The resonant frequency determined based on the amplitude variation curve of the voltage signal on the primary side of the voltage transformer; the determined resonant frequency. w aThis refers to the current frequency corresponding to a sudden change in the voltage signal on the primary side of the voltage transformer. The determination of this sudden change in the voltage signal on the primary side of the voltage transformer employs a slope threshold method. By pre-setting a voltage amplitude change slope threshold, the resonant frequency is automatically identified, avoiding subjective errors from manual determination and improving the accuracy of resonant frequency estimation.

[0043] A swept-frequency current signal is injected at the current signal injection terminal. By applying a continuously varying frequency current signal to the system under test, the system's response at different frequencies is excited, thereby detecting its inherent resonant characteristics. Specifically, in this embodiment, a programmable AC current source is used, with its output terminal connected to the neutral point of the secondary side of the voltage transformer, and its output current frequency is set to perform a linear sweep within a preset range. Alternatively, a precision current source controlled by a digital signal processor (DSP) can be used. This current source can generate an AC current with stable amplitude and a continuously varying frequency according to a specific pattern (such as from small to large), and inject it into the neutral point.

[0044] While injecting a swept-frequency current signal, the voltage signal on the primary side of the voltage transformer is monitored to obtain the system's voltage response under swept-frequency current excitation in real time. This can be achieved by connecting a high-precision voltage sensor or data acquisition (DAQ) module to the primary side of the voltage transformer, converting the voltage signal into a digital signal for recording and processing. Alternatively, a digital oscilloscope or spectrum analyzer can be used to directly capture and display the voltage waveform on the primary side of the voltage transformer, allowing for direct observation of its frequency variation. In the resonant state, the voltage or current of the circuit often exhibits significant nonlinear changes. Thus, the specific frequency at which the system resonates can be identified from the monitored voltage signal.

[0045] Thus, by injecting a swept-frequency current signal, the system is fully excited over a wide frequency range, ensuring the capture of the true resonant frequency and avoiding measurement errors caused by insufficient frequency matching. Simultaneously, real-time monitoring of the voltage signal on the primary side of the voltage transformer, using abrupt changes in the voltage signal as the resonance criterion, provides an intuitive and reliable method for identifying the resonant point. This reduces the stringent requirements for precise control of the signal source, simplifies the complexity of resonant point analysis, and enhances the system's immunity to power frequency interference.

[0046] In step S3, a specified frequency is injected into the neutral point on the secondary side of the voltage transformer. w b The current signal is used to introduce data at another frequency point for comparative analysis with the data at the intended resonant frequency point, thus eliminating the limitations of single-frequency measurement. This designated frequency... w b The current signal is also generated by a programmable AC current source, which adjusts the frequency to a preset specified frequency and injects it.

[0047] Specified frequency w b By the proposed resonant frequency w a Confirm, specify frequency w b With the intended resonant frequency w a To meet preset difference requirements, aiming to ensure a specified frequency. w b With the intended resonant frequency w a Sufficient differences exist between them to allow for effective comparative analysis while avoiding the recurrence of strong resonances. The preset difference requirement is: at a specified frequency... w b With the intended resonant frequency w a The difference range is 50Hz-200Hz, and the specified frequency... w b It shall not exceed the maximum value of the carrier communication frequency.

[0048] Specified frequency w b It can be set to a relative resonant frequency. w a Higher or lower by a fixed value (e.g., 50Hz or 100Hz higher), or higher by a certain percentage (e.g., 10% higher); or, a specified frequency. w b It can also be dynamically determined through an algorithm. In this method, due to the frequency of the second injected signal (i.e., the specified frequency) w b ) and the first injected signal (i.e., the intended resonant frequency) w a The larger the difference, the smaller the relative error in measuring the grounding capacitance current value; however, if a specific frequency is specified... w b An excessively small reverse impedance can also increase the error due to the primary excitation impedance of the voltage transformer. Therefore, at a specified frequency... w b With the intended resonant frequency w a It meets the preset difference requirement, and generally a specified frequency is set. w b Greater than the intended resonant frequency w a and specify the frequency w b It does not exceed the maximum value of the carrier communication frequency. In other words, w b exist[ wa , w max Within the specified range; and specify the frequency. w b With the intended resonant frequency w a The difference between them should be as large as possible to specify the frequency. w b It is generally set to the maximum value of the carrier communication frequency of the power distribution system. w max .

[0049] In steps S2 and S3, noise reduction processing is performed on the voltage and current values ​​on the primary side of the voltage transformer. In this embodiment, a systematic comprehensive noise reduction scheme of "hardware filtering + frequency-selective injection + software slope discrimination + real-time impedance compensation + three-phase vector synthesis" is adopted. The specific method is as follows: (1) Frequency-selective injection.

[0050] The 50Hz power frequency is the primary source of interference on the primary side of a voltage transformer. Its harmonic signals superimpose with the original voltage and current signals, causing signal distortion. This embodiment uses a weak signal injection technique at a different frequency to suppress interference at its source.

[0051] Based on the characteristics of the primary circuit, the sweep frequency range of the weak signal of different frequencies is set to 10Hz-1000Hz, and the sweep step size can be flexibly adjusted according to the interference intensity (usually 1Hz / step); the amplitude of the injected signal is controlled at 5mA-20mA to ensure that the signal strength is sufficient to realize subsequent detection, while avoiding the impact on the normal operation of the primary side.

[0052] (2) Hardware filtering.

[0053] Even after frequency injection, the signal may still carry external electromagnetic interference, power grid surges, and high-frequency noise. It needs to be physically filtered by a hardware filtering module to further purify the signal.

[0054] First, a dedicated filter grounding module is installed at the primary side grounding terminal of the voltage transformer. This module adopts a shielded design, which can conduct stray currents and electromagnetic interference in the primary side circuit through the grounding circuit, while suppressing the intrusion of external electromagnetic radiation into the circuit. The grounding resistance is strictly controlled within the specified range to avoid interference reflection caused by poor grounding.

[0055] Then, a signal filtering unit is connected in series at the signal acquisition end. This unit integrates three functions: low-pass filtering, surge suppression, and high-frequency attenuation. The low-pass filter can intercept high-frequency noise with a frequency higher than 1000Hz, the surge suppression can absorb the instantaneous interference signal brought by the power grid surge, and the high-frequency attenuation can further weaken the noise generated by external electromagnetic radiation.

[0056] After filtering is completed, the voltage and current signals processed by the hardware are collected and compared with the original signals from the preprocessing stage (voltage and current values ​​on the primary side of the voltage transformer). It is confirmed that the amplitudes of power frequency interference, high frequency noise, and surge signals have all been reduced to below the preset threshold. After ensuring that the hardware filtering effect meets the standard, the process proceeds to the next stage.

[0057] (3) Software slope discrimination.

[0058] Even after hardware filtering, a small amount of random noise (such as thermal noise or instantaneous fluctuations) may still remain in the signal. This type of noise is characterized by randomness, small amplitude but frequent fluctuations, and needs to be accurately filtered by software algorithms to retain the effective signal and remove the noise signal.

[0059] The voltage and current signals, after hardware filtering, are acquired by the acquisition module. These signals are then digitally converted from analog to processable digital signals, and extremum removal is performed to eliminate abnormal spikes. Based on the normal variation pattern of the primary voltage amplitude of the voltage transformer, a reasonable slope threshold is set (the threshold can be dynamically adjusted according to operating conditions). This threshold is used to judge the changing trend of the voltage amplitude curve—the amplitude change of the effective signal is regular, and the slope is relatively stable; the amplitude change of random noise is irregular, and the slope will abruptly exceed the threshold range. A software algorithm performs real-time slope discrimination on the voltage amplitude curve, focusing on identifying resonance points. Only abrupt inflection points with slopes within the threshold range are retained, while random noise points and fluctuation points with slopes exceeding the threshold are eliminated, ensuring that the filtered signal contains only effective voltage and current characteristics.

[0060] (4) Real-time impedance compensation.

[0061] The resistance of the primary circuit of the voltage transformer (denoted as ) R y The signal can fluctuate due to temperature changes and contact conditions, resulting in thermal noise and contact noise. This can cause drift in subsequent signal calculations and affect data accuracy.

[0062] The resistance of the primary circuit of the voltage transformer is collected in real time using a dedicated resistance detection module. R y The measurement frequency is kept consistent with the signal acquisition frequency (typically 10 times per second) to ensure timely capture of dynamic changes in resistance. Based on real-time measurements... R y The numerical value, combined with a preset reference resistance value, is used by a software algorithm to calculate the compensation coefficient, which varies with... R y The compensation is dynamically adjusted to ensure that the accuracy keeps pace with resistance fluctuations. The calculated compensation coefficients are substituted into the calculation model of the voltage and current signals to dynamically correct the acquired signals and offset any changes. Ry Thermal noise generated by fluctuations and computational drift caused by contact noise ensure the stability and accuracy of signal calculation results.

[0063] (5) Three-phase vector synthesis.

[0064] The primary side of a voltage transformer is a three-phase circuit. Due to factors such as unbalanced three-phase loads and wiring differences, asymmetrical noise exists in the three-phase voltage and current signals, resulting in significant deviations in the three-phase signals. Three-phase vector synthesis technology can be used to average the three-phase noise, further optimizing signal quality.

[0065] First, ensure synchronized acquisition time of the three-phase voltage and current signals to avoid vector synthesis errors caused by acquisition delays. After acquisition, extract the amplitude and phase information of the three-phase signals separately. Based on the principle of vector operation, perform vector summation on the three-phase voltage and current signals respectively. The asymmetrical noise between the three phases is canceled out by vector superposition. After vector synthesis, the forward and reverse noise cancel each other out, the remaining noise is averaged, and the amplitude is significantly reduced. The voltage and current signals after three-phase vector synthesis are processed to output the final noise-reduced signal. This signal has eliminated various interference components such as power frequency interference, electromagnetic interference, and random noise, and the three-phase signal is symmetrical and meets the accuracy standards, which can be directly used for the operation of subsequent metering and protection devices.

[0066] In steps S2 and S3, the system impedance at the corresponding frequency is calculated based on the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency or the proposed resonant frequency, specifically according to the following formula: ; In the formula, Z y The system impedance at the corresponding frequency; y = a or b, Z a The system impedance at the proposed resonant frequency, Z b The system impedance at a specified frequency; U y This represents the voltage value on the primary side of the voltage transformer at the corresponding frequency. I y This represents the current value on the primary side of the voltage transformer at the corresponding frequency. n The inductance coefficient of the voltage transformer; R y This represents the total circuit resistance on the primary side of the voltage transformer at the corresponding frequency. R yObtained through real-time measurement rather than a preset fixed value, it can dynamically compensate for errors caused by changes in resistance with frequency and temperature, solving the error problem caused by fixed resistance in traditional impedance calculation and further improving the accuracy of impedance calculation.

[0067] The voltage value on the primary side of the voltage transformer is the vector sum of the voltages of three parallel adjustable inductors and adjustable capacitors in series, rather than a simple arithmetic sum. A vector synthesis algorithm is used to eliminate measurement errors caused by minor asymmetries in the three-phase structure. Simultaneously, combined with the signal filtering function of the filtering grounding module, interference signals in the grounding loop are eliminated, ensuring the accuracy of the voltage measurement and providing reliable data support for system impedance calculation and ground capacitance calculation. The voltage of the adjustable inductor-adjustable capacitor series structure is calculated using the following formula: ; In the formula, U x The voltage is for an adjustable inductor-adjustable capacitor series structure. x =1, 2, or 3 U 1 represents the voltage in the first adjustable inductor-adjustable capacitor series structure. U 2 represents the voltage in the second adjustable inductor-adjustable capacitor series structure. U 3 represents the voltage in the third adjustable inductor-adjustable capacitor series structure; n The inductance coefficient of the voltage transformer; R x A resistor with an adjustable inductor-adjustable capacitor series structure; w y The frequency of the injected current signal, y = a or b ; L x It is an inductor with an adjustable inductor-adjustable capacitor series structure; it is adaptively adjusted and then locked according to the parameters of the cable under test. C x It is a capacitor with an adjustable inductor-adjustable capacitor series structure; it is adaptively adjusted and then locked according to the parameters of the cable under test. I x The current in the adjustable inductor-adjustable capacitor series structure is equal; the current in the three parallel adjustable inductor-adjustable capacitor series structures is equal; the current in the three parallel adjustable inductor-adjustable capacitor series structures is kept dynamically equal through the current sharing unit, which further improves the symmetry of the three-phase structure, reduces measurement error, and solves the measurement deviation problem caused by the uneven current in the traditional three-phase structure.

[0068] Therefore, the above-mentioned method for measuring the ground capacitance current of a 10kV cable first injects a sweep current signal with a stable amplitude and unique frequency into the neutral point of the secondary side of the voltage transformer, and uses the voltage value to determine the resonant frequency of the system. Then, a signal of a fixed frequency is injected, and based on the feedback voltage value and the system's state at the resonant frequency, the system's ground capacitance value is calculated.

[0069] In step S4, the capacitance to ground is calculated based on the proposed resonant frequency, the system impedance at the proposed resonant frequency, the specified frequency, and the system impedance at the specified frequency, according to the following formula: ; In the formula, C Capacitance to ground; k This is a correction factor; n The inductance coefficient of the voltage transformer; w a The proposed resonant frequency; w b For a specified frequency; Z a The system impedance at the proposed resonant frequency; Z b The system impedance at a specified frequency.

[0070] The above formula introduces a correction factor. k Correction factor k Based on the length, cross-sectional parameters, and ambient temperature of the cable under test, an adaptive calculation is performed to compensate for the influence of ambient temperature and cable laying method on capacitance measurement, thereby further improving measurement accuracy and solving the technical problems of traditional measurement methods ignoring environmental factors and having large measurement errors.

[0071] Correction coefficient k Calculate using the following formula: ; In the formula, k L This is a cable length correction factor. k T This is the correction factor for ambient temperature.

[0072] ; ; Where L is the actual length of the cable to be tested, in meters; L0 is the preset reference length, which is recommended to be 100 meters. This is the length influence coefficient. The value range is [0.0005, 0.002]; T is the actual ambient temperature, in °C; T0 is the preset reference temperature, recommended to be 25 °C; Temperature coefficient, unit: °C -1 ; The value is determined based on the material of the cable. Generally, it is 0.00393 for copper and 0.00403 for aluminum.

[0073] The improved capacitance calculation model is designed based on the dual-frequency impedance measurement principle. Compared with the traditional capacitance calculation model, it has been improved in many aspects, aiming to solve the technical problems of large measurement error and poor adaptability of the traditional model, and to achieve accurate calculation of the capacitance to ground of 10kV cables. Specific implementation details are as follows: This model abandons the limitations of traditional single-frequency measurement and adopts a predetermined resonant frequency. w a With a specified frequency w b The dual-frequency measurement scheme injects a current signal of the corresponding frequency into the neutral point of the secondary side of the voltage transformer, simultaneously acquires and measures the voltage and current values ​​of the primary side of the voltage transformer at two frequencies, and calculates the system impedance Z at the corresponding frequency. a Z b This model uses the voltage transformer inductance coefficient *n* to construct a basic calculation framework. Compared to traditional models that use a fixed, preset circuit resistance, this model incorporates the dynamically measured total resistance of the primary circuit of the voltage transformer in real time. R y (Total circuit resistance) can dynamically compensate for calculation errors caused by resistance variations with frequency and ambient temperature; a three-phase voltage vector synthesis algorithm is adopted, taking the voltage vector sum of three adjustable inductor-adjustable capacitor series structures as the calculation input, effectively eliminating measurement deviations caused by minor asymmetries in the three-phase structure; at the same time, a correction coefficient is introduced. k This coefficient is adaptively calculated based on the length, cross-sectional parameters, and ambient temperature of the cable under test, and is used to compensate for the influence of cable laying method and ambient temperature on capacitance measurement. Finally, by integrating dual-frequency impedance data, transformer ratio, real-time resistance, and correction coefficients, a complete improved capacitance calculation model is formed, which can effectively eliminate the interference of loop resistance, grounding resistance, and environmental factors, significantly improve the accuracy of ground capacitance measurement, and adapt to the measurement needs of 10kV cables with different parameter specifications.

[0074] The aforementioned method for measuring the ground capacitance current of a 10kV cable effectively solves the interference, accuracy, and safety issues of traditional methods by injecting current signals of different frequencies in stages and calculating the system impedance. The dual-frequency impedance measurement and calculation avoids the error accumulation that may occur with single-frequency measurements, reduces dependence on system parameter variations, and improves the robustness and accuracy of the calculation. Furthermore, the multi-frequency injection method, using an ungrounded system connection, provides strong adaptability.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A device for measuring the capacitance current to ground of a 10kV cable, characterized in that, include: A voltage transformer, the secondary side of which is connected to the cable under test, and a current signal injection terminal is reserved at the neutral point of the secondary side of the voltage transformer; An adjustable three-phase capacitor structure is introduced on the primary side of a voltage transformer. The adjustable three-phase capacitor structure includes three symmetrically connected adjustable inductor-adjustable capacitor series structures. In the adjustable inductor-adjustable capacitor series structure, the end of the adjustable inductor furthest from the adjustable capacitor is grounded, and the end of the adjustable capacitor furthest from the adjustable inductor is grounded. The adjustable inductor is matched with the primary side of the voltage transformer. In the three adjustable inductor-adjustable capacitor series structures, the nodes of the adjustable inductor and the adjustable capacitor are connected to the same common node.

2. The 10kV cable-to-ground capacitance current measuring device according to claim 1, characterized in that, The adjustable inductor-adjustable capacitor series structure also includes a programmable AC current source between the nodes of the adjustable inductor and the adjustable capacitor and the common node.

3. A method for measuring the capacitance current to ground of a 10kV cable, based on the 10kV cable capacitance current measuring device according to claim 1, characterized in that, Includes the following steps: Inject a current signal at the intended resonant frequency at the current signal injection terminal, and measure the voltage and current values ​​on the primary side of the voltage transformer at the intended resonant frequency. Calculate the system impedance at the proposed resonant frequency based on the voltage and current values ​​on the primary side of the voltage transformer at the proposed resonant frequency. A current signal of a specified frequency is injected at the current signal injection terminal, and the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency are measured; the system impedance at the specified frequency is calculated based on the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency; the specified frequency is determined by the proposed resonant frequency, and the specified frequency and the proposed resonant frequency meet a preset difference requirement; Calculate the capacitance to ground of the cable under test based on the proposed resonant frequency, the system impedance at the proposed resonant frequency, the specified frequency, and the system impedance at the specified frequency.

4. The method for measuring the capacitance current to ground of a 10kV cable according to claim 3, characterized in that, The determination of the proposed resonant frequency includes the following steps: A sweep frequency current signal is injected at the current signal injection terminal, and the voltage signal on the primary side of the voltage transformer is monitored at the same time, and the amplitude change curve of the voltage signal is recorded. The resonant frequency is determined based on the amplitude variation curve of the voltage signal on the primary side of the voltage transformer; the determined resonant frequency is the current frequency corresponding to the sudden change in the voltage signal on the primary side of the voltage transformer.

5. The method for measuring the capacitance current to ground of a 10kV cable according to claim 4, characterized in that, The amplitude of the swept current signal is stable between 5mA and 20mA, and the amplitude fluctuation does not exceed ±0.5mA, with the frequency continuously increasing from small to large.

6. The method for measuring the capacitance current to ground of a 10kV cable according to claim 3, characterized in that, The steps for calculating the capacitance to ground based on the proposed resonant frequency, the system impedance at the proposed resonant frequency, the specified frequency, and the system impedance at the specified frequency are as follows: ； In the formula, C Capacitance to ground; k This is a correction factor; n The inductance coefficient of the voltage transformer; w a The proposed resonant frequency; w b For a specified frequency; Z a The system impedance at the proposed resonant frequency; Z b The system impedance at a specified frequency.

7. The method for measuring the capacitance current to ground of a 10kV cable according to claim 3, characterized in that, The preset difference requirement is that the difference between the specified frequency and the proposed resonant frequency is in the range of 50Hz-200Hz, and the specified frequency does not exceed the maximum value of the carrier communication frequency.

8. The method for measuring the capacitance current to ground of a 10kV cable according to claim 3, characterized in that, The system impedance at the corresponding frequency is calculated based on the voltage and current values ​​on the primary side of the voltage transformer at the specified frequency or the proposed resonant frequency, according to the following formula: ； In the formula, Z y The system impedance at the corresponding frequency; y = a or b , Z a The system impedance at the proposed resonant frequency, Z b The system impedance at a specified frequency; U y This represents the voltage value on the primary side of the voltage transformer at the corresponding frequency. I y This represents the current value on the primary side of the voltage transformer at the corresponding frequency. n The inductance coefficient of the voltage transformer; R y This represents the total circuit resistance on the primary side of the voltage transformer at the corresponding frequency.

9. The method for measuring the capacitance current to ground of a 10kV cable according to claim 3, characterized in that, The voltage value on the primary side of the voltage transformer is the vector sum of the voltages of three adjustable inductors and adjustable capacitors connected in parallel.

10. The method for measuring the capacitance current to ground of a 10kV cable according to claim 9, characterized in that, The voltage of the adjustable inductor-adjustable capacitor series structure is calculated using the following formula: ； In the formula, U x The voltage is for an adjustable inductor-adjustable capacitor series structure. x =1, 2, or 3 U 1 represents the voltage in the first adjustable inductor-adjustable capacitor series structure. U 2 represents the voltage in the second adjustable inductor-adjustable capacitor series structure. U 3 represents the voltage in the third adjustable inductor-adjustable capacitor series structure; n The inductance coefficient of the voltage transformer; R x A resistor with an adjustable inductor-adjustable capacitor series structure; w y The frequency of the injected current signal, y = a or b ; L x An inductor with an adjustable inductor-adjustable capacitor series structure; C x A capacitor with an adjustable inductor-adjustable capacitor series structure; I x The current is for the series connection of an adjustable inductor and adjustable capacitor; the current is equal for three parallel series connections of an adjustable inductor and adjustable capacitor.

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