Medium-voltage cable frequency conversion resonance voltage withstanding test system and method

Abstract

The invention relates to the technical field of electrical equipment insulation tests, and discloses a medium-voltage cable variable-frequency resonance voltage-withstanding test system and a medium-voltage cable variable-frequency resonance voltage-withstanding test method. The method comprises the steps of configuring a test loop and setting related parameters, and presetting a target test voltage, test time, a system quality factor range, a local discharge capacity threshold and a current change rate threshold based on a cable model, a cable length, a laying environment temperature and an equivalent capacitance. And executing frequency scanning in a preset frequency range through an adaptive step length strategy, adjusting the step length according to the current response, identifying the resonant frequency and verifying the quality factor of the system. In the test process, the loop current change rate, the partial discharge capacity and the equipment temperature are monitored in real time, a protection mechanism is triggered according to abnormal characteristics, and insulation state diagnosis is completed. When the test is ended, voltage is reduced at a controllable rate, residual charges are dissipated, and a test report is automatically generated. The method improves the accuracy and safety of the test, and is suitable for various field conditions.

Description

Technical Field

[0001] This invention relates to the field of electrical equipment insulation testing technology, specifically to a medium-voltage cable frequency conversion resonant withstand voltage test system and method. Background Technology

[0002] Medium-voltage cables are a critical component of power systems, and their insulation performance directly affects the reliable operation of the power grid. During manufacturing, installation, and long-term operation, cables may develop insulation defects due to various factors, such as partial discharge, moisture intrusion, or mechanical damage. These defects may not be easily detected under normal operating conditions, but they can trigger faults under overvoltage conditions, leading to power outages. Therefore, regularly conducting withstand voltage tests on medium-voltage cables is an important means of ensuring power system safety. The withstand voltage test aims to simulate the overvoltage stress that the cable may experience during operation and verify whether its insulation strength meets the standard requirements.

[0003] Traditional cable withstand voltage testing methods mainly include DC withstand voltage testing and power frequency AC withstand voltage testing. DC withstand voltage testing was once widely used for field testing of medium-voltage cables, with advantages such as small equipment capacity and portability. However, the insulation stress distribution under a DC electric field differs significantly from that under AC operating conditions. Cable insulation is prone to space charge accumulation under DC voltage, which may lead to aging or damage of the insulation material, and even mask certain defects, such as water treeing. Furthermore, DC testing lacks sufficient sensitivity for detecting certain types of defects and may not effectively identify partial discharge problems under AC electric fields. With the development of cable technology, especially the widespread use of cross-linked polyethylene cables, the applicability of DC withstand voltage testing has been questioned, and many international standards have gradually restricted its use.

[0004] Power frequency AC withstand voltage tests more closely approximate the actual operating conditions of cables and can better reflect the insulation performance under an AC electric field. However, power frequency tests require the test power supply to provide a large reactive power. For medium- and long cables, the equivalent capacitance is large, requiring large-capacity power frequency test transformers and voltage regulating equipment, resulting in bulky and costly equipment and difficulties in on-site deployment. The implementation of power frequency tests is often limited, especially in urban power grids or confined spaces. To overcome the shortcomings of power frequency tests, the variable frequency resonant withstand voltage test method has emerged. This method adjusts the frequency of the test power supply to make the circuit operate in a resonant state, thereby using a reactor to compensate for the capacitive current of the cable, reducing the power supply capacity requirement, making the test equipment lightweight, and facilitating on-site application.

[0005] Existing frequency conversion resonant testing techniques typically employ a fixed-step frequency scanning method to locate the resonant point. However, cable parameters are affected by factors such as cable length and ambient temperature, potentially leading to inaccurate resonant frequency identification and low testing efficiency. Voltage control often uses an open-loop approach, which is susceptible to load variations during voltage boosting, resulting in poor voltage stability and potentially affecting the reliability of test results. Furthermore, existing methods are weak in real-time monitoring and protection, lacking comprehensive diagnostics of current change rate, partial discharge, and temperature. Delayed protection responses under abnormal conditions may lead to equipment damage or safety risks. Insulation condition assessment largely relies on manual post-test analysis, lacking sufficient automation and failing to meet the demands of modern power systems for intelligent testing. Summary of the Invention

[0006] The purpose of this invention is to provide a medium-voltage cable frequency conversion resonant withstand voltage test system and method to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides a medium-voltage cable frequency conversion resonant withstand voltage test system and method, the method comprising: Configure the test circuit and set the parameters, collect the model, length, laying ambient temperature and equivalent capacitance of the cable under test, and preset the target test voltage, test time, system quality factor range, partial discharge threshold and current change rate threshold according to the standard. Perform a frequency scanning operation, scan within a preset frequency range using an adaptive step size strategy, adjust the scanning step size based on changes in current response, identify the current peak point as the resonant frequency, and verify whether the system quality factor is within the preset range. A voltage stability test was conducted, with the voltage gradually increased from an initial low voltage to the target test voltage. A closed-loop control algorithm was used to adjust the voltage and frequency to keep the test voltage within the allowable fluctuation range. Implement real-time protection and condition diagnosis, monitor the circuit current change rate, partial discharge quantity and equipment temperature data, trigger the protection mechanism based on abnormal characteristics, and complete the insulation condition assessment; The test termination procedure is executed to reduce the voltage to a safe level at a controllable rate, dissipate residual charge, automatically generate a test report, and manage data storage.

[0008] Preferably, in the step of configuring the test circuit and setting parameters, the measurement accuracy of the equivalent capacitance is not less than ±0.02μF, the acquisition accuracy of the laying environment temperature is ±0.2℃, the system quality factor preset according to the standard is in the range of 25 to 65, the partial discharge threshold is 8pC, and the current change rate threshold is 4A / ms.

[0009] Preferably, in the step of performing the frequency scanning operation, the adaptive step size strategy includes two stages: preliminary scanning and fine scanning. The preliminary scanning scans within the frequency range with a step size of 3Hz. When the current growth rate of two consecutive steps exceeds 120% of the previous step size, it switches to fine scanning. The fine scanning scans within ±4Hz of the frequency locked by the preliminary scanning with a step size of 0.02Hz. The frequency point where the current maximum value is determined and the fluctuation is less than 0.6% is the resonant frequency.

[0010] Preferably, the initial scanning stage outputs a low voltage signal with an amplitude of 25% of the rated excitation voltage. If the system quality factor exceeds the preset range, the system automatically issues an alarm and provides adjustment suggestions.

[0011] Preferably, in the step of conducting the voltage stability test, the step voltage increase of each step is one-eighth of the target test voltage, and the duration of each step is 15 to 25 seconds. If the reactor temperature is detected to reach 55°C or the temperature rise rate exceeds 0.8°C / min during the voltage increase process, the duration is extended to 50 seconds and the cooling system is activated.

[0012] Preferably, the closed-loop control algorithm includes proportional-integral-derivative (PID) control and fuzzy logic control. When the absolute value of the voltage deviation does not exceed 0.6% of the target test voltage and the frequency drift is less than 0.15Hz, PID control is activated. The proportional coefficient ranges from 0.7 to 1.1, the integral time constant is from 0.2 to 0.4 seconds, and the derivative time constant is from 0.04 to 0.08 seconds.

[0013] Preferably, the input variables of the fuzzy logic control are voltage deviation and frequency drift, and the fuzzy subset is defined as negative large, negative small, zero, positive small, and positive large. The voltage adjustment and frequency adjustment are output through rule base reasoning, and the frequency adjustment step size is 0.02Hz to 0.06Hz.

[0014] Preferably, in the step of implementing real-time protection and status diagnosis, the protection mechanism includes a multi-level response. When the current change rate exceeds 4A / ms, the frequency converter power output is cut off, the main circuit switch is disconnected, and the energy release circuit is activated sequentially within 4ms. The insulation status assessment is based on partial discharge data. A discharge of less than 8pC is rated as good, 8pC to 40pC is rated as caution, and more than 40pC is rated as defect and the test is terminated. The partial discharge data were processed by wavelet transform and then decomposed and denoised using the sym6 wavelet basis in 5 layers to extract discharge feature values.

[0015] Preferably, in the step of executing the test termination procedure, the voltage reduction rate is 0.4 times the target test voltage per minute, the residual charge safety value is 30V, the generated test report includes resonant frequency, quality factor, voltage-current curve and partial discharge spectrum, and the data storage supports encryption and remote access.

[0016] Preferably, the present invention also includes a medium-voltage cable frequency conversion resonant withstand voltage test system, including a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that, when the processor executes the computer program, it implements the steps of the above-mentioned medium-voltage cable frequency conversion resonant withstand voltage test method.

[0017] Compared with the prior art, the beneficial effects of the present invention are: The medium-voltage cable frequency conversion resonant withstand voltage test method provided by this invention exhibits multiple advantages in terms of accuracy, safety, and adaptability by optimizing the frequency scanning strategy, voltage control mechanism, and condition diagnosis process. The adaptive step-size frequency scanning operation in the method can dynamically adjust the scanning step size according to the current response, quickly locating the resonant point within a preset frequency range. This strategy avoids the overscanning or underscanning problems that may occur with fixed step-size scanning, reduces frequency search time, and is particularly suitable for testing cables of different lengths and parameters. The resonant frequency identification is more accurate, and the verification of the system quality factor ensures that the test circuit is in optimal working condition.

[0018] During the voltage stability test, a stepped voltage boost combined with a closed-loop control algorithm was employed to achieve smooth regulation and stable maintenance of the test voltage. The closed-loop control adjusted the output voltage and frequency through real-time feedback, offsetting deviations caused by load fluctuations or environmental changes, ensuring that voltage fluctuations were strictly controlled within permissible limits. This control method reduced the risk of overvoltage, avoided uneven insulation stress due to voltage instability, and ensured the consistency of the test voltage, thus facilitating the acquisition of reliable insulation performance data. Simultaneously, the stepped design of the voltage boost process avoided voltage abrupt changes, reducing the impact on cable insulation and extending cable life.

[0019] Real-time protection and condition diagnosis mechanisms enhance test safety through multi-dimensional monitoring. Monitoring the rate of change of loop current can promptly identify abnormal current increases, such as increased partial discharge or precursors to insulation breakdown, triggering protective actions to prevent the accident from escalating. Continuous acquisition and analysis of partial discharge quantities helps to detect insulation defects early, preventing them from worsening during testing. Monitoring equipment temperature data prevents overheating and protects the test equipment itself. These monitoring data are integrated into the diagnostic logic to achieve online assessment of insulation condition, reducing reliance on offline post-test analysis and improving diagnostic efficiency.

[0020] The test termination procedure reduces voltage at a controlled rate and dissipates residual charge, avoiding the risks of induced overvoltage or residual charge caused by sudden voltage drops, ensuring operator safety and equipment integrity. The automatic test report generation function simplifies data management; test parameters, process data, and evaluation results are recorded and stored by the system for easy subsequent retrieval and trend analysis. This approach reduces human intervention and the possibility of operational errors. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the working principle of a medium-voltage cable frequency conversion resonant withstand voltage test system and method according to the present invention. Figure 2 This is a graph showing the relationship between voltage step-up and reactor temperature. Figure 3 The image shows a comparison of partial discharge quantity before and after wavelet noise reduction. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Please see Figure 1 This invention provides a frequency conversion resonant withstand voltage test system and method for medium-voltage cables. The method is implemented through the following steps: configuring the test circuit and setting parameters; collecting the model, length, laying environment temperature, and equivalent capacitance of the cable under test; and presetting the target test voltage, test time, system quality factor range, partial discharge threshold, and current change rate threshold according to standards; performing a frequency scanning operation, scanning within a preset frequency range using an adaptive step-size strategy, adjusting the scanning step-size based on current response changes, identifying the current peak point as the resonant frequency, and verifying whether the system quality factor is within the preset range; conducting a voltage stability test, gradually increasing the voltage from an initial low voltage to the target test voltage, and using a closed-loop control algorithm to adjust the voltage and frequency to maintain the test voltage within the allowable fluctuation range; implementing real-time protection and status diagnosis, monitoring the circuit current change rate, partial discharge, and equipment temperature data, triggering the protection mechanism based on abnormal characteristics, and completing the insulation status assessment; and executing the test termination procedure, reducing the voltage to a safe level at a controllable rate, dissipating residual charge, automatically generating a test report, and managing data storage. Example 1:

[0024] In configuring the test circuit and setting parameters, the measurement accuracy of the equivalent capacitance is controlled to be no less than ±0.02μF, the acquisition accuracy of the laying environment temperature is ±0.2℃, the system quality factor is preset to a range of 25 to 65 according to the standard, the partial discharge threshold is 8pC, and the current change rate threshold is 4A / ms. When performing frequency scanning, the adaptive step size strategy includes two stages: preliminary scanning and fine scanning. The preliminary scan scans within the frequency range in 3Hz steps. When the current growth rate of two consecutive steps exceeds 120% of the previous step, it switches to fine scanning. Fine scanning scans within ±4Hz of the frequency locked by the preliminary scan in 0.02Hz steps, determining the frequency point where the current maximum value and fluctuation is less than 0.6% as the resonant frequency. During the preliminary scan stage, a low voltage signal with an amplitude of 25% of the rated excitation voltage is output. If the system quality factor exceeds the preset range, the system automatically issues an alarm and provides adjustment suggestions.

[0025] In specific implementation, Example 1 of the medium-voltage cable frequency conversion resonant withstand voltage test method involves configuring test circuit parameters and performing frequency scanning operations. When configuring the test circuit parameters, the measurement accuracy of the equivalent capacitance is controlled to be no less than ±0.02μF, the acquisition accuracy of the laying environment temperature is ±0.2℃, the system quality factor is preset to a range of 25 to 65 according to the standard, the partial discharge threshold is 8pC, and the current change rate threshold is 4A / ms. The frequency scanning operation adopts an adaptive step-size strategy, including two stages: preliminary scanning and fine scanning. In some embodiments, the preliminary scanning is performed within a preset frequency range with a step size of 3Hz. When the current growth rate of two consecutive steps exceeds 120% of the previous step size, it switches to fine scanning. Fine scanning is performed within ±4Hz of the frequency locked by the preliminary scanning with a step size of 0.02Hz. The frequency point where the maximum current value and fluctuation is less than 0.6% is determined as the resonant frequency. The current growth rate is calculated using the formula... Where R represents the current growth rate, This represents the current value at step n. This represents the current value at step n-1.

[0026] It is understood that during the initial scanning phase, a low-voltage signal with an amplitude of 25% of the rated excitation voltage is output. If the system quality factor exceeds the preset range, the system automatically issues an alarm and provides adjustment suggestions. Optionally, the switching conditions of the adaptive step-size strategy are adjusted in real time based on changes in current response to ensure scanning efficiency. In some embodiments, the resonant frequency verification step of the frequency scanning operation includes multiple sampling and averaging to reduce measurement errors. It is understood that the calculation of the system quality factor depends on loop parameters and measurement data. Optionally, a high-precision frequency generator is used to implement step-size control during the fine scanning phase. Example 2:

[0027] In the voltage stability test, the step voltage increase is set to one-eighth of the target test voltage for each step, and the duration of each step is 15 to 25 seconds. If the reactor temperature reaches 55°C or the temperature rise rate exceeds 0.8°C / min during the voltage increase, the duration is extended to 50 seconds and the cooling system is activated.

[0028] In practical implementation, the stepped voltage ramp process during the voltage stability test has clearly defined parameter settings. Each ramp increment is set to one-eighth of the target test voltage, and each ramp duration is between 15 and 25 seconds. It is understood that the voltage ramp process requires real-time monitoring of the reactor's thermal state, and the reactor temperature monitoring data is directly used in the control logic. In some embodiments, when the monitoring system detects that the reactor temperature reaches 55°C or the reactor temperature rise rate exceeds 0.8°C / min, the system automatically extends the duration of the current voltage level to 50 seconds and simultaneously activates the forced cooling system for heat dissipation. Optionally, the adjustment of the duration is based on preventing reactor overheating and damage; the forced cooling system typically includes a fan or water cooling device.

[0029] In practice, the voltage ramp-up process from the initial low voltage to the target test voltage is continuous and automated, with a closed-loop control algorithm ensuring voltage stability. In some embodiments, after each voltage ramp-up stage, the system enters a voltage holding phase, during which loop parameters are continuously acquired to assess stability. It is understood that a reactor temperature reaching 55°C or a temperature rise rate exceeding 0.8°C / min is a critical condition for triggering protective delay operation. Optionally, extending the holding time to 50 seconds aims to provide sufficient cooling time for the reactor, thereby ensuring equipment safety and test continuity. Activating the cooling system is a coordinated action performed simultaneously with the extended holding time operation.

[0030] See Figure 2 This graph corresponds to the voltage stability test in the medium-voltage cable frequency conversion resonant withstand voltage test method. The horizontal axis represents the number of voltage boost stages, the left vertical axis represents the test voltage, and the right vertical axis represents the reactor temperature. This graph visually presents the relationship between the voltage boost process and the equipment's thermal state, demonstrating the design logic of the stepped voltage boost strategy in avoiding voltage surges and ensuring equipment safety. It also provides data support for the coordinated management of voltage and temperature in closed-loop control, directly reflecting the feasibility of the technical solution for the voltage stability test. Example 3:

[0031] The closed-loop control algorithm includes proportional-integral-derivative (PID) control and fuzzy logic control. PID is activated when the absolute value of the voltage deviation does not exceed 0.6% of the target test voltage and the frequency drift is less than 0.15Hz. The proportional coefficient ranges from 0.7 to 1.1, the integral time constant is from 0.2 to 0.4 seconds, and the derivative time constant is from 0.04 to 0.08 seconds. The input variables for fuzzy logic control are the voltage deviation and the frequency drift. The fuzzy subset is defined as negative large, negative small, zero, positive small, and positive large. The algorithm infers the voltage and frequency adjustment values ​​through a rule base, with a frequency adjustment step size of 0.02Hz to 0.06Hz.

[0032] In practical implementation, the closed-loop control algorithm relies on the coordinated operation of proportional-integral-derivative (PID) control and fuzzy logic control. Real-time monitoring data of voltage deviation and frequency drift serve as the reference for control input. When the absolute value of the voltage deviation does not exceed 0.6% of the target test voltage and the frequency drift is less than 0.15Hz, the system activates PID control. The proportional gain is set to a range of 0.7 to 1.1, the integral time constant to a range of 0.2 to 0.4 seconds, and the derivative time constant to a range of 0.04 to 0.08 seconds. These parameters are configured and adjusted via a digital signal processor. It can be understood that the activation condition of PID control is based on the judgment of the system's stable state, and the threshold values ​​for voltage deviation and frequency drift are designed to ensure smooth switching of control modes. In some embodiments, the execution cycle of the PID control algorithm is synchronized with the voltage sampling frequency to ensure real-time control response. Optionally, the specific values ​​of the proportional gain, integral time constant, and derivative time constant can be fine-tuned according to the loop impedance characteristics.

[0033] In practical implementation, fuzzy logic control serves as a supplement or alternative to proportional-integral-derivative (PID) control. The input variables for fuzzy logic control are explicitly defined as voltage deviation and frequency drift. The fuzzy subset is defined as five levels: negative large, negative small, zero, positive small, and positive large. Inference calculations are performed using a preset rule base, and the output variables are voltage adjustment and frequency adjustment. The step size of the frequency adjustment is set to 0.02Hz to 0.06Hz. It can be understood that the rule base of fuzzy logic control contains a series of if-then conditional statements used to map the relationship between the input fuzzy set and the output fuzzy set. In some embodiments, the inference process of fuzzy logic control employs the Mamdani fuzzy inference method and uses the centroid method for defuzzification to obtain precise control quantities. Optionally, output limiting mechanisms for voltage and frequency adjustments prevent overshoot. Fuzzy logic control is typically activated when system parameters fluctuate significantly or when PID control cannot meet accuracy requirements. Example 4:

[0034] In the implementation of real-time protection and condition diagnosis, the protection mechanism includes a multi-level response. When the current change rate exceeds 4A / ms, the inverter power supply output, the main circuit switch, and the energy release circuit are sequentially cut off within 4ms. The insulation condition assessment is based on partial discharge data. A discharge level less than 8pC is rated as good, 8pC to 40pC is rated as requiring attention, and a level exceeding 40pC is rated as defective, terminating the test. The partial discharge data is processed using wavelet transform and decomposed into five layers of noise using the sym6 wavelet basis to extract discharge feature values.

[0035] In practical implementation, the real-time protection and status diagnosis steps include a multi-level response protection mechanism and insulation status assessment based on partial discharge. When the monitoring system detects a loop current change rate exceeding 4A / ms, the protection mechanism will sequentially execute three actions within 4ms: cutting off the inverter power supply output, disconnecting the main circuit switch, and activating the energy release circuit. This series of actions is triggered sequentially by a high-speed programmable logic controller to ensure equipment safety. The insulation status assessment is based on real-time collected partial discharge data. The assessment criteria are: a partial discharge of less than 8pC is rated as a good state; a partial discharge between 8pC and 40pC is rated as a state of concern; and a partial discharge exceeding 40pC is rated as a defective state, and the entire test process is immediately terminated. It can be understood that the timing logic of the multi-level response protection mechanism is pre-set, and the monitoring of the current change rate uses high-frequency sampling technology. In some embodiments, signal processing technology is used to process the partial discharge data. The partial discharge data undergoes wavelet transform processing to improve the signal-to-noise ratio. Specifically, the sym6 wavelet basis function is used to perform a 5-level decomposition of the original signal to achieve noise reduction. Effective discharge feature values ​​are extracted from the processed signal for status judgment. Optionally, the number of decomposition levels and the type of wavelet basis for wavelet transform processing are optimized. A 5-level decomposition using the sym6 wavelet basis can effectively separate noise from real discharge pulses. Extraction of discharge feature values ​​involves multi-dimensional information such as pulse amplitude, phase, and frequency. It can be understood that activating the energy release circuit is to rapidly dissipate the electromagnetic energy stored in the system. In some embodiments, the acquisition of partial discharge data is completed using a high-frequency current transformer in conjunction with a dedicated acquisition card. Optionally, the results of the insulation condition assessment are displayed in real time on the human-machine interface and recorded in the test log.

[0036] See Figure 3, this figure corresponds to the technical link of real-time protection and condition diagnosis in the medium-voltage cable variable-frequency resonance withstand voltage test method. In the figure, the horizontal axis is the time point, the vertical axis is the partial discharge quantity, the orange curve is the discharge quantity before noise reduction, which contains a large amount of noise interference, with violent and irregular data fluctuations; the green curve is the discharge quantity after noise reduction, with a smooth curve and a clear trend. This figure visually verifies the effectiveness of the wavelet noise reduction technology. By removing noise, the true characteristics of partial discharge are clearly presented, providing a reliable data basis for subsequent insulation condition assessment based on the partial discharge quantity, reflecting the design value of signal processing technology in improving the accuracy of insulation diagnosis and achieving automatic high-precision insulation assessment, and is a direct manifestation of the feasibility of the technical solution in the real-time protection and condition diagnosis link. Embodiment 5:

[0037] In the steps of executing the test termination process, the voltage reduction rate is controlled at 0.4 times the target test voltage per minute, the residual charge safety value is 30V, and the generated test report includes the resonance frequency, quality factor, voltage-current curve, and partial discharge pattern. Data storage supports encryption and remote access.

[0038] In specific implementation, executing the test termination process involves the gradual reduction of voltage and subsequent system processing. The voltage reduction rate is controlled at 0.4 times the target test voltage per minute, and this rate is set in the control unit of the variable-frequency power supply through a program. The voltage drop process is linear. The safety value of the residual charge is set at 30V. When the system monitors that the loop voltage drops below 30V, it is determined that the residual charge has reached the safe level and subsequent physical disconnection operations are allowed. It can be understood that setting the voltage reduction rate of 0.4 times the target test voltage per minute is to prevent operating overvoltage caused by sudden voltage drop. In some embodiments, the voltage drop process is monitored in real time, and the deviation between its actual rate and the set value is recorded for report generation.

[0039] In specific implementation, the generation of the test report and data management are automatically completed. The content of the generated test report includes the resonance frequency, system quality factor, the curves of voltage and current changing with time, and the partial discharge pattern. It can be understood that the voltage-current curve reflects the complete dynamic relationship between voltage application and current response during the entire test process. The data storage system supports encryption processing of all test data. The encryption algorithm uses the standard AES-256 encryption method. At the same time, the storage system provides a remote access interface based on a security protocol. In some embodiments, the partial discharge pattern is presented in the form of a time-frequency analysis image, showing the statistical characteristics of the partial discharge signal. Optionally, the remote access function requires an authentication and authorization mechanism to ensure data security. The test report is automatically archived in the database in a structured document format. It can be understood that the resonance frequency and system quality factor are key parameters for evaluating the stability of the resonance point.

[0040] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for frequency conversion resonant withstand voltage test of medium-voltage cables, characterized in that, The method includes: Configure the test circuit and set the parameters, collect the model, length, laying ambient temperature and equivalent capacitance of the cable under test, and preset the target test voltage, test time, system quality factor range, partial discharge threshold and current change rate threshold according to the standard. Perform a frequency scanning operation, scan within a preset frequency range using an adaptive step size strategy, adjust the scanning step size based on changes in current response, identify the current peak point as the resonant frequency, and verify whether the system quality factor is within the preset range. A voltage stability test was conducted, with the voltage gradually increased from an initial low voltage to the target test voltage. A closed-loop control algorithm was used to adjust the voltage and frequency to keep the test voltage within the allowable fluctuation range. Implement real-time protection and condition diagnosis, monitor the circuit current change rate, partial discharge quantity and equipment temperature data, trigger the protection mechanism based on abnormal characteristics, and complete the insulation condition assessment; The test termination procedure is executed to reduce the voltage to a safe level at a controllable rate, dissipate residual charge, automatically generate a test report, and manage data storage.

2. The method for frequency conversion resonant withstand voltage test of medium-voltage cables according to claim 1, characterized in that, In the steps of configuring the test circuit and setting parameters, the measurement accuracy of the equivalent capacitance is not less than ±0.02μF, the acquisition accuracy of the laying environment temperature is ±0.2℃, the system quality factor preset according to the standard is 25 to 65, the partial discharge threshold is 8pC, and the current change rate threshold is 4A / ms.

3. The method for frequency conversion resonant withstand voltage test of medium-voltage cables according to claim 1, characterized in that, In the step of performing the frequency scanning operation, the adaptive step size strategy includes two stages: preliminary scanning and fine scanning. The preliminary scanning scans within the frequency range with a step size of 3Hz. When the current growth rate of two consecutive steps exceeds 120% of the previous step size, it switches to fine scanning. The fine scanning scans within ±4Hz of the frequency locked by the preliminary scanning with a step size of 0.02Hz. The frequency point where the current maximum value is determined and the fluctuation is less than 0.6% is the resonant frequency.

4. The method for frequency conversion resonant withstand voltage test of medium-voltage cable according to claim 3, characterized in that, The initial scanning stage outputs a low voltage signal with an amplitude of 25% of the rated excitation voltage. If the system quality factor exceeds the preset range, the system will automatically issue an alarm and provide adjustment suggestions.

5. The method for frequency conversion resonant withstand voltage test of medium-voltage cables according to claim 1, characterized in that, In the voltage stability test, the step voltage increase is one-eighth of the target test voltage, and the duration of each step is 15 to 25 seconds. If the reactor temperature reaches 55°C or the temperature rise rate exceeds 0.8°C / min during the voltage increase, the duration is extended to 50 seconds and the cooling system is activated.

6. The method for frequency conversion resonant withstand voltage test of medium-voltage cable according to claim 1, characterized in that, The closed-loop control algorithm includes proportional-integral-derivative (PID) control and fuzzy logic control. When the absolute value of the voltage deviation does not exceed 0.6% of the target test voltage and the frequency drift is less than 0.15Hz, PID control is activated. The proportional coefficient ranges from 0.7 to 1.1, the integral time constant is from 0.2 to 0.4 seconds, and the derivative time constant is from 0.04 to 0.08 seconds.

7. The method for frequency conversion resonant withstand voltage test of medium-voltage cable according to claim 6, characterized in that, The input variables for the fuzzy logic control are voltage deviation and frequency drift. The fuzzy subset is defined as negative large, negative small, zero, positive small, and positive large. The voltage adjustment and frequency adjustment are output through rule base reasoning, and the frequency adjustment step size is 0.02Hz to 0.06Hz.

8. The method for frequency conversion resonant withstand voltage test of medium-voltage cable according to claim 1, characterized in that, In the steps of implementing real-time protection and status diagnosis, the protection mechanism includes multi-level response. When the current change rate exceeds 4A / ms, the frequency converter power output is cut off, the main circuit switch is disconnected, and the energy release circuit is activated in sequence within 4ms. The insulation status assessment is based on partial discharge data. The discharge amount is rated as good if it is less than 8pC, rated as caution if it is between 8pC and 40pC, and rated as defective if it exceeds 40pC, and the test is terminated. The partial discharge data were processed by wavelet transform and then decomposed and denoised using the sym6 wavelet basis in 5 layers to extract discharge feature values.

9. The method for frequency conversion resonant withstand voltage test of medium-voltage cable according to claim 1, characterized in that, In the steps of executing the test termination procedure, the voltage reduction rate is 0.4 times the target test voltage per minute, the residual charge safety value is 30V, and the generated test report includes resonant frequency, quality factor, voltage and current curves and partial discharge spectrum. Data storage supports encryption and remote access.

10. A medium-voltage cable frequency conversion resonant withstand voltage test system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the medium-voltage cable frequency conversion resonant withstand voltage test method according to any one of claims 1 to 9.

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