A method of power cable withstand voltage test

Abstract

The present application relates to the field of cable withstand voltage test, particularly relates to a power cable withstand voltage test method, the present application is through the rough sweep positioning candidate interval, the fine sweep data, the secondary interpolation calibration three level resonance point search mode, combined with the cable total capacitance and the same type cable historical resonance data demarcate the scanning interval, solves the problem of low efficiency of full interval fine sweep, and can also make up for the defects of insufficient accuracy of experience estimation. The present application calculates the voltage compensation coefficient and the boost rate compensation value based on the test site temperature and humidity environment parameters, realizes the dynamic correction of the test voltage and the boost rate, and can eliminate the influence of temperature and humidity on the cable insulation dielectric strength. The present application constructs the correlation analysis mode of discharge sound and current data in the boost stage, locates the defect area through the decibel attenuation trend, verifies the insulation anomaly combined with the current fluctuation correlation, and accurately reduces the defect range to a single monitoring point or adjacent monitoring point interval.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of cable withstand voltage test, in particular to a power cable withstand voltage test method. BACKGROUND

[0002] Power cable is the core carrier of power system power transmission and distribution, and its insulation performance directly determines the safety, stability and reliability of power grid operation. As a key means to detect cable insulation defects and troubleshoot safety hazards, withstand voltage test plays an irreplaceable role in ensuring continuous power supply of power system and reducing the probability of fault outage.

[0003] For cable withstand voltage test, the prior art also proposes related cable withstand voltage test methods. For example, the Chinese patent application with publication number CN119780638A discloses a cable oscillation ultra-low frequency withstand voltage and partial discharge integrated test method and system. This technical solution optimizes the oscillation ultra-low frequency voltage signal by applying a manifold learning algorithm, realizes the integration of traditional dielectric loss test, withstand voltage test and partial discharge detection into a one-time joint test, and effectively solves the problems of prolonged power outage time, equipment redundancy and accelerated insulation degradation caused by multiple tests in the prior art. The modified sinusoidal wave oscillation ultra-low frequency voltage signal used by the signal excitation module significantly enhances the sensitivity of dielectric loss characteristics and partial discharge pulses while meeting the requirements of cable withstand voltage test, improving measurement accuracy and detection sensitivity.

[0004] The above-mentioned cable withstand voltage test mainly integrates dielectric loss test, withstand voltage test and partial discharge detection to improve test efficiency and optimize the sensitivity of dielectric loss characteristics and partial discharge pulses.

[0005] However, there are still some problems in the traditional withstand voltage test of the cable: (1) The existing technology uses a full-range fine sweep or an empirical frequency estimation method to find the resonance point, which is relatively rough. The full-range fine sweep has low efficiency, and the empirical estimation is prone to resonance point deviation due to cable parameter differences, which makes the test circuit unable to stabilize in the resonance state, affecting the stability of the test voltage and easily causing equipment overload risk. At the same time, the test voltage and the boost rate in the prior art are fixed values, which are not dynamically corrected in combination with environmental parameters such as test site temperature and humidity. In high temperature and high humidity environment, it is easy to miss the judgment of insulation latent defects due to insufficient test voltage, and in low temperature and low humidity environment, it is easy to damage the cable insulation layer due to high voltage and fast boost, resulting in distorted test results and safety hazards.

[0006] (2) The prior art only monitors a single electrical parameter such as current or voltage during the voltage boosting stage, and cannot distinguish between insulation defect discharge signals and environmental electromagnetic interference and device operation noise, which not only easily causes a qualified cable to be misjudged as unqualified, but also is difficult to accurately locate the specific position of the insulation defect, thereby reducing the accuracy of the test and the fault troubleshooting efficiency.

[0007] (3) The voltage withstand qualification determination of the prior art only refers to a single electrical signal parameter during the voltage maintaining stage, does not combine the insulation abnormality degree during the voltage boosting stage, cannot identify latent defects that have appeared weakly during the voltage boosting stage and gradually appear during the voltage maintaining stage, and simultaneously lacks quantitative analysis of the deviation degree of the electrical signal parameter, which easily causes subjective judgment to result in qualification determination deviation. SUMMARY

[0008] The present application solves the above technical problems, and the present application adopts the following technical scheme: a power cable voltage withstand test method, comprising: connecting a cable to be tested to a test device and forming a test loop, and uniformly setting a plurality of monitoring points according to the length of the cable.

[0009] The test voltage and the voltage boosting rate of the cable test are set, the test device is controlled to find a resonance point in a frequency scanning interval, and the test loop is in a resonant state.

[0010] The test loop is boosted at the voltage boosting rate; during the voltage boosting stage, the current data and discharge sound of each monitoring point are monitored synchronously, the abnormality degree of the voltage boosting stage is analyzed based on the monitoring data, and whether to continue boosting is determined according to the abnormality degree, if yes, boosting to the test voltage is performed and then voltage maintaining is performed, otherwise, boosting is stopped and the cable voltage withstand is determined to be unqualified.

[0011] During the voltage maintaining stage, the electrical signal parameters of each monitoring point of the cable are monitored, and the qualification of the cable voltage withstand test is determined based on the abnormality degree and the electrical signal parameters.

[0012] After the test is completed, the cable is fully discharged using a discharge rod.

[0013] The beneficial effects of the system are as follows: first, the present application finds a resonance point in three stages through coarse scanning positioning of a candidate interval, fine scanning of data, and twice interpolation calibration, combines the total capacitance of the cable with historical resonance data of the same type of cable to determine the scanning interval, solves the problem of low efficiency of full interval fine scanning, makes up for the defects of insufficient accuracy of experience estimation, ensures that the test loop is accurately and stably in a resonant state, and reduces the risk of overloading of the test device.

[0014] The present application calculates a voltage compensation coefficient and a voltage boosting rate compensation value based on the temperature and humidity environment parameters of the test site, dynamically corrects the test voltage and the voltage boosting rate, eliminates the influence of temperature and humidity on the insulation dielectric strength of the cable, and adapts the test parameters to the current test environment.

[0015] The application constructs the correlation analysis of the discharge sound and the current data in the boosting stage, locates the defect area through the decibel attenuation trend, verifies the insulation abnormality in combination with the current fluctuation correlation, can exclude the influence of the environmental interference signal, can also accurately narrow the defect range to a single monitoring point or an adjacent monitoring point interval, and improves the insulation defect identification accuracy and the fault positioning efficiency.

[0016] The application establishes the comprehensive judgment mode of the abnormal degree in the boosting stage and the comprehensive deviation degree in the pressure maintaining stage, performs the correlation analysis on the abnormal index in the boosting stage and the comprehensive deviation degree of the voltage, the current and the phase in the pressure maintaining stage, realizes the analysis on the cable insulation latent defect, and increases the accuracy degree of the cable qualification analysis. BRIEF DESCRIPTION OF DRAWINGS

[0017] The application is further illustrated by using the drawings, but the embodiments in the drawings do not constitute any limitation on the application, and other drawings can be obtained by the ordinary skilled in the art without creative labor on the basis of the following drawings.

[0018] Figure 1 It is a flow chart of the power cable withstand voltage test method of the application. DETAILED DESCRIPTION

[0019] The embodiments of the application are described in detail below. The embodiments described below are exemplary and are only used for explaining the application, and cannot be understood as the limitation on the application. The specific technology or condition not noted in the embodiments is carried out according to the technology or condition described in the literature in the field or according to the product instruction.

[0020] Reference Figure 1 A power cable withstand voltage test method, comprising: S100, connecting the cable to be tested to a test device and forming a test loop, and uniformly setting a plurality of monitoring points according to the length of the cable.

[0021] In one embodiment of the application, before the cable experiment is performed, the cable needs to be pre-discharged first, that is, the cable terminal is contacted by the grounded discharge rod to release the residual charge of the cable, and then the test device is checked to ensure that the test device is in good condition.

[0022] The specific operation steps of connecting the cable to be tested to the test equipment and forming a test loop are as follows: firstly, the output end of the variable frequency power supply is connected to the terminal of the excitation transformer through a copper bar or a special cable; then the excitation transformer is connected to the input end of the resonant reactor through a high-voltage insulation cable; the output end of the resonant reactor is connected to the high-voltage input end of the capacitive voltage divider through a high-voltage cable; the low-voltage signal end of the capacitive voltage divider is connected to the voltage acquisition interface of the test console through a shielded cable; the ground end of the capacitive voltage divider is temporarily grounded first, and then the high-voltage output end of the capacitive voltage divider is connected to the first end core wire of the cable to be tested through a special high-voltage terminal clamp; if the cable is multi-core, the non-test core wire needs to be short-circuited and grounded; the end core wire of the cable to be tested is connected to the special grounding electrode on site through an insulation cable. The above connection process is the prior art, and will not be described in detail here.

[0023] In an embodiment of the present application, the cable monitoring points are dynamically set according to the length of the cable, for example, one monitoring point is set at the first end, the middle end and the end of the cable respectively when the length of the cable is within 100 meters, and one monitoring point is set every 50 meters when the length of the cable is greater than 100 meters; each monitoring point is sleeved with a current transformer and placed with a sound sensor for respectively collecting the current signal and the discharge sound of the monitoring point.

[0024] S200, set the test voltage and the voltage boosting rate of the cable test, control the test equipment to find the resonance point in the frequency scanning interval, so that the test loop is in a resonant state.

[0025] Considering that the insulation performance of the cable is significantly affected by environmental conditions, temperature rise will reduce the insulation resistance, and humidity increase will accelerate the insulation aging, if only the rated voltage of the cable is used to set the fixed test voltage without considering the environmental deviation, the test voltage and the actual working condition will not match, in which case, the insulation defects may be missed due to insufficient voltage in high temperature and high humidity environment, and the cable insulation may be damaged due to excessive voltage in low temperature and low humidity environment, even causing safety risks in test.

[0026] Therefore, dynamically correcting the test voltage in combination with the environmental parameters is the premise to ensure the test accuracy and avoid safety hazards.

[0027] Based on this, in an embodiment of the present application, S210, the determination method of the test voltage is as follows: obtaining the environmental parameters of the test site, analyzing the voltage compensation coefficient based on the environmental parameters; the environmental parameters include the temperature and humidity of the test site.

[0028] The basic test voltage of the cable is determined according to the rated voltage of the cable.

[0029] The product of the voltage compensation coefficient plus 1 and the basic test voltage is taken as the test voltage.

[0030] It can be understood that the basic test voltage of the cable is a multiple of the industry standard, and the multiples of the basic test voltage of cables of different voltage grades are slightly different. For example, the basic test voltage of a 10KV cross-linked polyethylene cable is 2 times its rated voltage, and the basic test voltage of a low-voltage cable below 1KV is 2.5 times its rated voltage. The basic test voltage of the cable is set as a known technology in the industry, and will not be described in detail here.

[0031] As an embodiment of the present application, S211, the analysis method of the voltage compensation coefficient is to obtain temperature data and humidity data in the environmental parameters, and read the optimal working temperature and optimal working humidity of the test equipment. The temperature data is compared with the optimal working temperature, and the temperature influence degree is determined according to the temperature deviation. The humidity influence degree is determined in the same way. The optimal working temperature and the optimal working humidity are the optimal working temperature and the optimal working humidity of the test equipment, which are obtained from the product specification or the product nameplate calibration of the equipment manufacturer when the equipment is shipped.

[0032] Considering that the temperature and humidity values will affect the dielectric strength of common power cable insulation materials, generally, for every 1 degree Celsius increase in temperature, the dielectric strength of the cable decreases by about 0.5%-1%, and for every 1% increase in humidity, the dielectric strength decreases by about 0.2%-0.4%.

[0033] In the present application, the determination method of the temperature influence degree is: when the temperature deviation is within 3℃, it is determined that the temperature has no influence; when the temperature deviation is within 3℃-5℃, it is determined that the temperature has weak influence; when the temperature deviation is within 5℃-10℃, it is determined that the temperature has moderate influence; and when the temperature deviation is greater than 10℃, it is determined that the temperature has severe influence. Similarly, the determination method of the humidity influence degree is: when the humidity deviation is within 5%, it is determined that the humidity has no influence; when the humidity deviation is within 5%-10%, it is determined that the humidity has weak influence; when the humidity deviation is within 10%-20%, it is determined that the humidity has moderate influence; and when the humidity deviation is greater than 20%, it is determined that the humidity has severe influence. It should be noted that the lower limit of the above range is not included, and the upper limit of the range is included.

[0034] The compensation coefficient of temperature without influence and humidity without influence is 0, the compensation coefficient of temperature weak influence and humidity weak influence is 0.03, the compensation coefficient of temperature moderate influence and humidity moderate influence is 0.05, and the compensation coefficient of temperature severe influence and humidity severe influence is 0.08.

[0035] The compensation unit is determined according to upward deviation or downward deviation of temperature and humidity, that is, the test voltage needs to be increased for upward deviation, and the test voltage needs to be reduced for downward deviation, the corresponding specific compensation coefficients are determined according to the temperature influence degree and the humidity influence degree, and the final voltage compensation coefficient is obtained by mean calculation of the two compensation coefficients.

[0036] For example, the optimal working temperature of the experimental equipment of the 10KV cross-linked polyethylene cable is 20 DEG C, and the optimal working humidity is 50%; if the working environment temperature is 26 DEG C and the working environment humidity is 60%, the temperature deviation value and the humidity deviation value are 8 DEG C and 10% respectively, therefore, the compensation coefficient corresponding to the temperature influence degree of the working environment temperature is 0.05, the compensation coefficient corresponding to the humidity influence degree of the working environment humidity is 0.03, and the voltage compensation coefficient is the mean of the two, that is, 0.04; therefore, if the basic test voltage of the 10KV cross-linked polyethylene cable is 20KV, the test voltage thereof is 20.8KV.

[0037] The application introduces the voltage compensation coefficient based on the basic test voltage, on the one hand, the influence of temperature and humidity on insulation is corrected by the voltage compensation coefficient, so that the test voltage is fitted to the real-time working condition, and at the same time, the false judgment and the missed judgment caused by the fixed voltage are avoided, so that the defective cable is effectively identified and the qualified cable is normally passed; on the other hand, the upper and lower limit values are set to prevent excessive compensation, balance the test effectiveness and the cable protection, neither blindly increase the voltage nor reduce the standard, and the application is suitable for different climate conditions and different types of cable test scenes, and the basic rules do not need to be frequently adjusted.

[0038] Since the voltage rising rate is a key parameter for controlling the rhythm of voltage rising in the power cable withstand voltage test, and directly affects the test safety and the insulation defect identification effect, therefore, the setting based on the dynamic adjustment of humidity can make the cable rise voltage at a relatively reasonable speed, which is helpful for the accurate identification of cable defects.

[0039] Considering that the temperature affects the dielectric loss and the long-term voltage endurance in the process of cable voltage rising, and has weak influence on the voltage rising moment, but in the high humidity environment, the cable insulation surface is easy to condense water film or adsorb water vapor, forming a local conductive channel, leading to the increase of the instantaneous breakdown probability of insulation, therefore, the influence of temperature is negligible, and the humidity of the test environment is only considered in the process of voltage rising.

[0040] Based on this, in one embodiment of the application, S220, the specific setting step of the voltage rising rate is: calculating the deviation amount of humidity in the environmental parameter from the optimal working humidity.

[0041] The deviation amount is compared with the preset humidity deviation threshold range, if the deviation amount is less than the humidity deviation threshold range, the voltage rising is carried out according to the rated voltage rising rate of the cable.

[0042] If the deviation is within the humidity deviation threshold range, a first compensation amount is added based on the rated pressurization rate.

[0043] If the deviation exceeds the humidity deviation threshold range, a second compensation amount is added based on the rated pressurization rate.

[0044] It should be noted that the basic voltage rise rate of the cable is determined by the cable's test voltage. For example, the voltage rise rate of a 10KV cross-linked polyethylene cable is 2kV per second. In addition, the humidity deviation threshold range mentioned above is selected as 10%-20%, the first compensation is 0.03 times the rated voltage rise rate, and the second compensation is 0.05 times the rated voltage rise rate. Operators can make dynamic adjustments according to the actual test conditions.

[0045] This invention, through a voltage ramp setting method, on the one hand, dynamically corrects the compensation value to match the voltage ramp rhythm with the cable insulation layer's tolerance characteristics under the current humidity; on the other hand, it can avoid accidental insulation breakdown caused by excessively rapid voltage ramping in high humidity environments, thus protecting the test equipment and the cable itself.

[0046] Considering that the resonant point is the core parameter for achieving resonant voltage boost in the withstand voltage test of power cables, a stable test voltage can only be generated with a small equipment capacity when the test circuit is in a resonant state, thus avoiding voltage fluctuations or equipment overload in the non-resonant state.

[0047] S230, the specific steps for the test equipment to find the resonant point are as follows: S231, read the length and capacitance per unit length of the cable under test according to its model, multiply the two as the total capacitance of the cable and record it as C, combine it with the inductance parameter of the resonant reactor in the test equipment and record it as L, and then use the series resonant frequency formula... The theoretical resonant frequency is obtained. And based on this, the frequency scanning range is determined.

[0048] S232, within the frequency scanning range, the control test equipment uses a coarse scanning step size to sweep the frequency, collects the current signal of the test circuit in real time, synchronously monitors the phase difference between the output voltage of the frequency converter and the circuit current, and generates a frequency-current curve accordingly.

[0049] It should be noted that the coarse scan step mentioned above is set empirically, usually 1 / 5 to 1 / 10 of the frequency range width, to ensure coverage of key frequency points within the range.

[0050] S233 identifies the current peak in the frequency-current curve and sets the frequency corresponding to the current peak as the candidate resonant point.

[0051] S234, extracting the slope feature corresponding to the current peak on the frequency-current curve, and according to the slope feature, centering on the candidate resonance point, the resonance candidate interval is delimited; specifically, the current slope on the left side of the candidate resonance point and the current slope on the right side of the candidate resonance point on the frequency-current curve are read, and the change rate of the current per unit frequency is calculated, if the change rate is greater than 5A / Hz, it indicates that the resonance peak is sharp, if the change rate is less than 2A / Hz, it indicates that the resonance peak is flat, for the side of the sharp resonance peak, the candidate region is moved 5Hz to the side, for the side of the flat resonance peak, the candidate region is moved 8Hz to the side, for example, if the current slope on the left side of the candidate resonance point is flat and the current slope on the right side of the candidate resonance point is sharp, the resonance candidate interval is [candidate resonance point-8Hz, candidate resonance point+5Hz].

[0052] S235, in the resonance candidate interval, fine frequency sweeping is carried out with a fine scanning step smaller than the coarse scanning step, and the current value and the phase difference at each frequency are collected; it should be noted that the fine scanning step is 1 / 10 of the width of the resonance candidate interval.

[0053] S236, the data collected by fine frequency sweeping is analyzed by using the quadratic interpolation method to determine the frequency corresponding to the current peak; and the frequency is taken as the actual resonance point; specifically, the data with the phase difference closest to 0 (not less than 10 groups) in the fine scanning data is screened, and the frequency value that makes the current maximum is solved by using the quadratic interpolation formula on the screened data as the actual resonance point.

[0054] S2311, the specific determination method of the frequency scanning interval is: based on the theoretical resonance frequency and the historical resonance frequency of the same type of cable, a frequency offset coefficient is set; specifically, the historical resonance frequency of the same type of cable is read, the deviation rate of the historical resonance frequency from the theoretical resonance frequency is calculated, and the average deviation rate is obtained by taking the average, and the sum of the average deviation rate and the reference deviation rate is taken as the frequency offset coefficient; it should be noted that the reference deviation rate is a set value, which is valued in the range of 5%-10%, and the staff can set the value in this range, for example, the reference deviation rate in the present application is valued at 6%.

[0055] The product of the theoretical resonance frequency and the frequency offset coefficient is taken as the frequency offset, the initial frequency interval is determined, and the upper limit of the interval is the sum of the theoretical resonance frequency and the frequency offset, and the lower limit is the difference between the theoretical resonance frequency and the frequency offset.

[0056] The initial frequency interval is compared with the effective frequency interval of the test device, if the initial frequency interval is within the effective frequency interval, the initial frequency interval is taken as the frequency scanning interval, otherwise, the effective frequency interval of the test device is taken as the frequency scanning interval, it is necessary to point out that if the upper and lower limits of the initial frequency interval are lower than the lower limit of the effective frequency interval of the test device or higher than the upper limit of the effective frequency interval of the test device, the upper limit or the lower limit value of the test device is taken as the upper limit or the lower limit of the frequency scanning interval.

[0057] It is necessary to point out that the historical data should be selected from the same type and the same working condition of the cable to avoid the deviation rate calculation distortion caused by the working condition difference; at the same time, the offset coefficient should be a positive value to ensure that the interval is symmetrically centered on the theoretical frequency.

[0058] Compared with the traditional full-interval fine scanning or experience estimation method, the three-level calibration mode of the present application can accurately determine the resonance point through coarse scanning to narrow the range, fine scanning to refine the data and secondary interpolation calculation, ensure that the loop is accurately in the resonance state, and the present application can quickly locate the resonance candidate interval with a large step size, and then perform fine scanning, which is more efficient than full-interval fine scanning; the present application sets the scanning interval in combination with the total capacitance of the cable and the historical resonance data, which can adapt to cables of different voltage grades without frequent adjustment of the frequency scanning strategy.

[0059] S300, the test loop is boosted at a boosting rate; during the boosting stage, the current data and the discharge sound of each monitoring point are monitored synchronously, the abnormality degree of the boosting stage is analyzed based on the monitoring data, and it is judged whether to continue boosting according to the abnormality degree, if yes, the test voltage is boosted and then pressure maintaining is performed, otherwise, the boosting is stopped and it is determined that the cable is unqualified for pressure resistance.

[0060] The boosting stage is a key link for exposing the insulation defect of the power cable, the abnormality degree analysis of the present application quantitatively evaluates the risk by synchronously monitoring the discharge sound and the current data, and directly determines whether to continue boosting and the subsequent test direction.

[0061] If the above analysis is lacking, the cable may be broken due to environmental noise misjudgment, defect omission or excessive boosting, therefore, the analysis step is a technical link for ensuring the accuracy of the test, the safety of the equipment and the effectiveness of the defect positioning.

[0062] Considering that the insulation defect of the cable during the boosting stage will be accompanied by weak discharge sound and current fluctuation, but single parameter monitoring is easy to be disturbed, only monitoring the sound may misjudge the equipment running noise as defect discharge, and only monitoring the current may miss the small current fluctuation and obvious discharge defect, resulting in inaccurate judgment of the cable qualification. Through the above multi-parameter correlation analysis, the interference can be effectively eliminated to ensure the accuracy of defect identification.

[0063] In addition, if the abnormality degree is not analyzed in time, the voltage of the high abnormality risk cable continues to increase, which may cause the insulation defect to expand rapidly, and thus overcurrent of the test circuit, cable burning, device tripping and asset loss.

[0064] Therefore, as an embodiment of the present application, the specific analysis of the abnormality degree in the voltage increasing stage is as follows: S310, at each collection time period, the discharge sound of each monitoring point is collected synchronously, and the collected sound signal is filtered to obtain effective sound; specifically, the collected sound signal is processed by wavelet threshold filtering, that is, a corresponding threshold is set to remove 50Hz power frequency interference and low frequency background noise, and effective sound above 60dB is retained.

[0065] S320, the instantaneous decibel value and the duration of the effective sound signal of each monitoring point are extracted, and the decibel change curve of each monitoring point is constructed; the difference of the decibel values of each monitoring point at the same collection time period is calculated, and the attenuation trend of the decibel value with the position of the monitoring point is analyzed, because the sound of the defect point is the highest, the sound gradually attenuates to both sides, and thus the monitoring point corresponding to the effective sound is determined.

[0066] In the embodiment of the present application, S321, the specific way of determining the monitoring point corresponding to the effective sound is as follows: if the decibel value of a certain monitoring point is higher than the decibel values of all other monitoring points, and the difference between the decibel value and the decibel value of the adjacent monitoring point is greater than a first set decibel value, it is determined that the effective sound is close to the monitoring point.

[0067] If the difference between the decibel values of two or more adjacent monitoring points is less than a second set decibel value, and all are higher than other monitoring points, it is determined that the effective sound is located in the interval between the adjacent monitoring points; the setting of the above-mentioned first set decibel value and the second set decibel value is associated with the interval of the monitoring points; for example, when one monitoring point is set every 50 meters, due to the reason of sound attenuation, the sound attenuation is greater than 5 decibels beyond 50 meters, and therefore the first decibel value is set to 5 decibels, that is, the decibel value of a certain monitoring point is higher than the decibel value of the adjacent monitoring point by 5 decibels, and then the effective sound is near the monitoring point, if the difference between the decibel values of two or more adjacent monitoring points is less than 4 decibels, the effective sound is between the two adjacent monitoring points.

[0068] S330, the current data of each monitoring point is extracted, the fluctuation trend of the current data is analyzed, and based on the fluctuation trend of the current data of the monitoring point corresponding to the effective sound in the time period, it is determined that the effective sound is associated with the cable insulation abnormality.

[0069] It can be understood that the main purpose of the above steps is to distinguish the real insulation defect discharge sound and the environmental interference sound. When there is only effective sound and the corresponding current fluctuation of the monitoring point has time synchronization and amplitude correlation, it can be determined that the sound is caused by the cable insulation anomaly. If there is only sound without synchronous current fluctuation, or only current fluctuation without sound, it is determined as a disturbance signal and is not included in the abnormality degree calculation. Further, the key verification link of avoiding misjudgment of qualified cable as unqualified or missing defect cable directly determines the accuracy of subsequent abnormality degree evaluation.

[0070] S340, based on the associated effective sound and current data, the standard deviation of the effective sound decibel value and the standard deviation of the current value are calculated respectively, and the standard deviation of the effective sound decibel value and the standard deviation of the current value are linearly weighted. The calculation result is used as an abnormality index. Wherein, if the standard deviation of the decibel value of the effective sound is greater than the standard deviation of the current value of the corresponding monitoring point, the weight corresponding to the standard deviation of the decibel value of the effective sound is also greater than the weight corresponding to the standard deviation of the current value of the corresponding monitoring point, and the sum of the two weights is 1.

[0071] S350, the step of judging whether to continue to boost according to the abnormality degree is: if the abnormality degree of the monitoring point is lower than the first threshold value, it is determined that the cable state is normal, and the boosting is allowed to continue.

[0072] If the abnormality degree is between the first threshold value and the second threshold value, a warning prompt is sent, and the boosting is allowed to continue.

[0073] If the abnormality degree is higher than the second threshold value, the boosting operation is immediately stopped, the monitoring point is marked as a high-risk area, and it is determined that the cable test is unqualified.

[0074] For example, the first threshold value and the second threshold value are set based on historical data. Specifically, by collecting historical test data of the same type of cable under standard test conditions, the data at least includes the abnormality index calculated by each monitoring point in the boosting stage, and the corresponding conclusion of whether the cable finally passes the test; based on the final test conclusion, the historical abnormality index is classified into normal index set, warning index set and unqualified index set; based on the classified index set, the statistical characteristic value is calculated, including the maximum value of the normal index set, the minimum value and the maximum value of the warning index set, and the minimum value of the unqualified index set. According to the statistical characteristic value, the average value of the maximum value of the normal index set and the minimum value of the warning index set is taken as the first threshold value, and the average value of the maximum value of the warning index set and the minimum value of the unqualified index set is taken as the second threshold value.

[0075] The application can reduce the defect range to a single monitoring point or an adjacent monitoring point interval based on the attenuation trend of the decibel value with the position of the monitoring point, and then locate the abnormal point of the cable, and provide a basis for subsequent processing of the cable. The application can determine whether the cable meets the standard for subsequent voltage test by analyzing the abnormal degree, and prevent unqualified cables from subsequent testing to reduce the efficiency of the test.

[0076] Considering that the latent defect of the cable insulation will only produce weak electrical signal fluctuations under the dynamic high voltage in the voltage boosting stage, it is difficult to be effectively identified. The stable high voltage in the pressure maintaining stage will make the defect develop continuously, and the electrical signal parameter will be amplified, and finally be accurately captured. Therefore, the electrical signal parameter of the cable needs to be monitored and analyzed during the pressure maintaining test stage, and whether the cable is qualified needs to be determined.

[0077] In the embodiment of the application, S400, the electrical signal parameters of each monitoring point of the cable are monitored during the pressure maintaining stage, and the qualified condition of the voltage test of the cable is determined based on the abnormal degree and the electrical signal parameters.

[0078] The specific steps of determining the qualified condition of the voltage test of the cable are as follows: collecting the electrical signal parameters of each monitoring point, and the electrical signal parameters include voltage, current and their phase relationship.

[0079] By comparing the electrical signal parameters of each monitoring point with the standard threshold range, it is determined whether there is a deviation phenomenon. If the voltage, current and their phase relationship are not within the standard threshold range, the related electrical signal parameters of the cable have a deviation phenomenon.

[0080] If the electrical signal parameters of all monitoring points are within the standard threshold range, it is determined that the voltage test of the cable is qualified.

[0081] If the electrical signal parameters of a certain monitoring point exceed the threshold range, the cable area corresponding to the monitoring point is located, and is marked as a potential fault point.

[0082] The voltage performance of the cable is evaluated in combination with the abnormal degree and the deviation of the electrical signal parameters.

[0083] In other embodiments of the present application, according to the cable test standard, the voltage deviation degree of the cable in the pressure maintaining stage is not more than ±2% of its test voltage, the current deviation degree is not more than ±5% of its initial current, and the phase relationship deviation degree is not more than ±1 degree, and the deviation degrees of the voltage, the current and the phase relationship are calculated according to the monitored voltage, the current and the phase relationship and their corresponding deviation degrees, for example, the voltage deviation degree is calculated, the voltage deviation amount is obtained by multiplying the test voltage and the voltage deviation degree 2%, and then the absolute value of the difference between the test voltage and the measured voltage is taken as the voltage deviation amount, and the ratio of the voltage deviation amount to the voltage deviation amount is taken as the voltage deviation degree, and if the voltage deviation degree is more than 1, the voltage deviation phenomenon exists; similarly, the deviation degrees of the current and the phase relationship are calculated; the comprehensive deviation degree is obtained by averaging the voltage deviation degree, the current deviation degree and the phase relationship deviation degree, and if the cable comprehensive deviation degree is greater than 1 and the abnormality degree in the voltage boosting stage is normal, the position of the monitoring point is marked, and the cable at the position is manually checked to determine whether the cable is qualified according to whether the cable is broken down.

[0084] The present application adopts comprehensive analysis of multiple electrical signal parameters, and integrates the abnormality degree in the voltage boosting stage, thereby preventing the situation that the latent defect of the cable is delayed to be exposed, and preventing the risk of misjudgment of instantaneous interference, and increasing the accuracy of the cable withstand voltage test.

[0085] The present application quantifies the deviation degree of the electrical signal parameters through comprehensive analysis, so that the cable qualification determination is free of subjective deviation, and the qualification is converted into specific numerical comparison through deviation degree calculation; and through separate analysis of each monitoring point, the potential fault point can be accurately positioned, and the maintenance efficiency is increased.

[0086] S500, after the test is completed, the cable is discharged by using the discharge rod. When discharging the cable, the discharge rod end grounding clamp is reliably clamped on the special grounding electrode by holding the discharge rod insulating rod, then the discharge rod metal discharge head is close to the cable head end core terminal to keep the discharge head in contact with the head end core terminal, resistance pre-discharge is performed, after pre-discharge is completed, the cable is short-circuited and discharged, after discharging is completed, the voltage between the cable head end core and the grounding electrode is measured by using a multimeter until the residual voltage is less than 50V, and then the discharge operation on the cable is completed.

[0087] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application, which are still covered by the protection scope of the present application.

Claims

1. A method for withstand voltage testing of power cables, characterized in that, include: Connect the cable to be tested to the test equipment to form a test circuit, and set up multiple monitoring points evenly according to the length of the cable; Set the test voltage and voltage boosting rate for cable testing, and control the test equipment to find the resonant point within the frequency scanning range so that the test circuit is in a resonant state; The test circuit is boosted at a boost rate. During the boost phase, the current data and discharge sound at each monitoring point are monitored simultaneously. The degree of abnormality during the boost phase is analyzed based on the monitoring data. The degree of abnormality is then used to determine whether to continue boosting. If so, the voltage is held after boosting to the test voltage. Otherwise, the boosting is stopped and the cable is deemed to be unqualified in withstand voltage. During the pressure holding phase, the electrical signal parameters at each monitoring point of the cable are monitored, and the pass / fail status of the cable withstand voltage test is determined based on the degree of abnormality and the electrical signal parameters. After the test is completed, use a discharge rod to fully discharge the cable; The specific steps for the experimental equipment to find the resonance point are as follows: Based on the length and capacitance per unit length of the cable under test, the total capacitance of the cable is calculated. Combined with the inductance parameters of the resonant reactor in the test equipment, the theoretical resonant frequency is analyzed, and the frequency scanning range is determined accordingly. Within the frequency scanning range, the control test equipment uses a coarse scanning step size to sweep the frequency, acquires the current signal of the test circuit in real time, synchronously monitors the phase difference between the output voltage of the frequency converter and the circuit current, and generates a frequency-current curve accordingly. Identify the current peak in the frequency-current curve and set the frequency corresponding to the current peak as the candidate resonant point; Extract the slope feature of the current peak on the frequency-current curve, and based on the slope feature, define the candidate resonance interval with the candidate resonance point as the center; Within the candidate resonance range, a fine scanning step size smaller than the coarse scanning step size is used to perform a fine frequency sweep, and the current value and phase difference at each frequency are collected. The data collected by the fine frequency sweep was analyzed using the quadratic interpolation method to determine the frequency corresponding to the current peak value; and this frequency was taken as the actual resonant point. The specific method for determining the frequency scanning interval is as follows: Based on the theoretical resonant frequency and the historical resonant frequencies of similar cables, a frequency offset coefficient is set. The initial frequency range is determined by multiplying the theoretical resonant frequency and the frequency offset coefficient, with the theoretical resonant frequency as the center. The upper limit of this range is the sum of the theoretical resonant frequency and the frequency offset, and the lower limit is the difference between the theoretical resonant frequency and the frequency offset. The initial frequency range is compared with the effective frequency range of the test equipment. If the initial frequency range is within the effective frequency range, the initial frequency range is used as the frequency scanning range; otherwise, the effective frequency range of the test equipment is used as the frequency scanning range.

2. The method for withstand voltage testing of power cables according to claim 1, characterized in that, The test voltage is determined as follows: Obtain the environmental parameters of the test site, and analyze the voltage compensation coefficient based on the environmental parameters; Determine the basic test voltage of the cable based on its rated voltage; The product of the voltage compensation coefficient plus 1 and the basic test voltage is used as the test voltage.

3. The method for withstand voltage testing of power cables according to claim 1, characterized in that, The analysis method for the voltage compensation coefficient is as follows: Acquire temperature and humidity data from environmental parameters, and read the optimal operating temperature and humidity of the test equipment; The temperature data is compared with the optimal operating temperature, and the degree of temperature influence is determined based on the temperature deviation. Similarly, the degree of humidity influence is determined. The voltage compensation coefficient is determined based on the degree of influence of temperature and humidity.

4. The method for withstand voltage testing of power cables according to claim 3, characterized in that, The specific steps for setting the boost rate are as follows: Calculate the deviation of the humidity parameter from the optimal operating humidity. The deviation is compared with a preset humidity deviation threshold range. If the deviation is less than the humidity deviation threshold range, the voltage is increased according to the rated voltage increase rate of the cable. If the deviation is within the humidity deviation threshold range, then a first compensation amount is added based on the rated pressurization rate; If the deviation exceeds the humidity deviation threshold range, a second compensation amount is added based on the rated pressurization rate.

5. The method for withstand voltage testing of power cables according to claim 1, characterized in that, The specific steps for analyzing the degree of abnormality during the boost phase are as follows: During each acquisition period, the discharge sound of each monitoring point is collected synchronously, and the collected sound signals are filtered to obtain the effective sound. Extract the instantaneous decibel value and duration of the effective sound signal at each monitoring point, and construct the decibel change curve for each monitoring point; calculate the difference in decibel value between each monitoring point during the same acquisition time period, analyze the attenuation trend of the decibel value with the location of the monitoring point, and determine the monitoring point corresponding to the effective sound. Extract the current data from each monitoring point, analyze the fluctuation trend of the current data, and determine whether the effective sound is related to the cable insulation abnormality based on the fluctuation trend of the current data of the monitoring point corresponding to the effective sound in that time period. Based on the correlated valid sound and current data, the degree of anomaly at each monitoring point is calculated.

6. The method for withstand voltage testing of power cables according to claim 5, characterized in that, The specific method for determining the monitoring point corresponding to the valid sound is as follows: If the decibel value of a certain monitoring point is higher than the decibel values ​​of all other monitoring points, and the decibel difference between this decibel value and the adjacent monitoring points is greater than the first set decibel value, then it is determined that a valid sound is approaching the monitoring point. If there are two or more adjacent monitoring points with a decibel value difference less than the second set decibel value, and both are higher than other monitoring points, then the effective sound is determined to be located within the interval between the adjacent monitoring points.

7. The method for withstand voltage testing of power cables according to claim 1, characterized in that, The step of determining whether to continue increasing the pressure based on the degree of abnormality is as follows: If the degree of abnormality at the monitoring point is lower than the preset first threshold, the cable is determined to be in normal condition, and voltage increase is allowed. If the degree of abnormality is between the first threshold and the second threshold, a warning will be issued; If the degree of abnormality exceeds the second threshold, the voltage boosting operation will be stopped immediately, the monitoring point will be marked as a high-risk area, and the cable will be deemed unqualified for testing.

8. The method for withstand voltage testing of power cables according to claim 5, characterized in that, The specific steps for determining the pass / fail status of the cable withstand voltage test are as follows: The electrical signal parameters of each monitoring point are collected, including voltage, current and their phase relationship; By comparing the electrical signal parameters of each monitoring point with the standard threshold range, it can be determined whether there is any deviation. If the electrical signal parameters at all monitoring points are within the standard threshold range, the cable withstand voltage test is deemed qualified. If the electrical signal parameters at a certain monitoring point exceed the threshold range, the cable area corresponding to that monitoring point is located and marked as a potential fault point. The withstand voltage performance of the cable is evaluated by combining the degree of anomaly with the deviation of electrical signal parameters.

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