This scheme is applicable to the handover test, preventive test and post-repair verification test of 6kV to 220kV cross-linked polyethylene (XLPE) insulated power cables. It adopts the variable frequency series resonance test method and is formulated in accordance with GB 50150-2016 “Standard for Handover Test of Electrical Equipment in Electrical Installation Engineering”, DL/T 474.4-2018 “On-site Insulation Test Implementation Guidelines – AC Withstand Voltage Test” and DL/T 596-2021 “Regulations for Preventive Test of Electrical Equipment”, aiming to detect the air gap, moisture, aging and joint-making defects of the main insulation of the cable, and ensure that the insulation performance of the cable meets the operation requirements.
I. Test Principle
Based on the principle of series resonance circuit, the output frequency of the variable frequency power supply is adjusted to make the inductive reactance of the reactor in the test circuit equal to the capacitive reactance of the tested cable, and the circuit reaches the resonant state. At this time, the total impedance of the circuit is the smallest and the current is relatively large, and the high voltage several times that of the excitation voltage can be obtained at both ends of the cable, and the output voltage waveform is close to a sine wave (distortion rate ≤ 1%). When the test sample is broken down, the resonant condition is destroyed and the voltage drops automatically, which can effectively prevent the expansion of the accident. At the same time, the capacity requirement of the test power supply is much lower than that of the traditional power frequency withstand voltage test.
II. Test Preparation
(1) Confirmation of Test Object Parameters
Check the basic information of the tested cable in advance, including model and specification, rated voltage (U0/U), length, laying method, operation years and the type of this test (handover/preventive test), record the number and manufacturing process of the cable terminals and intermediate joints, and mark the position of the interconnection box for cross-interconnected cables.
(2) Selection and Inspection of Test Equipment
The core equipment is the variable frequency series resonance test device (including variable frequency power supply, excitation transformer, series reactor, and high-voltage capacitive voltage divider), the rated voltage and capacity of which should match the tested cable, with a quality factor (Q value) ≥ 30 and a frequency adjustment range of 20 to 300Hz. Auxiliary equipment includes 2500V/5000V megohmmeters, dedicated discharge rods, grounding electrodes, thermohygrometers, infrared thermal imagers, and partial discharge detectors (optional).
Equipment inspection should confirm that all equipment is undamaged in appearance, the connection terminals are firmly connected, the megohmmeter is calibrated and qualified, the protection functions of the resonance device (overvoltage, overcurrent, flashover, temperature rise protection) are normal, and the insulation resistance of the reactor is ≥ 1000MΩ.
(3) On-site Preparation and Safety Arrangement
Disconnect the connection between both ends of the tested cable and the power grid and distribution equipment, reliably ground the non-test phases, and ensure that the cable terminal heads are clean and dry, free of dirt, cracks and condensation. Define the test area, set up hard barriers, and hang “High Voltage Danger, No Entry” warning signs. The safety distance between the test area and the live body should meet the following requirements: 10kV ≥ 3 meters, 35kV ≥ 5 meters, 110kV and above ≥ 8 meters.
Set up an independent grounding electrode to ensure that the shells of the test equipment, capacitive voltage dividers, and cable shielding layers are grounded at one point, with a grounding resistance ≤ 4Ω. The test site should be equipped with safety protection equipment such as insulating gloves and insulating boots, and a dedicated person should be assigned for supervision. Personnel should be stationed at both ends of the cable.
The test environment should meet the following requirements: temperature -10℃ to 40℃, relative humidity ≤ 80%, and tests should not be conducted in rainy, foggy, snowy or windy weather. In coastal or high-humidity environments, the cable terminal heads should be wrapped with silica gel blankets.
(4) Pre-test Detection
Use a megohmmeter to measure the insulation resistance of each phase of the tested cable. For 10kV cables, the insulation resistance should be ≥ 1000MΩ, and for 35kV and above cables, it should be ≥ 10000MΩ. If the insulation resistance is not significantly low, proceed to the formal test; otherwise, insulation defects should be investigated.
III. Test Equipment Parameter Configuration
According to the voltage level and length of the cable, the reactors (series/parallel) should be reasonably combined to match the cable capacitance, ensuring that the resonant frequency is in the 30 to 75Hz section close to the power frequency. For cables of different voltage levels, the typical configurations are as follows: For 10kV (8.7/10kV) cables, the handover test parameters are 2.5U0 = 21.75kV for 60 minutes, and the pre-test parameters are 3.5U0 = 30.5kV for 5 minutes. The reactor configuration is recommended as two 10kV/1.5A reactors in parallel, with a resonant frequency range of 100 to 200Hz. For 35kV (26/35kV) cables, both the handover and pre-test parameters are 1.7U0 = 44.2kV, for 60 minutes and 30 minutes respectively. The reactor configuration is recommended as four 26kV/1A reactors in series, with a resonant frequency range of 40 to 80Hz. For 110kV (64/110kV) cables, the handover test parameters are 1.7U0 = 108.8kV for 60 minutes, and the reactor configuration is recommended as six 50kV/1A reactors in parallel, with a resonant frequency range of 20 to 50Hz.
Note: For short cables (less than 300m), if the capacitance is too small, an internal compensation capacitor in the device should be used to increase the resonant current and prevent protection misoperation. For long cables (more than 1km), tests should be conducted in sections, with each section not exceeding 600m.
IV. Test Operation Procedures
(1) Test Circuit Wiring
Connect the main circuit in series in the order of variable frequency power supply → excitation transformer → series reactor → test cable → high-voltage capacitive voltage divider → grounding electrode. Connect the voltage signal of the capacitive voltage divider to the variable frequency power supply. Ensure that all connection terminals are tight and not loose. After wiring, have two people cross-check to avoid incorrect or missing connections.
(2) Resonance Tuning
Turn on the variable frequency power supply, set the frequency scanning range to 20 to 300Hz, and adjust the output voltage to 10% to 30% of the test voltage. Slowly adjust the frequency. Observe the changes in circuit current and voltage. When the current is at its minimum and the voltage at both ends of the cable is relatively high, the circuit reaches the resonant state. Lock the resonant frequency and record the frequency and current values at this time. If the resonant point cannot be found, check the reactor combination method or whether the cable is open-circuited. After tuning, confirm that the voltage waveform distortion rate is ≤1%, and the baseline of the partial discharge detector (if used) is stable with no stray interference.
(3) Voltage Rise and Withstand Voltage Test
Slowly increase the voltage at a rate of 1 to 2kV/s, and monitor the test voltage, current, resonant frequency, and equipment temperature rise throughout the process. Rapid voltage increase or skipping voltage levels is strictly prohibited. After reaching the test voltage, maintain the specified withstand voltage time. During this period, continuously observe the following: ① Whether the voltage and current are stable without sudden rises or drops; ② Whether there is no discharge, abnormal sounds, or burnt smell from the cable and joints; ③ Whether the surface temperature rise of the reactor is <30°C and the temperature is ≤70°C; ④ Whether the partial discharge quantity is ≤5pC (handover test) / ≤10pC (pre-test). If any abnormality occurs during the withstand voltage test, immediately press the “emergency stop” button, stop the voltage increase, and reduce the voltage to zero. Do not handle faults under pressure.
(4) Voltage Reduction and Discharge
After the withstand voltage time is reached, slowly reduce the voltage to less than 10% of the test voltage at a uniform rate. Directly cutting off the power to reduce the voltage is strictly prohibited. Turn off the variable frequency power supply and use a dedicated discharge rod to fully discharge the test cable, reactor, and capacitive voltage divider multiple times. The discharge time should be no less than 5 minutes. Only after confirming that there is no residual charge can the test equipment and cable be touched.
(5) Re-test
After discharging, measure the insulation resistance of the cable again using a megohmmeter and compare it with the value before the test. If there is no significant decrease (decrease ≤5%), the single-phase reverse test is completed. Repeat the above operations for the other two phases in sequence.
V. Test Result Evaluation
(1) Qualification Criteria
During the withstand voltage test, there is no breakdown or flashover in the cable, the voltage and current are stable without abnormal fluctuations; the insulation resistance of the cable does not significantly decrease before and after the test, and the partial discharge quantity meets the specified requirements; there are no fault alarms from the equipment, and the reactors and excitation transformers have no overheating or abnormal sounds, etc., then it is judged as qualified.
(2) Unqualified Criteria and Handling
If any of the above conditions are not met, it is judged as unqualified. The cause should be identified and the necessary measures taken to rectify the issue. If the problem cannot be resolved, the cable should be replaced.
During the withstand voltage test, if there is a sudden drop in voltage, a sudden increase in current, or obvious discharge or breakdown sounds from the cable, it is determined that the insulation has broken down. The voltage value at the time of breakdown should be recorded, and the fault point (intermediate joint/terminal joint/cable body) should be located using the segmented test method. If the partial discharge exceeds the standard or the insulation resistance drops significantly, it is determined that the insulation has deteriorated. The cable should be dried, repaired or the defective part replaced, and the test should be repeated after the treatment is completed. If the equipment temperature rise exceeds the standard or the protection alarm is triggered during the test, the machine should be stopped immediately for cooling, and the equipment fault or test circuit problem should be investigated. The test can be resumed after the situation returns to normal.
VI. Special Scene Handling
Cross-Connected Cables: Before the test, the interconnection box should be disconnected. When testing each section, the non-tested sections should be temporarily grounded to avoid induced voltage (the induced voltage of the unconnected sections can reach 5kV or more). After the test is completed, the interconnection box wiring should be restored.
Old Cables (in operation for ≥8 years): The test voltage should be appropriately reduced (execute at 2.5U0) to prevent accelerated insulation deterioration. A 0.1Hz ultra-low frequency withstand voltage test can be conducted as a pre-inspection, followed by a series resonance test.
Cross-Sea/Long-Distance Cables: Before the test, the cable capacitance should be re-measured in sections using an LCR bridge. The number of reactor combinations should be adjusted considering the capacitance rise effect. Salt spray protection should be done during the test at the seaside, and an infrared thermal imager should be used to confirm that there is no condensation at the terminal joint.
GIS Terminal Cables: During the test, a voltage equalizing ring should be installed to reduce corona interference and ensure the accuracy of partial discharge detection. The connection part between the GIS terminal and the cable should be well sealed and protected.
Cables with Multiple Intermediate Joints: During the withstand voltage test, the joint parts should be closely monitored. If there is a sudden increase in partial discharge, the joint manufacturing defects (such as aging of the waterproof tape, stress cone not pushed in) should be immediately investigated.
VII. Safety Precautions
The “one person operates, one person supervises” system must be strictly followed throughout the test. All test personnel must wear insulating protective equipment. It is strictly prohibited to walk around in the test area at will, and non-test personnel are not allowed to enter.
The connection and disconnection of test equipment must be carried out after power off and discharge. It is strictly prohibited to connect or disconnect under pressure. The grounding device must be reliable, and false or missing connections are not allowed. If the inverter controller has been running continuously for more than 30 minutes, do not immediately turn off the power after stopping the machine. Keep the fan running for cooling. After the equipment triggers overvoltage or overcurrent protection, the control circuit power should be disconnected, and the problem should be investigated before restarting.
Smoking and open flames are strictly prohibited at the test site. Dry powder fire extinguishers should be provided. In case of thunder and rain, the test should be stopped immediately, all power sources should be cut off, and the test equipment should be reliably grounded. After the test is completed, all temporary connections should be removed, the original grounding and connection methods of the cable should be restored, and the test site should be cleaned up to ensure that the cable is ready for operation.
VIII. Test Records and Reports
All data during the test should be recorded in a timely and accurate manner, including basic cable information, test environment (temperature, humidity, weather), equipment configuration parameters, resonant frequency, test voltage, withstand voltage duration, insulation resistance value, partial discharge quantity, equipment temperature rise, and any abnormal phenomena (if any).
After the test is completed, a test report should be compiled. The report should include the test basis, test equipment, operation process, test data, result determination, and handling suggestions. Test waveforms and partial discharge spectra (optional) can be attached. The test records and reports should be signed and confirmed, and archived for future reference.
Post time: Jan-27-2026