1、 Preparation before the experiment

1. Verify the cable model, specifications, length, laying path, and terminal type, confirm its rated voltage (U ₀/U=8.7/10kV), insulation structure (mainly cross-linked polyethylene XLPE), and design allowable test voltage level;

2. Check the status of the terminal heads at both ends of the cable: clean and dry, without cracks or discharge marks, the stress cone is installed in place, and the outer sheath and metal shielding layer are reliably grounded;

3. Disconnect all electrical equipment connected to the tested cable (such as switchgear, transformers, lightning arresters, PTs, etc.), ensure cable isolation, and set clear disconnection signs and safety fences at each disconnection point;

4. Measure and record the main insulation resistance of the cable (using a 2500V megohmmeter), and the insulation resistance values between phases and each phase to ground should be ≥ 1000M Ω, and the absorption ratio R ₆₀/R ₁₅ should be ≥ 1.3; If there is moisture or local defects, sufficient drying or defect treatment must be carried out, and it is strictly prohibited to apply pressure with hidden dangers;

5. Check the test equipment: frequency conversion resonant voltage withstand device (recommended frequency range 30-300Hz), voltage divider, partial discharge detection unit, protective ball gap, grounding wire (copper cable with cross-sectional area ≥ 25mm ²), safety interlock system, and whether the overvoltage, overcurrent, and flashover protection functions are properly calibrated and within their validity period;

6. The on-site layout complies with the requirements of DL/T 596-2021 "Preventive Test Code for Power Equipment" and GB 50150-2016 "Handover Test Standard for Electrical Equipment Installation Engineering". The test area is equipped with hard isolation, warning lights, and sound and light alarm devices, and non test personnel are not allowed to enter.

2、 Experimental parameter setting

1. Test voltage value: Take 2.0U ₀ (i.e. 2 × 8.7kV=17.4kV) according to the handover test, with a duration of 60 minutes; According to the preventive test, take 1.6U ₀ (i.e. 13.92kV) for a duration of 60 minutes; It is strictly prohibited to use DC withstand voltage instead of AC withstand voltage (which can easily cause space charge accumulation and accelerate XLPE insulation aging);

2. Test frequency: Priority should be given to using the equivalent frequency band of 35-100Hz to avoid approaching the natural resonant frequency of the cable (usually 3-15Hz low frequency is prone to exciting sheath circulation, and>200Hz significantly increases dielectric loss). The frequency conversion resonant system should automatically tune to the main resonant peak with a Q value ≥ 20;

3. Boosting method: Starting from zero, uniformly boosting at a rate of no more than 2% of the test voltage per second, and maintaining voltage stability after reaching the specified value, with a fluctuation range of ≤± 1%;

4. Partial discharge monitoring: Simultaneously connect high-frequency current sensors (HFCT) or capacitive couplers, measure background noise ≤ 5pC at 1.2U ₀, and when pressurized to 1.4U ₀, the discharge amount should be<10pC, with no obvious increasing trend.

3、 Monitoring of the experimental process

1. Two person operation throughout the process: one person operates the boost, while the other monitors the instruments (voltage, current, frequency, partial discharge amplitude, waveform diagram) and equipment for abnormal noise, odor, smoke, and flashover phenomena;

2. Real time monitoring of circuit current: If the current suddenly increases by more than 10% of the rated excitation current or exhibits periodic pulse characteristics, immediately reduce the voltage and check for end corona, shielding disconnection, or outer sheath damage;

3. Observe the cable body: focus on inspecting the middle joint, terminal stress cone area, and areas with small bending radius for corona blue light, slight cracking sound, or ozone odor;

4. Record key data: Record voltage, current, partial discharge, environmental temperature and humidity (temperature should be 10-35 ℃, humidity ≤ 70%) every 10 minutes, and save original waveform screenshots and spectrograms.

4、 Post experimental processing

1. Voltage reduction must be fully discharged: first, slowly reduce the test voltage to<10% through the built-in resistor of the device, and then use a dedicated grounding rod to discharge the cable conductor, metal shielding layer, and armor layer phase by phase multiple times (each time ≥ 3 minutes), confirming that there is no residual charge before contact can be made;

2. Retest insulation resistance: After discharge is completed, measure again and the resistance value should be basically the same as before the test (deviation ≤ 15%). If there is a significant decrease, check for moisture or insulation damage;

3. Partial discharge trend analysis: Compare the partial discharge spectra before and after the experiment, identify typical defect patterns (such as internal air gap discharge showing a "rabbit ear" shape and surface discharge showing a "crescent" distribution), and form defect localization recommendations;

4. Prepare a formal report: clearly indicate the test basis standards, equipment information, environmental conditions, measured parameters, description of abnormal situations, conclusions (qualified/unqualified/conditionally passed), and handling suggestions (such as "B-phase terminal local emission exceeds the standard, it is recommended to replace the stress cone and retest").

5、 Mandatory Security Clause

1. All grounding must be 'single point grounding, double line grounding', with a grounding resistance of ≤ 4 Ω. Series grounding or natural grounding using the structure is strictly prohibited;

2. During the experiment, dedicated personnel must be assigned to guard both ends of the cable, ensuring smooth communication and equipped with walkie talkies and emergency power-off buttons;

3. When thunderstorms, heavy fog, wind speeds greater than level 5, or air humidity greater than 85%, the test shall be immediately suspended;

4. It is strictly prohibited to restore cable power transmission without acceptance signature and dispatch permission.