Before the ultra-low frequency withstand voltage test, the overcurrent protection value cannot be filled in randomly based on experience. The core basis is the capacitance of the tested cable and the test voltage. By mastering the formula I=2 π fCU, one can first calculate the approximate range of capacitive current on site and then set a reasonable protection value.
In the formula, I is the capacitive current of the test sample, f is the test frequency, C is the cable to ground capacity, and U is the test voltage. If the capacitance is measured in microfarads and the voltage is measured in kilovolts, the calculation results should be converted according to the units specified in the instrument manual. The lower the frequency, the smaller the current; The longer the cable, the greater the capacitance and the higher the current.
The instruction manual provides an estimated case of a 10kV cable: the rated line voltage of the cable is 10kV, the length is 4km, the single phase to ground capacitance is 0.21 µ F/km, the peak test voltage is 26kV, and the estimated leakage current at 0.1Hz is about 9.69mA. Taking the reliability factor k=1.5, the overcurrent protection value can be set to about 14.5mA. This value is not a universal answer for all cables, but tells us that parameter tuning must be based on the actual capacitance.
When calculating on site, first check the unit length capacitance corresponding to the cable model and length; When there is no reliable information, confirmation should be made through instruments or capacitance measurement methods, and direct testing should not be conducted when the capacitance is unclear. The capacitance of the test sample shall not exceed the rated load capacity of the equipment, and when it is below 0.05 µ F, a red compensation capacitor shall be connected in parallel as required.
If the boost voltage does not reach the target value before overcurrent shutdown, in addition to checking for insulation defects, it is also necessary to verify whether the frequency is too high, whether the sample capacity exceeds the limit, and whether the grounding is reliable. Calculating ahead will make on-site debugging much smoother.