The complete set of testing methods for frequency conversion series resonance test includes six methods: (1) cable test ultra-low frequency method, (2) cable test resonance withstand voltage method, (3) cable test oscillation voltage method, (4) cable test and inductive resonance pressure resistance method, (5) cable test parallel resonance method, (6) cable test series resonance method. The article mainly introduces three methods for cable AC withstand voltage test.

1、 Ultra-low frequency method

The commonly used method is the 0.1Hz withstand voltage test. Due to the large capacity of cables and transformers, power frequency testing is required, and a considerable testing power supply needs to be provided on site to provide reactive power to the cables. This method is not suitable for on-site use. Therefore, using ultra-low frequency as the testing power supply can greatly improve the capacity of the testing transformer. The theoretical capacity is 1/500 of the power frequency at 0.1hz, which is easy to implement on site. However, this method is not effective for detecting defects in the main and secondary insulation of extruded insulated cables such as cross-linked polyethylene (XLPE). At present, this method has been applied to the testing of medium and low voltage cables.

2、 Oscillation voltage method

DC charging cable, inductor discharge voltage resistance to achieve a certain gap to obtain a damping oscillation voltage, defect inspection of main insulated cables and accessories. This method is more effective than DC voltage testing, but there are still some issues. When one of the oscillating voltage attenuates, it is difficult to meet the needs of long cables, as these two voltage cables have higher frequencies of significant damage.

3、 Vibration withstand voltage method

The method of resonant pressure testing is to change the inductance and frequency in the testing circuit to make the circuit in a resonant state, which can meet the testing requirements of high voltage and high current. The resonant withstand voltage method is divided into frequency modulation and frequency modulation based on the adjustment method. According to the resonance mode, it can be divided into series resonance and parallel resonance.

1. The resonant withstand voltage of the inductive cable is achieved by adjusting the inductance of the loop reactor of the cable capacitor in the reactor and at the resonant frequency (50 Hz) according to the testing requirements.  

2. Frequency modulation cable resonance withstand voltage, by changing the output frequency of the test power supply, the fixed inductance reactor in the circuit can resonate with the test product to meet the testing requirements.

3. The cable and resonant series method, when the current of the test transformer meets the test requirements but the voltage does not reach the test voltage, the reactor and the test piece are tested in series. When the circuit is in a resonant state, the output voltage of the transformer Q times (Q is the circuit quality factor) can be generated on the test piece, and the power provided by the power supply is only the active power consumed in the circuit.  

4. Cable and resonant parallel method, when the main testing requirements for voltage transformer and current are met, the anti parallel electrical connection with the testing parameters is used to meet the testing requirements of the resonant circuit. When the induced current compensates for the capacitive current of the test reactor.