Series resonance (also known as resonance device) for AC withstand voltage tests includes three methods: ultra-low frequency test method, oscillation voltage test method, and resonance withstand voltage test method.
Test the ultra-low frequency method
In the common power frequency withstand voltage test, the cable has a large capacity, the test transformer also has a large capacity, and a considerable amount of test power needs to be provided on-site to supply reactive power to the cable. Therefore, the power frequency withstand voltage test is not suitable for on-site use. Thus, an ultra-low frequency should be adopted as the test power source. This not only reduces the capacity of the test transformer but also makes the on-site test operation simpler. However, the detection effect of insulation defects by this method is not very good, so this method is generally applied in medium and low voltage cable tests.
Test oscillation voltage method
The principle of the test is to charge the cable with a direct current. Once the voltage reaches a certain level, it discharges through the gap onto the resistor and inductor, resulting in a damping oscillation voltage. This voltage is used to check the insulation defects of the main insulation and accessories of the cable. This method is more effective than the direct current withstand voltage test. However, the oscillation voltage has attenuation and cannot meet the needs of long cables. Moreover, high-frequency voltage causes very significant damage to the cable. These are the problems with this method.
Resonance withstand voltage test method
This method can meet the requirements of high voltage and large current tests. The resonance withstand voltage method is classified into adjustable inductance type and adjustable frequency type according to the adjustment method, and into series resonance and parallel resonance according to the resonance mode. The adjustable inductance resonance withstand voltage is achieved by adjusting the inductance of the circuit reactor to make the reactor and the capacitance of the cable resonate under the industrial frequency (50Hz), thus meeting the test requirements; the adjustable frequency resonance withstand voltage is achieved by changing the output frequency of the test power supply, so that the fixed inductance of the reactor in the circuit and the test sample resonate to meet the test requirements.
Series resonance occurs when the current of the test transformer meets the test requirements but the voltage fails to reach the test voltage. In this case, the test is conducted by connecting the reactor in series with the test sample. When the circuit is in resonance, a transformer output voltage Q times (Q is the circuit quality factor) greater than the actual output voltage can be generated on the test sample. The energy supplied by the power supply is merely the active power consumed in the circuit. Parallel resonance method occurs when the voltage of the test transformer meets the test requirements but the current fails to meet the requirements. In this case, the test is conducted by connecting the reactor in parallel with the test sample to make the circuit parameters meet the resonance requirements. At this time, the inductive current of the reactor compensates for the capacitive current of the test sample.
Post time: Dec-23-2025