1. Ultra-low frequency 0.1Hz withstand voltage test
Due to the large capacitance of the tested XLPE cable, the capacity of the test transformer required for power frequency withstand voltage test equipment is also large, making the test equipment bulky and unsuitable for on-site use. Using 0.1Hz as the test power supply can theoretically reduce the capacity of the test transformer to 1/500, greatly reducing the weight of the test transformer, making it easier to move to the site for testing. Currently, this method is mainly applied to tests on medium and low voltage cables. Since the authenticity of test conditions is not as good as near power frequency AC voltage, and the voltage level is relatively low, it cannot be used for high-voltage cable tests of 110kV and above.
2. Oscillating voltage test
The oscillating voltage test uses a DC power supply to charge the cable. When the test voltage is reached, the discharge gap breaks down and discharges through the inductor coil, applying a damped oscillating wave voltage of a certain amplitude and frequency in the kHz range to the cable as another approach for the completion test method of extruded insulation cable lines. This method is more effective than the DC withstand voltage test, but compared with the power frequency voltage test, its effect in checking the main insulation and accessory defects of the cable is still not ideal. First, the wave attenuates severely, making it difficult to meet the needs of long cables; second, it increases partial discharge, causing greater damage to the cable.
3. Resonant withstand voltage test
The resonant withstand voltage test method changes the inductance and test frequency of the test system to make the circuit in a resonant state. In this way, most of the capacitive current on the test object in the test circuit cancels out with the inductive current on the reactor, and the energy supplied by the power supply is only the active power consumed in the circuit, which is 1/Q of the test object's capacity (Q is the resonance multiple of the system). Therefore, the capacity of the test power supply is reduced, and the weight is greatly reduced. The resonant withstand voltage test system is divided into two types according to the adjustment method: adjustable inductance type (VISR) and adjustable frequency type (VFSR).
The adjustable inductance resonant test system can meet the withstand voltage requirements, but due to its heavy weight and poor mobility, it is mainly used in laboratories. The variable frequency series resonant withstand voltage test uses the inductance of the reactor and the capacitance of the test object to achieve capacitive resonance, obtaining high voltage and large current on the test object. It is a new method and trend in high-voltage testing and has been widely applied domestically and internationally.
Variable frequency series resonance is a resonant current filtering circuit that can improve power supply waveform distortion, obtain a better sinusoidal voltage waveform, and effectively prevent harmonic peaks from causing false breakdown of the test object. Variable frequency series resonance operates in a resonant state. When the insulation point of the test object breaks down, the current immediately detunes, and the loop current rapidly drops to one-tenth of the normal test current. When flashover breakdown occurs, due to the loss of resonance conditions, in addition to the immediate drop of short-circuit current, the high voltage also disappears immediately, and the arc can be extinguished. The re-establishment process of recovery voltage is very long, and it is easy to disconnect the power supply before reaching the flashover voltage again, so it is suitable for insulation withstand voltage tests of high-voltage and large-capacity power equipment.
The variable frequency resonant test system can not only meet the withstand voltage requirements of high-voltage XLPE cables, but also has the advantages of light weight and good mobility, making it suitable for on-site testing. After analysis and comparison, we adopted the VFSR variable frequency series resonant complete test device produced by Shanghai Siyuan Electric Co., Ltd. This device uses a fixed reactor as the resonant reactor and achieves resonance by frequency adjustment, with a frequency adjustment range of 30-300Hz, which complies with the resonant withstand voltage test recommended in the "Preventive Test Procedures for Power Equipment" enterprise standard issued by China Southern Power Grid Company in 2004, which recommends a frequency range of 20-300Hz.
