Application of Serial Resonant in the Electric Test

Gang Li
HIMALAYAL - SHANGHAI - CHINA

Abstract: Serial resonant withstand voltage test uses reactor inductance and test equipment to generate serial resonance, obtaining high voltage and large current from test objects. Main tuning methods are variable inductance and variable frequency. The variable inductance adopts reactor with adjustable core air gap. Adjusting the inductance value makes circuit in power frequency resonant condition. Besides loud noise and complicated mechanical structure, the device is so heavy that it is difficult to carry. The variable frequency adopts the capacitor with fixed inductance, and adjusting exciting power source makes test circuit reach resonant. With small volume, light weight and high quality factor, it is easy to use and is gradually taking place of variable inductance.     

Key Words: Serial, Resonant, withstand Voltage, Test

Serial resonant is resonant current filtering circuit, which can improve the waveform distortion of power voltage, obtain excellent sinusoidal voltage waveform and effectively prevent harmonic peak from damaging test objects. In serial resonant condition, when breakdown of insulation weakness of test objects occurs, the circuit detunes and loop current reduces into one tenth of normal test current (1/Q) immediately. However, breakdown current rises ten times in a minute if adopting shunt resonance or test transformer to conduct voltage withstand test. There is hundredfold difference in breakdown current through comparing both. Therefore, withstand voltage of serial resonant not only effectively locates insulation weakness but also is free of burning fault points due to short circuit current. When the breakdown of flashover occurs, because of losing resonant condition, the high voltage disappears and arc extinguishes immediately besides reduction of short circuit current. In addition, it takes longer time to rebuild resonant voltage and it is highly possible that exciting power cuts off before reaching flashover voltage again. The voltage restoration waveform is an interval oscillation process of energy-accumulating. Its process is long and restoring over-voltage will never occur.