Varistor coupling method and recommendation parameters for combination wave test

Varistor coupling is recommended by International Telecommunication Union in combination wave test waveforms of surge protective devices (SPD), although the varistor coupling method and recommendation parameters for the combination wave test are not well understood. The waveform influencing characteristics introduced by varistor coupling are studied in detail in this paper. Low-voltage varistors, with maximum operating voltages ranging from 130 to 750 V, are adopted here in the OrCAD/PSpice environment to analyze the influence of varistor coupling on the output characteristic of the combination wave generator (CWG) in test levels of 0 to 20 kV. Simulation results show that, owing to varistor coupling, the reverse oscillation phenomena will arise in the open-circuit voltages (OCVs). Also owing to varistor coupling, compared with those generated directly by a CWG, the peak value, the front time, and the tail time for both OCV and short-circuit current will decrease, whereas the virtual impedance will increase. Moreover, the lower the test level, or the higher the maximum operating voltage of the varistor, the more serious this effect. Experiments have also been conducted here, and our simulation results are found to match the experimental ones well. It seems that, compared with the conventional capacitor coupling, the range of the voltage of power supply networks and the levels of combination wave test by varistor coupling have to be limited to a great extent. Varistor coupling for combination wave test of SPD should be only used in special cases and special test levels where there are no strict waveform limitations.