Study of the behavior of low voltage ZnO varistors against very fast transient overvoltages (VFTO)

Abstract

• Complete VFTO(s) generation and measurement system based on (MPC) and (TLT) technologies. • Five commercial ZnO varistors of the same reference were used in the characterization. • The influence of the applied overvoltage front time on the capacitive effect of the varistor is confirmed. To study the behavior of low voltage ZnO varistors against Very Fast Transient Overvoltage (VFTO), the authors have developed a complete VFTO(s) generation and measurement system based on magnetic pulse compression (MPC) and Transmission Line Transformer (TLT). This system has an output voltage of 24 kV, rise time of 32 ns, and a bandwidth of 10 MHz. The measurement system comprises a damped resistive voltage divider with an impedance of 1000 MΩ and a maximum bandwidth of 10 MHz. The experiment was performed with five commercial zinc oxide varistors with a nominal voltage of 150 V, each one was subjected to VFTO-type voltage pulses and, by Analysis of Variance (ANOVA), a single varistor was selected to perform the case study. This work presents an experimental methodology to observe the response of low voltage ZnO varistors against VFTO. This response is the delay of the residual voltage to the applied discharge current and indicates the poor performance of these devices against VFTOs. Our methodological proposal consists of a series of laboratory tests that will allow manufacturers and consumers to take corrective measures in case of malfunction, damage of the varistor material, or permanent degradation of the device.