Determination of Non-Inductive Residual Voltage for Steep (0.5 Microsecond Front Time) Impulse Currents on Station Class Surge Arrester Blocks

Abstract

Metal Oxide Surge Arrester (MOSA) has been an essential part of power system since years. It safeguards the substation/distribution line equipment by limiting the surge voltage to an acceptable value. This is realized by the nonlinear property of the Zinc Oxide blocks used inside the MOSA. Protection Margin (PM) of the power system is defined as the difference between the BIL of the power system equipment to be protected and the residual voltage of the MOSA. The residual voltage of the MOSA depends on the peak and rate of rise of the surge current, therefore faster the surge, higher the residual voltage across MOSA. Power system is subjected to fast transients caused due to current chopping, bushing flashovers inside the valve hall, restrike in circuit breaker contacts, etc. MOSA when subjected to steeper current surges observe higher residual voltage, which shrinks the PM and may even damage the equipment to be protected. Steep impulse residual voltage of MOSA comprises of a non-inductive part, which is the property of the MOSA block and an inductive component which is determined by the length of the MOSA. IS 15086–4, 2017 [1] defines steep current of virtual front time of 1 microsecond and virtual time to half of not more than 20 microseconds. The standard also defines the procedure to measure the noninductive component of the residual voltage of MOSA when subjected to steep current impulse. But, even steeper currents can occur in the power systems due to reasons mentioned earlier; therefore, it is important to know the non-inductive residual voltage of the MOSA when subjected to such steeper current impulses. Hence in this paper, a steeper current impulse of 0.5 microsecond virtual front time is generated and the non-inductive residual voltage drop across the MOSA block is estimated for various classifications of station class surge arrester blocks. Comparison of steep (0.5 microsecond front time) current impulse residual voltage and lightning current impulse residual voltage is also performed which will be useful in conducting proper insulation co-ordination study of critical installations.