Estimation of Critical Resistive Leakage Current of polymer housed ZnO surge arrester by electro-thermal modelling

Abstract

In this study, a MATLAB/Simulink software based electro-thermal model of the polymer housed, gapless, 60 kV ZnO surge arrester is used for estimating the ‘ Critical Resistive Leakage Current (CRLC)’. The electrical model is obtained by conducting experiments on a representative ZnO element at various voltages and temperatures which yields voltage–current–temperature (V–I–T) and power loss–voltage–temperature (P–V–T) characteristics. The thermal model represents the thermal properties of the arrester components as an electrical analogous circuit made of resistance & capacitance. The power loss calculated by the electrical model is the input to the thermal model. The electrical and thermal models run for numerous iterations until the arrester goes to thermal runaway or until it reaches a stable element operating temperature. The electro-thermal model is used to determine the two intersection points (stable operating point and the thermal instability point) of the power input and heat loss curves. The authors have proposed a new criteria based on detection of occurrence of the highest segmental slope inbetween the lower stable operating point and upper instability point on the power input curve (P–V–T characteristics) to estimate the CRLC. In order to sensitise the maintenance personnel about the level of deterioration and imminent failure of the arrester in the near future a warning in the form of the Observation Current (OBCUR) is also proposed. The easy to follow procedures for estimating the CRLC and OBCUR and the results obtained for two different arresters (of two different manufacturers) are presented in this paper. Keywords: Gapless ZnO surge arrester, ZnO element, Electro-thermal model, Power loss, Thermal runaway, Critical Resistive Leakage Current (CRLC)