Modelling of a 400-kV MSCDN Reactor for Computation of Voltage and Field Distributions During Switching Transients

In this paper, a model for the reactor of a 400-kV mechanically switched capacitor with damping network (MSCDN) based on an equivalent circuit representation is developed. The model is based on sub-dividing the physical reactor into sections which are sufficiently small to be represented by a lumpedparameter equivalent circuit. The circuit parameters are obtained for each section using analytical formulae based on the physical configuration of the reactor, the winding layout, and the insulation material. The model is then simulated in the ATP/EMTP program for the evaluation of transient voltage and field distributions along of the reactor. This helps in identifying possible failure scenarios which will allow designing measures to mitigate failures effectively during transients arising from switching operations. Further analysis of the results has revealed that there are substantial dielectric stresses imposed on the winding insulation that can be attributed to a combination of three factors. First, the surge arrester operation during the MSCDN energization, which causes steep voltage change at the reactor terminal. Second, the non-uniform voltage distribution, resulting in high stresses across the top inter-turn windings. Third, the rapid rate-of-change of voltage in the assumed worst-case reactor winding location. This is accompanied by a high dielectric (displacement) current through the inter-turn winding insulation. The results of this paper indicate that a synergistic effect of high electric field and high dielectric current occurring at worst energization, followed by the thermal effects of steady state operation may contribute to the failure of air-core reactors used on the 400-kV MSCDN.