COMPARATIVE ASSESSMENT OF ABNORMAL VOLTAGE STRESSES IN AN ISOLATED MUTUALLY COUPLED TRANSFORMER MODEL WINDING WITH AND WITHOUT VARISTOR

The dynamic performance of high voltage power transformer (HVPT) windings owing to the impact of high voltage surges has evoked the attention of insulation engineers for almost a century. As these very fast rising transient over voltages generate huge peak voltage stresses along the windings and may perhaps threaten its insulation. Accordingly a profound understanding of this is crucial for insulation engineers to design the winding insulation optimally and thinking of novel protective devices to be used to ensure that insulation failure does not take place when such high voltage surges strike the terminals of HVPT's. In this paper, a HVPT winding is being represented by an isolated single phase 12 coil section and 18 coil section mutually coupled model winding and shunted with metal oxide varistors to suppress the huge peak voltage stresses and their complex oscillations occurring due to these extremely fast rising lightning surges. A simple electrical model of a Zno varistor is presented and its excellent properties are being utilized for the transformer winding insulation protection against such fast rising lightning surges. Simulation studies have been conducted for comparative assessment of abnormal voltage stresses in an isolated mutually coupled transformer model winding energized with full a periodical and chopped a periodical lightning surge voltages without and with varistor for a distribution winding constant α=10.