A New Test Method for Assessing the Impact of Thermal Cycling on Hydrogenerator Stator Insulation

Abstract- A fast growing amount of renewable energy from wind or solar plants with their inherent production fluctuations re-quires conventional backup and storage capacity in order to keep grids stable. Hence, hydropower plants in Switzerland are in-creasingly selling peak energy in a liberalized European electrici-ty market. This development leads to higher start/stop-cycle frequencies of their hydrogenerators and some older machines are also subjected to more starts per year than they were origi-nally designed for. In consequence the high voltage insulation system in the stator conductor bars is stressed by a higher num-ber of thermal cycles which might shorten machine lifetimes sig-nificantly. In this paper a new test for assessing the impact of thermal cycling in modern mica/epoxy insulation systems is pre-sented. The new method isolates insulation aging exclusively caused by thermal cycling by the use of cycles with short heating intervals and a maximum temperature close to the glass transition point of the insulation system’s binder (epoxy) resin. The insulation aging condition is monitored with non-destructive measurements of the Dissipation Factor and Par-tial Discharges during, and a destructive voltage endurance test after completion of the thermal cycling. A first implementation of the new test method with 1.500 cycles resulted in no severely greater deterioration of the cycled group compared to an un-cycled group but shows the necessity of further investigation