A new test method for assessing the impact of thermal cycling on hydrogenerator stator insulation

A fast growing amount of renewable energy from wind or solar plants with their inherent production fluctuations requires conventional backup and storage capacity in order to keep grids stable. Hence, hydropower plants in Switzerland are increasingly selling peak energy in a liberalized European electricity 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 originally designed for. In consequence the high voltage insulation system in the stator conductor bars is stressed by a higher number of thermal cycles which might shorten machine lifetimes significantly. In this paper a new test for assessing the impact of thermal cycling in modern mica/epoxy insulation systems is presented. 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 Partial 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 uncycled group but shows the necessity of further investigation.