Investigation of damage-behavior in cast epoxy resin coils under thermal and electrical stress with the help of partial discharge (PD) measurements

Some results of step test investigations of the damage caused by thermal stress in cast epoxy resin coils are presented. The thermal stress is introduced from outside by way of a liquid heat transmission insulating medium as well as inside with help of current heat losses. The stress situation considered simulates the overcurrent and short circuit behavior of real dry-type transformers. With internal heating-up it is possible to investigate the superposition of thermal and mechanical stresses caused by overcurrent. Three different specimens are considered. These are laboratory model coils with about two and ten turns and industrially manufactured cast epoxy resin dry-type coils. The influence of the winding wire insulation is examined on laboratory model coils. Finally, the PD stress and PD damage behavior of simplified model arrangements is considered in comparison with casting epoxy resin dry-type transformers