A Comparison of Dielectric Characteristics and Voltage Endurance Life for Two High Voltage Coil Designs

Stator windings of large AC machines consist of multi- turn form wound coils insulated with a combination of materials of differing permittivity. A relatively simple and straightforward change in materials and processes can modify the voltage stress across the mainwall layers of dielectric materials comprising an AC stator coil. Finite element (FE) analysis showed a significant increase in voltage stress where the turn and mainwall insulation have different relative permittivity. The choice of materials and structure of the layered insulation system can modify the electric field distribution across the dielectric materials. Experiments on sample coils with two different 13.8-kV vacuum-pressure impregnation (VPI) insulation designs are described, along with the manufacture, preparation and processing of samples for screening and verification testing. Voltage endurance (VE) curves show an improvement in test life associated with the choice of material and manufacturing process used for dedicated turn insulation. Comparative results from diagnostic testing of the two systems are shown.