Stress grading system evaluation for a converter feed hydro generator winding

Converter feed synchronous generators have some important advantages in pump storage hydro power stations compared to conventional synchronous generators with fixed speed. Due to the converter voltage pulses with large du/dt and high frequency content the winding insulation is electrically and thermally stressed different than for power frequency. As known from MV motors especially the stress grading system is very sensitive regarding these voltage pulse trains. In this feasibility study the electric and thermal stress resulting of the stress grading system is analyzed via a FEM-model. This is done for different for voltage characteristics, i.e. sinusoidal and impulse voltages. The intention is to compare and assess the field strength distribution along the bar surface and the hot-spot temperature in the stress grading layer depending on the voltage signal. Finally, some conclusions and recommendations are given for converter feed windings in hydro-generators.