PD-Activity in Generator Stator Bar insulation versus Voltage Frequency and Temperature

Abstract

Condition assessment of generators is today performed at various voltage frequencies and test temperatures. The main purpose of this paper is to compare measured PD results from sections of generator bars with that of similar tests performed on laboratory made test objects with void enclosures of known geometry. All test objects were subjected to voltage frequency in the range from 0.1 to 50 Hz (to 300 Hz for the smaller laboratory made test objects) to a maximum voltage of 10 kV, and test temperature in the range of 20° to 155°C. The measured PD characteristics were compared to estimated values using an impedance abc-model, including both conduction and dielectric response. At temperatures above 130 °C the measured partial discharge inception value (PDIV) was found to decrease with increasing temperature, particularly so at the lowest test frequencies. A mechanism caused by increased conductivity and higher rate of polarization at increased temperatures. The magnitude of the measured apparent charge was varying, strongly indicating that several discharges are needed for a complete discharge of a void.