A study on PD activities of oil-impregnated paper under pulse voltages using gas analysis

Gassing problems and premature insulation failures in wind turbine transformers indicate reliability concerns about the insulation of such components. This fact necessitates investigating degradation of transformer insulation systems under high-dV/dt pulse voltages that exist in today's wind energy plants. For this purpose, PD activity, generally assumed to be the main long-term cause of insulation deterioration, needs to be analyzed. This work evaluates the effects of impulse voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of PD activities. PD parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD activities, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples reveals that a faster rate of rise increases PD energy as well as inception voltage. Results based on experiments conducted in small oil chambers show good correlation between the amount of hydrogen detected and PD energy level; hence it can be used as a benchmark for comparing PD energy levels under various impulse waveforms.