Partial discharge diagnosis on large power transformers

Phase resolved partial discharge measurements offer excellent possibilities to characterize the properties of defects in insulation materials. The appearance of the partial discharge pattern reflects the defect's geometry, the availability of the initial electron, and the properties of the contributing materials. With partial discharge measurements on large power transformers the complexity of the winding has a strong impact on the high frequency properties of the transformer and, thus, on the applicability and range of a charge impulse calibration in different frequency bands. Especially, when performing field measurements, using elevated frequencies offers sensitive partial discharge measurements even in electromagnetically polluted environments. However, the influence of effects such as attenuation, reflection, and cross coupling must be considered and evaluated both in the frequency and the time domain.