Partial Discharge Theory

2.8 Partial discharge Analysis

Partial discharge theory involves an analysis of materials, electric fields, arcing characteristics, pulse wave propagation and attenuation, sensor spatial sensitivity, frequency response and calibration, noise and data interpretation. It is obvious from the above that most plant engineers will not have the time, or available energy, to pursue such a course of study

In an effort to promote a better understanding of partial discharge (PD), this paper attempts to provide simplified models and relate the characteristics of these models to the interpretation of PD test results. First, we will present a few technical concepts relating to partial discharges. Partial Discharge can be described as an electrical pulse

This usually occurs on the insulation surface. These discharges can bridge the potential gradient between the applied voltage and ground by cracks or contaminated paths on the insulation surface

The above can be illustrated by development of a simplified model of the partial discharges occurring within the insulation system. Figure 2.8.2 – Surface Partial

This would occur when the internal arcing had carbonized to the point where the resistive component of the model was low enough to prevent a build-up of voltage across the void. This new low resistive component would also allow higher current flows, and additional heating and resultant insulation damage. The above model, including the resistive component correlates to the actual failure mode of a partial discharge void, with the resistive component passing more leakage current as the partial discharges increase with time. One form of this resistive component is visible tracking on the surface of insulation. An explanation of tracking, and how surface partial discharges are related to the development of tracking follow [5]: “Tracking damage has been traced entirely to the locally intense heat caused by leakage currents. These currents flow thru any contaminated moisture film on the bridging insulating surface. As long as this film is fairly broad and continuous, the heat associated with the leakage current is spread over a wide area and is dissipated. However, heating promotes film evaporation. This causes the film to break up into small pools or islands. Each break in the film tends to interrupt a segment of the leakage current, causing a tinyarc. Even though the arc is small, severe local heating results. The intense heat of the leakage current arc is sufficient to