Inner Partial Discharge in a Gas filled Void

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The initial field strength distribution in a gas solid insulation system is determined by the ratio of relative dielectric constants of the two dielectric materials.

Standard insulating gases always do have an relative dielectric constant of e_r=1. If molecules were polar, the resulting forces would turn them into liquids. Typical cast or impregnation resins as well as their composites with solid fillers have a e_r~4.

For this reason a gas void is loaded by a about four times higher field strength than the solid resin.

Fig 1 illustrates the situation for the case that no partial discharges occur.

If the inner dielectric strength E_i of the voids is exceeded an discharge can occur. Since for AC voltage the highest field strength stress occurs at the voltage peak, a first PD will also most likely at this point of AC period. Depending on the magnitude of this first discharge the qualitative field strength distribution is as follows - Fig 2

For space charges stick minutes or hours to a solid, the charges of the elapsed PD will also remain at their location until a high field strength will forced them to do






space charge based explanation for phenomena and respectively the phase histogram.