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 void is exceeded or the Paschen's Law is fulfilled respectively a discharge can occur. Since for AC voltage in general the highest field strength stress occurs at the voltage peak, a first PD will occur most likely at the 90 or 270° phase angle of the AC voltage period. The global field strength and the field strength resulting out of the space charge will add up vectorial. In any case the resulting field strength within the void will be significantly less. Depending on the magnitude of the discharge the qualitative field strength distribution is as follows - Fig 2.

In general space charges stick minutes or rather hours to a solid surface, since it is energetically a more stable status - see: Electronic Band Structure. Due to their local separation the charges do not recombine either. Therefore also the field strength dE_PD resulting out of the charge separation by the PD remains effective, independent of the following change of voltage and therefore global field strength. Depending on the nominal magnitude of the previous PD E_i of opposite polarity will be exceeded at an more early phase angle for a higher PD charge magnitude and a relatively late phase angle for a lower PD charge.

This depiction explains the phase angle of the typical phase resolved histogram for inner partials discharges of voids.