Demodulated Waveforms: Gap Discharge

Gap Discharge or Partial Discharge is a discharge activity across an insulation void or an air gap in a connection point. Mica insulation is very robust against partial discharge but cross-link polyethylene structures (such as cable insulation) are very susceptible to further damage from the partial discharge process itself. For discharges across a pit in a connection point it can escalate into an arcing pattern as the connection point gets deteriorated by the discharge action over time.

If the frequency band in this signature is noted in the low (exciter) frequency, it could be a normal diode firing pattern (depending on the number of spikes per sine wave structure). This can usually be listed in the “tone” pattern and the spikes will modulate based upon the voltage regulator control pattern.

If in the higher frequency band (as in the above signature), it could be developed by a gap in the ceramic insulator structure (bolt gap, insulator crack, space between the conductor and the insulator, etc.).

It could also be the start of a connection problem. Trend this pattern over time to see if it degrades into a multi-gap discharge pattern or arcing pattern.

The discharge noted can be stable or inconsistent, single or multiple discharges.

If the discharge is single and stable, then it is indicative of a minor pit or single void. Multiple and stable indicate multiple pits or voids but this is still a stable condition. Monitor these conditions to determine if it is degrading over time into a stronger multiple discharge pattern or an unstable signal.

If the discharge is unstable, then it is indicative of a condition such as a loose connection, loose bolt, insulation gap movement, or other condition that causes the gap structure size to change.

In the above case the motor appears to have a very low-level start to a connection gap discharge. The level of the frequency of interest was less than 100uV so no action was taken. But it was a type of signal we would monitor to see if the energy levels of the interference increased over time.

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