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Some Doubts of Partial Discharge(PD) Monitoring of HT/Transformer

07/13/2017 3:00 AM

Hi, I have some doubts of HT partial discharge(PD) based on some cases I have met. In theory, partial discharge (PD) is a localised dielectric breakdown (DB) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress, which does not bridge the space between two conductors.

So once the space exists, the PD should allways be there. That's why we are using the knowledge of partial discharge monitoring(PDM) to confirm the PD of HT/TF. But so far, we also met some cases which the monitoring system shows the PD is also a periodic appearance. It will occur during daytime(normally high load of HT/TF), then become cool down during night time(No Load). Some cases also show High PD at night and low PD at daytime, which also confuses me a lot.

So if the PD is due to High voltage, it should always exist, no matter how high load or high current it is, as long as the voltage is there.... Can anyone help me to solve this question? Is it possible that PD depends on period as well?

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#1

Re: Some doubts of Partial Discharge(PD) Monitoring of HT/Transformer.

07/13/2017 3:24 AM

<...Can anyone help...> The transformer's manufacturer should always receive the first phone call.

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#2

Re: Some doubts of Partial Discharge(PD) Monitoring of HT/Transformer.

07/13/2017 3:53 AM

Load affects temperature of HT, and dielectric strength changes with temperature.

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#3

Re: Some Doubts of Partial Discharge(PD) Monitoring of HT/Transformer

07/13/2017 10:38 PM

You have not told us how you are determining that PD is occurring, where it is occurring, and/or its magnitude. Also, your premise that "...once the space exists, the PD should allways (sic) be there...", requires refinement to allow for relative motion between the two surfaces.

What may be happening is that as the windings and/or the insulation heat up and cool down during the daily load cycling, the "space" opens and closes with the heat-related dimensional changes. Your description Of when the PD occurs seems to support that premise.

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#4

Re: Some Doubts of Partial Discharge(PD) Monitoring of HT/Transformer

07/14/2017 12:00 AM

You didn't mention the operating environment and whether it is a dry or oil-immersed system. Also, in addition to internal breakdown, PD can occur along insulating surfaces, particularly in the presence of atmospheric condensation. Surface contamination often enhances surface PD and tracking. And it may completely disappear after the water evaporates. This might explain why PD sometimes occurs only during cooler evening hours if the surface temperature of the affected insulatinn drops below the dew point.

PD is opportunistic. Although it generally becomes worse as the insulation system is progressively degraded, the process is not always continuous. PD may be also enhanced by repetitive thermal expansion and contraction cycles when gas-filled gaps are created in the insulation system. With thermally-induced changes in gap dimensions, the E-field may sometimes be above or below the partial breakdown threshold depending on temperature. The resulting PD signature can thus appear to be inconsistent and confusing.

The point is that the presence of PD - for any reason - is a definitely cause for concern. It's time to consult with the transformer manufacturer ASAP.

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#5

Re: Some Doubts of Partial Discharge(PD) Monitoring of HT/Transformer

07/14/2017 7:30 PM

If you have PD in a transformer, you have a *REAL* problem. It's not a matter of *IF* it's going to fail, it's only a matter of *WHEN*, and how much damage will occur when it does.

Have you put a picocoulomb meter on the DUT and actually measured the discharge?

Depending on the magnitude and how long it's been going on, it might still be possible to arrest the deterioration.

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