Partial discharge and Harmonic on dry type transformer 1

[PAP]

Hello all,

I have a problem of Harmonic in the system but the harmonic monitoring is still in process, however , my concern is the dry type transformer that supply the load have some partial discharge trace. Could anyone may suggest how harmonic can accelerate the partial discharge process in an dry type transformer? I also experiene on the HV coil of dry type transformer is cracked and failed. Its root cause of problem can not be identified yet, but suspect the harmonic problem. Our system cann't connect many steps of LV capacitor bank, because the harmonic will be boost up and capacitor bank will be damaged.

Thanks all for every helps.

 

[Skogsgurra]

PAP,

The problem you describe is not very common, but real.

I have had it on two different locations. One with 11 kV and one with 6 kV primary voltage. There are a few steps in this explanation. They involve how PD happens, how dry resin insulation fails, how ozone is created in the voids between copper and resin and how capacitive currents in the insulating system depends on frequency.

Partial Discharge happens when the V/mm exceeds a certain value. When it does, the oxygene molecules are separated into O- radicals, which combine with O2 to form O3, which is ozone - a gas that eats organic substances like insulating materials.

The voltage distribution in an insulating system is inversely proportional to the dielectric constants (epsilon) of the insulating material. Epoxy resin has a higher epsilon than air, so if there is an air pocket in the epoxy (or between epoxy and copper), the V/mm will be unexpectedly high. And ozone will be produced.

The ozone production is proportional to the current flowing in the void. And the current is proportional to voltage and frequency. Increasing frequency increases current flow and hence ozone production. The process is usually quite slow. It typically takes several years before the insulation fails, but I had one case where ringing from an CSI inverter for a large pump destroyed the insulation in a little more than a year.

To conclude:

1 High frequency components increase capacitive currents in the insulating system.

2 Capacitive currents produce ozone in voids.

3 Ozone destroys organic material.

4 It takes several years, but can happen in as short a time as a year if the frequency is high.

I have some recordings showing the problem. I think that you can find out how to contact me. Do that and I will send you the report.

Gunnar Englund

http://www.gke.org

 

[electricuwe]

What is the load on your transfomer ?

Harmonic problems may have totally different character depending on frequency and if voltage or current are distorted. Typical loads tend to cause typical harmonic issues.

For the cracks in the HV winding there might be other reasons:
harmonic currents tend to cause localized overheating if the transformer was not designed or choosen correctly for the specific application.