The power off Online PD surve

During a online pd survey, pd activity were measured on several cables in a substation. Due to the propagation effect off the PD pulses, we needed to do measurements on several cable circuits connected to the same switchgear in order to determine the source.

The difference between some of the measurements were minor, but we were able to determine that the activity came from one of the cable circuits.

The survey resulted in;

1) Replacement of one current transformer

2) Recommendations to replace two terminations in substation

3) Recommendation on new measurements on 50 kV transformer at an later stage to check how the activity trends

The analysis of the data revealed three different faults on this circuit.

1) Critical surface discharge at phase 1 (In blue, with correct PRPD)

2) Critical surface discharge at phase 2 (In red, the PRPD has a 120-degree phase-shift)

3) Suspicious PD activity on a 50/24 kV transformer

Filtered PRPD with just the PD activity from the terminations

Common for the surface discharge on phase 1 and 2 is that they have a symmetric

pd pattern, steep pd pulses with a high frequency content and high pd amplitude.

This indicates the presence of a critical pd phenomena like high frequency

surface discharges.

Filtered PRPD with just the PD activity from the transformer

For the suspicious activity on the transformer the pattern is more A-symmetric, but with steep pd pulses with a high frequency content and high pd amplitude. There are also two different sourses for the activity. This may indicate that there is something not working properly in the transformer. In oil-insulated components, it is important to look at the development of the activity over time. This is because the discharges can occur due to temporary air pockets which will "repair" itself.