Drying transformers

When Distribution Network Operator took its 132kV transformer in North London out of service it was with the intention of scrapping it and replacing it with a new one.

When the delivery time for the new transformer failed to meet up with the project timings a decision was made to get the old transformer refilled with oil and pressed into service. The only problem was it had been empty of oil for several months and despite being inside a transformer housing it had clearly picked up some water from normal atmospheric breathing and needed some treatment before it could be energised and put back into service.

EOS were approached and asked for advice and the following vacuum / heat treatment was undertaken

1. Reclaimed transformer oil was put into the transformer through a Mobile Processing Unit

2. Two days were spend circulating the oil through the MPU to raise the core temperature as high as possible –with lagging and the coolers isolated we got close to 80°C WTI

3. The oil was drained into a standby Hold On Site tanker (circulated to keep hot and dry)and a vacuum immediately applied to the transformer main tank

4. The vacuum, water removed and temperature were monitored for 12 hours and when the temperature fell to a pre-determined level the oil was replaced and step 2) repeated

5. Vacuum applied as step 4

This was repeated for 3 -4 cycles when electrical tests confirmed that sufficient dryness had been obtained.

The transformer was put back into service till the replacement could be installed a year later.

To discuss this case study further, or to explore other drying options please contact Andy Bartram