Oil reclamation service prolongs active life for transformers

ABB’s state-of-the-art on-line oil reclamation service has helped Deeside Power Station to ensure the reliability and long service life of its three vital GSU (generator step-up) transformers by removing high levels of acidity and harmful corrosive sulphur from their insulating oil.

Oil ReclaimationTaking an oil sample from a transformer
Acids form as a result of oil degradation due to oxidation, indicating that the transformer oil is ageing. Corrosive sulphur is also well known as a significant cause of premature transformer failure as it reacts with copper components to form highly conductive copper sulphide that deposits on windings and the paper insulation, causing a breakdown of insulating properties and resulting in shorting, arcing and even fires.

Deeside Power Station, which opened in 1994, is a 515 MW CCGT (combined cycle gas turbine) power station owned by International Power GDF Suez and sited by the river Dee in Flintshire, North Wales. It has three GSU transformers (two for each of the gas turbines and one for the steam turbine) that step-up the voltage produced by the generator units to the 400 kV required by the UK transmission system.

Oil tests carried out as part of a general maintenance programme had indicated increasing levels of acidity and the presence of corrosive sulphur. The oil in each of the three transformers had previously been passivated using 7 litres (100 ppm) of Irgamet 39. Subsequent tests showed an unexpectedly high consumption of the passivator so Deeside Power asked ABB’s specialist UK transformer repair and maintenance team to take further samples and recommend remedial measures. These new tests confirmed that the oil was in a condition where it needed reconditioning or replacement. The main aim was to remove the acidity and sulphur that were reducing the DP (Degree of Polymerization) value of the paper insulation, which is an important indicator of its remaining service life.

It was decided to use ABB’s unique on-line oil reclamation service to reclaim the oil rather than replacing it with new. This process enabled the acidity and sulphur to be removed over a period of time with the transformers remaining energized, so the power station could continue to operate at full capacity without the need for an outage.

Mobile oil reclamation rig

Oil RigThe oil is circulated through the mobile rig
The reclamation process, in which the oil is circulated continuously through ABB’s mobile oil reclamation rig, housed in a
40 foot trailer, took around 10 weeks for each transformer due to the large volume of oil to be processed – a 190 MVA GSU transformer, such as the units at Deeside Power, holds 70,000 litres. The progress of the reclamation process was monitored by checking the colour of the oil as it changed from a dark brown to a light straw colour. At this point the oil was sampled for laboratory testing to confirm the successful removal of acids and corrosive sulphur.

Reclamation is superior to an oil change
Initially, it might appear that carrying out a complete change of the transformer oil would be a faster and more efficient method of eliminating the by-products of oil degradation. However, extensive laboratory and field tests carried out by ABB have shown this offers only a short term benefit as the new oil soon becomes contaminated by the residual oil in the tank and the insulation. So, while the reclamation process is longer and slower, it provides a permanent long-term solution that restores the properties of the oil to very near those of new oil by removing acids, sludge and other degrading products like corrosive sulphur. This also has a beneficial effect on the ageing of the paper insulation.

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    Laboratory testing
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