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Field/Case Studies

Currently, the Centre conducts on-line monitoring of three power transformers operating at the Sponsoring utilities:

• 150 MVA mineral oil filled transmission transformer (2003 vintage)
• 10 MVA mineral oil filled underground distribution transformer (1952 vintage)
• 10/16 MVA vegetable oil filled distribution transformer (2005 vintage).

The 150 MVA transformer features a number of fibre-optic temperature sensors in the windings and RTD temperature probes in oil and air, and also on-line moisture and dissolved gas monitors installed. All these devices are being monitored using a DRMCC unit.

The 10 MVA underground unit features two moisture and temperature transmitters installed in the transformer tank and one in the air. The transformer is undergoing a few rounds of on-line dryout. Following each round of dryout, a new change is introduced into the transformer’s breathing system: a free-breathing system, a silica gel protected breathing system and a ground mounted rubber bag protected breathing system. Numerous oil tests are taken for chemical analysis before and after dryout. Absence of a change introduced into the winding clamping pressure by the on-line dryout is tested using the FRA measurement. A special portable unit for assessment of the effectiveness of on-line dryout in real time is being used.

The 10/16 MVA vegetable oil filled transformer features two moisture and temperature transmitters installed in the transformer tank. Moisture dynamics in the transformer was observed during the factory temperature rise test and then during the first years of operation.