Investigation of Urgent Service and Detoxification Requirements on 14 11kv Oil-Immersed Transformers Due to Transient Overvoltages Using Yateks Approach.

In this work, practical work to determine urgent servicing and detoxification requirements on 14 11kV oil-immersed transformers (TXs) due to transient overvoltages using the Yateks approach is presented. The practical work based on oil insulation evaluation is performed to investigate and ascertain which of the 1411kV oil-immersed TXs require priority servicing and detoxification. Several significant faults have existed in oil-immersed TXs over the last seven years and all these faults exhibited analogous attributes and originated under similar conditions. The first evaluations of the TXs demonstrated that insulation failure had transpired in districts in which no overvoltages were foreseen. Commonly, the breakdown happened for approximately 1 to 3 years of service. Shortly after being substituted with homogeneous TXs, these TXs failed repeatedly. The analyses of the retrieved equipment made it abundantly evident that the TXs’ breakdown was induced by transient overvoltage occurrence. This work presents oil analysis results obtained through tests that were based on a full-scale of 14 11kV oil-immersed TXs rated at 1250-2500kVA. The tests were conducted for a mill that consumes an average of 10MW of power through an 11kV distribution system. The TXs were tested in three conditions: TX oil analysis, internal operation (DGA), and paper coordination. The reported results and observations show that 30% of the TXs require preventive maintenance and 70% of the TXs require emergency maintenance. Therefore, this will aid optimize aspects that contribute to lower TX operating and service costs, and hence lower energy prices for power generation.