This month’s Featured Technical Paper is from the Proceedings of the 2023 International Conference of Doble Clients. The paper is titled: Winding Resistance Issue During Transformer Bushing Replacement.


During the replacement of low voltage (LV) bushings on a 140 MVA generator transformer, the measured winding resistance changed significantly. The winding resistance increased by 22% in one phase and 6% in the other two phases (all compared to the original factory acceptance test [FAT]), which is above the recommended IEEE limit of 5% increase. A sister transformer had a similar LV bushing changeout performed earlier with no change in winding resistance. The LV is delta connected with the delta made at the bushing connections. The LV is high current and has 4 large parallel winding leads joined to form the winding phase current and then connected with the leads of another phase to make the delta connection. The high winding resistances were confirmed with different test equipment. Investigations were done as follows: 1) Measure resistance of the bushings only, 2) disconnect the bushings and measure without the bushing in the circuit, 3) break the delta connection and measure each winding phase and, 4) measure each winding lead for all 3 winding phases. Eventually it was determined that the high resistance was caused where the 4 parallel winding leads were joined for one phase. The connection is made on a heavy copper plate by bolted connection, and copper surface corrosion caused a higher contact resistance. Even though this connection was not part of the bushing changeout, the connection was disturbed by physical movement of the copper plate during the bushing lead connection tightening. Once the copper plate and lead connectors were cleaned and everything was reconnected, the measured winding resistances were very close to the original factory acceptance test values. Calculations later showed that the added resistance at the winding lead connection was in the order of 35% of the winding resistance itself, which would have caused very high loss and heating in this small area if not fixed. The delta connection also made the winding resistance increase appear to be less severe due to the parallel phase paths softening the measured phase to phase increase. This case demonstrates the importance of winding resistance measurement, the need to investigate winding resistance differences to resolution and the subtleties of understanding a high winding resistance in a delta connection.

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