Toward Online Frequency Response Analysis of In-Service Power Transformers: A Single-Coil Three-Phase Parallel-Connected Sensing System

Winding deformation is an important contributor to power transformer faults and forced outages. Effective and mature online detection methods are still lacking. The online frequency response analysis (FRA) method is a promising candidate for further development. A major obstacle in online FRA is the strong interference posed by the high-amplitude power-frequency load current (hundreds of amperes) to the extremely weak FRA test signal at the sub-milliampere level. This paper proposes a single-coil, three-phase parallel-connected sensing system, which employs three identical coil-type current sensors. Its capability to suppress power-frequency interference while measuring high-frequency signals has been theoretically analyzed and experimentally validated. An online measurement system based on it was applicated on an operating 110 kV power transformer. Field measurements verified that the proposed single-coil sensing system suppresses power-frequency interference by a factor of 100. Furthermore, the new frequency response functions derived from the online current signals exhibited resonance features closely aligned with the conventional offline FRA method, confirming the method's stability and suitability for diagnosing winding deformation in in-service power transformers.