Saturation Curve Estimation of Three-Legged Three-Phase Transformers Using Inrush Current Waveforms

This paper proposes an innovative methodology to estimate the saturation curve of three-phase three-legged transformers, including deep saturation. Unlike other methodologies, the proposal only requires terminal measurements (only one three-phase inrush test and only one three-phase no-load test) without breaking the winding connections and without knowledge of the residual flux. No special tests with specific winding connections are necessary. This feature allows the methodology to be applied to any power transformer installed on the grid or in industrial facilities, not just limited to laboratory transformers. The estimation of the parameters related to deep saturation is based on the instantaneous reactive power during the inrush transient. The instantaneous reactive power shares common characteristics among all possible inrush transients at a given transformer, making it usable as a kind of signature. The proposed methodology differs from the literature ones in the no need to know the residual flux or the energization point-on-wave corresponding to the inrush test; therefore, inrush tests can be conducted without controlled switching. The methodology is validated through multiple laboratory measurements, demonstrating its effectiveness by showing close agreement between measured and estimated inrush currents and no-load hysteresis loops on a small transformer, as well as between measured and calculated inrush currents during the energization of a 7.5 MVA distribution transformer.