Transformer protection relay based on the induced voltages

The ratio of the induced voltages (IVs) of the primary and secondary windings is equal to the turns ratio during normal operating conditions, magnetic inrush, and over-excitation. It differs from the turns ratio during an internal fault. This paper describes a transformer protection relay based on the IVs. For a single-phase and a three-phase Y–Y transformer, the IVs are estimated. For a transformer with a delta winding, the differences of the IVs between the two phases are estimated to use the line currents, because the delta winding current are practically unavailable. The ratio of the matching IVs or the differences of the IVs is compared with the turns ratio for fault detection. The various test results clearly indicate that the algorithm successfully discriminates internal faults from magnetic inrush and over-excitation and is independent of the core characteristics. This paper concludes by implementing the algorithm into a prototype relay based on a digital signal processor.