Impact of Power Transformer Energization on Relay Protection Operation

One of the main causes of the incorrect operation of transformer relay protection are inrush currents. The transient inrush current occurs when energizing the unloaded transformer, and it is a consequence of the transformer core saturation. This paper presents analysis of transients caused by energization of 300 MVA three-phase autotransformer with stabilizing winding and voltage ratios 400/115/10,47 kV. Using the PAMSUITE Toolbox in EMTP, many simulations were performed to show the impact of model’s parameters on the amplitude and duration of inrush currents as well as the probability of 2nd harmonic differential current amplitude occurrence. Simulation results show that controlled switching reduces inrush currents amplitudes and increases 2nd harmonic content in differential currents. A Protection Toolbox in EMTP was used for modelling CT saturation and operation of transformer’s differential protection relay. Based on the simulation results the optimal differential protection settings were determined, and the probability of false relay protection operation was calculated.