Minimizing Inrush Current and Voltage Dip at Energization of Transformer

– This publication reports on a laboratory investigation of inrush currents at transformer energization, and the efficiency of using controlled switching as a means to minimize these. Investigations were performed in a 20 kV grid, where a transformer of 900 kVA, 20 kV / 690 V was energized and disconnected using a vacuum circuit breaker at the 20 kV level. Inrush currents ranging from a few amperes to close to 140 A were obtained (to be compared with the nominal current of the transformer which is 26 A rms at the 20 kV voltage level). Hence, a huge benefit can be obtained by choosing the optimal closing angle. The investigations were performed during different load conditions and the results revealed the optimal closing angle as well as the optimal type of closing operation (simultaneous or single pole operation).