Limiting Voltage Dips & Inrush Currents When Energizing Power Transformers

Energization of power transformers from the grid may lead to excessive voltage drop (dip) and inrush currents. A major breakthrough in Controlled Switching Device (CSD) technology has however been achieved to mitigate these transients while energizing power transformers through simultaneous pole operated switches (gang operated), including 3-pole medium voltage switchgears (MV- 600V similar to 69kV) and high voltage (HV > 69kV) circuit breakers (CBs). These "Point-On-Wave" devices mitigate the inrush current by controlling the energization instant of the power transformer relative to its core residual flux pattern. This paper covers the theory and case study of energizing power transformers using simultaneous 3-pole operated switches (3-p). It describes the basic theory to understand transformer inrush currents and then applies it to simultaneous energization of power transformers phases using gang operated switching apparatus. The negative effects of inrush currents are then described, including voltage dip. The transformer saturation is analyzed as a function of the transformer's residual flux, leading to a simple approach for the reliable prediction of inrush currents resulting from both random- and CSD controlled-operation of a 3-p switches. This concept is then verified by case studies, including the impact of transformer tap changers.