Research on controlled switching in reducing unloaded power transformer inrush current considering circuit breaker's prestrike characteristics

A random energization unload power transformers may cause large inrush currents and overvoltages, which have high magnitudes and rich harmonic components. These transients have undesirable effects, including potential damage or loss-of-life to the power transformer, protective relay misoperation, and reduced power quality on the system. Controlled switching is one of the most effective methods to eliminate inrush currents when energizing an unload transformer, and the performances of controlled energizing unload power transformers mainly depend on the optimum closing instants of circuit breakers. The influences of the prestrike characteristics of circuit breaker on controlled energizing unloaded power transformers are analyzed. And a math model of is given to determine the optimum closing instant in terms of the prestrike characteristics of circuit breakers. Experimental investigations are carried out in order to demonstrate the proposed method. The results show the validation to reduce the unloaded power transformer inrush currents.