Reduction of voltage stress and inrush current of power transformers using controlled switching

Random energisation of power transformers often causes significant inrush currents and steep switching voltage wave fronts that are responsible for several adverse effects like huge current forces in the transformer windings, dielectric stress of the transformer winding insulation and temporary overvoltages. Controlled switching taking into account the residual flux is able to eliminate these inrush currents, but significant voltage wave fronts emerge from the switching action in almost all cases. Thus, a new algorithm is presented that extends the existing one so that a combined reduction of all stresses can be achieved. Systematic energisation studies were carried out in EMTP-ATP to calculate the optimal closing times of a three-legged 400 kVA transformer. Thereby, an inrush current peak of 1 pu will be tolerated so that the voltage wave front amplitude can be reduced. Depending on the strategy for controlled switching, the maximum voltage wave front amplitude of all phases can be limited to 0.75 pu (“Delayed Closing”, reduction of 11.7 %) respectively to 0.7 pu for “Rapid Closing” (reduction of 22 %).