The basic principle of tile sum current method, illustrated in Fig. 1(a), is based on the fact that in a three-phase system, if the
system voltages are perfectly balanced and the bushings (or current transformers) are identical, the vector sum of the bushing currents will be zero (Fig. 1(b)). The bushing currents simply represent the leakage currents available at the capacitance or power factor taps of the bushings (Fig. 1(a))