Modelling of on-load tap-changer transformer with variable impedance and its applications

In power system fault analysis, an on-load tap-changer transformer is normally presented in terms of its leakage impedance at nominal tap position. However the operational position of an OLTCT is deviated from its nominal position, resulting in the actual impedance differing from its nominal value due to the different turns ratio. The variation of transformer impedance introduces error in fault calculations which may affect the setting of numerical protection relays and the selection of circuit breaker rating. This paper presents a new model of an on-load tap changer transformer to improve the accuracy of fault calculation.