Reverse Power Flow Capability of Tap-changers

INTRODUCTION SWITCHING SEQUENCE A single resistor on-load tap changer comprising fixed contacts F1 and F2, transitional arcing contact T, main arcing contact M and bridging resistor R is shown in Fig. 1. A command initiates the switching operation from normal running position 1 to position 2, such that contact T starts to move left towards contact F2 (Fig. 1a). As T approaches F2 a recovery voltage VR1 (equal to the tap step voltage VS) is established that gives rise to arcing. The moment T makes contact with F2 (Fig. 1b) a circulating current ICIRC (= VS/R) flows in the opposite direction to the direct load current IL. Contact M now starts to move away from F1 (Fig. 1c) creating a recovery voltage VR3 (= VS RIL) and an arc as the current (IL ICIRC) is interrupted. Further movement of M extinguishes the arc and approaching the already joined F2 and T will create (make) a current IL under recovery voltage VR4 (=RIL). The position where contacts F2, T and M are joined (Fig. 1d) is the last transitional arrangement preceding the second normal running position where contact T is separated from F2 and M without arcing since no current is interrupted (Fig. 1e). This paper presents an analysis of the reverse power flow capability of on-load tap-changers with the aim to find the penetration level of distributed generation that can be connected to the network. An in-detail analysis of the switching sequence during tap changing is presented first. The physical constraints that limit the switching process are then identified, and calculation of the reverse power flow capability is done by solving the optimisation problem.