The effects of unbalanced networks on synchronous and asynchronous machine transient stability

Untransposed lines and feeders, together with single-phase loads, impose permanent unbalanced conditions on the rest of the power system. Unbalanced phase voltages at the terminals of a machine can be represented as balanced sets of positive-, negative- and zero-sequence voltages. The negative-sequence voltage gives rise to negative-sequence current which creates excessive heating, particularly in the rotor circuits. Much attention has been paid to the heating aspect, and protection provided against this, by limiting the levels of negative-sequence current; however, power system stability studies, in particular for multimachine systems, have until now ignored the imbalance and its effects upon the dynamic response of the rotor speed of each machine. This paper reconsiders the transient stability of a single synchronous generator connected to an infinite bus and evaluates the significance of the error incurred by representing the unbalanced line and shunt load by an equivalent balanced system. The paper also reconsiders the transient stability of a multimachine system containing a synchronous generator and an induction motor when subjected to different types and degrees of imbalance and finds that significant errors are incurred under certain conditions.