UPFC operations under stressed system conditions

Power systems often work in stressed conditions nowadays. A unified power flow controller (UPFC) with its capability to regulate significant amounts of real and reactive power flow simultaneously and independently can, at its full capacity, exert effects on the system that are strong enough to be counted as transient events. In the worst scenario, improper operations of UPFC might jeopardize the system stability. In this paper, a mathematical representation is developed for a simple power system with UPFC installed. Insight into the mechanism of UPFC is obtained. Stability criteria are studied for three different UPFC control strategies: fixed parameters control (FPC), in phase voltage control (IPVC) and quadrature voltage control (QVC). It is found out that FPC always has the smallest stability margin, while QVC has the biggest stability margin at stressed conditions, and IPVC has a slightly better result at medium real power output level. Therefore, under stressed conditions, QVC should be employed whenever possible.