Direct stability analysis ofAC/DC power systems using a structure-preserving energy function (SPEF) is proposed in this paper. The system model considered retains the load buses thereby enabling the representation of nonlinear voltage dependent loads. TheHVDC system is represented with the same degree of detail as is normally done in transient stability simulation. The converter controllers can be represented by simplified or detailed models. Two or multi-terminalDC systems can be considered. The stability analysis is illustrated with a 3-machine system example and encouraging results have been obtained.

With the availability of high power semiconductor switches with turn-off capability, voltage source converter based controllers for power transmission system applications have become a reality. Prototypes of some second generation Flexible AC Transmission System (FACTS) controllers like TCSC and STATCON have been installed. This paper presents a review of the progress in FACTS. A generalized description of FACTS controllers is also presented.