Improvement of System Stability by Switched Series Capacitors

Improvement of transient stability of a 2-circuit ac transmission link by use of switched series capacitors is analyzed by equal-area diagrams for two kinds of disturbances: 1) a permanent fault on one ac circuit, cleared by opening a section of that circuit, without reclosure and 2) a permanent fault on a dc line operating in parallel with the ac line. In both cases, the aggregate megavar rating of series capacitors required for a given transient stability limit is shown to be smaller if some capacitors are switched than if all are unswitched. Moreover, the scheme with switched capacitors reduces the angular swing of the machines and the fluctuation of load voltages during the swing.