Emergency Control and Special Protection Systems InLarge Electric Power Systems

Abstract

This chapter addresses the topic of designing a corrective control strategy after large disturbances. When a power system is subjected to large disturbances such as simultaneous loss of several generating units or major transmission lines, and the vulnerability analysis indicates that the system is approaching a catastrophic failure, control actions need to be taken to limit the extent of the disturbance. In our approach, the system is separated into smaller islands at a slightly reduced capacity. The basis for forming the islands is to minimize the generation-load imbalance in each island, thereby facilitating the restoration process. An analytical approach to forming the islands using a two-time-scale procedure is developed. Then by exploring a carefully designed load-shedding scheme based on the rate of frequency decline, we limit the extent of the disruption and are able to restore the system rapidly. We refer to this corrective control scheme as “controlled islanding” followed by load-shedding based on the rate of frequency decline. A new class of controls called special protection systems is now finding greater acceptance in power systems with their enhanced need to provide reliability in a competitive restructured utility environment. These systems are triggered only when large disturbances occur. They provide a degree of safety in preventing cascading outages. The special protection systems be presented and their potential for greater use will be examined.