Practical protection and control strategies during large power-system disturbances

Disturbances in power systems can take the form of large and sudden changes, such as loss of a large generating unit or fault on a critical part of the system. In case of an emergency, control and/or protection actions are required to return the system back to a normal, secure state as soon as possible. Historically, only centralized control was used because only at this level were computers and communications economically justified. With the increased availability of sophisticated computer, communication and measurement technologies, more “intelligent” equipment can be used at the local level to improve the overall emergency response. In this paper, present design practices for emergency protection equipment are analyzed. Attributes of typical underfrequency load-shedding schemes are described. Implementation of the rate-of-change-of-frequency function is evaluated and adaptive features suggested. Intelligent emergency control strategies to prevent system islanding, frequency drop, and voltage instability are proposed