Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when primary relay fails to be operated during a fault. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. Combines protection, sensors, control power, and circuit breaker in a single package Typically added to a breaker close circuit to prevent accidental reclosure after a trip. Three fundamental components required for each circuit breaker. Applications of the concepts to accepted transmission line-protection schemes are also presented. Many important issues, such as coordination of settings, operating times, characteristics of.
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