A number of catastrophic failures have occurred in the past when synchronous generators have been accidentally energized while at standstill. Among the causes for such incidents were human errors, breaker flashover, or control circuitry malfunction.
A number of protection schemes have been devised to protect the generator against inadvertent energization. The basic principle is to monitor the out-of-service condition and to detect an accidental energizing immediately following that state. As an example, Fig. 9.23 shows an application using an over-frequency relay supervising three single phase instantaneous overcurrent elements. When the generator is put out of service or the overfrequency element drops out, the timer will pick up. If inadvertent energizing occurs, the overfrequency element will pick up, but because of the timer dropout delay, the instantaneous overcurrent elements will have the time to initiate the generator breakers opening. The supervision could also be implemented using a voltage relay.
Accidental energizing caused by a single or three-phase breaker flashover occurring during the generator synchronizing process will not be detected by the logic of Fig. 9.23. In such an instance, by the time the generator has been closed to the synchronous speed, the overcurrent element outputs would have been blocked.
A number of protection schemes have been devised to protect the generator against inadvertent energization. The basic principle is to monitor the out-of-service condition and to detect an accidental energizing immediately following that state. As an example, Fig. 9.23 shows an application using an over-frequency relay supervising three single phase instantaneous overcurrent elements. When the generator is put out of service or the overfrequency element drops out, the timer will pick up. If inadvertent energizing occurs, the overfrequency element will pick up, but because of the timer dropout delay, the instantaneous overcurrent elements will have the time to initiate the generator breakers opening. The supervision could also be implemented using a voltage relay.
Accidental energizing caused by a single or three-phase breaker flashover occurring during the generator synchronizing process will not be detected by the logic of Fig. 9.23. In such an instance, by the time the generator has been closed to the synchronous speed, the overcurrent element outputs would have been blocked.
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| FIGURE 9.23 Frequency supervised overcurrent inadvertent energizing protection. |