Schneider Electric 4000 manual April About Disturbance Monitoring, 115

63230-300-212	Chapter 9—Disturbance Monitoring
April 2001	About Disturbance Monitoring

A voltage sag is a brief (1/4 cycle to 1 minute) decrease in rms voltage magnitude. A sag is typically caused by a remote fault somewhere on the power system, often initiated by a lightning strike. In Figure 9–1,the utility circuit breaker cleared the fault near plant D. The fault not only caused an interruption to plant D, but also resulted in voltage sags to plants A, B, and C.

NOTE: The CM4000 is able to detect sag and swell events less than 1/4 cycle duration. However, it may be impractical to have setpoints more sensitive than 10% for voltage and current fluctuations.

Utility

Circuit Breakers

with Reclosers

1 Plant A

Utility

Transformer

2 Plant B

3 Plant C

X 4 Plant D

Fault

A fault near plant D, cleared by the utility circuit breaker, can still affect plants A, B, and C, resulting in a voltage sag.

Figure 9–1: A fault can cause voltage sag on the whole system.

System voltage sags are much more numerous than interruptions, since a wider part of the distribution system is affected. And, if reclosers are operating, they may cause repeated sags. The circuit monitor can record recloser sequences, too. The waveform in Figure 9–2shows the magnitude of a voltage sag, which persists until the remote fault is cleared.

Figure 9–2: Waveform showing voltage sag, which was caused by a remote fault and lasted five cycles.

© 2001 Schneider Electric All Rights Reserved 115