Schneider Electric 4000

63230-300-212 Chapter9—Disturban ceM onitoring

April 2001 About Disturbance M onitoring

115

A voltage sag isa brief (1/4 cycle to 1 minute) decrease in rms voltage

magnitude.A sag is typicallycaused by a remote fault somewhere on the

power system,often initiated by a lightning strike. In Figure 9–1, the utility

circuitbr eakercleared the fault near plant D.The fault not only caused an

interruptionto plant D, but also resultedin voltage sags to plants A, B,and C.

NOTE: TheCM 4000i s ableto detect sag and swell eventsless than 1/4 cycleduration. However,it may be impractical to havesetpoints more sensitivethan 10% for voltageand current fluctuations.

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

System voltagesags are much more numerous than interruptions, since a

wider part of the distributionsystem is affected. And, if reclosers are

operating,they may cause repeated sags. The circuit monitor can record

recloser sequences,too. The waveform in Figure9–2 shows the magnitude

of a voltagesag, which persists until the remotefault is cleared.

Figure 9–2: Waveform showing voltage sag, whichwas causedby a

remote fault and las ted five cycles.

Utility

Circuit Breakers

with Reclosers

Utility

Transformer

1 Plant A

2 Plant B

3 Plant C

4 Plant D

Fault

A faultnear plant D, clearedby the utility

circuit breaker, can stil l affect plants A, B,

andC, resulting in a voltagesag.