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Voltage variation (VAR)
4.3.3
Monitoring phase-to-phase voltage
(i)
Figure 4.3-4 illustrates that the twisted waveforms are observed at the local end. The
waveforms are twisted by the fault occurrence, and tripping & reclosing CB. Similar
waveforms will be observed at the remote end at the same time. If the power distribution has
the twisted phenomena occasionally, power system equipment could suffer from the
overvoltage induced by that.
Voltage
Figure 4.3-4 Three voltage waves twisted by fault, CB tripping and reclosing
The PWRQTY function can monitor the variation of waveform. When the variation is bigger
than the nominal value (100%), the PWRQTY function can issue detection signals†. During
the variation analysis, amplitude of the voltage‡ is compared with the nominal amplitude.
†Note: Signals will be issued from the detection logics. See Figure 4.3-5–Figure 4.3-7
about kinds of the signals issued.
Issuing signals
(ii)
Figure 4.3-4 shows that the twisted waveform are grouped into swell/dip/interrupt conditions.
VAR detection logics can inform the degree in accordance with the below cases:
Swell case:
Dip/Sag case:
Interrupt case:
Swell stages
If the amplitude is larger than the nominal (>100%), Swell logic can alarm SWL_DETECTED
signal together with three level signals: STAGE1, STAGE2, STAGE3. Figure 4.3-5 shows the
logic to detect swell stages. For example, STAGE2 alarm will be signaled when the amplitude
has raised for 0.6 sec.
Threshold setting [Swl-ALRM] defines a level for issuing; it has 110% default setting. The
Swell
Dip
Voltage amplitude is rising higher than the nominal voltage
Voltage amplitude is dropping lower than the nominal voltage
Power flow is interrupted (momentarily power failure)
- 558 -
6F2T0207 (0.01)
Interrupt
nominal
Time
GRE200 (5,6)
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