Under Frequency Protection Sequence Example
An Under Frequency protection sequence would be an Under Frequency trip followed by a Normal Frequency Close as depicted in the figure below.
Figure 175
Under Frequency Protection Sequence
Frequency/Time diagram
This sequence occurred with the following settings:
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System Frequency = 50.0 Hz
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Under Frequency Trip at = 48.0 Hz
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Under Frequency Normal at = 49.0 Hz
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And Dead-band = 0.2 Hz
Sequence of Events
The first time the frequency went below the Under Frequency Pickup (48.0 Hz) it caused a pickup, but it went back up above the Under Frequency Reset (48.2 Hz) and reset before the Under Frequency Trip After number of cycles had completed.
The next time the frequency went below 48.0 Hz it caused another pickup and this time stayed below 48.2 Hz long enough to cause an Under Frequency trip.
Sometime later the frequency went back up above the Under Frequency Normal At setting (49.0 Hz) which started the Normal Frequency Close After timer. Because the frequency then stayed above the Under Frequency Normal Reset value (48.8 Hz) until the Normal Frequency Close After time out expired, the switchgear would have closed.