5 INDEPENDENT R.O.C.O.F PROTECTION
Where there are very large loads, imbalances may occur that result in rapid decline in system frequency. The
situation could be so bad that shedding one or two stages of load is unlik
ely to stop this rapid frequency decline. In
such a situation, standard underfrequency protection will normally have to be supplemented with protection that
responds to the rate of change of frequency. An element is therefore required which identifies the high rate of
decline of frequency, and adapts the load shedding scheme accordingly.
Such protection can identify frequency variations occurring close to nominal frequency thereby providing early
warning of a developing frequency problem. The element can also be used as an alarm to warn operators of
unusually high system frequency variations.
5.1 INDEPENENT R.O.C.O.F PROTECTION IMPLEMENTATION
The device provides nine independent stages of protection. Each stage can respond to either rising or falling
frequency conditions. This depends on whether the fr
equency threshold is set above or below the system nominal
frequency. For example, if the frequency threshold is set above nominal frequency, the rate of change of frequency
setting is considered as positive and the element will operate for rising frequency conditions. If the frequency
threshold is set below nominal frequency, the setting is considered as negative and the element will operate for
falling frequency conditions.
The following settings are relevant for df/dt+t protection:
● df/dt+t (n) Status: determines whether the stage is for falling or rising frequency conditions
● df/dt+t (n) Set: defines the rate of change of frequency pickup setting
● df/dt+t (n) Time: sets the time delay
Chapter 11 - Frequency Protection Functions P14x
242 P14xEd1-TM-EN-1
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