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GEK-90636
CAUTION
It should be recogniz ed when applying the over frequenc y function in this
fashion that a loss of I)C to the relay will cause the output relay to reset, thus
enabling the associat ed off-line protectio n.
Load Restorat ion
If a load sheddin g program has been successf ully impleme nted, the system
frequency will stabilize , and through system control will recover to normal. The
recovery to normal is likely to be quite slow and may extend over a period of several
minutes . As the frequency approaches normal the restore function in the SFF relay
can be used to automat ically begin the load restorati on process. The amount of load
that can be restored is determin ed by the ability of the system
to
serve it. The
availabi lity of generati on, either locally
ol-
through system intercon nections ,
determin es whether or not the shed load can be successf ully restored .
A load
restorati on program usually incorporates substant ial time delay, which must be
provided by a timer external
to
the SFF relay. The amount of time delay to use is
related to the amount of time required to add generati on or
to
close tie lines during
emergency conditions. Also, both the time delay and the restorati on frequenc y
setpoint s should be staggere d so that all of the load is not reconnected at the same
time. Reconnecting loads on a distribut ed basis also minimizes power swings across
the system and thereby minimizes the possibility of initiatin g a new disturba nce.
Since the restorati on timers will be set for long time delays, it is essentia l that
they do not reset as a result of transien t system frequency oscillati ons that may
moment arily reset the restore output function. For this reason, the restore function
is provided with an adjustab le time delay reset before the contacts change state
whenev er the function is subjected to a frequency change from above the setpoint to
below the setpoint . This delay is adjustab le and should be kept very brief, only long
enough
to
ride over moment ary under frequency conditions.
The restore frequency setting of the function will most likely be at or very close to
rated frequency. If the continuo us variation s in normal frequency are such that the
restore function output relay will pick up and drop out continua lly, the life of the
relay may be shortene d. To prevent this, a contact converte r designat ed CC is
provided to control the restore output. CC is to be energized by an external device
after load sheddin g has occurred and when it is desired to restore load. CC should be
kept energized until all of the load that was shed has been restored at which time it
should be de-energized. In this way, the restore output relay will only be operated
after an under frequency condition has been detected and until the load that was shed
has been restored. During normal frequency conditions, CC will be de-energized and
the restore function will not operate due
to
minor changes in frequency.
6
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