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Underfrequency
Rate of change
of frequency
Recovery function
4
A protection stage has the function of an under-
frequency stage, when the setting value is be-
low the rated frequency. The setting value can-
not be the same as the rated frequency.
Each protection stage incorporates a rate of
change of frequency (df/dt) function, which can
be used alone or together with the frequency
function.
The operation principle of the rate of change of
frequency function is the same as that of the
overfrequency/underfrequency stage. This means
that if the frequency setting of this particular
stage is above the rated frequency, the rate of
change of the frequency has to be positive to
make the df/dt function become true. Corre-
spondingly, the rate of change of frequency has
to be negative, if the stage has the function of
an underfrequency stage. The same principle
also applies, when the df/dt function is used
alone, without the frequency function.
When the df/dt function is used alone, the stage
starts once the absolute value of the rate of
change of the frequency is greater than the set-
ting value of the stage. At the same time as the
stage starts, a start indicating code appears on
the display. Should the protection stage still be
In addition to the four protections stages the
module incorporates a recovery stage. This re-
covery stage can be used to control the desired
output relay when, after tripping of the protec-
tion stage, the frequency returns to normal and
remains within the setting range throughout the
operate time of the recovery function.
The permitted limits for recovery is defined as
a frequency window, the centre of which is the
programmed rated frequency of the module.
The limit to be set is the maximum permitted
frequency deviation from the rated frequency
± f
(f
) of the module.
n
r
The underfrequency stage operates in the same
way as the overfrequency stage, but starts, when
the frequency falls below its setting value.
operated, when the set operate time expires, it
trips, delivering the programmed trip signal.
When the df/dt function is used together with
the frequency function, the stage starts when
the operation criteria for both functions, i.e. the
df/dt function and the frequency function, are
fulfilled at the same time. Should the protec-
tion stage still be operated (both conditions for
operation fulfilled), when the set operate time
expires, the stage trips, delivering the configured
trip signal.
The rate of change of frequency function is rec-
ommended to be used together with the fre-
quency function.
The minimum trip time is 150 ms, when the
rate of change of frequency function is used. This
means that if the rate of change of frequency
function is enabled with the SGF switches, the
minimum operate time of the timer is 150 ms.
When the frequency function is used alone, the
minimum operate time is 100 ms.
The timer of the recovery function resets and
remains reset as long as one single protection
stage selected is in a tripped condition. At the
same time the normally activated recovery out-
put resets. After resetting, the operate time is
calculated for the time when the frequency re-
mains within the specified limits. When the
operate time elapses, the recovery function op-
erates and the output is activated. At the same
time, an operation code indicating activation
of the recovery function appears on the display.
An external control signal can be used to pre-
vent both recovery functions in progress and the
start of new ones.
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