Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
9.18
FREQUENCY UNITS (81)
The GARD 8000 has 3 units each for Underfrequency (81m), Overfrequency (81M) and Rate-of-
Change of Frequency (81D) for supervision or load shed applications.
The frequency is calculated from one voltage channel.
All frequency units have individual pick-up settings, delay time and drop-out time. In addition, there
are 3 settings common to all frequency units:
•
Undervoltage inhibit. When the voltage is below the set threshold (2 – 150 V), the frequency
elements are blocked.
•
Activation time (pick-up time) set in number of half cycles. This setting determines the number
of half cycles used for measuring the frequency before a frequency condition can be declared.
•
Reset time (drop-out time) set in number of cycles. This determines the number of cycles used
to determine that the frequency has returned to a non-faulted condition. When the frequency
units have picked up but not yet tripped, there could be a change in frequency of a short
duration and this setting prevents the units from dropping out for such a condition.
9.18.1
OVERFREQUENCY UNITS (81M)
The overfrequency units use phase A voltage to measure the frequency. They pick-up when the
frequency exceeds 100% of set value for the set number of half cycles, following a frequency
condition has been declared (set as a common setting). The drop-out is at 99.9% of set value for the
duration of the set drop-out time set for the individual frequency unit.
9.18.2
UNDERFREQUENCY UNITS (81M)
The underfrequency units use phase A voltage to measure the frequency. They pick-up when the
frequency is below 100% of set value for the set number of half cycles, following a frequency
condition has been declared (set as a common setting). The drop-out is at 100.1% of set value for the
duration of the set drop-out time set for the individual frequency unit.
9.18.3
CHANGE OF RATE OF FREQUENCY UNITS (81D)
The change of rate of frequency units use phase A voltage to measure the frequency. They have
settings for underfrequency and rate-of-change of frequency pick-up levels. The underfrequency
element picks up when the frequency is below 100% of set value the set the number of half cycles,
following a frequency condition has been declared (set as a common setting). The underfrequency
drop-out is at 100.1% of set value for the duration of the set drop-out time set for the individual
frequency unit. The rate of change of frequency element picks up when the measured rate of change
exceeds the set value + 0.05 Hz/s for set number of half cycles (set as a common setting), but a
minimum of 5 cycles. The reason is that while frequency can be calculated over one cycle, rate of
change of frequency needs 5 cycles for calculation. In case the set time is less than 10 half cycles, 5
cycles (10 half cycles) are still used.
August 1, 2009
9-68
973.334.3100
