7SR210 & 7SR220 Description of Operation
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3.16 Voltage Protection: Under/Over Frequency (81) – 7SR22
Six under/over frequency elements are provided in the 7SR22 relay.
The relay utilises fundamental voltage measurement values for this function. The frequency calculation is based
on the highest input voltage derived from the voltage selection algorithm.
Frequency elements are blocked if all phase voltages fall below the 81 U/V Guard setting.
The sense of the element (under-frequency or over-frequency) is set by the 81-n Operation setting.
81-n Setting sets the pick-up voltage level for the element.
An output is given after elapse of the 81-n Delay setting.
The 81-n Hysteresis setting allows the user to vary the pick-up/drop-off ratio for the element.
Operation of the under/over frequency elements can be inhibited from: -
Inhibit 81-n A binary or virtual input, function key or remote data Comms.
81-n U/V Guarded Under voltage guard element.
or
> <
General Pickup
81-n
Inhibit 81-n
81-n Hysteresis
81-n Delay
c
&
81-n Setting
81-n Operation
81-n Element
Enabled
Disabled
81-n U/V Guarded
Yes
&
Voltage
Selection
VL1
VL2
VL3
F
<
&
81 U/V Guard
Setting
<
<
Inhibit 81-n
General Pickup
81-n
Figure 3.16-1 Logic Diagram: Under/Over Frequency Detector (81)
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