Voltage Protection: Under/Over Frequency (81) - 7Sr12; Figure 3-30 Logic Diagram: Under/Over Frequency Detector (81) - Siemens 7SR12 Argus Manual

3.15 Voltage Protection: Under/Over Frequency (81) – 7SR12
Four under/over frequency elements are provided in the 7SR12 relay – 81-1, 81-2, 81-3 & 81-4.
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.
The relay utilises fundamental frequency RMS voltage for this function. All under/over frequency elements have a
common setting to measure phase to phase (Ph-Ph) or phase to neutral (Ph-N) voltage using the Frequency
Input mode setting.
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
81-n U/V Guarded
81-n Element
Enabled
Disabled
Inhibit 81-n
81-n U/V Guarded
Yes
81 U/V Guard
Setting
<
<
<
VL1
VL2
VL3

Figure 3-30 Logic Diagram: Under/Over Frequency Detector (81)

©2017 Siemens Protection Devices Limited
A binary or virtual input, or function key.
Under voltage guard element.
&
&
&
F
Voltage
Selection
7SR11 & 7SR12 Description Of Operation
81-n Operation
81-n Setting
81-n Hysteresis
c
> <
or
General Pickup
81-n Delay
81-n
Chapter 1 Page 57 of 88