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7SR158 Technical Manual
3.5
Voltage Protection: Under/Over Frequency (81)
Six under/over frequency elements are provided in the relay – 81-1, 81-2, 81-3, 81-4, 81-5 & 81-6.
All under/over frequency elements have a common Frequency Input Mode setting defining whether phase to
phase (Ph-Ph) or phase to neutral (Ph-N) voltages are used. The frequency calculation is based on the highest
input voltage derived from the voltage selection algorithm. The frequency of all VT inputs is monitored continually,
switching between reference voltages is instantaneous.
All elements can be 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
User Inhibit
Figure 3-5
Chapter 1) Page 26 of 38
A binary or virtual input, or function key.
Under voltage guard element.
Reylogic (graphical logic)
Logic Diagram: Under/Over Frequency Detector (81)
©2014 Siemens Protection Devices Limited
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