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3.6
Voltage Protection: Rate of Change of Frequency (81R)
Rate of Change of Frequency (ROCOF) is used to detect 'islanding' or loss of connection between a generator
and the main utility supply.
Six elements are provided 81R-1, 81R-2, 81R-3, 81R-4, 81R-5 & 81R-6.
All ROCOF elements have a common ROCOF 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 will be blocked if all voltages fall below the 81R U/V Guard setting.
81R-n Setting sets the pick-up frequency change for the element.
The direction of frequency change, Positive (increasing frequency), Negative (decreasing frequency) or Both,
being looked for by the element is set by the 81R-n Direction setting.
An output is given after elapse of the 81R-n Delay setting.
Operation of the voltage vector shift elements can be inhibited from:
Inhibit 81R-n
81R U/V Guard
User Inhibit
©2014 Siemens Protection Devices Limited
A binary or virtual input.
Under voltage guard element.
Reylogic (graphical logic)
Figure 3-6 Logic Diagram: Rate of Change of Frequency (81R)
Chapter 1) 7SR158 Description of Operation
Chapter 1) Page 27 of 38
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