Manual MRG3
SEG Electronics GmbH
DOK-TD-MRG3, Rev. D
23
4.4 Principle of frequency supervision
The MRG3 is used to generally protect electrical generators, consumers or electrical operating
equipment in general against over- or under frequency. For this purpose, the relay provides three
frequency elements f
1
- f
3
with a free choice of parameters, with pickup values and delay times that
can be independently adjusted.
The measuring principle of the frequency supervision is based in general on the time measurement
of complete cycles, whereby a new measurement is started at each voltage zero passage. The in-
fluence of harmonics on the measuring result is thus minimized.
Figure 4.3: Determination of cycle duration by means of zero passages.
In order to avoid false tripping resulting from interference voltages and phase shifts the relay works
with an adjustable measuring repetition. (refer to chapter 5.4.2).
At low voltages which, for instance, may occur during start-up of the generator, frequency tripping
is some-times not desired.
All frequency supervision functions can be blocked with the aid of an adjustable voltage threshold
UB in case the measured voltages value is below this value.