SEG Electronics GmbH
Manual MRG3
24
DOK-TD-MRG3, Rev. D
4.5 Mains decoupling
The MRG3 provides two different evaluation criteria for the mains decoupling: measurement of the
frequency gradient and supervision of vector surge. By means of parameter setting, it is possible to
select one of the two measurement methods.
4.5.1 Measuring of frequency gradients
Electrical generators running in parallel with the mains, e.g. industrial internal power supply plants,
should be separated from the mains as soon as possible when a failure in the intrasystem occurs
for the following reasons:
It must be prevented that the electrical generators are damaged due to asynchronous
recovery asynchrone, e.g. after a short interruption.
The industrial internal power supply must be maintained.
A reliable criterion for the detection of a mains failure is the measurement of the rate of change of
frequency df/dt. Precondition for this is a load flow via the mains coupling point. At mains failure the
load flow changing then spontaneously leads to an increasing or decreasing frequency. At active
power deficit of the internal power station a linear drop of the frequency occurs and a linear in-
crease occurs at power excess. Typical frequency gradients during application of "mains decou-
pling" are in the range of 0.5 Hz/s up to more than 2 Hz/s. The MRG3 detects the instantaneous
frequency gradient df/dt of each mains voltage period in an interval of one half period each.
Through multiple evaluation of the frequency gradient in sequence the continuity of the directional
change (sign of the frequency gradient) is determined. Because of this special measuring proce-
dure a high safety in tripping and thus a high stability against transient processes, e.g. switching
procedure are reached. The total switching off time at mains failure is between 60 ms and 80 ms
depending on the setting.
