14
TD_MRN3-3_08.06_GB
~
~
∆
U' = I
1
'
jX
d
I
1
'
U
P
U
1
'
Mains
Z
Figure 4.6: Equivalent circuit at mains failure
In case of mains failure or auto re-closing the genera-
tor suddenly feeds a very high consumer load. The ro-
tor displacement angle suddenly increases and the
voltage vector U
1
changes its direction (U
1
') (Fig. 4.6
and 4.7).
Figure 4.7: Change of the rotor displacement angle at sudden
generator load
Figure 4.8: Voltage vector surge
As shown in the time diagram the instantaneous value
of the voltage jumps to another value and the phase
position changes. This is named phase or vector
surge.
The
MRN3-3
measures the cycle duration. A new
measuring is started at each voltage zero passage.
The measured cycle duration is internally compared
with a quartz stable reference time and from this the
deviation of the cycle duration of the voltage signal is
ascertained. In case of a vector surge as shown in fig.
4.8, the zero passage occurs either earlier or later.
The established deviation of the cycle duration is in
compliance with the vector surge angle.
If the vector surge angle exceeds the set value, the re-
lay trips immediately.
Tripping of the vector surge is blocked in case of loss
of one or more phases of the measuring voltage.
Tripping logic for vector surge measurement:
The vector surge function of the
MRN3-3
supervises
vector surges in all three phases at the same time.
Tripping of the relay can be adjusted for an one
phase vector surge (more sensitive measurement). For
this the parameter 1/3 has to be set to "1Ph". When
the parameter 1/3 is set to "3Ph", tripping of the vec-
tor surge element occurs only if the vector surge angle
exceeds the set value in all three phases at the same
time.
