SEG Electronics GmbH
Manual MRG3
28
DOK-TD-MRG3, Rev. D
Figure 4.8: Voltage Vector surge
As shown in the voltage/time diagram the instantaneous value of the voltage jumps to another
value and the phase position changes. This instance is generally called phase or vector surge.
The MRG3 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 relay trips without delay.
Tripping of the vector surge is blocked in case of loss of one or more phases of the measuring volt-
age.
Tripping logic for vector surge measurement:
The vector surge function of the MRG3 supervises vector surges in all three phases at the same
time. Irrespective of this, tripping of the relay can be adjusted for an one phase vector surge (more
sensitive measurement). For this purpose, the parameter 1/3 has to be set to "1Ph". When the pa-
rameter 1/3 is set to "3Ph", tripping of the vector surge element occurs only if, in case of a vector
surge, the vector surge angle exceeds the set value simultaneously in all three phases at the same
time.
Application hint
Although the vector surge relay guarantees very fast and reliable detection of mains failures under
nearly all operational conditions of mains parallel running alternators, the following borderline cas-
es, however, have to be considered:
a) During mains failure: none or only insignificant change of power flow at the utility connection
point during mains failures.
This can occur in peak lopping operation or in CHP stations (Combined Heat and Power) where the
power flow between power station and the public grid may be very low. To allow the detection of a
vector surge at parallel running alternators, the load change must be at least 15 - 20% of the rated
power. If the active load at the utility connection point is regulated to a minimal value and a high re-
sistance mains failure occurs, then there are no vector surge nor power and frequency changes
and the mains failure is consequently not detected.
This can only happen if the public grid is disconnected near the power station and so the alterna-
tors are not additionally loaded by any consumers. At distant mains failures the synchronous alter-
nators are abruptly loaded by remaining consumers which leads directly to a vector surge and so
mains failure detection is guaranteed.
If such a situation occurs the following has to be taken into account:
