Manual MRG3
SEG Electronics GmbH
DOK-TD-MRG3, Rev. D
31
4.7.3 Earth-fault directional feature
A built-in earth-fault directional element is available for applications to power networks with isolated
or with arc suppressing coil compensated neutral point.
For earth-fault direction detection it is mainly the question to evaluate the power flow direction in
zero sequence system. Both the residual voltage and neutral (residual) current on the protected
line are evaluated to ensure a correct direction decision.
In isolated or compensated systems, measurement of reactive or active power is decisive for earth-
fault detection. It is therefore necessary to set the ER/XR-relay type to measure according to sin
or cos
methods, depending on the neutral-point connection method.
The residual voltage U
E
required for determining earth fault direction can be measured in three dif-
ferent ways, depending on the voltage transformer connections. (see Table 4.1).
Total current can be measured by connecting the unit either to a ring core C.T. or to current trans-
formers in a Holmgreen circuit. However, maximum sensitivity is achieved if the MRG3 protective
device is connected to a ring core C. T. (see Figure 3.4).
The pick-up values I
E>
and I
E>>
(active or reactive current component for cos
or sin
method) for
ER-relay types can be adjusted from 0.01 to 0.45 x I
N
.
Adjustment
possibility
Application
see chapter 5.6
Voltage transformer
connections
Measured
voltage at
earth fault
Correction
factor for
residual
voltage
„3pha“
3-phase voltage
transformer connected
to terminals A3-A4,
A5-A6, A7-A8
(
MRG3-IER
;
MRG3-IU0
;
MRG3-U0
)
Encoding of the termi-
nal strip X19 to 3PHA
3 x U
N
= 3 x U
1N
K = 1/3
„e-n“
e-n winding
connected to
terminals B1, A2
(
MRG3-IER
;
MRG3-IU0;
MRG3-U0
)
Encoding of the termi-
nal strip X19 to 1:1
U
N
=
3 x U
1N
K = 1 /
3
„1:1“
Neutral-point voltage
(= residual voltage)
terminals B1, A2
(
MRG3-IER
;
MRG3-IU0;
MRG3-U0
Encoding of the termi-
nal strip X19 to 1:1
U
1N
= U
NE
K = 1
Table 4.1: Connection of the voltage transformers
