Woodward
Manual MRI3-ITER GB
20
DOK-TD-MRI3 ITERE Rev.A
4.6 Earth-fault directional feature (ER-relay type)
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 relay to measure according to sin
or cos
me-
thods, depending on the neutral-point connection method.
The residual voltage UE required for determining earth fault direction can be measured in three
different ways, depending on the voltage transformer connections.
(refer to 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
MRI3-ITER protective device is connected to a ring core C. T. (see Figure 3.3).
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
Voltage transformer
connections
Measured
voltage at
earth fault
Correction
factor for
residual
voltage
„3pha“
3-phase voltage
transformer con-
nected to terminals
A3, A5, A7, A2
3 x U
N
= 3 x
U
1N
K = 1 / 3
„e-n“
e-n winding
connected to
terminals A3, A2
U
N
=
3 x U
1N
K = 1 /
3
„1:1“
Neutral-point voltage
(= residual voltage)
terminals A3, A2
U
1N
= U
NE
K = 1
Table 4.1: Connection of voltage transformers