18
TB MRI(K)3-C 03.02 E
4.6
Earth-fault directional feature
(ER/XR-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 ques-
tion to evaluate the power flow direction in zero se-
quence 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 de-
tection. It is therefore necessary to set the ER/XR-relay
type to measure according to sin
ϕ
or cos
ϕ
methods,
depending on the neutral-point connection method.
The residual voltage U
E
required for determining earth
fault direction can be measured in three different
ways, depending on the voltage transformer connec-
tions (refer to Table 4.2).
Total current can be measured by connecting the unit
either to a ring core C.T. or to current transformers in a
Holmgreen circuit. However, maximum sensitivity is
achieved if the
MRI(K)3-C
protective device is con-
nected to a ring core C. T. See Figure 3.4.
The pick-up values I
E>
and I
E>>
(active or reactive cur-
rent component for cos
ϕ
or sin
ϕ
method) for ER-relay
types can be adjusted from 0.01 to 0.45 x I
N
. For re-
lay type
MRI(K)3-XR
these pick-up values can be ad-
justed from 0.1 to 4.5% I
N
.
Adjustment
possibility
Application
Voltage transformer
connections
Measurd
voltage at
earth fault
Correction factor
for residual
voltage
„e-n“
3-phase voltage
transformer connected
to terminals A3, A5,
A7, A2
(MRI(K)3-ICER
;
MRI(K)3-ICXR
)
U
N
=
√
3 x U
1N
K = 1 /
√
3
„1:1“
Neutral-point voltage
(= residual voltage)
terminals A3, A4
(MRI(K)3-ICER
;
MRI(K)3-ICXR
)
U
1N
= U
NE
K = 1
Table 4.2: Connection possibility of the voltage transformers
