Woodward MRI3, Manual, Page: 24 / 68

Woodward Manual MRI3 GB
24 DOK-TD-MRI3 Rev.A
4.5 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 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
φ
or cos φ methods, depending on the neutral-point connection method.
The residual voltage UE required for determining earth fault direction can be measured in three dif-
ferent ways, depending on the voltage transformer connections.
(refer to Table 4.1: Connection of the voltage transformers). Total current can be measured by
connecting the unit either to a ring core C.T. or to cur-rent transformers in a Holmgreen circuit.
However, maximum sensitivity is achieved if the MRl1 protective device is connected to a ring core
C. T. (see Figure 3.2).
The pick-up values I
E>
and I
E>>
(active or reactive current 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 relay type MRI3-XR these pick-up val-
ues can be adjusted from 0.1 to 4.5% I
N
.
Adjust-
ment pos-
sibility Application Voltage transfor-
mer connections
Measured
voltage at
earth fault Correction fac-
tor for residual
voltage
„3pha“ 3-phase voltage
transformer con-
nected to terminals
A3, A5, A7, A2
(
MRI3-IRER; MRI3-
IER; MRI3-ER/XR
)
√3 x U
N
= 3 x
U
1N
K = 1/3
„e-n“ e-n winding con-
nected to terminals
A3, A2 (
MRI3-IER;
MRI3-ER/XR
)
U
N
= √3 x U
1N
K = 1/ √3
„1:1“ Neutral-point vol-
tage(=residual vol-
tage) terminals A3,
A2 (
MRI3-IER; MRI3-
ER/XR
)
U
1N
= U
NE
K = 1
Table 4.1: Connection of the voltage transformers