ABB RELION REX640 Technical Manual page 468

Section 4
Protection functions
462
when the compensated network is operated either in the
undercompensated or overcompensated mode.
For example, in a 15 kV compensated network, the magnitude of the earth fault current
of the protected feeder is 10 A (Rf = 0 Ω) and the magnitude of the network is 100 A
(Rf = 0 Ω). During an earth fault, a 15 A resistor is connected in parallel to the coil after
a 1.0 second delay. Compensation degree is overcompensated, K = 1.1.
During an earth fault in the forward direction, that is, inside the protected feeder, the
theoretical value for the measured admittance after the connection of the parallel
resistor can be calculated.
I j I 1 K
(
(
+ ⋅ ⋅ −
Rcc eTot
Yo
≈
U
ph
A j A
15 100 1 1 1 .
( (
+ ⋅ ⋅ −
=
15 k k V
3
GUID-6E24223A-E045-4FD3-9823-801F2972D98F V1 EN
Before the parallel resistor is connected, the resistive part of the measured admittance
is due to the leakage losses of the background network and the losses of the coil. As
they are typically small, the resistive part may not be sufficiently large to secure the
discrimination of the fault and its direction based on the measured conductance. This
and the rating and the operation logic of the parallel resistor should be considered
when setting the admittance characteristic.
When a high sensitivity of the protection is required, the residual
current should be measured with a cable/ring core CT, that is, the
Ferranti CT. Also the use of the sensitive Io input should be
considered. The residual voltage measurement should be done with an
open delta connection of the three single pole-insulated voltage
transformers.
The sign of the admittance characteristic settings should be
considered based on the location of characteristic boundary in the
admittance plane. All forward-settings are given with positive sign
and reverse-settings with negative sign.
Operation characteristic
After the admittance calculation is released, the calculated neutral admittance is
compared to the admittance characteristic boundaries in the admittance plane. If the
calculated neutral admittance Yo moves outside the characteristic, the enabling signal
is sent to the timer.
I
)
)
−
eFd
A
10
) )
−
j
1 73 . 2 31 .
(
≈ − ⋅
1MRS759142 C
mS
)
Technical Manual
(Equation 56)
REX640