ABB 615 series Technical Manual page 386

Section 4
Protection functions
380
When the network is fully compensated in compensated networks,
theoretically during a forward fault, the imaginary part of the measured
admittance equals the susceptance of the protected feeder with a negative
sign. The discrimination between a forward and reverse fault must
therefore be based on the real part of the measured admittance, that is,
conductance. Thus, the best selectivity is achieved 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 ground-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 a ground 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 a ground 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 gTot
Yo
≈
V
ph
15 A j 100 A 1 1 1 .
(
(
+ ⋅ ⋅ −
=
k k V
15 3
GUID-0A181CF5-7EBC-4DF6-BB02-FC76290044D1 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 IG 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.
I )
)
−
gFd
10 A
)
)
−
1 73 . j 2 31 .
(
≈ − ⋅
1MAC059074-MB A
milliSiemens
)
(Equation 35)
615 series ANSI
Technical Manual