ABB RELION REX640 - Page 497

Im(Y
o)
Re(Y
o)
Forward fault,
unearthed network:
Y
o ≈ j*(I
eTot
-I
eFd
)/U
ph
Forward fault, compensated network:
Y
o ≈ (I
rcc
+ j*(I
eTot
*(1-K) - I
eFd
))/U
ph
Under-comp. (K<1)
Resonance (K=1)
Over-comp. (K>1)
Reverse fault:
Y
o ≈ -j*I
eFd
/U
ph
Forward fault,
high resistance earthed network:
Y
o ≈ (I
Rn
+j*(I
eTot
-I
eFd
))/U
ph
~
~
~
A B C
E
A
E
B
E
C
I
o
Forward
Fault
L
cc
R
cc
I
eFd
I
eTot
(I
eTot
- I
eFd
)
Uo
R
n
Protected feeder
Background network
Y
Fd
Y
Bg
Figure 281: Admittance calculation during a forward fault
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
1MRS759142 F Protection functions
REX640
Technical Manual
497