ABB RELION Series - Page 339

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
GUID-5DB19698-38F9-433E-954F-4EBDBA5B63BD V1 EN
Figure 168: 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 must therefore be based on the real part of the measured
admittance, that is, conductance. Thus, the best selectivity is achieved
1MRS758755 A Section 4
Protection functions
REC615 and RER615 333
Technical Manual