ABB RELION RET670 Applications Manual page 438

Section 8
Impedance protection
432
grounded at both IEDs. The effect of zero sequence mutual impedance on possible
overreaching of distance IEDs at A bus is increased compared to non compensated
operation, because series capacitor does not compensate for this reactance. The reach
of underreaching distance protection zone 1 for phase-to-ground measuring loops must
further be decreased for such operating conditions.
A
j(X -X )
0L m0
jX
m0
j(X -X )
0L m0
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IEC06000628 V1 EN-US
Figure 220: Zero sequence equivalent circuit of a series compensated double
circuit line with one circuit disconnected and grounded at both IEDs
Zero sequence mutual impedance may disturb also correct operation of distance
protection for external evolving faults, when one circuit has already been disconnected
in one phase and runs non-symmetrical during dead time of single pole autoreclosing
cycle. All such operating conditions must carefully be studied in advance and
simulated by dynamic simulations in order to fine tune settings of distance IEDs.
If the fault occurs in point F of the parallel operating circuits, as presented in figure
221, than also one distance IED (operating in POTT teleprotection scheme) on parallel,
healthy circuit will send a carrier signal CSAB to the remote line end, where this signal
will be received as a carrier receive signal CRBB.
RAA I
FC1
X
F
RAB I
FC2
CSAB
ANSI06000629 V1 EN-US
Figure 221: Current reversal phenomenon on parallel operating circuits
It is possible to expect faster IED operation and breaker opening at the bus closer to
fault, which will reverse the current direction on the healthy circuit. Distance IED RBB
will suddenly detect fault in forward direction and, if CRBB signal is still present due
to long reset time of IED RAB and especially telecommunication equipment, trip its
related circuit breaker, since all conditions for POTT have been fulfilled. Zero
sequence mutual impedance will additionally influence this process, since it increases
the magnitude of fault current in healthy circuit after the opening of first circuit
breaker. The so called current reversal phenomenon may cause unwanted operation of
protection on healthy circuit and this way endangers even more the complete system
stability.
To avoid the unwanted tripping, some manufacturers provide a feature in their distance
protection which detects that the fault current has changed in direction and temporarily
-jX B
C
-jX
C
I RBA RAA I
FC1 FC1
RBB RAB I
FC2
CRBB CSAB
1MRK 504 163-UUS A
RBA
X
F
RBB
CRBB
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Transformer protection RET670 2.2 ANSI
Application manual