underreaching distance protection zone 1 for phase-to-ground measuring loops must
further be decreased for such operating conditions.
en06000628.vsd
jX
m0
j(X
0L
-X
m0
)
j(X
0L
-X
m0
)
-jX
C
-jX
C
A B
IEC06000628 V1 EN
Figure 100: 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
101,
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.
en06000629_ansi.vsd
RAA RBAI
FC1
I
FC1
X
F
RAB RBBI
FC2
CSAB CRBB
RAA RBAI
FC1
X
F
RAB RBBI
FC2
CSAB CRBB
ANSI06000629 V1 EN
Figure 101: 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 in 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
blocks distance protection. Another method employed is to temporarily block the signals
received at the healthy line as soon as the parallel faulty line protection initiates tripping.
The second mentioned method has an advantage in that not the whole protection is
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 201
Application manual
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