en06000628.vsd
jX
m0
j(X
0L
-X
m0
)
j(X
0L
-X
m0
)
-jX
C
-jX
C
A
B
IEC06000628 V1 EN-US
Figure 113: Zero sequence equivalent circuit of a series compensated double circuit line
with one circuit disconnected and earthed 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
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.vsd
RAA
RBA
I
FC1
I
FC1
F
RAB
RBB
I
FC2
CSAB
CRBB
RAA
RBA
I
FC1
F
RAB
RBB
I
FC2
CSAB
CRBB
IEC06000629 V1 EN-US
Figure 114: 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 blocked for
the short period. The disadvantage is that a local communication is needed between two
protection devices in the neighboring bays of the same substation.
Distance protection used on series compensated lines must have a high overreach to cover the
whole transmission line also when the capacitors are bypassed or out of service. When the
capacitors are in service, the overreach will increase tremendously and the whole system will
be very sensitive for false teleprotection signals. Current reversal difficulties will be
accentuated because the ratio of mutual impedance against self-impedance will be much
higher than for a non-compensated line.
If non-unit protection is to be used in a directional comparison mode, schemes based on
negative sequence quantities offer the advantage that they are insensitive to mutual coupling.
However, they can only be used for phase-to-earth and phase-to-phase faults. For three-phase
faults an additional protection must be provided.
1MRK 505 343-UEN B
Section 7
Impedance protection
179
Application manual
Содержание Relion 670 series
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