ABB REL670 2.2 IEC, Applications Manual

Page: 397 / 908

Zero sequence mutual impedance Z
m0
cannot significantly influence the operation
of distance protection as long as both circuits are operating in parallel and all
precautions related to settings of distance protection on series compensated line
have been considered. Influence of disconnected parallel circuit, which is earthed
at both ends, on operation of distance protection on operating circuit is known.
Series compensation additionally exaggerates the effect of zero sequence mutual
impedance between two circuits, see figure
216 . It presents a zero sequence
equivalent circuit for a fault at B bus of a double circuit line with one circuit
disconnected and earthed 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-earth 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-US
Figure 216: 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
217 , 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.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 217: 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,
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 391
Application manual