ABB REG670 Applications Manual page 252

Section 8
Impedance protection
246
Zero sequence mutual impedance Z
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 127. 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.
A
j(X -X )
0L m0
jX
m0
j(X -X )
0L m0
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IEC06000628 V1 EN
Figure 127: 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
128, 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
F
RAB I
FC2
CSAB
IEC06000629 V1 EN
Figure 128: 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
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
cannot significantly influence the operation of
m0
-jX B
C
-jX
C
I RBA RAA I
FC1 FC1
RBB RAB I
FC2
CRBB CSAB
1MRK 502 071-UEN -
RBA
F
RBB
CRBB
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Application manual