RFL Electronics RFL 9300 - Effect of Infeed;Outfeed; Strong Bus Tie; Figure 3-4. Effect of Infeed;Outfeed; Figure 3-5. Strong Bus Tie

3.4 EFFECT OF INFEED/OUTFEED
Consider the 3-terminal transmission line shown in Fig 3-4. Each of the terminals will pass current to the fault.
The impedance seen by relay Rr is:
Z’ = Zr + Zf ( 1 + Im/Ir )
Distance relays must be concerned about over-reaching or under-reaching for this fault. Current-based relays
base trip decision on the arithmetic sum of the currents (or total charge) entering/exiting the zone of protection.
Therefore they are not subject to the effects of infeed or outfeed currents supplied by a third terminal.
RM
R
R
R
L
Z
R
IR
IF
IM
Z
M
Figure 3-4. Effect of infeed/outfeed
3.5 STRONG BUS TIE
Consider the case where a strong bus tie exists between Bus M and Bus R as shown in Fig. 3-5. If a strong in-
ternal fault exists near Bus R and outfeed current can flow through terminal M. This would cause a distance or
directional relay at terminal M to see the fault as external. They will not clear the fault until relay R opens.
M
RR
R
L
RM
R
Figure 3-5. Strong bus tie
RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 4 (973) 334-3100