ABB RET670 - Page 184

In case of parallel lines, consider the influence of the mutual coupling according to
section "Parallel line application with mutual coupling" and select the case(s) that are
valid in the particular application. By proper setting it is possible to compensate for the
cases when the parallel line is in operation, out of service and not grounded and out of
service and grounded in both ends. The setting of ground-fault reach should be selected
to be <95% also when parallel line is out of service and grounded at both ends (worst
case).
Setting of overreaching zone
The first overreaching zone (normally zone 2) must detect faults on the whole
protected line. Considering the different errors that might influence the measurement in
the same way as for zone 1, it is necessary to increase the reach of the overreaching
zone to at least 120% of the protected line. The zone 2 reach can be even higher if the
fault infeed from adjacent lines at remote end is considerable higher than the fault
current at the IED location.
The setting shall generally not exceed 80% of the following impedances:
• The impedance corresponding to the protected line, plus the first zone reach of the
shortest adjacent line.
• The impedance corresponding to the protected line, plus the impedance of the
maximum number of transformers operating in parallel on the bus at the remote
end of the protected line.
Larger overreach than the mentioned 80% can often be acceptable due to fault current
infeed from other lines. This requires however analysis by means of fault calculations.
If any of the above gives a zone 2 reach less than 120%, the time delay of zone 2 must
be increased by approximately 200ms to avoid unwanted operation in cases when the
telecommunication for the short adjacent line at remote end is down during faults. The
zone 2 must not be reduced below 120% of the protected line section. The whole line
must be covered under all conditions.
The requirement that the zone 2 shall not reach more than 80% of the shortest adjacent
line at remote end is highlighted in the example below.
If a fault occurs at point F see figure
58, the IED at point A senses the impedance:
1
A B B
AF AC CF AC CF
A A
I I I
Z Z Z Z Z
I I
æ ö
+
= + × = + + ×
ç ÷
è ø
EQUATION302 V3 EN (Equation 77)
Section 3 1MRK504116-UUS C
IED application
178
Application manual