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reset when an external fault is followed by a
maximum load current flowing through the
line. The UPLC™ operates in the same man-
ner as when used with the current-only
scheme, except for the fault detection and arm-
ing techniques.
Two sequence current networks and two dis-
tance relays supplement the two overcurrent
fault detectors.
One sequence current network responds only
to negative and zero sequence currents, detect-
ing all phase-to-phase and ground faults (but
not three-phase faults). The output of this
adjustable network operates the conventional
overcurrent FD
and FD
l
2
two distance relays operate only for three-
phase faults. Thus, FD
provides the arming
2
function for all unbalanced phase and ground
faults, through the adjustable filter, and one of
the distance relays (21P) provides arming for
all three-phase faults.
The second and non-adjustable sequence cur-
rent network operates through the squaring
amplifier, providing the local square wave and
the carrier-keyed square wave required for
phase comparison. This signal is keyed by FD
and the second distance relay (21S) to provide
the carrier start functions. This second net-
work responds to positive, negative, and zero
sequence currents. Separate networks provide
greater sensitivity: with phase-to-phase faults,
for example, more than twice the sensitivity is
gained.
The setting coordination of FD
current units is the same as for the current-
only system. Settings for the two three-phase dis-
tance units are shown in Figure 3–32. Both 21S
and 21P distance relays must be set to overreach
both the local and remote terminal buses; 21S
must be set further than 21P, as shown.
May 2012
fault detectors. The
1
and FD
over-
l
2
X
Z C
Z A
Z C
Bus H
Z A
Bus G
X
Bus H
Z A
Z C
Z C
Z A
Bus G
Figure 3–32. Single Phase-Comparison Blocking,
Distance-Supervised Operation.
Chapter 3. Applications
21 S at Breaker
21 P at Breaker
R
21 S at Breaker
21 P at Breaker
R
Page 3–35
3
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Troubleshooting
