January 2016 Page 3–5
Chapter 3. Applications
3
tively than FD2 and permits the local square wave
s
ignal to key the “ON/OFF” carrier transmitter.
FD
2, set with a higher pickup than FD1, is used to
arm the system for tripping. For transmission lines
less than 100 miles long, the FD2 pickup is set at
125 percent of FD1. For lines longer than 100 miles,
the FD2 pickup is set at 200 percent of FD1. On a
three-terminal line, FD2 is set at 250% of FD1, pro-
vided the line length between any two breakers is
less than 100 miles. Phase-Comparison cannot
occur until FD
2 operates. The purpose of the two
fault detectors is to coordinate the comparison of
the local and remote square waves with the keying
of the carrier square wave. The carrier must be
started before the comparison is allowed to
ensure that the remote square wave has been
received.
The basic operation of the system is shown in
Figure 3–3, at the end of this section. FD
1
and FD
2
at both terminals operate for an internal fault (F
I
).
The square wave inputs to the AND from the local
currents are essentially in phase with those trans-
mitted via the channel from the remote terminal.
The local square wave turns the carrier “ON” and
“OFF” to provide the square wave receiver output
for the remote terminal.
A flip flop is energized if the inputs to the AND
continue for 4ms, providing a continuous trip out-
put supervised by FD
2 operation. The 4ms corre-
spond to a phase angle difference of 90°, on a 60-
Hz base, between the currents at the two terminals.
The currents at the two ends of the line may be out
of phase by up to 90° and still trip. This is a block-
ing system, since the receipt of a signal from the
channel prevents tripping. The carrier signal, there-
fore, does not have to be transmitted through the
internal fault. No received signal puts a “1” on the
AND input. With the remote terminals open, this
system provides sensitive instantaneous overcurrent
protection for the entire line. As is characteristic of
blocking systems, the channel is not required for
tripping on internal faults.
For an external fault, such as F
E in Figure 3–3,
blocking is essentially continuous, since the remote
wave input to the AND is out-of-phase with the
local square wave. The secondary ct currents are
essentially out-of-phase for an external fault. The
currents can, however, be in-phase by up to 90
° on
a 60-Hz base and still block.
3.1.2.2 Single Phase-Comparison Blocking,
Distance-Supervised
A distance-supervised scheme should be used if the
minimum internal three-phase fault current is less
than twice the maximum load current. Twice maxi-
mum load current allows FD
l to operate positively
on the minimum internal three-phase fault, yet reset
when an external fault is followed by a maximum
load current flowing through the line. The UPLC-
II™ operates in the same manner as when used with
the current-only scheme, except for the fault detec-
tion and arming techniques.
Two sequence current networks and two distance
relays supplement the two overcurrent fault detec-
tors.
One sequence current network responds only to
negative and zero sequence currents, detecting all
phase-to-phase and ground faults (but not three-
phase faults). The output of this adjustable network
operates the conventional overcurrent FD
l and FD2
fault detectors. The two distance relays operate only
for three-phase faults. Thus, FD2 provides the arm-
ing 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 current
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
1 and the second distance
relay (21S) to provide the carrier start functions.
This second network responds to positive, negative,
and zero sequence currents. Separate networks pro-
vide greater sensitivity: with phase-to-phase faults,
for example, more than twice the sensitivity is
gained.
The setting coordination of FD
l and FD2 overcur-
rent units is the same as for the current-only system.
Settings for the two three-phase distance units are
shown in Figure 3–4. Both 21S and 21P distance
relays must be set to overreach both the local and
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