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3.5
Protective Relay
Applications Using
ON/OFF Carriers
The UPLC™ carrier set is particularly suitable for
the following types of protective relay systems:
• Directional-Comparison Blocking
• Phase-Comparison Blocking
• Current Only
• Distance Supervised
3.5.1
Directional-Comparison
Blocking
The basic elements for directional-comparison
blocking systems are shown in Figure 3–26a and
Figure 3–26b. At each terminal, the phase and
ground trip units (P) must be directional and set to
overreach the remote terminal; that is, they must
be set to operate for all internal faults. Nominal
settings of the distance units are 120 to 150 per-
cent of the line. The start units (S) must reach far-
ther, or be set more sensitively, than the remote
trip units. Thus S
must be set more sensitively
1
than P
or reach farther behind bus G. Likewise,
2
S
must be set more sensitively than P
2
farther behind bus H. In any case, the S and P
relays should be similar in type. If the trip unit (P)
is a directional overcurrent ground relay, the start
(S) ground relay should be a similar non-direc-
tional overcurrent unit. The same principle applies
for the phase relays.
When the UPLC™ is set for ON-OFF power line
carrier applications, except for possible auxiliary
functions, no signal is normally transmitted,
since the S units operate only during fault condi-
tions.
Operation of the directional-comparison scheme
(shown in Figure 3–26a and Figure 3–26b) is
internal faults. Subscript 1 indicates relays at sta-
tion G for breaker 1; subscript 2, relays at station
H for breaker 2. (Figure 3–26c shows a solid-state
logic version of Figure 3–26b.)
The schemes shown are still widely used for their
flexibility and reliability. Since the communica-
tion channel is not required for tripping, internal
May 2012
faults that might short and interrupt the channel
are not a problem. Over tripping will occur, how-
ever, if the channel fails or is not established for
external faults within the reach of the trip fault
detectors. Since the carrier transmitter is normally
OFF, or non-transmitting, channel failure cannot
be detected until the system is tested or until an
external fault occurs. This limitation can be over-
come by using the optional checkback system
with the UPLC™ carrier.
A sample schematic for the KA-4 relaying system
is shown in Figure 3–27. A sample schematic for
a basic microprocessor relay system is shown in
Figure 3–28. Figures 3–29a, 3–29b & 3–29c illus-
trate some GE EM relay schematics.
3.5.2
Phase-Comparison Blocking
Basic elements of the phase-comparison systems
are shown in Figure 3–30. The system uses a com-
posite sequence current network to provide a sin-
gle-phase voltage output proportional to the posi-
tive, negative, and zero sequence current input.
or reach
1
Sensitivity to different types of faults depends on
the weighting factors or constants designed into
the sequence current network. Adjustments to the
network are provided.
A squaring amplifier in the controlling relay con-
verts the single-phase voltage output to a square
wave. The positive voltage portion corresponds to
the positive half-cycle of the filter voltage wave
and the zero portion corresponds to the negative
half-cycle. The square wave is used to key the
UPLC™, transmitting to the remote terminal. The
square wave from the remote terminal is com-
pared to the local square wave, which has been
delayed by an amount equal to the absolute chan-
nel delay time. This comparison of the local and
remote square waves at each terminal determines
whether a fault is internal or external.
Fault detectors are used to determine whether a
fault has occurred and to supervise tripping. The
fault detectors must be overreaching, i.e., set sen-
Chapter 3. Applications
Page 3–25
3
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