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SPECIAL CONSIDERATIONS IN APPLYING POWER LINE CARRIER FOR PROTECTIVE RELAYING Special Considerations in Applying Power Line Carrier for Protective Relaying


SPECIAL CONSIDERATIONS IN APPLYING POWER LINE CARRIER FOR
PROTECTIVE RELAYING
IEEE Power Systems Relaying Committee Special paper
Relaying Communications Subcommittee, Working Group H9
Membership of the working group:
M. P. Sanders, Chair
M. J. McDonald, Vice Chairman
J. Appleyard
E. Derencinovic
P. R. Drum
J.D. Huddleston III
D. Jamison
S. Khan
T. Lanigan
W.G. Lowe
B. A. Pickett
R.E. Ray
S.D. Rowe
T. Seegers
M. Simon
J. Soehren
S. Ward
J. A. Zipp


Introduction
Power line carrier has been applied to power lines in this country for over 80 years. In 1919, the
first system for voice-communication purposes was placed into service. By the late twenties,
power line carrier was used for pilot protective relaying, and has continued to provide this
service ever since. [B66]
With the explosion of fiber and the high bandwidth communication, many engineers surmise that
Power Line Carrier is a dying technology. To the contrary, as leased lines become difficult to
acquire and fiber optic channels expensive to install, Power Line Carrier is still the most
economic channel for dedicated protective relaying available. The media is readily available
the power line itself. With the addition of a line tuner, the CCVT (used for potential input to the
protective relay) can be used to couple the PLC signal to the power line. If the only
communication channel required is for protective relaying, then Power Line Carrier is the
obvious choice. Of course, maintenance costs should also be considered when determining the
type of channel to use. Some PLC maintenance will require taking the transmission line out of
service.
The goal in using communications for protective relaying is to provide simultaneous high-speed
clearing for all faults within a line section, including end-zone faults. A PLC channel can also be
used to provide remote tripping functions for transformer protection, shunt reactor protection
and remote breaker failure relaying.
There are many references available that discuss PLC applications. IEEE 643 IEEE Guide for
Power-Line Carrier Applications is a particularly good reference. In the bibliography section of
this paper, many other references are listed.
The intent of this paper is not to cover the actual channel application but circumstances that can
affect the performance of the protective relay system. Subjects such as a non-homogeneous
line, the use of capacitors and reactors in the line, transformer-tapped lines, special protection
applications, and the issue of non-licensure of PLC by the electric utility are covered. The intent
of this paper is to document important issues that should be considered when applying a PLC
channel to a protective relay system.
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Special Considerations in Applying Power Line Carrier for Protective Relaying

Optimal Transmitter / Receiver Signal Levels
An optimal PLC channel would be a two terminal line with a high impedance (i.e. 1500 ohm)
adjustable wide band trap at each end with enough bandwidth to block two pilot schemes and
with line tuners that perfectly matched the 50 ohm carrier equipment to the characteristic
impedance of the line. This system would have zero reflected power and would only have the
series line attenuation to reduce the signal to the remote end. Real life in the Power Line
Carrier world is not like that. We are faced with discontinuities in the line construction, which
presents a change to the characteristic impedance, lines with cable segments, tapped power
transformers, three terminal lines, and limited trap bandwidth at the geometric mean frequency
(GMF), as well as line tuner matching that is a compromise.
The line trap is a tuned circuit that presents a high impedance to carrier frequencies and a low
impedance to the 60 Hz power frequency. If the trap impedance is identical to the characteristic
line impedance of approximately 377 ohms

and assuming that the station bus impedance is
very low, under normal conditions there will be a 6 dB loss to the carrier signal due to half the
carrier signal current traveling through the trap into the system behind the protected line. If the
trap impedance is higher, more of the signal will travel to the lower impedance line segment and
eventually to the remote receiver.
The most important function of a line trap is to prevent the carrier signal from being diverted into
the fault on the out-of-zone line or bus section. If there was a line to ground fault on the bus
behind the protected line segment on the phase the carrier is applied, most of the signal would
be shorted to ground and the signal level would not be adequate at the remote end. On a
directional comparison blocking scheme, this would result in a remote end over-trip. With a line
trap between the fault and the injection point, the carrier signal would see the trap impedance in
series with the fault and the fault would have little or no affect on the carrier signal level.
Another function of the trap is to isolate the protected line from system changes that are in other
areas and not directly on the protected line. It should isolate any impedance changes due to
line and bus switching as well as when protective grounds are connected to the high voltage
system. It is common practice to install wave traps at each end of a two terminal line.
Three Terminal Lines
It is a recommended practice to install line traps at all three ends of a three terminal line. This
will prevent faults or discontinuities external to the line section from attenuating the carrier signal
transmitted to the remote ends. With the lines terminated into a line trap, reflected power will be
kept to a minimum.
Tapped Transformers
A transformer tapped to a line using a PLC channel for communications may cause attenuation
to the signal for several different reasons. The length of the tap line to the transformer may be
near the quarter-wavelength of the carrier, the transformer may present either a low or high
impedance to the signal which will create a reflected signal that will interfere with the main line
signal, or the transformer may even present a short circuit to the carrier signal.
Quarter Wavelength Spur When a tap line is not terminated into tuned carrier equipment or is
terminated into a power transformer, and is one quarter wavelength of the carrier frequency in
length or odd multiples thereof, the maximum out-of-phase reflected signal will occur. This
reflected energy will be out of phase with the transmitted signal and can cause cancelation of
the transmitted signal. To minimize this effect, line traps should be placed as close to the main
line as possible. The trap will attenuate the signal traveling into the tapped line as well as the
reflected energy returning from the tap section. The amount of isolation will be dependent on the
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Special Considerations in Applying Power Line Carrier for Protective Relaying

trap impedance with respect to the impedance of the main line. The higher the trap impedance,
the greater the signal attenuation will be.
Grounded Wye Connected Transformer Transformers with a grounded wye connection
provide a shunt path through interwinding and terminal to ground capacitances for the carrier
signal to ground. This type of connection will greatly effect the levels of the carrier signal. A trap
would usually be necessary when adding a tapped transformer with a Grounded Wye
connection.
Delta Connected Tapped Transformer Generally it has been found that delta connected
transformers will have less attenuating effect on the carrier signal than grounded wye
connections. Experience has shown that in some instances as many as three or four tapped
transformers may be added to a line section before the attenuation becomes so great that the
remaining signal is too low to be usable. The characteristic impedance of transformers can vary
greatly and is difficult to determine. A trap installed close to the main line will usually reduce the
attenuation effect caused by delta connected tapped transformers.
Some interesting results were found and described in a paper Carrier Frequency
Characteristics of Power Transformers written in 1951 [B152]. It was determined that
approximate capacitances at carrier frequencies between terminals and from terminals to
ground in power transformers could be calculated from data obtained by actual transformer
measurement. Measurements and calculations showed that two winding transformer terminal to
ground capacitive reactance is almost twice as high as the terminal to terminal capacitive
reactance for both delta connected and wye connected transformers. Of the ten power
transformers that were analyzed, the severest problem was caused by a 15,000 KVA 69 kV
delta-wye transformer which attenuated a phase to ground carrier channel by 7 dB. Another
interesting point was that the 115 kV wye-delta transformers that were analyzed attenuated a
line to ground carrier channel by only 1 2 dB. Some of the conclusions drawn by the findings
of this paper are:
1. Two-winding high-voltage power transformers in general