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Pilot Relaying
Abstract
In this paper, pilot relaying “a modified form of
differential relaying” is presented. The goal of the
paper is to explain the outlines of pilot relaying
fundamentals and its applications.
Keywords: Pilot relaying, pilot wires, power line
carrier, carrier current, microwave channel.
1 Introduction
Pilot relaying is an adaptation of the principles of
differential relaying for the protection of transmission
line sections. In a sense, it is a means of remote
controlling the circuit breakers. The term “pilot” means
that between ends of the transmissin line, there is
interconnecting channel of some sort over which
current and voltage information can be conveyed. Pilot
relaying is used when high speed protection is required
for all types of short circuits and for any fault location.
This type of protection generally communicates the
decision made by a local relay of one of the four types
( magnitude, directional, ratio, differential relays) to
relays at the remote terminals of a transmission line.
2 Pilot Relaying
Pilot relaying systems employ high-speed protective
relays at the line terminals in order to ascertain in as
short a time as possible whether a fault is within the
protected line or external to it. If the fault is internal to
the protected line, all the terminal breakers are tripped
practically simultaneously, thereby permitting highspeed reclosing. If the fault is external to the protected
line, tripping is blocked.
The combination of high speed tripping and high speed
reclosing has several advantages: (1) Decreased fault
damage to equipment (2) Improved power system
stability (3) Maximium return on the investment
depending on improved stability (4) Decreased effect
on nearby generation and load.
Pilot relaying is the best type of protection that can be
applied to transmission lines. It is inherently selective
and suitable for high speed operation.
Pilot relaying is used on some multiterminal lines
where high-speed operation is not essential but where
the configuration of the circuit makes it impossible for
distance relaying to provide even the moderate speed.
Also some lines are too short for any type of distance
relay. For such lines the ohmic errors would be so high
as compared with the ohms being measured that such
relaying would be impractical.
Critical loads may require high-speed tripping beyond
the capabilities of distance relays.
High-speed automatic reclosing of transmission line
breakers after they have tripped to clear a fault is
generally possible only with pilot relaying, because
only pilot relaying is able to cause all the line terminals
to trip at high speed and simultaneously. With such
high-speed tripping and reclosing, generators do not
have time to swing very far out of phase and therefore
no synchronism check is necessary before reclosing.
For these reasons it is suitable to use pilot relaying for
most high-voltage transmission lines and for many
subtransmission and distribution circuits.
“Pilot relaying is usually use to provide primary
protection for a line. Back-up protection may be
provided by a seperate sets of relays(step-distance
relaying) or the relays employed in the pilot may also
be used ” [1]
3 Pilot Channels
A communications channel is an integral part of a pilot
relaying system that is used to protect a transmission
line. “It is needed to deliver information from one
terminal of the line to the other, so that a fault location
can be determined.” [2] Similar information is also
conveyed in the opposite direction, allowing
simultaneous determination of the presence of an
internal fault.
The types of pilot channels available for protective
relaying are:
1)Pilot wires
2)Power line carriers
3)Microwave
3.1. Pilot Wires
Pilot wires are seperate wire circuits, generally
consisting of a two-wire circuit of the telephone-line
type, either open wire or cable; frequently such circuits
are rented from the local telephone company. In some
cases, the power systems may install their own wire
pilot circuits.
Figure 1. Schematic illustration of the circulating
current principle a-c wire-pilot relaying
“ The wire pilot channel, in all cases, is completely
isolated from the power conductor and provides direct
metallic connection between the terminals of the
protected line – it is suited for both tripping and
blocking pilots ” [3]
Having established that the purpose of a pilot is to
convey certain information from one end of a line
section to another in order to make selective tripping
possible, the next consideration is the use to be made
of the information. If the relaying equipment at one
end of the line must receive a certain signal or current
sample from the other end in order to prevent tripping
at the one end, the pilot is said to be a “blocking” pilot.
However, if one end cannot trip without receiving a
certain signal or current sample from the other end, the
pilot is said to be a “tripping” pilot.
Pilot wires can operate at power system frequency,
audio frequency tones or in dc. Dc wire pilot relaying
is not used than very special applications. “Ac wire
pilot relaying is the most closely akin to current
differential relaying.”[2] However, in modern ac-wire
pilot relaying, the magnitude of the current that flows
in the pilot circuit is limited and only a two-wire pilot
is required. These two features make ac wire pilot
relaying economically feasible over greater distances
than current-differential relaying.
In ac-wire pilot relaying we should know two new
terms to describe the principle of operation:
“circulating current” and “opposed voltage”. Briefly
“circulating current” means that current circulates
normally through the terminal CTs and the pilot, and
opposed voltage means that current does not normally
circulate through the pilot. The schematic illustrations
of the circulating-current and opposed-voltage
principles are given in Figure 1 and Figure 2.
Figure 2. Schematic illustration of the opposed-voltage
principle of a-c wire-pilot relaying
In both figures
current-balance type relays are
employed at each end. In Figure 2 the current
transformers are connected in such a way that the
voltages across the restraining coils at the two ends of
the pilot are in opposition for current flowing through
the line section as to a load or an external fault.
Consequently, no current flows in the pilot except
charging current, if we assume that there is no
unbalance between the CTs outputs. The restraining
coils prevent relay operation owing to such unbalance
currents. But should a short circuit occur on the
protected line section, current will circulate in the pilot
and operate the relays at both end. Current will also
flow through the restraining coils but in a proper
application, this current will not be able to prevent
relay operation; the impedance of the pilot circuit will
be the governing factor in this respect.
Short circuits or open circuits in the pilot wires have
opposite effects on the two types of relaying
equipment,as the accompanying table shows.
Effect of short
Effect of open
circuit
circuit
Opposed voltage
trip
block
Circulating current
block
trip
Ac-wire pilot relaying is inherently immune to power
swings or loss of synchronism than the dc.
In most cases, the application of wire pilots is limited
to relatively short lines. “With increasing lengths, the
shunt capacitance of the pilot pair, its resistance and
cost will increase.” [3]. The distance limitation on the
application of wire pilots is usually the attenuation of
the signal due to the distributed capacitance and
resistance rather than cost.
3.2. Power line carrier
Power line carriers use the protected transmission line
itself to provide the channel medium for transmission
of signals at frequencies of 30-300 kHz. These are the
most commonly used “pilots” for protective relaying.
Power line carrier relaying is also known as “carriercurrent pilot relaying”.
In power line carrier relaying, the carrier transmitterreceivers are connected to the transmission line by line
tuning equipment and coupling capacitor devices that
are also used for line voltage measurement. Line traps
tuned to the carrier frequency are located at the line
terminals as shown in Figure 3. They prevent an
external fault behind the relays from shorting out the
channel by showing a high impedance to the carrier
frequency and low impedance to the power frequency.
The radio frequency (RF) choke acts as a low
impedance to 50 Hz power frequency but as a high
impedance to the carrier frequency. Therefore, it
protects the equipment from high voltage at the power
frequency and at the same time limits the attenuation of
the carrier frequency.
“Since the carrier signal is propogated on the protected
line itself, there can be no complete assurance that the
signal will get from one terminal to the other when a
fault is present on the protected line section in the path
of the carrier signal. Hence it is better suited as a
blocking pilot as the protected line is intact for an
external fault when the carrier is transmitted.” [3]
Carrirer current pilot relaying is the best and most
commonly used kind of relaying for high voltage lines.
It is applicable in some form to any aerial line. It is
preferred to wire-pilot relaying because it is somewhat
more reliable and is more widely applicable.
Consisting entirely of terminal equipment, it is
completely under the control of user, as contrasted
with rented wire pilot.
Figure 3. One-line diagram of power line carrier for
pilot relay system. [1]
3.3. Microwave
Microwave channel is a radio channel which uses
beamed radio signals of very short wavelengths (high
frequencies in the range of 2-12 GHz), between lineof-sight antennas located at the terminals.
The basic microwave channel is subdivided or
multiplexed so that it can be used for a large number of
different functions simultaneously. When one of the
subcarrier channels of a microwave scheme is used for
protective relaying, it is usually modulated by
frequency shift audio tone equipment. The protective
relaying scheme operates in conjunction with the audio
tone equipment which in turn utilizes the microwave
subcarrier channel to propogate the tone signal to the
remote terminals of the protected line. “A continious
tone of one frequency, called the “ guard frequency ”, is
transmitted under normal (or nonfault) conditions.
When there is an internal fault, the audio tone
transmitter is keyed by the protective relaying scheme
so that its output is shifted from the guard frequency to
a trip frequency.” [3]
Unlike power line carrier the microwave signal is
propogated through the atmosphere. Such a system
requires that a straight line from one antenna to another
be above intervening objects; preferably above 50 feet.
This usually limits the distance between antennae to
about 20 to 50 miles depending on the topography of
the land. “Transmission of a microwave signal beyond
roughly 35 miles, requires a repeater terminal. This is
simply a receive, amplify and retransmit terminal.” [2]
A typical microwave configuration is shown in Figure
4. The signals to and from the relays may or may not
feed through the multiplexer. A higher level of
dependability is achieved for the very important
relaying function by working directly into the baseband
equipment.
on the protected line will not interfere with the
transmission of a remote-tripping signal. Occasionally,
microwave is useful where the attenuation would be
too high for carrier current, such as, on a power-cable
circuit.
A microwave pilot is used only when the relaying
equipment can share the channel with enough other
services ; it is not economically justifiable for relaying
alone if carrier current or wire pilot is applicable.
Microwave is entirely suitable although it is not as
reliable as carrier current for protective relaying
purposes. This is because of the complex circuitry and
the large number of tubes involved, and also because
of large number of services on the same microwave
channel.
4 Choice of Channels
Figure 4. Typical microwave configuration [2]
Figure 5 shows the application that takes advantage of
the reflective quality of the medium. A large billboard
type structure is used to deflect the microwave signals
to the desired path. The reflector terminal is entirely
passive, providing no signal amplification whatever.
Figure 6 provides a rough indication of the relative cost
of several types of channels. Pilot wire channel cost
increases linearly with distance, but it is the lowestcost channel for short distances up to 5 or 10 miles
beyond which a carrier-current pilot usually becomes
more economical. A single microwave channel is not
economically justifiable but as the diagram shows, the
incremental cost for additional channels is small.
Because the microwave is line-of-sight medium, and
terrain and earth contours limit the distance over
which transmission can take place reliably without
repeat terminals, there is a jump in cost at the limiting
distance.
“Power line carrier is not limited by distance, in
general, but the terminal cost is appreciable because
line traps, tuners, and coupling capacitors are
required.” [2]
Figure 5. Use of a reflector [2]
Microwave has certain theoritical advantages over
carrier –current because it is dissociated from the
power line, but its only real advantage is in connection
with remote tripping. Because the presence of a fault
Figure 6. Relative costs of different channel types [2]
5 Pilot Relaying Schemes
Pilot relaying systems use either phase comparison or
directional comparison to detect faults.
5.1 Directional Comparison
This type is usually applied over blocking type carrier
pilot channel. In its basic application, directional phase
and ground units sense the direction to a fault. They
compare the direction of power flow at the line
terminals. Power flow into the line at the terminals
indicates that there is an internal fault, and the line is
tripped. If power flows into the line at one end and out
at the other, the fault is considered external to the
protected line and tripping is blocked.
Consider the line shown in Figure 7. Assume that
directional relays are used and high-speed protection is
provided for the entire line by pilot relaying.
Therefore, both faults F1 and F2 are detected as internal
faults by the relays at B1 and B2 and cleared at high
speed. Here, both relays see the fault current flowing in
the forward direction. If this information is transmitted
to the remote ends over a pilot channel, it is confirmed
that the faults are the faults are indeed on the protected
line. Now assume that there is a fault at F3 . The relays
at B2 sees it as an external fault and the relays at B1 and
B3 see it as an internal fault. Upon receiving this
directional information at B1 , that relay will be able to
block tripping for the fault at F3 .
Figure 7. Example application of pilot relaying [ 1]
If there is sufficient mutual induction with another line,
directional comparison relaying using directionalground relays, is not applicable.
5.2 Phase Comparison
In the phase comparison, the phase angle of the current
entering a terminal is compared with the phase angle of
the current leaving the other terminal with the aid of
the communications channel. If these two currents are
essentially in phase, there is no fault in the protected
line section. If these two currents are essentially 1800
out of phase, a fault in the line is indicated.
Amplitude modulation is used in a phase comparison
system. “Modulation is of the all-or-nothing type,
producing half-cycle pulses of carrier signals
interspersed with half-periods of zero signals.” [1]
When the fault is external, carrier will be transmitted
during alternate half-cycles from each terminal and
carrier receiver at each terminal will receive the signal
continously (transmission from one end fills in the
gaps in the signals from the other and vice versa) so
that tripping is blocked. When there is an internal fault,
carrier is transmitted during the same half-cycle from
each terminal and each receiver will receive the carrier
only for half-cycle. During the remaining half cycle
tripping occurs at each terminal. This is shown in
Figure 8.
Figure 8. Phase comparison carrier scheme
[3]
The above scheme uses power line carrier but the same
scheme can also be applied to audio tones over a
microwave channel. The audio frequency equipment
replaces the carrier frequency equipment and the
microwave channel replaces the power line channel.
Phase comparison relaying equipment protects only
against the internal faults. No back-up protection is
provided for faults at adjacent lines and buses. “It
leaves the line completely unprotected during the
removal of carrier channel for periodic testing and
maintenance.” [3].
Phase comparison is not affected by mutual induction
from neighboring power circuits. This is an advantage
over directional comparison. “It also, inherently, is not
susceptible to incorrect operation because of power
system swings or instability” [2]
Since the phase comparion is made for currents alone,
no voltage input is needed for such systems. On the
other hand, directional comparison systems require
voltage inputs since an impedance calculation is
involved. “It should be noted that, if the
communication system should fail, the phase
comparison function becomes totally inoperative,
while the directional comparison system can be
designed to provide some distance relaying functions
as a second protective function” [4]
6 Conclusion
Pilot relaying is the best type for line protection. It is
used for high speed tripping at line terminals, therefore
in the case of a fault, minimum damage to equipment is
obtained. Pilot relaying uses different types of channels
for
communication and different schemes for
protection. The appropriate type of channel and
protective scheme must be chosen for different
situations. In the paper, pilot relaying advantages and
the use of different channels and protective schemes
have been given.
References
[1] Gonen, T., “Modern Power System Analysis.”
Wiley, New York, 1988.
[2] Elmore, A. W., “Pilot Protective Relaying.”
Marcel Dekker, New York, 1999.
[3] Rao, T. S., “Power System Protection.” Wiley,
1989.
[4] Phadke, A., “Computer Relaying for Power
Systems”