© OCT 2018 | IRE Journals | Volume 2 Issue 4 | ISSN: 2456-8880

Pilot Protection - An Overview

AJAKOR E. M1, EZECHUKWU A.O2
1,2
Department of Electrical Engineering, Nnamdi Azikiwe University, Awka, Anambra State, Nigeria

Abstract -- This paper presents the overview of pilot disturbance that occurs when a heavy load line is
protection. It explains the principle of operation, the pilot opened. For the protection system, the relaying system
communication channels, reason for chosen pilot must be selective and precautions are taken to ensure
protection technique and different zone of operation.
no operations are initiated by the relay logic or other
Finally dealt with Permissive Overreaching Transfer
means that would cause tripping of important lines or
Tripping (POTT) scheme.
other facilities when not absolutely necessary. Thus,
pilot protection is an adaptation of differential relay
Indexed Terms: distance protection, pilot wire, transfer principles that avoid the use of control cables between
tripping, zone of protection
terminals.

I. INTRODUCTION
Communications Channels use in pilot protection.
The term ‘pilot’ refers to a communication channel
between two or more ends of a transmission line to a) Power Line Carrier (PLC): Operates on radio
provide instantaneous clearing over 100% of the line. frequency signals over transmission lines in
Communication channels typically used include power the 10 to 490 kHz band.
line carrier, microwave, fibre optic, and b) Microwave: Operates at frequencies between
communication cable. 150 MHz and 20 GHz.
c) Fibre-optic links

Pilot protection for lines provides possibilities for high

speed simultaneous detection of phase and ground 1.1 Fiber communication channel in pilot
faults protection for 100% of the line segment being protection
protected from all terminals. It is a type of protection
for which quantities at the terminals are compared by The development of modern optical fiber
a communication channel rather than by a direct wire communication technology has become increasingly
interconnection of the relay input devices. The popularized due to its long-distance, large-capacity,
increment in time delays when using distance relays high-speed, and real-time synchronous data
becomes impractical, because of the distance between transmission. Fiber optic communication is applied in
several terminals. Pilot protection offers increased power protection because the appearance of digital
certainty when a fault is present in a system. communication technology makes information
Implementation of pilot protection in transmission exchange reliable and fast. Hence, proposes the
systems is widely used because of its adaptability. construction of an intercommunicated protection
Thus, pilot protection does not require any system. Pilot protection can improve relay reliability
coordination with protection in the adjacent system with communications between protections schemes.
unless additional backup is included. Fiber optic-based communications in pilot protection
systems can detect faults more rapidly with a low time
Pilot protection conveys three fundamental concepts
delay. With the implementation of fiber optics,
protective system design:
information exchange is not limited to the digital state
a) Selectivity value. A variety of information exchange by the same
b) Reliability communication channel provides sufficient
c) Security. information. Pilot protection can be implemented with
distance relays, which distinguish internal and external
This is especially important in extra high voltage
fault by comparing fault direction of fault distance on
(EHV) circuits because of a considerable system
both sides. The information exchange is logical instead

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of analog quantities. Therefore, in a pilot protection Justification of Pilot Protection on Transmission Lines
system, protection Intelligent Electronic Device (IED) The protection zone for a transmission line is unique
on each side of a transmission line collects information because the zone limits generally extend to
and calculates fault direction, fault distance, and other geographically separate locations. In addition to their
parameters based on local information and then sends relay sources, elements entirely at one location can
the results to the IED on the opposite side. have instantaneous tripping configured. In order to
affect high speed tripping for 100% of a transmission
The information exchange is voltage and currents line, each terminal of the protected line must
values, protection start-up signal, fault direction, and communicate with the other terminal(s) in some way.
distance information, fault phase selection When pilot protection is evaluated for implementation,
information, and breaker status. Reference concludes its goal is to improve system stability by fault clearing
that, besides providing better reliability and rapid in the shortest amount of time. From the perspective of
communication, the digital communication channel electric utilities, clearing time reduction improves
also provides the possibility for various and large stability, reduces equipment damage, and improves
amounts of synchronous electrical information power quality in addition to providing quality service.
exchange. With the aid of an optical digital channel,
multiple protection criteria can be executed to improve
Technical reasons to consider pilot protection:
the operation performance of traditional pilot
protection system which can complete various Cascading Issues: Protective relay with protected
functions such as relaying protection, auto reclosing, zones are configured with distance elements, and
measurement of transmission line parameters, and stepped distance schemes are coordinated in a
more functions within the unified pilot protection. cascading manner. Therefore, this configuration risks
triggering a chain of undesirable events, leading to
widespread blackout.
Pilot protections based on fiber communication
technology have become one of the primary forms of
transmission line. Consequently, many of these 1.2 Limit fault damage due to high current: Fault
configurations rely on differential protection, but currents can cause thermal and mechanical damage to
problems such as low sensitivity or poor reliability conductors and electrical equipment.
because of CT saturation and influence of large Need for high-speed reclosing: A system in
charging current because of line distributed equilibrium with no fault, mechanical power equals
capacitance for long transmission lines arise when electrical power, ignoring losses. When a fault occurs,
implementing differential protection. These equilibrium is disturbed and the synchronous machine
complications are seriously impairing and threatening accelerates. The positive sequence voltage
to the speed and sensitivity of conventional current immediately after the fault can be used to estimate the
differential protection. To reduce CT saturation and requirement for high-speed tripping. The accelerating
line distributed capacitance, an Enhanced power is proportional to the difference between pre-
Transmission Line Pilot Impedance (ETLPI) scheme fault and fault positive sequence voltages at the point
is adopted. ETLPI is defined as the ratio of voltage of fault. Thus, the smaller the positive sequence
difference of fault-superimposed components at both voltage, the faster the system accelerates and the faster
terminals of the protected line, which can be calculated the system needs to isolate the fault. Therefore, high
from real-time voltages and currents measurements speed reclosing is required.
synchronously transmitted from local terminal to Protection performance requirements for the line
remote terminal. When this model is implemented, the dictate the number of pilots schemes required. The
amplitude of ETLPI is greater than the amplitude of following are considerations to determine the number
the positive sequence impedance of the protected of required pilot systems:
segment of the line. ETLPI also effectively avoids
distributed capacitances and CT saturation. Therefore,
this scheme may suit larger transmission lines.

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Number of systems required: Where high speed

clearing is desired for faults anywhere on the line, but
time delayed tripping is acceptable under
contingencies.

Different voltage levels: Protection system

performance requirements can vary greatly and dictate
at what voltage level pilot channels are used. From 230
kV to 345kV, at least one pilot scheme is typically
present and, depending on system configuration, two
schemes often exist, in addition to direct transfer
tripping for the breaker. Above 345kV, at least two Figure 1: Permissive Overreaching Transfer Tripping
pilot schemes and a direct transfer trip for equipment (POTT) Scheme
failure are typically applied.

a) Pilot schemes
Regulatory/regional reliability council requirements:
Reliability councils sometimes dictate protection The non-pilot protection schemes have usually an
system performance requirements, the number and acceptable performance on short or medium lines.
type of pilot systems, and the channel required. However, for long lines which are mostly operating in
EHV or UHV levels and transmitting large electric
II. PROTECTION SCHEME power, the tripping time delays would cause severe
network stability problems due to the system
a) Permissive Overreaching Transfer Tripping acceleration. Also the huge fault currents could cause
Scheme dramatic damages for equipment’s. In such cases,
more complex transmission line protection schemes
In the Permissive Overreaching Transfer Tripping are required in order to perform a high speed tripping
(POTT) scheme, a distance element is set to reach in both ends of the line.
beyond the remote end of the protected line to send a
signal to a remote end. However, the received relay An alternative protective scheme which has been in
contact must be monitored by a directional relay use for protection of EHV/UHV transmission lines,
contact to ensure that tripping does not occur unless utilizes local information, as well as remote
the fault is within the protected section. information for a relaying decision. In this category
In Fig 2, the contacts of Zone 2 are arranged to the known as “pilot protection schemes “the relays
signal, and the received signal, supervised by Zone 2 installed at terminals, as shown in Figure 2, are able to
operation, is used to energize the trip circuit. The make a common decision about tripping the line in
scheme is known as a POTT. Since the signaling case of fault inceptions inside the protection zone.
channel is keyed by overreaching Zone 2 elements, the
scheme requires duplex communication channels.

To prevent the relay from operating under current

reversal conditions in parallel feeder circuit, a current
reversal guard timer must be used to restrain tripping
of the forward Zone 2 elements. Otherwise,
malfunction of the scheme may occur under current
reversal conditions. It is necessary only when the Zone
2 reach is set greater than 150% of the protected line Figure 2: single line diagram of pilot protection
impedance. scheme for transmission line

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The integrated communication system in pilot flowing to a node must be equal with the sum of the
protection schemes usually uses analog or digital currents leaving the same node”.
signals transmitting in DC or AC power frequencies or In external faults the same current is entering to
in higher frequency bands such as: protected zone and leaving it from the second end. But
in case of internal fault the current entering the
Audio frequencies of approximately from 20 to 20000 protected zone is not equal to the current which is
Hz, leaving the same zone. Therefore, this principal could
Power line carrier frequency in the range from 30 to be utilized in directional protection schemes for
600 kHz, protection of transmission lines.
Radio frequencies with practical limits of roughly 2) Phase Comparison Scheme
10kHz to 100,000MHz, In a phase comparison scheme the relay is able to
Microwave frequency bands loosely applied to radio distinguish an internal inception of the fault on
waves from 1000 MHz, protected transmission line by comparing the current
Visible light frequencies with nominal wavelength phase angle at one end with current phase angle at the
range of about 0.3μm - 30μm. second end. Where in case of the internal faults there
will be a notable phase difference. However, incorrect
Moreover, the communication link between the relays operation of the relay can happen by changing the
is usually chosen among the pilot wires, power system configuration which may affect the polarity of
transmission conductors, microwave-radio channels the quantities used for directional comparison.
or fiber-optic strands.
In pilot protection schemes depending on quantities Non-unit pilot protection schemes are sometimes also
which are communicated and used in relay decision referred to as "state comparison protection schemes”
making, as illustrated in Figure 2, the schemes or “open system” schemes. In these schemes direct
belonging to this group can be classified in two comparison of measurements is not performed
following major subgroups: between two ends and only logical status related to
Unit pilot protection schemes are sometimes also fault information is exchanged between protective
referred to as "analog comparison protection” or relays located at each end of transmission line.
"closed" schemes. In these schemes analog quantities Two important non-unit pilot protection schemes are
such as amplitude and/or phase information of power identified as distance and directional comparison
system in one end is compared with the other end. schemes. In such schemes the logical information
Two important unit pilot protection schemes are typically related to direction of the fault are sent over
identified as longitudinal differential and phase the communication link for a common relaying
comparison schemes. In such schemes the main decision. Therefore, there will be less dependency on
communicated information between the ends of the data synchronization comparing to unit protection
protected line are either amplitude and/or phase data schemes.
of the transmission line components. 1) Distance Scheme
Communication link between relays in pilot distance
In case of an internal fault the result of the compared schemes can eliminate the time delays for relay
data will be a differential value and for specific decision makings in case of occurrence of faults in
threshold values the relays in both terminals perform a second or even third zones for distance protection
relaying operation. Since there is an instantaneous schemes. Thus, the local relays can communicate with
comparison between the analog values, the the remote relay in order to make sure that the detected
information acquired from both relays needs to be time fault is located on protected zone. This provides fast
synchronized to guarantee the comparison of directional fault detection as well as opportunity of
measured data at same time instants from both ends. implementing the step distance relays in protection of
1) Longitudinal Differential Scheme long transmission lines.
The operation principle of the relay is expressed by 2) Directional Comparison Scheme
Kirchhoff’s first law that says: “the sum of the currents The relays in directional comparison pilot schemes,
such as directional over current relays, detect the

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direction to the fault at their local position and share Table 1: Protection zone
the information with the remote terminal relay. Zone 1 Forward 90% of the Line Instantaneous
Consequently, the overall functionality of the scheme Looking Protection
can be accomplished by a common decision from both Zone 2 Forward 120% of the Line comm. assisted
ends. Furthermore, the new generation of transmission Looking with Time Delay
line protection schemes called Ultra-High-Speed Zone 3 Backward 20% of the Line With Time
Looking Delay
relaying (UHS) schemes.

III. COMMUNICATION-BASED
Protection schemes based on fundamental frequency
PROTECTION SCHEME TYPES
components usually are referred as ‘conventional’ or
‘classical’ transmission protection schemes. In these
a) Permissive Overreaching Transfer Trip
schemes both high and low frequency components in
(POTT)
current and voltage quantities introduced by any
disturbances in the power system are considered as
noise to main signals and are used to be filtered out in
order to perform the analysis.
POTT Scheme Settings and Parameters.

This section presents the parameters, settings, and

zones of the POTT scheme of various protection
zones. Figure 3 shows one line diagram simplified and
its different zones of protection. In this case, three
zones of protection are evident, two of which are
forward looking and the remaining one is backward
looking. This scheme is also used as a reference for
design of the.

Trip =z2 element . RCVR

Permissive Overreaching TT
 Permissive signal must be detected from the
remote end for the communication-aided trip
 Absence of communication channel disables
the accelerated tripping

Permissive Overreaching TT Complications and

Concerns
Figure 3; three zone POTT scheme

In the three protection zones, the first zone is  Desensitization due to infeed
instantaneous protection and is set at 90% percent of  Dependability issue –failure to trip high
the total length of the transmission line. The second speed
zone is set at 120% percent of the total length of the  Current reversal
transmission line, overreaching the bus, and the third  Occurs in parallel lines with sequential
zone is set to protect 20% of the transmission line tripping
backward looking. Table 1 outlines protection zones
for the POTT scheme.

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 Security issue –coupled with long channel  Similar to POTT but permissive signal sent
reset times may cause trip of the healthy by under reaching Z1 elements
parallel line  At the receiving end, Z2 elements qualify the
permissive signal
Current Reversal. All Sources In  No problems with current reversal since Z1
doesn’t overreach

Directional Comparison Blocking Protective

Zones

Z2 at Breaker 1 picks up and sends permissive signal

to Breaker 2

 Zone 2 elements cover the entire line

 Z2 at Breakers 3 and 4 send permissive  Reverse Zone 3 elements must reach further
signals to each other than the opposite Zone 2 overreach
 Z1 at Breaker 4 trips instantaneously

Current Reversal System, after Breaker 4 Opens

Directional Comparison Blocking, Basic Logic

 In-section faults will not key transmitter and

both ends trip high-speed
 Current reverses through the healthy line
 Out-of-section fault will key the transmitter
 Z2 at Breaker 2 picks up
at the nearest end to block the trip at the
 If the permissive signal has not reset, Breaker
opposite end
2 trips on POTT

Current Reversal, Possible Solution

Directional Comparison Blocking complications
 Timer with instantaneous pickup and time
and Concerns
delayed dropout, initiated on reverse Z3
 Coordinating time at fault inception
 Delay trip with POTT until the timer drops
 Z3 faster than Z2, but channel delay time
out
reduces the margin
 Z must be slowed down
b) Permissive Under reaching Transfer Trip
 External fault clearing
(PUTT)

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 Z3 and Z2 race to drop out, if Z3 drops out

first Z2 over trips
 Z3 operates faster and drops slower
 Channel reset time helps
 Slower transmitter key dropout time helps.

REFERENCES

[1] W.A Pullman, ‘Transmission Protection

Overview’, Schweitzer Engineering
Laboratories, 2014, pp 19-30
[2] A.A. Chavez, J.L. Guardado, D. Sebastian
and E. Melgoza, ‘Distance protection
coordination using search methods’ IEEE
Latin America Transactions, March, 2008
Vol.6, No.1.
[3] S. H. Horowitz, A. G. Phadke and B. A. Renz,
“The Future of Power Transmission” IEEE
Power & Energy Magazine, March/April
2010
[4] J. D Glover, M.S. Sarma and T.J.Overbye
‘Powert System Analysis and Design’ 5th
Edition, 2011

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