Download Advanced Computer Architecture

Protection of Power
Systems
1. Introduction
Faults
 A fault is an inadvertent, accidental
connection, and flashover between the phase
wires or from the phase wires to ground.
 Short circuits (faults) occur in power systems
when equipment insulation fails, due to:



system overvoltages caused by lightning or
switching surges,
insulation contamination, or
other mechanical and natural causes.
 Careful design, operation, and maintenance
can minimize the occurrence of short circuits
but cannot eliminate them.
Lightning Striking Power Lines
 Fault currents can be several orders of
magnitude larger than normal operating
currents and, if allowed to persist, may cause
insulation damage, conductor melting, fire,
and explosion.
 Windings and busbars may also suffer
mechanical damage due to high magnetic
forces during faults.
 Clearly, faults must be quickly removed from
a power system.
 Standard EHV protective equipment is
designed to clear faults within 3 cycles.
 Lower-voltage protective equipment typically
operates within 5–20 cycles.
Types of Faults
 Most faults in an electrical utility system with
a network of overhead lines are one-phaseto-ground faults resulting primarily from
lightning-induced transient high voltage and
from falling trees and tree limbs.
 In the overhead distribution systems,
momentary tree contact caused by wind is
another major cause of faults.
 Ice, freezing snow, and wind during severe
storms can cause many faults and much
damage.
 These faults include the following, with very
approximate percentages of occurrence:




Single phase-to-ground: 70%–80%
Phase-to-phase-to ground: 17%–10%
Phase-to-phase: 10%–8%
Three-phase: 3%–2%
 Series unbalances, such as a broken
conductor or a blown fuse, are not too
common, except perhaps in the lower-voltage
system in which fuses are used for protection.
 Fault occurrence can be quite variable,
depending on the type of power system (e.g.,
overhead vs. underground lines) and the local
natural or weather conditions.
Protection
 Protection is defined as ‘‘the science, skill,
and art of applying and setting relays and/or
fuses to provide maximum sensitivity to faults
and undesirable conditions, but to avoid their
operation on all permissible or tolerable
conditions’’
 The basic idea is to define the undesirable
conditions and look for differences between
the undesirable and permissible conditions
that relays or fuses can sense.
 It is also important to remove only the faulted
equipment from the system while maintaining
as much of the unfaulted system as possible
in service, in order to continue to supply as
much of the load as possible.
 Although fuses and reclosers (circuit breakers
with built-in instrument transformers and
relays) are widely used to protect primary
distribution systems (with voltages in the 2.4–
46 kV range), we focus primarily in this
course on circuit breakers and relays, which
are used to protect HV (115–230 kV) and
EHV (345–765 kV) power systems.
Relay
 The Institute of Electrical and Electronic
Engineers (IEEE) defines a relay as:
 ‘‘an electric device that is designed to
respond to input conditions in a prescribed
manner and, after specified conditions are
met, to cause contact operation or similar
abrupt change in associated electric control
circuits.’’
 Inputs are usually electric, but may be
mechanical, thermal, or other quantities or a
combination of quantities.
 Relays are used in all aspects of activity:
home, communication, transportation,
commerce, and industry, to name a few.
 Wherever electricity is used, there is a high
probability that relays are involved.
 They are used in heating, air conditioning,
stoves, dishwashers, clothes washers and
dryers, elevators, telephone networks, traffic
controls, transportation vehicles, automatic
process systems, robotics, space activities,
and many other applications.
 In this course we focus on one of the more
interesting and sophisticated applications of
relays, the protection of electric power
systems.
Protective Relay
 The IEEE defines a protective relay as:
 ‘‘a relay whose function is to detect defective
lines or apparatus or other power system
conditions of an abnormal or dangerous
nature and to initiate appropriate control
circuit action’’
Fuse
 Fuses are also used in protection.
 IEEE defines a fuse as:
 ‘‘an over-current protective device with a
circuit-opening fusible pat that is heated and
severed by the passage of the overcurrent
through it’’
 Thus, protective relays and their associated
equipment are compact units connected to
the power system to sense problems.
 Protective relaying is a nonprofit,
nonrevenue-producing item that is not
necessary in the normal operation of an
electrical power system until a fault—an
abnormal, intolerable situation—occurs.
 A primary objective of all power systems is to
maintain a very high level of continuity of
service, and when intolerable conditions
occur, to minimize the extent and time of the
outage.
 Loss of power, voltage dips, and overvoltages
will occur, however, because it is impossible,
as well as impractical, to avoid the
consequences of natural events, physical
accidents, equipment failure, or misoperation
owing to human error.
Relay Technologies
 Electromechanical relays
 Analog type electronic relays
 Microprocessor-based electronic relays
Electromechanical Relays
 Originally, all protective relays were of the
electromechanical type.
 Electromechanical type relays are still in
widespread use and continue to be
manufactured and applied.
Electromechanical Relays
Analog Type Electronic Relays
 Analog type electronic relays using discreet
electronic components were introduced in the
1970s.
Solid State Relay (Static Relay)
Microprocessor-based Relays
 In recent years, microprocessor-based
electronic relays have been developed and
are being applied at an increasing rate.
 Microprocessor- based relays are sometimes
referred to as numerical type relays since the
analog inputs are converted to digital
numbers that are then processed within the
relay.
Numerical Relay
 Even with this trend toward the utilization of
micro processor -based relays, however, it
may be a long time before electromechanical
devices are completely replaced.
 With electronic relays, the protection
principles and fundamentals are essentially
unchanged as are the issues regarding
protection reliability.
Benefits of Microprocessor Type
Relays
 Microprocessor type relays do provide many
benefits:




Higher accuracy
Reduced space
Lower equipment and installation costs
Wider application and setting capabilities
 Other desirable supplemental features:







Control logic
Remote and peer-to-peer communications
Data acquisition
Event recording
Fault location
Remote setting
Self monitoring and checking
 Many modern micro processor relays utilize a
liquid crystal display (LCD) on the front panel.
 Such displays typically show setting,
metering, event, and relay self-test status
information.
 Relay settings can also be changed through
the LCD interface without the need for a data
terminal.
 Target information is typically displayed on
microprocessor relays with the use of LEDs
that identify the protective functions that had
operated to initiate tripping along with other
information such as the type of fault that had
been detected (i.e., A- phase-t o-ground),
recloser status, etc.
 Terminal blocks are normally provided on the
back of the relay for connecting the various
input s that are require d and outputs that are
provided by the relay.
 Communication ports are provided for
transmitting digital data.
Classification of Relays
 Relays may be classified in several different
ways, such as by function, input, performance
characteristics, or operating principles.
 Classification by function is most common.
 There are five basic functional types:
1. Protective.
2. Regulating.
3. Reclosing, synchronism check, and
synchronizing.
4. Monitoring.
5. Auxiliary.
Protective Relays
 Protective relays and associated systems
(and fuses) operate on the intolerable power
system conditions.
 They are applied to all parts of the power
system: generators, buses, transformers,
transmission lines, distribution lines and
feeders, motors and utilization loads,
capacitor banks, and reactors.
 For the most part, the relays discussed are
separate devices that are connected to the
power system through CT and VTs from the
highest system voltage (765 kV, at present)
down to service levels of 480 V.
 In general, distribution equipment below 480
V is protected by fuses or protection devices
that are integral with the equipment.
Regulating Relays
 Regulating relays are associated with tap
changers on transformers and on voltage
regulators of generating equipment to control
the voltage levels with varying loads.
 Regulating relays are used during normal
system operation and do not respond to
system faults unless the faults are left on the
system for too long. This is not normal.
Reclosing, Synchronism Check,
and Synchronizing Relays
 Relays of this type are used in energizing or
restoring lines to services after an outage,
and in interconnecting preenergized parts of
systems.
Monitoring Relays
 Monitoring relays are used to verify
conditions in the power system or in the
protective system.
 Examples in power systems are fault
detectors, voltage check, or directionalsensing units that confirm power system
conditions but do not directly sense the fault
or trouble.
 In a protection system, they are used to
monitor the continuity of circuits, such as pilot
wires and trip circuits.
 In general, alarm units serve as monitoring
functions.
Auxiliary Relays
 Auxiliary units are used throughout a
protective system for a variety of purposes.
 Generally, there are two categories: contact
multiplication and circuit isolation.
 In relaying and control systems there are
frequent requirements for:
1. more outputs for multiple tripping, alarms,
and operating other equipment, such as
recording and data acquisition, lockout, and
so on,
2. contacts that will handle higher currents or
voltages in the secondary systems, and
3. electrical and magnetic isolation of several
secondary circuits.
 The seal-in (CS ) relay of Figure 1.9 is an
auxiliary relay application.
 The trip and closing relays used with circuit
breakers are auxiliary relays.
Other Relay Classifications
 Protective relays classified by input are
known as current, voltage, power, frequency,
and temperature relays.
 Those classified by operating principle
include electromechanical, solid-state, digital,
percentage differential, multirestraint, and
product units.
 Those classified by performance
characteristics are known as distance,
reactance, directional overcurrent, inverse
time, phase, ground, definite, high-speed,
slow-speed, phase comparison, overcurrent,
undervoltage, overvoltage, etc.
Backup Protection
 Problems with the protection equipment itself
can occur.
 A second line of defense, called backup
relays, may be used to protect the first line of
defense, called primary relays.
 These can be at the same location (primary
backup), at the same station (local backup),
or at various remote stations (remote
backup).
 All three are used together in many
applications.
 In HV and EHV systems, separate current- or
voltage-measuring devices, separate trip coils
on the circuit breakers, and separate
batteries for the trip coils may be used.
Relay Coordination
 Also, the various protective devices must be
properly coordinated such that primary relays
assigned to protect equipment in a particular
zone operate first.
 If the primary relays fail, then backup relays
should operate after a specified time delay.