Upload
karan-tripathi
View
213
Download
0
Embed Size (px)
Citation preview
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 1/13
1
INTRODUCTION TO POWER SYSTEM PROTECTION
P. KaliappanRelay Testing Laboratory
Power Systems Division
Central Power Research InstituteBangalore
1.0 INTRODUCTION:
The Protective relay is to protect the system and its equipment from unnecessary
damage due to faults, such as short circuits; it is obvious that speed is important. The
damage caused by a short circuit is certainly minimized by clearing the fault quickly
and it is essential to maintaining system stability. It is to detect and locate a fault and
issue a command to the circuit breaker to disconnect the faulty element. They are in
effect, a form of insurance designed to maintain a high degree of service continuity
and limit equipment damage. They are also called as Silent Sentinels.
2.0 CLASSIFICATION OF RELAYS:
Relays can be divided into five functional categories:
a. Protective relays, which detect defective lines, defective apparatus, or other
dangerous or intolerable conditions. These relays can either initiate or permit
switching or provide an alarm.
b. Monitoring Relays, which verify conditions on the power system or in the
protection. These relays include fault detectors, alarm units, channel- monitoring
relays, synchronism verification, and network phasing. Power system conditions
that do not involve opening circuit breakers during faults can be monitored by
these relays.
c. Programming Relays, which establish or detect electrical sequences.
Programming relays are used for reclosing and synchronizing.
d. Regulating Relays, which are activated when an operating parameter deviates
from predetermined limits. Regulating relays function through supplementaryequipment to restore the quantity to the prescribed limits.
e. Auxiliary Relays, which operate in response to the opening or closing of the
operating, circuit to supplement another relay or device. These include timers,
contact-multiplier relays, sealing units, receiver relays, lock-out relays, closing
relays and trip relays.
In addition to these functional categories, relays may be classified by input, operating
principles or structure and performance characteristic:
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 2/13
2
i) Input
Current
Voltage
Power
Pressure
Frequency Temperature
Flow
Vibration
ii) Operating principle of structure
Percentage
Multi-restraint
Product
Solid state
Electromechanical
Thermal
The above classification and definitions are based on the ANSI Standard 37.90
(IEEE 313)
3.0 PROTECTIVE RELAYING SYSTEMS AND THEIR DESIGN:
Any protection scheme is required to safeguard the power system components. The
protection scheme basically consists of current transformers, voltage transformers,
Protective relay and circuit breaker. The Protective relay functions as a sensing
device. It senses the fault through Current/Voltage transformers determine its location
and finally it sends the command to the circuit breaker by closing its trip coil. The
circuit breaker after getting the trip command from protective relay disconnects the
faulty sections from the rest of the power system. Thus it is seen that the protective
relay, which is primarily the brain behind the whole scheme, plays a very important
role. Therefore proper care should be taken in selecting an appropriate protective
relay, which is reliable, efficient and fast in operation.
4.0 DESIGN CRITERIA:
4.1 Reliability:
A protective system must operate reliably when a fault occurs in its zone of
protection. The reliability of the protective relay system has two aspects:
dependability of operation and security from false operation. Dependability means
that each relay sends a trip signal when a fault is present in its zone. Security means
that no relay sends a trip signal if no fault is present in its zone. Since no human
invention is perfect, and the protective relay system is no exception, compromise
between dependability and security are inevitable. To achieve a high degree of
reliability, greater attention should be given to the design, installation, maintenance
and testing of the various elements of the protective system.
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 3/13
3
4.2 Selectivity:
Selectivity is the quality of a protective relay by which it is able to discriminate
between a fault in the protected section and the normal condition. Also, it should be
able to distinguish whether a fault lies within its zone of protection or outside the
zone. Sometimes, this quality of the relay is also called discrimination.
4.3 Sensitivity:
Sensitivity refers to the minimum level of fault current at which operation occurs. In
other words, it is fault setting. There is a difference between the sensitivity of a relay
and the sensitivity of a protection system. The sensitivity of a relay is expressed as the
apparent power in VA required to cause operation; thus 1 VA relay is more sensitive
than 3 VA relay.
4.4 Stability:
The term stability is often used to describe the quality of protective system by virtue
of which it remains inoperative under specified conditions usually associated with
high values of fault currents. Strictly speaking, it is a quality that only unit systems
can possess because they are required to remain inoperative under all conditions
associated with faults outside their own zone.
4.5 Speed:
A protective system should be fast enough to isolate the faulty element of the system
as quickly as possible to minimize damage to the equipment and to maintain system
stability. For modern power system, the stability criterion is very important and
hence, the operating time of the protective system should not exceed the critical
clearing time to avoid the loss of synchronism.
4.6 Cost:
Maximum protection at the lowest cost possible.
5.0 FACTORS INFLUENCING RELAY PERFORMANCE:
Relay performance is generally classified as:
Correct
Incorrect
No conclusion
Incorrect operation may be either failure to trip or false tripping. The cause of
incorrect operation may be, a) Wrong application, b) Incorrect settings, c) A
personnel error, or d) Equipment ma-function. Equipment that can cause an incorrect
operation includes CTs, VTs, circuit breakers, cable and wiring, relays, channels or
station batteries.
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 4/13
4
Incorrect tripping of circuit breakers not associated with the trouble area is often as
disastrous as a failure to trip. Hence, special care must be taken in both application
and installation to ensure against the possibility of incorrect tripping.
“No conclusion” is the last resort when no evidence is available for a correct or
incorrect operation. Quite often this is a personnel involvement.
6.0 ZONES OF PROTECTION:
A power system contains generators, transformers, bus bars, transmission and
distribution lines, etc. There is a separate protective scheme for each piece of
equipment or element of the power system, such as generator protection, transformer
protection, transmission line protection, bus bar protection, etc. Thus, a power system
is divided into a number of zones for protection. A protective zone covers one or at
the most two elements of a power system. The protective zones are planned in such a
way that they collectively cover the entire power system, and thus, no part of the
system is left unprotected. The different zone of protection is as given figure 1.
LINE ZONE
BUS ZONE BUS ZONE
LINE ZONE
BUS ZONE BUS ZONE
TRANSF ZONE
BUS ZONE
BUS
ZONES
TRANSF
ZONES
Figure 1.
7.0 PRIMARY AND BACK UP PROTECTION:
If a fault occurs in a particular, it is the duty of primary relays of that zone to isolate
the faulty element. The primary relay is the first line of defence. If due to any reason,the primary relay fails to operate, there is a back-up protective scheme to clear the
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 5/13
5
fault as a second line of defence. With proper design, installation and maintenance of
the relays, circuit breakers, trip mechanisms, ac and dc wiring, etc. a very high degree
of reliability can be achieved.
The back-up relays are made independent of those factors, which might cause primary
relays to fail. A back-up relay operates after a time delay to give the primary relaysufficient time to operate. When back-up relay operates, a larger part of the power
system is disconnected from the power source, but this is unavoidable. There are three
types of back-up relays:
Relay back-up
Breaker back-up
Remote back-up
Relay Backup: In this relaying scheme, the main relays, their CTs, PTs etc. are
duplicated
Breaker Backup: When a feeder breaker fails to trip on a fault, the feeder fault
becomes virtually a bus bar fault. In breaker back-up scheme, a time-delay relay is
operated by the main relay and it is connected to trip all other breakers on bus if the
proper breaker has not tripped within pre-set time.
Remote Backup: Remote back-up is provided by a relay on the next station towards
the source. This remote relay will trip in a delayed time if the breaker in the faulty
section has not tripped due to some reason. This is the most widely used form of
back-up protection.
8.0 ELECTRICAL POWER SYSTEM DEVICE NUMBERS AND FUNCTIONS:
Numbers, with appropriate suffix letters when necessary, refers to the device
switching equipment, according to the functions they perform.
These numbers are based on a system adopted as standard for automatic switchgear
by IEEE and incorporated in American standard C37.2-1970. This system is used in
connection diagrams, in instruction books and in specifications.
Device Number Definition Function
1 Master Element It is an initiating device, such as a Control switch,
voltage relay, float Switch etc which serves either
Directly or through such permissive Devices as
protective and time delay relays .to place an equipment
in or out of operation
2 Time Delay Starting
Or Closing Relay
It is a device which functions to give a desired amount
of time delay before of after any point of operation in a
switching sequence or protective relaying system,
except as specially provided by device function 48, 62
and 79 described later.
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 6/13
6
3 Checking or
Interlocking Relay
It is device which operates in response to the position
of a number of other devices ( or to a number of
predetermined conditions), in an equipment, to allow
an operating sequence to proceed , to stop , or to
provide a check of the position of these devices or of
these conditions for any purpose.4 Master Contactor It is a device, generally controlled by The device No.1
or equivalent, and the required permissive and
protective devices that serve to make and break the
necessary control circuits to place equipment into
operation under the desired conditions and to take it out
of operation under other or abnormal conditions.
5 Stopping Device It is a control device used primarily To shut down an
equipment and hold it out of operation. This device
may be manually or electrically actuated, but excludes
the function of electrical lockout (see device function
86) on abnormal conditions.
6 Starting Circuit
Breaker
It is a device whose principal Function is to connect a
machine to Its source of starting voltage
7 Anode Circuit Breaker It is one used in the anode circuits of a power rectifier
for the primary purpose of interrupting the rectifier
circuit if an arc back should occur.
8 Control Power
Disconnecting Device
It is a disconnecting device – such As a knife switch,
circuit breaker or pullout fuse block, used for the
purpose of connecting and disconnecting the source of
control power to and from the control bus or
equipment. Note: control power is considered toinclude auxiliary power, which supplies such apparatus
as small motors and heaters.
9 Reversing Device It is a used for the purpose of reversing A machine
field or for performing any other reversing function
10 Unit Sequence switch It is used to change the sequence in Which units may
be placed in and Out of service in multiples – unit
Equipment.
11 Reserved for future
Application
12 Over Speed Device It is usually direct connected Speed switch which
functions on Machine over-speed13 Synchronized
Speed Device
A Centrifugal speed switch, a slip Frequency relay, a
voltage relay, an Undercurrent relay or any type of
device that operates at approximately synchronous
speed of a machine
14 Under Speed Device It is device which functions when The speed of a
machine falls below a predetermined value
15 Speed or Frequency
Matching device
It is a device that functions to match and hold the speed
or the Frequency of a machine or of a system equal to,
or approximately equal to, that of another machine,
source or system.
16 Reserved for future
Application
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 7/13
7
17 Shunting or
Discharge switch
It serves to open or to close a shunting circuit around
any piece of apparatus (except a resistor), such as a
machine field, a machine armature, a capacitor or a
reactor. Note: this excludes devices which perform
such shunting operations as may be necessary in the
process of starting a machine by devices 6 or 42 orthey’re equivalent, and also exclude device 73 function
which serves for switching of resistors
18 Accelerating or
Decelerating Device
It is used to close or to cause the closing of circuits
,which are used to increase or to decrease the speed of
a machine
19 Starting – to-
Running transition
contactor
It is a device which operates to initiate or cause the
automatic transfer of machine from the starting the
running power connection.
20 Distance Relay It is a electrically operated, Controlled or monitored
valve in a fluid line. Note: The function of the valve may be indicated by
the use of the suffixes.
21 Distance Relay It is a device, which functions when the circuit
admittance impedance or reactance increases or
decreases beyond predetermined limits.
22 Equalizer circuit
breaker
It is a device to serves to control or to make and break
the equalizer or the current balancing connections for a
machine field, or for regulating equipment, in a
multiple-unit of the installation
23 Temperature
Control device
It is a device to raise or lower the temperature of
machine or other apparatus or any medium, when itstemperature falls below, or rises above, a
predetermined value.
Note: An example is a thermostat which switched on a
space heater in a switchgear assembly when the
temperature falls to a desired value as distinguished
from a device which is used to provide automatic
temperature regulation between close limits and would
be designated as 90T
24 Reserved for future
Application
25 Synchronizing or
Synchronism-device
It is a device that operates when two as circuits are
within the desired limits of frequency, phase angle and
Voltage to permit or to cause the paralleling of these
two circuits.
26 Apparatus
Thermal device
It is a device which functions when the temperature of
the shunt field or the armature winding of a machine,
or that of a load limiting or load shifting resistor or of a
liquid or other medium exceeds a predetermined value;
or if the temperature of the protected apparatus, such as
a power rectifier, or of any medium decreases below a
predetermined value.
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 8/13
8
27 Under voltage relay It is device which functions on a given value of under
voltage.
28 Flame detector It is a device that monitors the presence of the pilot or
main flame in such apparatus as a gas turbine or a
system boiler.
29 Isolating contactor It is a device used for disconnecting one circuit fromanother for the purpose of emergency operation,
maintenance, or test.
30 Annunciator relay It is a non-automatically reset device that gives a
number of separate visual indications upon the
functioning of protective devices and which may also
be arranged to perform a lockout function.
31 Directional Power relay It is a device which functions on a desired value of
power flow in a given direction, or upon reverse
power, like resulting from arc back in the anode or
cathode circuits of a power rectifier.
32 Separate
Excitation device
It connects a circuit such as the shunt field of a
synchronous converter during the starting sequence ,or
one which energises the excitation and ignition circuits
of a power rectifier
33 Position switch It makes or breaks a contact when the main device or
the piece of apparatus, which has no device function
Number, reaches a given position
34 Master Sequence
device
It is a device such as a motor-Operated multi-contact
switch, or the equivalent, or a programming device,
such as a computer, that establishes or determines the
operating sequence of the major devices in anequipment during starting and stopping or during other
sequential operations.
35 Brush-operating
Or slip-ring-short
Circuiting device
It is used for raising, lowering, or shifting, the brushes
of a machine, or for short circuiting its slip ring, or for
engaging or disengaging the contacts of a mechanical
rectifier.
36 Polarity or
Polarizing voltage
device
It operates or permits the operation of another device
on a predetermined polarity only or verifies the
presence of a polarising voltage in an equipment
37 Undercurrent or
Under power relay
It functions when the current or power flow decreases
below a predetermined value.38 Bearing protective
device
It functions on excessive bearing temperature, or on
other abnormal mechanical conditions, such as
Undue wear, which may eventually result in excessive
bearing temperature
39 Mechanical Condition
monitor
It is device that functions upon the occurrence of
abnormal mechanical conditions,(except that associated
with bearing as covered under device function 38),
such as excessive vibration , eccentricity, expansion,
shock, tilting or seal failure
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 9/13
9
40 Field relay It functions on a given or abnormally low value or
failure of mechanical field current, or an excessive
value of the reactive component of armature current in
an ac machine indicating abnormally low field
excitation.
41 Field circuit breaker It is a device which functions to apply or to remove thefield excitation of a machine
42 Running circuit breaker It is a device whose principal function is to connect a
machine to its source of running or operating voltage.
This function may also be used for a device, such as a
contactor, that is used in series with a circuit breaker or
other fault protecting means, primarily for frequent
opening and closing of the circuit.
43 Manual transfer
Or selector
Device transfer
It transfer the control circuits so as to modify the plan
of operation of the switching equipment or of some of
the devices
44 Unit sequence
Starting relay
It is a device, which functions to start the next available
unit in multiple-unit equipment on the failure or on the
non-availability of the normally preceding unit.
45 Atmospheric
Condition monitor
It is a device that functions upon the occurrence of an
abnormal atmospheric condition, such as damaging
fumes, explosive mixture smoke or fire.
46 Reverse – phase
Phase-balance
Current relay
It is a relay which functions when the poly-phase
currents are of reverse phase sequence, or when the
poly-phase currents are unbalanced or Contain negative
phase-sequence component above a given amount
47 Phase sequenceVoltage relay It functions upon a predetermined value of poly phasevoltage in the desired phase sequence
48 Incomplete
Sequence relay
It is a relay that generally returns the equipment to the
normal of off position and locks it out of the normal
starting , operating or stopping sequence is not properly
completed within a predetermined time, if the device is
used for alarm purpose only, it should preferably be
designated as 48A(alarm)
49 Machine or
Transformer,
Thermal relay
It is a relay that functions when the temperature of a
machine armature or other load carrying winding or
element of a machine or the temperature of a power
rectifier or power transformer (including a powerrectifier transformer) exceeds a predetermined value.
50 Instantaneous
Over current or
Rate of rise relay
It is a relay that functions instantaneously on an
excessive value of current, or on an excessive current
rise, thus indicating a fault in The apparatus of circuit
being protected.
51 AC time
Over current relay
It is a relay with either a definite or an inverse time
characteristic that functions when the current in an ac
circuit exceeds a predetermined value.
52 AC circuit breaker It is device that is used to close and interrupt an ac
power circuit under normal conditions or to interrupt
this circuit under fault or emergency conditions.
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 10/13
10
53 Exciter or
Generator relay
It is relay that forces the dc machine field excitation to
build up during starting or which functions When the
machine voltage has build up to a given value.
54 Reserved for
Future application
55 Power factor relay It is relay that operates when the power factor in an accircuit rises above or below a predetermined value.
56 Field application relay It is a relay that automatically controls the application
of the field excitation to an ac motor at some
predetermined point in the slip cycle.
57 Short circuiting relay It is primary circuit switching device that functions to
short circuit or to ground a circuit in response to
automatic or manual means.
58 Rectification
Failure relay
It is a device that functions if one or more anodes of a
power rectifier fail to fire, or to detect an arc-break or
failure of a diode to conduct or Block properly.
59 Over voltage relay It is relay that functions on a given value of over
voltage
60 Voltage or
Current balance relay
It is relay that operates on a given difference in voltage,
or current input or output of two circuits.
61 Reserved for
future application
62 Time – delay Stopping
or opening relay
It is a time delay relay that serves in conjunction with
the device that initiates the shutdown, stopping, or
opening operation in an automatic
63 Pressure switch It is switch , which operates on given values or on a
given rate of change of pressure64 Ground
Protective relay
It is a relay that functions on failure of the insulation of
a machine, transformer or of other apparatus to ground
m or on flashover of a dc machine to ground.
Note: this functions is assigned only to a relay which
detects the flow of current from the frame of am
machine or enclosing case or structure of a piece of
apparatus to ground, or detects a ground on a normally
ungrounded winding or circuit. it is not applied to
device connected in the secondary circuit or secondary
neutral of a current transformer, connected in the
power circuit of an normally grounded system65 Governor It is the assembly of fluid, electrical, or mechanical
control, equipment used for regulation the flow of
water, stream, or other medium to the prime mover for
such purpose as starting, holding or stopping speed
load.
66 Notching or
Jogging device
It functions to allow only a specified device, or
equipment, or a specified number of successive
operations within a given time of each other. It also
functions to energise a circuit periodically or for
fractions of specified time intervals , or that is used to
permit intermittent acceleration or jogging of a
machine at low speeds for mechanical positioning
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 11/13
11
67 AC directional
Overcurrent relay
It is a relay that functions on a desired value of ac over
current flowing in a predetermined direction.
68 Blocking relay It is relay that initiates a pilot signal for blocking of
tripping on external faults in a transmission line or in
other apparatus under predetermined conditions, or co-
ordinates with other devices to block tripping or to block re-closing on an out-step condition or on power
swings.
69 Permissive Control
device
It is generally a two position, manually operated switch
that in one position permits. The closing of an circuit
breaker, of an equipment into position prevents the
circuit breaker or the equipment from being operated
70 rheostat It is a variable resistance device used in an electrical
circuit which is electrically operated or has other
electrical accessories such as auxiliary position of limit
switches
71 Level switch It is a switch which operates on given values, or on a
given rate of change of level
72 DC circuit breaker It is used to close and interrupt a dc power circuit under
normal conditions or to interrupt this circuit under fault
or emergency conditions
73 Load resistor contactor It is used to shunt or insert a step of load limiting,
shifting , or indicating resistance in a power circuit , or
to switch a space heater in circuit , or to switch a light
,or regenerative load resistor of a power rectifier other
machine in and out of circuit
74 Alarm relay It consists a device other than the annuciator, ascovered under Device no.30 which is used to operate in
connection with a visual or audible alarm
75 Position Changing
mechanism
It is a mechanism that us used for moving a main
device from one position to another in an equipment as
for example , shifting a removable circuit breaker unit
to and from the connected ,disconnected and test
positions
76 DC over current relay It is a relay that functions when the current in a dc
circuit exceeds a given value
77 Pulse transformer It sis used to generate and transmit pulse over a
telemetering or pilot-wire circuit to the remoteindicating or receiving device.
78 Phase angle Measuring
or Out-of-step
Protective relay
It is a relay that functions at a predetermined phase
angle between two voltages of between two currents or
between voltage and current
79 AC re-closing relay It is a relay that controls the automatic reclosing and
locking out of an ac circuit interrupter
80 Flow switch It is a switch , which operates on given values , or on a
given rate of change, of flow
81 Frequency relay It is a relay that functions on a predetermined value of
frequency, either under/over on normal system
frequency or rate of change of frequency
8/13/2019 1_Introduction to Power System Protection
http://slidepdf.com/reader/full/1introduction-to-power-system-protection 12/13
12
82 DC re-closing It is a relay that controls the automatic closing and
reclosing of a DC circuit interrupter, generally in
response to load circuit conditions
83 Automatic Selective
Control or Transfer
relay
It is relay that operates to select automatically between
certain sources or conditions in an equipment, or
performs a transfer operation automatically84 Operating mechanism It is the complete electrical mechanism or servo-
mechanism, Including the operating motor, solenoids,
position switches, etc, for a tap change , induction
regulator or any similar piece of apparatus which has
no device function number
85 Carrier or pilot
Wire receiver relay
It is a rely that is operated or restrained by a signal used
in connection with carrier-current or dc pilot-wire fault
directional relaying
86 Locking out relay It is an electrically operated, hand or electrically reset,
relay that functions to shut down and hold an
equipment out of service on the occurrence of
abnormal conditions.
87 Differential
Protective relay
It is protective relay that functions on a percentage or
phase angle or other quantitative difference of two
currents or of some other electrical quantities
88 Auxiliary motor
Or motor generator
It is one used for operating auxiliary equipment such as
pumps, blowers, exciters , rotating magnetic amplifiers
, etc.
89 Line switch It is used as a disconnecting load interrupter , or
isolator switch in an as or dc power circuit in an ac or
dc power circuit when this device is electricallyoperated or has accessories , such as an auxiliary
switch, magnetic lock, etc
90 Regulating device It functions to regulate a quantity , or quantities , such
as voltage, current, power , speed , frequency,
temperature and load , at a certain value or between
certain (generally close) limits for machines, toe lines
or other apparatus.
91 Voltage Directional
relay
It is relay that operates when the voltage across an
open circuit breaker or contractor Exceeds a given
value in an given direction
92 Voltage and Powerdirectional relay
It is a relay that permits or causes the connection oftwo circuits when the voltage difference between them
exceeds a given value in an predetermined direction
and causes these two circuits to be disconnected from
each other when the power flowing between them
exceeds a given value in the opposite direction
93 Field changing
contactor
It functions to increase or decrease in one step the
value of field excitation on a machine
94 Tripping or trip
Free relay
It functions to trip a circuit breaker contactor, or
equipment or to permit immediate tripping by other
devices, or to prevent immediate re-closure of a circuit
interrupter in case it should open automatically even
though its closing circuit is maintained closed