During power system faults, devices
are used for fast isolation of affected equipment to save them from
damage.Special circuits called control
circuits are used to realize the above objective.Control circuits are used for other functions
besides switching on or off of circuit breakers and isolators as enumerated
below:
Voltage
raise or lower in tap changer device of power transformers.
Frequency
regulation and load control.
Power
system monitoring such as power factor control.
Alarm
and indication control.
Circuit
supervision.
Audio/visual
annunciation.
B.CONTROL SYMBOLS AND ALPHABETS
In order to make for easy
identification, symbols and alphabets are used for various devices in control
circuits.This method helps to simplify
the control drawings.Control symbols
and alphabets generally used are as shown in Table 1.A clear knowledge of these facilitates the
understanding of the control drawings.
CONTROL CIRCUIT SUPPLIES
To effect operation of control
circuits, external auxiliary power supplies are used.Two major sources of supplies are most common
namely:
·D.C.
supply
·A.C.
supply
D. C. SUPPLY
The major source of D. C. supply is
from a storage battery.The storage
battery types commonly used are:
(a)Lead
Acid Accumulator type
(b)Nickel
Cadmium type.
Auxiliary D.C. supply has standard
voltage ratings of 24V, 30V, 36V, 48V, 50V, 60V, 72V, 110V, 220V and 250V.Generally 110V is used for Trip/Close
control.In some cases a combination of
50V and 110V D.C. are used.In this case
the relay coil energizes an auxiliary interposing relay whose contacts make to
energize an 110V D.C. breaker trip/close coil which in turn opens/closes the
contacts of a breaker.
Standard ampere-hour ratings of
auxiliary D.C. supply are 45, 60, 100, 250, 500 and 1000AH.
The voltage rating and the
Ampere-Hour rating are decided by:
(i)The
size and capacity of the generating station and or substations.
(ii)The
busbar switching arrangement, which decides the number of circuit breakers and
isolators.
(iii)The
location of the control equipment in regard to the location of the controlled
apparatus i.e. the distance from the control room to the controlled apparatus.
In most 11KV, 33KV and 132KV
substations 110V DC batteries are
installed.In 330KV substations, both
50V DC and 110V DC batteries are used for control circuits.
The ampere-hour rating range between
100 and 250 AH.
A D. C. distribution panel is
generally associated with a D. C storage battery.The size of the panel depends upon the number
of individual circuits it serves.A
Non-fused breaker usually protects each sub-circuit of the distribution panel,
which trips as soon as a fault exists along the circuit being protected.
To protect the D. C. circuits from
ground fault, a ground fault relay is installed which usually flags whenever
there is a ground fault within any of the poles of the D.C circuits.For example, if there is a fault within the
positive pole of the
D. C. circuits, the D.C. ground
positive target of the ground fault relay will operate.The relay will not reset except the source of
the fault is cleared.In some cases, the
fault signal is wired to a visual alarm, which will indicate the actual pole
that is faulty.In some installations, a
switch is used to monitor the amount of voltage leaking to ground.Under normal conditions P-E and N-E voltages
are equal.But a pole loses the voltages
to ground if faulty.
A.C. SUPPLY
The A.C. supply for the control
circuits is obtained from a station auxiliary transformer.This, in the case of generating units, may be
directly connected to the generator terminals as unit auxiliary transformers.
A standby A.C generator is also used
as an alternate source of A.C. supply for control circuits.In stations where A.C. supply is to be
reliable, there could be two sources from which auxiliary supply is obtained
with an automatic change – over switch.In this case, if supply from one source fails then, supply from the
other source is readily available.The
alternative source could be another auxiliary transformer from a separate
source, D.C. motor, A.C. generator set, or battery inverter circuit.
In control circuits, A.C. supply
could serve the following purposes:
(a)Control panel illumination
(b)Control panel heater
(c)Breaker spring operating motor.
(d)Breaker control panel heater and
illumination.
(e)Control panel indication lamps
(f)Audio/visual
annunciation
(g)OLTC gear motor operation in power
transformers
(h)Position indication for tap – changer
progress.
TRIP CIRCUIT
The control circuit for the opening
of switchgear during normal operation or on fault is usually known as Trip
Circuit.
To ensure that this circuit does not
fail whenever a signal is sent to operate the breaker/disconnect switch, it is
being monitored continuously by a relay known as Trip Circuit Supervision
relay.The relay is wired in such a way
that the relay coil is energized as long as the trip circuit is healthy.If for any reason there is a fault within the
trip circuit causing a loss of D.C. supply, this relay de-energises causing the
mechanical target to flag, which will indicate, “Trip circuit faulty”.This relay is usually a self-reset relay,
which resets itself as soon as the D.C. supply is restored.D.C. supply can also be lost if the battery
charger is faulty or the D.C. fuse gets ruptured as a result of a short-circuit
fault within the D.C. circuit.
Relaying protection is used to
prevent or minimize damage to equipment and maintain continuous supply of
Electricity at barest minimum cost.The
need for relaying protection comes into play in providing the most efficient
protection for power system equipment.This can be very expensive.To
reduce such cost, a balance needs to be struck between the cost of the
protection and the degree of safety to the equipment.
The main purposes of relaying
protection are as stated below:
(i) To ensure uninterrupted power
supply.
(ii) To reduce equipment damage.
(iii) To maintain quality of service.
(iv) To guarantee safety of life and
property.
(v) To ensure operation of equipment
at peak efficiency.
The earliest method of protection was
the fuse. The fuse finds its use primarily in Distribution Circuits due to its
cheapness and simplicity.Its use in
system protection in the power industry is limited by the following disadvantages:
(a) The fuse is slow in operation
(b) Before power supply can be
restored the fuse has to be replaced
(c) The fuse is not selective or
discriminative in operation
(d) It cannot be used for very high
voltage protection.
As a result of the above
shortcomings, the use of fuses has generally been replaced with the protective
relays.
THE RELAY
This is an electrical device that
behaves in a prescribed way to an applied input so as to cause, by its contact
operation, abrupt changes in associated control circuits.
In protective relaying, there are
important parameters required for effective performance.They include:
(a)Sensitivity
A relay must be sensitive to the
least fault conditions for which it has been configured.
(b)Reliability
It must be relied upon at all times
to respond to any fault by relaying signals that will cause the faulty part to
be isolated.
(c)Selectivity
The relay must be able to
discriminate between faults and abnormal conditions.
(d)Simple
For a relay to be effectively used,
its construction and operation has to be simple in nature.
(e)Speed of Operation
To be able to prevent damage to the
associated equipment the relay is protecting, it must act fast before the
damage is done.
(f)Cost
The relay should not be so expensive
as to outweigh the benefit of using it to protect the associated equipment.
Fault Conditions
In power systems, faults occur as a
result of breakdown in equipment insulation.These faults can be categorised as follows:
Single
phase to ground fault
Double
phase to ground fault
Three
phase to ground fault
Phase
to phase fault
Three
phase fault.
The commonest, in occurrence, of the
above fault conditions, is the single phase to ground fault which is about 70%.
Damage to equipment can be caused by
other abnormal conditions in a power system.Such conditions are:
Over
heating
Over
voltage (surge)
Over
load
Fire
disaster
Unbalanced
loading
Loss
of synchronism
For the faults and abnormal
conditions enumerated above protective relays are
designed to isolate and reduce damage
to the system equipment.
RELAY
TYPES AND CLASSIFICATIONS
Relays are classified according to
the following:
·Input
- voltage, current, frequency.
·Operating
Principle – percentage or restraining.
·Function
- Monitoring, Regulating, Auxiliary, Programming or Protection.
·Performance
characteristics - Definite time, Inverse time or Distance.
·Structure
- Static, Electromechanical or Thermal
Sometimes relays are also classified
using a combination of the above terms, e.g. inverse time over current.
RELAY PERFORMANCE
Performance of relays can be
classified as:
(i)Correct
(ii)Incorrect
(iii)Inconclusive.
Incorrect Operation
This can be due to the following
factors:
(a)Poor Application
(b)Incorrect Relay Setting
(c)Personnel Error
(d)Equipment Malfunction
Incorrect tripping may be either
failure to trip or false tripping.
Failure to trip can be caused by
faulty associated instrument transformer, circuit breakers, control cables and
wiring and station batteries.
InconclusiveOperation
This is the last resort when no
evidence is available either for a correct or incorrect operation.Quite often, this is a personal involvement.
RELAY OPERATING TIME
Relays can be classified in terms of
their operating times as follows:
·High
Speed Relays - operate in less than three (3) cycles
·Slow
speed relays - operate in three (3) cycles or more
·Time
delay relays - have built in time delay facility to allow co-ordination with
other relays within the power system.
·Instantaneous
relays - have no deliberate time delay facility.They operate instantaneously.
ZONES OF PROTECTION
For effective protection of the
system with minimum part disconnected during fault, protection zones are mapped
out.These zones are created in such a
way that each overlap around an isolating device such as a circuit breaker.
This method guarantees total
protection of power system sub circuits.These zones follow common logical boundaries to cover such equipment as
Lines, Transformers, Buses, Generators, Motors and any combinations of the
above equipment.
For such boundaries to be genuine,
there must be:
Measuring
devices such as Current Transformer or Voltage Transformer.
Isolating
devices such as breakers.
Generator Protection Zone
Typical faults occur within
generators such as - Winding faults, Field Ground fault, A.C.
Over Voltage faults and Field Loss
faults.The protective relay, on occurrence
of any of these faults must act very fast to isolate the faulty part in order
to save both the life of the equipment and the personnel around it.
Transformer Protection Zone
In this zone, the usual faults that
can occur are as follows:
Winding faults, Phase to Ground
faults, Phase-to-Phase faults and Inter turn faults.
For these faults, differential
protection is the major type used for transformer protection.Oil/Winding temperature relays are also
provided along with Bucholz gas Alarm/Bucholz surge trip protection.
These four methods are used to
protect the transformer against faults within the windings.Over current/Earth fault protection are also
provided.
Adequate protection to the
transformer is cumbersome due to the following transformer constraints:
(i)There
is phase shift in star connected transformers.
(ii)There
are different voltage levels between the primary, the secondary and or the
tertiary side of the transformer.
(iii)There
is a high magnetizing inrush current especially during transformer energization.These currents show harmonic currents of the
2nd order and above.
In order to compensate for the above
constraints, transformer differential schemes are designed taking them into
account.
As an example:
(a)To prevent tripping of transformer on
high magnetizing inrush currents, a harmonic restraint device is embedded in
the differential relay, which prevents it from operating on inrush currents
during transformer energization.
(b)Matching current transformers are
used to correct the voltage level differences at both sides of the transformer
and also to correct the differences in transformer characteristics of the
current transformers on both sides of the transformer.
(c)Phase shift in the Star-Delta
windings of transformers are taken care of by connecting star winding CT’s in
Delta and Delta winding CT’s in Star.
In some cases the relay may be
unstable as a result of spill current on the Delta side of the transformer due
to zero sequence.These currents are
filtered out by wiring part of the matching C.T. winding to cancel itself out
of the delta side of the transformer.
The vector group of the transformer
windings also plays a prominent role when the differential relay is wired.To take care of this, the delta wiring of the
matching CT's must be wired according to the vector group of the winding as
stipulated by the manufacturer.
Details of transformer differential
are discussed fully in Chapter 9 on Power Transformer Protection.
Bus bar Protection Zone
The most common fault within this
zone is the phase to ground fault generally caused
by flash over on insulators as a
result of lightening.Other causes of
this flash over are:
ØCracked
insulators, birds and reptiles, dirty or broken insulators or animals that may
walk close to the bus.
The bus has several lines/feeders
tied to it.The current transformers on
the bus get easily saturated due to these lines.The usual type of protection for the bus zone
is the differential type.This method
compares the current entering the bus zone with that leaving it.
Current transformers installed on
each bus feed are used to make this comparison.Under a fault condition, the CT's on the faulted circuit get the sum of
all the currents from the other circuits.
Lines Protection Zone
The transmission and distribution lines
comprise the major means where by electric power is transported from generating
source to the points where the energy is to be used.These lines run into thousands of kilometers.
Faults occurring on power
transmission lines can be due to the following causes:
Lightning
Wind
Birds
Bush
fire
There are four types of line faults
namely:Line to Ground, Line to Line,
Double Line to Ground faults.
Transmission Line protection can be
classified as follows:
(a)Instantaneous/inverse time over
current (non-directional)
(b)Instantaneous/inverse time over
current (directional)
(c)Distance protection -
directional/inverse or instantaneous
(d)Pilot wire using communication
channels
(e)Current balance.
For effective line protection, the
different protection schemes must be properly coordinated.Distribution lines are adequately protected
using over current relays and fuses.
Distance protection is frequently
used for voltages of 66KV and above.The
scheme functions by comparing the system voltage and current and operates when
the voltage-current ratio is less than a pre-set value.
Thus V= IZ, where V, I, Z, are system voltage,
current and impedance respectively.
In normal operation, the system Z is
fixed.This value reduces or increases
depending upon an external or internal fault within the zone of
protection.Impedance diagrams are
usually used to show the characteristics of the distance relay and are usually
known as Mho characteristics.Distance relays
can be single-phase type or three-phase types.
In modern line distance protection,
tripping in remote stations is facilitated through the use of communication
channels.This method is called
carrier-assisted distance protection scheme.
PRIMARY AND SECONDARY RELAYS
Primary Relays are the first line of defense in the
system.They are generally high-speed
relays.The primary relay scheme is
designed to remove minimum equipment
from service.
Secondary Relays also called backup relays are
intentionally delayed in their operation so as to give the primary relays a
chance to operate first.The backup
relays scheme is independent of the primary relay scheme and operates if the
primary relay scheme fails to operate.The equipment removed from service by the backup protection is more of
the equipment including the faulty ones.Backup protection comes in overlapping zones.
ELECTRICAL POWER SYSTEM DEVICE
NUMBERS AND FUNCTIONS
The devices in switching equipment
are referred to by numbers with appropriate suffix letters when necessary,
according to the functions they perform.
These numbers are based on a system
adopted as standard for automatic switchgear by IEEE.
This system is used in connection
diagrams, in instruction books and in specifications.
Device Definitions and Functions
Number
1Master Element is the 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
equipment in or out of operation.
2Time Delay Starting or Closing Relay is a device which functions to give
a desired amount of time delay before or after any point of operation in a
switching sequence or protective relay
system except as specifically provided by Device Functions 48, 62 and 79
described later.
3Checking or Interlocking Relay is a device that 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.
4Master Contactor is a device, generally controlled by
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.
5Stopping device is a control device used primarily
to shut down equipment and hold it out of operation (this device may be
manually or electrically actuated but excludes the function of electrical lock
out (See Device Function 86 on abnormal conditions).
6Starting Circuit breaker is a device whose principal function
is to connect a machine to its source of starting voltage.
7Anode Circuit breaker is one used in the anode circuits of
power rectifiers for the primary purpose of interrupting the rectifier circuit
if an arc back should occur.
8Control Power Disconnecting Device is a disconnection device such as a knife switch, circuit breaker
or pull out 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 to include auxiliary power, which supplies
such apparatus as small motors and heaters.
9Reversing Device is used for the purpose of reversing
a machine field or for performing any other reversing functions.
10Unit Sequence Switch is used to change the sequence in
which units may be placed in and out of service in multiple unit equipment.
11RESERVED FOR FUTURE APPLICATION
12Over Speed device is usually a directly connected
speed switch, which functions on machine over speed.
13Synchronous Speed device such as a centrifugal speed switch,
a slip frequency relay, a voltage relay, an under current relay or any type of
device, that operates at an approximate synchronous speed of a machine.
14Under Speed device functions when the speed of a
machine falls below a predetermined value.
15Speed or frequency matching device 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.
16RESERVED FOR FUTURE APPLICATION
17Shunting or Discharge switch serves to open or to close a
shunting circuit around a 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 or their
equivalent, and also excludes Device 73 which serves for the switching of
resistors.
18Accelerating or Decelerating device is used to close or to cause the
closing of circuits, which are used to increase or to decrease the speed of a
machine.
19Starting to Running Transition
Contactor is a
device, which operates to initiate or cause the automatic transfer of a machine
from the starting to the running power connection.
20Electrically Operated Valve is electrically operated, controlled
or monitored valve in a fluid line.
Note:The function of the valve may be indicated by the use of suffixes.
21Distance Relay is a device which functions when the
circuit admittance, impedance or reactance increases or decreases beyond
predetermined limits.
22Equaliser Circuit breaker is a breaker, which serves to
control or to make and break the equaliser or the current balancing operations
for a machine field, or for regulating equipment in a multiple unit
installation.
23Temperature Control device which functions to raise or lower
the temperature of a machine or other apparatus or of any medium, when its
temperature falls below, or rises above, a predetermined value.
Note: An example is a thermostat, which
switches 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.
24RESERVED FOR FUTURE APPLICATION
25Synchronising or Synchronism Check device operates when two A.C circuits are
within the desired limits of frequency, phase angle or voltage to permit or to
cause the paralleling of these two circuits.
26Apparatus Thermal device functions when the temperature of
the shunt field or the armotisseur windings of a machine or that of a load limiting
or load shifting resistor or of a liquid of 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.
27Under Voltage relay is a device, which functions on a
given value of under voltage.
28Flame Detector is a device that monitors the
presence of the pilot or main flame in such apparatus as a gas turbine or a
steam boiler.
29Isolating Contactor is used expressly for disconnecting
one circuit from another for the purpose of emergency operation, maintenance or
test.
30Annunciator Relay 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 lock out
function.
31Separate Excitation device connects a circuit such as the shunt
field of a synchronous converter, to a source of separate excitation during the
starting sequence; or one, which energizes the excitation and ignition circuits
of a power rectifier.
32Directional Power relay is one which functions on a desired
value of power flow in a given direction, or upon reverse power resulting from arc back in the anode-cathode circuits
of a power rectifier.
33Position Switch makes or breaks contact when the
main device or piece of apparatus, which has no device function, reaches a
given position.
34Master Sequence device 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 an equipment during starting and stopping or both during other
sequential switching operations.
35Brush Operating or slip ring short circuiting
device is used for
raising, lowering or shifting the brushes of a machine, or for short-circuiting
its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.
36Polarity or Polarizing Voltage device operates or permits the operation of
another device on a predetermined polarity only or verifies the presence of a
polarizing voltage in an equipment.
37Undercurrent or under power relay functions when the current or power
flow decreases below a predetermined value.
38Bearing Protective device functions on excessive bearing temperature or on other abnormal
mechanical conditions, such as undue wear, which may eventually result in
excessive bearing temperature.
39Mechanical Condition monitor is a device that functions upon the
occurrence of an abnormal mechanical condition (except that associated with
bearings as covered under Device function 38) such as excessive vibration,
eccentricity, expansion, shock, tilting or seal failure.
40Field relay functions on a given or abnormally
low value or failure of machine field current, or an excessive value of the
reactive component of armature current in an A.C. machine indicating abnormally
low field excitation.
41Field Circuit breaker is a device, which functions to
apply, or to remove, the field excitation of a machine.
42Running Circuit breaker 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 or closing of the circuit.
43Manual Transfer or Selector device transfers the control circuits so as
to modify the plan of operation of the switching equipment or of some of the
devices.
44Unit Sequence starting relay 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.
45Atmospheric Condition monitor is a device that functions upon the
occurrence of an abnormal atmospheric condition such as damaging fumes,
explosive mixtures, smoke or fire.
46Reverse phase or Phase balance
current relay is a
relay which functions when the polyphase currents are of reverse phase
sequence, or when the polyphase currents are unbalanced or contain negative
phase sequence
components
above a given amount.
47Phase sequence Voltage Relay functions upon a predetermined value
of polyphase voltage in the desired phase sequence.
48Incomplete Sequence Relay is a relay that generally returns
the equipment to the normal, or off, position and locks it out if the normal
starting, operating or stopping sequence is not properly completed within a predetermined time. If the device is used for
alarm purposes only, it should preferably be designated as 48A (Alarm).
49Machine or Transformer Thermal Relay 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 power rectifying transformer) exceeds a predetermined value.
50Instantaneous over current or rate of riserelay is a relay that
functions instantaneously on an excessive value of current, or an excessive
rate of current rise, thus indicating a fault in the apparatus or circuit being
protected.
51A.C. Time Over current relay is a relay with either a definite or
inverse time characteristic that functions when the current in an A.C. circuit
exceeds a predetermined value.
52A.C. Circuit breaker is a device that is used to close
and interrupt an A.C. power circuit under normal conditions or to interrupt
this circuit under fault or emergency conditions.
53Exciter or D.C. generator relay is a relay that forces the D.C.
machine field excitation to build up during starting or which functions when
the machine voltage has built up to a given value.
54RESERVED
FOR FUTURE APPLICATION
55Power Factor Relay is a relay that operates when the
power factor in an A.C. circuit rises above or drops below a predetermined
value.
56Field Application Relay is a relay that automatically
controls the application of the field excitation to an A.C. motor at some
predetermined point in the slip cycle.
57Short circuit or grounding device is a primary circuit switching
device that functions to short circuit or to ground a circuit in response to
automatic or manual means.
58Rectification Failure Relay is a device that functions if one or
more anodes of a power rectifier fail to fire or to detect an arc back or
failure of a diode to conduct or block properly.
59Over voltage Relay is a relay that functions on a given
value of over voltage.
60Voltage or current balance Relay is a relay that operates on a given
difference in voltage, or current input or output of two circuits.
61RESERVED FOR FUTURE APPLICATION
62Time Delay stopping or opening relay is a time delay relay that serves in
conjunction with the device that initiates the shutdown, stopping or opening
operation in an automatic sequence.
63Pressure switch is a switch, which operates on given
values or on a given rate of change of pressure.
64Ground Protection Relay is a relay that functions on failure
of the insulation of a machine, transformer or of other apparatus to ground, or
on flash over of a D.C. machine to ground.
Note:This function is assigned only to a relay, which wired to operate the
relay in front is always equal to or less than the primary current required to
operate the relay behind it.
65Governor is the assembly of fluid, electrical
or mechanical control equipment used for regulating the flow of water, steam or
other medium to the prime mover for such purposes as starting, holding speed or
load or stopping.
66Notching or Jogging device functions to allow only a specified
number of operations of a given device or equipment or a specified number of
successive operations within a given time frame.It also functions to energise a circuit
periodically or for fractions of a specified time interval or that used to
permit intermittent acceleration or jogging of a machine at low speeds for
mechanical positioning.
67A.C Direction or Over-current Relay is a relay that functions on a
desired value of A.C over-current flowing in a predetermined direction.
68Blocking relay is a 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-operates with other
devices to block tripping or to block
re-closing
on an out of step condition or on power swings.
69Permissive control device is generally a two position manually
operated switch that in one position permits the closing of a circuit breaker
or the placing of equipment into operation and in the other position prevents
the circuit breaker or the equipment from being operated.
70Rheostat is a variable resistance device used
in an electric circuit, which is electrically operated or has other electrical
accessories, such as auxiliary position or limit switches.
71Level switch is a switch, which operates on given
values, or on a given rate of change of level.
72D.C Circuit breaker is used to close and interrupt a D.C
power circuit under normal conditions or to interrupt this circuit under fault
or emergency conditions.
73Load Resistor contactor 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 or other machine in and out of circuit.
74Alarm relay is a device other than an
Annunciator, as covered under Device No. 30, which is used to operate in
connection with a visual or audible alarm.
75Position Changing mechanism is a mechanism that is 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.
76D.C. Over-current relay is a relay that functions when the
current in a D.C circuit exceeds a given value.
77Pulse Transmitter is used to generate and transmit
pulses over a telemetering or pilot wire circuit to the remote indicating or
receiving device.
78Phase angle measuring or out of step
protective relay is
a relay that functions at a predetermined phase angle between two voltages or
between two currents or between voltage and current.
79A.C. Reclosing relay is a relay that controls the
automatic re-closing and locking out of an A.C. circuit interrupter.
80Flow Switch is a switch, which operates on given
values or on a given rate of change of flow.
81Frequency Relay is a relay that functions on a
predetermined value of frequency either under or over or on normal system
frequency or rate of change of frequency.
82D.C. Reclosing relay is a relay that controls the
automatic closing and re-closing of a D.C. circuit interrupter, generally in
response to load circuit conditions.
83Automatic Selective control or
Transfer relay is a
relay that operates to select automatically between certain sources or
conditions in an equipment or performs a transfer operation automatically.
84Operating mechanism is the complete electrical mechanism
or servo-mechanism, including the operating motor, solenoids, position
switches, etc., for a tap changer, induction regulator or any similar piece of
apparatus which has no device function number.
85Carrier or Pilot Wire Receiver Relay is a relay that is operated or
restrained by a signal used in connection with the carrier current or D.C pilot
wire fault directional relaying.
86Locking out relay 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.
87Differential Protective Relay is a protective relay that functions
on a percentage or phase angle or other quantitative difference of two currents
or of some other electrical quantities.
88Auxiliary motor or motor generator is one used for operating auxiliary
equipment such as pumps, blowers, exciters, rotating magnetic amplifiers etc.
89Line switch is used as disconnecting load
interrupter or isolating switch in an A.C or D.C power circuit when this device
is electrically operated or has electrical accessories such as an auxiliary
switch, magnetic lock etc.
90Regulating device 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, tie lines or other apparatus.
91Voltage directional relay is a relay that operates when the
voltage across an open circuit breaker or contactor exceeds a given value in a
given direction.
92Voltage and power directional relay is a relay that permits or causes
the connection of two circuits when the voltage difference between them exceeds
a given value in a 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.
93Field changing contactor functions to increase or decrease in
one-step the value of field excitation on a machine.
94Tripping or trip free relay 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.
DEVICES PERFORMING MORE THAN ONE FUNCTION
If
one device performs two relatively important functions in an equipment so that
it is desirable to identify both of these functions, this may be done by using
a double device function such as: 50/51 - An over-current relay with an
instantaneous element and an inverse element.
SUFFIX NUMBERS
If
two or more devices with the same function number and suffix letter (if used)
are present in the same equipment then these are distinguished as follows
52x-1, 52x-2, 52x-3 etc
SUFFIX LETTERS
Suffix
letters are used with device numbers for various purposes.The meaning of each suffix letter or
combination of letters should be clearly indicated in the legend on the
drawings or publications accompanying the equipment.This is to avoid possible confusion.These letters should be written directly after
the device function number to indicate that they are a part of the device.
Commonly used letters are as follows:
R-Raising relay or for remote operation
L-Lowering relay or for local operation
O-Opening relay or contactor
C-Closing relay or contactor
CS-Control Switch
CC-Closing Coil
TC-Trip Coil
PB-Push Button
G-Generator
T-Transformer
L-Line
F-Feeder etc
Example: 52 TC – Tripping coil
of the breaker.
REPRESENTATION OF DEVICE CONTACTS
There are almost in all electrical devices,
particularly in circuit breakers and relays, a set of contacts which are
normally open and another set of contacts which are normally closed.When the device operates, the contact
position reverses.Those normally open
become closed and vice versa.These are
generally indicated as ‘a’ and ‘b’ contacts.When the device has not operated or de-energised or open contacts, they
are designated thus: