119879279-132-Kv-Substation-Equipments.pdf

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SYMBOLS OF EQUIPMENTS USED IN A SUBSTATION

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CHAPTER-2
SUBSTATION

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1.INTRODUCTION
A substation may be defined as an assembly of apparatus, which transforms the
characteristics of electrical energy from one form to another form.
At generation stations the electric power is produced at low voltages. The electric power
should not be transmitted at these low voltages due to large amount of line losses and
economical reasons. To minimize the losses and for economical transmission the low
voltages are stepped up to high voltages and transmitted to far off place. The
consumers do not use such higher voltages directly and so they must be transformed to
low voltages for distribution purpose and done in these substations .So the substation
may be called as link between generation stations and consumers. The transmission
voltages are 66KV,110KV,132KV,220KV,400KV etc. The distribution voltages generally
used are 6.6KV,11KV and 33KV.
Substations usually contain transformers in order to change voltage levels; they are
connected to a "bus" via a circuit breaker. Specifically, substations are used for some or
all of the following purposes: connection of generators, transmission or distribution
lines, and loads to each other; transformation of power from one voltage level to
another; interconnection of alternate sources of power; switching for alternate
connections and isolation of failed or overloaded lines and equipment; controlling
system voltage and power flow; reactive power compensation; suppression of
overvoltage; and detection of faults, monitoring, recording of information, power
measurements, and remote communications. Minor distribution or transmission
equipment installation is not referred to as a substation.

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Substations present in the power system performs various operations depends on the
application such as stepping up the voltage, stepping down the voltage, high voltage
transmission and switching stations to route the power to desired load center.

Substations are installed to perform any of the following operations.
1. To switch ‘ON’ and ‘OFF’ the power lines,known as switching operation.

2. To transform voltage from higher to lower or vice versa,known as voltage
transformation operation.


3. To convert A.C, into D.C. or vice versa,known as power converting operation.

4. To convert frequency from higher to lower or vice-versa ,known as frequency
converting operation.


5. To improve the power factor by installing synchronous condenser at the end of
the line,known as power factor correction operation.

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1.1 classification of sub-stations

substations are classified according to service requirement and constructional
features .

1.According to service requirement: A substation may be called upon to change
voltage or improve power factor or convert a. c. power into d. c. power etc. According to the
service requirement, substations may be classified into
a. Transformer substation:
Those substations which change the voltage level of electric supply are called
transformer substations. These substations receive power at some voltage and deliver it at
some other voltage. Obviously, transformer will be the main component in such
substations. Most of the sub stations in the power system are of this type.
b. switching sub-station : These substations do not change the voltage
level i.e. incoming and outgoing lines have the same voltage. However ,they
simply perform the switching operations of power lines. A switching
substation is a substation which does not contain transformers and operates
only at a single voltage level. Switching substations are sometimes used as
collector and distribution stations. Sometimes they are used for switching the
current to back-up lines or for parallelizing circuits in case of failure

c. Power factor correction substation
These sub-station which improve the power factor of the system are
called power factor correction substation. These are generally located at
receiving end of transmission lines. These substations generally use
synchronous condensers as the power improvement equipment

d. Frequency changer sub-station :
Those sub-stations, which change the supply frequency, are known as
frequency change sub- stations . Such sub-station may be required for industrial utilization.
e. Converting sub-station :
Those sub-station which change a. c. power into d. c. power are called converting
s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding
etc.

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f. Industrial sub-station:
Those sub-stations, which supply power to individual industrial concerns are known
as industrial sub-stations.
2. According to constructional features : A substation has many components which
must be housed properly to ensure continuous and reliable service. According to
constructional features the substations are classified as :

 Indoor substation
 Outdoor substation
 Underground substation
 Pole-mounted substation

1. Indoor Sub-station :-
For voltage up to 11KV, the equipment of the s/s is installed indoor because of
economic consideration. However, when the atmosphere is contaminated with impurities,
these sub-stations can be erected for voltage up to 66KV.
2. Outdoor Sub-Station :-
Outdoor Substations are used for all voltage levels from 52 to 765 kV. They are built
outside cities, usually at points along the cross-country lines of bulk transmission systems.
They comprise switchgear like circuit breakers, disconnectors, instrument transformers,
power transformers, surge arrestors and bus bars. The control and protection equipment is
housed in central buildings or in small switching bay oriented containers in the
switchyard .
For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such
Voltage the clearances between conductor and the space required for switches, C.B. and
other equipment becomes so great that it is not economical to installed the equipment
indoor.

3. Under ground sub-station :-
In thickly populated areas, the space available for equipment and building is limited
and the cost of the land is high. Under such situations, the sub-station is created
underground. The design of underground s/s requires more careful consideration.

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 The size of the s/s should be as minimum as possible.
 There should be reasonable access for both equipment & personal.
 There should be provision for emergency lighting and protection against fire.
 There should be good ventilation

4. Pole-mounted sub-station :-
This is an outdoor sub-station with equipment installed overhead on double pole or 4-pole
structure. It is the cheapest from of s/s for voltage not exceeding 11KV (or 33KV in some
cases). Electric power is almost distributed in localities through such sub-station. The 11KV
line is connected to the T/F through gang isolator and fuses. The lighting arresters are
installed on the H.T. Side to protect the sub-station from lighting strokes. The T/F step
down voltage to 400 V, 3 phase, 4 wire supply. The voltage between any two lines is 400 V
& between line & neutral is 230V. The oil circuit breaker installed on the L.T. side
automatically Isolates the mounted sub-station.T/F are generally in the event of fault
generally 200 KVA T/F is used.

FUNCTIONS OF SUBSTATION:
1. Supply of required electrical power.
2. Maximum possible coverage of the supply network.
3. Maximum security of supply.
4. Shortest possible fault-duration
5. Optimum efficiency of plants and the network.
6. Supply of electrical power within targeted frequency limits, (49.5 Hz and50.5 Hz).
7. Supply of electrical power within specified voltage limits.
8. Supply of electrical energy to the consumers at the lowest cost

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CHAPTER-3
EQUIPMENTS IN A SUB-STATION

The main equipments in a sub-station are:
1. BUS BARS
2. INSULATORS
3. CIRCUIT BREAKER
4. INSTRUMENT TRANSFORMER
5. POWER TRANSFORMER
6. WAVE TRAP
7. SWITCH GEAR
8. ISOLATORS
9. MISCELLONOUS EQUIPMENTS
10. PROTECTIVE RELAY
11.

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1. BUS BARS
When a no. of lines operating at the same voltage have to be directly connected electrically,
bus-bars are used, it is made up of copper or aluminum bars (generally of rectangular X-
Section) and operate at constant voltage.
Duplicate bus-bar, generally it consist of two bus-bars a“ main” bus-bar and spare bus-
bar. The incoming and outgoing lines can be connected to either b/b. With the help of a bus-
bar coupler, which consist of a circuit breaker and isolators. However, in case of repair of
main bus-bar or fault accusing on it, the continuity of supply to the circuit can be maintain
by transforming it to the spare bus-bar for voltage exceeding 33KV, Duplicate bus-bar is
frequently used.

Bus bars , or buses, are conductors. Or group of conductors, that serve as a common
connection for two or more circuits. The bus bars experience forces when currents flow in
them. These forces can be great when short-circuit currents flow. A bus bar must be able to
withstand the forces caused by the flow of fault currents. Bur bars are the important
elements in the electrical substation. Bus bars acts as nodal point in the substation, which
connects different incoming and outgoing circuits.
Bus bars used in the substations are generally rectangular or circular cross section bars.
These bus bars can be either solid or hollow structures. Hollow circular cross section bus
bars are employs in EHV substations to reduce the corona effect.
An aluminum or copper conductor supported by insulators that interconnects the loads and
the sources of electric power in an electric power system. A typical application is the
interconnection of the incoming and outgoing transmission lines and transformers at an
electrical substation. Bus-bars also interconnect the generator and the main transformers in
a power plant. In an industrial plant such as an aluminum smelter, large bus-bars supply
several tens of thousands of amperes to the electrolytic process. See also Electric power
substation.
The size of the bus bar determines its application and the amount of current that it can carry
safely. They can be tubular, solid or flat depending on the application and to serve different
needs.
Various incoming and outgoing circuits are connected to bus bars. Bus bars receive power
from incoming circuits and deliver power to outgoing circuits.

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Bus bars are usually made up of Aluminum or copper and they are able to conduct
electricity to transmit power from the source of electric power to the load. They are usually
supported by insulators and conduct electricity within switchboards, substations or other
electric apparatus. Some typical applications of these devices can be to form the
interconnectedness of the incoming and outgoing electrical transmission lines and
transformers at an electrical substation; supplying huge amounts of amperes to the
electrolytic process in an aluminum smelter by using large bus bars and also interconnecting
generators to the main transformers in a power plant.

Different types of bus bar arrangements are employed based on the voltage, reliability of
the supply, flexibility in transmitting power and cost. The other aspects considering in
designing the bus bars arrangements are:
1. Simplicity in the design
2. Maintenance of different elements without interruption in the power supply
3. Future expansion feasibility
4. Economical in cost of installation and operation

 Different bus bar arrangements:
Some of the switching schemes are bus bar arrangements employed in the substations are
listed below:
1. Single Bus-bar arrangement
2. Double Main Bus-bar scheme
3. Main and Transfer bus-bar scheme
4. One and half breaker scheme
5. Ring Main arrangement scheme

1) Single Bus-bar scheme:

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This is the simplest bus bar scheme available which consists of single set of bus bars
connected to the generators, transformers and load feeders. All the feeders are connected
by circuit breaker and set of isolators. This arrangement helps to remove the connecting
elements (Generators, transformers, etc ) for maintenance by opening the circuit breaker
contacts and further opening the isolators. The entire Sub Station is lost in case of a fault on
the bus bar or on any bus bar isolator and also in case of maintenance of the bus bar.
Another disadvantage of this switching scheme is that in case of maintenance of circuit
breaker, the associated feeder has also to be shutdown.

Advantages:

1. This bus bar arrangement enjoys less cost of installation
2. Less maintenance
3. simple operation
Disadvantages:

1. Fault on the bus bar all the feeders connected to the bus bars should be disconnected
2. when Bus bar is under maintenance total supply and all feeders should be
disconnected
3. Least flexibility and reliability


2) DOUBLE BUS BAR ARRANGEMENT:

In this double bus bar arrangement , Each circuit can be connected to either one of these
bus bars through respective bus bar isolator. Bus coupler breaker is also provided so that
the circuits can be switched on from one bus to the other on load. This scheme suffers from
the disadvantage that when any circuit breaker is taken out for maintenance, the associated
feeder has to be shutdown.
This Bus bar arrangement was generally used in earlier 220 kV sub stations



3) Main and Transfer Bus bar Scheme:
Main and Transfer bus bar scheme is similar to single bus bar arrangement with
additional transfer bus connected. Tie circuit breaker is provided to tie both the main and
transfer bus. During normal operation all the circuits are connected to the main bus. When
circuit breaker connected to the circuit (transmission line) is required to trip for

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maintenance, tie circuit breaker connecting the main and transfer bus is closed. The relay
protection for the circuits connected to the transfer bus is taken care by the tie circuit
breaker.
Advantages:
1. Low initial cost
2. Any breaker can be taken of circuit for maintenance
Disadvantages:
1. Requires one extra breaker for bus tie
2. Switching is somewhat complicated when breaker is under maintenance




4) One and Half breaker Bus bar scheme:
In One and half breaker scheme, two circuits are connected between the three
circuit breakers. Hence One and Half breaker name was coined for this type of arrangement.
Under normal operating conditions all the breakers are closed and both the bus bars are
energized. Any Circuit fault will trip two circuit breakers and no other circuit will be affected
in this arrangement. When a bus bar fault occur only breakers adjacent to bus bars trips and
no circuit will loose power. Two bus bars can also be taken out of service without affecting
the power flow if the power source circuit ( alternator circuit) and receiving circuit
(transmission line) available in the same bay.
Advantages:
1. Most flexible operation possible
2. High reliability
3. Bus failure will not remove any circuit from service
Disadvantages:
1. High cost
2. Relaying is somewhat complicated since the middle breaker must responsible for both
the circuits on either direction and should operate

5) Ring bus bar scheme:
In this ring main bus bar scheme arrangement, breakers are connected in ring and
circuits are connected between the breakers. There will be same number of circuits as the
number of breakers in the arrangement. During normal operation all the breakers are

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closed. During circuit fault two breakers connecting the circuit trips. During breaker
maintenance the ring is broken but all the lines remain in service.
Advantages:
1. Low cost
2. Flexible operation for breaker maintenance
3. Any breaker can be taken out of service without interrupting load
4. Power can be fed from both the direction
Disadvantages:
1. Fault occur during maintenance will break the ring
2. Relaying is complex
3. Breaker failure during fault will trip one additional circuit

Major Type of Bus bar
The major types are
(1) Rigid bus-bars, used at low, medium, and high voltage
The rigid bus-bar is an aluminum or copper bar, which is supported by porcelain insulators.
The rigid bus-bar provides a quick and qualified installation, the necessary compensation of
linear thermal deformations of buses and minor errors in the installation of bus-bar
supports
(2) Strain bus-bars, used mainly for high voltage
The strain bus-bar is a flexible, stranded conductor which is strung between substation
metal structures and held by suspension-type insulators.
(3) Insulated-phase bus-bars, used at medium voltage
The insulated-phase bus-bar is a rigid bar supported by insulators and covered by a
grounded metal shield. The main advantage of this system is the elimination of short circuits
between adjacent phases.
(4) Sulfur hexafluoride (SF6)-insulated bus-bars, used in medium- and high-
voltage systems
The sulfur hexafluoride-insulated bus-bar is a rigid aluminum tube, supported by insulators
and installed in a larger metal tube, which is filled with high-pressure sulfur hexafluoride
gas.

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Advantages of Bus bar
View
• Fitting of any modular device with the same mounting height
• Components interchangeable at any time
• Compact energy distribution – up to max. 150A
• Full scope for the future
• Completely touch proof

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2.INSULATORS

Insulators are materials that resist the flow of electricity, so electricity does not easily pass
through. Examples are plastic, wood, rubber, cloth, air, glass. Some materials are better
electricity insulators than others. These are made up of porcelain or fiber glass. The most
commonly used material for the manufactures of insulators is porcelain.
An insulator type usually made of porcelain that can be stacked in a string and hangs from a
cross arm on a tower or pole and supports the line conductor.


Used for Insulation purpose. Different types of insulators are porcelain, Glass, Epoxy.
132kV Composite Substation Insulator:

Features:
1) End fittings are crimped onto fiberglass rod with voice-operated displacement type
crimper
2) The connection location between end fittings & rods is fused with sheds housing by the
overall high temperature vulcanized silicone rubber injection molding as it could minimize
the interface.

Materials:
1) Silicon Rubber housing

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2) ECR fiberglass reinforced epoxy resin rod Electromechanical

Specifications:
1).Rating voltage: 132kV
2).Specialized mechanical bending load:20kN
3).Structure height: 1200±2mm
4).Min arcing distance: 1024mm
5).Min creep age distance: 3150mm
6).Lightning impulse voltage: 450Kv
7).Power frequency wet withstand voltage: 185Kv

Function of an insulator:

An insulator is used to isolate current carrying components from conductive non current
carrying components. The insulator serves two purposes. They support the conductor and
confine te current to the conductor. Insulators are materials that resist the flow of
electricity. They support bus bars and provide insulation to the bus bars. Insulator prevents
the flow of current to the earth.

There are several types of insulators but the most commonly used are pin type, suspension
type, strain insulator and shackle insulator.
1.Pin type Insulators :
As the name suggests, the pin type insulator is secured to the cross-arm on the pole. There
is a groove on the upper end of the insulator for housing the conductor. The conductor
passes through this groove and is bound by the annealed wire of the same material as the
conductor.
Pin type insulators are used for transmission and distribution of electric power at voltages
upto 33 kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky
and hence uneconomical.

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Pin Type Insulator
2.Suspension Type Insulators :
For high voltages (>33 kV), it is a usual practice to use suspension type insulators shown in
Figure. consist of a number of porcelain discs connected in series by metal links in the form
of a string. The conductor is suspended at the bottom end of this string while the other end
of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low
voltage, say 11 kV. The number of discs in series would obviously depend upon the working
voltage. For instance, if the working voltage is 66 kV, then six discs in series will be provided
on the string.

Suspension Type insulator

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Suspension Type Insulator

3.Strain type Insulators:

When there is a dead end of the line or there is corner or sharp curve, the line is subjected
to greater tension. In order to relieve the line of excessive tension, strain insulators are
used. For low voltage lines (< 11 kV), shackle insulators are used as strain insulators.
However, for high voltage transmission lines, strain insulator consists of an assembly of
suspension insulators as shown in Figure. The discs of strain insulators are used in the
vertical plane. When the tension in lines is exceedingly high, at long river spans, two or more

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Strain Type Insulator

strings are used in parallel .


4.Shackle type Insulators:


shackle Type Insulator
In early days, the shackle insulators were used as strain insulators. But now a days, they are
frequently used for low voltage distribution lines. Such insulators can be used either in a
horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt
or to the cross arm.

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3.CIRCUIT BREAKERS


A circuit breaker is an automatically operated electrical switch designed to protect an
electrical circuit from damage caused by overload or short circuit. Its basic function is to
detect a fault condition and, by interrupting continuity, to immediately discontinue
electrical flow. Unlike a fuse, which operates once and then must be replaced, a circuit
breaker can be reset (either manually or automatically) to resume normal operation. Circuit
breakers are made in varying sizes, from small devices that protect an individual household
appliance up to large switchgear designed to protect high voltage circuits feeding an entire
city.

Circuit-breakers are used to make or break electric currents in circuits during normal
operation of the system, during system faults and during system disturbances. Most
highvoltage circuit breakers are physically located in the switchyard but are operated
frompanels provided in the control room.

A circuit-breaker is expected to have the following characteristics.

 It must be capable of closing on to and carrying full load currents.

 It must have an appropriate mechanism to automatically disconnect the
 load under prescribed conditions.

 It must be able to successfully interrupt short-circuit currents flowing
 through the lines controlled by it.
 The gaps between its contacts must not flash-over when the circuit breaker
 is open.
 The circuit breaker, when closed on to a circuit in which a fault exists,
 must be able to reopen to isolate the faulted section without being
 damaged.
 It must be capable of withstanding the flow of short-circuit currents until
 they are interrupted by an adjoining circuit breaker.
 It must be capable of withstanding the electro-magnetic forces and thermal
stresses caused by the flow of short-circuit currents.

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Circuit breaker
Different types of circuit breakers are used in the substations which depends upon
maximum voltage level, maximum continuous current carrying capacity and maximum
interrupting capacity.

 Low Voltage Circuit Breaker:

These breakers are made for direct current (DC) applications and are commonly used in
domestic, commercial, and industrial fields. They can be installed in multi-tiers in LV
switchboards or switchgear cabinets. Low voltage circuit breakers are usually placed in
draw-out enclosures that permit removal and interchange without dismantling the
switchgear. Miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) are
some common types of low voltage circuit breakers.
 Medium Voltage Circuit Breakers:
These breakers can be assembled into metal enclosed switchgear line used for indoor
applications, or as individual components for outdoor applications like substations. Medium
voltage circuit breakers use discrete current sensors and protection relays, and can be
attached into the circuit by bolted connections to bus bars or wires. Vacuum circuit
breakers, air circuit breakers and SF6 circuit breakers are some examples of medium voltage
circuit breakers.

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 High Voltage Circuit Breakers:
These breakers help in protecting and controlling electrical power transmission networks.
They are solenoid operated and are employed with current sensing protective relays that
function through current transformers.
 Magnetic Circuit Breakers :
These breakers use a three dimensional electromagnetic coil whose pulling force increases
with the current. The contacts are held closed by a latch so that when the current in the coil
goes beyond the rating of the circuit breaker, the coil's pull releases the latch which allows
the contacts to open with a spring action.
 Thermal Circuit Breakers:
These breakers employ heat to break the circuit current flow and consist of a bimetallic
strip, made of two types of materials welded together. At high heat levels, this strip bends
at an angle that pulls the lever down and breaks the connection between the circuit
breaker's contact plate and the stationary contact plate.
Rated circuit breakers, common trip breakers, Earth leakage circuit breakers are another
type. One of the most important difference between circuit breakers and fuses is that circuit
breakers can be reset either manually or automatically to resume normal operation,
whereas fuses once used, have to be replaced.

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4.INSTRUMENT TRANSFORMER

A current transformer (CT) is used for measurement of electric currents. Current
transformers, together with voltage transformers (VT) (potential transformers (PT)), are
known as instrument transformers.
Main information on the state of the power system and the substation equipment is
acquired by measuring various parameters. The measuring, monitoring, control and
protection devices measure and use parameters such as current. voltage. power factor.
frequency. active power, reactive power. direction of power flow. load balance. and phase
angles. These parameters are measured by using two types of analog sensors. current
transformers (CT) and voltage transformers (VT). These transducers provide the
instantaneous values of currents and voltages. The remaining parameters are derived from
these measurements. Older substations have instruments that are operated by analog
signals from the transducers. In modem substations, currents and voltages are acquired in
the form of quantized samples using analog to digital converters. The samples are then
processed by digital signal processors to estimate the desired parameters.

Instrument transformers are used for measuring voltage and current in electrical power
systems, and for power system protection and control. Where a voltage or current is too
large to be conveniently used by an instrument, it can be scaled down to a standardized low
value. Instrument transformers isolate measurement, protection and control circuitry from
the high currents or voltages present on the circuits being measured or controlled.

Current Transformer
A current transformer is essentially a step-down transformer which steps-down the current
in a known ratio, the primary of this transformer consist of one or more turn of thick wire
connected in series with the line, the secondary consist of thick wire connected in series
with line having large number of turn of fine wire and provides for measuring instrument,
and relay a current which is a constant faction of the current in the line. The current
transformer (CT) is often treated as a ‘‘black box.’’ It is a transformer that is governed by the
laws of electromagnetic induction:ε = k βAcNf
Where
ε = Induced voltage
β = Flux density
Ac = Core cross-sectional area
N = Turns
f = Frequency
k = Constant of proportionality

Current transformers are basically used to take the readings of the currents entering the
substation. This transformer steps down the current from 800 amps to 1 amp. This is
done because we have no instrument for measuring of such a large current. The main use of

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this transformer is (a) distance protection; (b) backup protection; (c) measurement.
A current transformer is a transformer designed to provide a current in its secondary coil
proportional to the current flowing in its primary coil.
When current in a circuit is too high to directly apply to measuring instruments, a current
transformer produces a reduced current accurately proportional to the current in the
circuit, which can be conveniently connected to measuring and recording instruments. A
current transformer also isolates the measuring instruments from what may be very high
voltage in the monitored circuit. Current transformers are commonly used in metering
and protective relays in the electrical power industry.
Current transformers are used extensively for measuring current and monitoring the
operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical
utility's watt-hour meter on virtually every building with three-phase service and single-
phase services greater than 200 amps.
The CT is typically described by its current ratio from primary to secondary. Often, multiple
CTs are installed as a "stack" for various uses. For example, protection devices and revenue
metering may use separate CTs to provide isolation between metering and protection
circuits, and allows current transformers with different characteristics (accuracy, overload
performance) to be used for the devices.



Current transformer

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Voltage Transformer:
It is essentially a step - down transformer and step down the voltage in known ratio. The
primary of these transformer consist of a large number of turn of fine wire connected across
the line. These secondary winding consist of a few turns and provides for measuring
instruments and relay a voltage which is known fraction of the line voltage.
Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed
to have an accurately known transformation ratio in both magnitude and phase, over a
range of measuring circuit impedances. A voltage transformer is intended to present a
negligible load to the supply being measured. The low secondary voltage allows protective
relay equipment and measuring instruments to be operated at a lower voltages.
Both current and voltage instrument transformers are designed to have predictable
characteristics on overloads. Proper operation of over-current protective relays requires
that current transformers provide a predictable transformation ratio even during a short-
circuit.

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT)
is a transformer used in power systems to step down extra high voltage signals and provide
a low voltage signal, for measurement or to operate a protective relay. In its most basic
form the device consists of three parts: two capacitors across which the transmission line
signal is split, an inductive element to tune the device to the line frequency, and
a transformer to isolate and further step down the voltage for the instrumentation or
protective relay. The tuning of the divider to the line frequency makes the overall division
ratio less sensitive to changes in the burden of the connected metering or protection
devices. The device has at least four terminals: a terminal for connection to the high voltage
signal, a ground terminal, and two secondary terminals which connect to the
instrumentation or protective relay. CVTs are typically single-phase devices used for
measuring voltages in excess of one hundred kilovolts where the use of wound primary
voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed
as a stack of smaller capacitors connected in series. This provides a large voltage drop across
C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are
used for filtering high frequency communication signals from power frequency. This forms
a carrier communication network throughout the transmission network.

The standards define a voltage transformer as one in which "the secondary voltage is
substantially proportional to the primary voltage and differs in phase from it by an angle
which is approximately zero for an appropriate direction of the connections.”

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Voltage transformer

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5.WAVE TRAP
Line traps prevent the transmission of high frequency carrier signals on high voltage
transmission lines to unwanted directions without a loss of energy at power frequency.
Line traps are a key component in PLC (Power Line Carrier) systems used for remote control
signals, voice communication, remote metering and control between substations in the
electrical T&D network. The Alstom Grid line trap is reliable and lightweight and requires
little or no maintenance. Line trap also is known as Wave trap.
Wave traps are provided in the substation because, there are multiple reasons for this,
depending on the configuration. If the wave traps are on different lines, they are likely
tuned to different carrier frequencies and are used to filter out the carrier for the line they
are installed on. If they are installed on the same line, there may be multiple carrier
frequencies used, or carrier is applied to multiple phases if on different phases.
Line Trap consists of Inductive coil usually connected in the outdoor yard incoming line. Line
traps are usually mounted above Capacitor Voltage Transformer (CVT) or on separate
structure.





30 kHz to 500 kHz frequency range
Operational up to 800 kV, the line trap can be used within the 30 kHz to 500 kHz frequency
range. It complies with IEC, ANSI or the equivalent standards.

Lightweight, reliable, and maintenance-free
Line traps are air core, dry types, with mounting flexibility and can withstand high, short-

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circuits. They are lightweight, can be used outdoors, and are doted with a reliable, open-
style and maintenance-free design. The line traps provide excellent cooling, are equipped
with extremely reliable tuning devices and have a self-resonance frequency greater than 50
kHz.

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6.POWER TRANSFORMER
Power transformer is a static electrical device, involving no continuously moving parts, used
in electric power systems to transfer power between circuits through the use of
electromagnetic induction. The term power transformer is used to refer to those
transformers used between the generator and the distribution circuits, and these
are usually rated at 500 kVA and above. Power systems typically consist of a large number of
generation locations, distribution points, and interconnections within the system or with
nearby systems, such as a neighboring utility. The complexity of the system leads to a
variety of transmission and distribution voltages. Power transformers must be used at each
of these points where there is a transition between voltage levels. Power transformers
are selected based on the application, with the emphasis toward custom design being
more apparent the larger the unit.
Power transformers are available for step-up operation, primarily used at the generator and
referred to as generator step-up (GSU)transformers, and for step-down operation, mainly
used to feed distribution circuits. Power transformers are available as single-phase or three-
phase apparatus. Transformer is a vital link in a power system which has made possible the
power generated at low voltages (6600 to 22000 volts) to be stepped up to extra high
voltages for transmission over long distances and then transformed to low voltages for
utilization at proper load centers. This flux induces an electro-motive force in the secondary
winding too. When load is connected across this winding, current flows in the secondary
circuit. This produces a demagnetizing effect, to counter balance this the primary winding
draws more current from the supply so that

IP NP=IS NS

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Electrical Power Transformer is a static device which transforms electrical energy from one
circuit to another without any direct electrical connection and with the help of mutual
induction between to windings. It transforms power from one circuit to another without
changing its frequency but may be in different voltage level.
Power transformer takes the AC mains (wall) supply voltage and converts it into one or
more AC voltages that are more convenient for our needs. For a valve amp this usually
means a low voltage for the heaters and a high voltage for the anode supply, at the very
least.
Working Principle of transform
The working principle of transformer is very simple. It depends upon Faraday's laws of
Electromagnetic Induction. Actually mutual induction between two or more winding is
responsible for transformation action in an electrical transformer.
er

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Faraday's laws of Electromagnetic Induction
According to these Faraday's laws,"Rate of change of flux linkage with respect to time is
directly proportional to the induced EMF in a conductor or coil".
An Ideal Transformer is an imaginary transformer which does not have any loss in it,
means no core losses, copper losses and any other losses in transformer. Efficiency of this
transformer is considered as 100%.

Types of Transformer
Transformers can be categorized in different ways, depending upon their purpose, use,
construction etc. The types of transformer are as follows,
• Step Up Transformer & Step Down Transformer - Generally used for stepping up and
down the voltage level of power in transmission and distribution power network.
Transformer & Single Phase Transformer - Former is generally used in three phase power
system as it is cost effective than later but when size matters it is preferable to use bank
of three Single Phase Transformer as it is easier to transport three single phase unit
separately than one single three phase unit.
• Electrical Power Transformer, Distribution Transformer & Instrument Transformer -
Transformer generally used in transmission network is normally known as Power
Transformer, distribution transformer is used in distribution network and this is lower
rating transformer and current transformer & potential transformer, we use for relay and
protection purpose in electrical power system and in different instruments in industries
are called Instrument Transformer.
• Two Winding Transformer & Auto Transformer - Former is generally used where ratio
between High Voltage and Low Voltage is greater than 2. It is cost effective to use later
where the ratio between High Voltage and Low Voltage is less than 2.
• Outdoor Transformer & Indoor Transformer - Transformers designed for installing at
outdoor is Outdoor Transformer and Transformers designed for installing at indoor is
Indoor Transformer.
EHV power transformers are usually oil immersed with all three phases in one tank. Auto
transformers can offer advantage of smaller physical size and reduced losses. The different
classes of power transformers are:
 Oil immersed, natural cooling
 Oil immersed, air blast cooling
 Oil immersed, oil circulation forced

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 Oil immersed, oil circulation forced, air blast cooling
Power transformers are usually the largest single item in a substation. For economy of
service roads, transformers are located on one side of a substation, and the connection to
switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take
precaution against the spread of fire. Hence, the transformer is usually located around a
sump used to collect the excess oil.
Transformers that are located and a cell should be enclosed in a blast proof room.



Power transformers

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7. SWITCH GEAR
One of the basic functions of switchgear is protection, which is interruption of short-circuit
and overload fault currents while maintaining service to unaffected circuits. Switchgear also
provides isolation of circuits from power supplies. Switchgear is also used to enhance
system availability by allowing more than one source to feed a load.
In an electric power system, switchgear is the combination of electrical disconnect
switches, fuses or circuit breakers used to control, protect and isolate electrical equipment.
Switchgear is used both to de-energize equipment to allow work to be done and to clear
faults downstream. This type of equipment is important because it is directly linked to the
reliability of the electricity supply.
The very earliest central power stations used simple open knife switches, mounted on
insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making
opening manually operated switches too dangerous for anything other than isolation of a
de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely
controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed
structure with electrically operated switching elements, using oil circuit breakers. Today, oil-
filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing
large currents and power levels to be safely controlled by automatic equipment
incorporating digital controls, protection, metering and communications.
High voltage switchgear was invented at the end of the 19th century for operating motors
and other electric machines. The technology has been improved over time and can be used
with voltages up to 1,100 kV.
Typically, the switchgear in substations is located on both the high voltage and the low
voltage side of large power transformers. The switchgear on the low voltage side of the
transformers may be located in a building, with medium-voltage circuit breakers for
distribution circuits, along with metering, control, and protection equipment. For industrial
applications, a transformer and switchgear line-up may be combined in one housing, called
a unitized substation or USS.
There are many different types of switch gears. To the common types of switchgears are
included vacuum switch gears, oil insulated switch gears ,and gas insulated switchgears.
Also there are simple open air switchgears.
The vacuum circuit Switch gear is the type of Switchgears that has minimal arcing. when the
arc is stretched to less than 2 to 3 mm ,it quenches. They are frequently used in modern
medium voltage switch gear of up to 35000 volts.
The oil insulated switchgear depends on the oil vaporization blast through its arc.
The gas insulated switchgear stretches the arc with a magnetic field. it also depends on the
dielectric strength of the gas to quench the stretched arc.

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The open air switch gear circuit breakers use compressed air to blow out the arc.whe the
displaced air is trying to escape, it blows out the arc.


Switch gear

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8.ISOLATORS


Isolating switches are also called isolators. disconnects, disconnecting switches.
Isolators are provided for isolation from live parts for the purpose of maintenance. Isolators
are located at either side of the circuit breaker. Isolators are operated under no load.
Isolator does not have any rating for current breaking or current making. Isolators are
interlocked with circuit breakers

Disconnector or isolator switch is used to make sure that an electrical circuit can be
completely de-energised for service or maintenance. Such switches are often found
in electrical distribution and industrial applications where machinery must have its source of
driving power removed for adjustment or repair. High-voltage isolation switches are used in
electrical substations to allow isolation of apparatus such as circuit
breakers and transformers, and transmission lines, for maintenance.


Function of Isolators :

 Isolating switches are used to disconnect circuit-breakers. sections of bus bars and
parts of the system.
 An isolator is used to open a circuit only after the flow of current has been
interrupted by another device.
 These switches are slow moving devices but are inexpensive compared to load
switches and circuit breakers.
 These switches are also used to transfer circuits from one bus bar to another and to
provide flexibility during system operation
 These switches must be able to:

 carry normal load currents continuously,
 carry fault currents until they are cleared by an interrupting device and,
 make and break small currents when the voltage difference across their
terminals is not significant.
 The open and closed status of an isolator can be visually verified. Often, video
cameras are
 placed at strategic locations in a switchyard to visually inspect the operation of an
isolator
 without leaving the control room. These cameras are remotely controlled and can be
 pointed towards selected equipment in the substation.


In substation , it is often desired to disconnect a part of the system for general
maintenance and repairs. This is accomplished by an isolating switch or isolator. An

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isolator is a switch and is design to often open circuit under no load, in other words,
isolator switches are operate only when the line is which they are connected carry
no load.


The major difference between an isolator and a circuit breaker is that an isolator is
an off-load device intended to be opened only after current has been interrupted by
some other control device. Safety regulations of the utility must prevent any attempt
to open the disconnector while it supplies a circuit.









For example, consider that the isolator are connected on both side of a cut breaker, if the
isolators are to be opened, the C.B. must be opened first.

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frequently used in different Electrical Isolators (Outdoor Isolators) are used to protect from
the direct contact of electricity. They are the very essential part of electrical equipments.
We offer different types of electrical Isolators which include Electric Fence Isolators and
Electric Current Isolators. All these types are of supreme quality. They are made shock proof
by special technology. They are being industries with total comfort. We are among the
leading Electric Current Isolators Suppliers in India who supply all the famous brands of
electric switches.

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MISCELLONOUS EQUIPMENTS
Metering and Indicating Instrument:

There are several metering and indicating Instrument (e.g.Ammeters, Volt-meters, energy
meter etc.) installed in a Sub-Station tomaintain which over the ckt quantities. The
instrument transformer areinvariably used with them for satisfactory operation.

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Surge Arrestors or Lightning Arrester:
A lightning arrester is a device used on electrical power systems to protect the insulation on
the system from the damaging effect of lightning. The typical lightning arrester also known
as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge
or switching surge travels down the power system to the arrester, the current from the
surge is diverted around the protected insulation in most cases to earth.

Surge Arresters or Lightning Arresters discharge the over voltage surges to earth and
protect the equipment insulation from switching surges and lightning surges. Surge
arresters are generally connected between phase conductor and ground. In a Substation
surge arrester is located at the starting of the substation as seen from incoming
transmission lines and is the first equipment of the substation. Surge arresters are also
provided near the transformer terminals phase to ground. Two type of surge arresters are
available 1) Gapped Arresters 2) Gapless Zinc – Oxide arresters.

Lightning Arrester or Surge Arreseter

Air-Break Switch
an electric switch in which the opening and closing of contacts and extinguishing of the
electric arc are accomplished by means of compressed air. An air-break switch consists of
three basic structural elements: a reservoir with a supply of compressed air, an arc
extinguisher, and an electropneumatic actuator.
The principal advantages of air-break switches lie in the fact that they are fireproof and
explosionproof, have rapid connect and disconnect operation, and are relatively simple in
design. The presence of equipment for the production and storage of compressed-air
supplies is a disadvantage. Air-break switches at currents up to 750 kV, which are generally
used at high-voltage power plants and substations, are manufactured in the USSR.
Modern air-break switches are provided with an enclosed isolating switch whose contacts
are housed in an insulated casing that fills with compressed air upon disconnection . Air-
break switches at currents of 110 kV and higher (up to 750 kV) are manufactured with air-
filled isolating switches..

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An air break switch comprising one or more fixed contacts and one or more movable
contacts which are movable between open and closed positions, and a coil which,
preferably in conjunction with…
The function of a switch is to open or close a path for electricity.
Low voltage kife switch
A knife switch has a simple handle that swings back and forth to open or close a circuit. The
switch itself is not covered with insulation. It is easy to see if the switch is open (on) or
closed (off). The knife switch in the photos below could be used to turn off a power tool or
other equipment.
A simple knife switch is good for doing experiments with electricity of low voltage. Dry cells
have low voltage and are often used for these experiments. If you touch the metal of the
switch, you should not get hurt when using very low voltage with dry cells!



Earth Switch:
Station Earthing System includes Earth Mat and Earth electrodes placed below ground level.
These Earth Mat and Earth electrode is connected to the equipment structures, neutral
points for the purpose of Equipment earthing and neutral point earthing.
Function earthing system is to provide low resistance earthing for
1. Discharging currents from the surge arresters, overhead shielding, earthing switches
2. For equipment body earthing
3. For safe touch potential and step potential in substation.

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Earth Switch is used to discharge the voltage on the circuit to the earth for safety. Earth
switch is mounted on the frame of the isolators. Earth Switch is located for each incomer
transmission line and each side of the bus bar section.

Battery
A battery is a device that converts the chemical energy contained in its active materials
directly into electric energy by means of an electrochemical oxidation-reduction
(redox)reaction. In the case of a rechargeable system, the battery is recharged by a reversal
of the process.
Batteries Room
Batteries are very important part of the grid. It works as a standby storage device, that
provides D.C power to the grid’s dc supply equipment in case of failure of A.C supply.
Different protection devices i.e relays, circuit breakers and other control equipment of relay
room, 11KVcontrol room, 132KV control room and yard operates on 110 D.C volt supply that
is normally supplied by a rectifier. In case of failure of A.C power batteries works as a
standby source of 110 D.C supply. No. of cells installed = 552 Volt/cell, 150 AH Total Output
Voltage = 110 Volt. Recommended Float Voltage = 202 Volt/cell at 25 CRecommended Boost
Voltage = 2.4 Volt/cell Minimum2.8 Volt/cell Maximum Total Float Voltage = 121 Volt


Capacitor
Capacitors are used in substation to improve power factor.

Series Reactors
Series reactors are used to limit short – circuit current and to limit current surges associated
with fluctuating loads. Series reactors are located at the strategic locations such that the
fault levels are reduced.

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Series Reactors
Lightning Protection:
Lightning protection is used to protect substation equipment from direct lightning strokes.
Lightning Masts are located at the outdoor yard. Overhead Shielding wires are used to cover
entire outdoor yard
.
Lightning Masts
Neutral Grounding Equipment:
Neutral Grounding Equipment are Resistors and reactors. They are used to limit the short
circuit current during ground fault. They are connected between neutral point and ground.

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FUSES
A fuse is a type of low resistance resistor that acts as a sacrificial device to provide over
current protection, of either the load or source circuit. Its essential component is a metal
wire or strip that melts when too much current flows, which interrupts the circuit in which it
is connected. Short circuit, overloading, mismatched loads or device failure are the prime
reasons for excessive current.
A fuse interrupts excessive current (blows) so that further damage by overheating or fire is
prevented. Wiring regulations often define a maximum fuse current rating for particular
circuits.Over current protection devices are essential in electrical systems to limit threats to
human life and property damage. Fuses are selected to allow passage of normal current plus
a marginal percentage and to allow excessive current only for short periods. Slow blow fuses
are designed to allow higher currents for a modest amount of time longer, and such
considerations are and were commonly necessary when electronics devices or systems
had electronic tube tech or a large number of incandescent lights were being powered such
as in a large hall, theater or stadium. Tubes and incandescent lights each have reduced
current needs as they heat up to operating temperatures for their internal resistance grows
as they are heated— the same physics principle causes the fuse material to melt,
disconnecting the circuit from power.

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.

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