Grid Transmission Substation
This report is an outcome of the contributions made by some
of the peoples. Therefore it is my sole responsibility to
acknowledge them. I am greatly thankful to the sincere
efforts made by Mr. R.B.Singh, J.E. (maintenance) without
whom this project would be abstract. I also thank the staff
of 220kV Grid Transmission Substation, Naubasta-Kanpur
who took out there precious time to tell me about the various
equipments. My special thanks is dedicated to Mr. Vijay
Kumar, J.E. (maintenance).
I would also mention the outstanding support given by my
parents who paved the way for me to overcome with this
Electrical & Electronics Engineering
Hindustan College of Science and Technology
This is to certify that ISHANK RANJAN, a student of
Hindustan College of Science and Technology pursuing
B.tech in Electrical & Electronics Engineering branch has
undergone industrial training at 220kV Grid Transmission
Substation, Naubasta-Kanpur under UPPTCL (Uttar Pradesh
Power Transmission Corporation Ltd) from 9th
of July, 2013
of August, 2013. And this project report is based on it.
His conduct was good during the entire period of training.
Mr. J.N. Prajapati Mr. R.B. Singh
A.E. (o & m) J.E. (m)
Mrs. Ranjana Srivastava
Mr. R.C. Srivastava
2.1.Types Of Substation
2.2.Components Of Substation
3.1.Aluminum In Place Of Conductors
3.2.Types Of Conductors
126.96.36.199. Capacitor Voltage Transformer
7.1.Types Of Circuit Breakers
7.1.1.Sulfur Hexafluoride H V Circuit Breaker
7.1.2.Carbon Di Oxide H V Circuit Breaker
9.Description Of Substation
9.1.1.Control Panel Section
9.1.2.Relay And Protection Panel Section
10.Some Full Forms Related To Substation
11.Components Used In Yard(220kV Substation ,Naubasta)
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UP Power Transmission Corporation Limited,
incorporated under the Companies Act 1956, was incorporated in
2006 with the main objective to acquire, establish, construct, take
over, erect, lay, operate, run, manage, hire, lease, buy, sell, maintain,
enlarge, alter, renovate, modernize, work and use electrical
transmission lines and/or network through extra high voltage, high
voltage and associated sub-stations, cables, wires, connected with
transmission ancillary services, telecommunication and telemetering
equipment in the State of Uttar Pradesh, India and elsewhere.
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A substation is a part of an electrical generation,
transmission, and distribution system. Substations transform voltage
from high to low, or the reverse, or perform any of several other
important functions. Between the generating station and consumer,
electric power may flow through several substations at different
Substations may be owned and operated by an electrical
utility, or may be owned by a large industrial or commercial
customer. Generally substations are unattended, relying on SCADA
for remote supervision and control.
A substation may include transformers to change voltage
levels between high transmission voltages and lower distribution
voltages, or at the interconnection of two different transmission
voltages. The word substation comes from the days before the
distribution system became a grid. As central generation stations
became larger, smaller generating plants were converted to
distribution stations, receiving their energy supply from a larger plant
instead of using their own generators. The first substations were
connected to only one power station, where the generators were
housed, and were subsidiaries of that power ratio.
As this project report is based on 220kV Grid Transmission
Substation, Naubasta, Kanpur; so the components used there are
Page | 3
2.1 Types of substation
A transmission substation connects two or more
transmission lines. The simplest case is where all transmission
lines have the same voltage. In such cases, substation contains
high-voltage switches that allow lines to be connected or isolated
for fault clearance or maintenance. A transmission station may
have transformers to convert between two transmission voltages,
voltage control/power factor correction devices such as
capacitors, reactors or static VAR compensators and equipment
such as phase shifting transformers to control power flow
between two adjacent power systems.
A distribution substation transfers power from the
transmission system to the distribution system of an area. It is
uneconomical to directly connect electricity consumers to the main
transmission network, unless they use large amounts of power, so
the distribution station reduces voltage to a level suitable for local
The input for a distribution substation is typically at least
two transmission or sub transmission lines. Input voltage may be, for
example, 115 kV, or whatever is common in the area. The output is a
number of feeders. Distribution voltages are typically medium
voltage, between 2.4 kV and 33 kV depending on the size of the area
served and the practices of the local utility. The feeders run along
streets overhead (or underground, in some cases) and power the
distribution transformers at or near the customer premises.
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In distributed generation projects such as a wind farm, a
collector substation may be required. It resembles a distribution
substation although power flow is in the opposite direction, from
many wind turbines up into the transmission grid. Usually for
economy of construction the collector system operates around 35 kV
and the collector substation steps up voltage to a transmission
voltage for the grid. The collector substation can also provide power
factor correction if it is needed, metering and control of the wind
farm. In some special cases a collector substation can also contain an
HVDC converter station.
2.2 Components of Substation
Various components are used at grid transmission
substations. These are as follows :-
(ii) Current Transformers
(iii) Potential Transformers
(iv) Power Transformers (Auto Transformer)
(v) Capacitive Voltage Transformers
(vi) Line Isolators
(vii) Bus Isolators
(viii) Lightning Arresters
(ix) Capacitor Bank
(x) Circuit Breakers
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In physics and electrical engineering, a conductor is an
object or type of material which permits the flow of electric charges
in one or more directions. For example, a wire is an electrical
conductor that can carry electricity along its length.
In metals such as copper or aluminium, the movable
charged particles are electrons. Positive charges may also be mobile,
such as the cationic electrolyte(s) of a battery, or the mobile protons
of the proton conductor of a fuel cell. Insulators are non-conducting
materials with few mobile charges and which support only
insignificant electric currents.
3.1 Aluminium in place of Copper:
a) Much lower cost
b) Lighter weight
c) Larger diameter
d) Lower voltage gradient less ionization/corona
3.2 Types of conductors used in 220kV substations are:-
a) AAC -> All Aluminium Conductors
b) AAAC -> All Aluminium Alloy Conductors
c) ACSR -> Aluminium Conductor Steel Reinforced
d) ACAR -> Aluminium Conductor Alloy Reinforced
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All Aluminium Conductors (AAC)
AAC are used primarily for overhead transmission and
primary and secondary distribution, where ampacity must be
maintained and a lighter conductor (compared to ACSR) is desired,
and when conductor strength is not a critical factor. Classes B and C
are used primarily as bus, apparatus connectors and jumpers, where
additional flexibility is required.
Aluminium 1350-H19 wires, concentrically stranded.
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All Aluminium-Alloy Conductor (AAAC)
Used as bare overhead conductor for primary and
secondary distribution. Designed utilizing a high-strength aluminium-
alloy to achieve a high strength-to-weight ratio; affords good sag
characteristics. Aluminium-alloy gives 6201-T81 gives AAAC higher
resistance to corrosion than ACSR.
Aluminium-alloy 6201-T81 wires, concentrically stranded.
Page | 8
Aluminium Conductor Steel Reinforced
Used as bare overhead transmission conductor and as
primary and secondary distribution conductor and messenger
support. ACSR offers optimal strength for line design. Variable steel
core stranding enables desired strength to be achieved without
• Aluminium 1350-H19 wires, concentrically stranded about a steel
core. Standard core wire for ACSR is class A galvanized.
• Class A core stranding is also available in zinc-5% aluminium -
mischmetal alloy coating.
• Additional corrosion protection is available through the application
of grease to the core or infusion of the complete cable
• ACSR conductor is also available in non-specular.
Page | 9
Names of ASCRs
S.No. Names Size(mm)*
No. Of strands / No. Of steel Strands/diameter of strands
Page | 10
Aluminium Conductor Aluminium Alloy
Used as bare overhead transmission cable and as primary
and secondary distribution cable. A good strength-to-weight ratio
makes ACAR applicable where both ampacity and strength are prime
considerations in line design; for equal weight, ACAR offers higher
strength and ampacity than ACSR.
Aluminium 1350-H19 wires, concentrically stranded about
an aluminium-alloy 6201-T81 core. Although the alloy strands
generally comprise the core of the conductor, in some constructions
they are distributed in layers throughout the aluminium 1350-H19
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A transformer is a static electrical device that transfers
energy by inductive coupling between its winding circuits. A varying
current in the primary winding creates a varying magnetic flux in the
transformer's core and thus a varying magnetic flux through the
secondary winding. This varying magnetic flux induces a varying
electromotive force (emf) or voltage in the secondary winding.
Transformers range in size from thumbnail-sized used in
microphones to units weighing hundreds of tons interconnecting the
power grid. A wide range of transformer designs are used in
electronic and electric power applications. Transformers are
essential for the transmission, distribution, and utilization of
Fig. Equivalent Circuit Diagram of an Ideal Transformer
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Instrument transformers are high accuracy class electrical
devices used to isolate or transform voltage or current levels. The
most common usage of instrument transformers is to operate
instruments or metering from high voltage or high current circuits,
safely isolating secondary control circuitry from the high voltages or
currents. The primary winding of the transformer is connected to the
high voltage or high current circuit, and the meter or relay is
connected to the secondary circuit.
Instrument transformers may also be used as an isolation
transformer so that secondary quantities may be used in phase
shifting without affecting other primary connected devices.
Types of Instrument Transformers:-
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Current transformers (CT) are a series connected type of
instrument transformer. They are designed to present negligible load
to the supply being measured and have an accurate current ratio and
phase relationship to enable accurate secondary connected
Current transformers are often constructed by passing a
single primary turn (either an insulated cable or an uninsulated bus
bar) through a well-insulated toroidal core wrapped with many turns
of wire. This affords easy implementation on high voltage bushings
of grid transformers and other devices by installing the secondary
turn core inside high-voltage bushing insulators and using the pass-
through conductor as a single turn primary.
A current clamp uses a current transformer with a split
core that can be easily wrapped around a conductor in a circuit. This
is a common method used in portable current measuring
instruments but permanent installations use more economical types
of current transformer.
Specially constructed wideband CTs are also used, usually
with an oscilloscope, to measure high frequency waveforms or
pulsed currents within pulsed power systems. One type provides an
IR voltage output that is proportional to the measured current;
another, called a Rogowski coil, requires an external integrator in
order to provide a proportional output.
The CT is typically described by its current ratio from
primary to secondary. A 1000:5 CT would provide an output current
of 5 amperes when 1000 amperes are passing through its primary
Page | 14
winding. Standard secondary current ratings are 5 amperes or 1
ampere, compatible with standard measuring instruments.
Burden and accuracy
Burden and accuracy are usually stated as a combined
parameter due to being dependent on each other.
Metering style CTs are designed with smaller cores and VA
capacities. This causes metering CTs to saturate at lower secondary
voltages saving sensitive connected metering devices from damaging
large fault currents in the event of a primary electrical fault. A CT
with a rating of 0.3B0.6 would indicate with up to 0.6 ohms of
secondary burden the secondary current will be within a 0.3 percent
error parallelogram on an accuracy diagram incorporating both
phase angle and ratio errors.
Relaying CTs used for protective circuits are designed with
larger cores and higher VA capacities to insure secondary measuring
devices have true representations with massive grid fault currents on
primary circuits. A CT with a rating of 2.5L400 would indicate it can
produce a secondary voltage to 400 volts with a secondary current of
100 amperes (20 times its rated 5 ampere rating) and still be within
2.5 amperes of true accuracy.
Care must be taken that the secondary winding of a CT is
not disconnected from its low-impedance load while current flows in
the primary, as this may produce a dangerously high voltage across
the open secondary (especially in a relaying type CT) and could
permanently affect the accuracy of the transformer.
Page | 15
High Voltage Types
Current transformers are used for protection,
measurement and control in high voltage electrical substations and
the electrical grid. Current transformers may be installed inside
switchgear or in apparatus bushings, but very often free-standing
outdoor current transformers are used.
In a switchyard, live tank current transformers have a
substantial part of their enclosure energized at the line voltage and
must be mounted on insulators. Dead tank current transformers
isolate the measured circuit from the enclosure. Live tank CTs are
useful because the primary conductor is short, which gives better
stability and a higher short-circuit current withstand rating. The
primary of the winding can be evenly distributed around the
magnetic core, which gives better performance for overloads and
transients. Since the major insulation of a live-tank current
transformer is not exposed to the heat of the primary conductors,
insulation life and thermal stability is improved.
A high-voltage current transformer may contain several
cores, each with a secondary winding, for different purposes (such as
metering circuits, control, or protection).
Fig. Equivalent Circuit Diagram of a Current Transformer
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Potential Transformer or Voltage Transformer are used
in electrical power system for stepping down the system voltage to a
safe value which can be fed to low ratings meters and relays.
Commercially available relays and meters used for protection and
metering, are designed for low voltage.
Potential transformers (PT) (also called voltage
transformers (VT)) are a parallel connected type of instrument
transformer. They are designed to present negligible load to the
supply being measured and have an accurate voltage ratio and phase
relationship to enable accurate secondary connected metering.
The PT is typically described by its voltage ratio from
primary to secondary. A 600:120 PT would provide an output voltage
of 120 volts when 600 volts are impressed across its primary winding.
Standard secondary voltage ratings are 120 volts and 70 volts,
compatible with standard measuring instruments.
Burden and accuracy
Burden and accuracy are usually stated as a combined
parameter due to being dependent on each other.
Metering style PTs are designed with smaller cores and
VA capacities than power transformers. This causes metering PTs to
saturate at lower secondary voltage outputs saving sensitive
connected metering devices from damaging large voltage spikes
found in grid disturbances.
A small PT (see nameplate in photo) with a rating of 0.3W,
0.6X would indicate with up to W load (12.5 watts ) of secondary
burden the secondary current will be within a 0.3 percent error
parallelogram on an accuracy diagram incorporating both phase
angle and ratio errors. The same technique applies for the X load (25
watts) rating except inside a 0.6% accuracy parallelogram.
Page | 17
Some transformer winding primary (usually high-voltage)
connection points may be labelled as H1, H2 (sometimes H0 if it is
internally designed to be grounded) and X1, X2 and sometimes an X3
tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3)
(and third (Z1, Z2, Z3) may also be available on the same voltage
transformer. The primary may be connected phase to ground or
phase to phase. The secondary is usually grounded on one terminal
to avoid capacitive induction from damaging low-voltage equipment
and for human safety.
Types of PTs
There are three primary types of potential transformers
(PT): electromagnetic, capacitor, and optical. The electromagnetic
potential transformer is a wire-wound transformer. The capacitor
voltage transformer (CVT) uses a capacitance potential divider and is
used at higher voltages due to a lower cost than an electromagnetic
PT. An optical voltage transformer exploits the electrical properties
of optical materials.
Capacitor Voltage Transformer
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.
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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
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.
Fig. Equivalent Circuit Diagram of CVT
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An autotransformer (sometimes called autostep down
transformer) is an electrical transformer with only one winding. The
"auto" (Greek for "self") prefix refers to the single coil acting on itself
and not to any kind of automatic mechanism.
In an autotransformer portions of the same winding act as
both the primary and secondary transformer. The winding has at
least three taps where electrical connections are made.
Autotransformers have the advantages of often being smaller,
lighter, and cheaper than typical dual-winding transformers, but
autotransformers have the disadvantage of not providing electrical
The primary voltage is applied across two of the
terminals, and the secondary voltage taken from two terminals,
almost always having one terminal in common with the primary
voltage. The primary and secondary circuits therefore have a number
of windings turns in common. Since the volts-per-turn is the same in
both windings, each develops a voltage in proportion to its number
of turns. In an autotransformer part of the current flows directly
from the input to the output, and only part is transferred inductively,
allowing a smaller, lighter, cheaper core to be used as well as
requiring only a single winding.
One end of the winding is usually connected in common
to both the voltage source and the electrical load. The other end of
the source and load are connected to taps along the winding.
Different taps on the winding correspond to different voltages,
measured from the common end. In a step-down transformer the
source is usually connected across the entire winding while the load
is connected by a tap across only a portion of the winding. In a step-
up transformer, conversely, the load is attached across the full
winding while the source is connected to a tap across a portion of
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An autotransformer does not provide electrical isolation
between its windings as an ordinary transformer does; if the neutral
side of the input is not at ground voltage, the neutral side of the
output will not be either. A failure of the insulation of the windings
of an autotransformer can result in full input voltage applied to the
output. Also, a break in the part of the winding that is used as both
primary and secondary will result in the transformer acting as an
inductor in series with the load (which under light load conditions
may result in near full input voltage being applied to the output) .
These are important safety considerations when deciding to use an
autotransformer in a given application.
Because it requires both fewer windings and a smaller
core, an autotransformer for power applications is typically lighter
and less costly than a two-winding transformer, up to a voltage ratio
of about 3:1; beyond that range, a two-winding transformer is
usually more economical.
In three phase power transmission applications,
autotransformers have the limitations of not suppressing harmonic
currents and as acting as another source of ground fault currents. A
large three-phase autotransformer may have a "buried" delta
winding, not connected to the outside of the tank, to absorb some
In practice, losses mean that both standard transformers
and autotransformers are not perfectly reversible; one designed for
stepping down a voltage will deliver slightly less voltage than
required if it is used to step up. The difference is usually slight
enough to allow reversal where the actual voltage level is not critical.
Like multiple-winding transformers, autotransformers
operate on time-varying magnetic fields and so will not function
Page | 21
Autotransformers are frequently used in power
applications to interconnect systems operating at different voltage
classes, for example 138 kV to 66 kV for transmission. Another
application is in industry to adapt machinery built (for example) for
480 V supplies to operate on a 600 V supply. They are also often
used for providing conversions between the two common domestic
mains voltage bands in the world (100-130 and 200-250).
On long rural power distribution lines, special
autotransformers with automatic tap-changing equipment are
inserted as voltage regulators, so that customers at the far end of the
line receive the same average voltage as those closer to the source.
The variable ratio of the autotransformer compensates for the
voltage drop along the line.
A special form of autotransformer called a zig zag is used
to provide grounding (earthing) on three-phase systems that
otherwise have no connection to ground (earth). A zig-zag
transformer provides a path for current that is common to all three
phases (so-called zero sequence current).
Fig. Equivalent Circuit Diagram of an Auto-Transformer
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A capacitor bank is a grouping of several identical
capacitors interconnected in parallel or in series with one another.
These groups of capacitors are typically used to correct or counteract
undesirable characteristics, such as power factor lag or phase shifts
inherent in alternating current (AC) electrical power supplies.
Capacitor banks may also be used in direct current (DC) power
supplies to increase stored energy and improve the ripple current
capacity of the power supply.
Single capacitors are electrical or electronic components
which store electrical energy. Capacitors consist of two conductors
that are separated by an insulating material or dielectric. When an
electrical current is passed through the conductor pair, a static
electric field develops in the dielectric which represents the stored
energy. Unlike batteries, this stored energy is not maintained
indefinitely, as the dielectric allows for a certain amount of current
leakage which results in the gradual dissipation of the stored energy.
The energy storing characteristic of capacitors is known as
capacitance and is expressed or measured by the unit farads. This is
usually a known, fixed value for each individual capacitor which
allows for considerable flexibility in a wide range of uses such as
restricting DC current while allowing AC current to pass, output
smoothing in DC power supplies, and in the construction of resonant
circuits used in radio tuning. These characteristics also allow
capacitors to be used in a group or capacitor bank to absorb and
correct AC power supply faults.
The use of a capacitor bank to correct AC power supply
anomalies is typically found in heavy industrial environments that
feature working loads made up of electric motors and transformers.
This type of working load is problematic from a power supply
Page | 23
perspective as electric motors and transformers represent inductive
loads, which cause a phenomenon known as phase shift or power
factor lag in the power supply. The presence of this undesirable
phenomenon can cause serious losses in terms of overall system
efficiency with an associated increase in the cost of supplying the
The use of a capacitor bank in the power supply system
effectively cancels out or counteracts these phase shift issues,
making the power supply far more efficient and cost effective. The
installation of a capacitor bank is also one of the cheapest methods
of correcting power lag problems and maintaining a power factor
capacitor bank is simple and cost effective.
One thing that should always be kept in mind when
working with any capacitor or capacitor bank is the fact that the
stored energy, if incorrectly discharged, can cause serious burns or
electric shocks. The incorrect handling or disposal of capacitors may
also lead to explosions, so care should always be exercised when
dealing with capacitors of any sort.
Fig. Capacitor Bank Installed In a Substation
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In electrical engineering, a disconnector or isolator switch
or disconnect 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.
Often the isolation switch is not intended for normal
control of the circuit and is used only for isolation; in such a case, it
functions as a second, usually physically distant master switch (wired
in series with the primary one) that can independently disable the
circuit even if the master switch used in everyday operation is turned
on. Isolator switches have provisions for a padlock so that
inadvertent operation is not possible. In high voltage or complex
systems, these padlocks may be part of a trapped-key interlock
system to ensure proper sequence of operation.
In some designs the isolator switch has the additional
ability to earth the isolated circuit thereby providing additional
safety. Such an arrangement would apply to circuits which inter-
connect power distribution systems where both end of the circuit
need to be isolated.
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.
Fig. An Isolator
Isolator Used In 33 kV Substations
Fig. A High Voltage Isolator
Page | 25
Used In 33 kV Substations
Page | 26
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 interrupt current 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.
The circuit breaker must detect a fault condition; in low-
voltage circuit breakers this is usually done within the breaker
enclosure. Circuit breakers for large currents or high voltages are
usually arranged with pilot devices to sense a fault current and to
operate the trip opening mechanism. The trip solenoid that releases
the latch is usually energized by a separate battery, although some
high-voltage circuit breakers are self-contained with current
transformers, protection relays, and an internal control power
Once a fault is detected, contacts within the circuit
breaker must open to interrupt the circuit; some mechanically-stored
energy (using something such as springs or compressed air)
contained within the breaker is used to separate the contacts,
although some of the energy required may be obtained from the
fault current itself.
Small circuit breakers may be manually operated, larger
units have solenoids to trip the mechanism, and electric motors to
restore energy to the springs.
The circuit breaker contacts must carry the load current
without excessive heating, and must also withstand the heat of the
arc produced when interrupting (opening) the circuit. Contacts are
made of copper or copper alloys, silver alloys, and other highly
conductive materials. Service life of the contacts is limited by the
Page | 27
erosion of contact material due to arcing while interrupting the
current. Miniature and molded case circuit breakers are usually
discarded when the contacts have worn, but power circuit breakers
and high-voltage circuit breakers have replaceable contacts.
When a current is interrupted, an arc is generated. This
arc must be contained, cooled, and extinguished in a controlled
way, so that the gap between the contacts can again withstand the
voltage in the circuit. Different circuit breakers use vacuum, air,
insulating gas, or oil as the medium the arc forms in.
Different techniques are used to extinguish the arc are :-
Lengthening / deflection of the arc
Intensive cooling (in jet chambers)
Division into partial arcs
Zero point quenching (Contacts open at the zero current time
crossing of the AC waveform, effectively breaking no load
current at the time of opening. The zero crossing occurs at
twice the line frequency i.e. 100 times per second for 50 Hz .
Connecting capacitors in parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the
contacts must again be closed to restore power to the interrupted
Types of circuit breakers
1. Sulphur hexafluoride (SF6) high-voltage circuit breakers
A sulphur hexafluoride circuit breaker uses contacts
surrounded by sulphur hexafluoride gas to quench the arc. They are
most often used for transmission-level voltages and may be
incorporated into compact gas-insulated switchgear. In cold climates,
supplemental heating or de-rating of the circuit breakers may be
required due to liquefaction of the SF6 gas. Issues related to SF6
Page | 28
Issue Related To SF6
The following issues are associated with SF6 circuit breakers:-
(a)Toxic lower order gases
When an arc is formed in SF6 gas small quantities of lower
order gases are formed. Some of these by-products are toxic and can
cause irritation to eyes and respiratory systems.
SF6 is heavier than air, so care must be taken when
entering low confined spaces due to the risk of oxygen displacement.
SF6 is the most potent greenhouse gas that the
Intergovernmental Panel on Climate Change has evaluated. It has a
global warming potential that is 23,900 times worse than CO2.
Alternatives to SF6 circuit breakers
Circuit breakers are usually classed on their insulating
medium. The follow types of circuit breakers may be an alternative
to SF6 types.
• Air blast
Fig. High Voltage SF6 Circuit Breaker
Page | 29
2. Carbon Dioxide (CO2) High-Voltage Circuit Breakers
In 2012 ABB presented a 72.5 kV high-voltage breaker
that uses carbon dioxide as the medium to extinguish the arc. The
carbon dioxide breaker works on the same principles as an SF6
breaker and can also be produced as a disconnecting circuit breaker.
By switching from SF6 to CO2 it is possible to reduce the CO2
emissions by 10 tons during the product’s life cycle.
Fig. High Voltage CO2 Circuit Breaker (maker ABB)
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Lightning arresters are protective devices for limiting
surge voltages due to lightning strikes or equipment faults or other
events, to prevent damage to equipment and disruption of service.
Also called surge arresters.
Lightning arresters are installed on many different pieces
of equipment such as power poles and towers, power transformers,
circuit breakers, bus structures, and steel superstructures in
Lightning, is a form of visible discharge of electricity
between rain clouds or between a rain cloud and the earth. The
electric discharge is seen in the form of a brilliant arc, sometimes
several kilometres long, stretching between the discharge points.
How thunderclouds become charged is not fully understood, but
most thunderclouds are negatively charged at the base and
positively charged at the top. However formed, the negative charge
at the base of the cloud induces a positive charge on the earth
beneath it, which acts as the second plate of a huge capacitor.
When the electrical potential between two clouds or
between a cloud and the earth reaches a sufficiently high value
(about 10,000 V per cm or about 25,000 V per in), the air becomes
ionized along a narrow path and a lightning flash results.
The conductor has a pointed edge on one side and the
other side is connected to a long thick copper strip which runs down
the building. The lower end of the strip is properly earthed. When
lightning strikes it hits the rod and current flows down through the
copper strip. These rods form a low-resistance path for the lightning
discharge and prevent it from travelling through the structure itself.
The lightning arrestor protects the structure from damage
by intercepting flashes of lightning and transmitting their current to
the ground. Since lightning strikes tend to strike the highest object in
the vicinity, the rod is placed at the apex of a tall structure. It is
connected to the ground by low-resistance cables. In the case of a
building, the soil is used as the ground, and on a ship, water is used.
A lightning rod provides a cone of protection, which has a ground
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radius approximately, equal to its height above the ground.
Surges due to lightning are mostly injected into the power
system through long cross-country transmission lines. Substation
apparatus is always well shielded against direct lightning strokes. The
protection of transmission lines against direct strokes requires a
shield to prevent lightning from striking the electrical conductors.
Terminal equipment at the substation is protected against by
surge diverters, also called surge arrester or lightning arresters. A
diverter is connected in parallel or shunt with the equipment to be
protected at the substation between the line and ground.
Ideally, it should
• become conducting at voltage above diverter rating
• become non conducting again when the line-to-neutral voltage
becomes lower than the design value. In other words, it should not
permit any power follow-on current;
• not conduct any current at normal or somewhat above normal
power frequency voltages.
Earthing screen and ground wires can well protect the
electrical system against direct lightning strokes but they fail to
provide protection against travelling waves, which may reach the
A lightning arrester or a surge diverter is a protective
device, which conducts the high voltage surges on the power system
to the ground.
Fig. Lightning Arresters Used In Substations
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Description of a Substation
It is divided into two parts:-
1) Panel Section
(a) Control Panel Section
(b) Relay & Protection Panel Section
(a) 220 kV Section
(b) 132 kV Section
(c) 33 kV Section
3) Battery Room(Extra)
Fig. A 220kV Transmission Substation
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1. Panel Section
It is a room which contains all types of panels:-
(a)Control Panel Section
(i) Control And Relay Panel
(iii) Bus Coupler Control Panel
(iv) Distribution Bus
(v) Remote Tap Changer
(vi) Auto Transformer On Load Tap Changing Control Panel
(vii) Direct Current Distribution Box
(viii) Float And Boost Charger
(ix) Capacitor Bank Panel
(x) Transformer H.V. and L.V. Side Control Panel
(xi) Triple Feeder
(xii) L.T. Distribution Board
(xiii) 40MVA Transformer
(b)Relay And Protection Panel Section
(i) Relay Panel
(ii) Protection Panel
(iii) Rotational Load Shedding
(iv) Line Protection Panel
(v) Transformer Control Panel
(vi) Apex Metering Panel
(vii) Auto Transformer PROIN
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Some Full Forms Related To Substations
S.No. Short Forms Full Forms
1. PLCC Power Line Carrier Communication
2. LA Lightning Arresters
3. CBT Capacitor Bank Transformer
4. CT Current Transformer
5. PT Potential Transformer
6. CVT Capacitive Voltage Transformer
7. LV Low Voltage
8. HV High Voltage
9. DCDB Direct Current Distribution Board
10. CTR Current Transfer Ratio
11. VTR Voltage Transfer Ratio
12. LSI Line Side Isolator
13. BSI Bus Side Isolator
14. CB Circuit Breaker
15. TI Tendom Isolator
16. BCT Base Current Transformer
17. MRI Meter Reading Instrument
18. OTI All Temp. Indicator
19. WTI Winding Temp. Indicator
20. kV Kilo Voltage
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Components Used In Yard
(220kV Substation Naubasta, Kanpur)
(1) Wave Trap (A Type Of Inductor)
(2) Two Transformer (160MVA)(220/132kV)
(3)One Auto Power Transformer (63MVA)(132/33kV)
(4)Two Auto Power Transformer (44MVA)(132/33kV)
(5)Two Auto Power Transformer (250kVA)(33/0.4kV)
(6)Two Capacitor Bank (5MVAR)
(8)Insulator Disc (To Isolate Pillar And Power Line Wire)
(9)Jumpher (A Small Piece Of Power Line Wire)
(10)Panther Wire(Used In 33kV And 132kV Power Line)
(11)Zebra, Moose And Dear Wires(Used In 220kV Power Line)
(12)Transformer Cooling By Mulsifier System
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Now I have studied a lot about the electrical transmission
system. One must have never thought that so many things are
required for just switching on a television or a refrigerator or say
an electric trimmer. The three wing of electrical system viz.
Generation, transmission and distribution are connected to each
other and that too very perfectly. Here man and electricity work
as if they are a family. Lots of labour, capital and infrastructure is
involved in the system just to have a single phase,220V,50Hz
power supply at our houses.
At last I would say...
Energy Saved Is Energy Produced