West Bengal State Electricity Transmission Company Limited
Under the guidance of
Mr. Debayan Mandal (assistant engineer,132/33kv saltlake sub-station)
(Calcutta Institute Of Technology)
1.132/33 KV SUB-STATION
Construction – Site Selection & Layout
Equipment in a 132KV Substation
7. Direct Lightning Stroke Protection
8. Line isolator
9. Wave Trap
10. Metering & Indicating Instrument
11. Single line diagram (SLD)
12 .Conclusion & Bibliography
“There are people, who, simply by being what they are, influence, encourage & inspire you
to do things you never thought yourself capable of doing….”
Among these are my teachers, friends & family members to whom I wish to extend my
gratitude on the event of completing my term project file.
Through the columns of this project file, I would like to take the opportunity to thank
Mr. Debayan Mandal(assistant engineer,132/33kv saltlake sub-station) for encouraging us in
doing the summer training.
I would also like to mention the support of my friends & family members for giving me
useful suggestions & contributed a lot to this file without whose endless efforts this
work would ever have been possible.
• An electrical substation is a subsidiary station of a transmission and distribution
system where voltage is transformed from high to low using transformers.
• Electric power may flow through several substations between generating plant and
consumer, and may be changed in voltage in several steps.
The present day electrical power system is a.c. i.e. electric power is generated,
transmitted and distributed in the form of Alternating current. The electric power is
produce at the power station, which are located at favorable places, generally quite away
from the consumers. It is delivered to the consumer through a large network of
transmission and distribution. At many place in the line of power system, it may be
desirable and necessary to change some characteristic (e.g. Voltage, ac to dc, frequency
p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-
station for example, generation voltage (11KV or 6.6KV) at the power station is stepped up
to high voltage (Say 220KV to 132KV) for transmission of electric power. Similarly near
the consumers localities, the voltage may have to be stepped down to utilization level.
This job is again accomplished by suitable apparatus called sub-station.
3.Site Selection & Lay out:-
220KV Sub-Station forms an important link between Transmission network and Distribution
network. It has a vital influence of reliability of service. Apart from ensuring efficient
transmission and Distribution of power, the sub-station configuration should be such that
it enables easy maintenance of equipment and minimum interruptions in power supply. Sub-
Station is constructed as near as possible to the load center. The voltage level of power
transmission is decided on the quantum of power to be transmitted to
the load center.
Selection Of Site:-
Main points to be considered while selecting the site for Grid Sub-Station are as follows:
i) The site chosen should be as near to the load center as possible.
ii) It should be easily approachable by road or rail for transportation of equipments.
iii) Land should be fairly leveled to minimize development cost.
iv) Source of water should be as near to the site as possible. This is because water is
required for various construction activities (especially civil works), earthing and for
drinking purposes etc.
v) The sub-station site should be as near to the town / city but should be clear of public
places, aerodromes, and Military / police installations.
vi) The land should be have sufficient ground area to accommodate substation equipments,
buildings, staff quarters, space for storage of material, such as store yards and store
sheds etc. with roads and space for future expansion.
vii) Set back distances from various roads such as National Highways, State Highways
should be observed as per the regulations in force.
viii) While selecting the land for the Substation preference to be given to the Govt. land
over private land.
ix) The land should not have water logging problem.
x) Far away from obstructions, to permit easy and safe approach /termination of high
voltage overhead transmission lines.
Equipment in a 132KV Substation:-
1)Bus-Bar- When a no. of lines operating at the same voltage have to be directly
connected electrically, bus-bar are used, it is made up of copper or aluminum bars
(generally of rectangular X-Section) and operate at constant voltage. The bus is a line in
which the incoming feeders come into and get into the instruments for further step up or
step down. The first bus is used for putting the incoming feeders in LA single line.There
may be double line in the bus so that if any fault occurs in the one the other can still
have the current and the supply will not stop. The two lines in the bus are separated by a
little distance by a Conductor having a connector between them. This is so that one can
work at a time and the other works only if the first is having any fault.
2)Insulators:- The insulator serves two purpose. They support the conductor (or bus bar)
and confine the current to the conductor. The most commonly used material for the
manufactures of insulators is porcelain. There are several type of insulator (i.e. pine
type, suspension type etc.) and there used in Sub-Station will depend upon the service
Pin Insulator is earliest developed overhead insulator, but still popularly used in power
network up to 33KV system. Pin type insulator can be one part, two parts or three parts
type, depending upon application voltage. In 11KV system we generally use one part type
insulator where whole pin insulator is one piece of properly shaped porcelain or glass. As
the leakage path of insulator is through its surface, it is desirable to increase the
vertical length of the insulator surface area for lengthening leakage path. In order to
obtain lengthy leakage path, one, two or more rain sheds or petticoats are provided on the
insulator body. In addition to that rain shed or petticoats on an insulator serve another
purpose. These rain sheds or petticoats are so designed, that during raining the outer
surface of the rain shed becomes wet but the inner surface remains dry and non-conductive.
So there will be discontinuations of conducting path through the wet pin insulator
In higher voltage like 33KV and 66KV manufacturing of one part porcelain pin insulator
becomes difficult. Because in higher voltage, the thickness of the insulator become more
and a quite thick single piece porcelain insulator can not manufactured practically. In
this case we use multiple part pin insulator, where a number of properly designed
porcelain shells are fixed together by Portland cement to form one complete insulator
unit. For 33KV tow parts and for 66KV three parts pin insulator are generally used.
In suspension insulator numbers of insulators are connected in series to form a string and
the line conductor is carried by the bottom most insulator. Each insulator of a suspension
string is called disc insulator because of their disc like shape.
In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size,
weight of the insulator become more. Handling and replacing bigger size single unit
insulator are quite difficult task. For overcoming these difficulties, suspension
insulator was developed.
The purpose of insulation coordination is to determine the necessary and sufficient
insulation characteristics of the various network components in order to obtain uniform
withstand to normal voltages and to overvoltages of various origins. Its final objective
is to ensure safe, optimized distribution of electrical power.
By optimized is meant finding the best possible economic balance between the various
parameters depending on this coordination:
Cost of insulation.
Cost of protective devices.
Cost of failures (operating loss and repairs) in view of their probability.
The first step towards removing the detrimental effects of overvoltages is to confront the
phenomena generating them: a task which is not always simple. Indeed, although equipment
switching overvoltages can be limited by means of suitable techniques, it is impossible to
have any effect on lightning.
It is thus necessary to locate the point of least withstand through which the current
generated by the overvoltage will flow, and to equip all the other network elements with a
higher level of dielectric withstand.
In gases, insulation withstand voltage is a highly nonlinear function of clearance. For
example, in air, a root mean square voltage stress of 300 kV/m is acceptable under 1 m,
but can be reduced to 200 kV/m between 1 and 4 m and to 150 kV/m between 4 and 8 m. It
should also be pointed out that this clearance is practically unaffected by rain. This
macroscopic behavior is due to the lack of uniformity of the electric field between
electrodes of all shapes and not to intrinsic gas characteristics. It would not be
observed between flat electrodes of «infinite» size (uniform field). Creepage distances of
busbar supports, transformer bushings and insulator strings are determined to obtain a
withstand similar to direct air clearance between two end electrodes when they are dry and
clean. On the other hand, rain and especially wet pollution considerably reduce their
In normal operating conditions, network voltage may present short duration power frequency
overvoltages (a fraction of a second to a few hours: depending on network protection and
operating mode). Voltage withstand checked by the standard one-minute dielectric tests is
normally sufficient. Determination of this category of characteristics is simple, and the
various insulators are easy to compare.
Study of insulation coordination of an electrical installation is thus the definition,
based on the possible voltage and overvoltage levels on this installation, of one or more
overvoltage protection levels. Installation equipment and protective devices are thus
chosen accordingly. Study of these «conditions» determines the overvoltage level to which
the equipment could be subjected during use. Choice of the right insulation level will
ensure that, at least as far as power frequency and switching impulses are concerned, this
level will never be overshot. As regards lightning, a compromise must generally be found
between insulation level, protection level of arresters, if any, and acceptable failure
In Sub-Station, 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
isolator is essentially a kniff Switch and is design to often open a circuit under no
load, in other words, isolator Switches are operate only when the line is which they are
connected carry no load. 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.
A circuit breaker is an equipment, which can open or close a circuit under normal as well
as fault condition. These circuit breaker breaks for a fault which can damage
other instrument in the station. It is so designed that it can be operated manually (or by
remote control) under normal conditions and automatically under fault condition. There are
mainly two types of circuit breakers used for any substations. They are (a) SF6 circuit
breakers; (b) spring circuit breakers. For the latter operation a relay wt. is used with a
C.B. generally bulk oil C.B. are used for voltage upto 66 KV while for high voltage low
SF6 C.B. are used. For still higher voltage, air blast vacuum or SF6 cut breaker are used.
The use of SF6 circuit breaker is mainly in the substations which are having high input kv
input, say above 132kv and more. The gas is put inside the circuit breaker by force ie
under high pressure.
When if the gas gets decreases there is a motor connected to the circuit breaker. The
motor starts operating if the gas went lower than 20.8 bar. There is a meter connected to
the breaker so that it can be manually seen if the gas goes low. The circuit breaker uses
the SF6 gas to reduce the torque produce in it due to any fault in the line. The circuit
breaker has a direct link with the instruments in the station,
when any fault occur alarm bell rings.
A protective relay is a device that detects the fault and initiates the operation of the
C.B. to isolate the defective element from the rest of the system”. The relay detects the
abnormal condition in the electrical circuit by constantly measuring the electrical
quantities, which are different under normal and fault condition. The electrical
quantities which may change under fault condition are voltage, current, frequency and
phase angle. Having detect the fault, the relay operate to close the trip circuit of C.B.
Power Transformer:- It is used for the transmission purpose at heavy load, high voltage
greater than 33 KV & 100% efficiency. It also having a big in size as compare to
distribution transformer, it used in generating station and Transmission substation at
high insulation level. They can be of two types: Single Phase Transformers and Multi Phase
Instrument Transformers:-The line in Sub-Station operate at high voltage and carry current
of thousands of amperes. The measuring instrument and protective devices are designed for
low voltage (generally 110V) and current (about 5A). Therefore, they will not work
satisfactory if mounted directly on the power lines. This difficulty is overcome by
installing Instrument transformer, on the power lines. There are two types o f instrument
i)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.
Current transformers are basically used to take the readings of the currents entering the
substation. This transformer steps down the current from 800 amps to1amp. This is done
because we have no instrument for measuring of such a large current. The main use of his
transformer is (a) distance protection; (b) backup protection; (c) measurement.
ii)Potential 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. The secondary way consist of a few turns and
provides for measuring instruments and relay a voltage which is known fraction of the line
Auto Transformers:- An autotransformer is an electrical transformer with only one winding.
The "auto" 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 sides of the 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 the
disadvantage of not providing electrical isolation.
On the basis of working:-On the above basis, transformers are of two types: Step up
Transformer and Step down Transformer.
Distribution Transformers:- A distribution transformer is a transformer that provides the
final voltage transmission in the electrical power distribution system, stepping down
voltage to the level used by customers.
Transformer faults and protection
1. There are different kinds of transformers such as two winding or three winding
electrical power transformers, auto transformer, regulating transformers, earthing
transformers, rectifier transformers etc. Different transformers demand different
schemes of transformer protection depending upon their importance, winding connections,
earthing methods and mode of operation etc.
2. Nature of Transformer Faults-
Although an electrical power transformer is a static device, but internal stresses arising
from abnormal system conditions, must be taken into consideration. A transformer generally
suffers from following types of transformer fault-
1. Over current due to overloads and external short circuits,
2. Terminal faults,
3. Winding faults,
4. Incipient faults.
All the above mentioned transformer faults cause mechanical and thermal stresses
inside the transformer winding and its connecting terminals. Thermal stresses lead
to overheating which ultimately affect the insulation system of transformer.
The general winding faults in transformer are either earth faults or inter-turns
faults. Phase to phase winding faults in a transformer is rare. The phase faults in
an electrical transformer may be occurred due to bushing flash over and faults in
tap changer equipment. Whatever may be the faults, the transformer must be isolated
instantly during fault otherwise major breakdown may occur in the electrical power
system. Incipient faults are internal faults which constitute no immediate hazard.
But it these faults are over looked and not taken care of, these may lead to major
faults. The faults in this group are mainly inter-lamination short circuit due to
insulation failure between core lamination, lowering the oil level due to oil
leakage, blockage of oil flow paths. All these faults lead to overheating. So
transformer protection scheme is required for incipient transformer faults also. The
earth fault, very nearer to neutral point of transformer star winding may also be
considered as an incipient fault. Influence of winding connections and earthing on
earth fault current magnitude. There are mainly two conditions for earth fault
current to flow during winding to earth faults,
1. A current exists for the current to flow into and out of the winding.
2. Ampere-turns balance is maintained between the windings.
The value of winding earth fault current depends upon position of the fault on the
winding, method of winding connection and method of earthing. The star point of the
windings may be earthed either solidly or via a resistor. On delta side of the transformer
the system is earthed through an earthing transformer. Grounding or earthing transformer
provides low impedance path to the zero sequence current and high impedance to the
positive and negative sequence currents.
Star Winding with Neutral Resistance Earthed
In this case the neutral point of the transformer is earthed via a resistor and the value
of impedance of it, is much higher than that of winding impedance of the transformer. That
means the value of transformer winding impedance is negligible compared to impedance of
earthing resistor. The value of earth current is, therefore, proportional to the position
of the fault in the winding. As the fault current in the primary winding of the
transformer is proportional to the ratio of the short circuited secondary turns to the
total turns on the primary winding, the primary fault current will be proportional to the
square of the percentage of winding short circuited. The variation of fault current both
in the primary and secondary winding is shown below.
Star Winding with Neutral Solidly Earthed
In this case the earth fault current magnitude is limited solely by the winding
impedance and the fault is no longer proportional to the position of the fault. The
reason for this non linearity is unbalanced flux linkage.
External Faults in Power Transformer
External Short - Circuit of Power Transformer
The short - circuit may occurs in two or three phases of electrical power system. The
level of fault current is always high enough. It depends upon the voltage which has been
short - circuited and upon the impedance of the circuit up to the fault point. The copper
loss of the fault feeding transformer is abruptly increased. This increasing copper loss
causes internal heating in the transformer. Large fault current also produces severe
mechanical stresses in the transformer. The maximum mechanical stresses occurs during
first cycle of symmetrical fault current.
Transient Surge Voltage:-
High voltage and high frequency surge may arise in the power system due to any of the
following causes, (a) Arcing ground if neutral point is isolated. (b) Switching operation
of different electrical equipment. (c) Atmospheric Lightening Impulse. Whatever may be the
causes of surge voltage, it is after all a traveling wave having high and steep wave form
and also having high frequency. This wave travels in the electrical power system network,
upon reaching in the power transformer, it causes breakdown the insulation between turns
adjacent to line terminal, which may create short circuit between turns.
Internal Faults in Power Transformer:-The principle faults which occurs inside a power
transformer are categorized as, (1) Insulation breakdown between winding and earth (2)
Insulation breakdown in between different phases (3) Insulation breakdown in between
adjacent turns i.e. inter - turn fault (4) Transformer core fault
1.Internal Earth Faults in Power Transformer:-
Internal Earth Faults in a Star Connected Winding with Neutral Point Earthed through an
In this case the fault current is dependent on the value of earthing impedance and is also
proportional to the distance of the fault point from neutral point as the voltage at the
point depends upon, the number of winding turns come under across neutral and fault point.
If the distance between fault point and neutral point is more, the number of turns come
under this distance is also more, hence voltage across the neutral point and fault point
is high which causes higher fault current. So, in few words it can be said that, the value
of fault current depends on the value of earthing impedance as well as the distance
between the faulty point and neutral point. The fault current also depends up on leakage
reactance of the portion of the winding across the fault point and neutral. But compared
to the earthing impedance,it is very low and it is obviously ignored as it comes in series
with comparatively much higher earthing impedance.
2.Internal Earth Faults in a Star Connected Winding with Neutral Point Solidly Earthed:-
In this case, earthing impedance is ideally zero. The fault current is dependent up on
leakage reactance of the portion of winding comes across faulty point and neutral point of
transformer. The fault current is also dependent on the distance between neutral point and
fault point in the transformer. As said in previous case the voltage across these two
points depends upon the number of winding turn comes across faulty point and neutral
point. So in star connected winding with neutral point solidly earthed, the fault current
depends upon two main factors, first the leakage reactance of the winding comes across
faulty point and neutral point and secondly the distance between faulty point and neutral
point. But the leakage reactance of the winding varies in complex manner with position of
the fault in the winding. It is seen that the reactance decreases very rapidly for fault
point approaching the neutral and hence the fault current is highest for the fault near
the neutral end. So at this point, the voltage available for fault current is low and at
the same time the reactance opposes the fault current is also low, hence the value of
fault current is high enough. Again at fault point away from the neutral point, the
voltage available for fault current is high but at the same time reactance offered by the
winding portion between fault point and neutral point is high. It can be noticed that the
fault current stays a very high level throughout the winding. In other word, the fault
current maintain a very high magnitude irrelevant to the position of the fault on winding.
Internal Phase to Phase Faults in Power Transformer:-Phase to phase fault in the
transformer are rare. If such a fault does occur, it will give rise to substantial current
to operate instantaneous over current relay on the primary side as well as the
Inter Turns Fault in Power Transformer:-Power Transformer connected with electrical extra
high voltage transmission system, is very likely to be subjected to high magnitude, steep
fronted and high frequency impulse voltage due to lightening surge on the transmission
line. The voltage stresses between winding turns become so large, it can not sustain the
stress and causing insulation failure between inter - turns in some points. Also LV
winding is stressed because of the transferred surge voltage. Very large number of Power
Transformer failure arise from fault between turns. Inter turn fault may also be occurred
due to mechanical forces between turns originated by external short circuit.
Transformers are totally enclosed static devices and generally oil immersed. Therefore
chances of fault occurring on them are very easy rare, however the consequences of even a
rare fault may be very serious unless the transformer is quickly disconnected from the
system. This provides adequate automatic protection for transformers against possible
faults. Various protection methods used for transformers are:-
Buchholz relay is a safety device mounted on some oil-filled power
transformers and reactors, equipped with an external overhead oil reservoir called
a conservator. The Buchholz Relay is used as a protective device sensitive to the effects
of dielectric failure inside the equipment. Depending on the model, the relay has multiple
methods to detect a failing transformer. On a slow accumulation of gas, due perhaps to
slight overload, gas produced by decomposition of insulating oil accumulates in the top of
the relay and forces the oil level down. A float switch in the relay is used to initiate
an alarm signal. Depending on design, a second float may also serve to detect slow oil
leaks. If an arc forms, gas accumulation is rapid, and oil flows rapidly into the
conservator. This flow of oil operates a switch attached to a vane located in the path of
the moving oil. This switch normally will operate a circuit breaker to isolate the
apparatus before the fault causes additional damage.
Conservator and Breather
When the oil expands or contacts by the change in the temperature, the oil level goes
either up or down in main tank. A conservator is used to maintain the oil level up to
predetermined value in the transformer main tank by placing it above the level of the top
of the tank. Breather is connected to conservator tank for the purpose of extracting
moisture as it spoils the insulating properties of the oil. During the contraction and
expansion of oil air is drawn in or out through breather silica gel crystals impregnated
with cobalt chloride. Silica gel is checked regularly and dried and replaced when
It has two meter which indicate the temperature of the oil and winding of main tank. If
temperature of oil or winding exceeds than specified value, relay operates to sound an
alarm. If there is further increase in temperature then relay completes the trip circuit
to open the circuit breaker controlling the transformer.
When the transformer is in operation heat is generated due to iron losses the removal of
heat is called cooling.
There are several types of cooling methods, they are as follows:
Air natural cooling:-In a dry type of self-cooled transformers, the natural circulation of
surrounding air is used for its cooling. This type of cooling is satisfactory for low
voltage small transformers.
Air blast cooling:-It is similar to that of dry type self-cooled transformers with to
addition that continuous blast of filtered cool air is forced through the core and winding
for better cooling. A fan produces the blast.
Oil natural cooling:-Medium and large rating transformers have their winding and core
immersed in oil, which act both as a cooling medium and an insulating medium. The heat
produce in the cores and winding is passed to the oil becomes lighter and rises to the top
and place is taken by cool oil from the bottom of the cooling tank.
Oil blast cooling:-In this type of cooling, forced air is directed over cooling elements
of transformers immersed in oil.
Forced oil and forced air flow (OFB) cooling:-Oil is circulated from the top of the
transformers tank to a cooling tank to a cooling plant. Oil is then returned to the bottom
of the tank.
Forced oil and water (OWF) cooling:-In this type of cooling oil flow with water cooling of
the oil in external water heat exchanger takes place. The water is circulated in cooling
tubes in the heat exchanger.
Oil Surge Relay:-The oil surge relay is connected in between oltc chamber and conservator
(separate for for oltc) with breather. a separation should be arrange such that the oil in
oltc chamber and transformer chamber should not be mixed together. in case any on load
tapchanger a problems developed during operation a gas is developed and that gas will
actute the oil surge relay and and the relay activates other circuits and main circuit ht
will be opned with alarm indication and flag indication
Oil Temperature Indicator : -The Oil Temperature Indicator (OTI) measures the Top oil
Temperature. It is used for control and protection for all transformers.
Winding Temperature Indicator : -The Winding is the component with highest temperature
within the transformer and, above all, the one subject to the fastest temperature increase
as the load increases. Thus to have total control of the temperature parameter within
transformer, the temperature of the winding as well as top oil, must be measured. An
indirect system is used to measure winding temperature, since it is dangerous to place a
sensor close to winding due to the high voltage. The indirect measurement is done by means
of a Built-in Thermal Image.Winding Temperature Indicator is equipped with a specially
designed Heater which is placed around the operating bellows through which passes a
current proportional to the current passing through the transformer winding subject to a
given load. Winding Temperature is measured by connecting the CT Secondary of the
Transformer through a shunt resistor inside the Winding Temperature Indicator to the
Heater Coil around the operating Bellows. It is possible to adjust gradient by means of
Shunt Resistor.In this way the value of the winding temperature indicated by the instrument
will be equal to the one planned by the transformer manufacturer for a given transformer
Pressure Relief Devices:- Pressure relief devices are used to provide a means of venting
excess pressure which could rupture a transformer. A pressure relief device is the last
line of defense for safety. If all other safety devices or operating controls fail, the
pressure relief device must be capable of venting excess pressure.
Magnetic Oil Gauge:-This device is used to indicate the position of transformer insulating
oil level in conservator of transformer. This is a mechanical device.
7. Direct Lightning Stroke Pretection:-
All equipments in a substation are susceptible to lightning strokes. They can be damaged
due the high voltage and current spikes generated in a lightning stroke. Thus a substation
is protected from lightning using mainly two methods, namely: Lightning masts and Shield
wires.There are some methods for calculating the maximum acceptable distance between the
masts or the shield wires but the mostly used method is the Principle of Rolling Sphere.
The radius of the sphere is determined by a predefined mathematical equation. This method
defines the protected region by rolling the virtual sphere on the region to be protected.
The region below the dotted line is protected from direct lightning. The radius of the
sphere is 1.2 times in case of lightning masts than that of shield wires. The shield wires
are generally fitted in a mesh so as to form a cover over the substation.
Lightning arrestors are used to bypass the spike generated by lightning to the ground
instead of the apparatus. A lightning arrestor has a block of a semiconducting material
such as silicon carbide or zinc oxide. Lightning arresters built for power substation use
are immense devices, consisting of a porcelain tube several feet long and several inches
in diameter, typically filled with discs of zinc oxide. A safety port on the side of the
device vents the occasional internal explosion without shattering the porcelain
cylinder.Lightning arresters are rated by the peak current they can withstand, the amount
of energy they can absorb, and the breakover voltage that they require to begin
conduction. They are applied as part of a lightning protection system, in combination with
air terminals and bonding.
Indirect Lightning Stroke and Switching Transient Protection:-
Indirect lightning strokes also pose a danger to the electrical equipments. Lightning
strokes falling on the ground create an electromagnetic field identical to a direct stroke
which induces a high voltage spike. This induced voltage produce a current spike which may
damage equipments. Switching in the bus produce transients which are also dangerous for
the equipments. These transients prevail comparatively for longer times than lightning
pulses. For this reason Surge Arrestors are used.
A surge arrester is a device to protect electrical equipment from over-voltage transients
caused by external (lightning) or internal (switching) events. Also called a surge
protection device (SPD) or transient voltage surge suppressor (TVSS), this class of device
is used to protect equipment in power transmission and distribution systems. (For consumer
equipment protection, different products called surge protectors are used.) The energy
criterion for various insulation materials can be compared by impulse ratio, the surge
arrester should have a low impulse ratio, so that a surge incident on the surge arrester
may be bypassed, to the ground instead of passing through the apparatus.
To protect a unit of equipment from transients occurring on an attached conductor, a surge
arrester is connected to the conductor just before it enters the equipment. The surge
arrester is also connected to ground and functions by routing energy from an over-voltage
transient to ground if one occurs, while isolating the conductor from ground at normal
operating voltages. This is usually achieved through use of a varistor, which has
substantially different resistances at different voltages.
Surge arresters are not generally designed to protect against a direct lightning strike to
a conductor, but rather against electrical transients resulting from lightning strikes
occurring in the vicinity of the conductor. Lightning which strikes the earth results in
ground currents which can pass over buried conductors and induce a transient that
propagates outward towards the ends of the conductor. The same kind of induction happens
in overhead and above ground conductors which experience the passing energy of an
atmospheric EMP caused by the lightning flash. Surge arresters only protect against
induced transients characteristic of a lightning discharge's rapid rise-time and will not
protect against electrification caused by a direct strike to the conductor.
Transients similar to lightning-induced, such as from a high voltage system's fault
switching, may also be safely diverted to ground; however, continuous overcurrents are not
protected against by these devices. The energy in a handled transient is substantially
less than that of a lightning discharge; however it is still of sufficient quantity to
cause equipment damage and often requires protection.
Without very thick insulation, which is generally cost prohibitive, most conductors
running more than a minimal distance, say greater than about 50 feet, will experience
lightning-induced transients at some time during use. Because the transient is usually
initiated at some point between the two ends of the conductor, most applications install a
surge arrester just before the conductor lands in each piece of equipment to be protected.
Each conductor must be protected, as each will have its own transient induced, and each
SPD must provide a pathway to earth to safely divert the transient away from the protected
component. The one notable exception where they are not installed at both ends is in high
voltage distribution systems. In general, the induced voltage is not sufficient to do
damage at the electric generation end of the lines; however, installation at the service
entrance to a building is key to protecting downstream products that are not as robust.
ZnO arrester in a porcelain enclosure for the 20 kV network.
Nonlinearity of resistances maintains a residual voltage which appears at the terminals of
the device, close to arcing level, since resistance decreases as current increases. A
variety of techniques have been used to produce varistor arresters and air gap protectors.
The most classical kind uses a silicon carbide (SiC) resistance. Some arresters also
contain voltage distribution systems (resistive or capacitive dividers) and arc blowing
systems (magnets or coils for magnetic blowing). This type of arrester is characterized
Its extinction voltage or rated voltage, which is the highest power frequency
voltage under which the arrester can be spontaneously de-energized. It must be
greater than the highest short duration power frequency overvoltage which could
occur on the network
Its arcing voltages according to wave shape (power frequency, switching impulse,
lightning impulse....); they are statistically defined
Its impulse current evacuation capacity, i.e. its energy dissipation capacity. Absorption
capacity is generally given by withstand to rectangular current waves.
Zinc Oxide (ZnO) Arresters: Made up only of varistors, they are increasingly replacing
nonlinear resistance arresters and air gap protectors.
Absence of air gap means that ZnO arresters are permanently conductive, but under
protected network rated voltage, have a very small earth leakage current (less than 10
mA). Their operating principle is very simple, based on the highly nonlinear
characteristic of ZnO varistors. This nonlinearity is such that resistance decreases from
1.5 M ohms to 15 ohms between operating voltage and voltage at rated discharging current.
The advantage of these arresters is their increased limitation and reliability compared
with silicon carbide arresters. Improvements have been made in recent years, in particular
in the thermal and electrical stability field of ZnO pellets on ageing.
8. Line isolator:-
Circuit breaker always trip the circuit but open contacts of breaker cannot be visible
physically from outside of the breaker and that is why it is recommended not to touch any
electrical circuit just by switching off the circuit breaker. So for better safety there
must be some arrangement so that one can see open condition of the section of the circuit
before touching it. Isolator is a mechanical switch which isolates a part of circuit from
system as when required. Electrical isolators separate a part of the system from rest for
safe maintenance works.
Wave trap is an instrument using for tripping of the wave. The function of this trap is
that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a
drum. It is connected to the main incoming feeder so that it can trap the waves which may
to the instruments here in the substation. Low pass filter when power frequency currents
are passed to switch yard and high frequency signals are blocked. Line Isolator with E.B.
isolate the line from Sub Station and earth, it under shut down.
10. Metering & Indicating Instrument:-
There are several metering and indicating Instrument (e.g. Ammeters, Voltmeters,energy
meter etc.) installed in a Substation to maintain which over the circuit quantities.The
instrument transformer are invariably used with them for satisfactory operation.
11. Single line diagram (SLD) :- A Single Line Diagram (SLD) of an Electrical System
is the Line Diagram of the concerned Electrical System which includes all the required
ELECTRICAL EQUIPMENT connection sequence wise from the point of entrance of Power up to
the end of the scope of the mentioned Work.
Now from this report one can conclude that electricity plays an important role in our
life. At the end of the training, I came to know about the various parts of substations
and how they are operated. Also I learnt about how transmission is done in various parts
of West Bengal.
As evident from the report, a substation plays a very important role in the transmission
system. That’s why various protective measures are taken to protect the substations from
various faults andits smooth functioning. West Bengal State Electricity Transmission
Company Limited takes such steps so that a uniform and stable supply of electricity can
reach in every part of this state.
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