Chapter 1

INTRODUCTION

Economics of generation of electrical energy and the huge demand of power in modern time
requires creation of bigger and bigger power houses there may be hydro system or atomic.
The power house may be away from the load centers, as in the case of hydro power station or
they may be nearer to load centers as in case of the steam power lines are necessary to
maintain a huge block of from source of generation to the lead centers to inter connected.
Power house for increased reliability of supply greater. The system stability lesser the
standby plant and hence cheaper the electrical energy in between the power houses and
ultimate consumers a number of transmission and switching station has to be created these
are generally known as "SUBSTATION".

Fig 1.1 KHUSHKHERA 220KV GSS

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Fig1.2: Single Line Diagram

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 Chapter 2
FEEDERS

Substation is situated nearby the Khushkhera. The substation is equipped with various
equipments and here are various arrangements for the protection purpose. At this substation
following feeders are established.
2.1Incoming Feeders
220 KV
1. 220 KV NEEMRANA
2. 220 KV BHIWADI
2.2Outgoing Feeders
132 KV
1. 132kV HONDA CK-1
2. 132KV HONDA CK-2
3. 132KV KUPURKOTKASIM

33 KV
1. NEELKANTH
2. SALARPUR
3. KHUSHKHERA
4. HONDA CHOWK
5. RATHI BAR
6. PARAMOUNT

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 Chapter 3
LIGHTENING ARRESTORS

Fig3.1: Lightning Arrester

An electric discharge between cloud and earth, between cloud and the charge centers of the
same cloud is known as lightening.
The earthen screens and the ground wires can well protect the electrical system against direct
lightening strokes but they fail to provide protection against traveling waves which may reach
the terminal apparatus. The lightening arrestors or the surge diverters provide protections
against such surges.

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 Fig 3.2: Field Of 220KV GSS Of Khushkhera

3.1 Thyrite Type:

Ground wire run over the tower provides an adequate protection against lighting and reduce
the induced electrostatic or electromagnetic voltage but such a shield is inadequate to protect
any traveling wave, which reaches the terminal of the electrical equipment, and such wave
can cause the following damage.

 The high peak of the surge may cause a flashover in the internal wiring thus it may spoil
the insulation of the winding .

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 The steep wave front may cause internal flash over between their turns of transformer.
 The stop wave front resulting into resonance and high voltage may cause internal or
external flashover causing building up the oscillator is the electrical operation.

Lightening arrestors are provided between the line and earth provided the protection against
traveling wave surge the thirties lightening arrestor are provided at GSS. This type of LA has
a basic cell made of thirties, which is a particular type of clay, mixed with carboren dum.
Thirties has a particular property of being insulator one voltage
At high voltage It will behave like a conducting material the electrical resistance of thyrite
depends upon the voltage each time the voltage is made twice the resistance decrease in such
a manner as to allow an increased current of 12.5 times the change in current is independent
of rate of application voltage and its instantaneous value.
A standard cell is rated for 1KV and is formed into a disc, which is sprayed on both the sides
of to give good contact with each disc. The dimensions of the discs are stacked i.e. 16 cm in
diameter and 17.5 cm thick these discs are stacked one upon each other and they are further
placed in to a porcelain container with a suitable arrangement of gap between them.
These gaps serves as the purpose of preventing any current flow during normal operating
voltage in case of any transients the gap are punctured. The Thyrite type arrestor will
discharge several thousands ampere without the slightest tendency of flashover on the edges
of most important of the advance is that there is absolutely no time lag in its performance.

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 Chapter 4
BUS BARS
If the bus bars are of rigid type (Aluminum types) the structure heights are low and minimum
clearance is required. While in case of strain type of bus bars suitable ACSR conductors are
strung / tensioned strain by tension insulator discs according to system voltages. In the widely
used strain type bus bars stringing tension is about 500-900 kg depending upon the size of
conductor used and tensioning is manual by means of rope pulleys or by pull lifts. It may also
with the help of tractors.
Here proper clearance would be achieved only if require tension is achieved. Loose bus bars
would effect the clearances when it swings while over tensioning may damage insulators.
Clamps or even effect the supporting structures in low temperature conditions. The clamping
should be proper, as loose clamp would spark under in full load condition damaging the bus
bars itself.
The bus bar is provided with lightening protection to safeguard the equipment against direct
stroke by providing aerial earth wire giving a protection at 30 degree i.e. height and earth
wire such that all the equipment and bus bar should be covered with in this 30 degree.

Bus-Bar Arrangement
1. Single bus arrangement.
2. Double bus bar arrangement.
3. Double bus bar arrangement with auxiliary bus.

Bus bar arrangement depends upon:

1. Interruption tolerable in the supply scheme.
2. Alternative supply arrangements in case of failure of Equipments.

4.1 SINGLE BUS BAR ARRANGEMENT:

This arrangement is simplest and cheapest. It suffers, however, from major defects.
a.) Maintenance without interruption is not possible.
b.) Extension of the sub station without a shut down is not possible.
The equipment connections are very simple and hence the system is very convenient to
operate. This scheme is not very popular for 33KV and above, except where the relative
importance of the sub station is less or the position of the sub station does not justify

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elaborate schemes. The indoor 11KV switchyards have quite often-single bus bar
arrangement.

4.2 DOUBLE BUS BAR ARRANGEMENTS:

This scheme has two bus bar so that:
a.) Each load may be fed from either bus.
b.) The load circuits may be divided in two separate groups if needed from operational
consideration. Two supplies from different sources can be put on each bus separately.
c.) Either bus bar may be taken out from maintenance and cleaning of insulators.

This arrangement is adopted frequently where the loads and continuity of supply is necessary.
In such a scheme a bus coupler breaker is mostly provided as it enables on load change over
from one bus to other. The normal bus selection isolators cannot be used for breaking load
currents. The arrangement does not permit breaker maintenance without causing stoppage of
supply.
4.3 MAIN BUS WITH AUXILIARY BUS (OR TRANSFER BUS):
The double bus bar arrangement provides facility to charge over to either bus to carry out
maintenance on the other but provide no facility to carry over breaker maintenance. The main
and transfer bus works the other way round. It provides facility for carrying out breaker
maintenance but does not permit bus maintenance. Wherever maintenance is required on any
breaker the circuit is changed over to the transfer bus and is controlled through bus coupler
breaker.

4.4 DOUBLE BREAKER:

This scheme is the modification of double bus bar scheme. In this arrangement the
maintenance of CB or isolator without an outage is possible, which is the main drawback of
double bus bar. This arrangement is costly so it varies for various generating stations.

4.5 MESH SCHEME:

This scheme is also known as ring bus bar. The main features of this scheme are:
1. It provides a double feed to each circuit i.e. opening of any breaker for maintenance does
not affect the supply to any circuit.
2. It provides breaker maintenance.

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3. All sections of conductor in the station are covered by the feeder protection and no. Of
separate protection needed.
4. It is cheaper than the double bus bar or main bus scheme.

The disadvantages of this scheme are:

1. If any breaker is to be taken under maintenance, then under this condition tripping of any
one circuit breaker may result in loss of supply to a no. of circuits.
2. Expansion of mesh is extremely difficult the scheme is limited to four or six circuits.

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 Chapter 5
ISOLATOR
Isolators are also called as disconnect switches or air break switches. Isolators are used to
isolate the bus when it is not in working condition. If the bus is to be shut down then it is
isolated from the main bus. The moving and fixed contacts is done so that all the three phase
of the isolator close and open simultaneously and there is a full surface contact between
moving and fixed contacts. The adjustment of the tendon pipes, leveling of post isolator,
stops Holts in the fixed contacts etc. are done for smooth operation of isolator. Isolators are
provided at both ends of the bus. There are ten isolators provided at 220KV substation.
Following type of isolator provided at 220KV substation. Following type of isolator are being
used in R.S.E.B-
a.) Isolator without earth blades.
b.) Isolator with earth blades.
c.) Tendon isolator.

5.1 Parts
(a) Contacts: Contacts are liberally rated and have been designed to with stand
electromagnetic stresses and preventing shattering at rated short time current. The contact
is made out of electrolytic copper, fixed in co housing.
(b) Switch plans: All the three phases of switch are cleaned open or closed simultaneously
with provision of adjustable tendon pipe connected to towers provided at the center
pedestal.

5.2. Insulators
Bus support insulators are porcelain or fiberglass insulators that serve to isolate the bus bar
switches and other support structures and to prevent leakage current from flowing through the
structure or to ground. These insulators are similar in function to other insulators used in
substations and transmission poles and towers.

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 Fig. 5.1 Bus Support Insulators-

5.2.1 SUSPENSION INSULATOR

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. Suspension insulators are used
for very high voltage systems when it is not practical or safe to use other types of insulators.
They have an advantage in that one or more of the insulators in a string can be changed out
without replacing the entire string.

5.3 EARTH SWITCH

Earthing is the achieved by means of earthed pivoted at the base. The earth contacts are fitted
at the main contacts either at the either at the back or right/left side. These are positively
mechanical interlocked.

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 Chapter 6
CIRCUIT BREAKER

The basic construction of any circuit breaker requires the separation of contacts in an
insulating fluid, which serves two functions here
1. It extinguishes the arc between the contacts when the circuit breaker opens.
2. It provides adequate insulation between the contact & from contact to earth.
The insulating fluid commonly used for C.B.
1. Air at atmospheric pressure.
2. Compressed air.
3. Oil that provides hydrogen for arc exciting.
4. Ultra high vacuum.
5. Sulphur hexa fluoride [SF6].

6.1. SF6 circuit breakers

Operate to switch electric circuits and equipment in and out of the system. These circuit
breakers are filled with compressed sulfur-hexafluoride gas, which acts to open and close the
switch contacts. The gas also interrupts the current flow when the contacts are open.

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 Fig 6.1 SF6 gas circuit Power breaker

6.1.1 PHYSICAL PROPERTIES OF SF6 GAS:

1. Colorless and odorless.
2. Non-toxic.
3. Non-inflammable.
4. State gas at normal temp & pressure.
5. Density heavy gas 5 times that of air 20 & atmospheric pressure.
6. Inert and thermally stable.

The heat transferability of SF6 gas is 2-5 times that of air at the same temperature & pressure
SF6 has low arc time constant. The time constant of medium is defined as the “medium time
between current zero and the instant the conductance of contract space reaches zero value".
Due to the electro negativity of SF6 dielectric strength is high.

6.1.2 CHEMICAL PROPERTIES OF SF6 GAS:

1. Stable up to 500` C.
2. Insert.
3. Electro negativity.
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4. Does not react with structural material up to 500` C.
5. During arc extinguishing process SF6 is broken down to some extent into SF4, SF2 the
product of decomposition recombine upon cooling to from the original gas.

6.1.3 CONSTRUCTION:
The cylindrical large size steel tanks are mounted horizontally parallel to each other. Each
tank consists of SF6 under pressure. The interruption is of multi break type & is placed along
the axis of each tank. The interruption assembly is supported inside the tank by the vertical
bushing, which are mounted near the end of each tank. Gas at high pressure is supplied to the
interrupter from a gas reservoir.
The bushing is also insulated with SF6 the conductor is in the form of copper tube supported
at both end by porcelain shields. SF6 gas is supplied from the high-pressure tanks. Shields are
provided with gasket seals to eliminate leakage of gas from beginnings.

6.1.4 DEMERITS OF SF6 CIRCUIT BREAKER:

 Sealing problems arise due to the type of the construction used.
 The presence of moisture in the system is very dangerous to SF6 circuit breaker.
 Arced SF6 gas is poisonous & should not be let out.
 The double pressure SF6 CB is cost liner due to complex gas system.
 The internal parts should be cleaned thoroughly during periodic maintenance
under clean dry environment.
 Dust of Teflon & sulfide should be removed.

S. No. Particulars Fuse Circuit Breaker

1 Functions It performs both of It performs the function of
function-detection as interruption only.
interruption
2 Operation Inherently complete Elaborate equipment, such as
automatic. relay
3 Operating Very small(say 0.002 Large(0.02-0.05)
Time second or so)
4 Breaking Small Very large
Capacity
Table6.1: Difference Between the Fuse And Circuit Breaker

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 Chapter 7
POWER TRANSFORMER

Power transformer are called autotransformer fitted with a load tap changer [OLTC]

7.1 Description Of Transformer

There are three power transformers of 220/132 KV 100MVA capacity each.
There are two transformers of 132/33 KV of 10/12.5 MVA & 16/20 MVA capacity.

RATING OF TRANSFORMER
MAKE CGL
TYPE OF COOLING ONAN/ONAF/OFAF
RATED VOLTAGE HV 220000 V
AT NO LOAD LV 132000 V
AMPERE HV 262.4 A LV 437.4 A

Fig7.1:Transformer

7.2 Tank

Small thanks are constructed from welded sheet steel ,and larger ones from plain boilerplates
.The lids may be cast iron, or waterproof gasket being used at the joints .The fittings include

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thermometer pockets, drain cock, rollers or wheels for moving the transformer into position,
eye bolts for lifting, conservators and breathers, cooling tubes are welded in, but separate
radiators are individually welded and afterwards bolted on .

7.3 Conservator

Conservator is required to take up the expansion and contraction of the oil to come in contact
with the air, from which it is liable to take up moisture. The conservator may consist of an
airtight cylindrical metal drum supported on the transformer lid or a neighboring wall, or of a
flexible flat corrugated dise drum. The tank is filled when cold and the expansion is taken up
in the conservator.

7.4 Transformer Oil

Oil in transformers construction serves the double purpose of cooling and insulating .in the
choice of oil for transformers use the following characteristics have to be considered.
Viscosity, insulating property, flash point, fire point, purity, slugging audity.

7.4.1 SYNTHETIC TRANSFORMERS OIL

These have been developed to avoid the risk of fire and explosion, present always with
normal mineral oils. Chlorinated biphenyl, a synthetic oil suitable for transformers is
chemically stable, non-oxidizing, rather voltage, and heavier than water. Its dielectric
strength is higher than that of mineral oil, and moisture has a smaller tendency to migrate
through it .the permittivity is 4.5, compared with an about 2.5.

7.5 Transformer Operation Noise

Under ‘no load condition’ the ‘hum’ developed by energized power a transformer originates
in the core, where the laminations tend to vibrate by magnetic forces. The essential factors in
noise production are consequently:

Magnectostriction i. e. the very small extension, with corresponding reduction of cross

section, of sheet steel strips when magnetized. The degree of mechanical vibration developed
by the laminations, depending upon the tightness of clamping, size, gauge, associated
structural parts etc.

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 Fig 7.2: Power Transformers In 220kV GSS khushkhera
7.6 Cores
Magnetic circuit is a three limbed care type construction; each limp being interleaved with
miter joints with top and bottom yokes the winding surrender with three limbs. The
lamination are made from high grade culled rolled grain oriented alloy steel. The insulation
on lamination is varnish.
The cooling ducts are provided parallel to the plane of laminations the yoke are clamped with
by means of clamping per sling plate.
They are clamped with bolts for lifting the core with 8 lighting blotters are provided insulated
from each other to withstand a pressure of 2KV, 50 c/s AC for one minute.

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 Chapter 8
CONTROL ROOM

Fig8.1: Control room of 220kV GSS

8.1 Annunciation:
In the control room the Annunciate the most compact in which probable faults at different
feeders and different feeders and different zone have written to inform the bulb behind the
structure when some faults is annunciate auxiliary relay. Relay’s first signal trip the circuit
breaker and signal goes to the auxiliary trip the relay, the relay send the signal to the
annunciate which give alarm and bulb is lighting up in front of the type of fault occurred.
The shift engineers come and receive the signal and see at which feeder the fault has come.
CB is been reset once again and see weather the relay trip or not if it is tripped then is leaved
as it is for repairing the damage.

8.2 Measuring Instrument Used :

 ENERGY METER to measure the energy transmitted energy meters are fitted to the
panel to different feeders the energy transmitted is recorded after one hour regularly for it
MWHr meter is provided.
 WATTMETERS: wattmeters are attached to each feeder to record the power exported
form GSS.
 FREQUENCY METER: to measure the frequency at each feeder there is the provision of
analog or digital frequency meter.

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 VOLTMETER: it is provided to measure the phase-to-phase voltage. It is also available
in both the forms analog as well as digital.
 KA METER: it is provided to measure the line current. It is also available in both the
forms analog as well as digital.
 MAXIMUM DEMAND INDICATOR: these are also mounted on the control panel to
record the average power over successive predetermined period.
 MVAR METER: it is to measure the reactive power of the circuit

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 Chapter 9
CAPACITIVE VOLTAGE TRANSFORMERS

9.1 Description:
The capacitive voltage transformer comprises of a capacitor divider with its associated
electromagnetic unit. The divider provides an accurate proportioned voltage, while the
magnetic unit transforms this voltage, in both magnitude and phase to convenient levels
suitable for measuring, metering, protection etc. All capacitor units have metallic bellows to
compensate the volumetric expansion of oil inside. The porcelain in multi unit stack, all the
potential points are electrically tied and suitably shielded to overcome the effect of corona
etc. Capacitive voltage transformers are available for system voltages of 33KV to 420KV.

9.2 Application:
1. Capacitive voltage transformers can be effectively as potential Sources for measuring,
metering, protection, carrier communication and other vital functions of an electrical
network.
2. CVT are constructed in single or multi unit porcelain housing with there associated
magnetic units. For EHV system cuts are always supplied in multi unit construction.
3. In case of EHV cuts the multi unit system has many advantages easy to transport and
storing, convenience in handling.
RATING OF CVT:
Type: CVE245/1050/50
YEAR: 2001
Frequency: 50 Hz
Capacitance C1: 4880pF
Capacitance C2: 44455pF
Equivalent 4400+10%pF
Capacitance:
Insulation level: 460/1050KV
Emu oil: 95+10%Kg

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9.3 potential transformers
The potential transformers are employed for voltages above 380 volts to feed the
potential coils of indicating and metering (voltmeters, watt meters, and watt-hour
meters) and relays. These transformers make the ordinary low voltage instruments
suitable for measurement of high voltage and isolate them from high voltage.
The primary winding of the potential transformer is connected to the main-bus-bars
of the switchgear installation and to the secondary winding; various indicating and
metering instruments and relays are connected.
When the rated high voltage is applied to the primary of a PT the voltage of 110
volts appears across the secondary winding. The ratio of the rated primary voltage
to the rated secondary voltage is known as turn 6r transformation ratio.
The potential transformers are rated for primary and secondary rated voltage, accu-
racy class, number of phases and system of cooling.

Fig 9.1: Capacitor Voltage Transformer

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 Chapter 10
RELAYS
Every electrical equipment needs portion the house wiring is protected by the fuses. Modern
generators are protected by complex protective schemes. The choice of protection depends
upon several aspects such as type and rating of protective equipments.
The location of relay is very important the protective relay may protect the concerned
equipment from the abnormal operating condition develops in protective relaying of that
equipment sense the abnormal condition and initiates the alarm and close the trip circuit of
CB and isolate the equipment from the supply.
The relays are compact self-contained device, which respond to an abnormal condition
whenever and abnormal condition is developed. The relay close there contacts thereby the
trip circuit of CB is closed current from the battery supply flows in the trip circuit [coil] of
breaker and breaker opens and the faulty part is disconnected from the supply. Besides relays
and CB there are several components in relaying schemes these includes potential
transformer protective fine relay time delay relay auxiliary relay secondary circuit and
accessories each equipment is important in protection relying in team work of their
components.
10.1 The function of protection relaying scheme includes following.

Fig 10.1: Basic of relay circuit

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1. To sound an alarm or close the trip coil of CB to disconnect the equipment in abnormal
condition, which includes overload under voltage temperature rise, unbalanced load
reserve power under frequency short circuit.
2. To disconnect the abnormally operating part to prevent subsequent fault as over load
protection of machine and prevent machine failure.
3. To disconnect the faulty part if a machine is connecte4d immediately after a winding fault
only a few coil may need replacement.
1. 4.To realize the effect of fault by disconnecting faulty part from healthy part causing least
disturbance to the healthy replacement.

4. To disconnect the faulty part quickly to improve the system stability service condition
and system performance.
The connections are divided into 3 main circuits consisting of
(i) Primary winding of the CT (current transformer) connected in series with
the main circuit to be protected
(ii) Secondary winding of the CT and the relay operating winding and
(iii) The tripping circuit.

Under normal operating conditions, the voltage induced in the secondary winding
of the CT is small and, therefore, current flowing in the relay operating coil is
insufficient in magnitude to close the relay contacts. This keeps the trip coil of the
circuit breaker delegated. Consequently, the circuit breaker contacts remain closed
and it carries the normal current. When a fault occurs, a large current flows through
the primary of the CT. This increases the voltage induced in the secondary and
hence the current flowing through the relay operating coil. The relay contacts are
closed and the trip coil of the breaker gets energized to open the breaker contacts.

The component of a substation which are provided with the protection schemes are:

 Power transformer.
 Transmission line feeder.
 Bus bars.

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10.1.1 POWER TRANSFORMER PROTECTION:
A power transformer is subjected to following faults
1. Over load and external short circuit.
2. Terminal fault
3. Winding fault
4. Incipient fault.

Winding and oil temperature indicator with alarm and trip contacts are provided. As soon as
the temperature of winding and oil exceed the predetermined value of contacts are bridged
reaches the spot valve the transformer is tripped the different protection which are provided
to a power transformer are
1. Bucholz relay
2. Over current and earth fault
3. Differential protection
4. Frame leakage protection

10.2 Buchholz Relay:

This relay is used for the protection of the transformer and is based upon the principle of a
gas operated relay since any internal fault inside the transformer will evaporate the oil due
top intense heat generated by short circuit current and will generate gases. This type of relay
can be fitted only to the transformers, which are equipped with conservator tank and the main
tank i.e. in the transformer pipe connecting. The two relays consists of an oil cum tuner with
the two internal floats which operates and Accurate mercury switches, which are in turn
connected to external to the external alarm and to the tripping circuit.
The relay is normally full of oil and floats remain engaged in seat due to buoyancy and floats
are made aluminum each one has a counter weight, which has mild steel coated with nickel.
The relay is also useful in indicating any loss of oil that a transformer may suffer because
heat loss of oil will cause oil level to drop the top float will indicate it by dropping and
shorting the alarm contact and if the oil level keeps on falling the lower float will affected
and will close the trip circuit of transformer.
The alarm circuit is made these by going a warning bell is advanced that a serious fault is
developing inside. The tank occurs the volume of gas generated is considerable which is
moving through the relay cause the gas surge flap valve to be defected. There by close the
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mercury contact switch and energizing the trip coil of CB and isolate the transformer from
the supply sample for analysis if gas from the top valve.

Fig 10.2: Buchholz relay at 220kV GSS

10.3 Over Current and Earth Fault Protection:

These protection schemes are provided against external short circuit and excessive load.
Commonly the over current and earth fault protect8ion is provided on the front side of
transformer and is made to trip both HV and LV breaker these protections serve only as
backup protection both for transformer internal and external fault.
The over current relay has a single element for each phase and earth fault has a single
element the connection for restricted earth fault protection on one winding of transformer
with a similar scheme. For a star connected winding three lines current transformer is
balanced against a CT the neutral connection. In case of delta connected winding the three
line CT are parallel and connected to across the earth fault relay some of them same in CT is
used for over current protection as well as earth fault protection.

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10.4 Frame Leakage Protection
This protection is known as tank protection. The transformer is lightly insulated from earth
by mounting it on a concrete plinth. The transformer tank is connected to earth fault relays.
Earth fault current due to insulation failure in any winding of the transformer will flow to
transformer elegizes here two relay operates.

10.5 Differential protection:

For the differential protection, different relay is used. Different protection scheme compares
quality derived from the input and output current of the protected circuit in such a way that
all the healthy system and protection in operative while for fault condition the balance is
disturbed and the protection operated.
The protection has many advantage over other protection over other protections the flash over
at the bushing are not adequately covered by other protective scheme also unless it. Involves
ground the differential relay scheme delete such fault and also on the lead between the CB
and power transformer provided. The current transformer are separately mounted and not in
the transformer bushing. In case of every serve internal faults differential faults
differential relay operates faster than the bucholz’s relay this control the external damages.
The differential damage protection response to a phase to phase faults with in the protection
zone. This generally comprises all equipments and connection between ct and all side of
transformer. It provides protection against turn to turn faults also. It operates on the principle
of the circulating current by comprising current in various winding through the media of CT’s
the ratio and connection of the CT’s on various winding r shown that secondary current are
equal in magnitude and phase in normal condition.
The polarity of CT is such that the relay receives the vector sum of the secondary current,
which should be zero for the normal condition and external fault condition. It however a fault
to earth or fault between corresponding CT will be disturbed and the relay will operate and
trip the CB.

10.6 Transmission Line Protection:

The protection scheme for transmission lines is differential relaying.

10.6.1 DIFFERENTIAL RELAYING:

In this type of relaying 6 pivot conductors would be required one for each phase CT and one
for neutral connection and two for the trip circuit but this scheme is costlier. A modified

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scheme is applicable only on parallel feeder. In this scheme the secondary current from CT’s
on the two circuits at the same end is compared. Under normal condition, each line will carry
equal current. In the event of fault the balance is disturbed that is why relay trips the CB.

10.6.2 DIRECTION BACKUP RELAYS:

These relays are used for the over current and earth fault of high voltage 132,220 KV sides.
This consists of main and backup scheme at standby. As carrier current protection of short
distance impedance, reactance and pilot relay for long short and very short distance
respectively.

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 Chapter 11
POWER LINE CARRIER COMMUNICATIONS:

As electronics plays a vital role in the industrial growth, communication is also a backbone of
any power station, communication between various generating and receiving station is very
essential for proper operation of power system. This is more so in case of a large
interconnected system where a control lead dispatch station has to coordinate the working of
various units to see that the system is maintained in the optimum working condition, power
line communication is the most econ90mical and reliable method of communication for
medium and long distance in power network.

11.1 Wave Trap:

Rejection filters are known as the line traps consisting of the parallel resonance circuit (L&C
in parallel) tuned to the carrier frequency are connected in series at each end of the protected
line such a CKT offers high impedance to the flow of carriers frequency current thus
reelecting the dissipation.

Fig 11.1: Wave Trap

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11.2 Coupling capacitor
The carrier equipment is connected to the transmission lines through the coupling capacitor,
which is such value that it offers a low reactance to the carrier frequency but high reactance
to the power frequency. For example 2000 pf capacitor offers 1.5 Mega ohm to 50 Hz and
150 ohm to 500 kHz. Thus coupling capacitor allows equipment but doesn’t allow 50 Hz
power frequency currents to enter the equipment’s. To reduce the impedance further a low
inductance is connected in series with the coupling capacitor to form a resonance at carrier
frequency.

11.3 Line Trap Unit

The line trap unit is inserted between bus bars and connection of coupling capacitor of the
line. It is parallel tuned circuit comprising L&C. It has a low impedance approximately .1
ohm to 50 Hz and high impedance to the frequency signals from entering the neighboring line
and the carrier currents flows only in the protected line.

11.4 Protection and Earthing

Once lightening and switching equipment and line cause voltages on power lines trap unit.
Nonlinear resistors in series with a protective gap are connected across the line trap unit and
inductor of coupling unit. The gap is adjusted to the spark at a set value of over voltage. Base
of coupling unit is earthen by earth rod in the vicinity to obtain a low earth resistance.

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 Chapter 12
CURRENT TRANSFORMER
As you all know this is the device which provides the pre-decoded fraction of the primary
current passing through the line /bus main circuit. Such as primary current 60A, 75A, 100A,
120A,150, 240A, 300A, 400A, to the secondary output of 1A to 5A.
Now a day mostly separate current transformers units are used instead of bushing mounting
CT’s on leveled structure they should be for oil level indication and the base should be
earthed properly. Care should be taken so that their should be no strain on the terminal.
When connecting the jumpers. Mostly secondary connections are taken to junction boxes
where star delta formation is connected for three phases and final leads taken to protection
/metering scheme. There should be no chance of secondary circuit remaining opens as it leads
to extremely high voltages which ultimately damage the CT itself.

Fig12.1 Current Transformers in 220kV GSSKhushkhera

12.1.1 Current transformers
can be used to supply information for measuring power flows and the electrical inputs for the
operation of protective relays associated with the transmission and distribution circuits or for
power transformers. These current transformers have the primary winding connected in series
with the conductor carrying the current to be measured or controlled. The secondary winding

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is thus insulated from the high voltage and can then be connected to low-voltage metering
circuits.
Current transformers are also used for street lighting circuits. Street lighting requires a
constant current to prevent flickering lights and a current transformer is used to provide that
constant current. In this case the current transformer utilizes a moving secondary coil to vary
the output so that a constant current is obtained.

Fig 12.2: Current Transformer

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 Chapter 13
DC SUPPLY SYSTEM OF G.S.S.

DC system has three main parts at G.S.S :-

(i) Battery Charger
(ii) Battery set
(iii) DC Distribution Board
Charger have two parts:-
(i) Float charger
(ii) Boost charger
1. Now a days Maintenance Free VRLA type battery set are used.
2. DC Distribution Board have Earth Fault Relay and various DC output
circuits.

Float Charger:-
1. 400V, 3- AC input through rotary switches and fuses is given to float CON1.
2. 1- AC input to motor control circuit which is connected to variac.
3. For proper output voltage regulation secondary of float transformer is connected in
series of secondary of boost transformer for necessary compensation.
4. Output is filtered by filter ckt and protected by HRC fuses.

BOOST CHARGER:-

1. 3- AC is given to CON2 through relay.

2. Boost transformer P1 have taps and its secondary has multiple taps which can be
used as per requirement through course and trip selector switches.
3. Output is protected by fuses and blocking diode, current is measured through shuts
of ammeter.

4. DC Power Contactor:-
1. When boost contactor ON then this contactor should be OFF. This is because of no
over voltage should come when set is boost charged.

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2. When boost is OFF then dc contactor is ON.

5 Battery Set:-
There is one battery set 220V DC of 110 batteries having 2V capacity of each cell.

Fig 13.1: Battery Of Relay

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 CONCLUSION

The training at 220KV Grid Sub Station was very useful. As it provide very useful
information about transmission and distribution of a system.

As we know that power as generated at 11kv. Than the voltage is stepped up for transmission
and collected at the outer parts of city. Like 220kv GSS Khushkhera is a primary sub station,
where the voltage is stepped down to 132kv from 220kv. This step down process involve use
of various equipments like lightening arrestor, transformer, C.T, P.T, etc.

From many of above instrument, the key instrument is “TRANSFORMER” as it step down
the voltage, also it is a very costly instrument so it is very important for us to protect it.

As for an electrical engineer, there are two main things one is generation and another is
transmission & distribution. In GSS we have seen the transmission and distribution part, that
how power is transmitted, what are the precautions to be taken while entering in a GSS, what
are the problems faced during transmission etc.

So it can be said that it is not useless to take training at GSS as it has its own importance, if
we go in a job of transmission of power then it is very useful. I also think that this is going to
help me further in the future.

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 REFERENCE

V.K.MEHTA “Principles of Power System” 225-230

B.R.GUPTA “Power System Analysis & Design” 325

J.B.GUPTA “POWER SYSTEM” 226,235

ROHIT MEHTA “Fundamentals Of Power System” 221-225

JOHN J. GRIN “Power System Analysis” 450-460

Web sites –

http://www.direct industry .com/products/transformers

http://Telk.com/circuit breaker
http://wikipedia.com/powe system
http://uneacadmy.com/power system
http://huskpowersystems.com/power system protection

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