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ABSTRACT

Power is an essential requirement for all facets of our life and has been

recognized as a basic human need. It is the critical infrastructure on which the

socio economic development of the country depends. the growth of the

economy and its global competitiveness hinges on the availability of reliable and

quality power at competitive rates. The demand power is enormous and is

growing steadily. In order to meet this demand, the power generating stations

have to transmit the power, which are located at hundreds of kilometers from the

load centers and due to these long distances we have to place the intermediate

substation in order to reduce the losses and to increase maximum power transfer

in the lines.

The generation for electricity in power generating station is done in

faraway places from consumers & load centers for various reasons reasons. The

generated voltage is stepped up at some places and stepped down at some places

i.e. at 11kv, 33kv it is stepped up to 132, 220, 400kv and transmitted to various

loads. at load centers the high voltage is stepped down to 11kv or 440 / 220v and

distributed to consumer. These voltage transformations are carried out at sub

stations.

The main equipment in substation is transformer, bus section, circuit

breaker, line going out of the substation. The protective relaying is necessary

with almost every electrical substation and no part of the substation is left

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unprotected. The choice of protection depends on different aspects such as type,

rating and protection equipment, location and cost.

The protection relaying scheme senses the abnormal condition of the

substation. Transient stability can be improved by means of protective relaying.

Faults cannot be avoided completely but they can be minimized. Hence relays

can play an important role in minimizing the faults.

In this project, we give an insight o the substation. it consists of various

fields which have been elaborated in a detailed manner. The necessity of a

substation, its features, classifications, specifications are dealt. The various

equipments used in a substation are explained.

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CHAPTER 1

INTRODUCTION

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Andhra Pradesh Transmission Corporation Ltd., has a great significance

of transmitting bulk power from four decades for which they have constructed

number of substations. Transmission substation is located between generating

power station and distribution substations.

Visakhapatnam consists of National thermal power corporation Ltd.,

Known as Simhadhri power plant. The plant generates 2*500MW power. This

total power is utilized in Andhra Pradesh only. So, AP TRANSCO established a

400 / 200kv transmission substation at KALAPAKA near NTPC.

This substation has been inaugurated by Honorable Former chief minister

Shri Nara Chandrababu Naidu in 2002. The substation is in collaboration with

J.B.I.C a Japanese company. The substation is spread over vast area of about 70

acres and the yard length is 900mts. According to area the substation is the

second largest in ASIA.

The equipment in this substation like transformers are manufactured by

CROMPTON GREAVES, Circuit breakers and Relays are manufactured by

ASIA BROWN BOVERI LTD.

It is feeded from NTPC by four incoming lines. The power is transmitted

to Vemagiri, Khammam and Power Grid. The 400kv is step down to 220kv and

transmits to Visakha dairy farm and Vizag switching station.

ORGANISATION OF THE MINI PROJECT REPORT:-

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The work presented in this report is organized in the following manner.

Chapter 2 deals with the (400/220 KV) substation which includes

introduction to substation, its necessity, classifications on various bases,

specifications, essential features & necessary structures within the

substation. The layout of 400/200KV Kalapaka substation is also

presented.

Chapter 3 deals with various elements that are used in the substation.

The substation equipment comprises of transformers, circuit breakers,

relays, lightning arresters, isolators, reactors, current transformers,

capacitor voltage transformers, wave trap, earthing system etc., the

ratings & specifications of the equipment used in Kalapaka substation

are also mentioned.

Chapter 4 deals with various protection schemes employed for the

protection of transformers & bus bars. The protection schemes for the

transformers includes buchholz relay, differential protection, over

fluxing protection, breaker failure relay protection & restricted earth

fault protection. The bus bar arrangements & protection schemes

includes back up protection & differential over current protection.

Chapter 5 deals with the various auxiliary equipments used in the

kalapaka substation. The auxiliary equipment includes battery operation

and maintenance, disturbance recorded & event logger.

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CHAPTER 2

400 / 220 KV

SUBSTATION

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2.1 INTRODUCTION

The assembly of apparatus used to change some characteristics (i.e., voltage A.C.

to D.C., frequency, power factor) of electric supply is called a substation. An

electrical substation is a subsidiary station of a electricity generation,

transmission and distribution system where voltage is transformed from high to

low or the reverser using transformers. Electric power may flow through several

substations between generating plant and consumer, and may be changed in

voltage in several steps.

There are several substations between generating station and final load points

Electrical substation receives power from the incoming lines and the power is

transferred at desired voltage by the transformer and is then supplied to the

outgoing lines.

The electrical sub stations at the various locations in network differ greatly in

their sizes, design, configuration and appearance. There are different works done

in a substation.

Switching Operation

Voltage Transformation Operation

Power Converting Operation

Power Factor Correction Operation.

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2.2 NECESSITY:

Between the generation of power and consumer point, transmission and

distribution exists. Generating stations are designed for generating bulk amount

of power and the Transmission takes place at high voltages to reduce the line

losses, where as consumer points are at low voltage. This high voltage is stepped

down to a lower voltage at different stages with the help of transformer and

switchgear in the substation.

2.3 CLASSIFICATION OF SUBSTATION.

The substation may be classified in numerous ways on the basis of

Nature of Duties

Service Rendered

Operating Voltage

Importance

Design

BASED ON THE NATURE OF DUTIES:

The substation based on the nature of duties may be classified into three types.

Step Up or Primary Substations

Primary Grid Substations

Step Down or Distribution Substations

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BASED ON THE NATURE OF OPERATING VOLTAGES:

High voltage Substation

Extra High Voltage Substation

Ultra High Voltage Substation

BASED ON SERVICE RENDERED:

Transformer Substation

Switching Substation

Converting Substation

BASED ON DESIGN:

Indoor Type Substation

Out Door Type Substation.

2.4 SPECIFICATIONS:

Specifications of substation denote the design requirement rating, technical

aspects regarding the substation and its associated electrical, mechanical, civil

and auxiliary sub system.

The sub stations are planned on the basis of economic, electrical and local

conditions at the project planning stage.

The substation requirements are influenced by network requirement in several

aspects including the following.

1. Insulation levels and insulation coordination

2. Fault levels and their coordination

3. Voltage control requirement of the network

4. Clearance and creep age distances

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5. Network monitoring and data communication between substation and

load dispatch centers.

6. Back up protection main protection zones maintenance zones

7. Switching sequence during normal emergency and post fault conditions.

ESSENTIAL FEATURES:

An AC substation has following parts.

AC switch yard

Control building

Low voltage and medium voltage AC system for auxiliaries

DC battery system and low voltage distribution system

Station mechanical, electrical and other auxiliaries

Civil work.

2.5 LAYOUT OF KALAPAKA SUBSTATION:

The first step in planning a substation layout is the preparation of a one line

diagram which shows in simplified from the switching and protection

arrangement required, as well as the incoming supply lines and outgoing feeders

or transmission lines. It is a usual practice by many electrical utilities to prepare

one line diagrams with principal elements (lines, switches, circuit breakers, and

transformers) arranged on the page similarly to the way the apparatus would be

laid out in the actual station.

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2.6 NECESSARY STRUCTURES:

The following structures are necessary in a conventional open terminal outdoor

substation.

Towers of incoming and outgoing transmission lines :

These are generally located outside the substation boundary adjacent to

the substation.

Tower and gantries for supporting strain insulators and flexible bus

bar :

These are used for mounting isolator, surge arrester and other equipment

suitable.

Towers and gaintries for supporting rigid tabular bus bar mounted

on post insulator.

The insulators are supported on horizontal beams. Supporting structures

of post insulator also support the tubular rigid bus bar, CTs, VTs,

insulators, CB and lime traps etc.,

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CHAPTER 3

ELEMENTS OF SUBSTATION

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3.1 INTRODUCTION:

Substations generally have switching, protection and control equipment

and one or more transformers. In a large substation, circuit breakers are used to

interrupt any short-circuits or overload currents that may occur on the network.

Smaller distribution stations may use recloser circuit breakers or fuses for

protection of distribution circuits. Substations do not usually have generators,

although a power plant may have a substation nearby. Other devices such as

power factor correction capacitors and voltage regulators may also be located at

a substation.

Substations may be on the surface in fenced enclosures, underground, or

located in special-purpose buildings. High-rise buildings may have several

indoor substations. Indoor substations are usually found in urban areas to reduce

the noise from the transformers, for reasons of appearance, or to protect

switchgear from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly grounded to

protect people from high voltages that may occur during a fault in the network.

Earth faults at a substation can cause a ground potential rise. Currents flowing in

the Earths surface during a fault can cause metal objects to have a significantly

different voltage than the ground under a persons feet; this tough potential

presents a hazard of electrocution.

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3.2. TRANSFORMER:

A Transformer is a static piece of apparatus by means of which electric

power in one circuit is transferred to another circuit without change in frequency.

The Transformer works on the principle of mutual inductance between

two circuits linked by a common magnetic flux.

Basically Transformers are of two types : ---

Core type

Shell type

CORE TYPE: The winding surrounds a considerable part of the core.

SHELL TYPE: The core surrounds a considerable portion of the winding.

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RATING OF A TRANSFORMER:

The mostly used power transformers in A.P. Region are

1. 400/220 KV 316 MVA Auto transformer

2. 220/132 KV 100 MVA Auto transformer

3. 220/33 KV 50 & 31.5 MVA transformer

4. 132/66 KV 40 & 27.5 MVA transformer

5. 132/33 KV 50, 31.5, 25, 16, 15 MVA transformers

6. 132/11 IV 16, 15 & 7.5 MVA transformers

7. 33/11 KV 8, 5, 3, 15 MVA transformers.

Most of the Power Transformer are of 132/11 KV and above are of star-

star vector grouping with the neutral solidly earthern.

In substation Auto transformers are preferred over two winding

transformer.

AUTO TRANSFORMER:

A transformer, in which a part of the winding is common to both the

primary and secondary circuits, is called an auto-transformer. In a two winding

transformer, primary and secondary windings are electrically isolated, but in an

auto-transformer the two windings are not electrically isolated.

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RATINGS OF AUTO-TRANSFORMER:

MAKE : M/s. CROMPTION GREAVES LIMITED, MUMBAI

CAPACITY : 315 MVA

RATING : 400/220/33 KV

WEIGHT : 273000 Kg.

WEIGHT OF OIL : 80,600 Lit.

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ADVANTAGES OF AUTO TRANSFORMER:

1. The weight of the (copper and aluminums) for any winding depends

Upon the cross sectional area and length of the conductor. Hence for

an auto transformer the weight and cost of the conductor required is

less compared to two winding transformer.

2. Owing to the reduction in conductor and core materials, the ohmic

losses in conductor and the core losses re lowered. Therefore, an auto

transformer has higher efficiency than a two winding transformer of

the same output.

3. Reduction in the conductor material means lower value of ohmic

resistance. A part of the winding being common, leakage flux and

therefore, leakage reactance is less. In other words, an auto

transformer has lower value of leakage impedance and has superior

voltage regulation then a two-winding transformer of the same output.

3.3 RELAYS:

A relay is an electrically operated switch. Many relays use an

electromagnet to operate a switching mechanism, but other operating principles

are also used. Relays find applications where it is necessary to control a circuit

by a low-power signal, or where several circuits must be controlled by one

signal. Solid state relays control power circuits with no moving parts, instead

using a semiconductor device to perform switching. Relays with calibrated

operating characteristics and sometimes multiple operating coils are used to

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protect electrical circuits from overload or faults; in modern electric power

systems these functions are performed by digital instruments still called

protection relays

Transistor relays are the most widely use static relays. In fact, when we

talk of static relay we generally mean transistor relays. The fact that the

transistor can be used both as an amplifying device and a switching device

makes this component suitable for achieving any functional characteristics.

The advantages of transistor relays can be summarized as follows:

1. The power consumption is low and hence provides less burden on CT and

PT as compare to the conventional electromechanical relays.

2. The relays are fast in operation.

3. No moving parts hence friction or contact troubles are absent and as a

result minimum maintenance is required.

4. The relays have greater sensitivity as amplication of signals can be obtainvery easily.

5. The relays has a high reset to pick up and the reset is very quick.

6. The use of printed circuits avoids wiring error and facilitates

rationalization of batch production.

7. It is possible to obtain wide range of characteristics approaching more or

less to the ideal requirements.

OPERATION:

When a current flows through the coil, the resulting magnetic field

attracts an armature that is mechanically linked to a moving contact. The

movement either makes or breaks a connection with a fixed contact. When the

current to the coil is switched off, the armature is returned by a force

approximately half as strong as the magnetic force to its relaxed position.

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Usually this is a spring, but gravity is also used commonly in industrial motor

starters. Most relays are manufactured to operate quickly. In a low voltage

application, this is to reduce noise. In a high voltage or high current application,

this is to reduce arcing. If the coil is energized with DC, a diode is frequently

installed across the coil, to dissipate the energy from the collapsing magnetic

field at deactivation, which would otherwise generate a spike of voltage and

might cause damage to circuit components. If the coil is designed to reenergized

with AC, a small copper ring can be crimped to the end of the solenoid. This

shading ring creates a small out of-phase current, which increases the minimum

pull on the armature during the AC Cycle.

By analogy with the functions of the original electromagnetic device, a

solid stage relay is made with a thyristor or other solid-stage switching device.

To achieve electrical isolation, a light emitting diode (LED) is used with a photo

transistor.

The type of relays used in Kalapaka substation is distance relays (REL 316

and REL 100).

DISTANCE RELAY

The impedance relays also called distance relays are employed to provide

protection to transmission lines connected in a network as they are economic and

possess several technical advantages. They are comparatively simple to apply.

Operate with extremely high speed and both primary and backup protection

features are inherent in them with power line carrier facilities and are suitable for

high sped re-closing. The impedance relay is made to respond to the impedance

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between the relay location and the point where fault is incident. The impedance

is proportional to the fault, and is therefore independent of the fault current

levels.

DISTANCE RELAYING PRINCIPLE

A distance relay compares the currents and voltages at the relaying points with

current providing the operating torque and the voltage provides the restraining

torque. In other words an impedance relay is a voltage restrained over current

relay.

Since the operating characteristics of the relay depend upon the ratio of voltage

and current and the phase angle between them their characteristics can be best

represented on an R-X Diagram where both V/I ratio and phase angle can be

plotted in terms of an impedance R + JX. Future the power system impedance,

loads, power swings etc., can also be plotted on the same R- X diagram.

Therefore response of a particular relay during power swing faults and other

system disturbances can easily be assessed.

TYPES OF DISTANCE RELAY:

1. Impedance relay

2. Reactance relay

3. Mho relay

4. Modified impedance relay

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IMPEDANCE RELAY:

Operation of the impedance relay is independent of the phase angle between

V and I the operating characteristics is a circle with its center at the origin

and hence the relay is non-direction.

THE REACTANCE RELAY

Reactance relay measures V/I sin O whenever the reactance measured by the

relay is less than the set value the relay operates.

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The resistance component of impedance has no effect on the operation of the

reactance relay. The relay responds solely to reactance component of

impedance. This relay is inherently non-directional. The relay is most suitable

to detect earth faults where the effect of are resistance is applicable.

MHO RELAY:

This is a directional impedance relay, also known as admittance relay. Its

characteristics on R-X diagram is a circle whose circumference passes

through the origin. This relay is inherently directional and it only operates

for fault in the forward direction.

MODIFIED IMPEDANCE RELAY:

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This is also known as mho relay whose characteristics enclose the origin on

R-X.

This off set mho relay has three main applications:

Bus-bar zone

Carrier starting unit in distance/carrier blocking schemes

Power swing blocking

3.4. CIRCUIT BREAKER

The function of the circuit breaker is to isolate the faulty part of the power

system in case of abnormal conditions.

A circuit breaker has two contacts, a fixed contact and a moving contact. Under

normal conditions these two contacts remain in closed position. When the circuit

breaker is required to isolate the faulty part, the moving contact moves away to

interrupt the circuit by separating the contacts. The flow of current is interrupted

results in the formation of arc between the contacts. The contacts are placed in a

closed chamber containing some insulation medium which extinguishes the arc.

ARC INTERRUPTION:

These are two methods of arc interruption:

1. High resistance interruption.

2. Current zero interruption

HIGH RESISTANCE INTERRUPTION:

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In this method of arc interruption its resistance is increased so as to reduce the

current to a value in sufficient to maintain the arc. The arc resistance can be

increased by cooling, lengthening, constraining and splitting the arc. This

method is not suitable for large current interruption.

CURRENT ZERO INTERRUPTION:

In case of AC supply the current passes through a zero point 100 times per sec at

the supply frequency of 50 Hz. This feature of arc is utilized for arc interruption.

The current is not interrupted at any point other than the zero current instant,

otherwise a high transient voltage will occur across the contact gap.

RESTRIKING VOLTAGE AND RECOVERY VOLTAGES

After the arc has extinguished the voltage across the breaker terminals do not

normalize instantaneously but it oscillates and there is a transient condition. The

transient voltage which appears across the breaker contact at the instant of arc

being extinguished is known as restriking voltage. The power frequency runs

voltage which appears across the arc is finally extinguished and transient

oscillations die out is called recovery voltage.

CLASSIFICATION OF CIRCUIT BREAKER:

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Depending on the arc quenching medium employed the following are important

types of circuit breaker.

1. Oil circuit breaker

2. Air blast circuit breaker

3. Sulphur hexa fluoride circuit breaker

4. Vacuum circuit breaker

RATING OF CIRCUIT BREAKER

Circuit breakers have the following important ratings.

1. Breaking capacity

2. Making capacity

3. Short time capacity

BREAKING CAPACITY

The braking capacity of the circuit breaker is of two types

1. Symmetrical breaking capacity

2. Asymmetrical breaking capacity

SYMMETRICAL BREAKING CAPACITY

It is the rms value of the AC component of the fault current, the breaker is

capable of breaking under specified condition of recovery voltage.

ASYMMETRICAL BREAKING CAPACITY:

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It is the rms value of the total current comprising of both AC and DC

components of the fault current that the circuit breaker can break under specified

condition of recovery voltage.

MAKING CAPACITY

The rated making current is defined as the peak value of the current

including the DC component in the first cycle at which a CB can be closed on to

a short circuit.

SHORT TIME CURRENT RATING:

The circuit breaker must be capable of carrying short circuit current for a

short period while another circuit breaker is clearing the fault. The rated short

time current is the rms value of the current that the circuit breaker can carry

safely for a specified short period.

RATED CURRENT AND FREQUENCY:

The rated current is the rms value of the current that the circuit breaker

can carry continuously without any temperature rise in excess of its specified

limit.

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The rated frequency is also mentioned by the manufacture. It is the

frequency at which the circuit breaker has been designed to be operated. The

standard frequency is 50 Hz. If a circuit breaker is to be used at a frequency

other than its rated frequency its effect should be taken into consideration.

OPENING AND CLOSING TIME:

Closing and opening time measurement of circuit breaker is done by

using digital time internal meter.

SULPHUR HEXAFLUORIDE (SF6) CIRCUIT BREAKER:

The type of circuit breaker being used in the substation is ASIAN

BROWN BOVERI made sulphur hexafluoride (SF6) circuit breaker.

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EXPLANATION:

Sulphur Hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc

extinguishing properties. The dielectric strength of the gas increases with

pressure and is more than of dielectric strength of oil at 3 kg/cm2 SF6 is being

widely used in electrical equipment like high voltage metal enclosed cables; high

voltage metal clad switchgear, capacitors, circuit breakers, current transformers,

brushings etc., The gas is liquefied at certain low temperature, liquefaction

temperature increases with pressure.

Sulphur Hexafluoride gas is prepared by burning coarsely crushed roll sulphur in

the fluorine gas, in a steel box, provided with staggered horizontal shelves, each

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bearing about 4 Kg of sulphur. The steel box is made gas tight. The gas thus

obtained contains other fluorides such as S2F10, SF4 and must be purified

Further SF6 gas generally supplier by chemical firms. The cost of the gas is

low if manufactured in large scale.

SF6 circuit breakers operate to switch electric circuits and equipment in and out

of the system. These circuit breakers are filled with compressed Sulphur

Hexafluoride gas which acts to open and close the switch contacts. The gas also

interrupts the current flow when the contacts are open.

During the arcing period SF6 gas is blown axially the arc. The gas removes the

heat from the arc by axial convection and radial dissipation. As a result, the arc

diameter reduces small during the decreasing mode of the current wave. The

diameter becomes small during the current zero and the arc is extinguished. Due

to its electro-negativity and low are timely constant, the SF6 gas regains its

dielectric strength is very high and the time constant is very small.

3.5 ISOLATOR

Isolating switches are employed only for isolating circuits. They ensure that the

current is not switched into the circuit until everything is in order.

Isolators or disconnecting switches are designed to operate under no load

condition. Isolators are employed in addition to circuit breaker and are provided

on each side of every CB to provide isolation. While opening a circuit the CB is

opened first, then the isolator. If an isolator is opened carelessly, when carrying

a heavy current, the resulting are could easily cause a flash over to ground. This

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may shatter the supporting insulator and may even cause a fatal accident to the

operator particularly in HV circuit. While closing a circuit the isolator is first

closed then circuit breaker. Isolators are necessary on the supply side of the CB

in order to ensure isolation of the CB from the live parts.

Isolators employed in power system are usually three pole isolator each having

three identical poles. Each pole consists of two or three insulators posts mounted

on a fabricated support. The fixed end has moving conducting rods which swing

apart and isolation is obtained.

The simulataneous operation of three poles is obtained by mechanical inter

locking of the three poles.

There are two types of isolators which are used in this substation.

They are

Horizontal break center rotating double break isolator

Pantograph isolator.

HORIZONTAL CENTER BREAK ISOLATOR

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In this type of construction there are two insulator stacks per pole. The two on

each side are fixed and one at the centre is rotating type. The center stacks can

swing about its vertical axis through about 90 degrees. The fixed contacts are

provided on the top of each of the insulator stacks on the side. In closed position

the contact shaft connects the two fixed contacts. While opening the central

stack rotates through 90 degrees.

PANTOGRAPH ISOLATOR:

While closing the pantograph the linkages are brought nearer by rotating the

insulator column. In closed position the upper two arms of the pantograph close

on the Over head station bus bar giving a grip. The current is carried by the

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upper bus bar to the lower bus bar thought he conducting arms of the pantograph

isolators. While opening, the rotating insulator columns are rotated about their

axes, thereby the pantograph blades, collapse in vertical plane and vertical

isolation is obtained between the lime terminals. And pantographs isolator

covers less area. Each pole can be located at a suitable point and the three poles

need not be in one line and can be located in a line at desired angle with the bus

axis.

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3.6 REACTORS:

Under light load or no load conditions the receiving end voltage is greater than

the sending end voltage due to capacitance effect, this is known as Ferranti

effect. To reduce this capacitance effect shunt reactors (Inductors) are placed at

the sending and receiving end terminals to absorb leading vars.

RATINGS OF 400 KV LINE REACTORS:

MAKE : BHEL, BHOPAL

CAPACITY : 63 Mvar

RATING : 420 Kv 87 A

WEIGHT : 121970 Kg.

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WEIGHT OF OIL : 39,730 Lit

3.7 LIGHTNING ARRESTERS:

Lightning arrester is a surge diverter and is used for the protection of

power system against the high voltage surges. It is connected between the line

and earth and so diverts the incoming high voltage wave to the earth.

Lightning arresters act as safely valves designed to discharge electric

surges resulting from lightning strokes, switching or other disturbances, which

would otherwise flash over the insulators or puncture insulation, resulting in a

line outage and possible failure of equipment. They are designed to absorb

enough transient energy to prevent dangerous reflections and to cut off the flow

of power frequency

Follow current at the first current zero. After the discharge of the transient

energy, arresters have the insulation that can break down the voltage being

independent of the steepness of the wave form.

Lightning protection by means of lightning arresters and gaps and over

head ground wire is a means of reducing outage and preventing damage to the

substation equipment from lightning disturbance. The amount and kind of

protection vary in different applications.

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The type of lightning arresters used in Kalpaka substation is HORN GAP

lightning arrester.

The horn gap consists of two horn shaped rods separated by a small

distance. One end of this connected to the line and the other to the earth with

or without a series resistance. The choke connected between the equipment

to be protected and the horn gap serves two purposes:

1. The steepness of the wave incident on the equipment to be protected

is reduced.

2. It reflects the voltage surge back on to the horn.

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Whenever the surge voltage exceeds the breakdown value of the gap a

discharge takes place and the energy content in the rest part of the wave is

by-past to the ground. An arc is setup between the gap, which acts like a

flexible conductor and raises upwards under the influence of the

electromagnetic forces, thus increasing the length of the arc which eventually

blows out.

3.8 CURRENT TRANSFORMER:

Protective relays in AC power system are connected in the secondary

circuit of current transformer and potential transformer. The design and use

of these transformers is quite different from that of well known transformer.

In current transformer primary current is not controlled by condition of the

secondary circuit. Hence primary current is a dominant factor. The current

transformers are classified into two groups depending on the usage of

secondary.

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Protective current transformer used in association with relay trip coil,

pilot wire etc.,

Measuring current transformer used in connection with ammeter, wattmeter

etc., as a rule, the ratio error is very important in protective current

transformer and phase angle error may be less. But current transformer and

voltage transformer comes under the INSTRUMENT TRANSFORMERS.

3.9CAPACITOR VOLTAGE TRANSFORMER:

Capacitor voltage transformer is used for line voltmeter, protective relays,

tariff meter etc,

The performance of capacitor voltage transformer is inferior to that of

electromagnetic voltage transformer. Its performance is affected by the supply

frequency, switching transient, magnitude of connected burden etc., the capacitor

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voltage transformer is more economical than an electromagnetic voltage

transformer when the nominal system voltage increases above 66 kV.

The carrier current equipment can be connected via the capacitor of the

capacitor voltage transformers. Thereby there is no need of separate coupling

capacitors.

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Capacitor type PT is used for voltages 66 kV and above. At such

voltages cost of electromagnetic type PTs tends to be too high. The capacitors

connected in series act like potential dividers provided the current taken by the

burden is negligible compared with the current passing through the series

connected capacitors. However, the burden current becomes relatively larger

and ratio error and phase error is introduced. Compensation is carried out by

tuning the reactor connected in series with the burden is adjustable to such a

value that at supply frequency it resonates with the sum of two capacitors. This

eliminates the error. The construction of capacitor type PT depends on the form

of capacitor voltage divider. Generally HV capacitor is enclosed in the porcelain

housing. A large sheet box at the base encloses the tuning coil intermediate

transformer.

3.10 WAVE TRAP:

Wave traps are used in power line communication. One of the devices

employed in power line carrier is the line trap, sometimes called a wave trap. A

wave trap is a parallel resonant circuit installed on the power line at a specific

frequency or frequencies. Properly tuned, the line trap shows its highest

magnitude of impedance, power frequency to pass. It is generally represented as

an inductor with a capacitor in parallel. Wave trap should be connected in series.

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RATINGS OF WAVE TRAP:

MANUFACTURER : AREVA Transformers Pvt. Ltd.,

INDUCTANCE : 1mH

FREQUENCY : 50 Hz

WEIGHT : 195 Kg

ISC : 20 KA/Sec

IR : 800 A

3.11 EARTHING SYSTEM:

The purpose of earthing is to safe guard against dangers of shock and fire etc., It

is essential to have good and effective earthing or grounding.

Station earthing system comprises of:

Ground mat risers, earthing strips, earthing spikes.

Over head earth wires for shielding against lightning strokes or

lightning masts.

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Galvanized steel structures or towers and gantries for support.

PLCC equipment including wave trap and tuning unit, coupling

capacitor etc.,

Power cables

Control cables for protection and control

Station lightning system.

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PURPOSE OF EARTHING:

An electric iron is connected to supply through 3-wire cable. The line and

neutral are connected to heating element where as the earth wire is connected to

the metal frame iron. The earth terminal is connected to the neutral earthing at

substation through mass of earth. Due to insulation damage or otherwise, the

line wire touches the metal frame of the iron is effectively gets connected to the

earth, if the person holds the iron the current will not pass through his body

because the resistance of the earth wire will be les than the human body

resistance. Hence the person will be protected from electric shock. Mean while

the current will pass through the low resistance path of the earth wire is not

provided then if the live wire accidentally makes contact with the iron frame the

latter will be at the same potential as the live wire. If a person holds the iron

unknowingly, the current flows through the body of the person which is very

dangerous to life.

The non-current carrying metallic parts in the neighborhood of electrical circuits

must be earthed, which ensures safety to human life. The non-current carrying

parts include the following:

1. Motor body, switch gear metal enclosure, transformer bank, conduits of

Wiring etc.

2. Supporting structures, towers, poles etc.,

3. Sheaths of cables

4. Body of portable equipment such as iron, oven, heater, kettle etc.,

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.

CHAPTER - 4

PROTECTION SCHEMES

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DIFFERENT PROTECTION SCHEMES

4.1 TRANSFORMER PROTECTION

The transformer is the main equipment which has to be protected mainly

in the power system.

Types of faults subjected to transformer.

1. Through Faults:

These are due to over load condition and external short circuits. Time graded

overloaded and earth fault relays are provided for external short circuit

conditions.

2. Internal Faults:

a. Electrical Faults: The faults which causes immediate serious damage such as

line to ground faults, Line to Line fault or short circuits between transformers

HV and LV Windings.

b. Incipient Faults: Which are initially minor faults, causing slowly developing

such as poor electrical connection of conductors or breakdown of insulation.

The following are the types of relays that employed for transformer protection:

a. Buchholz relay

b. Over fluxing relay

c. Differential relay

d. Breaker failure relay

e. Restricted earth fault relay

A) BUCHHOLZ RELAY:

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It is placed in between transformer of tank and conservator tank.

Whenever the fault on a transformer develops slowly, heat is produced locally,

which begins to decompose solid of liquid insulating materials and then to

produce inflammable gas and oil flow. Initially it sends an alarm signal

whenever a fault is detected; if the fault is left unaffected then it sends a trip

signal to the CB which opens the circuit.

SILICAGEL BREATHER:

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Expansion and contraction of oil cause breathing action. Any humidity in

the air is absorbed by the silica get dehydrating breather. An oil seal in the air

intake prevents the external moisture being absorbed when no breathing occurs.

The breather container is filled with silica gel crystals. It is arranged such that

the air breathed must pass through it.

The desiccant is impregnated with cobalt chloride and when those Silica

gel crystals are fully active, they have deep blue colour. If colour changes to a

whitish pink, they are then saturated with moisture and the charge contained

should be replaced by a new reactivated one.

(B) DIFFERENTIAL PROTECTION:

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The differential protection used for transformer is based on the principle

of current circulation. This type of protection is mostly used for transformers as

this respond not only to inter turn fault but also provide protection against phase-

to-phase fault.

If the current circulation principle is applied to transformer great care is to

be taken because of unequal voltage and different types of connections on the

primary and secondary sides of transformer resulting into different phase angles

and currents on both sides.

The relay use for differential protection must have time delay

characteristics in order to neutralize the unbalancing currents caused by the

switching surge of the magnetizing current. When transformer is energized that

is when the transformer is switched to supply the magnetizing currents may

assume very high values momentarily and may cause operation of relay, but such

peaks are generally transient.

The two basic requirements of differential relay connections must satisfy.

a. It must not be operated for external or load faults.

b. It must operate for internal faults.

c. The current flowing through operating coil of relay should be zero during

Normal operating condition and for external short circuit.

C. BREAKER FAILURE RELAY:

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Main protective schemes provided for lines, transformers, generators are

required to operate and clear the fault immediately, isolating the faulty section of

the system.

It is then important than the circuit breaker operates correctly, clearing the

fault quickly by tripping. However there is risk that breaker may not trip (either

due to mechanical sluggishness or due to inability to interrupt heavy fault

current) then the fault gets cleared by backup relays at remote sections.

Increase in power system complexity demands shorter faults clearing times. It is

therefore necessary to provide breaker failure relay. This scheme will isolate the

bus to which the local breaker backup relay (LBB) is connected faster. It

comprises of over load and earth load relays with a timer. The LBB relay is

energized by trip command of main protection schemes and thus initiates master

trip relay of the bus-bar protection scheme after elapsing of defined time. Then

the entire breakers connected to the bus get tripped, thus isolating faulty element.

(D) RESTRICTED EARTH FAULT PROTECTION:

This relay is operative only for the internal faults of the transformer and

thus fast operating time can be achieved.

1. An external fault on the star side will result in current flowing in line current

transformer of the effected phase and balancing current in the neutral CT and

current in the relay is zero and hence relay is stable. During an internal fault,

the line current on the line CT gets reversed and hence relay operates.

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2. The arrangement of residually connected CTs on the delta side of

transformer is only sensitive to earth faults on the delta side because the delta

winding blocks zero sequence currents.

(E) OVER FLUXING PROTECTION:

1. Over fluxing condition in a transformer can occur during system over voltage

and/or under frequency conditions (V/F).

2. The over fluxing condition does not call for high sped tripping. The tripping

can be delayed depending on the over flux withstand capability of transformer.

3. Relays with definite time delay (Nearly 30 sec.) and inverse characteristics

are being employed.

4.2 BUS BAR ARRANGEMENTS

In order to maintain system stability and minimize fault damage due to high fault

levels, instantaneous tripping for bus bar faults is necessary.

Bus bar protection scheme should be:

1. Completely reliable.

2. Absolutely stable for high faults

3. Selective

4. Accurate and fast operating.

GENERAL BUS BAR ARRANGEMENTS:

a. SINGLE BUS BAR ARRANGEMENTS:

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This has only single bus bar to which all lines transformers generators etc., are

connected. In the event of the fault on the bus bar entire bus has to be de-

energized and a major outage occurs.

b. SINGLE SECTIONALISED BUS BAR SCHEME

In this, main bus is divided into two sections with a circuit breaker. One

complete section can be taken out for maintenance or for break down works

without distribution continuity of other sections.

c. MAIN AND TRANSFER BUS BAR SCHEME:

With this arrangement, any line breaker (one at a tie) requiring

maintenance can be transferred to transfer bus. The feeder protection thus gets

transferred to trip bus couple breaker. On fault occurrence or maintenance,

entire bus becomes reenergized.

d. DOUBLE BUS BAR ARRANGEMENT:

It has the flexibility of transferring any line to any of the buses. On fault

occurrences or maintenance only one bus becomes dead. While other bus

remains in service.

e. DOUBLE BUS & TRANSFER BUS BAR ARRANGEMENT:

Combination of main, transfer bus and double buses arrangement

4.3 BUS BAR PROTECTION SCHEMES

Back up protection

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Differential over current protection

All the protection schemes must also be provided with a check system to

ensure that the protection responds only to earth faults occurring within the bus

zone not extraneous earth faults.

1) BACK UP PROTECTION

In principle it is a simplest of all to protect the buses with the aid of back

up protections of the connected supplying elements which should respond to any

fault appearing on the buses, when no separate bus protection is provided but

distance protection is provided for the feeders connected to the bus. It is possible

to cover the bus bars within zone to reach distance relays.

Distance protection is widely employed for the protection of transmission

lines. Hence it is quite economical to use the same for bus protection the draw

backs of this protection are : firstly delayed action, secondly disconnection of

more circuits in case there are two or more incoming lines and thirdly exact

discrimination not possible.

Bus back up protection may also mean that in case the breaker fails to

operate for a fault on the outgoing feeder, then it must be regarded as a bus fault.

It should then open all breakers on that bus such a backup protection can be

provided with appropriate time delay through a timer.

(2) DIFFERENTIAL OVER CURRENT PROTECTION

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It is based on the fact that any fault within electrical equipment would cause the

current entering it to be different from the current leaving it. By comparing the

two currents either in magnitude or in phase or in both, fault can be determined.

It is an attractive option if both the ends of the apparatus are located hears each

other.

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CHAPTER 5

AUXILIARY EQUIPMENT

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5.1 BATTERY OPERATION AND MAINTENANCE:

In case of failure of the supply, as a backup protection for the control room a

battery room is provided within the substation for its functioning.

At KALAPAKA substation 106 batteries with 1000ampere hour rating and

220volts and it have two types of charging.

1. Boost charge

2. Float charging.

Boost charging:

When there is a sudden drop in the voltage / also when there is considerable fall

in voltage the charger with automatically switch on to boost charging mode from

float charging mode.

Float charging:

This is a constant charging mode means these batteries will be constantly being

charged. When there is no load it takes less current. And these cells have to be

charged continuously so that they will not get discharged. As these has to supply

DC supply when there is a fail in AC supply.

There are two sources of batteries SOURCE I and SOURCE II for safety and

reliability that is if one of the sources fails then immediately source II will be

connected to I and this is called DC change over contractor.

5.2 DISTURBANCE RECORDED:-

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Behavior during recording:

The inducted 650 fault recorder continuously monitors all signals for trigger

conditions. The input values are continuously stores in a memory with the

capacity which equals the pre event history setting. Recording starts as soon as

a trigger condition exists. The even is then stored along with its pre event and

post event history.

This relay recorder is a fully electronics, digital, data acquisition module for

decentralized fault monitoring in electricity supply networks.

The indactic 650 fault recorders can either be used as a single acquisition unit

with 9 analog and 16 digital inputs, or several units may be combined possibly in

a decentralized set up, to form a logical acquisition station with a corresponding

larger number of inputs. Furthermore several such stations can be combined to

form an interconnected system. The acquisition units or stations communicate

among themselves via fast bus connections using co axial cables or fiber optic

cables.

5.3 EVENT LOGGER:

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Function:

The sequential events recorder is a versatile high speed distributed

microprocessor based data acquisition system design to monitor and record in

real time changes of state of customer supplied field inputs with an accuracy of

one millisecond.

The event information includes alarm and return to normal status; time in hours,

minutes, seconds and milliseconds. Input address and up to 60 characters of

descriptive legends. Even t information can be recorded on a printer or printer

terminal, displayed on a CRT monitor or transmitted to a remote computer or

distributed control system via RS 232 ASC II data links.

The SER is designed for optimum versatility and flexibility and flexibility with

minimum user hardware adjustments. The equipment operating configuration is

accomplished by means of key board entries employing user friendly software

commands. In addition, the operator can print various operating and

configuration states reports and initiate a system functional test.

CONCLUSION

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Electrical sub station consists of Transformers, bus bars, circuit breakers,

isolators, relays instrumental transformers and other equipments. The Protection

of a system is required when the short circuits and abnormal conditions often

occur on a power system.

If a fault occurs in an element of a power system an automatic protective devices

are needed to isolate the faulty element as quickly as possible to keep the healthy

section of system in normal operation.

The substation can be used for Switching Operation, Voltage Transformation

Operation, Power Converting Operation, and Power Factor Correction

Operation. The consumers can require different range of voltages for utilization,

so to achieve this sub stations are used.

The basis electrical quantities which are likely to change during the abnormal

conditions are current, voltage, phase angle and frequency. The protective relays

utilize one or more of these quantities to detect the abnormal conditions on a

power system.

Different types of protection schemes were employed for protection of

transformers, bus bars and equipment in substation. All schemes must to used

different types of protective relays.

BIBLIOGRAPHY

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Text book of power system engineering

M.L.Soni Gupta Bhatnagar, TataMc GrawHill publisher

Electrical power system

J.B. Gupta Khanna Publishers

ABB Relays manual

Protection system 400kv substation manual

http://en.wikipedia.org.