Visit ReportOn

500KV Grid Station NTDCL MultanMay 07th, 2015

Submitted By

Engr. Muhammad Ahmad

(Executive Engineer)

Submitted By

Muhammad Ikram

2K11-EPE-354

Raza Sikandar 2K11-EPE-316

Shahzad Hussain2K11-EPE-344

Muhammad Quddamah2K11-EPE-350

Electrical Engineering Department

NFC Institute of Engineering & TechnologyMultan

Visit Report on 500KV Grid Station Multan

OPENING

In the name of ALLAH, The Most Benevolent,

ever Merciful, All Praise be to ALLAH, Lord of

the whole world. Most Beneficent, ever Merciful,

King of the Day of Judgment, You Alone we

worship for, and to You Alone we turn for Help.

O’ GOD, Guide us to the Path that is straight.

The Path of those, who are blessed by you,

neither of those, who have earned Your Anger,

nor of those who have gone to astray.

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PREFACE

Concepts we build by studying theory in

classroom, & dimensions while observing &

analyzing the activities in real world. Practical

internship and research work on Technical

studies is an integral part of Engineering

program. To become an expert to understand

all concerning issues concerning Ethics, only

theoretical knowledge does not provide a

concrete base. Research work, report writing,

internship reports also considered a significant

task along with theoretical knowledge therefore

we were assigned a visit report on 500KV Grid

Station NTDCL Multan, so that we gain a clear

insight of the real world.

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Acknowledgement

We are indebted to most respected Executive

Engineer of protection Engr. Muhammad Ahmad,

for his able guidance, motivation and cooperation

that he expended to us during the visit program. We

wish to express our deep and sincere appreciation

and thankfulness to Engr. Muhammad Ahmad

Executive Engineer (P&I) for his able guidance,

whenever we needed them, he was ready to help us

by any means, indeed very kind and wonderful

human. Without his continuous help and

encouragement, this visit & this report would surely

not be able to take this present shape.

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List of Contents

OPENING..................................ERROR: REFERENCE SOURCE NOT FOUND

PREFACE.................................ERROR: REFERENCE SOURCE NOT FOUND

ACKNOWLEDGEMENT............ERROR: REFERENCE SOURCE NOT FOUND

LIST OF CONTENTS................ERROR: REFERENCE SOURCE NOT FOUND

EXECUTIVE SUMMARY.....................................................................................5

INTRODUCTION OF NTDC ...............................................................................6

NATIONAL GRID SYSTEM OF PAKISTAN .....................................................7

LOCATION OF 500KV GRID STATION.............................................................9

SINGLE LINE DIAGRAM OF 500KV G/STATION ..........................................10

FUNCTIONS OF GRID STATION.....................................................................11

TECHNICAL DATA OF GRID STATION..........................................................11

EQUIPMENTS USED IN 500KV G/STATION...................................................13

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EXECUTIVE SUMMARY

An electrical grid station is an interconnection point between two transmission ring circuits,

often between two geographic regions. They might have a transformer, depending on the

possibly different voltages, so that the voltage levels can be adjusted as needed.

Grid station regulates and controls the power between interconnected transmission lines to

increase the reliability of the power system. It receive power from the power station at extremely

high voltage and then convert these voltage to some low levels and supplied electric power to the

sub stations or to other grid stations at the same voltage level according to the requirements.

National grid system of Pakistan contains an interconnected group of transmission lines in a ring

system. It covers most of the power stations of the country in this single ring and supplied

electric power to the different areas of the country. Main function of the grid station is switching

between the connected line stations and the load centers. This report comprises on the basics of

the 500KV grid station. It includes the functions and necessary information about the elements of

the 500 KV grid station, NTDC, Multan.

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National Transmission and Dispatch Company (NTDC):

National Transmission & Dispatch Company (NTDC) Limited was incorporated on 6th

November, 1998 and commenced commercial operation on 24th December, 1998. It was

organized to take over all the properties, rights and assets obligations and liabilities of 220 KV

and 500KV Grid Stations and Transmission Lines/Network owned by Pakistan Water and Power

Development Authority (WAPDA).The NTDC operates and maintains nine 500 KV Grid

Stations, 4160 km of 500 KV transmission line and 4000 km of 220 KV transmission line in

Pakistan.

Grid and the Sub Station:

An electrical power substation is a conversion point between transmission level voltages (such as

500KV) and distribution level voltages (such as 11KV). A substation has one or more step-down

transformers and serves a regional area such as part of a city or neighborhood. Substations are

connected to each other by the transmission ring circuit system by equipments.

An electrical grid station is an interconnection point between two transmission ring circuits,

often between two geographic regions. They might have a transformer, depending on the

possibly different voltages, so that the voltage levels can be adjusted as needed.

The interconnected network of sub stations is called the grid, and may ultimately represent an

entire multi-state region. In this configuration, loss of a small section, such as loss of a power

station, does not impact the grid as a whole, nor does it impact the more localized

neighborhoods, as the grid simply shifts its power flow to compensate, giving the power station

operator the opportunity to effect repairs without having a blackout.

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National Grid System of Pakistan:

Electricity is generated at a voltage level of 11 KV at the largest hydral power station (Tarbela)

of the Pakistan and it steps up to the voltage level of up to 500 KV by using a unit transformer. A

complex distributed network of 500 KV transmission lines are present in the Pakistan (from

Peshawar to Karachi), the output of the unit transformer is given to these lines which then

supplied this power to all of the country with the help of their interconnected network of

transmission and distribution lines. In summer season, ice is reached in the Tarbela and

Mangla’s reservoirs after melting from northern areas. So in this season there is enough water for

the production of required electrical power and the generated electrical power is travel from

Tarbela to Karachi side. But in winter season, situation is opposite to the above. Water is not

enough to produce a required power, so the capacity of Tarbela station is somewhat reduced and

to compensate this reduced energy, the flow of electric power through the interconnected

network is changes its direction toward Tarbela from Karachi instead towards Karachi.

There is many station in our country but we consider only those have voltage level in between

220 and 500 KV. In National grid system of Pakistan, several power stations are connected in a

ring system and they supplied electric power to different areas of the country under the

supervision of WAPDA. All stations are transmitting their produced power to transmission line

and from the ring main system; all regional grids supplied power to their own areas. By

connecting several power stations into a single ring system, the system stability is increased.

Advantages of the Grid System

Any time electricity is available for the consumers at lower cost.

Flow of electrical energy is continuous and sure.

It is possible to fulfill the emergency demand of power.

Better regulation of the voltages.

Improved power factor

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It is possible to govern the generator according to the load.

Safe transmission system.

Reduced fault timings.

Controlled frequency range.

Disadvantages of the Grid System

Cost of the control system is increased and their maintenance is complicated.

Power system is affected from the environmental factors.

This system is unsafe during the war.

Extended system is going to complexity.

Due to the expensive equipments, additional load occurred on the consumers.

During short circuit condition it is impossible to maintain the continuity of power.

High initial and maintenance cost.

During load shedding, capacity of industries connected with the grid is reduced which

cause to industrial development problem.

For maintenance, qualified staff is required and for that reason our country has to spend

more money to call expert engineers from other countries.

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Locations of the Interconnected 500 KV Grid Stations of Pakistan

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Single Line Diagram of 500KV Grid Station New Multan

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Functions of a Grid Station:

A Grid Station has the following functions…

1 - Supply of required electrical power.

2 - Maximum possible coverage of the supply network.

3 - Maximum security of supply.

4 - Shortest possible fault-duration.

5 - Optimum efficiency of plants and the network.

6 - Supply of electrical power within targeted frequency limits, (49.5 Hz and

50.5 Hz).

7 - Supply of electrical power within specified voltage limits.

8 - Supply of electrical energy to the consumers at the lowest cost.

Technical data of the 500 KV/220KV grid station, NTDC, Multan:

Grid input

The 500KV grid station, NTDC Multan receives power at the voltage level of 500 KV from the

following generating stations.

The data of the Transmission lines with length & date of commissioning is as following.

Sr No Name of T/ Line Length (KM) Date of Energizing

01 Multan-Guddu 01 312 21-05-1991

02 Multan-Guddu 03 313 08-01-1991

03 Multan- M/ Garh 63.6 10-03-2000

04 Multan- Yousaf Wala 162 04-05-2001

05 Multan- Gatti 222 28-06-1995

06 Multan- Roush 62 24-02-1998

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Guddu Power station (2 Circuit)

Muzaffar Garh power Station (1 Circuit)

Rousch Power Station (1 Circuit)

Gatti Grid Station (1 Circuit)

Yousafwala Grid Station (1 Circuit)

Transmission Grid Exit (220KV)

The 500KV grid station, NTDC Multan has delivers/receive power at a voltage level of 220KV

to or from the following stations…

The data regarding 220KV transmission lines is given below.

Sr No Name of T/ Line Length (KM) Date of Energizing

01 Multan- S/ Road 01 188 12-10-2013

02 Multan- S/ Road 02 188 12-10-2013

03 Multan- NGPS 01 14 01-07-1991

04 Multan- NGPS 02 14 07-07-1991

05 Multan- M/ Garh 02 56 07-07-1991

06 Multan- M/ Garh 03 56 02-01-1995

07 Multan- M/ Garh 01 50 12-05-1995

08 Multan- M/ Garh 04 50 25-12-1988

09 Multan- KAPCO 03 102 10-12-1989

10 Multan- KAPCO 04 102 01-12-1989

11 Multan- KAPCO 06 102 10-02-1998

12 Multan- KAPCO 05 102 29-03-1988

13 Multan- Vehari 01 79 21-06-2008

14 Multan- Vehari 02 79 21-06-2008

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Kot Addu Power Station (4 Circuit)

Muzaffar Garh power Station (4 Circuit)

Vehari Substation (2 Circuit)

N.G.P.S Multan (2 Circuit)

Nishatabad (2 Circuit)

Equipments used in 500 KV Grid Station

Shunt Reactor:

Transmission cables have much higher capacitance to earth than overhead lines. Long

transmission lines for system voltages of 132 KV and more need shunt reactors. The same

goes for large urban networks to prevent excessive voltage rise when a high load suddenly

falls out due to a failure.

Shunt reactors contain the same components as power transformers, like windings, core,

tank, bushings and insulating oil and are suitable for manufacturing in transformer factories.

The main difference is the reactor core limbs, which have non-magnetic gaps inserted

between packets of core steel.

To stabilize the line voltage the line inductance can be compensated by means of series

capacitors and the line capacitance to earth by shunt reactors. Series capacitors are placed at

different places along the line while shunt reactors are often installed in the stations at the

ends of line. In this way, the voltage difference between the ends of the line is reduced both

in amplitude and in phase angle.

In this situation, the capacitance to earth draws a current through the line, which may be

capacitive. When a capacitive current flows through the line inductance there will be a

voltage rise along the line.

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3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-limbed core

there is strong magnetic coupling between the three phases, while in a 5-limbed core the

phases are magnetically independent due to the enclosing magnetic frame formed by the two

yokes and the two unwound side-limbs.

The neutral of shunt reactor may be directly earthed, earthed through an Earthing-reactor or

unearthed.

 

When the reactor neutral is directly earthed, the winding are normally designed with graded

insulation in the earthed end. The main terminal is at the middle of the limb height, & the

winding consists of two parallel-connected halves, one below & one above the main

terminal. The insulation distance to the yokes can then be made relatively small. Sometimes

a small extra winding for local electricity supply is inserted between the main winding &

yoke.

    

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Testing of reactors requires capacitive power in the test field equal to the nominal power of

the reactor while a transformer can be tested with a reactive power equal to 10 – 20% of the

transformer power rating by feeding the transformer with nominal current in short –circuit

condition.

The loss in the various parts of the reactor (12R, iron loss & additional loss) cannot be

separated by measurement. It is thus preferable, in order to avoid corrections to reference

temperature, to perform the loss measurement when the average temperature of the winding

is practically equal to the reference temperature.

Shunt reactors carry out different types of tasks:

They compensate the capacitive reactive power of the transmission cables, in particular in

networks with only light loads or no load.

They reduce system-frequency overvoltage when a sudden load drop occurs or there is no

load.

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They improve the stability and efficiency of the energy transmission.

Power line carrier line Traps:

Power line carrier communication (PLCC) is mainly used for telecommunication, tele-

protection and tele-monitoring between electrical substations through power lines at high

voltage, such as 110 kV, 220 kV, and 400 kV. PLCC integrates the transmission of

communication signal and 50/60 Hz power signal through the same electric power cable. The

major benefit is the union of two important applications in a single system.

In a PLCC system the communication is established through the power line. The audio

frequency is carried by a carrier frequency and the range of carrier frequency is from 50 kHz to

500 kHz. The modulation generally used in this system is amplitude modulation. The carrier

frequency range is allocated to include the audio signal, protection and the pilot frequency. The

pilot frequency is a signal in the audio range that is transmitted continuously for failure

detection.

The voice signal is converted/ compressed into the 300 Hz to 4000 Hz range, and this audio

frequency is mixed with the carrier frequency. The carrier frequency is again filtered, amplified

and transmitted. The transmission of these HF carrier frequencies will be in the range of 0 to

+32db. This range is set according to the distance between substations.

PLCC can be used for interconnecting PBXs. The electricity board in India has an internal

network PLCC between PBXs.

The purpose of PLC line traps

Provision of defined high voltage line impedances regardless of the configuration of the primary

system switchgear.

Prevention of signal losses due to propagation into other lines.

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Attenuation of RF signals from other parts of the power system, thus permitting multiple uses of

the same frequency bands.

PLC line traps are connected in series with the high-tension lines and must therefore be rated for

the maximum continuous load current and be able to withstand the maxi- mum fault current at the

place of

Installation. DLTC line traps fulfill all the RF requirements as well as all the power system

requirements of the latest IEC and ANSI recommendations.

Capacitance Coupled Voltage Transformer (CCVT):

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is

a transformer used in power systems to step down extra high voltage signals and provide a low

voltage signal, for measurement or to operate a protective relay. In its most basic form the device

consists of three parts: two capacitors across which the transmission line signal is split,

an inductive element to tune the device to the line frequency, and transformer to isolate and

further step down the voltage for the instrumentation or protective relay. The device has at least

four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two

secondary terminals which connect to the instrumentation or protective relay. CVTs are typically

single-phase devices used for measuring voltages in excess of one hundred kilovolts where the

use of voltage transformers would be uneconomical. In practice, capacitor C1 is often

constructed as a stack of smaller capacitors connected in series. This provides a large voltage

drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are

used for filtering high frequency communication signals from power frequency. This forms a

carrier communication network throughout the transmission network.

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Surge Arresters:

Each piece of electrical equipment in an electrical system needs to be protected from

voltage surges. To prevent damage to electrical equipment, surge protection

considerations are paramount to a well- designed electrical system. Modern metal

oxide arresters provide exceptional overvoltage protection of equipment connected to

the power system. The proper selection and application of the arrester, however,

involves decisions in several areas, which will be discussed in the paper. The original

lightning arrester was nothing more than a spark air gap with one side connected to a

line conductor and the other side connected to earth ground. When the line-to-ground

voltage reached the spark-over level, the voltage surge would be discharged to earth or

ground. The modern metal oxide arrester provides both excellent protective

characteristics and temporary overvoltage capability. The metal oxide disks maintain a

stable characteristic and sufficient non-linearity and do not require series gaps. Due to

the broad nature of this subject, this paper will concentrate on the application of the

gapless metal oxide arrester to circuits and systems rated 1000 V and greater.

A lightning arrester is a device used on electrical power systems to protect the insulation on the

system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for

power system protection since the mid 1970s. The typical lightning arrester also known

as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or

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switching surge travels down the power system to the arrester, the current from the surge is

diverted around the protected insulation in most cases to earth.

Circuit Breakers:

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical

circuit from damage caused by overload or short circuit. Its basic function is to detect a fault

condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike

a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either

manually or automatically) to resume normal operation. Circuit breakers are made in varying

sizes, from small devices that protect an individual household appliance up to

large switchgear designed to protect high voltage circuits feeding an entire city.

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Operation

All circuit breakers have common features in their operation, although details vary substantially

depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually

done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually

arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.

The trip solenoid that releases the latch is usually energized by a separate battery, although some

high-voltage circuit breakers are self-contained with current transformers, protection relays, and

an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit;

some mechanically-stored energy (using something such as springs or compressed air) contained

within the breaker is used to separate the contacts, although some of the energy required may be

obtained from the fault current itself. Small circuit breakers may be manually operated; larger

units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also

withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper

or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the

erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually

discarded when the contacts are worn, but power circuit breakers and high-voltage circuit

breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and

extinguished in a controlled way, so that the gap between the contacts can again withstand the

voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the

medium in which the arc forms. Different techniques are used to extinguish the arc including:

Lengthening of the arc

Intensive cooling (in jet chambers)

Division into partial arcs

Zero point quenching (Contacts open at the zero current time crossing of the AC

waveform, effectively breaking no load current at the time of opening. The zero crossing

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occurs at twice the line frequency i.e. 100 times per second for 50Hz and 120 times per

second for 60Hz AC)

Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore

power to the interrupted circuit.

Arc interruption

Circuit breakers are usually able to terminate all current very quickly: typically the arc is

extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon

age and construction of the device.

High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage breakers.

The definition of high voltage varies but in power transmission work is usually thought to be

72.5 kV or higher, according to a recent definition by the International Electro technical

Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current

sensing protective operated through current transformers. In substations the protective relay

scheme can be complex, protecting equipment and busses from various types of overload or

ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Bulk oil

Minimum oil

Air blast

Vacuum

SF6

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Some of the manufacturers are ABB, GE (General Electric) , AREVA, Mitsubishi Electric,

Pennsylvania Breaker, Siemens, Toshiba, BHEL and CGL.

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF 6 gas

to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking

mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage

AC circuit breakers are routinely available with ratings up to 765 kV. 1200KV breakers are

likely to come into market very soon.

High-voltage circuit breakers used on transmission systems may be arranged to allow a single

pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this

improves the system stability and availability.

Sulfur hexafluoride (SF6) high-voltage circuit-breakers

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to

quench the arc. They are most often used for transmission-level voltages and may be

incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or

de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.

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Air Plant System:

During repairing of different elements of grid station, air plant system is used. Air plant system

m contains a small size cylinder in which the air is stored after compression by the small size

compressor. During maintenance and routine cleaning of the different elements, compressed air

is through on the external surface of the particular element.

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Isolator Switch:

In electrical engineering, isolator switch is used to make sure that an electrical circuit can be

completely de-energized for service or maintenance. Such switches are often found in electrical

distribution and industrial applications where machinery must have its source of driving power

removed for adjustment or repair. High-voltage isolation switches are used in electrical

substations to allow isolation of apparatus such as circuit breakers and transformers, and

transmission lines, for maintenance. Often the isolation switch is not intended for normal control

of the circuit and is only used for isolation.

Isolator switches have provisions for a padlock so that inadvertent operation is not possible

(see: Lock and tag). In high voltage or complex systems, these padlocks may be part of

a trapped-key interlock system to ensure proper sequence of operation. In some designs the

isolator switch has the additional ability to earth the isolated circuit thereby providing additional

safety. Such an arrangement would apply to circuits which inter-connect power distribution

systems where both end of the circuit need to be isolated.

The major difference between an isolator and a circuit breaker is that an isolator is an off-

load device intended to be opened only after current has been interrupted by some other control

device. Safety regulations of the utility must prevent any attempt to open the disconnect or while

it supplies a circuit.

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Current Transformer:

In electrical engineering, a current transformer (CT) is used for measurement of electric currents.

Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are

known as instrument transformers. When current in a circuit is too high to directly apply to

measuring instruments, a current transformer produces a reduced current accurately proportional

to the current in the circuit, which can be conveniently connected to measuring and recording

instruments. A current transformer also isolates the measuring instruments from what may be

very high voltage in the monitored circuit. Current transformers are commonly used in metering

and protective relays in the electrical power industry.

The CT is typically described by its current ratio from primary to secondary. Often, multiple

CTs are installed as a "stack" for various uses. For example, protection devices and revenue

metering may use separate CTs to provide isolation between metering and protection circuits,

and allows current transformers with different characteristics (accuracy, overload performance)

to be used for the different purposes.

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Transformer:

A transformer is a device that transfers electrical energy from one circuit to another circuit

through inductively conductors—the transformer's coils. A varying current in the first or primary

winding creates a varying magnetic flux in the transformer's core and thus a varying field

through the secondary winding. This varying magnetic field induces a varying electromotive

force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and

electrical energy will be transferred from the primary circuit through the transformer to the load.

In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the

primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to

the number of turns in the primary (Np) as follows:

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By appropriate selection of the ratio of turns, a transformer thus allows an alternating current

(AC)voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by

making Ns less than Np.

In the vast majority of transformers, the windings are coils wound around a ferromagnetic

core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage

microphone to huge units weighing hundreds of tons used to interconnect portions of power

grids. All operate with the same basic principles, although the range of designs is wide. While

new technologies have eliminated the need for transformers in some electronic circuits,

transformers are still found in nearly all electronic devices designed for household ("mains")

voltage. Transformers are essential for high voltage power transmission, which makes long

distance transmission economically practical.

Basic principles

The transformer is based on two principles: first, that an electric current can produce a magnetic

field (electromagnetism), and, second that a changing magnetic field within a coil of wire

induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in

the primary coil changes the magnetic flux that is developed. The changing magnetic flux

induces a voltage in the secondary coil.

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Classification

Transformers can be classified in many different ways; list is:

By power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA;

By frequency range:  power-, audio-, or radio frequency;

By voltage class: from a few volts to hundreds of kilovolts;

By cooling type: air-cooled, oil-filled, fan-cooled, or water-cooled etc…

By application: such as power supply, impedance matching, output voltage and

current stabilizer, or circuit isolation;

By purpose:  distribution, rectifier, arc furnace, amplifier output, etc.;

By winding turns ratio: step-up, step-down, isolating with equal or near-equal

ratio, variable, and multiple windings.

By Supply: Single and three Phase transformers.

By special application: current and Voltage transformer.

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Auto Transformer:

The auto transformer is being used in 500KV/ 220KV grid station.

Fire Protection System:

Fire protection system is used in the electrical grid station to overcome the fire produced by any

cause, so the equipments will work under the safe condition and the chances of burning of the

electrical and other equipments used in the grid station is reduced. When the fire is produced by

any electrical or environmental reason, fire protection system will enables to control the

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produced fire. Different methods are used to make combustion impossible in the grid station, but

in 500 KV grid station, NTDC, Multan, water is primarily used in the fire protection system.

Nature of Fires

Three essentials are needed for the fire

Fuel

Oxygen

Heat

To bring fuel to its ignition point

Classification of Fires

Four classes of fires are…

Class A

Paper, wood, textiles and rubbish

Class B

Liquids such as alcohol, oil and grease

Class C

Electrical

Class D

Occur in certain metals like magnesium, sodium, potassium and titanium.

Principles of Extinguishing Fires

Cool the fuel below its ignition point

Remove the oxygen supply

Separate the fuel from the oxygen

Extinguishing Agents

Class A

Respond best to water or water type which lower the fuel below its ignition point.

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Class B

Respond to carbon dioxide, halogenated hydrocarbons, and dry chemicals, all of

which displace the oxygen supply making combustion impossible.

Protective Relays:

A relay is a device that “detects” the fault and “directs” the circuit breaker to isolate the

faulty part/equipment from the system.

Function of Relay

A relay performs three functions.

Sensing.

Comparing.

Tripping.

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It senses the “fault”. This is done by the relay to “respond” to the change if any, in the

currents passing through it.

It compares the current through it with the designed value of current. It responds only if the

current through it is different from its designed current rating.

If the current through it is different from its designed current rating, it sends information to

the circuit breaker for tripping.

Qualities of a Good Relay

In order to perform its function successfully, a relay should have the following qualities.

Selectivity.

Speed.

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Sensitivity

Reliability

Simplicity.

Economical

Relays Applications

Ground fault

Phase fault

Bus protection

Transformer protection

Transmission Line protection

Auxiliary

Bus Bars and Bus Coupler:

In electrical power distribution, a bus bar is a thick strip of copper or aluminum that conducts

electricity within a switchboard, distribution board, substation or other electrical apparatus. Bus

bars are used to carry very large currents, or to distribute current to multiple devices within

switchgear or equipment. For example, a household circuit breaker panel board will have bus

bars at the back, arranged for the connection of multiple branch circuit breakers. An aluminum

smelter will have very large bus bars used to carry tens of thousands of amperes to

the electrochemical cells that produce aluminum from molten salts.

When a number of generators or feeders operating at the same voltage have to be directly

connected electrically, bus bars are used as the common electrical component. The size of the

bus bar is important in determining the maximum amount of current that can be safely carried.

Bus bars can have a cross-sectional area of as little as 10 mm² but electrical substations may use

metal tubes of 50 mm in diameter (1,963 mm²) or more as bus bars.

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A bus bar may either be supported on insulators, or else insulation may completely surround it.

Bus bars are protected from accidental contact either by a metal earthed enclosure or by

elevation out of normal reach. Neutral bus bars may also be insulated. Earth bus bars are

typically bolted directly onto any metal chassis of their enclosure. Bus bars may be enclosed in a

metal housing, in the form of bus duct or bus way, segregated-phase bus, or isolated-phase bus.

Bus bars may be connected to each other and to electrical apparatus by bolted or clamp

connections. Often joints between high-current bus sections have matching surfaces that are

silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in

outdoor buses, corona around the connections becomes a source of radio-frequency interference

and power loss, so connection fittings designed for these voltages are used.

Following bus bar schemes are used in the field of electrical power system…

Single bus bar scheme

Sectionalizing bus bar scheme

Double bus bar scheme

Terminal section scheme

Main and transfer bus scheme

Ring bus scheme

Double Busbar Scheme

In 500KV grid station, the double bus bar and one & half circuit breaker scheme is being

used. The following diagrams are double bus bar and one & half circuit breaker scheme.

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Insulators:

Materials that do not have any free electrons. Because of this fact, they do not tend to share their

electrons very easily and do not make good conductors of electrical currents.

Electrical insulation is the absence of electrical conduction. Electronic band theory (a branch of

physics) says that a charge will flow if states are available into which electrons can be excited.

This allows electrons to gain energy and thereby move through a conductor such as a metal. If no

such states are available, the material is an insulator.

Most insulators have a large band gap. This occurs because the "valence" band containing the

highest energy electrons is full, and a large energy gap separates this band from the next band

above it. There is always some voltage (called the breakdown voltage) that will give the

electrons enough energy to be excited into this band. Once this voltage is exceeded, the material

ceases being an insulator, and charge will begin to pass through it. However, it is usually

accompanied by physical or chemical changes that permanently degrade the material's insulating

properties.

Materials that lack electron conduction are insulators if they lack other mobile charges as well.

For example, if a liquid or gas contains ions, then the ions can be made to flow as an electric

current, and the material is a conductor. Electrolytes and plasmas contain ions and will act as

conductors whether or not electron flow is involved.

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Insulators suffer from the phenomenon of electrical breakdown. When the electric field

applied across an insulating substance exceeds in any location the threshold breakdown field for

that substance, which is proportional to the band gap energy, the insulator suddenly turns into a

resistor, sometimes with catastrophic results. During electrical breakdown, any free charge

carrier being accelerated by the strong electric field will have enough velocity to knock electrons

from (ionize) any atom it strikes. These freed electrons and ions are in turn accelerated and strike

other atoms, creating more charge carriers, in a chain reaction. Rapidly the insulator becomes

filled with mobile carriers, and its resistance drops to a low level. In air, "corona discharge" is

normal current near a high-voltage conductor; an "arc" is an unusual and undesired current.

Similar breakdown can occur within any insulator, even within the bulk solid of a material. Even

a vacuum can suffer a sort of breakdown, but in this case the breakdown or vacuum arc involves

charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.

Different types of insulators are being used in the power transmission system and in the grid

stations. In grid stations, at extra high voltage, the bushing type insulators are mostly used.

Switch board and Control Room:

An electric switchboard is a device that directs electricity from one source to another. It is an

assembly of panels, each of which contains switches that allow electricity to be redirected. The

operator is protected from electrocution by safety switches and fuses.

There can also be controls for the supply of electricity to the switchboard, coming from a

generator or bank of electrical generators, especially frequency control of AC power and load

sharing controls, plus gauges showing frequency and perhaps a synchroscope. The amount of

power going into a switchboard must always equal to the power going out to the loads. Inside the

switchboard there is a bank of bus bars - generally wide strips of copper to which the switchgear

is connected. These act to allow the flow of large currents through the switchboard, and are

generally bare and supported by insulators.

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A control room is a room serving as an operations centre where a facility or service can be

monitored and controlled.

A control room can, at times, be designated as an area of refuge, particularly in high risk

facilities, such as a nuclear power station or a petrochemical facility, as an accidental fire can

have severe repercussions to the surrounding environment. As is typical for all areas of refuge,

occupants must be provided with guaranteed life support and guarantee of functionality of the

items they are intended to control for the anticipated design-basis fire event.

It is not unusual to provide control rooms with gaseous fire suppression systems to safeguard its

contents and occupants.

The primary equipment in control rooms is housed in multi-function cabinets. Since the control

equipment is intended to control other items in the surrounding facility, it follows that these

(often fire-resistance rated) service rooms require many penetrations. Due to routine equipment

updates, penetrates, such as cables are subject to frequent changes. It follows that an operating

control room maintenance program must include vigilant fire stop maintenance

for code compliance and for gaseous fire suppression systems to work as well. Due to the nature

of the sensitive equipment inside control room cabinets, it is useful to ensure the use of "T-

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rated" fire stops, that are massive and thick enough to absorb penetrate heat in an effort to reduce

heat transmission to the inside of the control room. It is also not uncommon to place control

rooms under positive air pressure to prevent smoke from entering. To put into nutshell, function

of the control room is to monitor, control, switching of the electrical power and to protect the

whole system from any harmful problem with the help of the associated electrical equipments

inside the control room.

Components of the Control Room

Protection Relays

Auto Transformer Bank (ATB) Panel

Bus Bar Panel

Shunt Reactor Panel

Rectifier’s

Fire Extinguisher

Battery Room

Isolator control panel

Circuit breaker control panel

Tape changer control panel

Lay out drawings

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