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INTRODUCTIONINTRODUCTION

TOTOELECTRICALELECTRICAL

POWERPOWER

DISTRIBUTIONDISTRIBUTION

byby

N.Ramchander,N.Ramchander,Asst Professor,Asst Professor,

EEEDEEED--BVRIT.BVRIT.

byby

N.Ramchander,N.Ramchander,Asst Professor,Asst Professor,

EEEDEEED--BVRIT.BVRIT.

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IntroductionIntroduction

HistoryHistory ofof ElectricityElectricity (General(General Concept)Concept)

CharacteristicsCharacteristics ofof PresentPresent DistributionDistribution SystemSystemDistributionDistribution SystemSystem LossesLosses

DistributionDistribution SystemSystem PlanningPlanning

FactorsFactors AffectingAffecting DistributionDistribution SystemSystem PlanningPlanning

LoadLoad ModelingModeling andand theirtheir CharacteristicsCharacteristics

ClassificationClassification ofof loadsloads andand theirtheir CharacteristicsCharacteristics

CONTENTS

CONTENTS

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INTRODUCTION

INTRODUCTION

Classical Structure of Power System

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Modern View of Power System

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Present Power Scenario in INDIA

Present Power Scenario in INDIA

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In INDIA, phenomenal expansion has taken place in the

in the area of power systems since last FIVE DECADES.

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For example generation capacity has grown at an

average rate of 12% and reached a level of more than1,57,000MW(Approx) as on today.

Similarly the development of transmission network

followed the growth, starting with a meager Tr.network,

with 132KV as the highest Tr. Voltage at the time of

Independence, today beside HVDE lines, we have

765KV as the highest Tr.voltage.

And five regional grids under operation and formation

of a National Grid inter connecting all the regional grids

in under progress (like National Highways) In our country out of total 5,87,258 villages as per 1991

survey more than 86% villages have been electrified, 13

states are 100% electrified.

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Distribution SystemDistribution SystemDistribution SystemDistribution System

The distribution system is particularly important to anelectrical utility for two reasons

1. its close proximity to the customers

2. its high cost of investment

Mainly distribution systems are two types

1. Primary Distribution (33KV/11KV)

2. Secondary Distribution (11KV/440V)

Household electricity is alternating current (AC)

Household voltages are typically 120V or 240V

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Distribution System Considerations

In determining the design of distribution systems, three

broad classifications of choices need to be considered:

1. The type of electric system: dc or ac, and if ac,

single- phase or poly phase.

2. The type of delivery system: radial, loop, or network.

Radial systems include duplicate and throwover systems.

3. The type of construction: overhead or underground.

DESIRED FEATURES

1. Safety

2. Smooth and Even Flow of Power

3. Economy

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Direct Current Systems

Alternating Current Single-Phase Systems1. Two-wire Systems2. Three-wire Systems

3. Series system

Alternating Current Two-phase Systems1. Four-wire Systems

2. Three-wire Systems

3. Five-wire Systems

Alternating Current Three-phase Systems1. Four-wire Systems

2. Three-wire Systems

Alternating Current Six-phase Systems

1. Six-wire Systems

2. Seven-wire Systems

Types of Electric Systems

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Direct Current Systems

Alternating Current Single-Phase Systems

1. Two-wire Systems

AC single-phase two wire system.

2. Three-wire Systems

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3. Series system

Alternating Current Two-phase Systems

1. Four-wire Systems

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2.Three-wire Systems

3.Five-wire Systems

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Alternating Current Three-phase Systems

1. Four-wire Systems

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2.Three-wire Systems

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Alternating Current Six-phase Systems

1. Six-wire Systems

2.Seven-wire Systems

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Types of Delivery Systems:

1. Primary distribution, which carries the load at higher

than utilization voltages from the substation (or other source)to the point where the voltage is stepped down to the value at

which the energy is utilized by the consumer.

Primary distribution systems include three basic types:

1. Radial systems, including duplicate and throwover systems 2. Loop systems, including both open and closed loops

3. Primary network systems

2. Secondary distribution, which includes that part of thesystem operating at utilization voltages, up to the meter at the

consumers premises.

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FKEE, UMP

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Principle of Operation

Torque producing mechanismTorque producing mechanismWhen a 3 phase statorwinding is connectedconnected to a 3

phase voltage supply, 3 phase current will flowflow inin thethe

windingswindings, hence the stator is energized.A rotating flux is produced in the air gap. The flux

induces a voltage Ea in the rotor winding (like atransformer).

The induced voltage produces rotor current, if rotor

circuit is closed.The rotor current interacts with the flux , producing

torque. The rotor rotates in the direction of the rotatingflux.

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FKEE, UMP

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Direction of Rotor Rotates

Q: How to change the direction of

rotation?

A: Change the phase sequence of the power supply.

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Induction motor speed

At what speed will the IM run?

Can the IM run at the synchronous speed, why?

If rotor runs at the synchronous speed, which is the

same speed of the rotating magnetic field, then therotor will appear stationary to the rotating magnetic field

and the rotating magnetic field will not cut the rotor. So,

no induced current will flow in the rotor and no rotor

magnetic flux will be produced so no torque isgenerated and the rotor speed will fall below the

synchronous speed

When the speed falls, the rotating magnetic field will cut

the rotor windings and a torque is produced

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Induction motor speed

So, the IM will always run at a speed lowerthan

the synchronous speed

The difference between the motor speed and the

synchronous speed is called the Slip

Where nslip= slip speed

nsync= speed of the magnetic field

nm = mechanical shaft speed of the motor

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The Slip

sync m

sync

n ns

n

!

Wheres is theslip

Notice that : if the rotor runs at synchronous speed

s = 0

if the rotor is stationary

s = 1

Slip may be expressed as apercentageby multiplying the above

eq. by 100, notice that the slip is a ratio and doesnt have units

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Problem(1)

(p)A 208-V, 10hp, four pole, 60 Hz, Y-connected

induction motor has a full-load slip of 5 percent

1. What is the synchronous speed of this motor?

2. What is the rotor speed of this motor at rated load?

3. What is the rotor frequency of this motor at rated

load?

4. What is the shaft torque of this motor at rated load?

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Solution

1.

2.

3.

4.

120 120(60)1800

4

e

sync

fn rp

P! ! !

(1 )

(1 0.05) 1800 1710

m sn s n

rpm

!

! v !

0.05 60 3r ef sf Hz! ! v !

260

10 746 /41.7 .

1710 2 (1/ 60)

out out

loadmm

P P

n

hp watt hpNm

X

[T

T

! !

v

! !

v v

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Problem(2)

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Example 1

1. A 4 poles, 3J Induction Motor operates from a supplywhich frequency is 50Hz. Calculate:

a. The speed at which the magnetic field is rotating

b. The speed of the rotor when slip is 0.04

c. The frequency of the rotor when slip is 3%.

d. The frequency of the rotor at standstill

SolutionSolution

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Previous Questions from IM

1. Explain the working principle of three phase

induction motor. 2. Explain with the help of suitable diagrams how

rotating magnetic field is produced

in a three phase induction motor.

3. Explain with neat sketches the construction of asynchronous machine.

4. A 6 pole induction motor is fed by three phase 50

HZ supply and running with a full load slip of 3%. Find the full load speed ofinduction motor and also

the frequency of rotor emf.

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