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Transformers & Electrical

Distribution SystemsHSC Module 9.3 Motors

& Generators

Copyright Jeff Piggott, 2003. All rights reserved.


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Objectives Discuss why some electrical appliances in

the home that are connected to the mainssupply use a transformer.

Identify some of the energy transfers and

transformations involving the conversion ofelectrical energy into more useful forms inthe home and industry

Analyse the impact of the development of

the transformer on society.


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Transformer:

Basic Structure A transformer consists of two or morecoils coupled magnetically by way of acore.

Side (coil) of transformer where source(or input) voltage/current is applied =primary coil.

Side (coil) of transformer where induced

(or output) voltage/current is produced =secondary coil.


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Transformer:

Principle of Operation A transformeroperates on theprinciple ofmutual

inductance ie. a changing current

in one coil(primary) induces

an emfin another(secondary) coil.


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Purpose and Principle of

the Transformer1. The changing currentin the primary coil, is

usually achieved by applying

an alternating voltage,

resulting in an alternating

current (AC)

AC in

put

AC output

2. As the alternating current changesmagnitude and direction, a magneticfield is produced, which changes in a

corresponding manner

3. The field from the primary coil isintensified and concentrated (alsoreferred to as increasing the flux

linkage) through thesecondary coil by an iron core

4.The changing flux through thesecondary coil, induces apotential difference across thesecondary coil


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Step-Up TransformerFlux. f

AC Input

Primary Coil SecondaryCoil

Core

AC Input

Flux. fAC Output(increased!)


Core

# turns on secondary > # of turn on primary

ns > np


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The Induction Coil Induction coil = step-up transformer with a muchgreater number of turns on the secondary

(~5 000) than on the primary (typically < 100).

Input voltage = 6V; Output voltage =~30 000V


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Operation of

an Induction CoilNOTE: Pulsed DC is used because the rate of change of fluxis much greater than that produced by 6 V AC.

+

I

iron cored coil

reed switch

DC supply

Electrical contact broken

as coil becomes magnetised- magnetic field starts tocollapse.

I

+

Field buildswhen currentflows in coil.


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Step-Down TransformerFlux. f

AC Outputdecreased


Core

AC Input

# turns on secondary < # of turn on primary

ns < np


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Provides a channel for magneticfields (enables redirection andstrengthening of magnetic field)f= total magnetic field lines (inWb)

B= flux density = # of field lines/unit area (in teslas, T)

Transformer Core

B = f / A


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Core Material Amount of flux produced in the core depends on aproperty of the core material - permeability, m,a constant for different types of material.

Materials that cause lines of flux to move further

apart ie. decrease flux density are calleddiamagnetic; those that concentrate flux by 1 10 times are called are called paramagnetic; andthose that concentrate flux by >10 times arecalled ferromagnetic.

Certain ferromagnetic materials, especiallypowdered or laminated iron, steel, or nickel alloys,have that can range up to about 1,000,000.


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Transformer Equation In ideal transformers, there is no power loss and powerinput to primary coil equals power output from secondarycoil.

The rate of change of flux in both coils is the same, =Df/Dt.

From Faradays Law (e=-Df/Dt)to:(i) the secondary coil: VS = nSDf/Dt..(1)(ii) the primary coil: VP = nPDf/Dt.(2)Dividing equation (1) by equation (2):

VP/VS = nP/nS


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Transformers and

Conservation of EnergyThe Principle of Conservation of Energy states that:Energy cannot be created or destroyed, merely changedfrom one form to another.This means that energy obtained from secondary coil, at

most (without heat losses), can only equal energy supplied toprimary coil. Also, since power = rate of supply of energy:Pprimary = PsecondaryBut P=VI, therefore:VPIP = VSIS

Combining this equation with the transformer equationgives:

IS/IP = nP/nS


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Eddy Currents Eddy currentsare induced currents that resultwhen there is a B field acting on part of ametal object and there is relative movementbetween the object and the field, such thatthe conductor cuts across magnetic flux lines.

Eddy currents arecircular currents.

They are an

application ofLenzs Law.

Eddycurrent

motion

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X

X X X X X X X X X X


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Eddy Currents Reduce

Transformer Efficiency Energy output of a real transformer is always less than theenergy input.

Energy losses occur because eddy currents induced in the

transformer core by the alternating current, result inresistive heat losses (the transformer core heats up).

Energy input Energy output

energy losses

Input240 V

Output12 V

transformer

The ratio of the energy output to the energyinput, expressed as a percentage is called theefficiency of the transformer.


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Core LaminationsSplitting the core intolaminations thin sheets reduces effects of eddy currents by restrictingthem to shorter pathways.

Laminated iron core

Insulatinglayers


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Effect of Core

Lamination ThicknessLamination Thickness(mm)

Eddy Current Losses

0.27 to 0.36 0.950.10 to 0.25 0.90

0.0508 0.85

0.0254 0.750.0127 0.50


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Transformers &

Electrical Distribution

In Australia, 23,000V AC generated, 330,000V or500 kV AC HV transmission line, 240VAC 50 Hz

end use single phase, 415VAC 50 Hz 2 and 3-phase.


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Electric Power Distribution

System - Structure The typical delivery system for the supply of electricalpower is based on central-station service.

Thepower generating stationproduces AC electricity Step-up transformersincrease the voltage level of the

electricity for bulk transmission Transmission linescarry large amounts of electricity across

the nation. Substation transformerslower voltage so that electricity

can be delivered to local homes and businesses. The electricity reaches the customer over a system of

distribution wires.


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Commercial Power

Generators Commercial powerstations use ACgenerators to produce

their electrical energy. AC generators are

preferred because:

(i) Easy to step up AC

emfs to higher voltagesfor transmission.

(ii) AC electricitytransmitted with lowenergy losses.


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Step-up Transformers at

Power Generation Plants Electricity generated at a power

station is usually produced at avoltage ranging from a few

hundred volts to tens of kilovolts.(Eraring power station at LakeMacquarie has four 660 MWgenerators with an output of 23kV).

It is transformed to 330 kV or500 kV for transmission over thedistribution grid.


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Transmission Grid

Conductors The transmission grid consists of high voltageoverhead lines and underground cable made ofeither copper or aluminium.

Copper is much heavier than aluminium so it is

used primarily in insulated wires and cables. Aluminium is suitable for transmission and

distribution and allows the use of much lighterand more economical support structures. Thetensile strength of pure aluminium is not high

enough for most applications so aluminium alloysor steel reinforced aluminium alloys are used.


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Electrical Transmission Lines

Insulation of WiresIn dry air, electrical sparkscan jump the followingdistances for the given

potential differences:

10 000 V --------- 1 cm

20 000 V --------- 2 cm

100 000 V ------- 10 cm

330 kV -------- 33 cm

*Distances smaller in very humid air


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High-Voltage Insulator

Prevents electrical sparksjumping from high voltage

lines to support poles or towers.

Insulators made of individual

sections: (i) Shape prevents build up

of dust or grime (which

conducts when it absorbs water)

(ii) Increases distance current

must flow over insulator surfaces,

so decreases chance of sparking.

StaticDischargers

Transmissioncable

Disk-shapedceramic/glassinsulators

Suspension insulatorfor 330 kV

transmission line


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Why Ceramic or

Glass Insulators? Glass and ceramics lack a crystalstructure - called amorphousmaterials.

To conduct electricity, a materialmust have "free" electrons (notthe same as excess electrons).

In glass and ceramics all of the

electrons are localised ie. bound toa nucleus, whereas in metals, someelectrons (free electrons) arenot bound to nuclei conduction.

Early glass

electrical insulator


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Electrical Transmission Lines Protection from Lightning Strike

Lightning usually strikeshighest point.

Electrical transmission

systems usually use a singlecable continuous earth line -running between poles & sittingabove the 4 transmission lines(3 phase lines and return

ground line)

Continuous earth linenormally carries no current -conducts charge from lightning

strike to earth.


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Powerline Energy Losses

Low resistance transmissioncables used so that resistiveheating and energy loss are

minimised.Power is transmitted at highvoltages [500 kV typical] , thusreducing the magnitude of the

current, I, flowing in the lines.P= VI

* Resistive heat losses:

Plost = I2R where I is small


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WARNING: Two Types

of Voltage to Consider There are two voltages to consider in mostelectrical transmission problems:

Floating voltage = voltage of transmission(energy per coulomb given to charges at theswitching yard).

This CANNOT be used in:P=VIto find power loss in wires!!

Voltage loss = difference in voltage at either

end of transmission line (energy per coulomb lostby charges during transmission). This is mosteasily found from V=IR

where I = current transmittedR =total resistance of transmission wire.


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Sample Problem

Power LossA generator produces 20 kW of power at 200 V. The 1.0km long transmission lines over which the power istransmitted have a total resistance of 0.50 W. Determinethe power lost in the lines and the voltage available at the

end of the lines.

Solution:(i) P = VI I = P/VI = 20000 / 200 = 100A.

Power lost in wires:P = I2R = (100)2 X 0.5 = 5 kW. (ii) Voltage loss during transmission: V= IR V = 100 x 0.5 = 50 V

Therefore, voltage available = 200 V 50 V = 150 V


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Superconducting

Transmission Lines Superconducting transmission cable is atechnology intended to increase transmissioncapability. High temperature superconductivity(HTS) cable has no resistance.

HTS has the potential to deliver twice the powercapacity with the same power loss and smallerdiameter as conventional cable.

One potential design which is well-suited forretrofitting in networks has an HTS conductor

enclosed in a cryogenic environment which iscovered by conventional room-temperaturedielectric. Prototype cable systems have been

developed in the US and actualsystems are expected there over thenext few years.


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Sub-Stations &

Local TransformersStep-down transformers are required at localsubstations to step down the very high voltagesfrom transmission lines to lower voltages (11 kV) for

suburban distribution. Finally, local transformersstep the voltages down further for domestic use(240 V).


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Household Uses of

Transformers Step-down transformers arefound in all electronic devices thatcan be run from the domestic 240 V

AC supply, since most electronicdevices require low voltages tooperate the semiconductorcomponents that they depend fortheir operation, for example, a

computer will include componentsthat run on 12V, 5V or 1.5 V.

If not AC, otherwise would have to be providedby batteries = high cost.

TVs need high voltages to function.


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Transformers & Electrical

Appliances in the HomeAppliances without

a transformerAppliances with a

transformer

kettle, hot waterheater, toaster, olderroom heaters, hairdryers, incandescentlights, old model

refrigerators, someclothes dryers

TV, stereo, computer, CDplayer, clock radio,fluorescent lights, homesecurity systems, microwaveoven, answering machines,

air conditioner, faxmachines, washing machines,microwave oven

Electronically operated domestic appliances require both astep-down transformer to change 240 volts to about 5 -

20 volts & a rectifier to change the low voltage AC to DC.


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Energy Transfers

in the Home (1)Much of the energy transferred in homes iselectrical energy. This is because electricalenergy is readily transferred as:

a)heat(thermal energy)b)lightc)soundd)kinetic energy(movement).Amount of electrical energy transferred depends

on:a)timeappliance is switched on;b) appliance powerratingW [work] = P [power] x t[time]


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Energy Transfers

in the Home (2)Household Appliance Energy Transformation

Television

Radio Blender

Air conditioner

Electric drill Hair dryer

VCR

Washing machine

Copy and complete the table below.


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Example:

Domestic Transformer For the transformer shown

here:

a) What is the ratio of the

number of turns on theprimary to the number ofturns on the secondarycoil?

b) Suggest a possible use

for this transformer.


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Transformers

Problem #1 A transformer has input voltage andcurrent of 12.0 V and 3.0 A. It has anoutput current of 0.75 A.

a) If there are 1200 turns on thesecondary coil, how many turns are onthe primary?

b) What is the output voltage?


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Transformers

Problem #2 An ideal transformer has 100 turns on theprimary coil and 2 000 turns on the secondarycoil. The primary voltage is 20 V. The currentin the secondary coil is 0.5 A.

a) What is the secondary voltage? b) What is the output power? c) What is the input power? d) What is the current flowing through the

primary coil?


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Impact of Transformers

on Society (1) The first practical transformer, using AC, wasdeveloped in 1883.

Prior to this, direct current was seen as being thelogical way to distribute energy using electricity.

AC triumphed, and by the early 1900s, its futureimpact on society was inevitable.

Transformers permitted the long-distancetransfer of electrical energy with low resistiveenergy losses.

Without the high voltages possible through theuse of transformers, the electrical wires required

to transmit large amounts of electricalenergy would have to have been toolarge to be practical.


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Impact of Transformers

on Society (2) Transformers were a key to establishing electricalenergy as the driving force behind technological andindustrial development in the 20th century.

Electrical energy rapidly became the means oflighting homes and cities, with its distribution

facilitated by the use of transformers. Electrically operated machines thus replaced less efficientmachines, resulting in the rapid growth of industry andcommerce.

Communication networks grew rapidly as a result ofelectrical energy and its intimate association with radio,then television and ultimately the computer revolution of

the late 20th century. Every home has dozens of appliances that make use oftransformers, permitting a host of electronicdevices to be operated from the mains.


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Effect of High Voltage

Power Lines on Humans

+

--

-

--

--

--+

+ ++

+ +

-

-

+

++

++

++

+-

- --- -

-

+

Alternating E field induces analternating current to flow inbodySign changes 100 times/ s.


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What are the Health

Implications? Studies still in progress At least one study has shown that

exposure to strong electric andmagnetic fields increases likelihoodof developing cancers and leukemia.


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Phasing and em Radiation

Exposure Levels

-

--

--

--

--+

+ ++

-

+

++

+

+ ++

+

-

+

Phasing assists to reduce the Efields when multiple power linesare present.Code related health effects

refer to wiring codes where theconductors are far apart.The closer the supply andreturn wires are together, thelower the fields due to phase

cancellation


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Phasing and em Radiation

Exposure Levels (cont) Phasing assists toreduce the B fieldswhen multiple powerlines are present aswith E-field.

Dynamic magneticfield causescurrents to flow in

a circular fashionwithin the body. They will reverse

100 times / second

Current direction

B Magnetic field arising from current

Induced

current in

body


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em Field Exposure Typical values: Under power line 10 microT and 10,000 V/m 10m from 12kV line 0.2-1 microT and 2-20

V/m Within home 150-0.02 microT depending on

proximity to electrical appliances >0.20 microT at 1m distance only for

washing machines,dishwashers, can openers,microwave ovens

Electric train ~ 60microT at seat

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