Transformers & Transformers & Electrical Distribution Electrical Distribution

SystemsSystems

Transformers & Transformers & Electrical Distribution Electrical Distribution

SystemsSystems HSC Module 9.3 Motors HSC Module 9.3 Motors

& Generators& Generators

Copyright Jeff Piggott, 2003. All rights reserved.

ObjectivesObjectives• Discuss why some electrical appliances

in the home that are connected to the mains supply use a transformer.

• Identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

• Analyse the impact of the development of the transformer on society.

Transformer: Transformer: Basic StructureBasic Structure

• A transformer consists of two or more coils coupled magnetically by way of a “core”.

• 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”.

Transformer: Transformer: Principle of Principle of OperationOperation

• A transformer operates on the principle of mutual inductance ie.

• a changing current in one coil (primary) induces an emf in another (secondary) coil.

Purpose and Purpose and Principle of the Principle of the TransformerTransformer

1. The changing current in the primary coil, is usually achieved by

applying an alternating voltage, resulting in an

alternating current (AC)AC input

AC output

2. As the alternating current changes

magnitude and direction, a magnetic

field is produced, which changes in a

corresponding manner

3. The field from the primary coil is intensified and concentrated

(also referred to as increasing the “flux linkage”) through the

secondary coil by an iron core

4.The changing flux through the

secondary coil, induces a potential difference across

the secondary coil

Step-Up TransformerStep-Up Transformer

Flux.

AC Input

Primary Coil

Secondary Coil

Core

AC Input

Flux.

AC Output(increased!)

Primary Coil

Secondary Coil

Core

# turns on secondary > # of turn on primary

ns > np

The Induction CoilThe Induction Coil• Induction coil = step-up transformer with a

much greater number of turns on the secondary (~5 000) than on the primary (typically < 100).

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

Operation of Operation of an Induction Coilan Induction Coil

NOTE: Pulsed DC is used because the rate of change of flux

is 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 to collapse.

I

+

–

Field builds when current flows in coil.

Step-Down Step-Down TransformerTransformer

Flux.

AC Outputdecreased

Primary Coil

Secondary Coil

Core

AC Input

# turns on secondary < # of turn on primary

ns < np

• Provides a channel for magnetic fields (enables redirection and strengthening of magnetic field)

= total magnetic field lines (in Wb)

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

Transformer CoreTransformer Core

B = / A

Core MaterialCore Material• Amount of flux produced in the core depends on

a property of the core material - “permeability”, , – a constant for different types of material.

• Materials that cause lines of flux to move further apart ie. decrease flux density are called “diamagnetic”; those that concentrate flux by 1 – 10 times are called are called “paramagnetic”; and those that concentrate flux by >10 times are called “ferromagnetic”.

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

Transformer Transformer EquationEquation

• In ideal transformers, there is no power loss and power input to primary coil equals power output from secondary coil.

• The rate of change of flux in both coils is the same, = /t.

• From Faraday’s Law (=- /t)to:

(i) the secondary coil: VS = nS /t……..(1)

(ii) the primary coil: VP = nP /t………….(2)

Dividing equation (1) by equation (2):

VP/VS = nP/nS

Transformers and Transformers and Conservation of Conservation of

EnergyEnergyThe Principle of Conservation of Energy states that: “Energy cannot be created or destroyed, merely changed from one form to another.” This means that energy obtained from secondary coil, at most (without heat losses), can only equal energy supplied to primary coil. Also, since power = rate of supply of energy:Pprimary = Psecondary But P=VI, therefore:VPIP = VSIS

Combining this equation with the transformer equation gives:

IS/IP = nP/nS

Eddy CurrentsEddy Currents• Eddy currents are induced currents that

result when there is a B field acting on part of a metal object and there is relative movement between the object and the field, such that the conductor cuts across magnetic flux lines.

• Eddy currents arecircular currents.

• They are an application of Lenz’s Law.

Eddy curren

t

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

Eddy Currents Eddy Currents Reduce Transformer Reduce Transformer

EfficiencyEfficiency• Energy output of a real transformer is always less than

the energy input.

• Energy losses occur because eddy currents induced in the transformer core by the alternating current, result in resistive 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 energy input, expressed as a percentage is called the efficiency of the transformer.

Core LaminationsCore LaminationsSplitting the core into laminations – thin sheets – reduces effects of eddy currents by restricting them to shorter pathways.

Laminated iron core

Insulating layers

Effect of Core Effect of Core Lamination Lamination Thickness Thickness

Lamination Thickness (mm)

Eddy Current Losses

0.27 to 0.36 0.95

0.10 to 0.25 0.90

0.0508 0.85

0.0254 0.75

0.0127 0.50

Transformers & Transformers & Electrical Electrical

DistributionDistribution

In Australia, 23,000V AC generated, 330,000V or 500 kV AC HV transmission line, 240VAC 50 Hz end use single phase, 415VAC 50 Hz 2 and 3-phase.

Electric Power Electric Power Distribution System - Distribution System -

StructureStructure• The typical delivery system for the supply of electrical

power is based on central-station service.• The power generating station produces AC

electricity• Step-up transformers increase the voltage level of

the electricity for bulk transmission• Transmission lines carry large amounts of electricity

across the nation.• Substation transformers lower voltage so that

electricity can be delivered to local homes and businesses.

• The electricity reaches the customer over a system of distribution wires.

Commercial Power Commercial Power GeneratorsGenerators

• Commercial power stations use AC generators to produce their electrical energy.

• AC generators are preferred because:

• (i) Easy to step up AC emfs to higher voltages for transmission.

• (ii) AC electricity transmitted with low energy losses.

Step-up Transformers Step-up Transformers at Power Generation at Power Generation

PlantsPlants

• Electricity generated at a power station is usually produced at a voltage ranging from a few hundred volts to tens of kilovolts. (Eraring power station at Lake Macquarie has four 660 MW generators with an output of 23 kV).

• It is transformed to 330 kV or 500 kV for transmission over the distribution grid.

Transmission Grid Transmission Grid ConductorsConductors

• The transmission grid consists of high voltage overhead lines and underground cable made of either 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 lighter and more economical support structures. The tensile strength of pure aluminium is not high enough for most applications so aluminium alloys or steel reinforced aluminium alloys are used.

Electrical Transmission Electrical Transmission Lines – Insulation of Lines – Insulation of

WiresWires

In dry air, electrical sparks can jump the following distances 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

High-Voltage High-Voltage InsulatorInsulator

• Prevents electrical sparks jumping 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.

Static Dischargers

Transmission cable

Disk-shaped ceramic/glass insulators

Suspension insulator for 330 kV

transmission line

Why Ceramic or Why Ceramic or Glass Insulators?Glass Insulators?

• Glass and ceramics lack a crystal structure - called amorphous materials.

• To conduct electricity, a material must have "free" electrons (not the same as excess electrons).

• In glass and ceramics all of the electrons are localised ie. bound to a nucleus, whereas in metals, some electrons (“free” electrons) are not bound to nuclei conduction.

Early glass electrical insulator

Electrical Transmission Electrical Transmission Lines – Protection from Lines – Protection from Lightning StrikeLightning Strike

• Lightning usually strikes highest point.

• Electrical transmission systems usually use a single cable – continuous earth line - running between poles & sitting above the 4 transmission lines (3 phase lines and return ground line)

• Continuous earth line normally carries no current - conducts charge from lightning strike to earth.

Powerline Energy Powerline Energy LossesLosses

• Low resistance transmission cables used so that resistive heating and energy loss are minimised.

• Power is transmitted at high voltages [500 kV typical] , thus reducing the magnitude of the current, I, flowing in the lines.

P= VI

* Resistive heat losses:

Plost = I2R where I is small

WARNING: Two WARNING: Two Types of Voltage to Types of Voltage to

ConsiderConsider• There are two voltages to consider in most

electrical transmission problems:• “Floating voltage” = voltage of

transmission (energy per coulomb given to charges at the switching yard).

• This CANNOT be used in: P=VI to find power loss in wires!!

• “Voltage loss” = difference in voltage at either end of transmission line (energy per coulomb lost by charges during transmission). This is most easily found from V=IR

where I = current transmitted R =total resistance of

transmission wire.

Sample Problem – Sample Problem – Power LossPower LossA generator produces 20 kW of power at 200 V. The

1.0 km long transmission lines over which the power is transmitted have a total resistance of 0.50 W.

Determine the 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

Superconducting Superconducting Transmission LinesTransmission Lines

• Superconducting transmission cable is a technology intended to increase transmission capability. High temperature superconductivity (HTS) cable has no resistance.

• HTS has the potential to deliver twice the power capacity with the same power loss and smaller diameter as conventional cable.

• One potential design which is well-suited for retrofitting in networks has an HTS conductor enclosed in a cryogenic environment which is covered by conventional room-temperature dielectric. Prototype cable systems have been

developed in the US and actual systems are expected there over the next few years.

Sub-Stations & Sub-Stations & Local TransformersLocal Transformers

Step-down transformers are required at local substations to step down the very high voltages from transmission lines to lower voltages (11

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

Household Uses of Household Uses of TransformersTransformers

• Step-down transformers are found in all electronic devices thatcan be run from the domestic 240

V AC supply, since most electronic devices require low voltages to operate the semiconductor components that they depend for their operation, for example, a computer will include components that run on 12V, 5V or 1.5 V.

If not AC, otherwise would have to be provided by batteries = high cost.TV’s need high voltages to function.

Transformers & Transformers & Electrical Appliances in Electrical Appliances in

the Homethe Home

Appliances without a

transformer

Appliances with a transformer

kettle, hot water heater, toaster, older room heaters, hair dryers, incandescent lights, old model refrigerators, some clothes dryers

TV, stereo, computer, CD player, clock radio, fluorescent lights, home security systems, microwave oven, answering machines, air conditioner, fax machines, washing machines, microwave ovenElectronically operated domestic appliances require

both a step-down transformer to change 240 volts to about 5 - 20 volts & a rectifier to change the low voltage AC to DC.

Energy Transfers Energy Transfers in the Home (1)in the Home (1)

Much of the energy transferred in homes is electrical energy. This is because electrical energy is readily transferred as:a) heat (thermal energy)b) lightc) soundd) kinetic energy (movement).Amount of electrical energy transferred dependson:

a) time appliance is switched on;b) appliance power rating W [work] = P [power] x t [time]

Energy Transfers Energy Transfers in the Home (2)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.

Example: Example: Domestic Domestic TransformerTransformer• For the transformer

shown here:

a) What is the ratio of the number of turns on the primary to the number of turns on the secondary coil?b) Suggest a possible use for this transformer.

Transformers Transformers Problem #1Problem #1

• A transformer has input voltage and current of 12.0 V and 3.0 A. It has an output current of 0.75 A. a) If there are 1200 turns on the secondary coil, how many turns are on the primary? b) What is the output voltage?

Transformers Transformers Problem #2Problem #2

• An ideal transformer has 100 turns on the primary coil and 2 000 turns on the secondary coil. The primary voltage is 20 V. The current in 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?

Impact of Impact of Transformers on Transformers on

Society (1)Society (1)• The first practical transformer, using AC, was

developed in 1883.• Prior to this, direct current was seen as being the

logical way to distribute energy using electricity.• AC triumphed, and by the early 1900s, its future

impact on society was inevitable.• Transformers permitted the long-distance transfer

of electrical energy with low resistive energy losses.

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

to transmit large amounts of electrical energy would have to have been too

large to be practical.

Impact of Impact of Transformers on Transformers on

Society (2)Society (2)• Transformers were a key to establishing electrical

energy as the driving force behind technological and industrial development in the 20th century.

• Electrical energy rapidly became the means of lighting homes and cities, with its distribution facilitated by the use of transformers.

• Electrically operated machines thus replaced less efficient machines, resulting in the rapid growth of industry and commerce.

• Communication networks grew rapidly as a result of electrical 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 of transformers, permitting a host of

electronic devices to be operated from the mains.

Effect of High Effect of High Voltage Power Lines Voltage Power Lines

on Humanson Humans

+

--- -

- - - - -++ +++ +

-

-

+++ +

+ + + +-- --- --

+

Alternating E field induces an alternating current to flow in bodySign changes 100 times/ s.

What are the Health What are the Health Implications?Implications?

• Studies still in progress• At least one study has shown that

exposure to strong electric and magnetic fields increases likelihood of developing cancers and leukemia.

Phasing and em Phasing and em Radiation Exposure Radiation Exposure

LevelsLevels

--- -

- - - - -++ ++

-

+++ +

+ + + +-

+

Phasing assists to reduce the E fields when multiple power lines are present.“Code” related health effects refer to wiring codes where the conductors are far apart.The closer the supply and return wires are together, the lower the fields due to phase cancellation

Phasing and em Phasing and em Radiation Exposure Radiation Exposure

Levels (cont)Levels (cont)• Phasing assists to

reduce the B fields when multiple power lines are present as with E-field.

• Dynamic magnetic field causes currents to flow in a circular fashion within the body.

• They will reverse 100 times / second

Current direction

B Magnetic field arising from current

Induced current in body

em Field Exposureem 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