Electrical EngineeringElectrical Engineering

The Inside StoryThe Inside Story

Overview of AC Power UnitOverview of AC Power Unit

Carla L. HoyerCarla L. HoyerE3E3

Texas A&M UniversityTexas A&M UniversitySummer 2003Summer 2003

AC Power Unit

Electrical Engineering: Inside Story

What is AC Current, Anyway?

How Do You Make AC Current?

Why Use AC Power?

PPPresentation

TransformerData SimScenario

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Presentation

AC PowerLab

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Field inCoil Lab

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Presentation

Field TripPower System

Great AC v. DCDebate

Assessment

Ocean: Unit TopicGreen: Teacher-Led PPPink: Student Activity and Assessment

Electrical EngineersElectrical Engineers

Improve our lives by:Improve our lives by:• Generating Electrical PowerGenerating Electrical Power• Electrical Power TransmissionElectrical Power Transmission• Electrical Power DistributionElectrical Power Distribution• Designing Electrical and Electronic Designing Electrical and Electronic

DevicesDevices• ComputersComputers• ResearchResearch

Texas A&M Power Engineering Texas A&M Power Engineering ResearchResearch

• Dr. Karen Butler-Purry, P.E.Dr. Karen Butler-Purry, P.E.

• Mirrasoul J. MousaviMirrasoul J. Mousavi

• Bill SpoonerBill Spooner

• Thomas TamezThomas Tamez

• Andre WilliamsAndre Williams

• Daniel LimbrickDaniel Limbrick

• Gaurav GargGaurav Garg

• Robert DavidsonRobert Davidson

• Sanjeev SrivastavaSanjeev Srivastava

• Torrey ThompsonTorrey Thompson

Research Problem: Can power Research Problem: Can power failures be failures be

predicted/prevented?predicted/prevented?•Failure of Failure of

Electrical Electrical CablesCables

•Failure of Failure of Electrical Electrical TransformersTransformers

Predicting Transformer Predicting Transformer Failure…Before its too LateFailure…Before its too Late

During the operation of the transformer, During the operation of the transformer, insulation inside deteriorates. When the gradual insulation inside deteriorates. When the gradual aging gets more severe, arcing discharge or aging gets more severe, arcing discharge or incipient fault may occur. This may cause a short incipient fault may occur. This may cause a short circuit between the adjacent turns of primary or circuit between the adjacent turns of primary or secondary winding leading to a catastrophic secondary winding leading to a catastrophic failure. This catastrophic failure may damage failure. This catastrophic failure may damage other equipment, buildings and even people near other equipment, buildings and even people near the transformer. Therefore, it is desirable to the transformer. Therefore, it is desirable to develop a method that detects any unusual develop a method that detects any unusual current activities in the primary or secondary current activities in the primary or secondary winding of the transformer before they become winding of the transformer before they become destructive and damage the transformer.destructive and damage the transformer.

Inside a TransformerInside a Transformer

Coiled Wires

Dielectric Insulation

What’s an Incipient Fault?What’s an Incipient Fault?

The situation of degraded The situation of degraded

insulation in the insulation in the

transformer transformer beforebefore short short

circuit and failure occurs is circuit and failure occurs is

referred to as an referred to as an Incipient Incipient

FaultFault..

What Causes Insulation What Causes Insulation Breakdown?Breakdown?

Thermal stressesThermal stressesInternal heating due to overloadsInternal heating due to overloads

Ambient temperatureAmbient temperature

Electrical stressesElectrical stressesExcessive Voltage gradientExcessive Voltage gradient

Mechanical stressesMechanical stressesAssembly configurationAssembly configuration

Short circuit and centrifugal forcesShort circuit and centrifugal forces

VibrationVibration

MoistureMoisture

Top 5 Reasons to Research Top 5 Reasons to Research Predictors of Transformer Predictors of Transformer FailureFailureWhy Detect Incipient Faults?Why Detect Incipient Faults?

– To improve the reliability of power To improve the reliability of power systemssystems

– To provide early warning of To provide early warning of electrical failure electrical failure

– To reduce unplanned outagesTo reduce unplanned outages– To enhance the public safetyTo enhance the public safety– TO SAVE $$$$ MILLIONS TO SAVE $$$$ MILLIONS

Test Setup for Insulation Test Setup for Insulation ExperimentsExperiments

BNC Adapter

DC SupplyConstant Resistors

Rheostat

Power Supply

Electrode system

Meter

What Do We need to Know What Do We need to Know About to Understand About to Understand Transformer Failure Transformer Failure

Research?Research?• Power =VI=IPower =VI=I22RR• What is Alternating Current? What is Alternating Current? Comparison to DCComparison to DC• How do you make Alternating How do you make Alternating

Current?Current? Electromagnetism and Induction Electromagnetism and Induction• Why do we use Alternating Current?Why do we use Alternating Current?

TransformersTransformers

So, What is AC Power, So, What is AC Power, Anyway?Anyway?

AC and DC Power – what’s AC and DC Power – what’s the difference?the difference?

AC and DC Power – What’s the AC and DC Power – What’s the difference?difference?

• DC is the kind of Electrical Current DC is the kind of Electrical Current found in Batteries.found in Batteries.

• DC stands for Direct CurrentDC stands for Direct Current

• AC is the kind of Electrical Current AC is the kind of Electrical Current found in the outlets of homes and found in the outlets of homes and businessesbusinesses

• AC stands for Alternating CurrentAC stands for Alternating Current

AC and DC Power – What’s the AC and DC Power – What’s the Difference?Difference?

Batteries are a source Batteries are a source of DC Powerof DC Power

• To be spontaneous, To be spontaneous, ∆G must be Negative∆G must be Negative

• ∆∆G = -nFG = -nFεε

• So, So, ε ε has to be + has to be + for ∆G to be for ∆G to be negative, and negative, and electrons to moveelectrons to move

ε°= +1.0

ε°= -0.5

Can electrons go back and Can electrons go back and forth between + and – poles in forth between + and – poles in

batteries?batteries?• From – to + poles, From – to + poles, εε

= +1.0V – (-0.5V) = = +1.0V – (-0.5V) = +1.5V, so electrons +1.5V, so electrons will will move move spontaneously from spontaneously from anode to cathodeanode to cathode

• ∆∆G = -nFG = -nFεε = -nF(+1.5V), so = -nF(+1.5V), so ∆ ∆G <0. G <0.

ε°= -0.5

ε°=+1.0

ε = +1.5V

Can electrons go back and Can electrons go back and forth between + and – poles in forth between + and – poles in

batteries? batteries? NO!NO!• From + to - poles, From + to - poles, εε

= -0.5V – (+1.0V) = -= -0.5V – (+1.0V) = -1.5V, so electrons 1.5V, so electrons will not will not move move spontaneously from spontaneously from cathode to anodecathode to anode

• ∆∆G = -nFG = -nFεε = -nF(- = -nF(-1.5V), so ∆G >0. NO 1.5V), so ∆G >0. NO GO!GO!

ε°=+1.0

ε°= -0.5

ε = +1.5V

AC and DC Power – what’s the AC and DC Power – what’s the difference?difference?• So, in DIRECT CURRENT, the electrons So, in DIRECT CURRENT, the electrons

move move DIRECTLYDIRECTLY from the anode to the from the anode to the cathodecathode

• The current flows from the cathode (+) to The current flows from the cathode (+) to the anode (-) – opposite the electron flowthe anode (-) – opposite the electron flow

• DIRECT CURRENT PRODUCES A ONE-DIRECT CURRENT PRODUCES A ONE-WAY CURRENT FLOW. THERE CAN BE WAY CURRENT FLOW. THERE CAN BE NO BACK-AND-FORTH!NO BACK-AND-FORTH!

DC Current is a One-Way StreetDC Current is a One-Way Street

Let’s do some Science…Let’s do some Science…

Alternating Current LabAlternating Current Lab

DC Power Supply ResultsDC Power Supply Results

DC Power Supply ResultsDC Power Supply Results

DC Voltage, Current and Light

Intensity are INDEPENDENT of

time

AC Power Supply ResultsAC Power Supply Results

AC and DC Power – what’s the AC and DC Power – what’s the difference?difference?

•In DC Power, current can In DC Power, current can only move in one directiononly move in one direction

•In AC Power, the current In AC Power, the current alternatesalternates direction direction

Next Class: How do they get Next Class: How do they get AC current to ‘cha-cha’?AC current to ‘cha-cha’?

So, How Do You Make So, How Do You Make Current Alternate?Current Alternate?

The Electron Cha-ChaThe Electron Cha-Cha

And Magnetic MagicAnd Magnetic Magic

Electricity & MagnetismElectricity & Magnetism• Two Fields, 90 Degrees apartTwo Fields, 90 Degrees apart

• MOVINGMOVING electrons (Current) in a wire electrons (Current) in a wire produce a Magnetic Field around wireproduce a Magnetic Field around wire

• Unit of Magnetic Field Strength is the Unit of Magnetic Field Strength is the TesslaTessla

• A stronger Magnetic field is produced A stronger Magnetic field is produced if the wire is Coiledif the wire is Coiled

• Strongest Magnetic field produced if Strongest Magnetic field produced if wire coiled around conductorwire coiled around conductor

Seeing is Believing…Create a Seeing is Believing…Create a Magnetic Field around a WireMagnetic Field around a Wire

Photo of Lab SetupPhoto of Lab Setup

Magnetic Field v. # of TurnsMagnetic Field v. # of Turns

The AC GeneratorThe AC Generator

• http://www.micro.magnet.fsu.edu/elehttp://www.micro.magnet.fsu.edu/electromag/java/generator/ac.htmlctromag/java/generator/ac.html

Why is Household Current Why is Household Current AC instead of DC?AC instead of DC?

Electromagnetic Induction Electromagnetic Induction and The Transformerand The Transformer

What You Pay for is POWERWhat You Pay for is POWER

• Recall:Recall:

Power (watts) = Power (watts) = VVI I

Ohm’s Law: V = Ohm’s Law: V = IRIR (In AC, V= (In AC, V=IZIZ))

Substituting: P= Substituting: P= IRIRII

Simplifying: P= ISimplifying: P= I22RR

P = f (I,R)P = f (I,R)

Imagine Your Imagine Your Neighborhood…Neighborhood…

• Needs 120 V Needs 120 V

• Needs 1000 ampere of Needs 1000 ampere of Current to Avoid BrownoutCurrent to Avoid Brownout

• Power = VI = 120,000 Power = VI = 120,000 wattwatt

• The Power Plant Generator The Power Plant Generator is 20 miles Awayis 20 miles Away

• The electricity is sent on a The electricity is sent on a line with a resistance of line with a resistance of 0.1 Ohm/mile0.1 Ohm/mile

How Much Voltage has to How Much Voltage has to Leave the DC Power Plant?Leave the DC Power Plant?

Due to the Resistance in the Transmission Due to the Resistance in the Transmission Line, The voltage (Line, The voltage (∆∆V=IR) will drop during V=IR) will drop during the trip:the trip:

Voltage Sent = Volts Lost + Volts Needed Voltage Sent = Volts Lost + Volts Needed

= IR + Volts Needed= IR + Volts Needed

= (1000amp)(.1ohm/mi)(20mi)= (1000amp)(.1ohm/mi)(20mi)+120V+120V

= 2000V + 120 V= = 2000V + 120 V= 2120V2120V

How Much DC Power is Lost on How Much DC Power is Lost on the Trip to Your Neighborhood?the Trip to Your Neighborhood?

Power Lost = Power Sent - Power Received Power Lost = Power Sent - Power Received

Power Received = VI = (120V)(1000amp)Power Received = VI = (120V)(1000amp)

= 120,000 watts= 120,000 watts

Power Sent = VI=(2120V)(1000amp)Power Sent = VI=(2120V)(1000amp)

= 2,120,000 watts= 2,120,000 watts

Power Lost = 2,120,000 watt -120,000 wattPower Lost = 2,120,000 watt -120,000 watt

= 2,000,000 watt = 2,000,000 watt ( 94% lost!)( 94% lost!)

We lose our DC Power over We lose our DC Power over Distance!! Distance!! ∆V = IR∆V = IR

What Can We Do?What Can We Do?

• Put an electric power plant on every Put an electric power plant on every street?street?

• We don’t have 90We don’t have 90°°F superconductors – F superconductors – all wires will have resistance- no way all wires will have resistance- no way out.out.

• The problem is Current – the higher The problem is Current – the higher the current, the greater the voltage the current, the greater the voltage drop and power lossdrop and power loss

• Ideas?...Ideas?...

Transformer Lab Set-upTransformer Lab Set-up

What if, somehow…What if, somehow…

• We sent the 120,000 watts of power at We sent the 120,000 watts of power at 60000V and 2 amps, then somehow 60000V and 2 amps, then somehow transformedtransformed it into 120V and 1000 amp at it into 120V and 1000 amp at your subdivision?your subdivision?

∆∆V = IR = (2amp)(0.1ohm/mi)(20mi)V = IR = (2amp)(0.1ohm/mi)(20mi)

= only 4V lost= only 4V lost

Power loss = (4V)(2amp)= 8 watts Power loss = (4V)(2amp)= 8 watts

NEGLIGIBLE POWER LOSS WITH LOW AMPSNEGLIGIBLE POWER LOSS WITH LOW AMPS

Induction and the Induction and the TransformerTransformer

2amp

60,000V AC1000amp

120V AC

The Relative Number of Turns Dictates the Output Current and Voltage

Induction Doesn’t Happen with Induction Doesn’t Happen with DCDC

• To get induction, there has to be a To get induction, there has to be a CHANGING magnetic fieldCHANGING magnetic field

• With DC, current and voltage are With DC, current and voltage are constant, so the magnetic field strength constant, so the magnetic field strength doesn’t changedoesn’t change

• With AC, the magnetic field is always With AC, the magnetic field is always changingchanging

AC Allows Efficient TransmissionAC Allows Efficient Transmission

Where does DC fit into the Where does DC fit into the Real World?Real World?

• PortabilityPortability

• Smooth OutputSmooth Output

• Safety?? – The Great AC v. DC Debate:Safety?? – The Great AC v. DC Debate:

Westinghouse, Edison and the Electric Westinghouse, Edison and the Electric ChairChair

• The Houston METRO Rail System is 7.5 The Houston METRO Rail System is 7.5 miles long and runs on 750VDC miles long and runs on 750VDC overhead wires. overhead wires.

Light Rail Field TripLight Rail Field Trip

Find Out:Find Out:

• Why engineers chose DC over AC?Why engineers chose DC over AC?

• How they avoid huge power losses over How they avoid huge power losses over the 7.5 mile run?the 7.5 mile run?

• How does the electrical power get the How does the electrical power get the train car moving?train car moving?

• Do the cars’ lights and air conditioning Do the cars’ lights and air conditioning run on DC from the cable?run on DC from the cable?

Incipient Fault Research Incipient Fault Research ScenarioScenario• Students Receive Scenario SheetStudents Receive Scenario Sheet

• Review Sample Trace (Next Slide)Review Sample Trace (Next Slide)

• After 3 minutes, “Any Questions?”After 3 minutes, “Any Questions?”

• Verbal Strategic InstructionsVerbal Strategic InstructionsFind specific pattern unique to failing research Find specific pattern unique to failing research

transformer firsttransformer first

THE NOTES ARE IMPORTANTTHE NOTES ARE IMPORTANT

Then, Find that pattern in live dataThen, Find that pattern in live data

• Handout Data PacketsHandout Data Packets

• Record Return TimesRecord Return Times

• Think-aloud DebriefingThink-aloud Debriefing

Debriefing/Think-AloudDebriefing/Think-Aloud

Debriefing/Think AloudDebriefing/Think Aloud