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Electrical Engineering. The Inside Story. Overview of AC Power Unit. Carla L. Hoyer E3 Texas A&M University Summer 2003. Ocean: Unit Topic Green: Teacher-Led PP Pink: Student Activity and Assessment. Electrical Engineers. Improve our lives by: Generating Electrical Power

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electrical engineering

Electrical Engineering

The Inside Story

overview of ac power unit

Overview of AC Power Unit

Carla L. Hoyer


Texas A&M University

Summer 2003


Ocean: Unit Topic

Green: Teacher-Led PP

Pink: Student Activity and Assessment

electrical engineers
Electrical Engineers

Improve our lives by:

  • Generating Electrical Power
  • Electrical Power Transmission
  • Electrical Power Distribution
  • Designing Electrical and Electronic Devices
  • Computers
  • Research
texas a m power engineering research
Texas A&M Power Engineering Research
  • Dr. Karen Butler-Purry, P.E.
  • Mirrasoul J. Mousavi
  • Bill Spooner
  • Thomas Tamez
  • Andre Williams
  • Daniel Limbrick
  • Gaurav Garg
  • Robert Davidson
  • Sanjeev Srivastava
  • Torrey Thompson
research problem can power failures be predicted prevented
Research Problem: Can power failures be predicted/prevented?
  • Failure of Electrical Cables
  • Failure of Electrical Transformers
predicting transformer failure before its too late
Predicting Transformer Failure…Before its too Late

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

inside a transformer
Inside a Transformer

Coiled Wires


what s an incipient fault
What’s an Incipient Fault?

The situation of degraded insulation in the transformer before short circuit and failure occurs is referred to as an Incipient Fault.

what causes insulation breakdown
What Causes Insulation Breakdown?

Thermal stresses

Internal heating due to overloads

Ambient temperature

Electrical stresses

Excessive Voltage gradient

Mechanical stresses

Assembly configuration

Short circuit and centrifugal forces



top 5 reasons to research predictors of transformer failure
Top 5 Reasons to Research Predictors of Transformer Failure

Why Detect Incipient Faults?

  • To improve the reliability of power systems
  • To provide early warning of electrical failure
  • To reduce unplanned outages
  • To enhance the public safety
test setup for insulation experiments
Test Setup for Insulation Experiments


BNC Adapter

Constant Resistors

DC Supply

Power Supply


Electrode system

what do we need to know about to understand transformer failure research
What Do We need to Know About to Understand Transformer Failure Research?
  • Power =VI=I2R
  • What is Alternating Current?

Comparison to DC

  • How do you make Alternating Current? Electromagnetism and Induction
  • Why do we use Alternating Current? Transformers
so what is ac power anyway

So, What is AC Power, Anyway?

AC and DC Power – what’s the difference?

ac and dc power what s the difference
AC and DC Power – What’s the difference?
  • DC is the kind of Electrical Current found in Batteries.
  • DC stands for Direct Current
  • AC is the kind of Electrical Current found in the outlets of homes and businesses
  • AC stands for Alternating Current
ac and dc power what s the difference16
AC and DC Power – What’s the Difference?

Batteries are a source of DC Power

  • To be spontaneous, ∆G must be Negative
  • ∆G = -nFε
  • So, ε has to be + for ∆G to be negative, and electrons to move

ε°= +1.0


ε°= -0.5


can electrons go back and forth between and poles in batteries
Can electrons go back and forth between + and – poles in batteries?
  • From – to + poles, ε = +1.0V – (-0.5V) = +1.5V, so electrons will move spontaneously from anode to cathode
  • ∆G = -nFε

= -nF(+1.5V), so

∆G <0.




ε = +1.5V

ε°= -0.5

can electrons go back and forth between and poles in batteries no
Can electrons go back and forth between + and – poles in batteries? NO!
  • From + to - poles, ε = -0.5V – (+1.0V) = -1.5V, so electrons will not move spontaneously from cathode to anode
  • ∆G = -nFε = -nF(-1.5V), so ∆G >0. NO GO!




ε = +1.5V


ε°= -0.5

ac and dc power what s the difference19
AC and DC Power – what’s the difference?
  • So, in DIRECT CURRENT, the electrons move DIRECTLY from the anode to the cathode
  • The current flows from the cathode (+) to the anode (-) – opposite the electron flow
dc current is a one way street
DC Current is a One-Way Street

Is Alternating Current

also a one-way street?

Let’s do some Science…

Alternating Current Lab

dc power supply results23
DC Power Supply Results

DC Voltage, Current and Light Intensityare INDEPENDENT of time

ac and dc power what s the difference25
AC and DC Power – what’s the difference?
  • In DC Power, current can only move in one direction
  • In AC Power, the current alternates direction

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

so how do you make current alternate

So, How Do You Make Current Alternate?

The Electron Cha-Cha

And Magnetic Magic



Magnetic Field

Electric Field


electricity magnetism
Electricity & Magnetism
  • Two Fields, 90 Degrees apart
  • MOVING electrons (Current) in a wire produce a Magnetic Field around wire
  • Unit of Magnetic Field Strength is the Tessla
  • A stronger Magnetic field is produced if the wire is Coiled
  • Strongest Magnetic field produced if wire coiled around conductor
the ac generator
The AC Generator
  • http://www.micro.magnet.fsu.edu/electromag/java/generator/ac.html
why is household current ac instead of dc

Why is Household Current AC instead of DC?

Electromagnetic Induction and The Transformer

what you pay for is power
What You Pay for is POWER
  • Recall:

Power (watts) = VI

Ohm’s Law: V = IR (In AC, V=IZ)

Substituting: P= IRI

Simplifying: P= I2R

P = f (I,R)

imagine your neighborhood
Imagine Your Neighborhood…
  • Needs 120 V
  • Needs 1000 ampere of Current to Avoid Brownout
  • Power = VI = 120,000 watt
  • The Power Plant Generator is 20 miles Away
  • The electricity is sent on a line with a resistance of 0.1 Ohm/mile
how much voltage has to leave the dc power plant
How Much Voltage has to Leave the DC Power Plant?

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

Voltage Sent = Volts Lost + Volts Needed

= IR + Volts Needed

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

= 2000V + 120 V= 2120V

how much dc power is lost on the trip to your neighborhood
How Much DC Power is Lost on the Trip to Your Neighborhood?

Power Lost = Power Sent - Power Received

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

= 120,000 watts

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

= 2,120,000 watts

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

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

we lose our dc power over distance v ir
We lose our DC Power over Distance!! ∆V = IR

What Can We Do?

  • Put an electric power plant on every street?
  • We don’t have 90°F superconductors – all wires will have resistance- no way out.
  • The problem is Current – the higher the current, the greater the voltage drop and power loss
  • Ideas?...
what if somehow
What if, somehow…
  • We sent the 120,000 watts of power at 60000V and 2 amps, then somehow transformed it into 120V and 1000 amp at your subdivision?

∆V = IR = (2amp)(0.1ohm/mi)(20mi)

= only 4V lost

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


induction and the transformer
Induction and the Transformer


60,000V AC


120V AC

The Relative Number of Turns Dictates the Output Current and Voltage

induction doesn t happen with dc
Induction Doesn’t Happen with DC
  • To get induction, there has to be a CHANGING magnetic field
  • With DC, current and voltage are constant, so the magnetic field strength doesn’t change
  • With AC, the magnetic field is always changing

AC Allows Efficient Transmission

where does dc fit into the real world
Where does DC fit into the Real World?
  • Portability
  • Smooth Output
  • Safety?? – The Great AC v. DC Debate:

Westinghouse, Edison and the Electric Chair

  • The Houston METRO Rail System is 7.5 miles long and runs on 750VDC overhead wires.
light rail field trip
Light Rail Field Trip

Find Out:

  • Why engineers chose DC over AC?
  • How they avoid huge power losses over the 7.5 mile run?
  • How does the electrical power get the train car moving?
  • Do the cars’ lights and air conditioning run on DC from the cable?
incipient fault research scenario
Incipient Fault Research Scenario
  • Students Receive Scenario Sheet
  • Review Sample Trace (Next Slide)
  • After 3 minutes, “Any Questions?”
  • Verbal Strategic Instructions

Find specific pattern unique to failing research transformer first


Then, Find that pattern in live data

  • Handout Data Packets
  • Record Return Times
  • Think-aloud Debriefing

Current Spike

Irregular Waveform

Helpful Notes

debriefing think aloud

Unique Pattern- Irregular Waveform

followed by spike