Subject : Physics
Author : Carla L. Hoyer
Objectives: Overview of AC Power Unit
Topics Covered :
Electrical Engineering: Inside Story
What is AC Current Anyway?
How do you Make AC Current?
Why Use AC Power?
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
•
Dr. Karen Butler-Purry, P.E.
•
Mirrasoul J. Mousavi
•
Bill Spooner
•
Thomas Tamez
•
Andre Williams
•
Daniel Limbrick
•
Gaurav Garg
•
Robert Davidson
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Sanjeev Srivastava
•
Torrey Thompson

Research Problem:
Can power failures be predicted/prevented?
Failure of Electrical Cables Failure of Electrical Transformers
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
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?
Thermal stresses

Internal heating due to overloads

Ambient temperature
Electrical stresses

Excessive Voltage gradient
Mechanical stresses

Assembly configuration

Short circuit and centrifugal forces

Vibration
Moisture

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
–
TO SAVE $$$$ MILLIONS
Test Setup for Insulation Experiments
BNC
Adapter
DC
Supply
Constant
Resistors
Rheosta t
Power
Supply
Mete r
Electrode system
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

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
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
Can electrons go back and forth between + and
– poles in batteries?
ε°=+1.0
ε = +1.5V
ε°= -0.5
•
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.
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!

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
•
DIRECT CURRENT PRODUCES A ONE-WAY CURRENT FLOW. THERE CAN
BE NO BACK-AND-FORTH!
DC Current is a One-Way Street
Is Alternating Current also a One-Way street?
Let’s do some Science…..Alternating Current Lab
Transformer Lab Setup

DC Power Supply Results
DC Voltage, Current and Light Intensity are INDEPENDENT of time
AC Power Supply Results

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?
The Electron Cha-Cha
And Magnetic Magic
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

Magnetic Field v. # of Turns
The AC Generator http://www.micro.magnet.fsu.edu/electromag/java/generator/ac.html
Electromagnetic Induction and the Transformer
Why is Household Current AC instead of DC?
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…
•
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?
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?
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
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?...
Transformer Lab Setup (Previously Pictured)
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
NEGLIGIBLE POWER LOSS WITH LOW AMPS

Induction and the Transformer
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?
•
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
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
•
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
THE NOTES ARE IMPORTANT
Then, Find that pattern in live data
•
Handout Data Packets
•
Record Return Times
•
Think-aloud Debriefing

Debriefing/Think-Aloud

Debriefing/Think Aloud