Electrical Engineering
The Inside Story
Overview of AC Power Unit
Carla L. Hoyer
E3
Texas A&M University
Summer 2003
AC Power
Unit
Electrical
Engineering:
Inside Story
What is AC
Current, Anyway?
PP
Presentation
Transformer
Data Sim
Scenario
How Do You Make
AC Current?
PP
PP
Why Use
AC Power?
PP
Presentation
Presentation
Presentation
AC Power
Lab
Field in
Coil Lab
Field Trip
Power System
Assessment
Ocean: Unit Topic
Green: Teacher-Led PP
Pink: Student Activity and Assessment
Great AC
v. DC
Debate
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
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• 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?
• 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
Coiled Wires
Dielectric Insulation
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
Rheostat
BNC Adapter
Constant Resistors
DC Supply
Power Supply
Meter
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
So, What is AC Power,
Anyway?
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
Difference?
ε°= +1.0
ε°= -0.5
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?
• From – to + poles, ε =
+1.0V – (-0.5V) =
+1.5V, so electrons
will move
spontaneously from
anode to cathode
ε = +1.5V
• ∆G = -nFε
= -nF(+1.5V), so
∆G <0.
ε°=+1.0
ε°= -0.5
Can electrons go back and forth
between + and – poles in
batteries? NO!
• From + to - poles, ε = ε°=+1.0
ε = +1.5V
ε°= -0.5
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 BACKAND-FORTH!
DC Current is a One-Way Street
Let’s do some Science…
Alternating Current Lab
DC Power Supply Results
DC Power Supply Results
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
Seeing is Believing…Create a
Magnetic Field around a Wire
Photo of Lab Setup
Magnetic Field v. # of Turns
The AC Generator
• http://www.micro.magnet.fsu.edu/electro
mag/java/generator/ac.html
Why is Household Current AC
instead of DC?
Electromagnetic Induction and
The Transformer
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 Set-up
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
2amp
1000amp
60,000V AC
120V AC
The Relative Number of Turns Dictates the
Output Current and Voltage
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
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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