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Introduction to Energy Utility Industry
Module-04b
Power Distribution- Transformers
Jerry Bernardini
Community College of Rhode Island
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Distribution References
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“Transformers: Basics, Maintenance, and Diagnostics” U.S. Department of
Interior, 2005, pp. 1-26
“Transformers”, IEEE Electrical Engineering Training Series,
http://www.tpub.com/neets/book2/5.htm
“Basic Power Transformers”,
http://www.bristolwatch.com/ele/transformers.htm
“Single Phase vs. Three Phase Power Systems”,
http://www.federalpacific.com/university/transbasics/chapter3.html
“Distribution Transformers, Chapter-15”, http://books.mcgrawhill.com/downloads/products/0071467890/0071467890_ch15.pdf
“An Introduction to Electric Power Distribution”, second edition, Wayne
Beaty; pp. 35-104
“Basic Electric Power Distribution”, third edition, Anthony J. Pansini; pp.
55-88
“Electric Power Industry Overview 2007” , eia
http://www.eia.gov/cneaf/electricity/page/prim2/toc2.html
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Simplified Electric Power System
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Transformer Importance
• Generator step-up (GSU) transformers represent the second largest
capital investment in Reclamation power production—second only
to generators.
• Reclamation has hundreds, perhaps thousands, of transformers, in
addition to hundreds of large GSU transformers.
• The total investment in transformers may well exceed generator
investment
• A transformer has no internal moving parts, and it transfers energy
from one circuit to another by electromagnetic induction.
• Transformers are typically used because a change in voltage is
needed.
• Power transformers are defined as transformers rated 500 kVA and
larger. Generator step-up transformers and transmission line stepdown transformers
• Transformers smaller than 500 kVA are generally called distribution
transformers.
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Transformer Operation
• The transformer is just two (or more) inductors, sharing a
common magnetic path
• The iron core is a more efficient magnetically than air
• When an electric current is passed through a coil of wire,
a magnetic field is created - this works with AC or DC
• Only with AC will we get transformer action.
• The iron core is typically made of laminated iron (to
reduce (Eddie current losses)
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Small Transformer Construction and Losses
• There are three general types of power
losses in transformers:
• Hysteresis: iron core in the transformer
resists the change in magnetic fields
caused by the AC current.
• Eddy currents: small current flow
induced in the iron core of the
transformer. Solution-laminated sheets
bonded together.
• Copper Loss = I*I*R where I is the
current flowing in the conductor and R
the resistance of the conductor.
• Good transformer is often over 95%
efficient.
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Transformer Turns Ratio and Voltage
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A coil (the primary) is connected to an AC voltage source
The flux induced into the core is coupled through to the secondary, a voltage is induced into the
winding, and a current is produced through the load.
The primary will induce a magnetic field into the core in sympathy with the current produced by the
applied AC voltage.
The magnetic field is concentrated by the core, and nearly all of it will pass through the windings of
the secondary
The magnitude of the voltage in the secondary is determined by the "turns ratio" (N)
N = Ts / Tp
Vs = Vp * N
N = Vs / Vp Is = Ip / N
For an efficient transformer Primary power equals secondary power Pp = Ps Tp
Vs Is = Vp Ip
Is / Ip = Vp / Vs
Is / Ip = / N
Is = Ip / N
Example of step-up transformer:
For Tp= 100 turns Ts =400 turns N= 4
For Vp=120 volts Vs = (120)(4)=480 volts
For Ip=20 amps Is = (20) / (4)=5 amps
Reverse the connection of the transformer:
For Tp= 400 turns Ts =100 turns N= 1/4
For Vp=120 volts Vs = (120)(1/4)=30 volts
For Ip=20 amps Is = (20) / (1/4)=80 amps
This is now a step-down transformer
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Transformer Types
Step-Down Transformer
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Distribution Transformer
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Transformer Basics
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Inside a Transformer
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Current Transformer
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Potential Transformer
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Autotransformers
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Regulators
Three Phase Regulator
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Single Phase Regulator
Regulator Dial
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Regulator Controls
• Load tap changer
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Load Tap Changer Transformer
• Contacts
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