Power Transformers

Lesson #1

Power Transformers

Presented by: Ralph Fehr, Ph.D., P.E.

University of South Florida

Topics – Power Transformer

• Fundamental

• Design

Objectives

• Recall basic transformer fundamentals and applications

• Identify the major components of transformers

Transformer Costs• 69/13.09 kV, 20 MVA - $300,000• 69/13.09 kV, 50 MVA - $450,000

• 115/13.09 kV, 20 MVA - $325,000• 115/13.09 kV, 50 MVA - $425,000

• 230/13.09 kV, 50 MVA - $475,000

• 230/69 kV, 250 MVA - $1,600,000

Over 900 Power Transformers in PE – FL

Transformer Age

•Typical Transformer Failure Rates:– Generator Transformers: 2.5% +– HV Auto-Transformers: 1.5 – 2%– T&D Transformers: 1 – 1.5%– Others <1%

•Average Age of a Power Transformer Is 30 Years– Typical Life Of A Generator Transformer Is 18-24 Years– Typical Life Of A T&D Transformer Is 40-60 Years

Voltage ratio = V1/ V2 =n1/n2

Current ratio = I1/ I2 = n2/n1

Impedance ratio = Z1/ Z2 = (n1/n2)2

Basic Transformer Equations

Transformer - A 4-terminal device comprised of 2 or more magnetically coupled coils.

Transformer - Typically used to change the ratio of voltage to current (maintaining constant power) from a source to a load.

+

Vs

-

ZL

Z1 Z2

Primary Coil Secondary Coil

Transformer Fundamentals

Ideal transformer – Both primary and secondary coils couple the same flux, assume the polarities of the voltages and currents result in flux in the same direction, then the ratio of voltages can be described:

Ideal transformer – No power loss in the magnetic circuit or the coils, then the ratio of currents can be described:

nN

N

V

V

1

2

1

2

ˆ

ˆ

dt

dNv

dt

dNv

2211 and

2211 iviv nN

N

V

V

I

I

1

2

1

2

2

1

ˆ

ˆ

ˆ

ˆ

Transformer Fundamentals

Primary Secondary Load

Primary Secondary Load

Transformer - Step-up / Step-down

Step-up (VLOAD > VSOURCE)

Step-down (VLOAD < VSOURCE)

Transformer – Components

Turn ratio of the transformer = NP / NS

The voltage ratio is the same as the turns ratio:

VP / VS = NP / NS

VP = primary voltage, VS = secondary voltage

NP = number of turns of wire in the primary

NS = number of turns of wire in the secondary

Transformer - Voltage Ratio

Transformer - Components

Laminated Iron Core

Captures the electromagnetic field produced by the primary windings and induces (directs) this field to the secondary windings.

Cores were made of iron and now steel because this material carries and confines the magnetic flow better than air.

Transformer - Components

Two types of core designs

Core form - Windings surround the laminated core steel.

Shell form - Laminated steel core surrounds the windings.

CORE TYPE SHELL TYPE

Transformer - Components

Primary Winding

Provide the medium for flow of alternating current that produces the flux that magnetizes the core steel.

Secondary Winding

Provide the medium for flow of alternating current that is induced by the flux radiating from the magnetized core steel.

Windings were made of copper with oil paper insulation.

Core Design - Core Form

Core Form vs. Shell Form

• Core Form

– As needed insulation

• Special Designs for HV

– Cylindrical Design

• Good Short Circuit Strength

• Cannot Parallel Groups

– Continuous Windings

• Shell Form

– Insulation Laden

• Easy to Implement HV Design

– High Volts per Turn

• Heavy Core

• Less Copper

– Able to Parallel Groups

• Short Circuit Strength

– Labor Intensive

• High Cost

• In Comparing Designs With Equivalent Performance, The Shell Form Unit Weighs More, Thus Requiring More Material

In General, Shell Form Are More Suited For GSU And Core Form Are More Suited For Autotransformer Designs

The Majority Of Transformers Manufactured Today Are Core Form