single and three phase transformer

8/13/2019 Single and Three Phase Transformer

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Lecture 5

Single and Three Phase Transformers


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Single Phase and three phase Transformers

In the class the following topics will be covered:

1) A comprehensive overview of transformer, essentials, and how does it operate2) Explain ideal and real transformers construction, equivalent circuit, phasor

diagrams

3) Transformer Design Equations, Performance of transformers,

4) Solving examples on these topics

By the end of this class you should be able to

1) Understand electromagnetism, induction and how these apply to transformer

operation2) Understand how to analyze, and find the performance of Transformer

3) Design, calculate performance, and identify different types of Single phasetransformers


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Introduction

For transmission and distribution networks to transfer large amounts of alternating

current electricity over long distances with minimum losses and least cost, differentvoltage levels are required in the various parts of the networks.

For example, the transfer of electricity efficiently over a long transmission line requires

the use of high voltages. At the receiving end where the electricity is used, the high

voltage has to be reduced to the levels required by the consumer.

Transformers enable these changes in voltage to be carried out easily, cheaply and

efficiently.

A transformer used to increase the voltage is called a "step up" transformer, while that

used to decrease the voltage is called a "step down" transformer.


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Introduction

What is a transformer?

A device for increasing or decreasing an AC voltage Power Transformers, TV sets to provide High Voltage to picture tubes,

portable electronic device converters, transformers on the pole, etc. are few

examples

A transformer consists of two coils of wires known as primary and secondary

windings

The two coils can be interwoven or linked by a laminated soft iron core to reduce

eddy current losses


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Basic Components of Transformers

A transformer consists of two coils electrically separate but linked by a common magnetic circuit of

low reluctance formed by a laminated soft iron core. If one coil (the primary coil) is connected to an

AC supply, an alternating magnetic flux is set up in the iron core.

This alternating magnetic flux passes through the secondary coil and induces and alternating voltage

in the secondary coil.

The magnitude of the secondary voltage is directly proportional to the ratio of the number of turns in

the secondary and primary windings and to the primary voltage.

Figure 1


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How do transformers work?

A changing current through a coil of wire can create a changing magnetic

field.

Currents can be induced in other wires by these changing magnetic field.

Therefore, the primary coil current must have AC.

The iron core of the transformer is not required but it does increases the

efficiency a great deal.


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Transformer Equation

The transformer equation does not work for DC current since there is nochange of magnetic flux

If NS>NP, the output voltage is greater than the input so it is called a step-up transformer while NS


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When an AC voltage is applied to the primary, thechanging B it produces will induce voltage of the same

frequency in the secondary wire

So how would we make the voltage different? By varying the number of loops in each coil

From Faradays law, the induced emf in the secondary is

The input primary voltage is

Since dB/dt is the same, we obtain

dt

dNV Bss

=

s s

p p

V NV N

=dt

dNV Bpp =

Transformer Equations


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Transformer Equations

Transformer Ratio, this is the most important relationship by which mostof the transformer variables are governed and it is denoted (a)

Number of Turns in the Primary Coil

Number of Turns in the Secondary Coila=

p

s

Na

N

=


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Induction and the Transformer

2amp

60,000V AC

1000amp

120V AC

Remember that the Relative Number of Turns Dictates the Output Current and Voltage

as seen above and by Transformer equations

Figure 2


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Transformers Types

Figure 3 Single phase transformer construction

A) Core type B) Shell type

Transformers are manufactured in many types the most widely used in power systems are classified

with their core types as seen below. Core type where each of the windings are wound on one leg of

the core, while the shell core type, in which both windings are wound on the same leg. Each type

has its own advantages and disadvantages. Core type is very reliable and easy to maintain, but take

larger space, however, shell type is smaller but not reliable.


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Ideal Transformer: Transformer Equations

Transformer Ratio, this is the most important

relationship by which most of the transformer variables aregoverned and it is denoted (a)

Number of Turns in the Primary Coil

Number of Turns in the Secondary Coila=

p

s

Na

N

=


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Ideal Transformer

p

s

s

p

s

p

I

I

E

E

N

Na ===

Number

of Turns

Coil

VoltagesCoil

Currents

This is the most known relationships that relates the induction between the two coils

(primary/Secondary) of an ideal transformer to turns ratio and their currents


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Ideal Unloaded Transformers

Winding resistances are zero, no leakage inductance and iron loss

Magnetization current generates a flux that induces voltage in both windings

2

mpp

N

E

=

2

mss

N

E

=

Figure 4 Current, voltages and flux in an unloaded ideal transformer


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Ideal loaded transformer

Loaded transformer , means current will be also feeding the load and thatsecondary side voltage, current (load Side) will be calculated using the

previous relations

Figure 5 Currents and fluxes in a loaded ideal transformer


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TransformersRemember that the following are mentioned before we bring them here to

emphasize their importance

Turn ratio is:

The input and output complex powers are equal

Notice the direct relations between number of turns and voltages and reciprocal relations

with currents.

s

p

s

p

E

E

N

N

Ta ===

** IESSIE sssppp ===

p

s

s

p

I

I

E

ETa ===


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Ideal Transformers Equivalent Circuit

Usually we find that ideal transformer symbol and its variables as depicted in figure 7

Figure 7 Equivalent circuit of an ideal transformer

Is

Ip= I

s/T

Es= Vs

Vp= E

p= T E

s

Vp Ep

T


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Transformers

Ideal transformer Transferring impedances through a transformer

S

S

S

S

P

PP

I

V

I

V

I

VZ 2T

T

T=

==

loadP ZZ2

T=

Vac

Zload

T

VP

VS

IP

IS

a) Original circuit with ideal

Figure 8-a Thvenin equivalents of transformer circuit


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Transferring impedances through a transformer

Vac

T2Zload

VP

IP

b) Equivalent circuit when secondary

impedance is transferred to primary side

and ideal transformer eliminated

(c) Equivalent circuit when primary source is

transferred to secondary side and ideal

transformer eliminated

Figure 8 b-c Thvenin equivalents of transformer circuit

We have seen how turns ratio (a) or (T) relates to voltages and currents, the presence of suchideal transformer relations facilitates the development of real transformers. This is to say itsequivalent circuit and using such relations to transfer from primary to secondary and viceversa. To transfer impedances of either side then the square of the turns ratio is used.


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Example 1

A 200 kV A, 6600 V/400 V, 50 Hz single-phase transformer

has 80 turns on the secondary. Calculate:

(i) the appropriate values of the primary and secondary currents;

(ii) the approximate number of primary turns


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Solution

V1/V2=N1/N2=a

N1=V1N2/V2= (a) N2

6600/400 (80)= 1220 turns

Transformer rating=Primary Voltage x Primary current

= Secondary Voltage x Secondary Current

For Primary 200 KVA = 6600 v (I1)

I1 = 200KVA/6600v = 30.3 A

For Secondary 200 KVA= 400 I2

I2 = 200 KVA/400v = 500 A


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Quiz

A single-phase transformer has 1000 turns on the primary and 200 turns on the secondary. The

no-load current is 3 A at a power factor of 0.2 lagging.

Calculate the primary current and power factor when the secondary current is 280 A at a power

factor of 0.8 lagging. Assume the voltage drop in the windings to be negligible.

Answer: 58.3 A, 0.78 pf lagging


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This lecture covers the first essentials that one should know on

transformer operation and ideal transformers. Next lecture we will go

over the practical transformer operation, equivalent circuit and

modeling

single and three phase transformer

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