R03EN0567 Chap03 Transformer

8/12/2019 R03EN0567 Chap03 Transformer

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EN0567 Power Machines and Renewable Energy

KDU College Penang



Explain principles of Transformer1

Topic Learning Outcome

Describe types and constructions of Transformer2

Describe the Ideal Transformer and Real Transformer with loses3

Define Transformer Efficiency and Voltage Regulation4

5

6

7

8

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Explain Transformer Impedance Transformation

Explain Transformer Equivalent Circuits

Explain Transformer Open and Short Circuit Tests

Tutorial 3



Transformer The transformer is not Electric Machine but is an electrical

device that is closely related. They operates on the same

principles as generators and motors, i.e. actions of magnetic

field.

Transformer converts AC electrical energy at one voltage level

to AC electrical energy at another voltage level.

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Use of transformers

Common transformers

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Principles of transformer AC (Alternating Current) is selected because it can be readily stepped

(transformed) upward or downward in voltage by means oftransformers.

Transformers are inductances coupled together by their mutual

magnetic fields, or Mutual Inductance. (work by Michael Faraday 1791-1867)

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Principles of transformer (contd)

•A varying current in the first or primary winding

creates a varying magnetic flux in the transformer's

core and thus a varying magnetic field through the

secondary winding.

•This varying magnetic field induces a varying

electromotive force (emf), or Voltage in the

secondary winding.

•This effect is called MUTUAL INDUCTION.

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Principles of transformer (contd) Definition :

A transformer can be defined as a device that transfer powerfrom its primary circuit to its secondary circuit with very little loss.

Power Line Symbols

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Principles of transformer (contd) Lenz’s Law states that the emf induced opposes the changes that produce it.

In other words, the polarity of the induced Voltage is such that the voltage

would produce a current (through an external resistance) that opposes the

original change of flux linkage.

The induced Voltage (V) in a circuit depends upon:a)The amount of flux linkage (λ ), and

b) Rate of change of the flux linkage (dλ /dt )

Note: λ = N.Φ ,where

N is the number of turns of the coil

Φ is magnetic flux (Weber) passing through surface bounded

by the coil.

V = dλ /dt =N.dФ/dt

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Principles of transformer (contd)

•When the flux linking a coil changes, avoltage is induced in the coil.

•The polarity of the voltage is such that

if a circuit is formed by placing aresistance across the coil terminals, the

resulting current produces a field that

tends to oppose the original change in

the field.

Note: B is Magnetic Flux Density,

which is defined as flux per unit area.

Unit measurement is Tesla or Weber/m2.EN0567 PM & RW9



Construction of transformer

A simple transformer consists of two windings verytightly coupled together, usually with an iron core, but

electrically insulated from each other.

It is desirable to get the magnetic flux coupling betweenthe two windings as high as possible.

The degree of coupling is called coupling efficiency. i.e.

how many percent of the flux lines from the primarywinding cut the secondary winding.

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Construction of transformer

Iron Core has a relative permeability (µr) in the

order of 5000 to 6000 times compares to Air.EN0567 PM & RW11



Construction of transformer (contd) Both coils are usually wound on an iron core so that

the path of the flux lines can be controlled and kept

where desired. A special alloy of silicon steel is

commonly used for transformer cores.

Types of core:

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Construction of transformer (contd)

Small

Transformer

Power

Transformer

AutoTransformer EN0567 PM & RW13



Autotransformer

Autotransformer are best suited is we need to change the voltage levels by only a small

amount. E.g. 230V to 240V, 13.2kV to 13.8kV.

Not all the power travelling from primary to secondary goes through the windings. As

a result, Autotransformer can handle much more power than conventional transformer.

Typically autotransformer of the same power rating as conventional one could be 5x

smaller and also much less expensive.

Main disadvantage is that the electrical isolation of the two sides is lost. EN0567 PM & RW14



Autotransformer

Step-down Step-up

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Construction of transformer (contd)

•It is possible to use 3 single-phase transformers or one common-core three-phase transformer.

•The latter saves space and cost but not flexible and gives higher down-time.

http://upload.wikimedia.org/wikipedia/commons/3/3b/Three-phase_transformer_EI_core_flux_animation_full_pulse.gif

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Ideal Transformer Assuming an ideal (no loss) transformer:

A)Voltage Ratio

B)Current Ratio

C)Power:

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Transformer Losses

1. Copper Loss : Loss from resistance of (copper) wire in windings of

transformer. L= I²R

2. Flux Leakage Loss: flux lines that leak from windings or core such

that they do not link between primary and secondary.

3. Hysteresis Loss: When current reverses, there is energy needed to

reverse the magnetic alignment of the core. This energy is not

available in the secondary, hence loss.

4. Eddy Current Loss: The core of the transformer conducts electricityand acts like a single-turn shorted secondary. This effect can be

minimized by using high resistivity material and laminating the core.

Note: items 3 and 4 are called Iron Loss

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Transformer Efficiency

Note that a transformer operating at full load is one that is delivering into a load the

specified Volt-Amperes; Actual Output Power will depend on the power factor.

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Transformer Voltage Regulation

The secondary voltage of a transformer decreases as the load

is increased, because of the increase of the losses that occur.

%100 X

V

V V VR

load full

load full load no

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Impedance Transformation

Thus a transformer added in between the source and load

alters the apparent impedance of the load. This is useful for

Impedance Matching for maximum power transfer.

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Transformer Equivalent Circuits

Full

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Transformer Equivalent Circuits

Exact

Sim plified

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How to measure Equivalent Circuit parameters

Open Circuit Test:

1)Secondary open-circuit2)Primary connected to a full rated AC line voltage

3)I2=0, I1 is small. Effects of L1, R1, L2, R2 are negligible

4)Essentially all the input voltage is dropped across Lm and R c

5)Hence measured values reflect effect of Lm and R c6)Measure input Voltage, Current and Power

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Open Circuit Test

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How to measure Equivalent Circuit parameters

Short Circuit Test:

1)Secondary is short-circuited2)Primary connected to a fairly low voltage AC source

3)Adjust input voltage until current in secondary equals to its rated value

4)Measure input Voltage ( Vsc ), Current and Power.

5)Since Vsc is low, effects of Lm and R c are negligible

6)Essentially all the input voltage is dropped across R1, R2, L1and L2

7)Hence measured values reflect effect of those series impedances. EN0567 PM & RW27



Short Circuit Test

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Example: The equivalent circuit impedances of a 20-kVA, 8000/240-

V, 60-Hz transformer are to be determined. The open-circuit test and

the short-circuit test were performed on the primary side of thetransformer, and the following data were taken:

Find the impedances of the approximate equivalent circuit referred

to the primary side, and sketch that circuit.

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The resulting simplified equivalent circuit. Note that

R eq = R 1 + a2R 2

Xeq = X1 + a2

X2EN0567 PM & RW32



Transformer is suited for power source transformation

In power systems, transformers are mainly used to step-up the

voltage to levels suitable for transmission or distribution and to step

down to levels as required by individual consumers.

For the same power flow, increasing the voltage by m-times itsformer value reduces the current flow to (1/m) its former value.

Hence, voltage drop (IZ) and power loss (I2R ) are reduced to (1/m)

and (1/m2) times the former values, respectively.

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Transformer is suited for power source transformation

Applications of transformers:(i)Change voltage level: e.g. generator transformer 20 kV to 400

kV, distribution transformer 11 kV to 400V three-phase and 230V

single-phase, rectifier transformer to set DC level at rectifier output.

(i)‘Safety’ applications: i.e. step voltage down to a safe level and‘isolate’ or separate secondary side from primary side. Also, the

neutral point of a star connected three-phase winding provides an

earthing point.

(i)Matching of voltage magnitude, phase or impedance: e.g.

applications which require connection of different circuits operating

independently.

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Question & Answer Session

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Tutorial 3: Q1: A single-phase power system is shown below. The power source feeds a

100-kVA 14/2.4-kV transformer through a feeder impedance of 40.0 + j 150Ω . The

transformer's equivalent series impedance referred to its low-voltage side is 0. 12 +

j0.5Ω. The load on the transformer is 90 kW at 0.85 PF lagging and 2300 V.

(a) What is the voltage at the power source of the system?

(b) What is the voltage regulation of the transformer?

(c) How efficient is the overall power system?

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Tutorial 3: Solution to Q1 (a):

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Tutorial 3: Solution to Q1 (b) & (c):

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Tutorial 3:

Q2: A 13.2 kV single-phase generator supplies power to a load through a

transmission line. The load's impedance is Zload = 500 36.87 o , , and thetransmission line's impedance is Zline = 60 53 .1o , .

(a) If the generator is directly connected to the load (Figure next page), what is the ratio

of the load voltage to the generated voltage? What are the transmission

losses of the system?(b) If a 1:10 step-up transformer is placed at the output of the generator and a 10: 1

transformer is placed at the load end of the transmission line, what is the new

ratio of the load voltage to the generated voltage? What are the transmission

losses of the system now? (Note: The transformers may be assumed to be ideal.)

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Tutorial 3:

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Tutorial 3: Solution to Q2 (a):

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Tutorial 3: Solution to Q2 (b):

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R03EN0567 Chap03 Transformer

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