1 Chapter 4. Transformer. 2 Transformer- Introduction Two winding transformers Construction and...

Chapter 4.

Transformer

Transformer- Introduction

Two winding transformers Construction and principles Equivalent circuit Determination of equivalent circuit

parameters Voltage regulation Efficiency Auto transformer 3 phase transformer

Varieties of transformers

Transformer is a device that makes use of the magnetically coupled coils to transfer energy

It is typically consists of one primary winding coil and one or more secondary windings

The primary winding and its circuit is called the Primary Side of the transformer

The secondary winding and its circuit is called the Secondary Side of the transformer

If one of those winding, the primary, is connected to an alternating voltage source, an alternating flux will be produced. The mutual flux will link the other winding, the secondary, and will induced a voltage in it.

Transformers are adapted to numerous engineering applications and may be classified in many ways:

Power level (from fraction of a volt-ampere (VA) to over a thousand MVA),

Application (power supply, impedance matching, circuit isolation),

Frequency range (power, audio, radio frequency (RF))

Voltage class (a few volts to about 750 kilovolts) Cooling type (air cooled, oil filled, fan cooled, water

cooled, etc.) Purpose (distribution, rectifier, arc furnace, amplifier

output, etc.).

Power transmission

Transformer

4.1 Construction

Transformer- construction

Basic components of single phase transformer

N1 N2Supply Load

Primary winding Secondary winding

Laminated iron core

Single phase transformer construction

A) Core type B) Shell type

PrimaryWinding

SecondaryWinding

Multi-layerLaminatedIron Core

2H1 H2

WindingTerminals

4.2 Ideal Transformer

Transformer

The emf which induced in transformer primary winding is known as self induction emf as the emf is induced due to to flux which produced by the winding itself.

While the emf which induced in transformer secondary winding is known as mutual induction emf as the emf is induced due to to flux which produced by the other winding.

Transformer

Acording to Faraday’s Law, the emf which induced in the primary winding is,

e1 = dt

Since the flux is an alternating flux,

tmak sin e1 = dt

tdN mak )sin(

tN mak cos1

Transformer

where,

tfN mak cos21

tE cosmax1

max1E fN 2max1=

EE rms fN max144.4

Transformer

Similarly it can be shown that,

E2 rmsfN max244.4

k is transformation

ratiok is transformation ratio

TransformerThe voltage ratio of induced voltages on the secondary to primary windings is equal to the turn ratio of the winding turn number of the secondary winding to the winding turn number of the primary winding. Therefore the transformers can be used to step up or step down voltage levels by choosing appropriate number their winding turns. In power system it’s necessary to step up the output voltage of a generator which less than 30kV to up 500kV for long distance transmission. High voltage for long distance power transmission can reduce current flow in the transmission lines, thus line losses and voltage drop can be reduced.

Current, voltages and flux in an unloaded ideal transformer

Transformer- Ideal Transformer

Winding resistances are zero, no leakage inductance and iron loss

Magnetization current generates a flux that induces voltage in both windings

E2 = V

Transformer

Transformer on no load.

Loaded transformer

V1E2E1

When a load is connected to the secondary output terminals of a transformer as shown in Figure 4.5, a current I2 flows into the load and into transformer secondary winding N2. The current I2 which flowing in N2

produces flux Φ2 which opposite –by Lenz’s law- to the main magnetic flux Φ in the transformer core. This will weaken or slightly reduce the main flux Φ to Φ’.

Loaded transformer

V1E2E1

The reduction of main flux Φ –by Faraday’s law- could also reduce the induced voltage in primary winding E1. Consequently E1 is now smaller than the supply voltage V1, then the primary current would be increased due to that potential differences. Therefore on loaded transformer, the primary current has an additional current of I1’.

Loaded transformer

V1E2E1

The extra current I1’ which flowing in the primary winding N1 produces flux Φ1 which naturally react according to Lenz’s law, demagnetize the flux Φ2. Therefore the net magnetic flux in the core is always maintained at original value, it is the main flux Φ (the flux which produced by the magnetizing current).

Loaded transformer

V1E2E1

The magneto motive force (mmf) source N2I2 at the secondary winding produces flux Φ2, while the mmf N1I1‘ produces flux Φ1. Since the magnitude of Φ1

equal to magnitude of Φ2 and the reluctance seen by these two mmf sources are equal, thus

N1I1‘ = N2I2

Currents and fluxes in a loaded ideal transformer

Loaded transformer

mIm + I 1

V 1E 1

Turn ratio If the primary winding has N1 turns and

secondary winding has N2 turns, then:

The input and output complex powers are equal

** IESSIE 222111

Functional description of a transformer:

When a = 1 Isolation Transformer

When | a | < 1 Step-Up Transformer Voltage is increased from Primary side to secondary side

When | a | > 1 Step-Down Transformer Voltage is decreased

from Primary side to secondary side

Transformer Rating Practical transformers are usually

rated based on: Voltage Ratio (V1/V2) which gives

us the turns-ratio Power Rating, small transformers

are given in Watts (real power) and Larger ones (Power Transformers) are given in kVA (apparent power)

Example 4.1 Determine the turns-ratio of a 5 kVA

2400V/120V Power Transformer Turns-Ratio = a = V1/V2 = 2400/120

= 20/1 = 20 This means it is a Step-Down

transformer

Example 4.2

A 480/2400 V (r.m.s) step-up ideal transformer delivers 50 kW to a resistive load. Calculate:

(a) the turns ratio, (0.2)(b) the primary current, (104.17A)(c) the secondary current. (20.83A)

Transformer- Ideal Transformer Nameplate of transformer

Equivalent circuit

I2I1 = I2 /T

E2 = V2

V1 = E1 = T E2

Equivalent circuit of an ideal transformer

Transferring impedances through a transformer

Equivalent circuit of an ideal transformer

loada ZZ 21

Vac Zload

Vac a2ZloadV1

Vac/k ZloadV2

a) Equivalent circuit when secondary impedance is transferred to primary side and ideal transformer eliminated

b) Equivalent circuit when primary source is transferred to secondary side and ideal transformer eliminated

Thévenin equivalents of transformer circuit

Transformer- practical transformer

Practical Transformer

4.3 Equivalent Circuits

Transformer

Transformer- equivalent circuit

Development of the transformer equivalent circuits

The effects of winding resistance and leakage flux are respectively accounted for by resistance R and leakage reactance X (2πfL).

In a practical magnetic core having finite permeability, a magnetizing current Im is required to establish a flux in the core.

This effect can be represented by a magnetizing inductance Lm. The core loss can be represented by a resistance Rc.

Transformer- practical equivalent circuit

Rc :core loss component, Xm : magnetization component, R1 and X1 are resistance and reactance of the primary windingR2 and X2 are resistance and reactance of the secondary winding

The impedances of secondary side such as R2, X2 and Z2 can be moved to primary side and also the impedances of primary side can be moved to the secondary side, base on the principle of:

The power before transferred = The power after transferred.

I22R2 = I1’ 2R2’

Therefore R2’= (I2/ I1’ ) 2 R2

= a2R2

X1 I2R’2

V2' = a V2 , I1' = I2/aX2' = a2 X2 , R2' = a2 R2

a = N1/N2

The turns can be moved to the right or left by referring all quantities to the primary or secondary side.

The equivalent circuit with secondary side moved to the primary.

Transformer- Approximate equivalent circuit

For convenience, the turns is usually not shown and the equivalent circuit is drawn with all quantities (voltages, currents, and impedances) referred to one side.I

X1 R’2X’2

Transformer- equivalent circuit

Example 4.3

A 100kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistance are 0.3 ohm and 0.01 ohm respectively and the corresponding leakage reactances are 1.1 ohm and 0.035 ohm respectively. The supply voltage is 2200V. Calculate:

(a) the equivalent impedance referred to the primary circuit (2.05 ohm)

(b) the equivalent impedance referred to the secondary circuit

4.4 Determination of Equivalent Circuit

Parameter

Transformer

1. No-load test (or open-circuit test).

2. Short-circuit test.

Transformer- o/c-s/c tests

The equivalent circuit model for the actual transformer can be used to predict the behavior of the transformer.

The parameters R1, X1, Rc, Xm, R2, X2 and N1/N2 must be known so that the equivalent circuit model can be used.

These parameters can be directly and more easily determined by performing tests:

No load/Open circuit test Provides magnetizing reactance (Xm) and core

loss resistance (RC) Obtain components are connected in parallel

Short circuit test Provides combined leakage reactance and

winding resistance Obtain components are connected in series

Transformer- open circuit test

No load/Open circuit test

AX1 R1

X m R c

Equivalent circuit for open circuit test, measurement at the primary side.

Simplified equivalent circuit

Transformer- open circuit test

Open circuit test evaluation

ococococ

0 sincos

Transformer- short circuit test

Short circuit test Secondary (normally the LV winding) is

shorted, that means there is no voltage across secondary terminals; but a large current flows in the secondary.

Test is done at reduced voltage (about 5% of rated voltage) with full-load current in the secondary. So, the ammeter reads the full-load current; the wattmeter reads the winding losses, and the voltmeter reads the applied primary voltage.

Transformer- short circuit test Short circuit test

X1R1 X2P sc

Equivalent circuit for short circuit test, measurement at the primary side

a2R2X1R1 a2X2

Simplified equivalent circuit for short circuit test

Short circuit test

Xe1Re1

I scVsc

Simplified circuit for calculation of series impedance

11 RaRRe

11 XaXX e

Primary and secondary impedances are combined

Short circuit test evaluation

Equivalent circuit obtained by measurement

X e1 R e1

Equivalent circuit for a real transformer resulting from the open and short circuit tests.

Example 4.4Obtain the equivalent circuit of a 200/400V, 50Hz1-phase transformer from the following test

data:-

O/C test : 200V, 0.7A, 70W - on L.V. sideS/C test : 15V, 10A, 85W - on H.V. side

(Rc =571.4 ohm, Xm=330 ohm, Re=0.21ohm, Xe=0.31 ohm)

Transformer – voltage regulation

Voltage Regulation

Most loads connected to the secondary of a transformer are designed to operate at essentially constant voltage. However, as the current is drawn through the transformer, the load terminal voltage changes because of voltage drop in the internal impedance.

To reduce the magnitude of the voltage change, the transformer should be designed for a low value of the internal impedance Zeq

The voltage regulation is defined as the change in magnitude of the secondary voltage as the load current changes from the no-load to the loaded condition.

I2’ I2

V12’=V20

R1’ R2

Rc’ Xm’

Ze2 = R1’ + R2 + jX1’ + jX2

= Re2 + jXe2

I2’ I2

V12’=V20

R1’ R2

Rc’ Xm’

Applying KVL, V20 = I2 (Ze2 ) + V2 = I2 (Re2 + jXe2 ) + V2

Or V2 = V20 - I2 (Ze2 )

V20 = I2 (Ze2 ) + V2 = I2 (Re2 + jXe2 ) + V2

Voltage drop = AM = OM – OA

= AD + DN + NM

AD = I2 Re2 cosθ2

DN=BL= I2 Xe2 sinθ2

Applying Phytogrus theorem to OCN triangle.

(NC)2 = (OC)2 – (ON)2

= (OC + ON)(OC - ON) ≈ 2(OC)(NM)

Therefore NM = (NC)2/2(OC)

NC = LC – LN = LC – BD

= I2 Xe2 cosθ2 - I2 Re2 sinθ2

2222222

sincos

RIXI ee

2222222

sincos

RIXI ee

AM = AD + DN + NM = I2 Recosθ2 + I2 Xe2 sin θ2 +

thus,votage regulation = (AM)/V20 per unit

In actual practice the term NM is negligible since its value is very small compared with V2. Thus the votage regulation formula can be reduced to:

Voltage regulation =

222222 sincos

XIRI ee

Transformer- voltage regulation

The voltage regulation is expressed as follows:

VVregulationVoltage

V2NL= secondary voltage (no-load condition)

V2L = secondary voltage (full-load condition)

For the equivalent circuit referred to the primary:

VVregulationVoltage

V1 = no-load voltage

V2’ = secondary voltage referred to the primary (full-load condition)

Consider the equivalent circuit referred to the secondary,

I2' R1'

X1' R2X2

sinXIcosRIregulationVoltage

222222

(-) : power factor leading(+) : power factor lagging

Consider the equivalent circuit referred to the primary,

211211

sinXIcosRIregulationVoltage ee

X1 R2'X2

(-) : power factor leading(+) : power factor lagging

Example 4.5 Based on Example 4.3 calculate

the voltage regulation and the secondary terminal voltage for full load having a power factor of

(i) 0.8 lagging (0.0336pu,14.8V) (ii) 0.8 leading (-0.0154pu,447V)

Transformer- Efficiency

Losses in a transformer Copper losses in primary and

secondary windings Core losses due to hysteresis and

eddy current. It depends on maximum value of flux density, supply frequency and core dimension. It is assumed to be constant for all loads

As always, efficiency is defined as power output to power input ratio

The losses in the transformer are the core loss (Pc) and copper loss (Pcu).

lossesP

)P(powerinput

)P(poweroutput

222222

ec RIPcosIV

Efficiency on full load

where S is the apparent power (in volt amperes)

scocFLFL

Efficiency for any load equal to n x full load

where corresponding total loss =

scocFLFL

scoc PnP 2

Example 4.6 The following results were obtained on a 50

kVA transformer: open circuit test – primary voltage, 3300 V; secondary voltage, 400 V; primary power, 430W.Short circuit test – primary voltage, 124V;primary current, 15.3 A; primary power, 525W; secondary current, full load value. Calculate the efficiency at full load and half load for 0.7 power factor.

(97.3%, 96.9%)

For constant values of the terminal voltage V2 and load power factor angle θ2 , the maximum efficiency occurs when

If this condition is applied, the condition for maximum efficiency is

that is, core loss = copper loss.

222 ec RIP

Transformer- Auto transformer

It is a transformer whose primary and secondary coils are in a single winding

Autotransformer

Transformer- Auto transformer Same operation as two windings

transformer Physical connection from primary

to secondary Sliding connection allows for

variable voltage Higher kVA delivery than two

windings connection

Transformer- Auto transformer Advantages:

A tap between primary and secondary sides whichmay be adjustable to provide step-up/down capability

Able to transfer larger S apparent power than the two winding transformer

Smaller and lighter than an equivalent two-winding transformer

Disadvantage: Lacks electrical isolation

A Step Down Autotransformer:

A Step Up Autotransformer:

Transformer- Auto transformer Example 4.7

An autotransformer with a 40% tap is supplied by a 400-V, 60-Hz source and is used for step-down operation. A 5-kVA load operating at unity power factor is connected to the secondary terminals.

Find: (a) the secondary voltage, (b) the secondary current, (c) the primary current.

Solution

Three phase transformers The three-phase transformer can be built by:

the interconnection of three single-phase transformers using an iron core with three limbs

The usual connections for three-phase transformers are: wye / wye seldom used, unbalance and 3th

harmonics problem wye / delta frequently used step down.(345 kV/69 kV) delta / delta used medium voltage (15 kV), one of the

transformer can be removed (open delta)

delta / wye step up transformer in a generation station For most cases the neutral point is grounded

Transformer -3 phase transformer

Analyses of the grounded wye / delta transformer

Each leg has a primary and a secondary winding.

The voltages and currents are in phase in the windings located on the same leg.

The primary phase-to- line voltage generates the secondary line-to- line voltage. These voltages are in phase

VAN VB N VC N

VabVbc Vca

VB C Vbc

Three phase transformer

Transformer

Transformer Three phase transformer

Transformer Construction

Iron Core The iron core is made of thin

laminated silicon steel (2-3 % silicon)

Pre-cut insulated sheets are cut or pressed in form and placed on the top of each other .

The sheets are overlap each others to avoid (reduce) air gaps.

The core is pressed together by insulated yokes.

Transformer Construction Winding

The winding is made of copper or aluminum conductor, insulated with paper or synthetic insulating material (kevlar, maylard).

The windings are manufactured in several layers, and insulation is placed between windings.

The primary and secondary windings are placed on top of each others but insulated by several layers of insulating sheets.

The windings are dried in vacuum and impregnated to eliminate moisture.

Small transformer winding

Iron Cores

The three phase transformer iron

core has three legs.

A phase winding is placed in each leg.

The high voltage and low voltage windings are placed on top of each other and insulated by layers or tubes.

Larger transformer use layered construction shown in the previous slides.

Three phase transformer iron core

The dried and treated transformer is placed in a steel tank.

The tank is filled, under vacuum, with heated transformer oil.

The end of the windings are connected to bushings.

The oil is circulated by pumps and forced through the radiators.

Three phase oil transformer

The transformer is equipped with cooling radiators which are cooled by forced ventilation.

Cooling fans are installed under the radiators.

Large bushings connect the windings to the electrical system.

The oil is circulated by pumps and forced through the radiators.

The oil temperature, pressure are monitored to predict transformer performance.

Three phase oil transformer

Dry type transformers are used at medium and low voltage.

The winding is vacuumed and dried before the molding.

The winding is insulated by epoxy resin

The slide shows a three phase, dry type transformer.

Dry type transformer

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