Download - Transformer Design 1

7/22/2019 Transformer Design 1

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TRANSFORMER

DESIGN

Er. T.K.RAMACHANDRAN , B.E., M.B.A


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TRANSFORMER DESIGN

TOPICS COVERED Aim Of Design ..

Magnetic Circuit ..

Magnetic Materials ..

Magnetization Characteristics ..

Silicon Steel Types

Operating Flux Density

Heating and Cooling ...

Ventilation .


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TRANSFORMER DESIGN

TOPICS COVERED

Types of Transformer .

Stepped core and yoke . Tank and accessories ..

Buchholz Relay .

Bushings


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Aim of Design :

Should be carried out based on the givenspecification

Using available materials economically

To achieve

* LOWER COST

* LOWER WEIGHT

* REDUCED SIZE

* BETTER OPERATING PERFORMANCE


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Aim of Design :

The Magnetic circuit design for variouselectrical machines, to achieve a reasonable

high flux in various parts of the magnetic

circuit without any increased losses.

The Magnetic circuit calculations largely

depends on the relationship betweenmagneto motive force and flux in various

magnetic partsof the circuit.


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Magnetic circuit of a Transformer

Single Phase Core Type Three Phase Core Type


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Should be quite familiar with

Various electrical materials , magnetic,

conductors , insulators and their properties.

Properties of magnetic and electrical fields.

Laws of governing electrical circuits.

Laws of governing electromagnetic induction.

Calculation of magnetic circuits.

Construction and behavior under working

condition.


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Magnetic materials

Design of electrical circuit ( winding ) anddielectric circuit ( insulation ) is mainlydependent upon limitations imposed by themagnetic materials.

Sizeof the magnetic frame will depend uponthe flux density at which the magneticmaterial can be worked.

Best magnetic materials for electricalmachines are those, worked at higher fluxdensities.


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Magnetic materials Qualities

HIGH MAGNETIC PERMEABILITYso that even aweak current flowing in the electromagnet canset up large fluxesin its core.

HIGH ELECTRICAL RESISTIVITY to reduce the

eddy current losses occuring in the magneticmaterial, achieved by core with laminations,insulated from each other by varnish. At higherfrequencies thickness of lamination must be

reduced. HYSTERESIS LOOPIt should be narrow and must

have small area to reduce the hysteresis loss.


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Magnetic materials Characteristics

Magnetization curves

Plotted in terms of Magnetic flux density in

Tesla and the magnetizing force per unit

length in ampere turns per metre. Most

essential one for the calculation of ampere

turns needed for the magnetic circuit.


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Magnetization

Curves

ofDifferent

Magnetic

Materials


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Magnetic materials Characteristics

Iron Loss curves

Curves for the total iron losses per kg of the

magnetic material Vs flux density are needed

during calculations of an electrical machine to

find out the total iron losses occuring in a

particular part of the machine, corresponding

to the operating flux density of that part.


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Silicon Steel ( Sheet steel )

The Magnetic properties of steel are greatlyimproved by adding a certain percentage of

silicon.

Addition of silicon increases the electricalresistivity of steel , it reduces eddy current

losses.

Addition of silicon above 1.8% increasesmagnetic permeability of the material.


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Addition of large percentage deteriorates the

mechanical properties.

Silicon contentin the order from 0.3% to 4.0%,Depends up on type of the machines.

Maximum silicon steel lamination thickness

from 0.35 mm to 0.5 mm.


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HOT ROLLED SILICON STEEL DYNAMO GRADE

Rotating Machines

silicon content level .Lohys..0.3 %, ( Iron Loss 3.57 w/kg )

Special Lohys.0.85%, ( Iron Loss 2.84 w/kg )

and42Quality..2.5 % ( Iron Loss 2.25 w/kg )


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HOT ROLLED SILICON STEEL , TRANSFORMER

GRADE

Small Transformers

silicon content level .

Grade 92 4.0 %, ( Iron Loss 2.02 w/kg )



Grade 74 4.0 %. ( Iron Loss 1.63 w/kg )


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COLD ROLLED GRAIN ORIENTED SILICON STEEL ,

TRANSFORMER GRADE

Medium and Large Transformers

silicon content level .

Grade 51 .. 3.1 %, ( Iron Loss 1.11 w/kg )




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Magnetic circuit of a Transformer


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Operating Flux Density

CORE AREA..

Hot Rolled Silicon Steel 1.1 to 1.4 Tesla

Cold Rolled Grain Oriented Silicon Steel 1.5 to

1.7 Tesla

YOKE AREA..

Hot Rolled Silicon Steel 1.0 to 1.2 TeslaCold Rolled Grain Oriented Silicon Steel 1.3 to

1.5 Tesla


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Heating and Cooling

Conducting Material

Magnetic Material

Insulating Material . Most Sensitive to

temperature rise.

Because 10C rise in temperature in anelectrical machine reduces the life of

insulation used there in, by 50%.


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Heating and Cooling

Insulating Material . Most Sensitive to

temperature rise.

Resistivity of the insulating material decreases.

Electric Strength Decreases.

Dielectric Loss angle increases.

Dielectric losses increases. Tensile strength decreases.


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Heating and Cooling

The total lossesoccurring in an electrical

machine is converted into heat and as a result,

the various parts of the machine are heated

and their temperature rises above theAmbient temperature.

The various parts of the machine dissipatesheat by radiation, convection andconduction.


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Heating and Cooling

In Transformersthe heat dissipated by conductionis negligible.

the heat dissipated by convection is maximum.

the heat dissipated by radiation is medium.

NATURAL CONVECTION

ARTIFICIAL CONVECTION


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Heating and Cooling

NATURAL CONVECTIONLiquid or the Gas particles near heated surface

of the machine becomes lighter. Lighter liquid

is carried away by the circulating currentproduced by the heavier fluid. Such a

continuous processes, by which heat transfer

from surface to liquid then to atmosphere isknown as natural convection.


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Heating and Cooling

ARTIFICIAL CONVECTION

Machines of large sizes, heat is removed by

forced circulation of cooling medium.

Cooling of transformer by blasting air on theexternal radiators,

Cooling of turbo-alternators by Hydrogen

under pressure.


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Heating and Cooling Curves


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Curve A A AShows the heating and cooling curves when the machine

working continuously on full load.

Curve B . B B

Shows heating and cooling condition of the machine, when it

operated on full load for a period of say 20 min. followed by a

stationary period sufficiently long so as to cool the machine.

Curve C . C C

Indicates heating and cooling conditions in case of intermittent

load cycle of 15 min. on full load followed by 25 min. on no

load, the cycle being repeated indefinitely.


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STANDARD RATINGS .Continuous ratingGives output without exceeding the specified

temperature.

Short time ratingOperated for specific period without exceeding the

specified temperature.

Intermittent periodic ratingOperated for short period followed by short periodof rest or no load and without exceeding the

specified temperature.


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Ventilation of Static Machines Transformer being a static machine, cooling is

more difficult compared to a rotatingmachine. Because no relative motion to assist

in the circulation of ventilating air.

The Iron losses/Kg. of magnetic material andthe copper losses/Kg. of the conducting

material are nearly equal for pow er

transformer. The cooling surface of the tank will not be in

a position to dissipate all the heat generated

by the losses.


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Ventilation in

Transformer

*Duct betweencore and windings.

*Ducts between windings.

*Ducts in the coreand yoke in case of

Large Transformers.

*Ducts in the windings forguided circulation of oil.


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Ventilation in

Transformer

The transformer attainsfinal temperature

depending on the cooling

system and the amount of

heat generated. Final

temperature, when theheat generated is equal to

the heat dissipated.


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Ventilation in Transformer

Heat transferred from interior of the magnetic

core and the windings to the external surface

in contact with oil by conduction.

Heat transfer by convection from the surface

of the core and winding to the oil.

Heat transfer by convection from oil to the

tank walls and the radiators.

Transfer Heat from wall radiators to the

atmosphere by convection and radiation.


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Types of Transformer

Position of the windings with respect to core

CORE TYPE and SHELL TYPE

Transformation ratio

STEP-UP and STEP-DOWN

Types of service

DISTRIBUTION and POWER


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Winding encircle the core .. Core type

transformer

core encircle the Winding .. Shell type

transformer


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COMPARISON :

* Construction:core type assembled anddismantled more easily compared to shell type.

* Repair : repairs can be carried out in a better way

compared to shell type. Windings is accessable.* Cooling : cooling of winding is better in core

type, because windings encloses the core, in

shell type cooling of core is better.* Leakage Reactance : shell type is smaller

compared to core type, better linkage between

L.V. and H.V. side.


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COMPARISON :* Mechanical forces: shell type transformer

are more robust mechanically compared to

core type, because of better support towindings against to e.m.fs.

* Application : shell type transformers is

better for low voltages and large output. In

core type for high voltage and

comparatively smaller output.


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Stepped Core and Yoke


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Oil ducts provide along the

lamination ( longitudinal ducts )

Lateral ducts


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In general circular coils are used , for better

mechanical strength , theoretically circular

core but complicated for production.

Stepped core is generally used.

Small transformer square core is used.


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Tank and Accessories

Conservator

Voltage rating of 6 K.V and

output rating of 25 K.V.A equipped with it.

Breather

contains calcum chloride or silica gel.

Buchholz Relay

Bushings


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Buchholz Relay


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Bushings


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THANK YOU

Er. T. K. RAMACHANDRAN. B.E., M.B.A.

Download - Transformer Design 1

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