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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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    Silicon Steel ( Sheet steel )

    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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    Silicon Steel ( Sheet steel )

    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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    Silicon Steel ( Sheet steel )

    HOT ROLLED SILICON STEEL , TRANSFORMER

    GRADE

    Small Transformers

    silicon content level .

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

    Grade 86 4.0 %, ( Iron Loss 1.89 w/kg )

    Grade 80 4.0 %, ( Iron Loss 1.76 w/kg )

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

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    Silicon Steel ( Sheet steel )

    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 )

    Grade 46 3.1 %, ( Iron Loss 0.97 w/kg )

    Grade 41 3.1 %, ( Iron Loss 0.89 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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    Heating and Cooling Curves

    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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    Heating and Cooling Curves

    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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    Types of Transformer

    Winding encircle the core .. Core type

    transformer

    core encircle the Winding .. Shell type

    transformer

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    Types of Transformer

    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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    Types of Transformer

    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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    Types of Transformer

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    Stepped Core and Yoke

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    Stepped Core and Yoke

    Oil ducts provide along the

    lamination ( longitudinal ducts )

    Lateral ducts

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    Stepped Core and Yoke

    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.