TransformersApplied Electricity EE 2850*

ContentsIntroductionWorking principleEmf equationTurns ratioEquivalent circuit & phasor diagramVoltage regulationLosses & efficiencyCoolingTests on transformersThree phase transformersOther types of transformers

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1)Introduction*A static device, which transfers electrical energy from one circuit to another circuit, without change of frequency

Types of transformers:Power transformers (Single-phase or 3-phase)- Used in power transmission and distribution systems - Step-up or step-downCurrent transformersVoltage transformersAuto-transformers

2) Working Principle(Single-phase power transformers)*2 windingsPrimary - connected to the alternating voltage sourceSecondary - delivers the power to the load

Ideal Transformer*Secondary is open circuitedIt does not supply any energy to the load

Transformer Taps*Voltage drops in transmission linesSecondary voltage is considerably less

Taps are provided on the primary windingTaps enable to change the turns ratio -> to raise the secondary voltage

Automatic taps Maintain the secondary voltage within 2% of its rated value

3)emf Equation*Sinusoidal primary current produces a sinusoidal flux

emf induced in the primary winding

maximum value of e1

effective value of the primary emf

Similarly, the effective value of the secondary emf

4)Turns Ratio*Stepped up and down transformer depends on the turns ratio

For an ideal transformer,Magneto-motive force due to primary current = Magneto-motive force due to secondary current

5)Equivalent Circuit and Phasor Diagram*Practical Transformers

Windings of practical transformers have resistance

Core is not infinitely permeable

Flux is not completely transferred to the secondary

Iron cores produce eddy-current and hysteresis losses

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Rm (core loss resistance) - represents iron lossesXm (magnetizing reactance) - measure of the permeability of the transformer core

Practical Transformer

Equivalent Circuit & Phasor Diagram*

Referring Impedances*

Resistance in the primary becomes , when referred to the secondary

Resistance in the secondary becomes , when referred to the primary

*Referred to primary

Referred to secondary

Equivalent Circuit

*Referred to primary

Referred to secondaryApproximate Equivalent Circuit

Example 1*A 100kVA single phase transformer has 400 turns in the primary and has 80 turns in the secondary. In primary side X1=1.1 and R1=0.3. In secondary side, X2=0.035 and R2=0.01. Supply voltage is 2200V. Determine the equivalent impedance referred to primary.

6)Voltage Regulation*

When the transformer is loaded -> secondary voltage changes due to drop in the internal impedance

Voltage regulation is a term that is used to identify this variation

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*For leading power factor, the sign is negative.

Example 2*A 100kVA single phase transformer has 400 turns in the primary and has 80 turns in the secondary. In primary side X1=1.1 and R1=0.3. In secondary side, X2=0.035 and R2=0.01. Supply voltage is 2200V. Calculate the voltage regulation and secondary terminal voltage at full load at power factor of 0.8 lagging.

7)Losses & Efficiency*

Efficiency is a function of secondary current

For maximum efficiency;

8)Cooling*

Indoor transformers below 200kVA Cooled by the natural flow of the airPlaced inside a metallic housing which is having ventilating louvers

Larger transformers Built the same wayForced circulation of clean air is provided

Distribution transformers below 200kVA Immersed in mineral oil and enclosed in a steel tankHeat is dissipated by radiation and convection to the outside air

*Large distribution transformersExternal radiators are added to increase the cooling surface of the oil filled tankOil circulates around the transformer windings and moves through the radiatorHeat is released to the surrounding air

For still higher ratingsCooling fans blow air over the radiators

For transformers in the megawatt rangeCooling may be effected by an oil-water heat exchanger

9)Tests on Transformers*Open Circuit Test

High voltage side kept openWattmeter reading is the core loss

*Short Circuit Test

Low voltage side short circuitedWattmeter reading is the copper loss

Example 3*A 2400V/240V, 24kVA transformer gave the following results. High voltage side open- 240V, 2A, 200WLow voltage side closed- 100V, 10A, 150WCalculate the voltage regulation if it is supplying an load of 0.8 power factor lagging at full load.Calculate operating efficiency and maximum efficiency.

10)Three Phase Transformers*Made either by:3 single phase transformers connected in a 3phase bank 3 windings are wound on a common core1st approach is cheaper, smaller and lighter

Three-phase transformer connectionsDepending on the connections of primary and secondary coils; Y-YY--Y-

Y-Y connection*

Rarely used for large amount of power transmissionCan be used for special types of applications

Y- connection*

Used at substations where the voltage is stepped downSecondary coil current is 57.7% of the load current

-Y connection*

Used at the generator station, where the voltage is stepped up

- connection*

If one transformer is removed, the remaining two are able to supply three-phase power to the load terminalsKnown as open delta or V-V connection

11) Other types of Transformers*Auto TransformerA small rating transformer with variable outputUsed in educational laboratoriesHas one winding -> common for primary and secondaryAdvantages:Cheaper initial investmentLow leakage reactanceLow lossesRequires low excitation current

*2)Instrument Transformers

Voltage TransformerRatio of primary to secondary voltage is knownUsed to measure the voltage on transmission lines

Current TransformerRatio of primary to secondary current is knownUsed to measure the current in a line

*3)Toroidal Current Transformers

Used when line current exceeds 100ALaminated ring shaped core that carries the secondary windingPrimary - a single conductor passes through the center of the ringSimple and inexpensiveUsed in low and medium voltage indoor installationsUsed in circuit breakers

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