Power systems - the power transformer

8/10/2019 Power systems - the power transformer

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The Power Transformer


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Transformer BasicsWhat is a transformerTransformer Basic TheoryPower TransformerConstruction of a Power Transformer

Ideal TransformerPractical TransformerThe Equivalent Circuit of a Power TransformerThe Power Transformer Phasor DiagramWinding Connections & Vector Groups

Protection of a Power Transformer


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Transformer Basics

What is a transformer It may be defined as a piece of static apparatus, which by e.m. indu

transforms a.c. voltage &b current between 2 or more windings @ frequency, and usually at different values of V & I.

In its basic form, it consists of 2 coils wound on a single magnetic

Transformer Basic Theory Faraday Newman Laws: Statements Basic Principle of operation: by mutual induction between 2 or mo

windings, which are linked by a common B-field, have energy tranbetween the 2 ccts without direct electrical or galvanic actio


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Transformer BasicsWhat is a transformer

Basic Circuits : (a) Tx on No Load, (b) Tx on Load The coupling efficiency & hence depends to a large exte

degree to which the common B- field links both circuits, i.e deof magnetic leakage. For this reason, the use of materials with , mainly steel based reduces the reluctance .


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The Power TransformerTransformer Basics

Tx Operation (ctd) On Load - see fig.02

I2 creates a flux 2 which opposes m . The demagnetizing effeweakens the balance between V 1 & E1, V1 E1. To compensatweakening, I 1 must increase appropriately so as to produce a flux

whichis = & opposite to 2 , i.e.

1 = 2 For any load

1 = 2This is known as the ampere turns balance


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Transformer BasicsBasic Tx Equation It relates the induced e.m.f E & m . = If the supply voltage is sinusoidal, K = 4.44, so = . Can re- write equn thus: = .

Tx Relations It can be shown that = =

To maintain magnetic balance in the circuit, the secondary ampereturns must be balanced by the opposite polarity ampere primary windings, i.e. =


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Transformer BasicsTx Relations (ctd)

Combining the above =

= =


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The Power TransformerThe Power TransformerWhat is a power tx

The term Power Transformer generally refers to all types of transformers exclcontrol txs .

Function: Transfers electrical energy from 1 voltage level to another: Can have step up or step- down tx : turns ratio

The need of a power tx in a power system

The tx allows electrical energy to be supplied to the load @ a pre-determined vollevel. Thus the coupled energy is transformed from 1 level to another It serves as an economical means of transporting the product to the con

(consumers) are very far away from the generating stns (the factory), i.e. Power Losses Safety to the consumer


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The Power TransformerThe Power TransformerPower Transformer Classification (ctd)

Application(a) Gen Tx(b) Grid - Tx(c) Distribution

Note :

(i) Power txs refers to the above(ii) have other types of txs(d) Control : VT, CT(e) isolation tx(f) communications

Construction: (a) Core, (b) Shell


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Transformer Construction Basics Core Tank Windings: Primary and Secondary Cooling Medium Bushings Reservoir etc


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The Power TransformerThree Phase Transformer Winding Connections

Wye Wye (Y - Y) Delta Delta ( - ) Wye Delta (Y - ) Delta Wye ( - Y)

Wye - Delta - Wye (Y - - Y) : tertiary winding Auto TxNote :1) Advantages & disadvantages See Notes2) Double Wound (2 winding)Tx vs Series (One) Winding Tx


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Three Phase Transformer Vector Groups Star - Star Connection: (Yy0) Delta Delta Connection: (Dd0) - Exercise Wye Delta Connection: (Yd11) Delta Wye Connection: (Dy11) Table


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The Power TransformerThe Ideal Transformer

What is an ideal tx? Hypothetical: Loss free

Pin = PoutWinding Resistance = 0

Eddy Current Loss = 0Hysteresis Loss = 0No flux leakage

We use the ideal tx model to approximate the behaviour of a real txpower system


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The Power TransformerThe Ideal 1 Phase Transformer : No LoadSteady State Behaviour

Property 1 : An ideal tx is lossless (hypothetical)MeaningWinding Resistance = 0: Rp = Rs = 0Eddy Current Loss = 0Hysteresis Loss = 0Pin = Pout

Voltage EquationsThe Transformer E.M.F Equation


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The Power TransformerThe Ideal 1 Phase Transformer : No Load

Property 2 : In an ideal tx all flux links all turns of all windings, i.flux leakage

Meaning Induced e.m.fs Transformation ratio: a - definition Equivalent Circuit on no load No Load current Voltage equations Phasor Diagram


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The Power TransformerThe Ideal 1 Phase Transformer : On Load

Demagnetizing Effect of i2

due N2i2

Additional current in N 1 to neutralize demagnetizing effect of i 2( ) = ( )

Balance of mmfs: ( ) = ( ) Property 3 : In an ideal tx, the permeability is infinite ( = )Meaning = ( ) = ( )

( )( )

=

=

Power Invariance Principle: For an ideal tx, Pin = Pout, i.e. VARs are cons

i.e. CLE


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The Power TransformerThe Ideal 1 Phase Transformer : On Load

Property 4 : Load reflected to the source: If an ideal tx isconnected to a load impedance Z, the apparent, Z, presented to thsource is

=


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The Power TransformerThe Real/Practical 1 Phase Transformer

Tx Losses C 0

Stray inductance: this affects the induced emfs


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The Power TransformerThe Real/Practical 1 Phase Transformer : Effect of Windin

Resistance Losses Distortion in the output voltage


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The Power TransformerThe Steady State Behaviour Real/Practical 1 Phase Tra

Real Tx on No - Load Equivalent Circuit Circuit Parameters: Primary: = Secondary: = Phasor Diagram


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The Power TransformerThe Steady State Behaviour Real/Practical 1 Phase Tra

Real Tx On- Load Equivalent Circuit Equivalent circuit referred to the primary Primary: = ( ) Secondary: = Phasor Diagram


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The Power TransformerTransformer Parameters

R1 R2 X1 X2 Rm Xm


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The Power TransformerDetermination of Transformer Parameters

Open Circuit (OC) Test Also called No Load Test Test performed by applying rated voltage at terminals of 1 wdg (of

V. wdg) while the terminals of the other wdg is kept open. This shouresult in rated voltage appearing on the HV wdg. Current drawn to blow s.t. I 2R loss 0 & power measured is almost all from core los

Aim is to determine (i) P c, (ii) Rm, (iii) Xm and a Measurements: (i) V 1 & V2, (ii) INL, (iii) Pc = = + Calculations


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The Power TransformerDetermination of Transformer Parameters

Short Circuit (S.C.) Test Test performed by shorting out one wdg (often LV) and apply

voltage at the other wdg (often HV) & adjusted until rated curr(IFL) flows in the HV wgd, at which time rated current should the LV wdg

Following are measured: (i) V sc, (ii) Isc, (iii) Psc Aim: To determine (i) R eq , (ii) Xeq , (iii) Load loss (P L,FL) Calculations


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The Power TransformerTransformer Performance

This addresses (i) Tx Efficiency and Tx V.R. Tx Efficiency What is the P in for a required P out ? = =

=

=

+

+ + = Maximum Efficiency: P cu = Pc


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The Power TransformerTransformer Performance

Tx Voltage Regulation (V.R.) Defined as the change in magnitude of the sec voltage from N.

condition to F.L. expressed as a %age of F.L. value Expression

Transformer Cooling ONAN ONAF ODAF OFAF, etc


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The Power TransformerTransformer Ratings (Specifications)

This gives nominal operating parameters These are No. of phases Frequency Winding Connections and Vector Group

Method of cooling Rated Apparent Power Voltage Ratio Tappings Rated voltage and current @ primary & secondary


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The Power TransformerTransformers Operating in Parallel

UJ APK campus 11 kV network Conditions (Some) The e.m.fs should be = in magnitude & phase ( vector

order to circulating currents & permit good load sharing The per unit impedances should be = in magnitude & have the

same angle, i.e. X/R ratios should be the same

h f


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The Power TransformerTransformers Protection

Bucholtz Winding Temperature Oil temperature E/F O/C, etc

Power systems - the power transformer

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