AY-2021-2022

DEPARTMENT OF ELECTRICAL ENGINEERING

SEMESTER-5^TH

SUB-(EC-II)

TOPIC-THREE PHASE TRANSFORMER

Er. SAILESH BARIK

THREE PHASE SYSTEM

BASICS

Line voltage VL= voltage between lines

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Phase voltage Vph= voltage between a line and neutral

THREE PHASE SYSTEM

BALANCED STAR

Line Voltage,

VL=√3Vph

Line current, IL = Iph

THREE PHASE SYSTEM

BALANCED DELTA

Line Voltage VL= Vph

Line current IL = √3 Iph

THREE PHASE TRANSFORMERS

Almost all major generation & Distribution Systems in the world are three phase ac systems

Three phase transformers play an important role in these systems

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3 phase transformers can be constructed from

• 3 single phase transformers

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(b) 2 single phase transformers

(c ) using a common core for three phase windings

3 phase Transformer connections

By connecting three single phase transformers

• Star- Star connection
• Delta- Delta connection
• Star – Delta connection
• Delta – Star connection

Star- Star connection

• This connection satisfactory only in balanced load otherwise neutral point will be shifted.

Star- Star connection

Advantages

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1.Requires less turns per winding i.e. cheaper

Phase voltage is 1/√3 times of line voltage

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2.Cross section of winding is large i.e. stronger to

bear stress during short circuit

Line current is equal to phase current

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3. Less dielectric strength in insulating materials

phase voltage is less

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Star- Star connection

Disadvantages

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1.If the load on the secondary side unbalanced

then the shifting  of neutral point is possible

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2.The third harmonic present in the alternator

voltage may appear on the secondary side. This

causes distortion in the secondary phase

voltages

3. Magnetizing current of transformer has 3^rd

harmonic component

Delta - Delta connection

• This connection is used for moderate voltages

Delta - Delta connection

Advantages

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1. System voltages are more stable in relation to

unbalanced load

2. If one t/f is failed it may be used for low power

level i.e. V-V connection

3. No distortion of flux i.e. 3^rd harmonic current not

flowing to the line wire

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Delta - Delta connection

Disadvantages

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1.Compare to Y-Y require more insulation.

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2. Absence of star point i.e. fault may severe.

Star- Delta connection

• Used to step down voltage i.e. end of transmission line

Star- Delta connection

Advantages

1. The primary side is star connected. Hence fewer number of turns are required. This makes the connection economical

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• The neutral available on the primary can be earthed to avoid distortion.

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• Large unbalanced loads can be handled satisfactory. 

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Star- Delta connection

Disadvantages

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The secondary voltage is not in phase with the primary. (30 ⁰ phase difference )

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Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.

Delta - Star connection

• This connection is used to step up voltage ie. Beginning of high tension line

Delta - Star connection

Features

• secondary Phase voltage is 1/√3 times of line voltage.

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• neutral in secondary can be grounded for 3 phase 4 wire system.

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• Neutral shifting and 3^rd harmonics are there.

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• Phase shift of 30⁰ between secondary and primary currents and voltages.

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THANK YOU