part 9 emachines EEP 3243

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ELECTRICAL MACHINE

EEP 3243

Lt Cdr Ong Khye Liat RMN

24 Feb 2010

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3-Phase Transformers

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Introduction

3 phase o/p voltage depends upon the turns ratio andtype of connection ( or Y).

In making the various connections, it is important to

observe transformer polarities. Wrong polarity maycause short-circuit or unbalance line voltage andcurrent.

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Cont.

3 phase transformer can produce a phase shiftbetween the 3 phase i/p voltage and 3 phase o/pvoltage. The amount of phase shift depends again

upon the turn ratio and how the primaries andsecondaries are interconnected.

Phase shift feature enables to change the number ofphase (2-phase, 6-phase or 12-phase).

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Voltage and Current Relationships in 3

Phase current = line current. Phase current = line current/3Phase voltage = line voltage/3 Phase voltage = line voltage

Same total apparent power

Voltage across the elements are 120o out of phase

Current across the elements are 120o out of phase

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Type ofConnection

Delta-delta Connection

Delta-wye Connection

Wye-delta Connection Wye-wye Connection

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Delta-delta Connection

Terminal H2 of eachtransformer is connectedto terminal H1 of the next

transformer. Similarly to the terminal X2

and X1.

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Cont. The voltages between the

respective incoming andoutgoing transmission lines arein phase.

The line currents are 3

timesgreater than the respectivecurrents Ip and Is flowing in theprimary and secondary windings.

The power rating of the 3 phase

transformer bank is 3 times therating of a single transformer.

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Delta-wye Connection

Terminal H2 of eachtransformer is connectedto terminal H1 of the next

transformer. All terminal X2 are joined

together, creating acommon neutral N.

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Cont.

Voltage across each primarywinding is equal to theincoming line voltage.

Outgoing line voltage is3

times the secondary voltageacross each transformer.

The line currents in phase A,Band C are 3 times the

currents in the primarywindings.

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Cont.

The line currents in phase 1,2and 3 are the same ascurrents in the secondary

windings. This connection produces a30o phase shift between theline voltages of the incoming

and outgoing transmissionlines (E12 is 30o ahead of EAB).

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Wye-wye Connection

VLL on each side transformer is 3 times thenominal voltage of the single-phase transformer.

Special precautions have to be taken to preventsevere distortion of the VLN.

There is no phase shift between the incoming andoutgoing transmission line voltages.

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Distortion of the VLN

Without grounding the neutral terminals, the wye-wyeoperation is satisfactory only when the 3 phase load isbalanced. The electrical insulation is stressed only to

about 58% of the line voltage while most transformerare designed to operate at or above the knee of thecurve, such design causes the induced emfs andcurrents to be distorted.

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Cont.

To prevent the distortion, we can connect/ground theneutral of the transformer primary and source.

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Cont.

Or provide each transformer with a tertiary windingwhich connected in delta.

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Wye-delta Connection

The currents and voltages in a wye-delta connectionare identical to those in the delta-wye connection.

Also produces a 30o phase shift between the incomingand outgoing lines voltages.

Very suitable for step-down application.

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Wye-delta Connection

The distortion in the waveform of the induced voltage isnot as drastic as wye-wye connection because the

distorted currents in the primary give rise to acirculating current in the delta connected secondary.

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PROBLEM

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ENDOF PART 9

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part 9 emachines EEP 3243