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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