Transformers and power distribution

7/29/2019 Transformers and power distribution

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PowerPointPresentation

Chapter 4Transformer Connections

Voltage Transformation

Three-Phase Circuits

Delta Connections Wye Connections

Delta-to-Wye Connections Wye-to-Wye

Connections Wye-to-Delta Connections

Delta-to-Delta Connections Open DeltaConnections Delta Connection with High Leg to

Ground Three-Phase Transformer Polarity


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The low-voltage winding ofa standard distribution

transformer connected to a

single-phase circuit isnormally made with two

equal coils. These sections

are arranged so that they

may be connected in series

or parallel.
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Transformer polaritymay be subtractive or

additive.


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The polarity of atransformer may be

determined using a

known transformeror with AC voltage.


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A delta connection hasthe wires from the ends

of each coil connected

end-to-end to form aclosed loop.


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Studying the current ina delta-connected

system at both 0 and

90

illustrates thecurrent flow in a delta-

connected system.


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A wye connection hasone end of each coil

connected together

and the other end ofeach coil left open for

external connections.


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Studying the current ina wye-connected

system at both 0 and

90

illustrates thecurrent flow in a wye-

connected system.


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A 3-phase delta-to-wye connection is

often used for

distribution where afour-wire secondary

distribution circuit is

required.


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Three single-phasetransformers may

be connected in a

wye-to-wyeconnection to form

a 3-phase

transformer.


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A wye-to-deltaconnection permits

single-phase and 3-

phase loads to be

drawn simultaneouslyfrom the delta-

connected secondary

at the same voltage.


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In a wye-to-deltaconnection of three single-

phase transformers, one

unit may be disconnected

from the circuit and

service maintained with

the secondary operating

in open delta connection

at 57.7% of normal bank

capacity.


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In a delta-to-deltaconnection, the

voltage of each

transformer is equal to

the 3-phase linevoltage and the

current in each of the

transformers is only

57.7% of the line

current.


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Grounding one of themidpoints of the

windings provides 240

V/120 V, single-phase,3-wire service for

lighting applications.


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An open deltaconnection makes

3-phase power

available anywhere

along thedistribution line with

the use of only two

transformers rather

than the usual

three.


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A deltaconnection with a

high leg to ground

provides 240 V,3-phase power to

equipment and

120 V power for

lighting and

receptacles.


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The phasesequence on the

secondary of a

three-phasetransformer is the

same as the

phase sequence

on the primary.


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Transformers areclassified into

connection groups

according to theirphase shift.


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Transformers and power distribution