ee207 electrical power - lecture 5

7/30/2019 EE207 Electrical Power - Lecture 5

1/12

EE207 Electrical Power

Lecture 5

Three phase transformers


2/12

Rajparthiban Kumar EE207 Electrical Power 2

Three Phase Transformers

A three phase transformer can be built in one of

two ways:

Connecting a bank of three single phase transformers

Constructing a three phase transformer on a common

magnetic structure

There are four possible connections for a three

phase transformer:

Y-Y: the primary and secondary of the three phase trans.

are connected in Y configuration. This connection israrely used because of problems with exciting current

and induced voltages.


3/12


-: primary and secondary connected as delta.This has the advantage of being able to operate

even if one of the phases is removed for repair.The remaining two phases can deliver 3 powerat reduced ratings of 58%.

Y-: primary is connected as Y and secondaryas . Commonly used to step down a highvoltage to a lower voltage.

-Y: primary is connected as and secondary asY. commonly used to step up the voltage.

The following shows the schematics of differentconfigurations of three phase transformers.



4/12


Three phase Transformers

a

1

b

2

c

3

1'

2'

3'

A

B

C

a

1

b

2

c

3

1'

2'

3'

A

B

C

a

1

b

2

c

3

1'

2'

3'

A

B

C

a

1

b

2

c

3

1'

2'

3'

A

B

C

Y-Y -

Y--Y


5/12



Y-Y Single phase

Relationships

sp

ls

LP

ls

LP

ZaZ

tiontransformaimpedanceThe

aIIand

VV

NNa

2

:

133

21

=

===

A

B

C

12

3

VA1

12

3

VlsVLNp

VLp

ILp

Vls

a

b

c

Ils

N1:N2

3LPV

3

lsV

ILP Ils


6/12


- Single Phase

Relationships


A

B

C

VLp

ILp

Vls

a

b

c

Ils

3LPI 3lsI

N1 N2

3

LPV

3

lsV

ILP Ils

3

2:

3

1 NN

sp

ls

LP

ls

LP

ZaZ


aI

I

andV

V

N

N

a

2

:

1

3

3

2

1

=

===


7/12


Y- Single Phase

Relationships


sp

ls

LP

ls

LP

ZaZ


aI

I

andaN

N

V

V

2)3(

:

3

1

332

1

3

3

====

============

A

B

C

1

23

VA1VLNp

VLp

ILp

Vls

a

b

c

Ils

3lsI

N2

3

LPV

3

lsV

ILP Ils

3

2:1

NN


8/12


-Y Single Phase

Relationships


sp

ls

LP

ls

LP

Za

Z

tiontransformaimpedanceTheaI

Iand

a

N

N

V

V

2)

3

(

:

3

32

31

3

3

====

============

12

3

Vls

Vls

a

b

c

Ils

A

B

C

VLp

ILp3

LPI

N1

3

LPV

3

lsV

ILP Ils

2:

3

1N

N


9/12


Phase Shift (Y-)


A

B

C

12

3

VA1VLNp

VLp

ILp

Vls

a

b

c

Ils3

lsI

N2

van

-vnb

vbn

vcn-vna

-vnc

bnbc vv 3====

=30=30=30=30

120o

120o

120

o

vab

vbc

vca

=30=30=30=30

120o

120o

120o

Note that for Y-transformer, the

primary line voltage isleading the secondaryline voltage by 30o.


10/12


Phase Shift (-Y)


Note that for -Ytransformer, the

primary line voltage islagging the secondaryline voltage by 30o.

1

23

Vls

Vls

a

b

c

Ils

A

B

C

VLp

ILp3LPI

N1

vab

vbc

vca

=30=30=30=30

120o

120o

120o

van

-vnb

vbn

vcn-vna

-vnc

bnbc vv 3====

=30=30=30=30

120o

120o

120o


11/12


Changing load into equivalent Y

A Balanced load can be converted into

equivalent Y as:


3

====Z

ZY

VL

IL

3====

ZZY

3

====Z

ZY

3

====

Z

ZYVL

IL

Z Z

Z


12/12


Example:

Three 1, 50 kVA, 2300/230V, 60 Hztransformers are connected to form a 3 4000/230V transformer. The equivalent impedance of eachtransformer referred to the low voltage side is:

0.012+j0.016. The 3 transformer supplies a 3load, 120kVA, 230V, p.f. 0.85 (lagging).

Draw a schematic diagram showing the transformerconnection.

Determine the transformer windings current.

Calculate the voltage regulation.


ee207 electrical power - lecture 5

Documents