ThreePhase_Tx

8/2/2019 ThreePhase_Tx

1/34

slide 1

Electrical Machin es 1

IVE(TY)IVE(TY)

Department of EngineeringDepartment of Engineering

Elect r ical Machines 1

Hour 13


2/34

slide 2


IVE(TY)IVE(TY)


A set of three similar single phase transformers maybe connected to form a three-phase transformer(three-phase transformer bank). The primary andsecondary windings may be connected in either star

or delta configurations

Thr ee Phase Tr ansf or mer (sect . 2.6)


3/34

slide 3


IVE(TY)IVE(TY)


Ease of transportationInefficient magnetic circuit, less efficient

Higher capital cost than a single one

1-phase of the transformer at fault, the other

two are not affected

Thr ee-phase Bank


4/34

slide 4


IVE(TY)IVE(TY)


Usually 3-limb core structure

5-limb core may be used to reduce the overallheight of a 3-limb core

Magnetic flux shares the magnetic circuit

Fault on one-phase very likely affects the othertwo

Thr ee-phase Tr ansf or mer (sect . 2.6.2)


5/34

slide 5


IVE(TY)IVE(TY)


Providing voltage to auxiliary power purposes in

the substation or to supply local distributionsystem

Power factor correction

Delta connected tertiary windings will provide acircuit for the third harmonics of the excitingcurrent

Ter t iar y Winding

A third set of windings usually exists in 3-phasetransformer and used to:


6/34

slide 6


IVE(TY)IVE(TY)


1. HV winding has a capital letter. The letters used

are A B C.

2. LV winding has a small letter. The letters usedare a b c.

3. Letters are the same for all windings on the

same limb.

BSEN60076 (BS 171)Ter minal Mar king Syst em


7/34

slide 7


IVE(TY)IVE(TY)


4. A third winding, known as a tertiary winding, is

prefixed with the number 3 followed by the capital

letter for that limb.

BSEN60076Ter minal Mar king Syst em


8/34

slide 8


IVE(TY)IVE(TY)


5. A suffix number indicates the potential or polarity of

the terminal; with the number 1 being the lowest

potential and the number increasing for tappings up

the winding to the highest potential, as shown.



9/34

slide 9


IVE(TY)IVE(TY)


6. The phase shift through a transformer refers the

same alphabetical letters of the output and input

connections to the windings. The phase shift is theangular displacement of the output with respect to

the input.



10/34

slide 10


IVE(TY)IVE(TY)


BSEN60076 recognises only 4 phase-shifts and it isconvenient to use a clock face number reference

instead of degrees to describe the phase shift.

St andar d Phase-shif t s


11/34

slide 11


IVE(TY)IVE(TY)


Gr oup I Tr ansf or mer


12/34

slide 12


IVE(TY)IVE(TY)




13/34

slide 13


IVE(TY)IVE(TY)




14/34

slide 14


IVE(TY)IVE(TY)


Gr oup I I Tr ansf or mer


15/34

slide 15


IVE(TY)IVE(TY)


Gr oup I I Tr ansf or mer


16/34

slide 16


IVE(TY)IVE(TY)


Gr oup I I I Tr ansf or mer


17/34

slide 17


IVE(TY)IVE(TY)


Gr oup I I I Tr ansf or mer


18/34

slide 18


IVE(TY)IVE(TY)


Gr oup I V Tr ansf or mer


19/34

slide 19


IVE(TY)IVE(TY)


Let t er Descr ipt ion f or Var iousWinding Met hods


20/34

slide 20


IVE(TY)IVE(TY)


Example

State the BSEN60076 description, using appropriate numbers

and letters, of the following transformers and also state thephasor group number for each.

(a) Delta/star with a 30 lead phase shift. HV winding connected

in delta.

(b) Star/star with a 0 phase shift. HV winding connected in star.

(c) Star/delta with a 30

lag phase shift. HV winding connectedin star.

(d) Zigzag/star with a 30 lag phase shift. HV winding connectedin zigzag.


21/34

slide 21


IVE(TY)IVE(TY)


Answer t o Example

IIIZy 1(d)

IIIYd 1(c)

IYy 0(b)IVDy 11(a)

Phasor Group

Number

Transformer

Description


22/34

slide 22


IVE(TY)IVE(TY)


Elect r ical Machines 1

Hour 14


23/34

slide 23


IVE(TY)IVE(TY)


Tur n Rat io/ Volt age Rat io/Cur r ent Rat io of 3-ph Tx

1. STAR/delta (Yd)

2. DELTA/star (Dy)

3. DELTA/delta (Dd)

4. STAR/star (Yy)


24/34

slide 24


IVE(TY)IVE(TY)


1. STAR/ delt a (Yd )

1

2

2

1

2

1

2

1

.N3

N

.a3

1

I

I;

N

.N3.a3

V

V====


25/34

slide 25


IVE(TY)IVE(TY)


2. DELTA/ st ar ( Dy )

1

2

2

1

2

1

2

1

N

.N3

a

3

I

I;

.N3

N

3

a

V

V====


26/34

slide 26


IVE(TY)IVE(TY)


3. DELTA/ delt a ( Dd )

1

2

2

1

2

1

2

1

N

N

a

1

I

I;

N

Na

V

V====


27/34

slide 27


IVE(TY)IVE(TY)


4. STAR/ st ar (Yy )

1

2

2

1

2

1

2

1

N

N

a

1

I

I;

N

Na

V

V====


28/34

slide 28


IVE(TY)IVE(TY)


Par allel Oper at ion


29/34

slide 29


IVE(TY)IVE(TY)


Condit ion f or Par allel Oper at ion

The transformers must have the same phasedisplacement between primary and secondaryline voltage i.e. they must belong to the samegroup number

The phase sequence must be the same

The line-voltage ratios must be the same

The per-unit impedance of the transformersshould be the same


30/34

slide 30


IVE(TY)IVE(TY)


Har monics in Tr ansf or mer

Harmonics in transformer occur due to the effectof saturation and hysteresis which are to producenon-sinusoidal current if the applied voltage issinusoidal. Upon saturation, the flux waveform isflat topped and contains mainly 3rd harmoniccomponent.


31/34

slide 31


IVE(TY)IVE(TY)


Ef f ect s of har monic cur r ent s

(i) Additional I2R losses due to circulatingcurrents .

(ii) Increased iron loss in core.

(iii) Magnetic interference with protectivegear and communication circuits.


32/34

slide 32


IVE(TY)IVE(TY)


Ef f ect s of har monic volt ages

(i) Increased dielectric stress.

(ii) Electric field interference withcommunication circuit.

(iii) Harmonic resonance may occurbetween the inductance of transformerwindings and the capacitance of a

feeder to which it is connected.


33/34

slide 33


IVE(TY)IVE(TY)


I n 3-ph Supply Syst em (1/ 2)

Star-connected with isolated neutral the linevoltage contains no 3rd harmonic component and3rd harmonic current is precluded.

Star connected with neutral wire, the 3rd

harmonic currents can flow and still no 3rd

harmonic line voltages.


34/34

slide 34


IVE(TY)IVE(TY)


I n 3-ph Supply Syst em (2/ 2)

Delta Connected, the 3rd harmonic emfs aroundthe 3-phase windings are summed, which causesa circulating 3rd harmonic current around the 3-phase windings.

But the line voltages across any two lines containno 3rd harmonic component, since they are shortcircuited by the windings.

ThreePhase_Tx

Documents

Transcript of ThreePhase_Tx