part 10 emachines EEP 3243

8/14/2019 part 10 emachines EEP 3243

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ELECTRICAL MACHINE

EEP 3243

Lt Cdr Ong Khye Liat RMN

2 Mac 2010

1



RECAP/ADDITION

2



Voltage and Current Relationships in 3Ø

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

Phase 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

3



Type of Connection

Delta-delta Connection

Delta-wye Connection

Wye-delta Connection Wye-wye Connection

4



Open Delta Connection

I t is mainly used in emergency situation when 1 of the delta-delta connection becomes defective and has to be removed.

5



Cont.

Open delta connectionis seldom used as theload capacity of the

installed transformer bank is only 86.6% of the installed

transformer capacity.

6



PROBLEM 1

7



3-Phase Transformer

T ransformers for 3-phase circuits can be constructed in

2 ways.

± 3 single phase transformer connected in a 3 phasebank.

± Make 3 phase transformer consisting of 3 sets of windings wrapped on a common core.

8



Cont.

T he majority of three phase transformers aremostly in the form of core-type construction. Asmall or medium rated transformer is usually of

three-legged core-type.

9



The schematic of three phase

core-type magnetic paths U nder balanced condition,the currents in 3 phases areequal in magnitude, with

angles 120° apart. Accordingly, the vector formfluxes in three phases are120° apart and summed to

zero at the yoke. T here is no need of a return path for the flux.

10



Cont. I f there is some unbalance in the

terminal voltage, the sum of the 3 phase fluxes (residual flux), will not be zero and it has to return

through a path out of thetransformer magnetic core.

T his means the residual flux at thetop yoke has to pass through a

huge air gap and the tank to thebottom yoke.

11



Cont. T he path through the air gap and

the tank has low permeability and, thus, high magnetic reluctance.

I n order to overcome theshortcoming of three-legged transformers in unbalanced condition, a fourth leg is added tothe transformer core.

12



Four-legged core-type transformer

T he fourth leg provides a path for thezero-sequence magnetic flux.

For a four-legged transformer, thezero sequence magnetic reluctance issmall and the zero sequenceimpedance is large.

13



Cont. T he top and bottom yokes

generally are designed tocarry one per unit of zerosequence flux in the fourth

leg. I n order to reduce the height

of the yoke on a large rated transformer for

transportability, a fifth leg isadded to the four-legged transformer.

14



Five-legged core-type transformer

T he zero sequence performance of afive legged transformer is similar tothat of a four-legged transformer,which is approximately the positivesequence leakage impedancebetween the windings until theapplied voltage saturates either theyokes or the end legs.

15



Cont.

16



Cont.

A 3-phase transformer always cheaper and smaller than 3 single phase transformer for the samecapacity.

17



3 Phase Transformer Calculation

S ame way as for a single-phase transformer. S teps as following:

± Assume that the primary and secondary windings areboth connected in wye. I n wye connection theimpedance per phase is understood to be the line-to-neutral impedance. T he voltage per phase is simply theline voltage divided by ¥3 and the current per phase is

equal to the line current.

18



Cont.

Consider only single phase of this transformer bank. Primary voltage of this single phase transformer is theline-to-neutral voltage of incoming line.

S econdary voltage of this transformer is line-to-neutral

voltage of the outgoing line. T he nominal power rating of this transformer in the

rating of the 3 phase transformer bank. T he load on this transformer is the load on the

transformer bank.

19



PROBLEM 2

20



The per-unit system for

3P

hase Transformers S olving circuits containing transformers can be

quite a tedious operation because of the need torefer all the different voltage levels on different sides of the transformers in the system to acommon level.

T he per unit system can simplify the calculation.

21



Cont.

T he per unit system can simplify the calculation.

22



Cont.

A transformer has no effect on the base apparent power of the system, since the apparent power intoa transformer equals the apparent power out of thetransformer.

T he per unit system applies just as well to 3 phasetransformers as to single phase transformer.

23



Cont.

I f the total base voltampere value of thetransformer bank is called S base, then the basevoltampere value of one of the transformer, S 1Ø,base,

is

24



Voltage Regulation

With primary voltage held constant at its rated value,the voltage regulation (in %) is

Where

E NL = secondary voltage at no load

E FL = secondary voltage at full-load

25



Cont.

I f the transformer equivalent circuit is in the per-unit system, then voltage regulation can beexpressed as

26



Cont.

T he relationship between the base linevoltage/current and base phase voltage/current of the transformer depends on the connection of windings.

I f the windings are connected in delta, V L,base = V Ø,base

and I L,base = ¥3 I Ø,base.

While the windings are connected in wye,

V L,base = ¥3 V Ø,base and I L,base = I Ø,base.

27



PROBLEM 3&4

28





Cont.

As we slide contract P form phase B toward phase C,voltage E AP changes both in

amplitude and phase. T his simple phase shifter

only can only be used insmall load application.

30



Cont.

By moving the contract P, we obtain the sameopen circuit voltages

and phase shiftswhether theautotransformer delivers a current to the

load or not.

31



Cont.

Contract P 1, P 2 and P 3move in tandem enablesto create a 3 phase

source whose phaseangle changes stepwiseand the voltagesmagnitude vary from E line

to 0.5 E line.

32



Phase Shift Transformer

A special 3 phase autotransformer shifts the phaseangle between incoming and outgoing line without changing the voltage ratio.

T he angle may be leading or lagging, and isusually variable between zero and 20 o.

33



Cont.T he basic power rating of this transformer depends upon the apparent power carried by the transmission line and the phase shift. For angles

lessthan 20 o,

S T = 0.025 S L max

where,

S T = basic power rating of 3 phase transformer bank (VA),

S L = apparent power carried by the transmission line (VA),

max = maximum transformer phase shift ( o ),

0.025 = an approximate coefficient.

34



Cont.

T he basic principle of obtaining a phase shift is toconnect 2 voltages in series that are generated by 2 different phases.

35



Cont.

T hus, voltage E 1b generated by phase B is connected in series with E PN generated by phase A. T he values of E PN and E 1b are selected so that the output voltage is

equal to the input voltage while obtaining the desired phase angle.

36



3 Phase to 2 Phase Transformation

T he voltages in a 2 phase system are equal but

displaced by 90 o.

37



Cont.

U se a single phase autotransformer having taps at 50% and 86.6%T he ratio of transformation is fixed and given by E AB /E AT =

100/86.6 = 1.1547 .

38



Cont. U se scott connection

± Consists of 2 identical single phasetransformers, one having a 50% tapand other 86.6% tap on primary.

± T he ratio of transformation is givenby E AB /E 12 .

± Has advantage of isolating the 3 phase and 2 phase systems.

39



END OF PART 10

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part 10 emachines EEP 3243