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Chapter: Electrical machines - Synchronous Generator

Important Short Questions and Answers: Synchronous Generator

Electrical machines - Synchronous Generator - Important Short Questions and Answers: Synchronous Generator

SYNCHRONOUS GENERATOR

 

1.             What do you mean by the salient-pole type rotor?

 

Salient - pole type rotor means a low and moderate speed rotor having large diameter and small axial length with projected poles coming out of the rotor frame the outer surface of which almost follows the inner cylindrical surface of the stator frame.

 

2.             Define voltage regulation of an alternator

 

The voltage regulation of an alternator is defined as the increase in terminal voltage when full load is thrown off, assuming field current and speed remaining the same.

 

Percentage regulation = (E0 – V) /V x 100 

E0 = No load terminal voltage

V = Full load rated terminal voltage.

 

3. What are the advantages of having rotating field system?

1.   Better insulation   

2. Ease of current collection

3.   Increased armature tooth strength.

4. More rigid construction

5. Reduced armature leakage reactance.

6. Lesser number of sliprings.

7.  Lesser rotor weight &intertia   

8.  Improved ventilation & heat dissipation.

 

4.             Why is EMF method called Pessimistic method?

 

The value of voltage regulation obtained by EMF method is always more than the actual value, therefore it is called Pessimistic method.

 

5.             Why is MMF method called Optimistic method?

 

Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method.

 

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6.             Compare salient pole rotor & smooth cylindrical rotor.

 

Salient Pole Rotor

1 Large diameter and short axial

2. Used for low speed alternators

3.Has projecting poles

4. Needs damper windings

5.Windage loss is more

 

Cylindrical Rotor

1 . Small diameter and long axial length, length

2. Used for high - speed turboalternators

3. No projecting poles

4. Does not need damper windings.

5. Windage loss is less

 

7.How is the armature winding in alternators different from those used in dc machines?

The armature winding of the alternator is placed in the stator, but the in case of dc machines, armature winding is placed in rotor.

 

8. What are the methods by using zero p.f. lagging curve can be obtained?

Zero power factor characteristic of an alternator gives the variation of terminal voltage with field current, when the alternator is delivering its full rated current to a zero power factor (lagging) load. This characteristic is obtained by running the machine at synchronous speed and connecting a purely inductive

 

3phase load to its terminals. The load is varied in steps and at each step the field current is adjusted, so that the armature current is equal to its rated value.

 

9.         What are squirrel-cage windings of alternators? How and why are they used?

 

Damper windings are squirrel cage windings of the alternators.

This winding is placed in rotor pole shoes

 

10. Write down the equation for frequency of emf induced in an Altenator.

 

Frequency of emf induced in an Alternator,f ,expressed in cycles per second or Hz, is given by the following equation

 

F = (PN)/120 Hz,

 

Where 

P- Number of poles 

N-Speed in rpm

11. What are the advantages of salient pole type construction used forSynchronous machines?

Advantages of salient-pole type construction are :  They allow better ventilation

The pole faces are so shaped that the radial air gap length increases from the pole center to the pole tips so that the flux distribution in the air-gap is sinusoidal in shape which will help the machine to generate sinusoidal emf

Due to the variable reluctance the machine develops additional reluctance power which is independent of excitation

 

12. Name the types of Alternator based on their rotor construction.

 

Alternators can be classified into the following two types according to its rotor construction 

Smooth cylindrical type alternator

Salient pole alternator

13.Why is short pitch winding preferred over full-pitch winding ?

 

Advantages

Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated.

 

Conductor material , copper , is saved in the back and front end connections due to less coil-span.

 

Fractional slot winding with fractional number of lots/phase can  be used which in turn reduces the tooth ripples.

 

Mechanical strength of the coil is increased.

 

14. Define winding factor.

The winding factor Kd is defined as the ratio of phasor addition of emf  induced in all the coils belonging to each phase winding to their arithmetic addition.

 

15. Why are Alternators rated in kVA and not in kW?

The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class

 

Apart from the constant loss incurred in Alternators is the copper loss, occurring in the 3 -phase winding which depends on I2 R, the square of the current delivered by the generator.

 

As the current is directly related to apparent - power delivered by the generator , the Alternators have only their apparent power in VA/kVA/MVA as their power rating.

 

16.What is the necessity for predetermination of voltage regulation?

 

Most of the Alternators are manufactured with large power rating ,hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

 

17. Name the various methods for predetermining the voltage regulation of 3-phase Alternator.

 

The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators

 

 Synchronous impedance / EMF method  Ampere-turn / MMF method

Potier / ZPF method

 

 

18. What are the advantages and disadvantages of estimating the voltage regulation of an Alternator by EMF method?

 

  Simple no load tests (for obtaining OCC and SCC) are to be conducted   Calculation procedure is much simpler

Disadvantages:

The value of voltage regulation obtained by this  method is always higher than the actual value

 

19. What are the test data required for predetermining the voltage regulation of an Alternator by MMF method?

Data required for MMF method are :

 

  Effective resistance per phase of the 3-phase winding R   Open circuit characteristic (OCC) at rated speed/frequency   Short circuit characteristic (SCC) at rated speed/frequency

 

22. State the condition to be satisfied before connecting two alternators in parallel.

 

The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars.

 

The terminal voltage magnitude of the incoming Alternator must be made equal to the existing Alternator or the bus-bar voltage magnitude.

 

The phase sequence of the incoming Alternator voltage must be similar to the bus-bar voltage. The frequency of the incoming Alternator voltage must be the same as the bus-bar voltage.

 

23.List the factors that affect the load sharing in parallel operating generators?

 

The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors

  Prime-mover characteristic/input

 

Excitation level and

Percentage synchronous impedance and its R/X ratio

 

24.Why almost all large size Synchronous machines are constructed with rotating field system type?

 

The following are the principal advantages of the rotating field system type construction of Synchronous machines:

 

The relatively small amount of power, about 2%, required for field system via slip-rings and brushes. For the same air gap dimensions, which is normally decided by the kVA rating, more space is available in the stator part of the machine for providing more insulation to the system of conductors, especially for machines rated for 11kV or above. Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.

 

Stationary system of conductors can easily be braced to prevent deformation. It is easy to provide cooling arrangement for a stationary system of conductors.

 

Firm stationary connection between external circuit and system of conductors enable he machine to handle large amount of volt-ampere as high as 500MVA.

 

 

 

25.Why do cylindrical Alternators operate with steam turbines?

 

 

Steam turbines are found to operate at fairly good efficiency only at high speeds. The hig h speed operation of rotors tends to increase mechanical losses and so the rotors should have a smooth external surface. Hence, smooth cylindrical type rotors with less diamete rand large axial length are used for Synchronous generators driven by steam turbines with either 2 or4 poles.

 

 

26.Which type of Synchronous generators are used in Hydro-electric plants and why?

 

As the speed of operation is low for hydro turbines used in Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilatio nandalso have other advantages over smooth cylindrical type rotor.

 

 

27.How does electrical degree differ from mechanical degree?

 

Mechanical degree is the unit for accounting the angle between two points based on their mechanical or physical placement. Electrical degree is used to account the angle between two points in rotating electrical machines. Since all electrical machines operate with the help of magnetic fields,theelectricaldegreeisaccountedwithreferencetothemagneticfield.180 electrical degree is accounted as the angle between adjacent North and South poles.

 

 

28.What is distributed winding?

 

When coil-sides belonging to each phase are housedor distributed in more than ones lot under each pole regi on then the winding is called distributed winding. A full pitch coil has width of coil other wise called coil-span as 180º-angle between adjacent slots in electrical degree and x=1,2,3…

 

 

29.Definewinding factor.

 

ThewindingfactorKdisdefinedastheratioofphasoradditionofemfinducedinallthe coilsbelonging toeach phasewinding to theirarithmeticaddition.

 

 

30.What are the causes of changes in voltage in Alternators when loaded?

 

VariationsinterminalvoltageinAlternatorsonloadconditionareduetothefollowing three causes:

□                                   Voltage variation due to the resistance of the winding, R

□                                   Voltage variation due to the leakage reactance of the winding, Xt

 

 

31.What is meant by armature reaction in Alternators?

 

Theinteractionbetweenfluxsetupbythecurrentcarryingarmatureandthemainis defined asthe armature reaction.

 

 

32.What do you mean by synchronous reactance?

 

Synchronous reactance Xs=(Xl+Xa) The value of leakage reactance Xl

 

Is constant for a machine based on its construction. Xa depends on saturating condition of the machine. It is the addition of Xa, which represent the armature reaction effect between two synchronously acting magnetic fields that makes the total reactance Xa to be called synhronous reactance.

 

 

33.What ismeant by synchronous impedance of an Alternator?

 

 

The complex addition of resistance, Rand synchronous reactance, jXs can be represented together by a single complex impedance Zs called synchronous impedance.

 

In complex form Zs = (R + jXs )In polar form Zs = |Zs| <Where |Zs|= ¥_52 + X2 s)

 

 

34.What is meant by load angle of an Alternator?

 

The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , U during the load condition of an Alternator is called load angle.

 

 

35.Why is the stator core of Alternator laminated?

 

The stator core of Alternator is laminated to reduce eddy current loss.

 

36.State the condition to be satisfied before connecting two alternators in parallel

 

The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars.

 

□                                   The terminal voltage magnitude of the incoming Alternator must be made equal to the existing Alternator or the bus-bar voltage magnitude.

 

□                                   The phase sequence of the incoming Alternator voltage must be similar to the bus-bar voltage.

 

□                                   The frequency of the in coming Alternator voltage must be the same as the bus-bar voltage.

 

37.How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators?

 

The correctness of the phase sequence can be checked by looking at the three sets of lamps connected across the3-pole of the synchronizing witch. If the lamps grow bright and darkinunisonitisan indication of the correctness of the phase sequence. If on the other hand, they become bright and dark one after the other, connections to any two machine terminals have to be interchanged after shutting down the machine.

 

 

38.Whataretheadvantagesanddisadvantagesofthreedarklampsmethodofsynchronizing?

 

 

 

Advantages:

□                                   The synchronous switch using lamps is inexpensive

 

□                                   Checking for correctness of the phase sequence can be obtained in a simplemannerwhichisessentialespeciallywhentheAlternatorisconnectedforthefirst time or for fresh operation after disconnection .

 

 

Disadvantages:

 

□  The rate of flickering of the lamps only indicates the frequency difference between the bus-barand the incoming Alternator. The frequency of the in coming Alternator in relation to the bus-bar frequency is not available.

 

 

39.Howsynchronoscope is used for synchronizing Alternators?

 

Synchronoscope can be used for permanently connected Alternators where the correctness of phase sequence is already checked by other means. Synchronoscope  is capable of rotating in both directions. The rate of rotation of the pointer indicates the amount of frequency difference between the Alternators. The direction of rotation indicates whether incoming Alternator frequency is higher or lower than the existing Alternator. The TPSTs witch is closed to synchronize the incoming Alternator when the pointer faces the top thick line marking.

 

 

40.Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance?

 

The presence of more resistance in the Synchronous generators will resist or oppose their synchronous operation. More reactance in the generators can cause good reaction between the two and help the generatorstoremainin synchronisminspite of any disturbance occurring in any one of the generators.

 

 

41.List the factors that affect the load sharing in parallel operating generators?

 

The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors

 

□                                   Prime-mover characteristic/input

□                                   Excitation level and

□                                   Percentage synchronous impedance and its R/X ratio

 

 

42. How does the change in prime mover input affect the load sharing?

 

An increase in prime-mover input to a particular generator causes the active

 

power shared by it to increase and a corresponding decrease inactive- power shared by other generators. The change in reactive power sharing is less appreciable. The frequency of the bus-bar voltage will also subjected to slight increase in value.

 

 

43. How does change in excitation affects the load sharing?

 

The decrease in excitation in one generator causes the reactive power shared byitto decrease and a corresponding increase in reactive-power shared by other generators. The change inactive-power sharing is less appreciable. There will be a slight decrease in terminal voltage magnitude also.

 

44.What steps are to be taken before disconnecting one Alternator from parallel operation?

 

The following steps are to be taken before disconnecting one Alternator from parallel operation

 

 The prime-mover input of the outgoing generator has to be decreased and that of other generators has to be increased and by this the entire active-power delivered by the outgoing generator is transferred to other generators.

 

□                                   The excitation of the outgoing generator has to be decreased and that of other generators have to be increased and by this the entire re active-power delivered by the outgoing generator is transferred to other generators.

 

□                                   After ensuring the current delivered by the outgoing generator is zero,it has to be disconnected from parallel operation.

 

45.What is meant by infinite bus-bars?

 

The source or supply lines with non-variable voltage and frequency are called infinite bus-bars. The source lines are said to have zero source impedance and infinite rotational inertia.

 

46.How does increase in excitation of the Alternator connected to infinite bus-bars affect this operation?

 

Increase in excitation level of the synchronous generator will effectively increase the reactive component of the current supplied by the generator and hence the active power delivered.

 

47.Upon what factors does the load angle depend?

 

Angle is positive during generator operation and negative during motor operation.

 

48.An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?

 

The nature of the load is of leading power factor ,load consisting of resistance and capacitive reactance.

 

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