Can transformers be used in parallel?

 

Figure: Transformers in Parallel

Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other. For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have an impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.

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37 Interview Questions and Answers on DC Generators - Part-3

Q. 1. How may the number of parallel paths in an armature be increased?

Ans. By increasing the number of magnetic poles.

Q. 2. How are brushes connected in a d.c. generator?

Ans. Usually, all positive brushes are connected together and all the negative brushes together (figure-a.).

Q. 3. What is meant by armature reaction?

Ans. It is the effect of armature magnetic field on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:

1. It demagnetises or weakens the main flux and
2. It cross-magnetizes or distorts it.

Figure - (a)

Q. 4. What is the effect of this distortion on the operation of the machine?

Ans. It acts as a magnetic drag on the armature which consequently requires more power to turn it.

Q. 5. How can field distortion be remedied?

Ans. By using compensating windings which are embedded in the slots in the pole-shoe and are connected in series with the armature.

Q. 6. What is meant by normal neutral plane?

Ans. It is a plane which passes through the axis of the armature perpendicular to the magnetic field of the generator when there is no flow of current through the armature.

Q. 7. What is the importance of this plane in the working of the machine?

Ans. It gives the position where brushes would be placed to prevent sparking during the operation of the generator where the main pole field not distorted by armature field and were there no self-induction in the coils.

Q. 8. How do you differentiate between normal neutral plane?

Ans. The NNP is the position of zero induction and hence minimum sparking assuming no field distortion i.e. on no-load.  It is perpendicular to magnetic axis.  NA is the position of zero induction and hence minimum sparking with distorted field i.e. when generator is on load.

Q. 9. How do you define ‘commutating plane?

Ans. It is the plane which passes through the axis of the armature and through centre of contact of the brushes as shown in Figure (b).

Q. 10. What is the angle of lead?

Ans. It is the angle between the NNP and the commutating plane.

Q. 11. What affects this angle?

Ans. For sparkless commutation, the angle of lead varies directly with load. Its value can be kept small by making main pole field considerably more powerful than the armature field.

Q. 12. What is the best way of minimizing eddy currents in an armature?

Ans. Lamination.

Q. 13. How should the armature be laminated for the purpose?

Ans. It should be laminated at right angle to its axis.

Q. 14. How does field distortion affect communication?

Ans. The neutral plane no longer coincides with the normal neutral plane but is advanced by a certain angle in the direction of rotation of the armature.

Q. 15. Should the brushes of a loaded generator be placed in the neutral plane?

Ans. No.

Q. 16. Why not?

Ans. The brushes must be advanced by a certain angle (called brush lead) beyond the neutral plane to prevent sparking.

Q. 17. What causes sparking at the brushes?

Ans. It is due to the self-induction of the coil undergoing commutation.

Q. 18. What is the standard direction of rotation of the d.c. generators?

Ans. Clockwise when viewed from the end opposite to the driven end.

Q. 19. What is meant by build-up of a generator?

Ans. It means the gradual increase in the generator voltage to its maximum value after the generator is started from rest.

Q. 20. How should a generator be started?

Ans. It is usually brought up to speed with the help of the driving engine called prime-mover.

Q. 21. How should a shunt or compound generator be started?

Ans. Such machines excite best when all switches controlling the external circuit are open.

Q. 22. How about a series generator?

Ans. In this case, the external circuit must be closed otherwise the generator will not build-up.

Q. 23. What is the procedure for shunting down a generator?

Ans. First, the load should be gradually reduced, if possible, by easing down the driving engine, then when the generator is supplying little or no current, the main switch should be opened. When the voltmeter reads almost zero, then brushes should be raised from the commutator.

Q. 24. What are the indications and causes of an overloaded generator?

Ans. A generator is said to be overloaded if a greater output is taken from it that it can safely carry. Overloading is indicated by (i) excessive sparking at brushes and (ii) overheating of the armature and other parts of the generator. Most likely causes of overloading are :

1. Excessive voltage-as indicated by the voltmeter or the increased brilliancy of the pilot lamp. This could be due to over-excitation of field magnets or too high speed of the engine.
2. Excessive current–which could be due to bad feeding of the load.
3. Reversal of polarity–this happens occasionally when the series or compound-wound generators are running in parallel.  Polarity reversal occurs during stopping by the current from the machines at work.
4. Short-circuit or ground in the generator itself or in the external circuit.

Figure - (b)

Q. 25. Mention and explain the various causes for the failure of the generator to build up.

Ans. Principal causes due to which a generator may fail to excite are :

1. Brushes not properly adjusted–if brushes are not in their proper positions, then whole of the armature voltage will not be utilized and so would be insufficient to excite the machine.
2. Defective contacts-unclean contacts may interpose large resistance in the path of the exciting current and reduce it to such a small value that it fails to excite the machine.
3. Incorrect adjustment of regulators–in the case of shunt and compound generators, it is possibly that the resistance of field regulator may be too high to permit the passage of sufficient current through the field windings.
4. Speed too low–in the case of shunt–and compound-wound generators, there is certain critical armature speed below which they will not excite.
5. Open-circuit–in the case of series machines.
6. Short-circuit–in the generator of external circuit.
7. Reversed field polarity–usually caused by the reversed connections of the field coils.
8. Insufficient residual magnetism–The trouble normally occurs when the generator is new. It can be remedied by passing a strong direct current through the field coils.

Q. 26. How do we conclude that connections between field coils and armature are correct?

Ans. If the generator builds up when brought to full speed. If it does not, then connections are reversed.

Q. 27. When a generator loses its residual magnetism either due to lighting or short circuit, how can it be made to build up?

Ans. By temporarily magnetisng the main poles with the help of current from an external battery.

Q. 28. Can a generator be reversed by reversing the connections between the armature and field coils?

Ans. No, because if these connections are reversed, the generator will not build up at all.

Q. 29. Will a generator build up if it becomes reversed?

Ans. Yes.

Q. 30. Then, what is the objection to a reversed generator?

Ans. Since the current of such a reversed generator is also reversed, serious trouble can occur if attempt is made to connect it in parallel with other machines which are not reversed.

Q. 31. What are the two kinds of sparking produced in a generator?

Ans. One kind of sparking is due to bad adjustment of brushes and the other due to bad condition of the commutator. The sparking of the first are bluish whereas those of the other are reddish in colour.

Q. 32. What is the probable reason if sparking does not disappear in any position when brushes are rocked around the commutator?

Ans. 

1. The brushes may not be separated at correct distance.
2. The neutral plane may not be situated in the true theoretical position on the commutator due to faulty winding.

Q. 33. What is the permissible rise of temperature in a well-designed generator?

Ans. 27°C above the surrounding air.

Q. 34. What are the causes of hot bearings?

Ans. 

1. Lack of oil 
2. Belt too tight 
3. Armature not centered with respect of pole pieces
4. Bearing too tight or not in line.

Q. 35. What causes heating of armature?

Ans. 

1. Eddy currents.
2. Moisture which almost short-circuits the armature.
3. Unequal strength of magnetic poles.
4. Operation above rated voltage and below normal speed.

Q. 36. What is the commutator pitch of a 4-pole d.c. armature having 49 commutator bars?

Ans. Yc  = (49 ± 1)/2 = 24 or 25.

Q. 37. Will it make any difference if lower figure of 24 is selected in preference to other?

Ans. Yes.  Direction of armature rotation would be reversed.

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Source: A Textbook of Electrical Technology by BL Theraja, A K Theraja, Sedha R S_Volume_II

Read More:

1. 16 Common Interview Questions and Answers in Generators - Part-1
2. 21 Common Interview Question and Answers in AC Generator - Part-2

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What is Power Plant? What are the different types of Power Plants?

Power Plant:

A power plant, also referred to as a generating station, power plant, powerhouse, or generating plant may be defined as a machine or assembly of equipment that generates and delivers a flow of mechanical or electrical energy. The main equipment for the generation of electric power is generated. When coupling it to a prime mover runs the generator, the electricity is generated.

Figure: 2x120 MW Gas Turbine Peaking Power Plant, Siddhirganj, Bangladesh.

Classification of Power Plants:

a) Conventional

1. Steam Engines Power Plants
2. Steam Turbine Power Plants
3. Diesel Power Plants
4. Gas Turbine Power Plants
5. Hydro-Electric Power Plants
6. Nuclear Power Plants

b) Non-conventional

1. Thermo-electric Generator
2. Therm-ionic generator
3. Fuel-cells Power Plants
4. Photovoltaic solar cells Power System
5. MHD Power Plants
6. Fussion Reactor NPP Power System
7. Biogas, Biomass Energy Power system
8. Geothermal Energy
9. Wind Energy Power System
10. Ocean Thermal energy conversion (OTEC)
11. Wave and Tidal Wave
12. Energy Plantation Scheme

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Draw the Schematic Arrangement of Steam Power Station.

Figure: Schematic Arrangement of Steam Power Station.

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How nuclear power plants generate electricity?

Figure: Nuclear power plant.

Nuclear power plants generate electricity in much the same way that other thermal power plants generate electricity. The combustion of a fuel is used to generate heat, the heat is used to create steam, and the steam is used to spin turbines, which in turn generate electricity. The difference with nuclear power plants is that instead of using the combustion of a fuel to generate heat, they use nuclear fission to generate heat. Nuclear fission in simple terms is the splitting of large atoms into smaller atoms; this process releases vast amounts of energy.  

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What types of protection are available for nuclear power plants against the effects of external hazards?

There are four main protection methods, which are as follows:

• Design of essential equipment and structures to withstand hazards;

• Physical protection of the plant against the effects of a hazard, for example, by ensuring that it is suitably housed;

• Redundancy, diversity, segregation, and separation of essential systems;

• Systems to mitigate the effects of hazards, such as fire-fighting equipment.

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What kind of hazards to nuclear power plants occur that originate outside the plants?

External hazards originate outside the site, and may affect the plant or station as a hole. The following external hazards have to be considered and, where appropriate for the particular circumstances of the site, taken into account in the plant design:

• Earthquakes.

• Extreme weather, for example, extreme air or sea temperatures, wind speed, rain or snow, lightning, tornados, or hurricanes.

• Site flooding, for example by tidal waves.

• Aircraft crashes.

• Ground settlement and subsidence.

• Hazards from off-site industrial activity.

• Dangerous substances.

• Sabotage.

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12 Most Common Interview Questions and Answers on Transformer - Part 2

Q. 1: What is induction?

A. 1: The process by which one conductor produces, or induces, a voltage in another conductor, even though there is no mechanical coupling between the two conductors.

Q. 2: What is inductance?

A. 2: The property of a coil in a circuit to oppose any change of existing current flow.

Q. 3: What is self-inductance?

A. 3: The inducing of an emf within the circuit itself, caused by any change of current within that circuit. This induced emf is always in a direction opposite to the applied emf, thus causing opposition to any change in current within the circuit itself.

Q. 4: What is mutual inductance?

A. 4: The linkage of flux between two coils or conductors, caused by the current flowing within one or both of the coils or conductors.

Q. 5: Name several methods by which an emf can be generated.

A. 5: By conductors being cut by-

• A magnetic field (as in generators) 
• Chemical reactions (as in batteries)
• Heat (as in thermocouples)
• Crystal vibration (as in piezoelectricity) and 
• Friction (as in static electricity).

Q. 6: What is a transformer?

A. 6: A device that transforms electrical energy from one or more circuits to one or more other circuits at the same frequency but usually at a different voltage and current. It consists of a core of soft-iron laminations surrounded by coils of copper-insulated wire.

Q. 7: Draw a diagram of two coils, such as the coils of a transformer winding, and indicate the self-inductance and the mutual inductance.

A. 7: Self-inductance is produced within the primary coil, and mutual inductance exists between the two transformer coils, as shown bellow this Figure:

Figure: Self-inductance and mutual inductance in the coils of a Transformer.

Q. 8: What always surrounds a conductor when a current flows through it?

A. 8: A magnetic field.

Q. 9: There are two basic types of transformers. What are they?

A. 9: The isolation type, in which the two windings are physically isolated and electrically insulated from each other, and the autotransformer type, in which there is only one coil with a tap or taps taken off it to secure other voltages (the primary is part of the secondary and the secondary is part of the primary).

Q. 10: What is an oil-immersed transformer?

A. 10: The core and coils are immersed in a high-grade mineral oil, which has high dielectric qualities.

Q. 11: What is Transformer Oil?

A. 11: Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Transformer oil is most often based on mineral oil, but alternative formulations with better engineering and/or environmental properties are growing in popularity.

Q. 12:  Why is oil used in a transformer?

A.12: To increase the dielectric strength of the insulation, to keep down the possibility of arcing between coils, and to dissipate heat to the outer case so that the transformer can carry heavier loads without excessive overheating.

Courtesy: 

1. Book of Paul Rosenberg
2. Wikipedia
3. Internet.

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Read More: 10 Most Common Interview Questions and Answers on Transformers - Part-1         

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What is the difference between a dc motor and a dc generator?

Fundamentally, there is none. A dc motor will generate electricity if driven by some prime mover, and a dc generator will act as a motor if connected across a dc source.

Read More:  If a dc motor is connected across an ac source, what will be the result? Why?

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What are the conditions for parallel operation of alternators?

Figure: Parallel operation of shunt generators.

To synchronize AC generators, several important factors must be checked:

• Equal terminal voltages- The terminal voltage of the incoming machine must be approximately equal to bus-bar voltage. This is obtained by adjustment of the incoming generator’s field strength.

• Equal frequency- The frequency of the incoming machine must be equal to those bus-bars. This is obtained by adjustment of the incoming generator’s prime-mover speed.

• Phase voltages in proper phase relation- The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e. the phase voltages of the incoming machine and the bus-bar should be in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation (the bus-bars) and the incoming machine.

Note: The above figure shows the shunt generator No.1 is connected to the bus-bars and delivering load. The shunt Generator No.2 is connected in parallel to the Generator No.1. When the load on the generator No.1 increases beyond its rated capacity, immediately the second shunt generator operate and wish the first generator to come across the raised load demand.

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21 Common Interview Questions & Answers on AC Generators - Part-2

Figure: Three Phase Generator Output

Q1. Magnetic induction occurs when there is relative motion between what two elements?

A1. A conductor and a magnetic field.

Q2. What is the part of an alternator in which the output voltage is generated?

A2. Armature.

Q3. What are the two basic types of alternators?

A3. Rotating armature and rotating field.

Q4. What is the main advantage of the rotating field alternator?

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

Q5. Most large alternators have a small dc generator built into them. What is its purpose?

A5. To provide dc current for the rotating field.

Q6. How are alternators usually rated?

A6. Kilovolt-amperes (volt amperes).

Q7. What type of prime mover requires a specially designed high-speed alternator?

A7. Steam turbine.

Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?

A8. Internal combustion engines, water force and electric motors.

Q9. What does the term single phase indicate?

A9. One voltage (one output).

Q10. In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?

A10. In series.

Q11. What determines the phase relationship between the voltages in a two-phase ac generator?

A11. Placement of armature coils.

Q12. How many voltage outputs are available from a two-phase three-wire alternator?

A12. Three.

Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?

A13. C is 1.414 times greater than A or B.

Q14. In a three-phase alternator, what is the phase relationship between the individual output voltages?

A14. Each phase is displaced 120º from the other two.

Q15. What are the two methods of connecting the outputs from a three-phase alternator to the load?

A15. Wye and Delta.

Q16. Ships’ generators produce 450-volt, three-phase, ac power; however, most equipment uses 117volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?

A16. Three single-phase, delta-delta, step-down transformers.

Q17. What two factors determine the frequency of the output voltage of an alternator?

A17. Speed of rotation and number of poles.

Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

A18. 120 Hz.

Q19. The variation in output voltage as the load changes is referred to as what? How is it expressed?

A19. Voltage regulation. As a percentage.

Q20. How is output voltage controlled in practical alternators?

A20. By varying the voltage applied to the field windings.

Q21. What generator characteristics must be considered when alternators are synchronized for parallel operation?

A21. Output voltage, frequency, and phase relationships.

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1. 16 Common Interview Questions and Answers in Generators - Part-1
2. 37 Interview Questions and answers on DC Generators - Part-3

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