Synchronous Generator Interview Questions


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Synchronous Generator Interview Questions

Q : Why AC generator is called Alternator?

An alternator is such a machine which produces alternation electricity. It is a kind of generator which converts mechanical energy into alternating electrical energy.

Q: Why Alternator is called Synchronous generator?

Because it rotates at a constant (i.e.synchronous speed ) no matter what the load on alternator is.For instance if its a 2 pole machine it will rotate at 3000 rpm to produce a frequency of 50 Hz in India.

Q: Why a 3-phase synchronous machine will always run at synchronous speed?

Because of the magnetic coupling between the stator poles and rotor poles the motor runs exactly at synchronous speed.

Q. What are the essential features of synchronous machine ?

i. The rotor speed is synchronous with stator rotating field.

ii. Varying its field current can easily vary the speed.

iii. It is used for constant speed operation.

Q:Why almost all large size Synchronous machines are constructed with rotating field system type?

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

  1. Small size

  2. low centrifugal forces

  3. easy to insulate field winding

  4. Easy construction

  5. require less number of slip rings

  6. better ventilation

  7. higher speed

  8. reduce sparking problems

  9. higher capacity

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

F = (PN)/120 Hz,Where P- Number of poles, N-Speed in rpm

Q: How are alternators classified?

According to shape of field system

· Salient pole type

· Smooth cylindrical type

Q:Compare salient pole rotor with cylindrical pole rotor.

Topic Salient pole Cylindrical pole
Poles Projected type Non projected
Air gap Non uniform Uniform
Diameter Large Small
Axial length Small Large
Mechanical strength Low High
Speed rating Low High
Prime mover Water turbine Steam turbine
Overall size High Less
Damper winding Required Not required
application Hydropower plant Steam power plant

Q. Why do cylindrical Alternators operate with steam turbines?

A: Steam turbines are found to operate at fairly good efficiency only at high speeds. The high 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 diameter and large axial length are used for Synchronous generators driven by steam turbines with either 2 or 4 poles.

Q: Which type of pole generators are used in Hydro-electric plants and why?

A: 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 ventilation and also have other advantages over smooth cylindrical type rotor.They have more diameter and less axial length.

Q.State three important features of turbo alternator rotors.

  1. The rotors of turbo alternators have large axial length and small diameters.

  2. Damping torque is provided by the rotor itself and so there is no necessity

for additional damper winding.

  1. They are suitable for high speed operations and so number of poles is

usually 2 or 4.

Q: What are the advantages of salient pole type of construction used for synchronous machines?

  1. They allow better ventilation.

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

3 Due the variable reluctance, the machine develops additional reluctance power, which is independent of excitation.

Q.What are the prime movers used for a) Salient pole alternator, b) Non- salient pole alternator.

The prime movers used for salient pole alternators are water wheels likeKaplan

turbine, Francis turbine, Pelton wheel etc., and diesel or petrol engines.

The prime movers used for non-salient pole alternators are steam turbines and gas turbines.

Q.Mention the uses of damper windings in a synchronous machine?

  1. Damper winding is used to reduce the oscillations developed in the rotor

    of alternator when it is suddenly loaded.

  2. The damper winding is used to start the synchronous motor as an

    induction motor.

Q: Why is the stator core of Alternator laminated?

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

Q: 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, the electrical degree is accounted with reference to the magnetic field. 180 electrical degree is accounted as the angle between adjacent North and South poles.

Q: What is the relation between electrical degree and mechanical degree?

Electrical angle = (P/2) x Mechanical angle

Where P is number of poles

Q: Define pole pitch.

It is centre to centre distance between two adjacent poles. Two poles are responsible for 360 degree electrical of induced emf so one pole is responsible for 180 degree electrical of induced emf. Hence 180 degree electrical is called 1 pole pitch.

Q: What is short pitch winding?

If coil pitch is equal to less than pole pitch it is called short pitch winding

Q: What is full pitch winding?

If coil pitch is equal to pole pitch it is called full pitch winding

Q. Define pitch factor or coil span factor.

The pitch factor is defined as the ratio of vector sum of emf induced in a coil to

arithmetic sum of emf induced in the coil

OR

It is the ratio of resultant emf induced when coil is short pitch to the resultant emf induced when the coil is full pitched.

Q: Why is short pitch winding preferred over full-pitch winding ?

Advantages

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

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

· Higher efficiency: As the high frequency harmonics are eliminated, the eddy current and hysteresis losses are reduced which increases overall efficiency

Q: What is distributed winding?

When coil-sides belonging to each phase are housed or distributed in more than one slot under each pole region then the winding is called distributed winding

Q: What is concentrated winding?

If all the conductors or coils belonging to a phase are placed in one slot under every pole then winding is called concentrated winding.

Q. Define distribution factor or winding factor or breadth factor or spread factor

The distribution factor is defined as the ratio of vector sum to arithmetic sum of emf induced in the conductor of one phase spread.

OR

It is ratio of resultant emf induced when coils are distributed to the resultant emf induced when coils are concentrated.

Q: what is slot angle β?

The phase difference contributed by 1 slot in electrical degree is called slot angle.

Slot angle β - angle between adjacent slots in electrical degree

=180/n

=180/(slots/pole)

Q : Write down the formula for distribution factor

Kd = sin (mβ/2) / msin(β/2)

where

m - number of slots/pole/phase

Slot angle β - angle between adjacent slots in electrical degree =180/(slots/pole)

Q: Which type of winding is preferred for armature windings of alternator? Why?

Double layer, short pitched and distributed type of armature winding. It eliminates high frequency harmonics hence it makes induced emf more sinusoidal.

Q: 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 I2R, 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.

Q. Why alternators are rated in KVA?

The KVA rating of ac machine depends on the power factor of the load. The

power factor in turn depends on the operating conditions. The operating

conditions differ from place to place. Therefore the KVA rating is specified for all ac machines.

Q: What is meant by armature reaction in Alternators?

The effect of armature flux on main flux

Q: Explain armature reaction of an alternator and its effect on main flux at various power factor.

  1. zero lagging pf: armature flux lags main flux by 180 . Hence effect is demagnetizing i.e the main flux is reduced

  2. lagging pf: it has 2 components demagnetizing and cross magnetizing. Demagnetizing reduce flux and cross magnetizing distorts flux.

  3. unity pf: armature flux lags main flux by 90 degree. Hence effect is completely cross magnetizing

  4. leading pf: it has 2 components magnetizing and cross magnetizing. magnetizing increase flux and cross magnetizing distorts flux.

  5. zero leading pf: Armature flux is in phase with main flux. Effect is completely magnetizing.

Q: What are the causes of changes in voltage in Alternators when loaded?

When alternator is loaded the armature carries load current. Due to this load current the voltage drops occur in alternator. Hence terminal voltage decreases due to following voltage drops:

  1. Voltage drop due to the resistance of the winding, R

  2. Voltage drop due to the leakage reactance of the winding, Xl

3· Voltage drop due to the armature reactance, Xa

Q: What is synchronous reactance ?

It is the combination of leakage reactance and armature reactance

Q: What is synchronous impedance?

It is the combination of synchronous reactance and armature resistance.

Q: What is meant by synchronous impedance of an Alternator?

The combination of armature resistance and synchronous reactance is known as synchronous impedance

Q. 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 generators to remain in

synchronism in spite of any disturbance occurring in any one of the generators

Q: 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.

Q. 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.

Q. Define the term voltage regulation of Alternator.

The voltage regulation of an Alternator is defined as the change in terminal

voltage from no-load to load condition expressed as a fraction or percentage of

terminal voltage at load condition ; the speed and excitation conditions

remaining same.

Q.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.

Q. 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

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

Advantages:

· 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

Q. Why is the synchronous impedance method of estimating voltage regulation considered as pessimistic method?

Compared to other methods, the value of voltage regulation obtained by the

synchronous impedance method is always higher than the actual valuei.e actual value is lower than the results obtained and therefore this method is called the pessimistic method.

Q. In what way does the ampere-turn method differ from synchronous impedance method?

The ampere-turn /MMF method is the converse of the EMF method in the

sense that instead of having the phasor addition of various voltage

drops/EMFs, here the phasor addition of MMF required for the voltage drops

are carried out. Further the effect of saturation is also taken care of.

Q. 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

Q. Why is the MMF method of estimating the voltage regulation considered as the optimistic method?

In this method the actual value is more than the result obtained.

Q: What is need of parallel operation of alternators?

  1. Optimum utilization of alternators as most of the power stations are interconnected by grid.

  2. Reduced operating cost: number of alternators operating at a time can be changed depending on load demands.

  3. Easy maintenance: Alternator may be shut down for maintenance

  4. Continuity and reliability

Q. 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.

. Zero phase difference

Q. 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.

Q.What are the methods of checking the phase difference?

  1. All Dark lamp method

  2. Two bright one dark lamp method

  3. Synchroscope

Q What is synchronizing?

The operation of connecting an alternator in parallel with another alternator or with common bus bars is known as synchronizing.

Q:What happens in all dark lamp method ?

If frequency and phase difference of incoming alternator and busbar are not same then lamps connected across incoming alternator will flicker. The flickering decreases as frequency will become near to each other by adjusting the speed by prime mover.

Q. What are the advantages and disadvantages of three dark lamps method of synchronizing?

Advantages:

· The synchronous switch using lamps is inexpensive

· Checking for correctness of the phase sequence can be obtained in a simple manner which is essential especially when the Alternator is connected for the first time or for fresh operation after disconnection

Disadvantages:

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

Q What is a synchroscope?

Synchroscope is an instrument, which shows the phase relationship of emf of the incoming alternator. It also indicates whether the incoming alternator is running slow or fast.

Q. How synchronoscope 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.

The rotation of pointer is proportional to the difference in two frequencies. If the pointer rotates in clockwise direction it indicates the incoming alternator is running fast. If the pointer rotates in anticlockwise direction it indicates the incoming alternator is running slow. If the difference in frequency is large the pointer will oscillate

Q:Why synchroscope method is more better compared to others?

It is very accurate as it doesnot require personal judgement

Q. 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

Q: What happens if excitation of alternator is changed keeping speed of prime mover constant ?

The emf induced will be changed and hence terminal voltage will also change. The terminal voltage will increase with increase in excitation and vise versa.

Q: What happens if speed of prime mover is changed keeping excitation of alternator constant?

If speed of prime mover is changed, the speed of alternator also changes hence frequency also changes. The frequency increase with increase in steam input and hence speed and vise versa.

Q. 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 reactive-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.

Q. 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 power supplied by the generator keeping steam input constant.

Q. How does increase in steam input of the Alternator connected to infinite bus-bars affect this operation?

KW output power of alternator increase with increase in steam input keeping excitation constant.

Q. Discuss how the ventilation and cooling of large high speed alternator is carried out.

For high speed alternator two cooling methods are available and they are

conventional cooling and direct cooling.

In conventional cooling methods, radial and axial ventilating ducts are provided in the core. Cooling is performed by forced circulation of air or hydrogen at a pressure higher than atmosphere.

In direct cooling methods, cooling ducts are provided in the stator and rotor slots or conductor itself will be in the form of tubes. Coolants like water or oil or hydrogen are circulated in the ducts to remove the heat directly from the conductors.