Two Marks Questions with Answers

Two Marks Questions with Answers

Q. l What is the need of parallel operation of alternators ?

Ans. :

1. Due to parallel operation, it is possible to operate only few number of alternators as required by the load demand so that alternators can be operated at full load capacity to have better efficiency. This reduces the cost considerably.

2. From ease of maintainance and ispection of power stations, the number of alternators operating in parallel is important.

3. Several power stations can be interconnected due to parallel operation of alternators which is economical and advantageous.

4. The continuity and reliability of the supply can be maintained at better level due to parallel operation of alternators.

Q. 2 What are the conditions for parallel operation of alternators ? AU : May-04, 13, Dec.-05, 09, 10, 16

Ans. :

1. The terminal voltage of the alternators must be same.

2. The frequency of the alternators must be same. This necessitates that speeds must be properly adjusted (f = PN/120).

3. With respect to the external load, the phase of alternator voltages must be identical with each other. Alternately we can say that phase sequence of the alternator voltages must be same.

Q. 3 State the characteristics of two alternators working in parallel.

Ans. :

1. The induced emfs in the two alternators are in the same phase as far as external load or circuit is considered.

2. In the local circuit, the two emfs are in phase opposition.

3. If no load is connected to the alternators only circulating current I_SY will flow

in the circuit which is given by, 

Q. 4 List out two advantages of synchronizing lamps used for synchronizing. AU : May-05

Ans. :

The advantages of synchronizing lamps are,

1. It provides sharp and accurate method for synchronization.

2. No extra device is required for synchronization which is not the case with method of synchronoscope as phase sequence indicator is additionally required.

3. All the conditions that are required for synchronization can be simultaneously checked with the use of lamps.

Q. 5 Explain what happens when the excitation of synchronous machine connected to an infinite bus is varied, the load torque remains constant.

Ans. :

The synchronous alternator supplies a lagging power factor current when overexcited and a leading power factor current when underexcited when excitation is varied with the load torque remains constant. While the synchronous motor draws a leading power factor current when overexcited and a lagging power factor current when underexcited when excitation is varied with the load torque remains constant.

Q. 6 Represent the power/power angle curve of a synchronous machine.

Ans. :

The power/power angle curve of a synchronous machine is shown in the Fig. 3.1.

As power angle increases, the power increases till power angle becomes 90° after which machine loses its synchronism. Practically machine is operated with power angle much less than 90°.

Q. 7 Specify the condition beyond which the synchronous machine is said to be out of step ?

Ans. :

The electrical power in synchronous machine is given by P = EV / X_s sin- sin δ .

The maximum power occurs at δ = 90°, beyond this point, the machine fails to operate and falls out of step, loosing synchronism.

Q. 8 Write down the conditions to be satisfied for proper synchronization of alternators.

Ans. :

1. The terminal voltage of the incoming machine must be same as that of bus bar voltage.

2. The frequency must be same as that of the incoming machine as well as that of the bus bar. This necessitates that speed must be properly adjusted (f = PN/120).

3. With respect to the external load, the phase of alternator voltage must be identical with that of the bus bar voltage. Alternately we can say that phase sequence for the two voltages must be same. 

Q. 9 List the factors that affect the load sharing in parallel operating generators.

Ans. :

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 :

1. Prime-mover characteristic or input

2. Excitation level and

3. Percentage synchronous impedance and its R/X ratio.

Q.10 What is meant by compounding curve in synchronous generator ? AU : May-08

Ans. :

If synchronous generator is supplying power at constant frequency to a load whose power factor is constant then curve showing variation of field current versus armature current when constant power factor load is varied is called compounding curve for alternator.

Q.11 What is the purpose of conducting slip test? AU : Dec.-11

Ans. :

The slip test is conducted to obtain the quadratue axis (X) and direct axis (X) reactances of a synchronous machine.

Q. 12 Define synchronization.

Ans. :

The process of switching of an alternator to another alternator or with a common bus bar without any interruption is called synchronization.

Q.13 Draw the typical set up for the synchronization of alternators.

Q.14 Give the expressions to calculate direct and quadrature axis reactances by slip test.

Ans. :

The expressions to calculate direct and quadrature axis reactances by slip test are,

Q. 15 What is synchronizing power of an alternator ?

Ans. :

When a machine connected to infinite bus bar, the power which automatically comes into play and accelerates the machine which is retarding and decelerates the machine which is accelerating is called synchronizing power. This power keeps the machine in step with infinite bus bar.

Q. 16 State the disadvantages of lamps dark method of synchronization.

Ans. :

1. When there is large difference between the two voltages, the lamps become dark as voltage across them is about half of their rated voltage. Hence faulty synchronization may be done in the dark period which may damage the machine.

2. This method does not given idea about how much the machine is slow or fast. 

3. Due to large voltage, lamps may burnout as lamp ratings are normally low. Hence extra trasnsformer is required to step down the voltages which increases the cost and complexity.

Hence generally lamps bright and dark method is used.

Q. 17 What is meant by alternator on infinite bus-bars ?

Ans. :

When the number of alternators are interconnected forming a system in which terminal voltage is constant irrespective of the changes in the load, then the system is called infinite busbars. Any alternator connected to such system is called alternator connected to infinite busbars.

Q. 18 Two reaction theory is applied only to salient pole machines. State the reason. AU : Dec.-14, May-18

Ans. :

In nonsalient pole rotor, the rotor is smooth cylindrical with an uniform air gap. Hence its reactance is constant irrespective of rotor position, in salient pole rotor, the air gap is nonuniform and its reactance changes as per the rotor position, due to nonuniform reluctance of the magnetic path. Hence the armature mmf is divided in two parts namely i) In the direction of axis of salient pole rotor called direct axis component and ii) In the direction perpendicular to the axis of salient pole rotor called quadrature axis component. This is not possible in cylindrical rotor hence two reaction theory is applicable to salient pole alternators.

Q. 19 Distinguish between transient and sub-transient reactances.

Ans. :

The transient and sub - transient reactances are considered when there occurs fault on an alternator. When an alternator is short circuited then the currents in all the three phases rise rapidly to high value. During first two cycles, the short circuit current is high and reactance is least. This reactance is called sub - transient reactance denoted as X". After first sub - transient cycles (2 or 3), change in the short circuit current is less rapid. This is called transient state. The reactance in this state is called transient reactance denoted as X'.