1. Why are alternators put in parallel ?
Ans: For high efficiency of operation, reliability , convenience and economy in maintenance and repair and possibility of additions to plant capacity with the growth in load on the power station.
2. What is mean by synchronizing of alternators ?
Ans: The process of connecting an alternator in parallel with another alternator or with the common bus bars is called synchronizing of alternators.
3. What are the conditions for parallel operation of alternators ?
Ans: Terminal voltage, frequency and the phase of incoming machine must be the same as that of the bus bars(or other alternators already operating in parallel)
In case of 3-phase alternators, an additional requirement to be met is that the phase sequence if the incoming machine must be the same as that of the bus bars.
4. What are the two methods by which two alternators are put in parallel ?
Ans: Synchronizing lamp method and synchro scope method
5. A machine with a large air gap has a higher synchronizing power. Why ?
Ans: A large air gap offers a large reluctance to the path of the flux produced by the armature mmf and thus reduces the effect of armature reaction. this results in a small value of synchronous reactance and therefore a higher synchronizing power because synchronizing power inversely proportional to the synchronous reactance.
6. What is the effect of increasing the steam admission to the prime mover connected to an alternator ?
Ans: By increasing the steam admission to the prime mover of an alternator will load in further.
7. How can the distribution of load between two alternators operating in parallel be varied ?
Ans: Distribution of load between two alternators operating can be varied by adjusting the governor control. one prime mover governor is opened while the other is closed simultaneously so as to maintain the system frequency constant.
8. What is the effect of varying excitation of an alternator running in parallel with an other alternator ?
Ans: Variation in excitation of an alternator running in parallel with an other alternator causes variation in watt less component and, therefore , of reactive kva without affecting the sharing of the KW load or power.
9. What is mean by infinite bus bars ?
Ans: Infinite bus bars represent a system of large capacity whose frequency and the phase and magnitude of the voltage are not affected even if there is a variation of excitation.
10. Will an increase in the excitation of an alternator connected to an infinite bus increase its real power generation ?
Ans: No
11. Hoe will the power angle and power factor of a 3-phase alternator supplying power to an infinite bus at constant excitation, change if the steam input is reduced ?
Ans: both power angle and power factor will decrease if the alternator was supplying load to the infinite bus at lagging power factor.
12. Do the generators driven by the steam turbines have a tendency to hunt ?
Ans: No
13. How hunting is reduced in alternators ?
Ans: The hunting in alternators is reduced by employing heavy flywheels, putting dash-pots on the engine governors, and to use amortisseur or squirrel cage windings on the surface of the rotors.
14. Two reaction theory is applied only to salient pole machines. State the reason.
Ans: A multipolar machine with cylindrical rotor has a uniform air-gap and, therefore, its reactance remains the same, irrespective of the spatial position of rotor. Because of non- uniformity of the reluctance of the magnetic paths, the mmf of the armature is divided into two components viz.(1) a direct- acting component along the field pole axis, called the direct- axis and ( 2) quadrature- component along the axis passing through center of the inter polar space, called the quadrature- axis. these facts form the basis of the two- reactance theory applied to the salient pole machines.