Interconnected Systems Interview Questions and Answers:
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 meant by synchronizing of alternators ?
Ans. The process of connecting an alternator in parallel with another alternator or with the common bus-bars is called the synchronizing.
3. What are the conditions for parallel operation of alternators ?
Ans. Terminal voltage, frequency and the phase of the 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 of the incoming machine must be the same as that of the bus-bars.
4. What is the effect of varying excitation of an alternator running in parallel with another alternator ?
Ans. Variation in excitation of an alternator running in parallel with another alternator causes variation in wattless component and, therefore, of reactive kVA, without affecting the sharing of the kW load or power.
5. 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 it further.
6. How can the distribution of load between two alternators operating in parallel be varied ?
Ans. Distribution of load between two alternators operating in parallel 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.
7. What is the effect of increasing the driving torque of the prime mover of one of the two alternators operating in parallel ?
Ans. By increasing the driving torque of the prime mover of one of the two alternators operating in parallel, it will be further loaded and other will be relieved of its load. If the output of the alternator, whose driving torque has been increased, becomes more than the total load supplied, then the other alternator will run as a synchronous motor.
8. What is meant 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 or power of a synchronous machine connected to it.
9. How 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.
10. At what power angle a synchronous generator will develop maximum power ?
Ans. Synchronous generator will develop maximum power when power angle δ = θ, internal angle where θ = Tan-1 Xs/Re
11. What is the effect of changing the excitation of a synchronous machine connected to infinite bus-bars and supplying load at lagging pf ?
Ans. When the excitation of an alternator connected to infinite bus-bars and supplying load at lagging pf is increased, the alternator falls back (i.e., load angle decreases), power factor further drops and armature current increases due to reduced power factor, active component being constant.
12. What is the effect of changing the torque of the prime mover of an alternator connected to infinite bus-bars ?
Ans. By increasing the torque of the prime mover of one alternator, it is further loaded and an equivalent load is removed from the other unit or units with which the machine is paralleled. If the output of the alternator, whose prime mover torque has been increased, become more than the total load being supplied, then the other machine or machines will operate as synchronous motor(s).
13. What is meant by the synchronous capacity of an interconnector ?
Ans. The maximum power transferred is referred to as the synchronous capacity of the interconnector and it is defined as the kW transmitted per radian of the displacement between the two voltages of the power stations.
14. What is the drawback of shunt capacitor/reactor voltage control ?
Ans. The drawback of this method of voltage control is that as the voltage falls the correction VARs also falls i.e., when it is most needed, its effectiveness falls. Similarly, on light loads when the corrective VARs requirement is comparatively less, the capacitor output is large.
15. Why it is necessary to maintain the frequency of the power system constant ?
Ans. The reasons for maintaining the frequency of the power system constant are as follows :
- The frequency control keeps the balance between generation and absorption of real power and thus makes the operation of power station in parallel satisfactory.
- The speed of synchronous and induction motors, extensively used in industry as prime movers, depends upon supply frequency (the synchronous speed being equal to 120 f/P where f is supply frequency and P is the number of poles) and so change in supply frequency causes variations in speed of motors of consumers-not desirable for consumers, particularly the process industries depending on constant speed drives.
- The variation in supply frequency beyond permissible limits also affects the performance of electric motors.
- The extensive use of synchronous clocks establishes a strong requirement for maintaining supply frequency constant in order to have correct timing from such clocks.