Hydroelectric Power Plants Interview Questions and Answers:
1. What is power generating station ?
Ans. Power generation station generates electrical energy by conversion of energy available in different forms from natural sources such as kinetic energy of blowing winds, pressure head of water, chemical energy of fuels (either in solid, liquid or gaseous form) and nuclear energy of radioactive substances.
2. What is a hydrological cycle ?
Ans. Hydrological cycle is a process by which moisture from the surface of water bodies covering the earth’s surface transferred to the land and back to the water bodies again. The input to this cycle is the solar energy. Due to this water from the water bodies evaporates and on cooling these water vapours form clouds. Further cooling makes the clouds to fall down in the form of rain, snow, hail or sleet etc. known as precipitation. Precipitation includes all water that falls from the atmosphere to the earth’s surface in any form.
3. What is the meaning of basic rule curve ?
Ans. Basic rule curve represents accumulation of water in reverse order of time of deficiency between firm load and energy available from stream flow in critically dry periods.
4. On what factors does the power output of a hydro power plant depend ?
Ans. The power developed by a hydro plant depends upon the discharge and head and is directly proportional to their (discharge and head) product.
5. What is the estimated hydro potential in India ?
Ans. About 84,000 MW.
6. What are the factors of selection of site for hydro power plant ?
Ans. The factors governing the site selection for a hydroelectric plant are : (i) Availability of water (ii) Water storage (iii) Water head (iv) Distance from load centre (v) Accessibility of the site (vi) Water pollution, (vii) Sedimentation (viii) Large catchment area (ix) Availability of land (x) Possibility of stream diversion.
7. What is meant by catchment area ?
Ans. The catchment area is the area, bounded by watersheds, which drains into a stream or river across which the dam has been built at a suitable place.
8. What is hydrograph ?
Ans. A hydrograph is a graphical representation between discharge or flow with time.
9. What is a mass curve ?
Ans. Mass curve is a curve which indicates the total volume of run-off in m3 up to a certain time.
10. Between which quantities is mass curve plotted ?
Ans. The abscissae for the mass curve may be a day, month or year and ordinate may be plotted in terms of second-metre-day or second-meter-month which means the flow collected at the rate of 1 cubic metre per second for one day or one month respectively. It indicates the total volume of run-off in m3 up to a certain time.
11. What is the use of mass curve ?
Ans. Mass curve is used in determination of the capacity of the storage reservoir in hydro projects.
12. What for pondage is provided ?
Ans. In case power plant is away from the storage reservoir, a small pond is provided near the power plant in order to meet the hourly changes in power demand.
13. What for dam is provided ?
Ans. The functions of a dam in hydro power plants are to raise the water surface of the stream to create an artificial head and also to provide the pondage, storage or the facility of diversion into conduits.
14. What civil engineering works are necessary for a hydroelectric project ?
Ans. The civil engineering works for an hydroelectric project include reservoir dam, forebay, spillway, intake structures, surge tank, penstock, tailrace etc.
15. What is forebay and what for it is provided ?
Ans. The forebay serves as a regulating reservoir storing water temporarily during light load period and providing the same for initial increase on account of increasing load during which water in the canal is being accelerated. In short a forebay may be considered as an enlarged body of water just above the intake to store water temporarily to meet the hourly load fluctuations. This may either be a pond behind the diversion dam or an enlarged section of a canal spread out to accommodate the required widths of intake.
16. What is meant by the phenomenon “water hammer” ?
Ans. A reduction in load on the generator causes the governor to close the turbine gates and thus create an increased pressure in the penstock. This may result in water hammer phenomenon and may need pipe of extraordinary strength to withstand it otherwise the penstock may burst.
17. What are the functions of surge tank in hydro plants ?
Ans. Surge tank is provided to absorb sudden changes in water requirements and reduce water hammer and negative pressure in penstock.
18. How a surge tank helps in reducing water hammer effect?
Ans. For reducing water hammer effect, some means are required to be provided for taking the rejected flow. This may be accomplished by providing a small storage reservoir or tank (open at the top) for receiving the rejected flow and thus relieving the conduit pipe of excessive water hammer pressure. This storage reservoir, called the surge tank is usually located as close to the power plant as possible, preferably on ground to reduce the height of the tower.
19. What is draft tube and why is its installation necessary ?
Ans. An airtight pipe of suitable diameter attached to the runner outlet and conducting water down from the wheel and discharging it under the surface of the water in the tailrace is known as draft tube.
If there is no draft tube and the water discharges freely from the turbine exit, then the turbine operates under a head equal to the height of the headrace water level above the runner exit. By installing draft tube, the operating head is increased by an amount equal to the height of the runner outlet above the tailrace. This creates a negative pressure head at the runner exit. This makes it possible to install the turbine above the tailrace without loss of head.
20. How are hydroelectric power plants classified ?
Ans. The hydroelectric power plants may be classified according to
- the extent of water flow regulation available
- the availability of water head and
- the type of load they supply.
Apart from above classification, hydroelectric power plants are also classified on the basis of installed capacity such as large, medium, small, mini and micro hydro power plants.
21. What are the different types of turbines ?
Ans. Turbines employed in hydroelectric power plants are Pelton wheel, Francis turbine and Kaplan turbine.
22. How are hydro turbines classified ?
Ans. There are many ways of classifying water or hydraulic turbines, for example, according to the type of flow of water, according to action of water on moving blades, according to head and quantity of water available, after the name of originator and according to the specific speed of the machine.
23. What is specific speed of a turbine ?
Ans. The specific speed ns of a turbine characterizes the turbine’s shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type. The specific speed is the speed with which the turbine turns for a particular discharge Q, with unit head and thereby is able to produce unit power.
24. What are the advantages of reaction turbine over the Pelton wheel with respect to efficiency, size, cost and maintenance ?
Ans. As compared to Pelton wheel a Francis turbine offers the advantage of high efficiency at full load and at 75% of full load. Since this turbine can be designed for a higher speed than Pelton wheel, the dimensions and weights are smaller together with the reduced generator cost.
25. For what deflectors are provided in Pelton wheels ?
Ans. Deflectors are provided in Pelton wheels to control the speed.
26. How are reaction turbines distinguished from the impulse type ?
Ans. The reaction type turbines are distinguished from the impulse type by having a runner that always functions within a completely water-filled casing.
27. Compare Kaplan and Francis turbines.
Ans.
- Kaplan turbine is more efficient, more compact in cross section and has higher rotational speed to that of an identical Francis turbine.
- Kaplan turbine has less frictional loss, higher (2 to 3 times more), specific speed, and lesser number of runner blades.
- In Kaplan turbine runner the water strikes the turbine blades axially while in Francis turbine runner receives the water radially.
- Francis turbines are used in medium head plants while Kaplan turbines are used in low head plants.
28. Which turbine is ideally suited to pumped storage plants ?
Ans. Kaplan turbine is ideally suited to pumped storage plants because its runner is capable of reverse operation as a pump.
29. Why is governing of turbines essential ?
Ans. In order to have electrical output of constant frequency it is necessary to maintain speed of the alternator driven by the turbine constant. This is achieved by controlling the flow of water entering the turbine by the automatic adjustment of guide vanes in case of reaction turbines and of the nozzle needle in the case of impulse turbines. Such an operation of speed regulation is called the governing, and it is attained automatically by means of a governor. In case of impulse turbine the governor also operates the auxiliary relief valves or jet deflectors.
30. On what factors does the selection of a water turbine depend ?
Ans. The selection of a water turbine depends upon various factors such as working head, available discharge, speed, output and nature of load.
31. Why are small hydroelectric power plants becoming more and more popular ?
Ans. Small hydroelectric power plants are becoming more and more popular because they offer several advantages such as simple operation, more reliable, minimal maintenance, quick construction, simple civil engineering and small financial needs.
32. What is hydro potential in India ?
Ans. India is blessed with immense amount of hydroelectric potential and ranks 5th in terms of exploitable hydro potential on global scenario. According to the accessment made by CEA, India is endowed with economically exploitable hydro power potential to the tune of 148,700 MW of installed capacity.
In addition, 56 number of pumped storage projects have also been identified with probable installed capacity of 94,000 MW. In addition to this, hydro potential from small, mini and micro schemes has been estimated as 6,782 MW from 1512 sites. Thus, in totality India is endowned with hydro potential of about 250,000 MW.
33. What is the estimated potential of small hydro power schemes in India ?
Ans. About 15,000 MW.
34. Can an old steam power plant be an alternative to a pumped storage plant for peak load operations ?
Ans. No, because it is a costly proposition and moreover it (steam power plant) needs large starting time and shutdown time