Selection of Power Plant Interview Questions and Answers:
1. Why are diesel power plants rarely used ?
Ans. Diesel power plants have limited field of application as emergency power plants in private industries and other installations to supply power for short intervals in the event of failure of power supply from grid. This is because the escalating costs of fuel oil have made operation of diesel power plants very uneconomical.
2. What are factors affecting selection of plant location ?
Ans. The factors affecting selection of plant location are : availability of fuel and its cost, availability of land and its cost and local taxes, cost of transmission of energy, requirement of space, availability of site for water power, storage space for fuel, transportation facilities, availability of cooling water, degree of reliability, pollution and noise, disposal of ash, nature of load etc.
3. Is cost of fuel directly proportional to power output ?
Ans. No doubt, the cost of fuel increases with the increase in power output but does not follow directly linear law i.e., the cost of fuel is not directly proportional to output. Fuel cost is directly proportional to the power output in addition to losses occurring in the conversion process.
4. Why is availability of cooling water significant for a steam power plant ?
Ans. Large quantity of water is required in a steam power plant. It is required (i) to raise the steam in boilers (ii) for cooling purposes such as in condensers (iii) as a carrying medium such as disposal of ash and (iv) for drinking purposes.
In steam power plants approximately 1.26 x 106 kcals of heat per MW per hour has to be disposed off in the condenser. In case of direct circulation from the source of water 120 m3 of water is required per MW per hour for this purpose. In case cooling towers are used about 2.4 m3 of water per MW per hour will be required in addition as make-up water. With pond cooling slightly large quantity of water is required to meet evaporation and drift losses etc. but cooling effect is better. The efficiency of direct cooled plant is about 0.5% higher than that of the plant cooled by cooling towers. This means a saving of about Rs 7.5 lacs per year in fuel cost for a 2,000 MW station.
Hence availability of cooling water is significant for steam (coal based) power plants.
5. Why are fuel transport facilities important in case of coal based steam power plants ?
Ans. Central coal based steam power plants need huge quantity of coal every day. The site should be such that coal can be transported easily from mines to power plant.
6. Why are nuclear power plants located away from thickly populated area ?
Ans. There should be a reasonable distance between the nuclear power plant and the nearest populated area from the point of view of safety as there is a danger of presence of radioactivity in the atmosphere near the plant.
7. Which type of plant has highest capital cost ?
Ans. Nuclear power plant has highest capital cost because of heavy investment on building a nuclear reactor.
8. What is the largest size of thermal unit being used in India ?
Ans. The largest size of thermal unit being used in India is 660 MW in Mundra thermal power station (Gujarat).
9. Why large sized units are preferred in steam power plants ?
Ans. Large size units have lower capital cost per kW, need less floor area, require less operating labour and have better efficiency. Thus, large sized units are economical both from the point of view of initial investment and low operating cost.
As the size of plant increases, it may be worth considering the use of larger size of generating units in the power station, if they are available and load curve permits their use.
10. What are the advantages of large size plants ?
Ans. Large sized plants have many advantages (mostly economic). Some of the costs are hardly affected by the plant size. The cost of office space, shops, docks and landscaping can be spread over more capacity. Coal handling equipment, cooling facilities and other appurtenances can also be operated at lesser cost per kWh generated in case of larger plants. Broadly speaking, a large sized plant will produce electricity at a lower cost.
11. What is spinning reserve ?
Ans. The spinning reserve is the extra generating capacity that is as available by increasing the power output of generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to the turbine rotor.
12. What is non-spinning reserve ?
Ans. The non-spinning or supplemental reserve is the extra generating capacity that is not currently connected to the system but can be brought on line after a short delay. In isolated power systems, this typically equates to power available from fast-start generators. However, in interconnected power systems, this may include the power available on short notice by importing from other systems or retracing power that is currently being exported to other systems.
13. Distinguish between operating reserve and spinning reserve ?
Ans. The operating reserve refers to capacity in service in excess of peak load. It is provided for regulation within the hour to cover minute to minute variations, load forecasting errors, loss of equipment and maintenance of equipment.
The operating reserve is made up of the spinning reserve as well as the non-spinning or supplemental reserve.
The spinning reserve is the extra generating capacity that is available by increasing the power output of generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to the turbine rotor.
14. Give the operating efficiencies of different power plants.
Ans. The operating efficiencies of hydro, coal-based steam, nuclear, diesel, gas turbine, wind turbines, power plants, solar thermal systems and geothermal systems are respectively 85-90, 32-42, 30-35, 35-42, 32-38, 30-45, 12 and 35 per cent.