Combined Operation of Power Plants Interview Questions and Answers:

1. Why is it necessary to operate hydro and steam plants in combination ?

Ans. It is necessary to operate hydro and steam plants in combination because such a combination provides (i) flexibility of operation, (ii) reliability of power supply, (iii) reduced reserve capacity, (iv) better utilization of hydro power and (v) better economy.

2. What do you understand by base load and peak load ?

Ans. The unvarying load, which occurs almost the whole day on the power plant is called the base load whereas the various peak demands of the load over and above the base load of the power plant is called the peak load.

3. What do you understand by ‘peak load plant’ and ‘base load plant.

Ans. The power plant that supplies various peak demands of the load over and above base load of the power system is known as peak load plant while the plant supplying base load (unvarying load) of the system is known as base load plant.

4. What are the requirements of plant so that it is suitable for base load operation ?

Ans. The power plants to be employed as base power plants should have (i) low operating cost (ii) capability of working continuously for the long periods (iii) requirement of few operating personnel and their repair should be economical and speedy.

5. What are the requirements of power plants that are suitable for peak load operation ?

Ans. The power plants to be employed as peak power plants should have the capability of quick start, synchronization and taking up of system load and quick response to load variations.

6. Why the hydroelectric power plants are suited to both base load and peak load operations ?

Ans. The hydroelectric power plants are suited to base load operation because their operating costs are very low and suited to peak load operation because they can be started, synchronized and loaded very quickly and sudden variations of load demand can be met easily.

7. Why is it necessary to operate nuclear power plant as base load plant supplying load at load factor higher than 0.8 ?

Ans. The operating cost is quite low and once the installation is completed, the loading of the power plant will have no effect on the generation cost. Therefore, a nuclear power plant is always operated as a base load plant. The nuclear power plants are usually not operated at a load factor less than 0.8.

8. Explain the function of power grid.

Ans. Power grid is an interconnection of major power plants by means of EHV transmission lines. As far as power sector is concerned, India is divided in five zones/regions—Northern, Southern, Eastern, Western and North-Eastern regions. Each of these regions has a grid network of its own which is managed at regional load dispatch centres. These grids are further interconnected to form National Power Grid. The functions of national power grid include supplying of power to power deficit areas by evacuating power from power surplus areas. It is also responsible for maintaining frequency and voltage levels. Another function is to prevent complete outage in an area in case of failure of power plants of the concerned area.

9. Why is it advantageous to interconnect various power plants ?

Ans. Some of the advantages of interconnected power system are:

(i) Reduced plant reserved capacity, (ii) Increased reliability of supply, (iii) Increased effective capacity of the power system, (iv) Economical operation, (v) Exchange of peak loads, (vi) Possibility of using obsolete plants. (vii) Reduced capital cost and (viii) Saving in operating costs.

10. What is run-off river plant ?

Ans. Some hydro power plants are so located that the water is taken from the river directly, and no pondage or storage is possible. Such plants are called the run-off river power plants without pondage. Such plants can use water only as and when available; these cannot be used at any time at will or fit any desired portion of the load curve.

The usefulness of run-off river power plants is increased by pondage. Pondage refers to storage at the plant which makes it possible to cope, hour to hour, with fluctuations of load throughout a week or some longer period depending on the size of pondage.

11. Why is it necessary to operate run-off river plant in combination with steam plant ? How are they operated in rainy season and dry season ?

Ans. The run-off river power plant has a small pondage and uses water as it is available. The run-offs of river vary widely during the year-very large in rainy season and very low in dry season. As such the firm capacity of such plants is very low. So, in order to increase utility of such plants, it becomes necessary to operate it in combination with steam plant. The run-off river power plant can he employed as a base load power plant (up to its installed capacity) during rainy seasons (periods of high stream flows) while the steam power plant may supply the peak load. During dry season (periods of lean flow) the steam power plant is used to supply the base load and run-off river power plant for supplying peak load. Thus the amount of load carried by the steam plant can be adjusted to conform with the available river flow. Such a plan of operation results in a greater utilisation factor of the river flow and a saving in the amount of fuel consumed in the steam plant.

12. What is the meaning of basic rule curve ?

Ans. The basic rule curve is that curve which represents accumulations of water, in reverse order of time, of the deficiency between firm load and energy available from the stream flow in critically dry periods.

13. What is meant by incremental water rate ?

Ans. Incremental water rate is a change in output power due to corresponding change in input water flow.

14. Discuss the methods used for computing the generation schedules in a combined hydrothermal system.

Ans. The various methods for computing the generation schedules in a combined hydro thermal system are as follows:

1. Constant Hydro Generation: In constant hydro generation, we keep hydro generation constant throughout the operating period at such a value that the desired quantity of water is consumed. The balance load (varying from time to time) is supplied by the thermal plants.
2. Constant Steam:  In constant steam generation, we keep steam generation constant throughout the operating period and the balanced load is supplied by the hydro plants. The thermal generation is kept such that the hydro plants would consume the desired quantity of water over the operating period.
3. Maximum Hydro Efficiency Method: In this method, the hydro plants are operated at the point of maximum efficiency during peak load periods. During the off-peak hours hydro generation is scheduled so as to consume desired quantity of water and operate hydro plants as near the point of maximum efficiency as possible. The balance load is supplied by thermal plants.

The scheduling of thermal plants, for supplying the balance load, is accomplished either by equal incremental cost criterion or by solution of co-ordination equations (including transmission losses).