Combined Operation of Various Power Plants

In this article we will discuss about the combined operation of various power plants.

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. The utility of such plants can be considerably increased by operating them in combination with steam power plants.

The run-off river power plant can be 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 to 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.

Combined Operation of Storage Hydro Plant and Steam Power Plant:

The hydroelectric power plant with ample storage has more firm power. The operating cost of hydro power plants are very low and hydro energy can be produced at very little incremental cost. In a combined operation of storage hydro power plant and steam power plant, the generation of thermal power should be displaced by available hydro power so that maximum decrement production costs will be realised at the steam plant.

The extent to which it can be achieved depends upon the availability of water, storage facilities available and the chronological load curve of the system. During the different times of the year, the hydro power and steam power plants can be operated as base load and peak load plants and vice versa depending upon the factors mentioned above. The hydro and thermal generation should be so scheduled as to keep the generation cost per unit minimum. This can be achieved by making optimum use of available water.

The operation of a combined system is cumbersome owing to the fact that most of the hydro projects are multipurpose ones and are to be operated under very unique restrictions of flood control irrigation etc., in addition to generation of power. Moreover, the general pattern and average amounts of run-offs are almost known from past history, but the daily patterns can be quite different from estimated ones. The matter’s become worse owing to unpredictable evaporation and seepage losses.

The aim of the combined operation of hydro and thermal power stations is to keep the total operating cost minimum and simultaneously to meet the total demand and losses of the power system at any instant. It means that the available water should be put to an optimum use.

Optimisation of hydro power generation involves quite a few variables like operation of various hydro plants at points of their best efficiency, optimisation of available head and reducing the spilling of water to the minimum. The operation of different hydro plants at the points of their best efficiency is concerned with hourly variations in system load demand, division of load between various plants etc.

The optimisation of water head and minimisation of spilling of water are contradictory to each other. Reducing the spilling of water needs storage space in reservoir i.e., the water head is less than the maximum. Spilling of water also increases the tail water elevation and thereby reduces the effective water head. With a reasonably high development of storage and where a considerable down-stream is developed, reducing the spilling of water is found to be more significant.

i. Long Term Hydro-Thermal Co-Ordination:

When the hydro plant capacity in a system is ample and the reservoirs are full, the starting of thermal power plants should be delayed as long as possible. Normally, the thermal plants should supply part of load throughout the year and they should supply the deficit during dry seasons. They should be operated for sufficient time for the required purpose. It may also be possible as an alternative to supply all the load by hydro plants for some time and thus delay the starting of thermal power plants.

When the storage in the reservoirs approaches a minimum level, the water discharge should be reduced and the thermal plants started. In determining the minimum zone of the storage reservoirs, it is necessary to have a detailed statistical analysis of their in-flow, evaporation losses, etc. The minimum zone corresponds to the minimum water level in the reservoirs below which the hydro plants cannot be operated from the reservoir water supply.

The minimum level is a function of the time of the year, as depicted in Fig. 10.5. This curve is usually known as basic rule curve and the area under this zone is called the minimum zone. The basic rule curve represents accumulations of water, in reverse order of time, of the deficiency between the firm load and energy available from the stream flow in critically dry periods. As soon as the reservoir storage tends to reach the basic rule curve, the thermal plants should be started and run at maximum output in order that the firm load is supplied and reservoir does not further deplete.

There is another curve, known as no-spill curve, which represents the accumulations, in reverse order of time, of the surplus between the firm load and the energy available from stream flow in the wettest period. There is no risk of spill if the storage contents are below the ‘no-spill rule curve’. Efforts should be to keep the reservoir contents up to the no-spill curve as far as possible so as to provide additional head without any risk of spill.

The economic long-term hydro-thermal scheduling using large storages is a very complicated problem. A large number of variable factors are involved, resulting in many sets of nonlinear equations. It is not possible to solve such problems by long-hand methods. However, the development of fast computers made it possible to solve a number of complicated equations quickly, as well to arrange programming of the economic working of the power plants.

In general, there are two extreme possibilities. The hydro plant is operated at maximum discharge until the allotted water is consumed, after which the hydro plant is shut down for the rest of the day and the remaining load is supplied by the thermal plant(s). An alternative is to keep the hydro plant shut down and inoperative as long as possible, and to put it in commission only when there is enough time left during the day to consume the allotted quantity of water.

Another method is to reduce the total thermal cost by changing the hourly division of load between the thermal and hydro plants using incremental programming. The incremental value of the water should be determined for the purpose. A number of iterations on computer help in finally obtaining the optimum hydro plants schedule.

ii. Short Term Hydro-Thermal Co-Ordination:

When both thermal and hydro plants are available in a power system they should be scheduled over a given period to achieve maximum economy by minimizing the fuel cost of the thermal- plant units when using desired amounts of water for the hydro plant units.

The following information’s is required for the study:

1. Load cycle.

2. Incremental fuel costs of steam power plants.

3. Expected water inflow during the period of operation.

4. Hydro-generation as a function of head and discharge.

5. Reservoir elevation as a function of storage that is function of inflow, water use, time, evaporation, seepage etc.

6. Incremental transmission loss characteristics.

7. Water intake losses.

8. Tail race elevation as a function of water discharge.

If it is assumed that hydro plants will operate at constant heads and that the thermal plants will operate with the minimum cost of fuel during the year; the economical schedule can be worked out by using co-ordination equations.

Knowing the amount of water that would be available for use at a hydro plant r_j can be chosen. Using this value, co­ordination equation is solved for the economic scheduling of plants in the system. Equations can be solved for different values in the above equations are assumed to be linear functions; an approximate solution can be had.

Another method of solution would be equaled incremental- plant cost method.

Sometimes, the practice is to operate hydro plants near the points of maximum efficiency except when surplus water is available. When hydro plants are operated in this way, the thermal plants can be scheduled either by equal incremental fuel costs or by solution of co-ordination equations.

Combined Operation of Wind Power Unit and Other Power Plants:

The wind power units cannot provide firm power. However, these units can be employed in conjunction with steam and hydro power plants.

i. Combination of Wind and Steam Power:

By operating wind power units in conjunction with a steam power plant supplying major portion of the demand of the power system, there can be savings in fuel. For its economic justification the cost of wind energy may be compared with the cost of the generating additional units from the existing steam plants. Wind power would not be economical beyond the total incremental cost of generating in the steam station. If the steam power station is less efficient and operates at low load factors, it might be economical to add wind power unit(s) to the system.

ii. Combination of Wind and Hydro Power:

The firm capacity of an electrical power system can be increased by operating the wind power unit(s) in combination with storage hydro power plants, particularly if there is diversity between the occurrence of the rain and wind. During wind season, the wind power can be supplied to the electrical power system and thus the load on the hydro plants can be reduced. This will result in the conservation of stored water which can be used afterwards more beneficially.