Combined Operation of Power Plants: Top 5 Benefits

The following points highlights the five benefits of combined operation of power plants. The benefits are: 1. Flexibility of Operation 2. Reliability of Supply 3. Reserve Capacity 4. Better Utilisation of Hydro Power 5. Economic Benefits.

Benefit # 1. Flexibility of Operation:

A fact that has a very important bearing on the cost of electrical energy is that, not only is the demand higher in winter than that in summer, but it also varies throughout the 24 hours of each day, being lowest during the night and rising during the day with peaks at certain times. The varying load on a system calls for solution of some very complex operating problems.

The system requirement is met by loading the most efficient plant in order of merit of operating cost—the power plants with the minimum operating cost are put on load for the maximum time, plants with higher operating cost are only loaded when required by the load demand on the system and those with the maximum operating cost are kept in reserve and would operate only in emergency.

The major drawback of a steam power plant is that it requires appreciable time to start up, synchronise and take up load. It is also necessary to keep the boilers hot throughout the day and night in order to keep the stresses and strains, caused by the temperature variations, to a minimum. It means a continuous unavoidable loss of energy (1 to 2.5% of full- load consumption).

On the other hand, a hydroelectric power plant can be started, synchronised and loaded very quickly and has the advantage of low operating cost. Such a power plant can also meet the sudden variations of load demand easily and so it can be profitably employed both for supplying base loads as well as peak loads.

Nuclear power plant is not suitable for variable load operation as the reactors cannot be easily controlled to respond quickly to load variations. Such a plant is used as base load plant only operating at high load factors of over 0.8. A gas turbine power plant having the advantage of low initial cost and low start up time is ideally suited for supplying peak loads. Diesel power plant, because of its uneconomical operating cost, is operated only when necessary i.e., in emergency.

Thus if various types of power plants are operated in combination, the operation will become flexible i.e., the steam power plants and nuclear power plants are operated to supply base loads, the hydroelectric power plants to supply base loads or peak loads depending upon the availability of water, gas turbine power plants for meeting with peak demand of the system and existing diesel power plants in emergency only.

Benefit # 2. Reliability of Supply:

The reliability of a steam power plant depends upon the coal supply that of a hydro-plant on the stream flow. Due to greater diversity, a combined operation of various types of power plants would be more reliable than the individual steam or hydropower system.

Benefit # 3. Reserve Capacity:

In any electric power system, the system has to have a certain reserve capacity in order to meet any unforeseen excess power demand and to take care of the forced outages of units and schedule maintenance of units. In combined operation of several plants the reserve capacity required is reduced. In a combined system the effect of all the diversities may result in a still larger benefit from the point of view of reserve capacity.

Benefit # 4. Better Utilisation of Hydro Power:

In combined power system the hydro power can be utilised in a more effective way. During periods of heavy run-off, the hydro power plants can be operated to supply base load resulting in conservation of fossil fuels and during periods of draught the steam power plants can be used as base load plants to allow water to be stored in reservoirs and thus make the capacity of the hydro plants effective.

Benefit # 5. Economic Benefits:

We can divide the countries of the world in three groups as regard power potential.

First group of countries (such as Norway, Sweden) is that which have bulk of hydro power resources but fossil fuel is to be imported. Second group of countries (such as UK) is that which have very limited hydro power resources and have to depend mainly on thermal generation. Third group of countries (such as India, USA) is that which have to exploit both hydro and thermal resources to meet the power requirements.

In case of countries, falling under group 1, there might be various plant sites and the comparison will have to be made in terms of cost of producing equivalent electric power and its transmission to load centre. Some sites will be suitable for base load power plants and the others might be suitable for peak load service. The cost of generation of electrical energy at various sites will have to be studied for various load factors. The load factor to be adopted will be that which will provide the lowest overall cost of generation.

There is one drawback of the above system. The maximum run-off may not occur during the periods of maximum demand and for the hydro plant to supply the whole demand throughout the year; storage will have to be built to cover the seasonal variations of run-off. This storage may be either impracticable or very costly to provide. The storage requirements can be reduced by using thermal generation for supplying peak loads. The load factor of the thermal plant will be high only for a small period and its annual load factor will be low.

In case of countries falling in second group i.e., for the countries having very limited hydro power sources and abundant coal reserves, it will be economical to develop the hydro power plants to meet peak load requirements. Such hydro power plants will run at low load factors and enable the thermal power plants to run at high load factors in an economic manner.

It is noteworthy that in hydro plants, the cost of storage is the major item in the total cost and this item does not depend much on the annual load factor, therefore, the capital cost will be reduced for low load factor than for high load factor. It is found that incremental costs resulting from reduced load factor i.e., enhanced amount of installed capacity is less than the capital cost of raising the installed capacity of a thermal plant.

An advantage of increasing capacity of hydro plant is that the spoilage can be reduced at the time of high run-off by operating the hydro plant for longer periods and thereby saving the fuel cost in the thermal power plants.

For the countries falling in third group, proper co­ordination between hydro and thermal power plants is very essential. The hydro plants must be planned to generate enough electric power during adverse periods so that these can meet the needs of that portion of load curve to which they have been assigned. It will result in conserving the fossil fuels to the maximum extent.

The thermal power plants should be set up to enhance the firm capacity of the power system. The run-off river power plants would generally meet the entire or part of the base loads. The remaining demand can be met by judicious mix of reservoir type hydro plants, coal-based thermal plants and nuclear plants.

For any particular scheme, there must be some ratio of the hydro plant to the total demand which gives the lowest annual cost of electric generation. In regions where both cheap fuel and favourable hydro sites are available, this ratio can be 25 to 40. In regions, where fuel is costly and the suitable hydro sites are available, this ratio will be much higher. But for regions where fuel is cheap and the cost of development of hydro power plant is high, the ratio may be quite low.