Plants are classified as follows according to the quantity of water available :
- Run off river plants without pondage
- Run off river plants with pondage.
- Reservoir plants.
1. Run off river plants without pondage
In this type of hydro-electric plant, water from the river is not stored in the pond. It uses the water from the river directly. Hence, it can use water when it is available in the river. The capacity of such plants primarily depends on the rate of flow of water, hence during the rainy season some quantity of water will go as waste, while during low run-off periods i.e. summer season the capacity of the plant is reduced.
2. Run off river plants with pondage
Run-off river plants have limitations in its use. The usefulness of such plants can be increased by storing river water in ponds. Due to pondage, it is possible to store water in ponds during the off- peak periods and use the same during the peak periods of load. It will be possible to cope with hour -to -hour fluctuations of load throughout the week or longer period. When sufficient capacity of pondage is provided, it will increase the firm capacity of the plant. Such type of plants can be used on peak load part of load curve.
3. Reservoir plants
Such type of plants have a reservoir of such a capacity that the storage of water is permitted in them from rainy season to dry season. The water stored in reservoir in this fashion is available to the plant with control as required. Such plant has better capacity and can be used efficiently throughout the year. Their firm capacity is more and they can be used as base-load plant or as a peak-load plant as is required by the system. They can operate on any portion of the load curve Majority of hydro-plants are of this type.