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Hydro-electric Power Plant

2.2. Hydro-electric Power Plant

2.2.1. Classification

Hydro-electric power stations may be classified as follows:

A. According to availability of head

1. High head power plants

2. Medium head power plants

3. Low head power plants.

B. According to the nature of load

1. Base load plants

2. Peak load plants ..

C. According to the quantity of water available

1. Run-of-river plant without pondage

2. Run-or-river plant with pondage

3. Storage type plants

4. Pump storage plants

5. Mini and micro-hydel plants.

A. According to Availability of Head;

The following figures give a rough idea of the heads under which the various types of plants work:

(i)                           High head power plants …… 100 m and above

(ii)                        Medium head power plants …… 30 to 500

(iii)                      Low head power plants …… 25 to 80 m.

Note. It may be noted that figures given above overlap each other. Therefore it is difficult to classify the plants directly on the basis of head alone. The basis, therefore, technically adopted is the specific speed of the turbine used for a particular plant.

1. High head power plants;

Water is usually stored up in lakes on high mountains during the rainy season or during the season when the snow melts. The rate of flow should be such that water can last throughout the year.

Fig. 2 shows high head power plant layout. Surplus water discharged by the spillway cannot endanger the stability of the main dam by erosion because they are separated. The tunnel through the mountain has a surge chamber excavated near the exit. Flow is controlled by head gates at the tunnel intake, butterfly valves at the top of the penstocks, and gate valves at the turbines. This type of site might also be suitable for an underground station. 4

Fig. 2. High head power plant layout. The main dam, spillway, and power
house stand at widely separated locations. Water flows from the reservoir
through a tunnel and penstock to the turbines.


The Pelton wheel is the common prime-mover used in high head power plants.


2. Medium head power plants:


Refer Fig. 3. This type of plant commonly uses Francis turbines. The forebay provided at the beginning of the penstock serves as water reservoir. In such plants, the water is generally carried in 5

Fig. 3. Medium head power plant layout.


open canals from main reservoir to the forebay and then to the powerhouse through the penstock. The forebay itself works as a surge tank in this plant.


3. Low head power plants:


Refer Fig. 4. These plants usually consist of a dam across a river. A sideway stream diverges from the river at the dam. Over this stream the power house is constructed. Later this channel joins the river further downstream. This type of plant uses vertical shaft Francis turbine or Kaplan turbine. 6

Fig. 4. Low head power plant layout.


B. According to the Nature of Load:


1. Base load plants:


The plants which cater for the base load of the system are called base load plants. These plants are required to supply a constant power when connected to the grid. Thus they run without stop and are often remote-controlled with which least staff is required for such plants. Run-of-river plants without pondage may sometimes work as base load plant, but the firm capacity in such cases will be very much less.


2. Peak load plants:


The plants which can supply the power during peak loads are known as peak load plants. Some of such plants supply the power during average load but also supply peak load as and when it is there, whereas other peak load plants are required to work during peak load hours only. The run-of-river plants may be made for the peak load by providing pondage.


C. According to the Quantity of Water Available:


  1. 1.   Run-of-river plants without pondage. A run-of-river plant without pondage, as the
    name indicates, does riot store water and uses the water as it comes. There is no control on flow of water so that during high floods or low loads water is wasted while during low run-off the plant capacity is considerably reduced. Due to non-uniformity of supply and lack of assistance from a firm capacity the utility of these plants is much less than those of other types. The head on which these plants work varies considerably. Such a plant can be made a great deal more useful by providing sufficient storage at the plant to take care of the hourly fluctuations in load. This lends Some firm capacity to the plant. During good flow conditions these plants may cater to base load of the system, when flow reduces they may supply the peak demands. Head water elevation for plant fluctuates with the flow conditions. These plants without storage may sometimes be made to supply the base load, but the firm capacity depends on the minimum flow of river. The run-of-river plant may be made for load service with pondage, though storage is usually seasonal.
  2. 2.   Run-of-river plant with pondage. Pondage usually refers to the collection of water behind a dam at the plant and increases the stream capacity for a short period, say a week. Storage mean collection of water in upstream reservoirs and this increases the capacity of the stream over an extended period of several months. Storage plants may work satisfactorily as base load and peak load plants.


This type of plant, as compared to that without pondage, is more reliable and its generating capacity is less dependent on the flow rates of water available.


  1. 3.   Storage type plants. A storage type plant is one with a reservoir of sufficiently large size to permit carry-over storage from the wet season to the dry season, and thus to supply firm flow substantially more than the minimum natural flow. This plant can be used as base load plant as well as peak load plant as water is available with control as required. The majority of hydro-electric plants are of this type.
  2. 4.   Pumped storage plants. Refer Fig. 5. Pumped storage plants are employed at the places where the quantity of water available for power generation is inadequate. Here the water passing through the turbines is stored in ‘tail race pond’. During low load periods this water is pumped back to the head reservoir using the extra energy available. This water can be again used for generating power during peak load periods. Pumping of water may be done seasonally or daily depending upon the conditions of the site and the nature of the load on the plant. 7

    Fig. 5. Pumped storage plant.

    Such plants are usually interconnected with steam or diesel engine plants so that off peak capacity of interconnecting stations is used in pumping water and the same is used during peak load periods. Of course, the energy available from the quantity of water pumped by the plant is less than the energy input during pumping operation. Again while using pumped water the power available is reduced on account of losses occuring in prime-movers.