Classification of Hydroelectric Power Plants

The hydroelectric power plants may be classified according to:- A. Classification According to the Extent of Water Flow Regulation Available B. Classification According to Availability of Water Head C. Classification According to Type of Load Supplied D. Classification of Hydroelectric Power Plants Based on Installed Capacity.

A. Classification According to the Extent of Water Flow Regulation Available:

According to the extent of water flow regulation available the hydroelectric power plants may be classified into: 

(1) Run-off river power plants without pondage.

(2) Run-off river power plants with pondage.

(3) Reservoir power plants.

1. Run-Off Power Plants without Pondage:

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. In such plants there is no control on flow of water.

During high flow and low load periods, water is wasted and during the lean flow periods the plant capacity is very low. As such these plants have a very little firm capac­ity. At such places, the water is mainly used for irrigation or navigation and power generation is only incidental. Such plants can be built at a considerably low cost but the head available and the amount of power generated are usually very low.

During floods, the tail water level may become excessive rendering the plant inoperative. The main objec­tive of such plants is to use whatever flow is available for generation of energy and thus save coal that otherwise be necessary for the steam plants. During the high flow periods such plants can be employed to supply a substantial portion of base load.

2. Run-Off River Power Plants with Pondage:

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. With enough pondage, the firm capacity of the power plant is increased.

Such type of power plants can be used on parts of the load curve as required, within certain limitations and is more useful than a plant without pondage. Such power plants are comparatively more reliable and its generating capacity is less dependent on available rate of flow of water. Such power plants can serve as base load or peak load power plants depending on the flow of stream.

During high flow periods these plants may be used as base load and during lean flow periods these plants may be used to supply peak loads only. When providing pondage, tailrace condition should be such that floods do not raise the tailrace water level, thus reducing the head on the plant and impairing its effective­ness. Such plants offer maximum conservation of coal when operated in conjunction with steam power plants.

3. Reservoir Power Plants:

When water is stored in a big reservoir behind a dam, it is possible to control the flow of water and use it most effectively. Storage increases the firm capacity of the plant and it can be used efficiently through­out the year. Such a plant can be used as a base load or as a peak load plant as per requirement. It can also be used on any portion of the load curve in a grid system. Most of the hydroelectric power plants everywhere in the world are of this type.

B. Classification According to Availability of Water Head:

According to availability of water head the hydroelectric power plants may be classified into:

(a) Low Head

(b) Medium Head and

(c) High Head Power Plants.

Though there is no definite line of demarcation for low, medium and high heads but the head below 30 metres is considered low head, the head above 30 metres and below 300 metres is considered as medium head and above 300 metres is considered as high head.

(a) Low Head Hydroelectric Power Plants:

A typical low head installation on a river consists essentially of a dam across the stream to back up the river and create a fall, the water flowing through the turbines and remerging  the river below the dam. A dam or barrage constructed across the river creates the necessary head. The power plant is located near the dam and therefore, no surge tank is required. Either one half of the barrage has regulating gates for discharge of surplus water while the plant is in front of second half or the plant is constructed by the side of the river.

In low head power plants Francis, propeller or Kaplan turbines are employed. Since for given output, large quantity of water is required, head being low, therefore pipes of large diameter and short length are required in low head plants. Structure of such plants is extensive and expensive. Genera­tors employed in such plants are of low speed and large diameter.

(b) Medium Head Hydroelectric Power Plants:

In these power plants, the river water is usually tapped off to a forebay on one bank of the river as in case of a low head plant. From the forebay the water is led to the turbines through penstocks. The forebay provided at the beginning of penstock serves as a water reservoir for such power plants.

In these plants, water is usually carried in open channel from main reservoir to the forebay and then to the turbines through the penstock. The forebay itself serves as the surge tank in this case. In these plants horizontal shaft Francis, propeller or Kaplan turbines are used. The arrangement is shown in Fig. 2.14.

(c) High Head Hydroelectric Power Plants:

If high head is available, a site may be chosen, where a stream de­scending a steep lateral valley can be dammed and a reser­voir for storage of water is formed. A pressure tunnel is constructed between reservoirs to valve house at the start of penstock to carry water from reservoir to valve house.

Surge tank (a tank open from the top) is built just before the valve house so that the severity of water hammer effect on penstock can be reduced in case of sudden closing of fixed gates of the water turbine. Surge tank also serves as a ready reservoir from which the turbine can draw water temporarily when there is sudden increase in demand.

The valve house consists of main sluice valves and automatic isolating valves, which operate on bursting of penstock and cut off further supply of water to penstock. Penstocks are pipes and carry the water from the valve house to the turbines.

For heads above 500 m Pelton wheels are used while for lower heads Francis turbines are employed. The generators used are of high speed and small diameter. Penstocks are of large length and comparatively smaller cross section.

C. Classification According to Type of Load Supplied:

According to the load supplied hydroelectric power stations may be classified into:

(a) Base Load,

(b) Peak Load, and

(c) Pumped Storage Plants for the Peak Load.

(a) Base Load Plants:

The plants, which can take up load on the base portion of the load curve of the power system, are called the base load power plants. Such plants are usually of large capacity. Since such plants are kept run­ning practically on block load (i.e., the load that is practi­cally constant), load factor of such plants is therefore high. Run-off river plants without pondage and reservoir plants are used as base load plants.

Plants having large storage can best be used as base load plants and particularly in rainy seasons, when the water level of the reservoir will be raised by rain water. For a plant to be used as base load plant, the unit cost of energy generated by the plant should be low.

(b) Peak Load Plants:

Plants used to supply the peak load of the system corresponding to the load at the top por­tion of the load curve are called the peak load plants. Run­off river plants with pondage can be employed as peak load plants. If the pondage is enough, a large portion of the load can be supplied by such a plant if and when required. Res­ervoir plants can of course be used as peak load plants also. Peak load plants have large seasonal storage. They store water during off-peak periods and are operated during peak load periods. Load factor of such plants is low.

(c) Pumped Storage Plants for the Peak Load:

This is a unique design of peak load plant.

D. Classification of Hydroelectric Power Plants Based on Installed Capacity:

Apart from above classification, hydroelectric power plants can be classified, on the basis of installed capacity, as large, medium, small, mini, and micro hydro power plants. Generally the mini, micro, and pico hydro come under the subcategory of small hydro plants.

These are briefly described as below:

Apart from the above said classification, there is also a class of very large hydro power plants coming up with capacity ranging from more than 5,000 MW up to 10,000 MW due to the large scale investment and better technology available. However, as far as small hydro is concerned the upper and lower limit varies from country to country while defining the small hydro. There is a general tendency all over the world to define small hydro by power output. Different countries are following different norms keeping the upper limit ranging from 5 to 50 MW.