Components of Hydro Power Plant | Electricity | Energy Management

The main components of hydro power plant could be listed as: 1. Forebay and Intake Structures 2. Head Race or Intake Conduits 3. Surge Tank 4. Turbines and Generators 5. Power House 6. Tail Race and Draft Tube.

Component # 1. Forebay and Intake Structures:

As the name suggests forebay is an enlarged body of water in front of intake. The reservoir acts as forebay when penstock takes water directly from it. When canal leads water to the turbines the section of the canal in front of turbines is enlarged to create forebay. The forebay temporarily stores water for supplying the same to the turbines.

The water cannot be allowed to pass as it comes in the reservoir or the canal. At intake gates are provided with hoist to control the entry of water. In front of the gates trash racks are provided to prevent debris, trees, etc. from entering into the penstock. Rakes are also provided to clean the trash racks at intervals.

Component # 2. Head Race or Intake Conduits:

They carry water to the turbines from the reservoir. The choice of open channel or a pressure conduit (Penstock) depends upon site conditions. The pressure conduit may be in the form of a flared intake passage in the body of the dam or it may be a long conduit of steel or concrete or sometimes a tunnel extending for few kilometres between the reservoir and the power house.

The pressure conduit does not follow the ground contours and any gradient is given to suit the site conditions. The velocity of water in the power conduit is also higher than in the open channel. Upto about 60 metres head the velocity may range between 2.5 to 3.0 m/sec. For higher heads the velocity may be still higher.

Sometimes it is convenient or economical to adopt open channel partly or wholly as the main conduit. The head race canal may lead water to the turbines or to the penstocks and is usually adopted in low-head installations where head losses are relatively important. The advantage of an open channel is that it could be used for irrigation or navigation purposes.

Component # 3. Surge Tank:

A surge tank is a storage reservoir fitted at some opening made on a long pipe line or penstock to receive the rejected flow when the pipe line is suddenly closed by a valve fitted at its steep end, see Fig. 20.5. A surge tank, therefore, relieves the pipe line of excessive pressure produced due to its closing, thus eliminating the positive water hammer effect.

It is done by admitting in the surge tank a large mass of water which otherwise would have flown out of the pipe line, but returns to the tank due to closure of pipe end.

It also serves the purpose of supplying suddenly an additional flow whenever required by the hydraulic prime movers at any instant. The surge tank is mostly employed in a water power plant or in a large pumping plant to control the pressure variations resulting from rapid changes in the flow.

In the case of water power plant, when there is sudden reduction of load on the turbine, it becomes necessary for the governor to close the turbine gates for adjusting the flow of water in order to keep the speed of the turbine constant. However, the water is already on its way to the turbine. When the turbine gates are closed, the moving water has to go back. A surge tank would then act as a receptacle to store the rejected water and thus avoids water hammer.

On the other hand when there is an immediate demand on the turbine for more power, the governor re-opens the gates in proportion to the increased load, thus, making it necessary to supply more water.

For a long pipe it takes a considerable time before the entire mass of water can be accelerated. The surge tank which is generally located near the turbine will meet the suddenly increased demand of water till such time the velocity in the upper portion of the line assumes a new value.

Similarly for a large pumping plant with a long delivery pipe, a surge tank can also be employed to control the pressure variations on the delivery side, which result due to sudden shut down or starting up of a pump.

When the pump is started, most of the initial flow from the pump enters the surge tank thus reducing the water hammer effect in the delivery pipe. On the other hand when the pump is shut down suddenly, the surge tank provides extra space to accommodate water which would come back, thus relieving water hammer pressure.

Functions of Surge Tank:

The surge tank thus serves the following purposes:

(i) Control of pressure variations resulting from rapid changes in pipe line flow, thus eliminating water hammer effect.

(ii) Regulation of flow in power and pumping plants by providing necessary accelerating or retarding head.

Location of Surge Tank:

Theoretically a surge tank should be located as close to a power or pumping plant as possible. The ideal place in case of power plant is at the turbine inlet, but it is seldom possible in case of medium and high head installations because it will have to be made very high. In order to reduce its height, it is generally located at a junction of pressure tunnel and a penstock (see Fig. 20.5) or on the mountain side.

Component # 4. Turbines and Generators:

Turbine converts hydraulic energy into mechanical energy. The mechanical energy developed by a turbine is used in running an electric generator. It is directly coupled to the shaft of the turbine. The generator develops electric power. A turbine consists of a wheel called runner. The runner is provided with specially designed blades or buckets. The water possessing large hydraulic energy strikes the blades and the runner rotates.

Water turbines may be classified under two types, namely:

(i) Impulse or velocity turbines, and

(ii) Reaction or pressure turbines.

(i) Impulse Turbine:

In the impulse turbine, all the available potential energy or head is converted into kinetic energy or velocity head by passing the water through a contracting nozzle or by guide vanes before it strikes the buckets. The wheel revolves free in air and water is in contact with only a part of wheel at a time. The pressure of water all along is atmospheric.

In order to prevent splashing and to guide the water discharged from the buckets to the tail race, a casing is provided. An impulse turbine is essentially a low-speed wheel and is used for relatively high heads. Pelton wheel, Turgo impulse wheel and Girard turbine, are some types of impulse turbine. In the Pelton wheel water strikes the runner tangentially.

(ii) Reaction Turbine:

In a reaction turbine, only part of the available potential energy is converted into velocity head, at the entrance to the runner. The balance portion remains as a pressure head. The pressure at the inlet of the turbine is much higher than the pressure at the outlet. It varies throughout the passage of water through the turbine. Mostly the power is developed by the difference in pressure acting on front and back of runner blades. Only small part of power comes from the dynamic action of velocity.

Since the water is under pressure, the entire flow from head race to tail race takes place in a closed system.

Francis and Kaplan turbines are two important types of reaction turbines. In Francis turbine there is inward radial flow of water. In modern Francis turbine the flow enters radially inward but leaves in parallel direction to shaft at centre. It is called mixed flow.

In Girard, propeller and Kaplan turbines the flow is axial or parallel to the axis of the turbine shaft.

Choice of Turbine:

Selection of a suitable type of turbine depends primarily upon the available head and the quantity of water required.

The turbines may be classified as follows with reference to type of power plant:

a. Low head turbine (less than 30 m);

b. Medium head turbine (30 to 160 m);

c. High head turbine (upto and over 1000 m).

Low head turbines are Propeller turbine and Kaplan turbine. These turbines use large quantity of water. Medium head turbines are modern Francis turbines. Impulse turbines are high head turbines. These turbines require relatively less quantity of water.

It has been found from experience that there is a range of head and (Ns) specific speed at which each type of turbine is most suitable. Certain curves representing Ns versus H have been drawn from experimental data for all types of water turbines. The head H being known from the field data, an appropriate value of Ns can be determined from the above curves.

However, for a particular head different values of specific speed may be available. The selection of right type of specific speed is a matter of experience. It is a common practice to select a high specific speed runner which is always economical because the size of the turbo-generator as well as that of the power house will become smaller.

High specific speed is essential where head is low and output (BHP) is large, because otherwise the rotational speed of the turbine will be very low which means cost of turbo-generator and power house will be high. On the other hand there is practically no need of choosing a high value of specific speed for high head installations, because even with low specific speed, high rotational speed can be attained with medium capacity plants.

Component # 5. Power House:

The purpose of the power house is to support and house the hydraulic and electrical equipment.

The power house is readily divided into two parts as follows:

(a) The substructure to support the equipment and to provide the necessary water-ways.

(b) The superstructure or building to house and protect the equipment.

Substructure:

The substructure may form an integral part of the dam and intake structure. In other cases the substructure may be remote from the dam, the dam intake and power house being entirely separate structures. The substructure is built exclusively of concrete and is enforced with steel where necessary.

In modern practice the hydroelectric generating units are nearly always placed in a single row approximately normal to the direction of flow through the power house. This arrangement not only avoids greater complication in the water passage ways, but also facilitates the handling of equipment by a travelling crane.

The substructure is thus divided into a series of bays, one bay for each main unit, one for the exciter units, unless the exciters are mounted on the main units and an entrance or a working bay. In addition sufficient space is provided for housing various accessories and also for operation and maintenance.

For large installations requiring both an operating and maintenance force the switch board may be located at any convenient place. For smaller stations where only one or two men are required for operation and maintenance the switch board is generally located as near as possible to the generators and at the centre of the maintenance activities. In the large plants the switch board is often remote from the generating equipment and is placed in its own operating room. In times of operating emergency there is a distinct advantage in this remoteness and isolation.

Foundation Requirements:

The problem of a suitable foundation for the power house depends on the foundation material. Where rocky ledge is available within moderate depth, the foundation should always be carried to it. With an earth foundation where the soil capacity it definitely limited, the whole base should be made to bear the weight. Bearing capacity of earth foundation depends upon its composition, the extent to which it is confined, and its moisture content.

Super-Structure:

The generating room, the main portion of the power house, contains the main units and their accessories, and usually there is a power or hand operated overhead crane which spans the width of the power house. The switch board and operating stand are usually near the middle of the station, either at floor level or, for better visibility, on the second floor or at a level above the main floor.

Usually an auxiliary bay or section of the powerhouse will be required upstream from the main units for the switches, bus connections, and outgoing lines. If transformers are located inside the station, these will also be in the auxiliary bay, commonly at floor level and shut off from the main floor by steel doors or shutters.

A travelling crane is an important part of the powerhouse equipment. It spans the width of the house and travels for its entire length. In fixing the elevation of the crane rail above the floor, it is essential that sufficient headroom be provided for lifting and carrying along any of the various machine parts.

Component # 6. Tail Race and Draft Tube:

The channel into which the turbine discharges in case of impulse wheel and through draft tube in case of reaction turbine is called a tail race.

The suction pipe or draft tube is nothing but an airtight tube fitted to all reaction turbines on the outlet side. It extends from the discharge end of the turbine runner to about 0.5 metres below the surface of the tail water level. The straight draft tube is generally given a flare of 4 to 6 degrees to gradually reduce the velocity of water. The suction action of the water in this tube has same effect on the runner as an equivalent head so that the turbine develops the same power as if it were placed at the surface of the tail water.

The tail race of the impulse wheel is commonly an approximately rectangular passage, running from a point under the wheel to a point outside the power house foundations where it enters the exit channel or the river. Because of the small discharge of the impulse wheel, as well as higher allowable velocity, the tail race passage is much smaller than that of the reaction turbine.

In case of the reaction turbine the width of the tail race channel under the power house depends upon the unit spacing and thickness of piers and walls between the unit bays. The depth of the tail race channel depends upon the velocity which is generally taken to be about 1 metre per second.

Where the power house is close to the river, the tail race may be the river itself. In other case a tail race channel of some length may be provided to join the turbine pit with the river.