Principles of Hydropower Engineering (Part - 4) - Civil Engineering (CE) PDF Download

Hydropower plant scheme layout 

Typical components of a hydroelectric plant consist of the following:

1. Structure for water storage and/or diversion, like a dam or a barrage.
2. A head-race water conveying system like a conduit (penstock) or an open channel to transport water from the reservoir or head-water pool up to the turbines.
3. Turbines, coupled to generators
4. A tail race flow discharging conduit of open channel that conveys the water out of the turbine up to the river. 

Although the above components are common for all hydropower development schemes the general arrangement for high and medium head power houses are more or less similar. The low head power plants, which are usually of run-of-power type schemes, have a slightly different arrangement as mentioned in the paragraphs below. 

High and medium head development    

Usually, there could be two types of power scheme layout: 

• Concentrated fall schemes
• Diversion schemes

In the concentrated fall type projects, the powerhouse would be built at the toe of a concrete gravity dam, shown as a schematic view in Figure 7 and sectional view in Figure 12. This type of project development is suitable for medium head projects since a high head project would require an enormous concrete gravity dam, which is generally not adopted. A medium or high head project with an earthfill or rockfill dam may have an isolated or off-stream power house as shown in Figure 13. Here, the water is conveyed to the turbines via penstocks laid under, or by-passing, the dam. Spillways are provided separately to take care of floods. A distinction of such projects is that it consists of a long system of water conduits. Surge tanks are sometimes provided at the end of the conduits to relieve them of water hammer, which is the very high pressure developed by causing the stoppage of flow too suddenly at the turbine end. 

In the diversion type of layout, the diversion could be using a canal and a penstock (Figure 18) or a tunnel and a penstock (Figure 19). The former is usually called the Open-Flow Diversion System and the latter Pressure Diversion System. 

Figure 18 Hydroelectric Project Based On Open Flow Diversion Scheme

1-Dam .2-Intake Diversion Conduit. 3-Head Pond. 4- Spillway . 5- Power House.6- Tailvace. 7-Penstocks. 8-Reserwair

Figure 19. Hydroelectric Project Using A Pressure Diversion System
 1-Watercourse. 2-Dam. 3- Intake Structures. 4-Diversion Tunnel. 5- Surge Tank. 6-Penstock Fork House 7-Penstocks.8- Penstocks Support.9- Power House. 10- Power Line

In fact, the combination of open channel and pressure conduit and penstock may be done in a variety of ways shown in Figure 20. 

Figure 20. Diversion Hydro Rower Project Based On Open Channel And Pressure Flow Systems
(A)    Long Canal And Short Surface Penstock Along Stfiaight River Reach
(B)    Same As (A) Bui In A Curved River Reach
(C)    Sectional View Along Water Conducting System For (A) And (B)

Figure 20. Diversion Hydro Power Project Based On Open Channel And Pressure Plow Systems
 (D)    Short Canal And Long Surface Penstock
 (E)    Sectional View For (D)

Figure 20. Diversion Hydro Power Project Based On Open Channel And Pressure Flow Systems
 (F)  Head Race Tunnel And Penstock In A Curves River Reach
 (G)  Sectional View For {F}

There could be totally underground projects which consist of only pressure system of water conveyance. A variety of such projects are shown in Figure 21. This type of project layout may be termed as underground diversion schemes where even the power house is built underground.   

Figure 21. Underground project with (a),(b) and (c) pressure diversion system, and (d),{e) and (f) open flow diversion system;
 1-intake structure; 2- surge tank: 3-tailrace pressure tunnel; 4- power house; 5-penstock G-mtake open flow tunnel; 7-tailrace open flow tunnel :8-inlake pressure tunnel: 9--head pond

Low head development 

Here too, two types of layouts may be possible:

• In-stream scheme
• Diversion scheme 

In the in-stream type of project, the powerhouse would be built as a part of the diversion structure, as shown in Figure 2(a) or a general detailed view in Figure 6. A typical layout of such a power house and its cross section is shown in Figure 22. 

FIGURE 22. A TYPICAL LOW-HEAD IN STREAM POWER HOUSE
 (a) plane ;(b) sectional elevation of the powerhouse; 1-earth dam.2-over flow dam
 3- power house; 4-lock;5-spillways in power house; 6- navigable canal dike downstream of dam;
 7-output dike; 8- left bank clearing; 9- electrical switch yard

In the diversion type of scheme, there has to be a diversion structure as well as a diversion canal, as shown in Figure 2(b). The power house may be located at some convenient point of the canal, that is, at its upstream end, middle, or at the downstream end. The location of the power house depends upon conditions such as hydrological, topographical, geological, environmental economic conditions. The ground water table has to be taken into account.

Position of power houses  

As might have been noticed from the layouts, there could be a variety of position for the power house with respect to natural ground level.

IS: 4410(Part10)-1988 differentiates between the following types of power stations, which may be constructed as per site conditions:

1. Surface power station or over ground power station: A power station which is constructed over the ground with necessary open excavation for foundations. Typical examples may be seen from Figs. 7, 11 or 12.

2. Underground power station: A power station located in a cavity in the ground with no part of the structure exposed to outside. A typical example of this type is shown in Figure 23. 

FIGURE 23 Underground power Iwuse of Sardar Sarovar Dam project

3. Semi-underground power station: A power station located partly below the ground level and followed by a tail race. 

Electrical terms associated with hydropower engineering  

Electrical power generated or consumed by any source is usually measured in units of Kilowatt-hour (kWh). The power generated by hydropower plants are normally connected to the national power grid from which the various withdrawals are made at different places, for different purposes. The national power grid also obtains power generated by the non-hydropower generating units like thermal, nuclear, etc. The power consumed at various points from the grid is usually termed as electrical load expressed in Kilo-Watt (KW) or Mega-Watt (MW). The load of a city varies throughout the day and at certain time reaches the highest value (usually in the evening for most Indian cities), called the Peak load or Peak demand. The load for a day at a point of the national power grid may be plotted with time to represent what is known as Daily Load Curve. Some other terms associated with hydropower engineering are as follows: 

Load factor 

This is the ratio of average load over a certain time period and the maximum load during that time. The period of time could be a day, a week, a month or a year. For example, the daily load factor is the ratio of the average load may be obtained by calculating the total energy consumed during 24 hours (finding the area below the load vs. time graph) and then divided by 24. Load factor is usually expressed as a percentage

Installed capacity 

For a hydro electric plant, this is the total capacity of all the generating units installed in the power station. However all the units may not run together for all the time. 

Capacity factor

This is the ratio of the average output of the hydroelectric plant for a given period of time to the plant installed capacity. The average output of a plant may be obtained for any time period, like a day, a week, a month or a year. The daily average output may be obtained by calculating the total energy produced during 24hours divided by 24. For a hydroelectric plant, the capacity factor normally varies between 0.25 and 0.75. 

Utilization factor 

Throughout the day or any given time period, a hydroelectric plant power production goes on varying, depending upon the demand in the power grid and the power necessary to be produced to balance it. The maximum production during the time divided by the installed capacity gives the utilization factor for the plant during that time. The value of utilization factor usually varies from 0.4 to 0.9 for a hydroelectric plant depending upon the plant installed capacity, load factor and storage. 

Firm (primary) power 

This is the amount of power that is the minimum produced by a hydro-power plant during a certain period of time. It depends upon whether storage is available or not for the plant since a plant without storage like run-of-river plants would produce power as per the minimum stream flow. For a plant with storage, the minimum power produced is likely to be more since some of the stored water would also be used for power generation when there is low flow in the river. 

Secondary Power 

This is the power produced by a hydropower plant over and above the firm power.