1. CHAPTER ONE
Classification of Hydroelectric Power Plants
Hydropower is the harnessing of flowing water using the dam or other
diversion structures for the purpose of producing electrical energy
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2. Classification of Hydroelectric Power Plants
The hydroelectric power plants may be classified
according to:-
Classification According to the Extent of Water
Flow Regulation Available
Classification According to Availability of Water
Head
Classification According to Type of Load Supplied
Classification of Hydroelectric Power Plants Based
on Installed Capacity.
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3. A. Classification According to the Extent of Water
Flow Regulation Available
Run-off river power plants without pondage.
Run-off river power plants with pondage.
Reservoir power plants.
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4. 1. Run-Off Power Plants without Pondage
Generates the power from direct water from
river
Such plants can use water only as available
No control on flow of water.
During high flow and low load periods water
is wasted .
During the low flow periods the plant capacity
is very low.
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5. During the floods time the tail water level may
become excessive because of that the plant is
inoperative.
During the high flow periods such plants can
be employed to supply a substantial portion of
base load.
The capacity of such plant is low.
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6. 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 cover hour to hour
fluctuations of load
With enough pondage, the firm capacity of
the power plant is increased.
Such power plants can serve as base load or
peak load power plants depending on the flow
of stream.
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7. During high flow periods these plants used as
base load
During low flow periods these plants may be
used to supply peak loads only
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8. 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
Most of the hydroelectric power plants
everywhere in the world are of this type
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10. Classification According to Availability of Water Head
There is no definite line of demarcation for low,
medium and high heads.
a. Low Head Power Plants, <30m
b. Medium Head Plants , 30m<h<300m
c. High Head Power Plants,>300m
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11. (a) Low Head Hydroelectric Power Plants
A low head dam is constructed across the stream of the
river to the purpose of back up the river and creates a
fall of water through the turbine.
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.
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.
Generators employed in such plants are of low speed
and large diameter.
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12. (b) Medium Head Hydroelectric Power Plants
In these power plants, the river water is usually tapped
off to a fore bay on one bank of the river as in case of a
low head plant.
The fore bay provided at the beginning of penstock
serves as a water reservoir for such power plants
From the fore bay the water is led to the turbines
through penstocks.
In these plants, water is usually carried in open channel
from main reservoir to the fore bay and then to the
turbines through the penstock.
The fore bay itself serves as the surge tank in this case
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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 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.
Penstocks are pipes and carry the water from the valve house to the
turbines.
The generators used are of high speed and small diameter.
Penstocks are of large length and comparatively smaller cross section.
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15. C. Classification According to Type of Load Supplied:
a. Base Load
b. 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.
Unvarying load, which occurs almost the
whole day on the power plant, is called the
base load.
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16. Such plants are usually of large capacity. Since such plants
are kept running the load that is practically constant
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
For a plant to be used as base load plant, the unit cost of
energy generated by the plant should be low.
The power plants to be employed as base power plants
should have low operating cost, Capability of working
continuously for the long periods, requirement of few
operating personnel and their repair should be economical
and speedy.
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17. (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.
The various peak demands of the load over and above the base load of the power
plant is called the peak load
Run - off 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.
The power plants to be employed as peak power plants should have the capability
of quick start, synchronization and quick response to load variations.
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18. Classification of Hydroelectric Power Plants Based on Installed Capacity
Very large hydro power plants coming up with
capacity ranging from more than 5,000 MW up to
10,000 MW.
Small hydro is concerned the upper and lower
limit varies from country to country.
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19. Pumped Storage Plants
A pumped storage plant stores power in the form
of potential energy of water.
The scheme consists of a storage reservoir at a
higher level and turbine-pump installation.
The scheme is used to generate power only
during the peak hours of demand.
During off-peak hours, pumping the water from
the low reservoir back to the storage reservoir at
higher elevations and water is thus stored.
This stored water is utilized again during hours of
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20. Cont.
For the purpose of cost reduction use the
newly developed machine which is designed
to work as turbine as well as pump.
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21. The necessary head can be created in
different ways
1. Building a dam across a stream to hold back water and
release it through a channel conduit or a tunnel.
2. Divert a part of the stream by creating a low-head diversion
structure like barrage
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22. Cont.
In the diversion type of layout, the diversion
could be using a canal and a penstock or a
tunnel and a penstock for divert the portion of
river.
The diversion type is usually called the open-
flow diversion system and the latter pressure
diversion system.
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23. Open flow and Pressure diversion
channel
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24. Cont…
The main parts of a high-head diversion canal
type plant are
a. the weir
b. the canal intake
c. he head race
d. the head pond with spillway and gate or valve
chamber
e. penstock
f. powerhouse
g. tailrace.
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25. Multistage (Cascade) hydroelectric system
A series of integrated power developments
along the same watercourse is called a
multistage hydroelectric system .
Each portion of the river with a power plant of
its own is referred to as a stage.
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26. Assignment 1
1. List the source of electric power generation in Ethiopian
with its installed capacity?
2. List hydropower plant station in Ethiopian with its
installed capacity?
3. How much percent of the hydropower plant contribute for
the total power demand in Ethiopia? Describe using pie
chart.
4. How many hydropower potential is available in Ethiopian
for the production of electric energy from water.
5. How much percent of hydropower plant is developed in
Ethiopia while compare to the hydropower generating
capacity?
Sub date: Thursday(15/09/2011) - 5:30 LT
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27. • The Government of Ethiopia has focused on
the construction and expansion of
various power generating projects to deliver
reliable electricity. Approximately 90% of the
installed generation capacity is
from hydropower while the remaining 8% and
2% is from wind and thermal sources
respectively.Nov 5, 2018
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