This document outlines various ways to classify hydroelectric power plants. It discusses classification based on: 1) the quantity of water available and ability to regulate flow, including run-of-river without pondage, run-of-river with pondage, storage, and pumped storage. 2) The available head height, including high, medium, and low head. 3) The nature of the load, including base load and peak load plants. 4) Whether the plant is on or off the transmission grid. 5) The plant's capacity. 6) The purpose of the plant. 7) The hydrological relationship between plants, including single stage and cascade systems.

1. Chapter Two
Classification of Hydroelectric
Power Plants

2. Outline
o Capacity of Water Flow Regulation
o Availability Of Head
o Power Plant Capacity
o The nature of load characteristics
o Based on transmission system(interconnection)
o According to purpose
o According to hydrological relation
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3. 2.1 Based on Quantity of water available (water
flow regulation)
1. Run off river plants without pondage
2. Run off river plants with pondage
3. Storage Type Plants(Impoundment plants)
4. Pumped storage plants
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4. 1. Run off river plants without pondage
o It doesn’t store water and uses as the water comes.
o 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.
o Generating capacity primarily depending on the rate of flow of water
o During dry periods the power production will be very poor, since the water flow rate
will be low.
o Due to non uniformity of supply and lack of assistance from a firm capacity the
utility of these plants are much less than those of other types
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5. 2. Run off river plants with pondage
o Pondage: the collection of water behind a dam at the plant and increases the stream
capacity for a short period say a week.
o Storage : collection of in up stream reservoirs and this increases the capacity of the
streams over an extended period of several months.
o Usefulness of run off river is increased by pondage
o These plants may work satisfactorily as base load and peak load plants.
o As compared to that without pondage, is more reliable and its generating capacity is
less dependent on the flow rates of water available.
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6. 3. Storage type plants(Impoundment plants)
o This type of plant, water is stored behind the dam and water is available throughout the
year even in dry season
o Storage type plant is one with 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 the minimum natural flow.
o This plant can be used as base load plant as well as peak load plant as water is available
with control as required.
o The majority of hydroelectric power plants in our country are of this type.
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7. 4. Pumped storage plant
 Are employed at the places where the quantity of water available for power generation
is inadequate
 Water passing through the turbine is stored in the tail race pond. During low load
periods this water is pumped back to the head reservoir, which can be used during
peak load plants.
 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.
 Usually interconnected with steam or diesel power plants.
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8. Cont.…..
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9. 2.2 Based on the Availability of Head
 Water must fall from a higher elevation to a lower one to release its stored energy.
 The difference between these elevations (the water level in forebay and tail bay) is
called head.
 Three categories of hydro power plant based on design head values are
❖High-head (100m and above)
❖Medium-head (30m to 100m)
❖Low-head (less than 30m)
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10. 1. High Head Power Plants
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11. Cont.…..
 A hydel plant with a water head of more than 100 meters is termed a high head plant.
 In this case, the water from the main reservoir is carried through tunnels up to the surge tank, from
where it is taken through the penstock.
 Since the water head is very high the effect of water hammer is too severe in such plants, it is essential
to provide a surge tank in the water line at appropriate location.
 The surge tank takes care of the increasing and decreasing water levels during the low demand and high
demand periods, respectively.
 The Francis and Pelton wheel turbines are most suitable for high head plants.
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12. 2. Medium Head Power Plants
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13. Cont.…..
 A hydel plant with a water head of in the range of 30 to 100 meters is termed as a
medium head power plant.
 The forebay provided at the beginning of the penstock serves as water reservoir.
 The water is generally carried in open canals from main reservoir to the forebay
and then to the power house through the penstock
 The forebay itself acts as the surge tank, and hence receives the excess water
during the low demand periods.
 This type of plant commonly uses Francis turbines. 13

14. 3. Low Head Hydel Power Plant
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15. Cont.…..
 A plant with a water head of less than 30 meters is termed a low head plant.
 No surge tank is required for this plant, dam itself discharge the surplus water
from the river
 In such plants, a small dam is constructed across a river to obtain the necessary
water head.
 The excess water is allowed to flow over the dam, while the water head is made
use to run a hydraulic turbine.
 This type of plant uses vertical shaft Francis or Kaplan turbine.
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16. 2.3 According to the Nature of Load
1. Base load plants :
o These plants are required to supply constant power in the grid.
o They run continuously without any interruption and are mostly remote controlled.
o e.g. storage plants, run-off river plants without pondage.
2. Peak load plants:
o Plants which supply the power during peak loads are known as peak load plants.
o They only work during certain hours of a day when the load is more than the average.
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17. Cont.…..
e.g. pumped storage plants, run-off river plants with pondage.
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18. 2.4 Based on Transmission System (Interconnection)
A. Isolated (off-grid) power plants
o Whenever a hydropower plant is set up in a remote area in order to meet the local demands
then such a hydropower plant is known as Isolated(off-grid) System.
B. Interconnected (on-grid) power plants
o Whenever the hydropower plant is set up to meet the demands of areas which are at a fair
distance from the plant, then the transmission of power takes through the grid system. Such a
setup is referred to as Connected to grid.
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19. 2.5 Plant capacity
i. Large-hydro
➢> 100 MW feeding into a large electricity grid
ii. Medium-hydro
➢15 - 100 MW usually feeding a grid
iii. Small-hydro
➢1 - 15 MW - usually feeding into a grid
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20. Cont.…..
iv. Mini-hydro :0.1 - 1 MW Either stand alone schemes or more often feeding
into the grid
v. Micro-hydro: From 5 - 100 kW Usually provided power for a small
community or rural industry in remote areas away from the grid.
vi. Pico-hydro: < 5 kW Remote areas away from the grid.
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21. 2.6 Classification According to Purpose
A. Single Purpose:
o When the whole soul purpose of a project is to produce electricity
then such a project is known as a Single Purpose Hydro Power
Project.
B. Multi Purpose :
o When the water used in hydropower project is to be used for
other purposes is known as Multi Purpose Hydro Power Project.
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22. Purpose of multipurpose hydro project
o For generation of electricity.
o For irrigation of agricultural land.
o For navigation.
o For fisheries and tourism.
o For flood control.
o For domestic water supply.
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23. 2.7 Classification According to Hydrological Relation
 Single Stage: When the run off from a single
hydropower plant is diverted back into river or
for any other purpose other than power
generation, the setup is known as Single Stage.
 Cascade System: When two or more
hydropower plants are used in series such that
the runoff discharge of one hydro power
plant is used as an intake discharge of the
second hydro power plant such a system is
known as CASCADE hydropower plant.
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Figure-(a) single stage hydropower development scheme
(b) cascade or multistage hydropower system

24. Questions?
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