Hydro power plant

What is Hydropower Plant?
Hydropower plant uses hydraulic energy of water to produce electricity. The power
obtained from this plant is termed as hydroelectric power. Nearly 16% of total power used
by the world is represented by hydropower.
There are several types of hydropower plants classified on different characteristics. But
for every hydropower plant some important principal components are needed and those
are explained here.

Components of a Hydropower Plant
The major components of a hydroelectric plant are as follows.
1. Forebay
2. Intake structure
3. Penstock
4. Surge chamber
5. Hydraulic turbines
6. Power house
7. Draft tube
8. Tailrace

1. Forebay
A forebay is a basin area of hydropower plant where water is temporarily stored before
going into intake chamber. The storage of water in forebay is decided based on required
water demand in that area. This is also used when the load requirement in intake is less.
We know that reservoirs are built across the rivers to store the water, the water stored on
upstream side of dam can be carried by penstocks to the power house. In this case, the
reservoir itself acts as forebay.
2. Intake Structure
Intake structure is a structure which collects the water from the forebay and directs it into
the penstocks. There are different types of intake structures are available and selection of
type of intake structure depends on various local conditions.
Intake structure contain some important components of which trash racks plays vital role.
Trash racks are provided at the entrance of penstock to trap the debris in the water.
If debris along with water flows into the penstock it will cause severe damage to the wicket
gates, turbine runners, nozzles of turbines etc. these trash racks are made of steel in rod
shape. These rods are arranged with a gap of 10 to 30 cm apart and these racks will separate
the debris form the flowing water whose permissible velocity is limited 0.6 m/sec to 1.6
m/sec.
In coldweather regions, there is chance of formation of ice in water, to prevent the entrance
of ice into the penstocks trash racks heated with electricity and hence ice melts when it
touches the trash racks.
Other than trash racks, rakes and trolley arrangement which is used to clean the trash racks
and penstock closing gates are also provided in intake structure.

3. Penstock
Penstocks are like large pipes laid with some slope which carries water from intake
structure or reservoir to the turbines. They run with some pressure so, sudden closing or
opening of penstock gates can cause water hammer effect to the penstocks.
So, these are designed to resist the water hammer effect apart from this penstock is similar
to normal pipe. To overcome this pressure, heavy wall is provided for short length penstock
and surge tank is provided in case of long length penstocks. Steel or Reinforced concrete
is used for making penstocks. If the length is small, separate penstock is used for each
turbine similarly if the length is big single large penstock is used and at the end it is
separated into branches.
4. Surge Chamber
A surge chamber or surge tank is a cylindrical tank which is open at the top to control the
pressure in penstock. It is connected to the penstock and as close as possible to the power

house. Whenever the power house rejectedthe water load coming from penstock the water
level in the surge tank rises and control the pressure in penstock. Similarly, when the huge
demand is needed in power house surge tank accelerates the water flow into the power
house and then water level reduces. When the discharge is steady in the power house, water
level in the surge tank becomes constant. There are different types of surge tanks available
and they are selected based on the requirement of plant, length of penstock etc.
5. Hydraulic Turbines
Hydraulic turbine, a device which can convert the hydraulic energy into the
mechanical energy which again converted into the electrical energy by coupling the
shaft of turbine to the generator. The mechanism in this caseis, whenever the water
coming from penstock strike the circular blades or runner with high pressure it will
rotate the shaft provided at the center and it causes generator to produce electrical
power.
Generally hydraulic turbines are of two types namely
 Impulse turbine
 Reaction turbine

Impulse turbine is also called as velocity turbine. Pelton wheel turbine is example
for impulse turbine. Reaction turbine is also called as pressure turbine. Kaplan
turbine and Francis turbine come under this category.
6. Power House
Power house is a building provided to protect the hydraulic and electrical equipment.
Generally, the whole equipment is supported by the foundation or substructure laid for the
power house. In case of reaction turbines some machines like draft tubes, scroll casing etc.
are fixed with in the foundation while laying it. So, the foundation is laid in big dimensions.
When it comes to super structure, generators are provided on the ground floor under which
vertical turbines are provided. Besides generator horizontal turbines are provided. Control
room is provided at first floor or mezzanine floor.

7. Draft Tube
If reaction turbines are used, then draft tube is a necessary component which connects
turbine outlet to the tailrace. The draft tube contains gradually increasing diameter so that
the water discharged into the tailrace with safe velocity. At the end of draft tube, outlet
gates are provided which can be closed during repair works.
8. Tailrace
Tailrace is the flow of water from turbines to the stream. It is good if the power house is
locatednearer to the stream. But, if itis located far away from the stream then it is necessary
to build a channel for carrying water into the stream.
Otherwise the water flow may damage the plant in many ways like lowering turbine
efficiency, cavitation, damage to turbine blades etc.

This is because of silting or scouring caused by unnecessary flow of water from power
house. Hence, proper design of tailrace should be more important.
TYPES OF PROJECT
Capacity, unit size and selection of Equipment, their Characteristics and
Specifications for design of hydro power station depends upontype of hydroelectric
Development and classification with respectto head and size. There are three main
types of hydropower schemes that can be categorized in terms of how the flow at a
given site is controlled or modified. These are:
1. Run-of-river plants (no active storage); and
2. Plants with significant storage
3. Pumped storage
In a run-of-river project, the natural flow of the river is relatively uncontrolled. In a
storage project, the filling and emptying of the impounded storage along with the
pattern of the natural stream flow controls the flow in the river downstream from the
storage impoundment.
Run-of-river plants can be located at the downstream end of a canal fall, openflume,

or pipeline diverting the stream’s flow around a water supply dam or falls. The
available flow governs the capacity of the plant. The plant has little or no ability to
operate at flow rates higher than that available at the moment.
In a conventional plant, a dam, which stores water in a reservoir or lake
impoundment, controls the river flows. Water is released according to electric,
irrigation, water supply, or flood control needs. Constructing a dam and storage
reservoir can increase the percentage of time that a project can produceagiven level
of power. Base load plants those operated at relatively constant output-may have
either a small capacity relative to the river flow or may have a significant storage
reservoir.
Storage reservoirs can be sized for storing water during wet years or wet seasons.
Alternatively, they canbesized to provide water forweekly ordaily peak generation.
A storage reservoir allows using available energy that might otherwise be wasted as
spill.
Plants with storage at both head and tailrace are pumped storage project.
Run of the River Schemes or Diversion Schemes
This type of development aims at utilizing the instantaneous discharge ofthe stream.
So the discharge remain restricted to day to day natural yield from the catchments;
characteristics of which will depend on the hydrological features. Diurnal storage is
Sometime provided for optimum benefits. Development of a river in several steps
where tail race discharges from head race inflows for downstream power plants
forms an interesting variation ofthis caseand may require sometimes specialcontrol
measures.
Small scale powergeneration also generally fall in the category and may have special
Control requirement especially if the power is fed into a large grid.
Storage Schemes
In such schemes annual yield from the catchment is stored in full or partially and
then released according to some plan for utilization of storage. Storage may be for
single purpose such as power development or may be for multipurpose use which
may include irrigation, flood control, etc. therefore, design of storage works and
releases from the reservoir will be governed by the intended uses of the stored water.
If the scheme is only for power development, then the best use of the water will be

by releasing according to the power demand. Schemes with limited storage may be
designed as peaking units. If the water project forms a part of the large grid, then the
storage is utilized for meeting the peak demands. Such stations could be usefully
assigned with the duty of frequency regulation of the system.
There are two main types of pumped storage plants:
i) Pumped-storage plants and
ii) Mixed pumped-storage plants.
Pump Storage Scheme
Like peaking, pumped storage is a method of keeping water in reserve for peak
period powerdemands. Pumped storage is water pumped to a storage poolabove the
powerplant at a time when customer demand for energy is low, such as during the
middle of the night. The water is then allowed to flow back through the turbine-
generators at times when demand is high and a heavy load is place on the system.
The reservoir acts much like a battery, storing power in the form of water when
demands are low and producing maximum power during daily and seasonal peak
periods. An advantage of pumped storage is that hydroelectric generating units are
able to start up quickly and make rapid adjustments in output. They operate
efficiently when used for one hour or several hours.
Because pumped storage reservoirs are relatively small, construction costs are
generally low compared with conventional hydropower facilities.
Pump-storage plants:
In this type only pumped storage operation is envisaged without any scope for
conventional generation of power. These are provided in places where the run-off is
poor. Further, they are designed only for operation on a day-to-day basis without
room for flexibility in operation.

Principle
The basic principle of pumped storage is to convert the surplus electrical energy
available in a system in off-peak periods, to hydraulic potential energy, in order to
generate power in periods when the peak demand on the system exceeds the total
available capacity of the generating stations.
By using the surplus scheme electrical energy available in the network during low
demand periods, water is pumped from a lower pond to an upperpond. In periods of
peak demand, the power station is operated in the generating mode i.e. water from
the upper pond is drawn through the same water conduit system to the turbine for
generating power.
Hydropower schemes of the pumped-
storage type are those which utilize the
flow ofwater from a reservoir at higher
potential to one at lower potential
(Figure 6d). A typical schematic view
of such a plant is shown in Figure 9.
The upper reservoir (also called the
head-water pond) and the lower
reservoir (called the tail-water pond)
may both be constructed by providing
suitable structure across ariver (Figure
10). During times of peak load, water
is drawn from the head-water pond to
run the reversible turbine-pump units
in the turbine mode. The water
released gets collected in the tail-water
pond. During off-peak hours, the reversible units are supplied with the excess

electricity available in the power grid which then pumps part of the water of the tail-
water pond back into the head-water reservoir. The excess electricity in the grid is
usually the generation of the thermal power plants which are in continuous running
mode. However, during night, since the demand of electricity becomes drastically
low and the thermal power plants can not switch off or start immediately, there a
large amount of excess power is available at that time.
Mixed pumped-storage plants: In this type, in addition to the pumped storage
operation, some amount of extra energy can be generated by utilizing the additional
natural run-off during a year.
These can be designed for operation on a weekly cycle or other form of a longer
period by providing for additional storage and afford some amount of flexibility in

There are many factors that need to be considered. Primarily hydroelectric power (HEP)
works by using the kinetic energy of falling water. The greater the fall or head, the more
power that is potentially available. Therefore, mountainous areas, such as Nepal, have
potential for HEP. In addition to head height, catchment characteristics need to be known,
for example area of the river basin, valley slope, rainfall and river discharge.
The power generated from a hydropower scheme is given by the following formula
Power = η ρ g Q H
Where:
η is the ‘water to wire’ efficiency – typically (70-80% for a small scheme).
ρ is the density of water (1,000 kg/m3).
g is acceleration due to gravity (9.81 m/s2).
Q is the flow through the turbine (m3/s).
H is the Net Head (m) i.e. including head losses.

Power is proportional to the head and the flow. In general, the physical size and, hence, to a
large extent, costofthe turbine is governed by its design flow, rather than the hydraulic head.
For example, a 100kW scheme at 300m head will require about 40 litres/s, whilst a 3m head
will require a flow of 4,200 litres/s (assuming 80% efficiency).
Therefore, a high head scheme is preferable to a low head scheme and head is more important
than flow (although clearly flow is also important!). But in both cased hydroelectric turbines
are available for high head or low head situations.
From the equation Power = ηρgQH
there are two critical parameters in determining the potential power from a site; the hydraulic
head and the flow in the river. However, the flow in the river will tend to vary, and often
substantially vary, on a day by day basis. It is of great importance to know what kind of
variation there is over a period of a year
P = 9.81QH [kW] The above expression gives the theoretical power of the selected
river stretch at a specified discharge. In order to evaluate the potential of power that
may be generated by harnessing the drop in water levels in a river between two
points, it is necessary to have knowledge of the hydrology orstream flow of the site,
since that would be varying everyday. Even the average monthly discharges over a
year would vary. Similarly, these monthly averages would not be the same for
consecutive years. Hence, in orderto evaluate the hydropowerpotential ofa site, the
following criteria are considered:
1. Minimum potential power is based on the smallest runoff available in
the stream at all times, days, months and years having duration of 100
percent. This value is usually of small interest
2. Small potential power is calculated from the 95 percent duration
discharge
3. Medium or average potential power is gained from the 50 percent
duration discharge

4. Mean potential power results by evaluating the annual mean runoff.
Since it is not economically feasible to harness the entire runoff of a river
during flood (as that would require a huge storage), there is no reason for
including the entire magnitude of peak flows while calculating potential
power or potential annual energy.
Types of Loads
ResidentialLoad
This type of load includes domestic lights, power needed for domestic appliances
such as radios, television, water heaters, refrigerators, electric cookers and small
motors for pumping water.
CommercialLoad
It includes lighting for shops, advertisements and electrical appliances used in
shops and restaurants, etc.
Industrial Load
It consists of load demand of various industries.
Municipal Load
It consists of street lighting, power required for water supply and drainage
purposes.
Irrigation Load
This type of load includes electrical power needed for pumps driven by electric
motors to supply water to fields.
TractionLoad
It includes terms, cars, trolley, buses and railways.

Load Curve
A load curve (or load graph) is a graphic record showing the power demands for
every instant during a certain time interval. Sucha record may cover1 hour, in which
case it would be an hourly load graph; 24 hours, in which case it would be a daily
load graph; a month in which case it would be a monthly load graph; or a year (7860
hours), in which caseit would bea yearly load graph. The following points are worth
noting
:
(i) The area under the load curve represents the energy generated in the period
considered.
(ii) The area under the curve divided by the total number of hours gives the average
load on the power station.
(iii) The peak load indicated by the load curve/graph represents the maximum
demand of the power station.
Significance of Load Curves:
 Load curves give full information about the incoming and help to decide the
installed capacity of the power station and to decide the economical sizes of
various generating units.
 These curves also help to estimate the generating cost and to decide the
operating schedule of the power station, i.e. the sequence in which different
units should be run.

Load Duration Curve : A load duration curve represents re-arrangements of all the
load elements of chronological load curve in order of descending magnitude. This
curve is derived from the chronological load curve. Figure shows atypical daily load
curve for a power station.
It may be observed that the maximum load on power station is 35 kW from 8 AM to
2 PM. This is plotted in Figure .Similarly, other loads of the load curve are plotted
in descending order in the same figure. This is called load duration curve

The following points are worth noting :
(a) The area under the load duration curve and the correspondingchronological load
curve is equal and represents total energy delivered by the generating station.
(b) Load duration curve gives a clear analysis of generating power economically.
Proper selection of base load power plants and peak load power plants becomes
easier.
Dump Power
This term is used in hydroplants and it shows the power in excess of the load
requirements and it is made available by surplus water.
Firm Power
It is the power which should always beavailable even under emergency conditions.
Prime Power
It is the power which may be mechanical, hydraulic or thermal that is always
available for conversion into electric power.
Cold Reserve
It is that reverse generating capacity which is not in operation but can be made
available for service.
Hot Reserve
It is that reserve generating capacity which is in operation but not in service.
Spinning Reserve

It is that reserve generating capacity which is connected to the bus and is ready to
take the load.
Connected Load
The connected load on any system, or part of a system, is the combined continuous
rating of all the receiving apparatus on consumers’ premises, which is connected to
the system, or part of the system, under consideration.
Demand
The demand of an installation or system is the load that is drawn from the source of
supply at the receiving terminals averaged over a suitable and specified interval of
time. Demand is expressed in kilowatts (kW), kilo volt-amperes (kVA), amperes
(A),or other suitable units.
Maximum Demand or Peak Load
The maximum demand of an installation or system is the greatest of all the demands
that have occurred during a given period. It is determined by measurement,
according to specifications, over a prescribed interval of time.
Demand Factor
The demand factor of any system, or part of a system, is the ratio of maximum
demand of the system, a part ofthe system, to the total connected load of the system,
or of the part of the system, under consideration. Expressing the definition
mathematically,
Demand factor = Maximum demand / Connected load
Load Factor
The load factor is the ratio of the average power to the maximum demand. In each
case, the interval of maximum load and the period over which the average is taken
should be definitely specified, such as a “half-hour monthly” load factor. The proper
interval and period are usually dependent upon local conditions and upon the

purpose for which the load factor is to be used. Expressing the definition
mathematically,
Load factor = Avg Load / Maximum demand
Diversity Factor
The diversity factor of any system, or part of a system, is the ratio of the maximum
power demands of the subdivisions of the system, or part of a system, to the
maximum demand of the whole system, or part of the system, under consideration,
measured at the point of supply. Expressing the definition mathematically,
Diversity factor = Sum of individual maximum demands / Maximum demand of
entire group
Utilization Factor
The utilization factor is defined as the ratio of the maximum generator demand to
the generator capacity.
Plant Capacity Factor
It is defined as the ratio of actual energy produced in kilowatt hours (kWh) to the
maximum possible energy that could have been produced during the same period.
Expressing the definition mathematically
Plant capacity factor = ( E / C* t)
where, E = Energy produced (kWh Plant Economy ) in a given period,
C = Capacity of the plant in kW, and
t = Total number of hours in the given period.
Plant Use Factor
It is defined as the ratio of energy produced in a given time to the maximum possible
energy that could have been produced during the actual number of hours the plant
was in operation. Expressing the definition mathematically,
Plant use factor = E / C*t'
where, t' = Actual number of hours the plant has been in operation.

Plant Capacity Factor :
It is the ratio of the average loads on a machine or equipment to the rating of the
machine or equipment, for a certain period of time considered. Since the load and
diversity factors are not involved with „reserve capacity‟ ofthe power plant, a factor
is needed which will measure the reserve, likewise the degree of utilization of the
installed equipment. For this, the factor “Plant factor, Capacity factor or Plant
Capacity factor” is defined as,
Plant Capacity Factor = (Actual kWh Produced)/(Maximum Possible Energy that
might have produced during the same period)
Thus the annual plant capacity factor will be, = (Annual kWh produced)/[Plant
capacity (kW) × hours of the year]
The difference between load and capacity factors is an indication of reserve
capacity.
Load Factor :
It is defined as the ratio of the average load to the peak load during a certain
prescribed period of time. The load factor of a power plant should be high so that
the total capacity of the plant is utilized for the maximum period that will result in
lower costof the electricity being generated. It is always less than unity. High load
factor is a desirable quality. Higher load factormeans greater average load, resulting
in greater number of powerunits generated fora given maximum demand. Thus, the
fixed cost, which is proportional to the maximum demand, can be distributed over a
greater number ofunits (kWh) supplied. This will lower the overall costofthesupply
of electric energy.

Utility Factor:
It is the ratio of the units of electricity generated per year to the capacity of the plant
installed in the station. It can also be defined as the ratio of maximum demand of a
plant to the rated capacity of the plant. Supposingthe rated capacity of a plant is 200
mW. The maximum load on the plant is 100 MW at load factor of 80 per cent, then
the utility will be = (100 × 0.8)/(200) = 40% Plant Operating Factor It is the ratio
of the duration during which the plant is in actual service, to the total duration ofthe
period of time considered.
Demand Factor :
The actual maximum demand of a consumer is always less than his connected load
since all the appliances in his residence will not be in operation at the same time or
to their fullest extent. This ratio of' the maximum demand of a system to its
connected load is termed as demand factor. It is always less than unity.
Diversity Factor:
Supposing there is a group of consumers. It is known from experience that the
maximum demands of the individual consumers will not occurat one time. The ratio
ofthe sum of the individual maximum demands to the maximum demand ofthe total
group is known as diversity factor. It is always greater than unity. High diversity
factor (which is always greater than unity) is also a desirable quality. With a given
number of consumers, higher the value of diversity factor, lower will be the
maximum demand on the plant, since, Diversity factor = Sum of the individual
maximum Demands/Maximum demand of the total Group So, the capacity of the
plant will be smaller,
resulting in fixed charges.

Load Curve:
It is a curve showing the variation of powerwith time. It shows the value ofa specific
load for each unit of the period covered. The unit of time considered may be hour,
days, weeks, months or years.
Firm Power :
It is the power, which should always beavailable even under emergency conditions.
Prime Power It is power, may be mechanical, hydraulic or thermal that is always
available for conversion into electric power.
For Example: In Case of hydro power plant with reservoir, the firm power is that
power which a hydro electric plant supplies for 95% of the time. However, it is not
necessary that firm power should beproduces throughout the year & available under
emergency conditions.
Plant Use Factor :
This is a modification of Plant Capacity factor in that only the actual number of
hours that the plant was in operation is used.
Thus Annual Plant Use factor is, = (Annual kWh produced) / [Plant capacity (kW)
× number of hours of plant operation]
Load Curve:
The Load Curve is a Graph, which represents load onthe generation station (the load
is in kW/MW) recorded at the interval of half hour or hour (time) against the time in
chronological order.
The Load Curve gives following Information:
The daily load curve shows the variation of load on the power station during
different hours of the day.
The area under the daily load curve gives the number of unit generated in the day.
Unit generated/day= Area (in kWh) under daily load curve.
The highest point on the daily load curve represents the maximum demand on the
station on that day.

The area under the daily load curve divided by the total number of hours gives the
average load on the station in that day.
The load curves helps in selecting the size & number of generating units.
The load curve helps in preparing the operation schedule of the station.
The curve which gives idea of load of a whole day with respectto time (24 Hours or
12 Hours of the day) is known as daily load curve. T
he monthly load curve can be obtained from the daily load curve of the month. For
this purpose, average values of power over a month at different times of the day are
calculated.
The yearly load curve is obtained by considering the monthly load curve of that
particular year.
The yearly load curve is generally used to determine annual load factor.
Load Duration Curve:
Definition: When the load elements of a load curve are arranged in the order of
descending magnitudes, the curve thus obtained is called a load duration curve. The
load duration curve is obtained from the same data as load curve but the ordinate
representing the maximum load is represented to the left and the decreasing loads

are represented to the right in the descending order.
From the above figure (i) shows the daily load curve, the daily load duration curve
can be readily obtained from it.
From fig (ii), it is clear from the daily load duration curve that the magnitudes of
load elements are in descending order. The magnitudes are 20MW for 8 Hours,
15MW for 4 hours & remaining 5 MW from 12 hours.
The Load Duration Curve gives following information:
The load duration curve readily shows the number of hours during which the given
load has prevailed.
The area under daily load duration curve (in kWh) will give the units generated on
that day.
The load duration curve, which helps to give information about annual load duration
curve.
Average Demand or Load:
Definition: The average of loads occurring on the power station in a given period
(day or month or year) is known as average load or average demand.

The Average Demand is Calculated by using given formula:
Maximum Demand (MD)
Definition: It is the greatest demand of load on the Power Station during a giving
period is known as Maximum Demand
We know that, the load on every power station in not constant. The load varies from
time to time. The variation ofload on the power station is depends uponthe demand
ofload with respectto time. Consider, the abovefigure, the figure X-axis Represents
Time in Hours & Y-axis represents Load in MW. In this figure, at every two hours
give information about how much load generated. Out of the 6MW load generated
during evening period. So that maximum Demand is 6MW. The Knowledge of
Maximum Demand is very important as it helps in determining the installed capacity
of the power station.
Surplus (secondary) power:
All the power available in excess of firm power.
 Secondary power cannot be relied upon.
 Its rate is usually less than that of firm power.

 It can be generated ~9 to 14 hours/day

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