2. Syllabus
Unit No. 02: Loads on Power Plant
• Introduction, classification of loads on power plant,
Different load curves and load factors, Effect of
variable load on power plant, design & operation,
comparison of the thermal, hydroelectric, nuclear and
diesel power plants. (Numerical treatment)
• Classification of plants, Requirements of peak load
plant, Pumped storage plants, Compressed air storage
plants, Load sharing between base load & peak load
power stations.
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3. About Title of Subject
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• Loads: Load refers to the amount of power in the electrical
grid. The load must be balanced so that supply continuously
meets demand levels which are subject to variations and
changes that impact the grid. Thus, load is separated into
base load, and peak load.
• Power Plant: It is an industrial facility that generates
electricity from primary energy.
• Base load is the minimum
level of electricity demand
required over a period of 24
hours. It is needed to provide
power to components that keep
running at all times (also referred
as continuous load).
• Peak load is the time of high
demand.
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An electrical load is a device or an electrical component that
consumes electrical energy and convert it into another form of
energy. Electric lamps, air conditioners, motors, resistors etc.
are some of the examples of electrical loads.
The various Types of Load in Power System are :
• Domestic load
• Commercial load
• Industrial load
• Municipal load
• Irrigation load
5. Importance of Chapter
• It represents the maximum power consumed by
the load during a specific period of time. It also
represents the maximum power generated by a power
station during that period of time (neglecting the
transmission losses). Knowing the
Maximum Load is important as it helps in determining
the capacity of the power plant.
• This chapter is important to know about various load on
plant, different load curves their factors. Also it is
important to know various plants and their comparison
study and related calculation of loads.
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Classification of Base load Power plants
The plant supplying “Base Load”
1. Hydroelectric Power Plants:
Operating cost is minimum.
Maintenance cost is lower.
Initial cost is high
Can’t be located near to load.
2. Steam Power Plants:
Capital cost is more
Can be located near to load center.
Maintenance requirement is slightly higher.
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Classification of Base load Power plants
The plant supplying “Base Load”
3. Run of river Plants:
use of water when it comes.
4. Nuclear Power Plant:
Initial Cost is High
Limited availability of fuel
Whenever they are constructed they are used as base load plants due to
constant supply.
5. Wind and Solar:
Depend randomly on wind and solar rays
6. Nuclear Power Plants:
they are not used as peak load plants.
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Classification of Peak load Power plants
The plant supplying “Peak Load”
1. Hydroelectric Plants:
can be considered as peak load plant.
It can be easily started.
No fuel required.
2. Diesel Power Plants:
can be used for peak load plants
Easily start from cold conditions.
Number of auxiliaries required is limited, so less maintenance.
They have good overload capacity.
They have high thermal efficiency therefore less operating cost
than steam power plants.
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Types of Loads
A device which taps electrical energy from the electric power system
is called a load on the system. The load may be resistive (e.g., electric
lamp), inductive (e.g., induction motor), capacitive or some
combination of them. The various types of loads on the power system
are :
(i) Domestic load. Domestic load consists of lights, fans,
refrigerators, heaters, television, small motors for pumping water etc.
Most of the residential load occurs only for some hours during the
day (i.e., 24 hours) e.g., lighting load occurs during night time and
domestic appliance load occurs for only a few hours. For this reason,
the load factor is low (10% to 12%).
(ii) Commercial load. Commercial load consists of lighting for shops,
fans and electric appliances used in restaurants etc. This class of load
occurs for more hours during the day as compared to the domestic
load. The commercial load has seasonal variations due to the
extensive use of air conditioners and space heaters.
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iii) Industrial load
Industrial load consists of load demand by industries. The magnitude
of industrial load depends upon the type of industry. Thus small scale
industry requires load upto 25 kW, medium scale industry between 25kW
and 100 kW and large-scale industry requires load above 500 kW. Industrial
loads are generally not weather dependent.
(iv) Municipal load
Municipal load consists of street lighting, power required for water
supply and drainage purposes. Street lighting load is practically constant
throughout the hours of the night. For water supply, water is pumped to
overhead tanks by pumps driven by electric motors. Pumping is carried out
during the off-peak period, usually occurring during the night. This helps to
improve the load factor of the power system.
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(v) Irrigation load
This type of load is the electric power needed for pumps
driven by motors to supply water to fields. Generally this
type of load is supplied for 12 hours during night.
(vi) Traction load
This type of load includes tram cars, trolley buses, railways
etc. This class of load has wide variation. During the morning
hour, it reaches peak value because people have to go to their
work place. After morning hours, the load starts decreasing
and again rises during evening since the people start coming
to their homes.
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Definition of Load Curve:-
The curve showing the variation of load on the power
station with respect to time.
The power demands of different consumers vary in
accordance with their activities.
The result of this variation in demand is that load on a
power station is never constant rather it varies from time
to time. Daily Load curve
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TYPE OF LOAD CURVES
Daily load curve –Load variations during the whole
day
Monthly load curve – Load curve obtained from the
daily load curve
Yearly load curve - Load curve obtained from the
monthly load curve
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IMPORTANCE OF DAILY LC
The daily load curves have attained a great importance in generation as
they supply the following information readily :
The daily load curve shows the variations of load on the power station
during different hours of the day.
The area under the daily load curve gives the number of units generated
in the day.
Units 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 the day.
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IMPORTANCE OF DAILY Load Curve
The ratio of the area under the load curve to the total area of
rectangle in which it is contained gives the load factor.
The load curve helps in selecting the size and number of
generating units.
The load curve helps in preparing the operation schedule of
the station.
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Variable Loads on Power Plant:
• The load on power station varies time to time due to uncertain
demands from customer and is known as variable load on station.
•A power station is designed to meet requirements customer.
•An ideal power plant is designed to operate the equipments and
machines constant magnitude and steady conditions.
•But such steady load on power station will not remain constant it
varies w.r.t. Time.
•The customer needs the load as per their demands and activities.
•The load demand of one customer at any time is different than
other.
•The result is that load on power station varies time to time
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Effect Variable Loads on Power Plant Design
& Operation:
• Need Additional Equipments
• Increase in Production Cost
• In variable loads generator must able to take variable load as
quick as possible without change in voltage & frequency.
• When load on generator increases its first effect is to slow
down rotor & prime mover T.F. To reduce the frequency.
•Frequency Stabilizer are used to maintain the it constant.
•The design of thermal power plant for variable load is more
difficult as compare to diesel or hydraulic power plant and it is
always desirable to operate thermal power plant as Base Load
Plant.
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Requirement of Peak Load Plant:
• Peaking power plants, also known as Peaker plants, and
occasionally just “Peakers", are power plants that generally run
only when there is a high demand, known as peak demand, for
electricity.
•To cater the demand peaks, peak load power plants are used.
They are started up whenever there is a spike in demand and
stopped when the demand recedes.
•Examples of gas load power plants are: Gas plant.
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Requirement of Pumped storage plants :
• A pumped-storage plant is designed with two reservoirs – upper and lower.
Like every other hydroelectric plant, a pumped-storage plant generates electricity
by allowing water to fall through a turbine generator.
Need:
Pumped storage
projects store and
generate energy by
moving water between
two reservoirs at
different elevations.
It is the most
competitive and
reliable way of storing
electricity, enabling
both the efficient use
of surplus energy and
the returning of a
significant amount of
energy back on the
grid.
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Requirement of Compressed Air Storage Plants :
• Compressed-air energy storage (CAES) plants operate by using motors to
drive compressors, which compress air to be stored in suitable storage
vessels. The energy stored in the compressed air can be released to drive an
expander, which in turn drives a generator to produce electricity.
•When the energy is needed, this compressed air is then released into
turbine generators so it can be used as electricity again.
•The five key benefits of CAES include energy savings, better air quality,
improved pressure stability, reduced maintenance costs, and enhanced
compressor service life.
•Instead of compressing the air with valuable gas, lower cost excess energy can
be used during off peak periods
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Load Duration Curve:
• Load duration curve indicates the variation of the load, but with
the load arranged in descending order of magnitude.
• Load duration curve give the number of hours for which a
particular load lasts during a day.
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• The load duration curve gives the minimum load present
throughout the specified period.
•It authorizes the selection of base load and peak load power plants.
•Any point on the load duration curve represents the total duration
in hours for the corresponding load and all loads of greater values.
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Load Sharing between Base Load Plants &
Peak Load Plants :
• If load is supplied from one power plant only then in such case
Installed capacity of power plant = Peak load or more
•Such power plant will be uneconomical since the peak load is
occurred only for short period of time.
•Therefore, the capacity equal to difference between base load and
peak load would remain idle for major portion of year.
•Hence such a power plant load should not be supplied from single
power plant.
•There should be some power stations supplying the base load and
some power plants which should be supplying peak load power
demand.
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Important Definitions related with Load curves & Load
Factors :
1. Connected Load
It is the sum of continuous ratings of all the equipments connected to
supply system.
2. Maximum Demand
It is the greatest demand of load on the power station during a given
period.
3. Demand Factor
It is the ratio of maximum demand on the power station to its
connected load.
4.Average Load
It is 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.
Demand Factor = Maximum Demand/Connected Load
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Important Definitions related with Load curves :
5. Load factor :
The ratio of average load to the maximum demand during a given period is
known as Load Factor
41. Prof. Y. M. Khedkar
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6. Diversity Factor :
It is the ratio of sum of individual maximum demands to the maximum
demand on the power Station.
7. Capacity Factor
It is the ratio of actual energy produced to the maximum possible
energy that could have been produced during a given period.
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8. Plant Use Factor
It is the ratio of KWhr generated to the product of plant capacity and the
number of hours for which the plant was in operation
9. Units generated/annum = Average load (in kW) × Hours in a year
= Max. demand (in kW) × L.F. × 8760
10. Reserve capacity = Plant capacity − Max. demand
