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CHAPTER 1
INTRODUCTION
1.1 Current and Future Trends in the Energy scenario
The high Indian population coupled with increase in industrial growth has
resulted in an urgent need to increase the installed power capacity. Energy ‘self-
sufficiency’ was identified as the major driver for new and renewable energy in
the country in the wake of the two oil shocks of the 1970s. Owing to problems
related to uncertainty in pricing & supply, Commission for Additional Sources
of Energy in the Department of Science & Technology was established in 1981
& India became the first country to set up a ministry for renewable energy.
Currently research in the field of solar, wind, biomass etc is being extensively
carried out. The solar insolation level in India is very high, thereby attracting
the interest of government and private players in solar energy generation. A
35,000 km² area of the Thar Desert has been set aside for solar power projects,
sufficient to generate 700 to 2,100 Gigawatts. Under the Jawaharlal Nehru
National Solar Mission, plans have been made to generate 1,000 MW of power
by 2013 and up to 20,000 MW grid-based solar power by 2020.
Wind power started to gain momentum during the 1990s and India currently
stands at the fifth spot for production of wind energy in the world. Wind energy
contributes to over 6% of the country’s total installed capacity, with the state of
Tamil Nadu contributing the highest. Hydro electric power projects, although
antiquated now, are still constructed as they are clean and green. The first hydro
plant was constructed over Niagara Falls with barely a capacity of a few
kilowatts and today, there are plants with capacities as high as gigawatts. The
different generating modes of hydro plants are using Dams, Pumped storage,
Run-of-the-river, Tide etc. India is blessed with immense amount of hydro-
electric potential and ranks 5th in terms of exploitable hydro-potential on global
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scenario. The installed capacity as on 30th June 2011 is approximately 37,367.4
MW which is 21.53% of total Electricity Generation in India. India also has
geothermal units in its northern states, biomass generation scheme for rural use
among other notable renewable resources. It has been estimated that the energy
which can be generated from biomass obtained from cattle residue and molasses
is as high as 25GW, given the fact that India has 28% of world’s cattle
population making it the highest in the world in terms of livestock.
1.2 Challenges in grid connection of Renewable Resources
The challenges in grid connection of renewable resources arise mainly due to
their intermittent nature. This results in increased frequency deviations which is
not desirable. This is so due to:
AC machines at the utility, for instance an industrial exhaust fan, a home
appliance like juicer might underperform at frequency other than the
nominal frequency. Hence frequency is maintained virtually at a constant
value.
Electronic equipments depend on frequency for their timing.
Microcontroller based device like oven and washing machine needs
accurate timing for good performance.
Transformers are frequency sensitive and may get overloaded during high
deviations.
The auxillary electric motors in a power plant which feed fuel to boiler or
remove ash may underperform thereby reducing the output of the power
plant which is undesirable considering the high demand for quality
power.
Due to these challenges, one may be inclined to feel that renewable resources
have more disadvantages than the advantages it offers. Also, in the case of
certain renewable resources like wind, it is assumed that additional plant must
be held in readiness to account for times when energy production from wind
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ceases. But this is a misconception since such a problem would exist only in
case of a standalone system (or islanding). When connected to a grid, the
changes in output of wind farm will be accounted the same way as the changes
in load are accounted for, by conventional plants. Hence the deviation in power
output of renewable resources has low impact in the requirement of reserves.
Other challenges include high installation cost, requirement of clearance of area
as in case of wind mills.
1.3 Wind Energy
Wind energy is seen as one of the most promising resources for the future. It is
the fastest growing renewable resource. The differential heating of the earth’s
surface drives wind hence wind energy is as an indirect solar energy. It has been
estimated that between 18 to 68TW can be extracted from winds which is
considerably higher than the total installed capacity in the world which makes it
easier to tap. Several countries have already achieved relatively high levels of
wind power penetration, such as 21% of stationary electricity production
in Denmark, 18% in Portugal, 16% in Spain, 14% in Ireland and 9%
in Germany in 2010. As of 2011, 83 countries around the world are using wind
power on a commercial basis. India has installed capacities of over 15000MW
most of which is in Tamil Nadu. Nowadays, offshore wind mills are gaining
popularity. Offshore wind power refers to the construction of wind farms in
bodies of water to generate electricity from wind. Better wind speeds are
available offshore compared to on land, so offshore wind power’s contribution
in terms of electricity supplied is higher.
Grid management of Wind Farms
Induction generators, often used for wind power, require reactive
power for excitation so substations used in wind-power collection systems
include substantial capacitor banks for power factor correction. Different types
of wind turbine generators behave differently during transmission grid
disturbances, so extensive modeling of the dynamic electromechanical
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characteristics of a new wind farm is required by transmission system operators
to ensure predictable stable behavior during system faults. In particular,
induction generators cannot support the system voltage during faults, unlike
steam or hydro turbine-driven synchronous generators. Doubly fed
machines generally have more desirable properties for grid interconnection.
Transmission systems operators will supply a wind farm developer with a grid
code to specify the requirements for interconnection to the transmission grid.
This will include power factor, constancy of frequency and dynamic behavior of
the wind farm turbines during a system fault.
Capacity Factor
The capacity factor of a generator is usually defined as the ratio of its yearly energy output to the output it would have produced if it operated continuously at its nameplate rating. Since
wind speed is not constant, a wind farm's annual energy production is never as
much as the sum of the generator nameplate ratings multiplied by the total hours
in a year. In other words, its capacity factor is low ranging between 20-40%.
Wind Turbine
Due to the many advantages of doubly fed Induction Machines, wind turbines
nowadays use DFIG instead of Induction Generators. DFIG consists of a wound
rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The
stator winding is connected directly to the 50 Hz grid while the rotor is fed at
variable frequency through the AC/DC/AC converter. Their advantages include:
• DFIG technology allows extracting maximum energy from the wind for
low wind speeds.
• Minimizes mechanical stresses on the turbine during gusts of wind.
• Ability for power electronic converters to generate or absorb reactive
power, thus eliminating the need for installing capacitor banks.
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• DFIG wind turbine can supply considerably greater kinetic energy than a
fixed speed wind turbine.
1.4 Hydro Energy
It is a clean energy harnessed from the kinetic energy of water. Hydropower is
produced in 150 countries, with the Asia-Pacific region generating 32 percent of
global hydropower in 2010. The capacity of plants constructed shows an
increasing trend. There are now three hydroelectricity plants larger than 10
GW, the Three Gorges Dam in China, Itaipu Dam in Brazil, and Guri Dam in
Venezuela. The different turbines used for generation are Pelton, Francis,
Kaplan, Crossflow, Tyson, Jonval etc. The deciding factor for selecting turbines
is the water head. Hydro electric energy is preferred due to its many advantages.
Flexibility:
Hydro is a flexible source of electricity since plants can be ramped up and down
very quickly to adapt to changing energy demands.
Low power costs:
The major advantage of hydroelectricity is elimination of the cost of fuel. The
cost of operating a hydroelectric plant is nearly immune to increases in the cost
of fossil fuels such as oil, natural gas or coal, and no imports are needed.
Suitability for industrial applications:
While many hydroelectric projects supply public electricity networks, some are
created to serve specific industrial enterprises. Dedicated hydroelectric projects
are often built to provide the substantial amounts of electricity needed
for aluminium electrolytic plants, for example.
Reduced CO2 emissions:
Since hydroelectric dams do not burn fossil fuels, they do not directly
produce carbon dioxide.
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1.5 Project Description
It is evident from the above discussion that growing energy needs can be met
only if renewable energy resources penetrate in the grid. This makes the study
of the same in grid systems important. Here, the grid system is first studied
under the impact of conventional resources alone. With changes in load,
frequency deviations occur. This is controlled using load frequency control
(LFC). The same grid is then studied with the penetration of hydro and wind
farms. Load frequency control is applied again to control frequency deviation.
The system has been modeled using Matlab R2010a. Transfer functions are
used to represent the different components of the power system and load has
been modeled separately. As load is dynamic in nature, a step change in load
will not be sufficient to model the real time system. Hence a random load with
suitable variance is applied to the units. Responses are recorded for
Conventional system i.e. only thermal units
Conventional system with penetration of hydro power plant
Conventional system with penetration of wind farms
System consisting of thermal, wind and hydro plant
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CHAPTER 2
ISSUES IN LOAD FREQUENCY CONTROL
2.1 Issues in Wind EnergyThe wind speed at a given location is continuously varying. There are changes in the annual mean wind speed from year to year ( annual ) changes with season ( seasonal ), with passing weather systems ( synoptic ), on a daily basis ( diurnal ) and from second to second ( turbulence ).All these changes, on their different timescales, can cause problems in predicting the overall energy capture from a site (annual and seasonal), and adversely affect the local electricity network to which the wind turbine is connected. The other obvious disadvantages are the higher costs of constructing and operating wind turbines offshore, and the longer power cables that must be used to connect the wind farm to the terrestrial power grid.
2.1.1 Local Impacts
Locally, wind power has an impact on the following aspects of the power
system:
• circuit power flows and busbar voltages
• protection schemes, fault currents, and switchgear rating
• power quality
– harmonic voltage distortion
– voltage flicker.
2.1.2 System-wide Impacts
In addition to the local impacts, wind power also has a number of system-wide
impacts as it affects the following :
• power system dynamics and stability
• reactive power and voltage support
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• frequency support.
2.1.3 Wind Generators Compared with Conventional Power Plant
There are significant differences between wind power and conventional
synchronous central generation:
• Wind turbines employ different, often converter-based, generating systems
compared with those used in conventional power plants.
• The prime mover of wind turbines, the wind, is not controllable and fluctuates
stochastically.
• The typical size of individual wind turbines is much smaller than that of a
conventional utility synchronous generator.
2.2 Wind Turbines
Wind turbines produce electricity by using the power of the wind to drive an electrical generator. Wind passes over the blades, generating lift and exerting a turning force. The rotating blades turn a shaft inside the nacelle, which goes into a gearbox. The gearbox increases the rotational speed to that which is appropriate for the generator, which uses magnetic fields to convert the rotational energy into electrical energy. The power in the wind than can be extracted by a wind turbine is proportional to the cube of the wind speed and is given in watts by P= (ρAν 3 Cp)/2 where ρ is the air density, A is the rotor swept
area, ν is the wind speed and C p is the power coefficient. A maximum value of
Cp is defined by the Betz limit, which states that a turbine can never extract
more than 59.3% of the power from an air stream. In reality, wind turbine rotors
have maximum Cp values in the range 25–45%.It is also conventional to define
a tip-speed ratio, λ, as λ = (ωR)/v where
ω = rotational speed of rotor
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R = radius to tip of rotor
ν = upwind free wind speed, ms-1.
The tip-speed ratio, λ, and the power coefficient, Cp, are dimensionless and so
can be used to describe the performance of any size of wind turbine rotor. The
maximum power coefficient is only achieved at a single tip-speed ratio and for a
fixed rotational speed of the wind turbine this only occurs at a single wind
speed. Hence, one argument for operating a wind turbine at variable rotational
speed is that it is possible to operate at maximum Cp over a range of wind
speeds. The power output of a wind turbine at various wind speeds is conventionally described by its power curve.The power curve has three key points on the velocity scale:
• Cut-in wind speed – the minimum wind speed at which the machine will
deliver useful power.
• Rated wind speed – the wind speed at which rated power (maximum power) is
obtained.
• Cut-out wind speed – the maximum wind speed at which the turbine is
allowed to deliver.
2.3 Wind Turbine Architecture
There are a large number of choices of architecture available to the designer of a wind turbine. However, commercial designs for electricity generation have now converged to horizontal axis, three-bladed, upwind turbines. The largest machines tend to operate at variable speed whereas smaller, simpler turbines are of fixed speed. Modern electricity-generating wind turbines now use three-bladed upwind rotors, although two-bladed, and even one-bladed, rotors were used in earlier commercial turbines. Reducing the number of blades means that the rotor has to operate at a higher rotational speed in order to extract the wind energy passing through the rotor disk. Although a high
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rotor speed is attractive in that it reduces the gearbox ratio required, a high blade tip speed leads to increased aerodynamic noise and increased blade drag losses. Most importantly, three-bladed rotors are visually more pleasing than other designs and so these are now always used on large electricity-generating turbines.
2.3.1 Fixed-speed Wind Turbines
Fixed-speed wind turbines are electrically fairly simple devices consisting of an
aerodynamic rotor driving a low-speed shaft, a gearbox, a high-speed shaft and
an induction (sometimes known as asynchronous) generator. It consists of a
squirrel-cage induction generator coupled to the power system through a turbine
transformer, refer figure 2.1. The generator operating slip changes slightly as
the operating power level changes and the rotational speed is therefore not
entirely constant. However, because the operating slip variation is generally less
than 1%, this type of wind generation is normally referred to as fixed speed.
Fig. 2.1 Schematic of a fixed-speed wind turbine
2.3.2 Variable-speed Wind Turbines
As the size of wind turbines has become larger, the technology has switched
from fixed speed to variable speed and the reduction in mechanical loads
achieved with variable-speed operation. Currently the most common variable-
speed wind turbine configurations are as follows:
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• Doubly fed induction generator (DFIG) wind turbine
• Fully rated converter (FRC) wind turbine based on a synchronous or induction
generator.
a) Doubly Fed Induction Generator (DFIG) Wind Turbine
It uses a wound-rotor induction generator with slip rings to take current into or
out of the rotor winding and variable-speed operation is obtained by injecting a
controllable voltage into the rotor at slip frequency. The rotor winding is fed
through a variable-frequency power converter, typically based on two AC/DC
IGBT-based voltage source converters (VSCs), and linked by a DC bus (refer
Fig. 2.2). The power converter decouples the network electrical frequency from
the rotor mechanical frequency, enabling variable-speed operation of the wind
turbine. The generator and converters are protected by voltage limits and an
Fig. 2.2 Typical configuration of a DFIG Turbine
over current ‘crowbar’. A DFIG system can deliver power to the grid through the stator and rotor, while the rotor can also absorb power. This depends on the rotational speed of the generator. If the generator operates above synchronous speed, power will be delivered from the rotor through the converters to the
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network, and if the generator operates below synchronous speed, then the rotor will absorb power from the network through the converters. Normally Back-to-Back VSCs are used in DFIG.
b) Fully Rated Converter (FRC) Wind Turbine
This type of turbine may or may not include a gearbox and a wide range of
electrical generator types can be employed, for example, induction, wound-rotor
synchronous or permanent magnet synchronous. As all of the power from the
turbine goes through the power converters, the dynamic operation of the
electrical generator is effectively isolated from the power grid. The electrical
frequency of the generator may vary as the wind speed changes, while the grid
frequency remains unchanged, thus allowing variable-speed operation of the
wind turbine. The power converter is able to generate or absorb reactive power
independently.
Fig. 2.3 Typical configuration of a fully rated converter connected wind turbine
Variable speed operation requires a frequency conversion through a power
electronic converter, a process that reduces somewhat the overall efficiency.
This is one reason why large modern wind turbines are designed to operate at
variable speed .This is attractive from an integration perspective as the rotor has
inertia available to absorb or release energy when accelerating or decelerating
respectively, thus smoothing short term variations in wind speed. Consequently
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its electrical power output varies less and can be more easily accommodated by
the electrical system.
Synchronous Generator Use in renewable energy:
Used in wind power mainly in its ‘ring’ forms for gearless coupling to a wind turbine.
Variable speed provided through a DC link power electronic interface.
Used in water power when reactive power control is required.
Induction Generator use in renewable energy: Used extensively with a gear box in wind Power. Variable speed provided with power electronics in the
rotor wound form. Used in water power with gearbox.
2.3.3 Fixed Versus Variable Speed – Dynamics
The vast majority of the world’s electricity is generated by synchronous
machines directly connected to their respective power systems. This
configuration works very well when the prime mover (usually an engine or a
steam, gas or water turbine) provides a steady torque to the generator. Wind,
however, is turbulent and this translates directly into fluctuations in Drive train
torque. Using a directly connected synchronous generator in a wind turbine
would form too rigid a coupling between the mechanical and electrical systems.
Wind gusts would cause large mechanical stresses in the turbine and, depending
on the nature of the electrical grid, large fluctuations in the power fed into the
electrical system.
The main advantages of variable speed wind turbines in terms of dynamics are:
• The total inertia of the aerodynamic rotor, the gearbox (if there is one) and the
electrical generator act as an energy buffer, smoothing out the wind turbulence.
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Transient torques and rapid variation in electrical power as well as stresses in
the drive train are greatly reduced.
• Lower structural loads and lighter foundations are other advantages of
particular importance in offshore applications.
Additional advantages are:
• The power electronics may also be capable of regulating the reactive power flow in the network.• Noise is reduced, when operating at low wind speeds.The drawbacks of variable speed are the extra complexity of the generator (in some schemes) and of the necessary power electronic hardware, all of which increase cost and possibly reduce reliability. To allow for wind gusts, virtually all wind turbines have generators or generator systems that provide some degree of speed variation.
2.4 Hydro Power Generation2.4.1 Issues in Hydro Power Generation
Ecosystem damage and loss of land:Large reservoirs required for the operation of hydro electric power station result in submersion of extensive areas upstream of the dams, destroying biologically rich and productive low land and riverine valley forests, marchland and grasslands. The loss of land is often exacerbated by the fact that reservoirs cause habitat fragmentation of surrounding area.
Siltation and flow shortage:
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When water flows, it has the ability to transport particles heavier than itself downstream. This has a negative effect on dams and subsequently their power stations, particularly those on rivers or within catchment areas with high siltation. Siltation can fill a reservoir and reduce its capacity to control floods along with causing additional horizontal pressure on the upstream portion of the dam.
Relocation:Another disadvantage of hydroelectric dams is the need to relocate the
people living where the reservoirs are planned.
Failure risks:Because large conventional dammed-hydro facilities hold back large
volumes of water, a failure due to poor construction, terrorism, or other
cause can be catastrophic to downriver settlements and infrastructure.
2.4.2. Comparison with other methods of power generation
Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion
and also avoids the hazards of coal mining along with its indirect health effects.
Compared to wind farms, hydroelectricity power plants have a more predictable
load factor. If the project has a storage reservoir, it can generate power when
needed. Hydroelectric plants can be easily regulated to follow variations in
power demand.
CHAPTER 3
MODELING OF THERMAL, HYDRO AND WIND SYSTEM
3.1 Frequency deviation with penetration of renewables
Load changes result in frequency deviation. With the anticipated rise in the
penetration of variable renewables, power systems will be required to
accommodate increasing second to second imbalances between generation and
demand requiring enhanced frequency control balancing services. It is not
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desirable to operate machines in frequency other than nominal value as it leads
to various problems.Also energy from renewable sources should generally be
used as fully as possible whenever available.Hence the method of LFC has to be
implemented to maintain frequency within tolerable limits. Apart from
uncontrollability owing to intermittent nature, other challenges include high
installation costs, high clearance of land (for wind).
Conventional steam generation plant assists the network frequency stability at the onset of a sudden imbalance of demand over supply by slowing down. Wind turbines respond differently. The stored energy is in the rotor inertia and fixed speed turbines will provide a limited benefit from their inertia provided that the voltage and frequency remain within their operating limits. Variable - speed wind turbines will not normally provide this benefit as their speed is controlled to maximize the energy production from the prevailing wind. Large wind turbines are now almost always of the variable speed type and as they increasingly displace conventional generation the total system inertia from such generation will decrease. Consequently the rate of change of frequency and the depth of the frequency dip caused by a sudden loss of generation will both increase. However, variable speed wind turbines could be controlled in principle to provide a proportionately greater inertial energy to the system than conventional plant of the same rating. Such sophisticated control arrangements to support system functions are likely to be requested by utilities as wind penetration increases.3.2 Load Frequency Control
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LFC, as the name signifies, regulates the power flow between different areas
while holding the frequency constant. We can therefore state that the LFC has
the following two objectives:
- Hold the frequency constant (Δf = 0) against any load change
- Each area must maintain the tie-line power flow to its pre-specified value.
3.2.1 Load Frequency Control Methods
The different methods for LFC are Flat Frequency Control (FFC), Tie Line Bias
Control (TBC), and Flat Tie Line Control (FTC). In FFC, some areas act as load
change absorbers and others as base load. The advantage is the higher operating
efficiencies of the base load as they run at their maximum rated value at all
times. But the drawback here is the reduced number of areas absorbing load
changes which makes the system more transient prone. In FTC, load changes in
each area are controlled within the area thereby maintaining tie line frequency
constant. In TBC, the load change in one area is shared by other areas through
signals from central control system.TBC has been adopted here with simulation
being batch controlled for improved performance. The wind system discussed
uses Doubly Fed Induction Generator (DFIG) due to its many advantages like
independent real and reactive power control; its generation capacity is 150MW.
The hydro system with droop compensation is considered which has a
generation capacity of 2400MW. The four thermal units have a net generation
capacity of 6100MW.
3.3 Thermal System
The four thermal power plants constitute the thermal unit. The plants have been
modeled using transfer function. Speed governor, turbine and generator
constitute the various parts.
3.3.1 Speed Governor System
From literature, we have,
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∆ Y E (s )=[∆ PC (s )− 1R
∆ F ( s )]×(K sg
1+T sg s) (3.1)
Where∆ PC=command signal
∆ F=¿frequency change
∆ Y E=changes in steam valve opening
R=speed regulation of the governor
Ksg=gain of speed governor
Tsg=time constant of speed governor
Equation 3.1 is represented in the form of a block diagram in Fig. 3.1
∆ PC(s) ∆ Y E(s)
∆ F (s)
Fig. 3.1 Governor transfer function model
This model has been adopted in the simulation.
3.3.2 Turbine Model
The dynamic response of steam turbine is related to changes in steam valve
opening ∆ Y E in terms of changes in power output. The dynamic response is
largely influenced by two factors, (i) entrained steam between the inlet steam
valve and first stage of the turbine, (ii) the storage action in the reheater which
causes the output of the low pressure stage to lag behind that off high pressure
stage. Thus, the turbine transfer function is characterized by two time constants.
It is assumed that the turbine can be modeled to have a single equivalent time
constant. Figure 3.2 shows the transfer function model of a steam turbine.
Typically the time constant Tt lies in the range 0.2 to 2.5 sec.
∆ Y E(s) ∆Pt(s)
Fig. 3.2 Turbine transfer function model
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K sg
1+T sgs1R
KT
1+T t s
3.3.3 Generator Load Model
The increment in power input to the generator-load system is
∆PG - ∆PD
Where ∆PG = ∆Pt, incremental turbine output power.This increament in power
input to the system is accounted for in two ways:
(i) Rate of increase of stored kinetic energy in the generator rotor.
(ii) Load changes sensitive to frequency.
Based on these, the following equation is obtained.
∆ F (s )=∆ PG (s )−∆ PD ( s )
B+ 2 Hf °
s = [∆ PG ( s)−∆ PD (s )¿×(
K ps
1+T ps s) (3.2)
Where Tps= 2 HBf °
=¿ power system time constant
Kps = 1B = power system gain
∆ PD (s )
∆ PG ( s) + - ∆ F (s )
Fig. 3.3 Block diagram of generator load model
From the above analysis, the complete block diagram for the thermal system
can be constructed by combining the block diagram of individual components is
shown in Figure 3.4. Typical values of time constants of load frequency control
system are related as Tsg<Tt<<Tps.
Y E(s) ∆ Pt (s)=∆ Pt (s) ∆ PD (s )
∆ PC(s) + + -
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K ps
1+T ps s
K ps
1+T ps sKT
1+T t sK sg
1+T sgs
- ∆ F (s)
Fig. 3.4 Complete block diagram
The system parameters for the four thermal areas is tabulated in Table 3.1.
Table 3.1Thermal System parameters
Area Rated Power (MW)
B = ∂ PD
∂ f
(pu MW/Hz)
Inertia Constant (H)
Kps= 1B Tps= 2H
Bf °
TP 1 2000 .01 6 100 24
TP 2 1000 .02 5 50 10
TP 3 600 .033 4 30 5.33
TP 4 2500 .008 6 125 30
3.4 Hydro System
Hydro plants contribute nearly 21% of generation of electric power in India.
The essential elements of the hydraulic plant are reservoir, penstock, gate,
turbine and generator. Hydraulic turbines are of two basic types: impulse and
reaction turbines. The impulse-type (Pelton wheel) is used for high heads 300
metres or more. The gravitational power energy of the water dammed becomes
its kinetic energy. The water spurt transmits its kinetic energy to the bun, where
it is transformed instantaneously into mechanical energy. In reaction turbine the
pressure within turbine is above atmospheric; the energy is supplied by the
wáter in both kinetic and potential (pressure head) forms. The water first passes
from a spiral casing through stationary radial guide vanes and around its entire
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1R
periphery. The gates control wáter flow. There are two sub categories of
reaction turbines: Francis and Propeller.
Francis turbine
In this type of turbine, wáter flows through guide vanes impacting on the
runner tangentially and exiting axially.
Propeller turbine
It uses propeller type wheels. Either fixed blades or variable pitch blades
may be used. The variable pitch blade propeller turbine, commonly
known as Kaplan Wheel, has high efficiency at all loads.
Typical range of heads
Kaplan 2 < H < 40 (H = head in meters)
Francis 10 < H < 350
Pelton 50 < H < 1300
The representation of the hydraulic turbine and water column in stability
studies is usually based on following assumptions:
1) The hydraulic resistance is negligible.
2) The penstock pipe is inelastic and the water is incompressible.
3) The velocity of the water varies directly with the gate opening and with
the square root of the net head.
4) The turbine output power is proportional to the product of head and
volume flow.
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Fig. 3.5 Block diagram of Hydraulic Unit
Hydro plants are modeled the same way as thermal except for the following
differences:
• The input to the turbine is water instead of steam.
• Inertia of the system is higher than thermal due to high inertia of water
column.
• Initial Droop Characteristics owing to reduced pressure on turbine on
opening the gate valve has to be compensated.
Requirement for transient droop:
Hydro turbines have peculiar response due to water inertia; a change in gate
position produces an initial turbine power change which is opposite to that
sought. For stable control performance, a large transient (temporary) droop with
a long resettling time is therefore required in the forms of transient droop
compensation as shown in Fig. 3.6. The compensation limits gate movement
until water flow power output has time to catch up. The result is governor
exhibits a high droop for fast speed deviations and low droop in steady state.
Typical Values for Hydro Modeling [3] are given below (refer Fig. 3.7)
Rp=0.05 Tg=0.2s M=6.0s D=1.0
Tw=1.0s Rt=0.38 Tr=5.0s Capacity = 2400MW
Where Rp = permanent droop
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Tg = main servo time constant
D = change in load with respect to frequency
M=2H where equals inertia constant
Tw = water starting time (time required for a head H to accelerate water in
penstock from standstill to velocity U)
Typically Tw at full load lies between 0.4s-0.5s
R t = temporary droop
Tr = reset time.
Fig. 3.6 Block Diagram of Hydro Plant
3.5 Wind System
A wind farm consisting of Doubly-fed induction generator (DFIG) wind turbine
is considered. DFIG consists of a wound rotor induction generator and an
AC/DC/AC IGBT-based PWM converter. The stator winding is connected
directly to the 50 Hz grid while the rotor is fed at variable frequency through the
Fig. 3.7 Typical Configuration of DFIG Turbine
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AC/DC/AC converter. The wind speed is maintained constant at 11 m/s. The
control system uses a torque controller in order to maintain the speed at 1.2 pu.
Parameters of wind farm
Power capacity of wind farm: 150MW
Nominal wind speed: 11m/s
Turbine initial speed: 1.2pu
Inertial constant of wind turbine: 4.32 s
Generator power: 1.5MW
Stator Voltage: 575V
Frequency: 50Hz
Advantages:
Ability for power electronic converters to generate or absorb reactive
power, thus eliminating the need for installing capacitor banks.
The DFIG technology allows extracting maximum energy from the wind
for low wind speeds by optimizing the turbine speed, while minimizing
mechanical stresses on the turbine during gusts of wind.
In variable speed DFIG wind turbines, which operate at defined torque, the
damping contribution of the generator is low because the torque no longer
varies rapidly as a function of the rotor speed. Also, active damping techniques
are often used to stabilize the mechanical systems of large variable-speed wind
turbines. Recently, power system stabilizers (PSSs) have been proposed to
enable DFIG wind turbines to contribute positively to network damping. If any
of the frequencies of mechanical vibrations of the rotor structure lies within the
bandwidth of the PSS, then resonance or adverse control loop interaction may
arise, which will affect the performance of both the mechanical & electrical
systems of the wind turbine.
CHAPTER 4
24
LOAD FREQUENCY FOR MULTIAREA SYSTEM
4.1 Multiarea System for Load Frequency Control
An extended power system can be divided into a number of load frequency
control areas interconnected by means of tie lines. The control objective now is
to regulate the frequency of each area and to simultaneously regulate the tie line
power as per inter-area power contracts. As in the case of frequency,
proportional plus integral controller will be installed so as to give zero steady
state error in the tie line power flow as compared to the contracted power. It is
conveniently assumed that each control area can be represented by an
equivalent turbine, generator and governor system. Symbols used with suffix 1
refer to area 1 & those with suffix 2 refer to area 2 and so on. Incremental tie
line power out of area 1is given by
∆ Ptie , 1=2 π T 12(∫∆ f 1 dt−∫∆ f 2 dt) (4.1)
Similarly, the incremental tie line power out of area 2 is given by
∆ Ptie , 2=2π T 21(∫∆ f 2 dt−∫∆ f 1 dt) (4.2)
Where T12 = synchronizing coefficient
f1 & f2 represent frequency of the respective area.
∆ Ptie , 2 ( s)=−2 π a12T 12
s[∆ F1 ( s )−∆ F2 (s )] (4.3)
Equation 4.3 can be represented by Fig. 4.1
∆ Ptie , 1(s) ∆ Ptie , 2(s)
∆ F1 ( s )+ - ∆ F2(s )
Fig. 4.1 Tie line power flow
25
-a122π T12
s
In the case of an isolated control area, ACE (area control error) is the change in
area frequency which when used in integral control loop forced the steady state
frequency error to zero. In order that the steady state tie line power error in a
two area control be made zero another integral control loop (one for each area)
must be introduced to integrate the incremental tie line power signal and feed it
back to the speed changer, as shown in Fig. 4.2
Y E(s) ∆ Pt(s)=∆ Pt (s) ∆ PD (s )
+ + -
- ∆ F (s) ∆ PC(s)
Frequency sensor
Fig. 4.2 Proportional plus Integral load frequency control
This is accomplished by a single integrating block by redefining ACE as a
linear combination of incremental frequency and tie line power. Thus for
control area 1
ACE1 = ∆ Ptie , 1+b1 ∆ f 1 (4.4)
Taking laplace transform,
ACE1(s) = ∆ Ptie , 1(s)+b1 ∆ F1(s ) (4.5)
Similarly, for control area n,
ACEn(s) = ∆ Ptie , n(s )+bn ∆ Fn(s) (4.6)
Combining the basic block diagrams of multiple control areas, with ∆ PC 1 (s ) to
∆ PCn ( s) generated by integrals of respective ACEs (obtained through signals
representing changes in tie line power and local frequency bias) and employing
the block diagram of Fig. 4.1, we easily obtain the composite block diagrams.
26
K ps
1+T ps s
1R
KT
1+T t sK sg
1+T sgs
K
−k i
s
4.2 LFC for Thermal System
The four thermal systems have been combined and the composite block diagram
is simulated in Simulink/Matlab R2010a as shown in Fig. 4.3.
Fig. 4.3 Thermal System
Let the loads ∆ PD1to ∆ PD4 be simultaneously applied in control areas 1 to 4
respectively. The system parameters of 4 area system are given in table 4.1The
frequency deviation versus time scale of 4 thermal areas for step load change
and random load variation is shown in Fig. 4.3.1 and Fig. 4.3.2
27
0 200 400 600 800 1000-4
-3
-2
-1
0
1
2
3
4x 10
-3
Time (s)
Cha
nge
in F
requ
ency
Frequency Deviation Vs Time (s) for Thermal System
Fig. 4.3.1 Response for Fixed load
0 200 400 600 800 1,000-1
-0.5
0
0.5
1
Time(s)
Cha
nge
in fr
eque
ncy
Frequency Deviation Vs Time(s) for Thermal System
Fig. 4.3.2 Response for Random load
4.3 LFC for Thermal and Hydro System
The four thermal systems along with the hydro unit are combined and the
composite block diagram is simulated in Simulink/Matlab R2010a as shown in
Fig. 4.4.
28
Fig. 4.4 Hydro and Thermal System
Frequency deviation versus Time for Integrated Thermal and Hydro system for
step load change and random load variation is shown in Fig. 4.4.1 and Fig. 4.4.2
29
0 200 400 600 800 1000-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
Time (s)
Chan
ge in
Fre
quen
cy
Frequency Deviation Vs Time(s) for Hydro & Thermal System
Fig. 4.4.1 Response for Fixed load
0 200 400 600 800 1000-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
Time (s)
Cha
nge
in F
requ
ency
Frequency Deviation Vs Time(s) for Hydro & Thermal System
Fig. 4.4.2 Response for Random load
4.4 LFC for Thermal and Wind system
The non conventional wind energy generator should act in conjunction with
conventional power generator for reliable power generation as wind generation
is intermittent in nature. The Fig. 4.5 shows the integration of thermal with wind
turbine.
30
Fig. 4.5 Thermal and Wind System
Frequency deviation versus Time for Thermal and Wind system for step load
change and random load variation is shown in Fig. 4.5.1 and Fig. 4.5.2
0 100 200 300 400 500-1
-0.5
0
0.5
1x 10
-4
Time(s)
Chan
ge in
Fre
quen
cy
Frequency Deviation vs Time(s) for Wind & ThermalSystem
Fig. 4.5.1 Response for Fixed Load
31
0 100 200 300 400 500-1
-0.5
0
0.5
1x 10
-4
Time(s)
Chan
ge in
Fre
quen
cy
Frequency Deviation Vs Time(s) for Wind & ThermalSystem
Fig. 4.5.2 Response for Random Load
4.5 LFC for Thermal, Hydro and Wind System
In order to compensate the intermittent nature of renewable, grid connection of
the same is imperative for reliable power generation.
Fig. 4.6 Structure of Power System
It is possible to divide an extended power system into sub areas in which the
generators are tightly coupled together so as to form a coherent group, i.e. all
the generators respond in unison to changes in load or speed changer settings.
32
Central Control SystemTP 1TP 2TP 3TP 4HydroWind
Such a coherent area is called a control area in which the frequency is assumed
to be the same throughout in static and dynamic conditions. For the purpose of
developing a suitable control strategy, a control area can be reduced to a single
speed governor, turbo-generator and load system. Here a power system
consisting of four thermal areas (TP1-TP4), a hydro area and a wind farm is
considered and the same is controlled by a central control system (refer Fig.
4.6). By batch control, the load is divided amongst various power plants in the
ratio of their capacities by central control system and the corresponding Matlab
code is given in Fig. 4.7.
Fig. 4.7 Matlab code for control system
33
This entire power system is modeled using Simulink/Matlab R2010a as shown
in Fig. 4.8.
Fig. 4.8 Simulink model of Power System
The four thermal areas and the hydro unit are combined together in the
‘Thermal & Hydro’ subsystem (Fig. 4.9). The output ∆ F of this subsystem gets
reflected in the grid voltage. DFIG wind farm draws supply for stator from grid
and the changing wind speeds has an impact on its output. Power from the
DFIG is fed to the grid via stator and rotor depending on the wind speed. Higher
the wind speed, higher is the power output, rotor feeds power; lower the wind
speed, power output is low, hence rotor draws power from grid to have constant
power flow through stator. The output of wind farm is sent to the central control
system to calculate the load distribution over thermal station. Random load of
1.7pu with maximum variation of 0.8pu (corresponds to 4250MW ± 2000MW)
is considered here.
34
1/R4
1/R2
delPtie
1/R5
Governor
b1
HYDRO SYSTEM
THERMAL SYSTEM
1
Out1
Continuous
powergui
-5.5
s
control ler 5
-.09
s
control ler 4
-K-
b5
-K- b4
-K- b3
-K-
-K- b1
-K- a15
-.8 a14
-.5 a13
.24 a12
XY Graph
1
.5s+1
Turbine4
1
.6s+1
Turbine3
1
.8s+1
Turbine2
1
.4s+1
Turbine1
1
.2s+1
Transient Droop
simout
To Workspace
1 Pref
-s+1
.5s+1
HydroTurbine
1
.4s+1
Governor4
1
.5s+1
Governor3
1
.7s+1
Governor2
1
.3s+1
Governor1
100
24s+1
Generator4
50
10s+1
Generator3
30
5.33s+1
Generator2
125
30s+1
Generator1
1
6s+1
Generator
.33
.33
.33
Delay5
Delay3
Delay2
Delay1
Dealy4
-.09
s
Controller 3
-.09
s
Controller 2
-.09
s
Controller 1
5s+1
38s+1
Compensation
Clock1
.05
s
2HT12/s
.33 1/R1
201/R
5
In5
4
In4
3
In3
2
In2
1
In1
Fig. 4.9 Thermal and Hydro Subsystem
Frequency deviation versus Time for Integrated Thermal, Hydro and Wind
system for step load change and random load variation is shown in Fig. 4.9.1
and Fig. 4.9.2
35
0 20 40 60 80 100-0.1
-0.05
0
0.05
0.1
Time (s)
Chan
ge in
Fre
quen
cy V
s Tim
e (s
)
Frequency Deviation vs Time(s) for the Integrated System
Fig. 4.9.1 Response for Fixed Load
Real time systems are best described by introducing random load variation. The
response for random load variation is shown in Fig. 4.17.
0 20 40 60 80 100-1
-0.5
0
0.5
1x 10
-3
Time(s)
Chan
ge in
Fre
quen
cy
Frequency Deviation Vs Time(s) for the Integrated System
Fig. 4.9.2 Response for Random Load
36
CHAPTER 5
ECONOMIC LOAD DISPATCH
5.1 Introducing Economic Dispatch
In power generation our main aim is to generate the required amount of
power with minimum cost.
Economic load dispatch means that the generator’s real and reactive
power is allowed to vary within certain limits so as to meet a particular
load demand with minimum fuel cost.
This allocation of loads is based on some constraints.
5.2 Different Constraints in Economic Load Dispatch
There are two different types of constraints namely Inequality and Equality
constraints.
5.2.1 Inequality Constraints
Voltage constraints
Vmin ≤ V ≤ Vmax
δmin ≤ δ ≤ δmax
where V = Voltage , δ = Load angle.
Generator constraints
KVA loading of generator should not exceed prescribed value
a) Pmin ≤ P ≤ Pmax where Pmin is governed by thermal condition and Pmax
is limited by flame instability in boiler.
b) Qmin ≤ Q ≤ Qmax where Qmax is limited by overheating of the rotar and
Qmin is limited by stability limit of the machine
Running spare capacity constraints
37
This constraint is needed to meet forced outage of one or more alternators in
the system and also unexpected load on the system
Transmission line constraints
The flow of power through transmission line should less than its thermal
capacity
Transformer tap set
For autotransformer tap‘t’ should be between 0 & 1
For two winding transformer tappings ‘t’ should be between 0 & n, where n =
transformation ratio.
5.2.2 Equality constraints
Real power
Pp= Vp Σ Ypq Vq cos(θpq-(δp+δq))
Reactive power
Qp= Vp Σ Ypq Vq sin(θpq-(δp+δq))
5.3 Operating cost of thermal plant
The factors influencing power generation at minimum cost are operating
efficiencies of generators, fuel cost, and transmission losses.
The most efficient generator in the system does not guarantee minimum
cost as it may be located in an area where fuel cost is high.
If the plant is located far from the load center, transmission losses may be
considerably higher and hence the plant may be overly uneconomical.
The input to the thermal plant is generally measured in Btu/h, and the
output is measured in MW.
In all practical cases, the fuel cost of generator can be represented as a
quadratic function of real power generation
38
C = aPg2+bPg+c
In all practical cases, the fuel cost of generator can be represented as a quadratic
function of real power generation
Fig. 5.1 Heat rate curve Fig. 5.2 Fuel cost curve
Incremental Fuel Cost Curve
By plotting the derivative of the fuel-cost curve versus the real power we get the
incremental fuel-cost curve
dC/dPgi = 2aP + b
The incremental fuel-cost curve is a measure of how costly it will be to produce
the next increment of power.
5.4 Economic Dispatch Neglecting Losses
It is the simplest economic dispatch problem
Assume that the system is only one bus with all generation and loads
connected to it
A cost function Ci is assumed to be known for each plant
The problem is to find the real power generation for each plant such that the
objective function (i.e., total production cost) as defined by the equation
C = aPg2+bPg+c
Is minimum subjected to constraint, Pg = Pd .
The power system model simulated is shown in Fig. 5.3
39
Fig. 5.3 Economic Dispatch Model
The response obtained is shown in Fig. 5.4 and Fig. 5.5
40
0 20 40 60 80 100-4
-3
-2
-1
0
1
2
3
4x 10
-4
Time (s)
Freq
uenc
y D
evia
tion
Frequency Deviation Vs Time(s)
Fig. 5.4 Frequency Deviation with Economic Dispatch
0 20 40 60 80 1000
0.2
0.4
0.6
0.8
1
Time (s)
Load
Dis
tribu
tion
(p.u
.)
Load Distribution (p.u.) Vs Time (s)
TP1TP2TP3TP4
Fig. 5.5 Load distribution to thermal power plants
41
0 20 40 60 80 1000
100
200
300
400
500
600
700
800
900
1000
Time (s)
Cos
t
Cost Vs Time (s)
Fig. 5.6 Cost Vs Time graph
42
CHAPTER 6
CONCLUSION AND FUTURE SCOPE
Load frequency control becomes more important, when a large amount of renewable power supplies like the photovoltaics and wind power generation are introduced. In this paper Load Frequency Control with considerable penetration of renewable has been analysed. It is observed that frequency deviation is within tolerable limits for the cases studied namely
Thermal + Hydro Thermal + Wind Thermal + Hydro + Wind
The frequency deviation versus time curve has been observed for two types of loads
Fixed Load Random Load
To represent real time scenario in the best possible way, random loads are used. The loads are distributed among different units using Tie Line Bias Control method of LFC as it gives minimal frequency deviation.Economic dispatch has been implemented in order to save fuel costs.The project can be further extended by studying real and reactive power flow with penetration of renewable. Also the system performance in the event of fault can be analysed.
43
REFERENCES
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2. Daniel Karlsson “Temporary Primary Frequency Control Support by
Variable Speed Wind Turbines—Potential and Applications” IEEE
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3. P. Kundur, Power System Stability and Control. New York: McGraw-
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44
9. Y. Eito, T. Matubara, M. Yamamoto, A. Mizutani, K. Shimizu, M. Kato,
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45
