The
working principle of a wind turbine encompasses two conversion processes, which
are carried out by its components, the rotor that extracts kinetic energy from
the wind and converts it into a generator torque and the generator that
converts this torque into electric power and feeds it into the grid.
Origin of wind
The earth is formed of highly varied surfaces and
when solar radiations reach the earth, it creates temperature, density and
pressure differences. This causes the development of the wind.
NON-CONVENTIONAL SOURCES
While fossils fuels will be the main fuels for
the thermal power there is a fear that they will get exhausted eventually in
next century therefore many countries are trying systems based on
non-conventional and renewable sources. These are Solar, Wind, Sea, Geothermal
and Biomass. Because if we take solar power on earth it is 10 6watts.The
total world demand is 10 13 watts, If we utilize 5% of the solar
energy, it will be 50 times what that world require. If we consider the wind
potential it is estimated to 1.6*10 7M.W, which is same as world
energy consumption. So the development of non-conventional energy source is
very; economical. While fossils fuels will be the main fuels for the thermal
power there is a fear that they will get exhausted eventually in next century
therefore many countries are trying systems based.
ADVANTAGES OF WIND ENERGY OVER OTHER NON-CONVENTIONAL SOURCES
·
It is available through out the day unlike solar
energy.
·
After solar energy it is the second largest
source of non-conventional source of energy
·
In India during the mid summer due lack of hydel
power generation which is one of the main source of energy there is desperate
need for energy. This can be meet to some extent by wind energy as there are
very high winds during this period
·
By using photo voltaics, the power generated is
dc .So it must be converted to ac to feed it to grid. But by using wind energy
we can directly produce ac.
·
In coastal areas, the cost of power generation
from wind has become lower than diesel power and compared to thermal power.
From the study of
wind distribution, it is estimated that about 27% of the land surface is
exposed to an annual wind speed higher than 18.36kmph at 10m above the surface.
GENERATING SYSTEM
A
wind turbine is a complex system in which knowledge from the areas of the
aerodynamics and mechanical, electrical and control engineering is applied
For
the generating system, nearly all wind turbines currently installed use either
one of the following systems.
1. Squirrel cage
induction generator
2. Doubly fed
induction generator
3. Direct drive
synchronous generator
In which first one is a fixed speed or constant
speed one while others are variable speed turbine
1. SQUIRREL CAGE
INDUCTION GENERATOR
- It is the
oldest one.
- It consists of
a conventional, directly grid coupled squirrel cage induction generator.
- The slip and the
rotor speed varies with the amount of power generated
- Its draw back
is it always consumes reactive power, which is undesirable in most of the
cases, particularly in the case of large turbines and weak grid.
- It can be always be partly or fully compensated by capacitors in order to achieve a power factor close to one.
2.
DOUBLY FED INDUCTION GENERATOR
- It is a
variable speed turbine
- In this case a
back-to-back voltage source converter feeds the three-phase rotor winding.
So the mechanical and electrical rotor frequencies are decoupled and the
electrical stator and rotor frequency can match, independently of the
mechanical rotor speed.
3.
DIRECT DRIVE SYNCHRONOUS GENERATOR
1. LOCAL IMPACTS.
Local impacts of wind power are impacts that
occur in the (electrical) vicinity of a wind turbine or wind farm and can be
attributed to a specific turbine or farm. Local impacts occur at each turbine
are largely independent of the over all wind power penetration level in the
system as a whole.
Local impacts are
- Branch flows and node voltages.
- Protection schemes, fault currents
and switch gear ratings.
- Harmonic distortion.
- Flicker
BRANCH FLOWS AND NODE VOLTAGES.
The
way in which wind turbines locally affect the node voltages depends on speed of
the turbine used .the squirrel cage induction generator in constant speed
cannot affect node voltages by adopting the reactive power exchange with the
grid. For this additional equipment for generating controllable amounts of
reactive power would be necessary. On the other hand variable speed turbines
have, at least theoretically, the capability of varying reactive power to
affect their terminal voltage, but this depends on the rating of the controllers
of the power electronic converter.
PROTECTION
SCHEMES, FAULTS CURRENTS AND SWITCH GEAR
RATINGS
Protection
schemes and switchgear ratings must be checked when connecting new generation
capacity. These are independent of the prime mover of the generator. The
contribution of wind turbines to the fault currents also differs between the
three main wind turbine types. Constant speed turbines are based on a directly
grid coupled squirrel cage induction generator. They therefore contribute to
the fault current and relay on conventional protection schemes. Turbines based
on the doubly fed induction generator also contribute to the fault current.
However, the control system of power
electronic converter that controls the rotor current measures fault currents
very quickly. Due to the sensitivity of power electronics to over currents,
this wind turbine type is currently quickly disconnected when a fault is
detected. Wind turbines with a direct drive generator hardly contribute to the
fault current because the power electronic converter through which the
generator is connected to the grid is not capable of supplying a fault current.
HARMONIC
DISTORTION
It
is mainly an issue in the case of variable speed turbines because this contains
power electronic devices, which are sources of harmonics. Harmonics cause over
heating of transformer and generators. This also cause increase in currents
through shunt capacitors. Thus leading to failure of such capacitors.
A
practical solution would be to provide shunt filters at the PCC of non-linear
loads and reduce the harmonic currents flowing all over the network. This would
result in lower voltage distortion. In the case of modern power electronic
converters with their high switching frequencies and advanced algorithms and
filtering techniques, harmonic distortion should not be a principal problem.
Well-designed, directly coupled synchronous and asynchronous generators hardly
emit harmonics
FLICKER.
Flicker
is a specific property of wind turbines. Wind is a quite rapidly fluctuating
prime mover. In constant speed turbines, prime mover fluctuations are directly
translated into output power fluctuation, because there is no buffer between
mechanical input and electrical output. Depending on the strength of the grid connection,
the resulting power fluctuations can result in grid voltage fluctuations, which
can cause unwanted and annoying fluctuations in bulb brightness. This problem
is referred to as flicker.
In
general, no flicker problem occur with variable speed turbines, because in
these turbines wind speed fluctuations are not directly translated into output
power fluctuations. The rotor inertia acts as an energy buffer.
SYSTEM IMPACTS
System-impacts
are the impacts that affect the behavior of the system as whole. They are an
inherent consequence from the application of wind power but cannot be
attributed to individual turbines or farms. They are strongly related to the
wind power penetration level in the system, that is the contribution of wind
power to actual load.
1. Power
system dynamics and stability
2. Reactive power and voltage control.
3. Frequency control and load dispatching of
conventional units.
1.
POWER SYSTEM DYNAMICS AND STABILITY.
In
order to investigate the impact of wind power on power system dynamics and
stability, adequate wind turbine models are essential. Squirrel cage induction
generator used with constant speed turbine can lead to voltage and rotor speed
instability. During a fault, they accelerate due to the unbalance between
mechanical power extracted from the wind and electrical power supplied to grid.
When the voltage restores, they consume much reactive power, impeding voltage
restoration. When the voltage returns to normal value quickly, the wind
turbines continue to accelerate and to consume large amounts of reactive power.
This eventually leads to voltage and rotor speed instability. Withvariable
speed turbines, the sensitivity of the power electronics to over currents
caused by voltage drops can have serious consequences for the stability of
power systems.
During high-speed winds, the turbine speed
exceeds its limit. This will cause
1. very high
fluctuations in voltage.
2. very high
fluctuations in frequencies.
3. It may damage
the rotor.
These problems to some extent can be solved by
1. By using some
governing mechanism to operate gear mechanism to control the speed of the rotor
of the wind turbine.
2. By using
computer techniques we may control the speed of the turbine or disconnecting
the turbine from generator during high-speed winds.
3. By connecting parachutes to the rotor for
blades
