Abstract :
Even though
the production of wind power is problematic, it is not a factor to consider due
to large energy crisis. The wind potential in India is about 20,000 MW.But we
presently achieved is just a fraction of total potential. If we utilize the
potential up to some more extent the energy crisis will be reduced. Still
research is going on to design efficient wind turbines. This paper deals with
wind power generation and the problems that arise in generation. As energy
crisis is very high in case of developing countries like India, there came
urgent need to look for other sources of energy that are clean and pollution
free as conventional sources cause much pollution.
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.
GENERATION
OF POWER FROM WIND
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.
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
3. DIRECT
DRIVE SYNCHRONOUS GENERATOR
In this
case generator is completely decoupled from the grid by a power electronics
converter connected to the stator winding.
The direct
drive generator is excited using an excitation winding or permanent magnets.
But
directly grid coupled synchronous generators are not used in wind turbines due
to unfavorable dynamic characteristics. When used in combination with a
fluctuating prime movers cause high structural loads and a risk of instability
during wind gusts which is also a problem.
IMPACTS
Impacts can
be classified mainly into two types.
1. Local
impacts.
2. Systems
impacts.
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.
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.
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