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WIND POWER GENERATION
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. This paved path for non-conventional sources. Of all the renewable energy sources; the one that has matured to the level of being a utility generation source is wind energy .It is estimated that wind potential is 1.6*10 7MW which is same as world energy requirement. But the only problem is that wind speed is highly fluctuating. So many problems arise during power generation. So we mainly concentrate on the problems occurred during generation and how they can be rectified. The problems faced are due to local impacts and system impacts. Local impacts deals with the impacts that occur in the vicinity of the wind turbine or wind farm. System impacts are the impacts that affect the behavior of the system as a whole. Using modern power electronics and special type of wind turbines that suit to the conditions can solve local impacts. Designing turbines to withstand voltage variations of certain magnitudes can rectify system impacts to some extent. Controlling the rotor speed by gear mechanism can rectify problems due to high wind or computer aided techniques.
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.
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.
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
• 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 can be classified mainly into two types.
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
1. Branch flows and node voltages.
2. Protection schemes, fault currents and switch gear ratings.
3. Harmonic distortion.
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.
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 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 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.