25-06-2014, 02:37 PM
Wind Power Generation
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Abstract:
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 deal 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.
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.
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.
FREQUENCY CONTROL AND LOAD DISPATCHING OF CONVENTIONAL UNITS.
The impact of wind power on frequency control and load dispatching is caused by the fact that the prime mover of wind power is uncontrollable. Therefore, wind power hardly ever contributes to primary frequency regulation. Further, the variability of the wind on the longer term tends to complicate the load dispatching with the conventional units that remain in the system, as the demand curve to be matched by these units is far less smooth than would be the case without wind power. This heavily affects the dispatch of power from the conventional generators.
Note that the aggregate short term output power fluctuations of a large number of wind turbines are very smooth and are generally not considered as problem. The impact of wind power on frequency control and load dispatching becomes more severe. The higher the wind power penetration level is. The higher the wind power penetration, the larger the impact of wind power on the demand curve faced by remaining conventional units. It is however, impossible to quantify the wind power penetration level at which system wide effects start to occur because of the differences in demand curve and network topology between various power systems.
The above impacts are solved to some extent. But there is no proper solution to the problems caused by high-speed winds.
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
CONCLUSION:
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.