19-09-2014, 02:02 PM
WIND NON-CONVENTIONAL ENERGY
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INTRODUCTION
Wind energy is a reliable, natural and renewable electrical power supply. The high installed capacity of today’s wind turbines and decreasing plant costs have shown that wind power can be competitive with conventional, more heavily polluting, fuels in the long term. Wind power growth with a 20% annual rate has experienced the fastest growth among all renewable energy sources science five years ago. It is predicted that by 2020 up to 12% of the world's electricity will have been supplied by wind power. Figure-1 shows the global cumulative installed capacity and global annual installed capacity.
In terms of wind power generation technology, as a result of numerous technical benefits (higher energy yield, reducing power fluctuations and improving var supply) the modern MW-size wind turbines always use variable speed operation which is achieved by electrical converters .These converters are typically associated with individual generators and they contribute significantly to the costs of wind turbines. Between variable speed wind turbine generators doubly fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) with primary converters are emerging as the preferred technologies.
As a result of large-scale wind power generation, interconnecting large wind farms to power grids and the relevant influences on the host grids need to be carefully investigated. Wind farms are now required to comply with stringent connection requirements including reactive power support, transient recovery, system stability and voltage/frequency regulation. Further to increase the maximum power extraction the variable speed generators are employed. These variable speed generators necessitate a AC-DC-AC conversion systems. The generator side converter controls the electromagnetic torque, and therefore the extracted power, while the grid side converter controls both the DC link voltage and the power factor. Moreover, when designing the control strategy, it seems that the generator-side converter must control the extracted power as it is located closer to the incoming power. Hence, the grid-side converter would control the DC voltage.
Fulfilling the new grid codes constitutes one of the main challenges for the wind power industry. There are ride through requirements. Enhancing the operation of wind turbines in front of the grid faults is mandatory requirement.
The wind turbines must stay connected to the grid during grid disturbances. They should continuously feed the reactive power in addition to limited active power. In modern wind turbines the increasing integration of power electronics enable to control the behavior of wind generation system under faulty scenarios.
INTRODUCTION
Wind is abundant almost in any part of the world. Its existence in nature caused by un even heating on the surface of the earth as well as the earth’s rotation means that the wind resources will always be available. The conventional ways of generating electricity using non renewable resources such as coal, natural gas, oil and so on, have great impacts on the environment as it contributes vast quantities of carbon dioxide to the earth’s atmosphere which in turn will cause the temperature of the earth’s surface to increase, known as the green house effect. Hence, with the advances in science and technology, ways of generating electricity using renewable energy resources such as the wind are developed. Nowadays, the cost of wind power that is connected to the grid is as cheap as the cost of generating electricity using coal and oil. Thus, the increasing popularity of green electricity means the demand of electricity produced by using non renewable energy is also increased accordingly
1 Features of wind power systems:
There are some distinctive energy end use features of wind power systems
Most wind power sites are in remote rural, island or marine areas. Energy requirements in such places are distinctive and do not require the high electrical power.
A power system with mixed quality supplies can be a good match with total energy end use i.e. the supply of cheap variable voltage power for heating and expensive fixed voltage electricity for lights and motors.
Rural grid systems are likely to be weak (low voltage 33 KV). Interfacing a Wind Energy Conversion System (WECS) in weak grids is difficult and detrimental to the workers’ safety.
There are always periods without wind. Thus, WECS must be linked energy storage or parallel generating system if supplies are to be maintained.
2.2 Power from the Wind:
Kinetic energy from the wind is used to turn the generator inside the wind turbine to produced electricity. There are several factors that contribute to the efficiency of the wind turbine in extracting the power from the wind. Firstly, the wind speed is one of the important factors in determining how much power can be extracted from the wind. This is because the power produced from the wind turbine is a function of the cubed of the wind speed. Thus, the wind speed if doubled, the power produced will be increased by eight times the original power. Then, location of the wind farm plays an important role in order for the wind turbine to extract the most available power form the wind. The next important factor of the wind turbine is the rotor blade. The rotor blades length of the wind turbine is one of the important aspects of the wind turbine since the power produced from the wind is also proportional to the swept area of the rotor blades i.e. the square of the diameter of the swept area. Hence, by doubling the diameter of the swept area, the power produced will be four fold increased. It is required for the rotor blades to be strong and light and durable . As the blade length increases, these qualities of the rotor blades become more elusive. But with the recent advances in fibre glass and carbon-fibre technology, the production of lightweight and strong rotor blades between 20 to 30 meters long is possible. Wind turbines with the size of these rotor blades are capable to produce up to 1 megawatt of power. The relationship between the power produced by the wind source and the velocity of the wind and the rotor blades swept diameter is shown below.
Number of rotor blades:
The three bladed rotors are the most common in modern aero generators. Compared to three bladed concepts, the two and one bladed concepts have the advantage of representing a possible saving in relation to cost and weight of the rotor. However, the use of fewer rotor blades implies that a higher rotational speed or a larger chord is needed to yield the same energy output as a three bladed turbine of a similar size. The use of one or two blades will also result in more fluctuating loads because of the variation of the inertia, depending on the blades being in horizontal or vertical position and on the variation of wind speed when the blade is pointing upward or downward. Therefore, the two and one bladed concepts usually have so-called teetering hubs, implying that they have the rotor hinged to the main shaft. This design allows the rotor to teeter in order to eliminate some of the unbalanced loads. One bladed wind turbines are less widespread than two–bladed turbines. This is because they in addition to a higher rotational speed, more noise and visual intrusion problems, need a counter weight to balance the rotor blade
ADVANTAGES OF WIND ENERGY
Although residential wind turbines and their energy source, the wind, have a few downsides, wind energy is an abundant and renewable resource. We won’t run out of wind for the foreseeable future, unlike oil and natural gas. Small-scale wind energy could also help decrease our reliance on declining and costly supplies of oil — if electricity generated by wind is used to power electric or plug-in electric hybrid cars and trucks, displacing gasoline, which is refined from oil. Wind could even eventually reduce our dependence on nuclear power as well. In the developed countries, nuclear power plants generate about 20 percent of the nation’s electricity. Although wind energy does have its impacts, it is a relatively benign technology compared to conventional sources of electricity. It could help all countries create cleaner and safer energy at a fraction of the environmental cost of conventional electrical energy production. Wind energy can help nations reduce global warming and devastating changes in our climate. Wind can also help homeowners and businesses do their part in solving other costly environmental problems such as acid rain. Another benefit of wind energy is that, unlike oil, coal and nuclear energy, the wind is not owned by major energy companies or controlled by foreign nations. An increasing reliance on wind energy could therefore ease international political tension. Reducing our reliance on Middle Eastern oil could reduce costly military operations aimed, in part, at stabilizing a region where the largest oil reserves reside. Wind is also a free resource. The cost of wind is not subject to price increases.
A wind- and solar-powered future might be one subject to less inflation. This is not to say that wind energy will be free of price increases. While the fuel itself (the wind) is free, the price of wind generators is likely to increase. That’s because it takes energy to extract and process minerals to make the steel and copper needed for wind turbines and towers. It also takes energy to make turbines and towers and ship and install them. As the price of conventional fuels and raw materials increases, the cost of wind energy also will go up. Yet another advantage of wind-generated electricity is that it uses existing infrastructure, the electrical grid, and existing technologies. A transition to wind energy could occur fairly seamlessly. Thanks to generous tax credits and other financial incentives, individuals in rural areas with good wind resources can meet all or part of their energy needs at rates that are often competitive with conventional sources. In remote locations, wind or wind and solar electric hybrid systems can be cheaper than conventional power
CONCLUSION
The output power of direct-driven wind energy conversion system (WECS) will be limited during grid voltage sag, but the operation of wind turbine will not be influenced by adding damp load in DC-side, then the capability of low voltage ride through (LVRT) may be enhanced. Full power back-to-back PWM converter is adopted in direct-driven variable-speed constant frequency WECS using permanent magnet synchronous generator (PMSG), which can effectively improve system efficiency. The effects of symmetrical and unsymmetrical fault are investigated. The whole system modeling of converters is realized. Then the simulation model of converters,and the characteristic during grid voltage sag is simulated; the validity of the model and analysis is verified by simulation, and the results show that direct-driven WECS have good fault ride through capability.
In the case of symmetrical and unsymmetrical fault conditions in grid network, the ac voltage of PCC can meet the basic requirements of grid network concerning voltage. On the other hand, according to simulation results, variable speed driven PMSG is not significantly impacted in abnormal conditions. So it can remain connecting to the grid during and after grid short circuit fault. The inverter is controlled so as to meet IEEE standard 1547 requirements for connection of distributed generation to the local grid. The simulated results validate the proposed topology.