WIND POWER - Electrical Engineering Gate

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Monday, October 13, 2014


Wind power is the conversion of wind energy into a useful form of energy, such as using: wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships.
A large wind farm may consist of several hundred individual wind turbines which are connected to the electric power transmission network. Offshore wind farms can harness more frequent and powerful winds than are available to land-based installations and have less visual impact on the landscape but construction costs are considerably higher. Small onshore wind facilities are used to provide electricity to isolated locations and utility companies increasingly buy surplus electricity produced by small domestic wind turbines

Wind power, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation and uses little land.[2] Any effects on the environment are generally less problematic than those from other power sources. As of 2011, 83 countries around the world are using wind power on a commercial basis.[3] As of 2010 wind energy was over 2.5% of total worldwide electricity usage, growing at more than 25% per annum. The monetary cost per unit of energy produced is similar to the cost for new coal and natural gas installations.[4] Although wind power is a popular form of energy generation, the construction of wind farms is not universally welcomed due toaesthetics
Solar electromagnetic radiation heats our atmosphere unevenly.This 
process, coupled with the Earth rotation and different rates of heating of land and water create unequal air pressure distributions, which contribute to the flows of air. Note that any moving object has kinetic energy, which is proportional to the mass of the object and the square of its speed. The energy in the wind is due to the moving air molecules. Thus, when a wind is formed, a portion of the sun's irradiance (about 1-2% of what reached the Earth) is converted to the energy of the moving air. Therefore wind energy is often referred to as a converted form of solar energy which has been in use for several millennia. It is one of the five main alternative power sources. Originally people used wind to power sailing ships. Later they began using it in grinding or milling turbines. For more than a century it has also been used for generation of electricity. Electricity powered by the moving air is casually referred to as wind power, and a device that converts air movement into electricity is called wind generator.

Such machine comes in various sizes, from those with rotors a few feet in diameter for sailboat battery charges to utility-grade systems with rotors measuring hundreds of feet across. Depending on the rotor's size, their peak output ratings can range anywhere from several watt to 2.5 megawatts

In short, a wind generator operates like inverse fan. Its main components are rotor, alternator, and structural support

The rotor assembly includes the aerodynamically designed blades for transferring kinetic air energy to the rotation of the shaft. Most modern wind turbines have two to five blades. The rotor is connected to the main shaft of the alternator. The alternator in turn converts the mechanical energy of the spinning shaft into electricity. It contains one or several permanent magnets or electromagnets that spin inside or outside stationary coils of wire. Small windpower systems, such as homemade designs normally use permanent magnets. When the air hits the blades, they spin the shaft with the magnets, which creates an alternating magnetic field in the stationary coils. This alternating magnetic field induces voltage in the coils according to Faraday's law (for more information on how it works see operation of generators). Depending on the design, the alternator rotor can operate either at variable speed which varies with the wind or at relatively constant speed.

The voltage level produced by an alternator is determined by the number of turns in its coils, the strength of the magnetic field, and the rotation speed. In small wind power generators suitable for home use, the blade rotor usually drives the magnet rotor directly. Larger systems may have a gearbox that converts the low speed high torque incoming rotation of the blade rotor to high speed low torque rotation of the alternator rotor.

The alternator usually generates 3-phase AC output with variable amplitude and frequency. To be usable it first has to be rectified. The rectified DC voltage can be used for battery charging. If its level is sufficiently high it can power appliances that can operate on DC, such as light bulbs and space heaters. However to power conventional household appliances or an entire home, DC voltage has to be converted to a regulated fixed frequency AC. Small systems for residential use are usually sold with a rectifier and a charge controller. A DC-AC inverter and a battery may be optional.

The amount of wind power generation is proportional to the cube of the air speed. For example, when air speed doubles, the available power increases by a factor of 8. Because air speeds increase with height, the turbines are normally mounted on tall towers to capture the maximum amount of air movement. In general, the higher the tower, the more electricity the system can generate. Nevertheless, only a small portion of the air energy can be captured. The typical efficiency of the blades is 25-45%, alternators-- 65-80%. As the result, an average overall efficiency of a home wind turbine is about 20%. Available annual wind resources of course vary with the location. Different geographical areas have different average air speeds and power densities

The transmission network usually consists of high to very high voltage power lines designed to transfer bulk power from major generators to areas of demand; in general, the higher the voltage, the larger the transfer capacity. Only the largest customers are connected to the transmission network..
Transmission network voltages are typically above 100 kV. The networks are designed to be extremely robust, so they can continue to fulfil their function even in the event of several simultaneous network failures. Failure of a single element, such as a transformer or transmission line, is referred to as an 'N-1' event, and transmission systems should be capable of withstanding any such event. More complex cases of simultaneous failures of multiple elements (for example, the failure of a transmission line when a parallel line has been disconnected for maintenance), can be termed 'N-2' or similar. Transmission systems should also be capable of withstanding any such credible combinations..
Transmission consists mainly of overhead lines. Although underground lines offer the advantage of being less visually intrusive and raising less environmental objections, they incur higher initial investment costs and have a lower transmission capacity.
Transmission systems are operated by transmission system operators (TSOs), or independent system operators (ISOs). Responsibility for constructing or owning the physical network may belong to other organizations..
Transmission systems are actively managed through power system control centres, also known as dispatch centres. Balancing power entering and leaving the high voltage network, and reconfiguring the network to cope with planned and forced outages, is a 24-hour 
Distribution networks are usually below 100 kV and their purpose is to distribute power from the transmission network to customers. At present, with the exception of wind and other renewable power plants, little generation is connected to distribution networks, but this is changing rapidly, for

example in Germany and Denmark

Generation connected to distribution networks is often termed “embedded generation” or “distributed generation.” Distribution networks are less robust than transmission networks and their reliability decreases as voltage levels decrease. For example, a connection at 33 kV could expect to lose only a few minutes of connection per year on average, whereas a low-voltage connection at 230 V for an individual domestic consumer in a rural area would, on average, expect to lose at least an hour. As with transmission networks, distribution networks are operated (in by Distribution System Operators (DSOs).

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