18-07-2012, 10:53 AM
Modeling and Control of a Variable Speed Wind Energy Conversion System Based on an Induction Machine
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Introduction
Wind Energy Conversion Systems (WECS) are on an established technology for power generation [10] and they are evolving from being an alternative energy source to providing additional functions such as reactive power supply, voltage control and active power regulation [13]. Such functions are possible owing to the improvement of the WECS components and to increasing sophistication of the control system [5]. For wind energy conversion system WECS, recent research has been focused chiefly on permanent magnet synchronous machines PMSM or doubly fed induction machines DFIM as generator of electrical power. Advantages have been variously attributed to high power density for PMSM and reduced rating of power converters for DFIM. However, the PMSM suffers from high cost of materials and manufacturing. In DFIM schemes, the slip-ring brush contacts pose a serious maintenance and reliability problem, especially for remote installation. Squirrel-cage induction machines still have an edge over these approaches in terms of cost and ruggedness. Squirrel cage generators with shunt passive or active VAR (Volt Ampere Reactive) generators was proposed in [9], which generate constant frequency power through a diode rectifier and line commuted thyristor inverter. The control systems for the operation of indirect rotor flux oriented vector controlled induction machines for variable speed wind energy applications are discussed in [2]-[11]. Sensorless vector control scheme suitable to operate cage induction is considered and a fuzzy control system is used to drive the WECS to the point of maximum energy capture for a given wind velocity.
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.
At the end of 2010, worldwide nameplate capacity of wind-powered generators was 197 gigawatts (GW).[3]
Energy production was 430 TWh, which is about 2.5% of worldwide electricity usage;[3][4] and has doubled in the past three years. Several countries have achieved relatively high levels of wind power penetration, such as 21% of stationary electricity production in Denmark,[3] 18% in Portugal,[3] 16% in Spain,[3] 14% in Ireland[5] and 9% in Germany in 2010.[3] As of May 2009, 80 countries around the world are using wind power on a commercial basis.[4]
Large-scale wind farms are connected to the electric power transmission network; smaller facilities are used to provide electricity to isolated locations. Utility companies increasingly buy back surplus electricity produced by small domestic turbines. Wind energy, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, and produces no greenhouse gasemissions during operation. The construction of wind farms is not universally welcomed because of their visual impact, but anyeffects on the environment from wind power are generally less problematic than those of any other power source.
Distribution of wind speed
Distribution of wind speed (red) and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado. The histogram shows measured data, while the curve is the Rayleigh model distribution for the same average wind speed.
The strength of wind varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To assess the frequency of wind speeds at a particular location, a probability distribution function is often fit to the observed data. Different locations will have different wind speed distributions. The Weibull model closely mirrors the actual distribution of hourly wind speeds at many locations. The Weibull factor is often close to 2 and therefore a Rayleigh distribution can be used as a less accurate, but simpler model.
Because so much power is generated by higher wind speed, much of the energy comes in short bursts. The 2002 Lee Ranch sample[13] is telling;[dubious – discuss] half of the energy available arrived in just 15% of the operating time.[citation needed] The consequence is that wind energy from a particular turbine or wind farm does not have as consistent an output as fuel-fired power plants.[14]
Electricity generation
Typical components of a wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position
In a wind farm, individual turbines are interconnected with a medium voltage (often 34.5 kV), power collection system and communications network. At a substation, this medium-voltage electric current is increased in voltage with a transformer for connection to the high voltageelectric power transmission system.
The surplus power produced by domestic microgenerators can, in some jurisdictions, be fed into the network and sold to the utility company, producing a retail credit for the microgenerators' owners to offset their energy costs.[15][16]
Grid management
Induction generators, often used for wind power, require reactive power for excitation so substations used in wind-power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behaviour during system faults (see: Low voltage ride through). In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators. Doubly-fed machines generally have more desirable properties for grid interconnection[citation needed]. Transmission systems operators will supply a wind farm developer with agrid code to specify the requirements for interconnection to the transmission grid. This will include power factor, constancy of frequency and dynamic behavior of the wind farm turbines during a system fault.[17][18]
Capacity factor
Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favourable sites.[19] For example, a 1 MW turbine with a capacity factor of 35% will not produce 8,760 MW•h in a year (1 × 24 × 365), but only 1 × 0.35 × 24 × 365 = 3,066 MW•h, averaging to 0.35 MW. Online data is available for some locations and the capacity factor can be calculated from the yearly output.[20][21]
Unlike fueled generating plants, the capacity factor is affected by several parameters, including the variability of the wind at the site, but also the generator size- having a smaller generator would be cheaper and achieve higher capacity factor, but would make less electricity (and money) in high winds.[22] Conversely a bigger generator would cost more and generate little extra power and, depending on the type, may stall out at low wind speed. Thus an optimum capacity factor can be used, which is usually around 20-35%.
Conclusion
In this paper, a control strategy of the WECS was proposed. The control has permitted to regulate the DC link voltage and the active and reactive power injected to the grid.
The proposed control strategy was therefore implemented for three cases that are first a wind turbulent speed, second the bidirectionality of the GSC converter and finally the behavior of the WECS during a fault of the grid voltage dip. Simulation results show the effectiveness of the proposed techniques.