Wind Turbines Induction Generators (Doc)
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Added on 2020-05-16
Wind Turbines Induction Generators (Doc)
Added on 2020-05-16
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Running head: Wind Turbines Induction generators 1Induction Generator schemes for wind turbinesAuthors’ nameUniversity affiliation
Wind Turbines Induction Generators 2AbstractOver the year’s wind energy has been exploited and is perceived to be the fastest growing alternative source of energy. Induction generators are considered as the best type of generators because of their variable speed, lower maintenance requirements and improved quality of power. There exist two main induction generators used in wind turbine systems namely the SCIG or the squirrel cage induction generator and DFIG in full as the doubly-fed induction generator. SCIG is a straightforward way of wind conversion normally employed inareas with less emphasis on control of power flow and high efficiency. Direct connection to grid would however lower the power output and turbine utilization due to the low range of speed variation. DFIG on the other hand has increased quality of power and high efficiency of wind energy. This makes it appropriate for large-MW applications. In this paper, an overview of the SCIG its application and the DFIG with its power flow diagram are discussed.
Wind Turbines Induction Generators 3Cage Induction GeneratorPart AThe classical ‘Danish Concept’ describes a wind turbine consisting of a stall-controlled rotor with three blades, a gearbox, pole-switched asynchronous generator with squirrel cage rotor and a direct mains grid coupling. In order to limit the inrush currents that are produced when the generator is coupled with the grid, soft-starters are used between the generator and the grid during the starting period.Explain the operation of the above scheme with the aid of diagrams, equivalent circuits and equationsAs shown in the figure above, the SCIG uses a capacitor bank and a power converter the formers’ purpose being holding the magnetising current. The SCIG is connected to the grid directly via a transformer, a gear box also being incorporated in the scheme. This type of configuration is also known as the fixed speed wind generator because it only operates in thinrange around the synchronous speed or else, explained as the speed of the SCIG changing insignificantly due to variation in wind speeds that causes generator slip[ CITATION Qiu18 \l
Wind Turbines Induction Generators 41033 ]. The soft starters are used to reduce current surge whilst attaining the right synchronism between the grid and generator. These generators have a steep torque speed characteristic and so wind power variations are directed to the grid. During the wind turbines grid connection these transients are vital especially where the in-rush current is high. In order to limit this, the process of SCIG-grid connection is handled gradually [ CITATION YAN09 \l 1033 ]. The starting characteristics of a SCIG is as follows;The induction generator is fed by a 3-phase voltage which creates a three phase current resulting in a magnetic field in rotating motion. A voltage is then induced in the rotor bars which creates huge circulating currents to its end rings and bars. The rotor bars are occupied in the stators magnetic field and are thus exposed to a strong mechanical force. According to Lenz law, the rotor will tend to move in the same revolving field direction because of the torque produced by the rotor bars. The rotor speed increases gradually to synchronous speed but never surpasses it. If it is lower, then the flux will not cut the rotor bars making the force acting on the bars be zero because of absence of current and voltage. Consequently making the rotor to slow down. The rotor speed is always a fraction less of the synchronous speed [ CITATION RBR17 \l 1033 ]. Mathematically the turbine characteristics can be obtained as follows;The winds K.E is absorbed by the rotor converted to mechanical power and the output is given by:Pωt=12ρπR2V3=12ρπR2(Rλ)3ωR3With R being the radius of the blade, ρ being the density of air in (kg/M3), ωR is the speed of rotor in rad/s, V is the speed of wind in (m/s) and λ is the ratio of turbine tip-speed and is expressed as
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