Speed Control of Induction Machine in Wind Energy Systems project work is based on a new control scheme for applications requiring cubic power characteristics. The system under consideration is a generating unit in which the prime mover is a wind turbine whose power –speed curves are cubic in nature. It is immaterial whether the system is stand-alone or grid-connected. In the former case, the generated power goes into a dc load and a battery backup.

In the latter case, it is inverted and fed to the grid. An IGBT based PWM converter is used in the stator circuit of a squirrel cage induction machine to extract maximum power from the shaft of the turbine. Since the converter can supply variable frequency as well as voltage, low slip operation over a wide speed range is possible as a result slip losses are negligible. To extract maximum power the control strategy should be independent of air velocity.

Since the peak power varies with air velocity, some systems which track peak power rely on air velocity measurement and works as long as air velocity varies slowly, but fails when it varies rapidly, since the peak point shifts with air velocity variation and confuses the controller. Hence the proposed scheme avoids pneumatic measurements. When the peak power points for various air velocities are joined, they are found to lie practically on a cubic curve, i.e., peak power is proportional to the cube of the shaft speed at which it occurs. By varying the stator voltage in such a proportion to square of the stator frequency, the pull out torque is made to vary as square of speed and the corresponding power to vary as cube of speed.

Refer to:

Ultra Sonic Wind Measurement Project Report

This ensures low slip operation throughout. Moreover, it is shown that only under this strategy does the slip speed at which the peak turbine power occurs become independent of air velocity. The optimum value of slip speed can be obtained by conducting experiment at any one air velocity. Thus, a new scalar control strategy   requires a cage rotor induction machine, is rugged, cheap and easy to implement the control logic is suggested. Open loop experiments are carried out in the laboratory to observe the proposed concept. Results of the experiments and relevant waveforms are presented, they are observed to be in good agreement with the simulation results.

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