Maximum Power Control of Grid Connected Cycloconverter Excited Induction Machine for the Wind Energy Conversion

Abstract

In the direct drive (gear less) wind energy conversion systems, the squirrel cage newlineinduction machine is operated as generator by maintaining the rotational speed higher newlinethan synchronous speed. The power conversion by wind turbine depends on the ratio of newlineblade tip speed to the wind speed, known as tip speed ratio. Hence, for variable wind newlinespeed, the rotational speed of wind turbine is adjusted corresponding to optimum newlinerotational speed, so that the tip speed ratio can be maintained at its optimum position at newlinewhich wind turbine extracts maximum power from the available wind. To adjust the newlinerotational speed of wind turbine, the induction machine is excited at variable frequency newlinecorresponding to optimum rotational speed continually for variable wind speed. The newlinevariable frequency control scheme requires consistently change in excitation voltage to newlinemaintain volt per hertz ratio constant at which the airgap flux remains constant at its rated newlinevalue. newlineDC link inverter or cycloconverter can adjust the magnitude and frequency of newlineexcitation voltage of the induction machine. The cycloconverter becomes first choice in newlinedirect drive conversion system due to its inherent capability of bidirectional power flow newlineand large power handling capability at low frequency. In this research work, newlinecycloconverter is used to supply excitation of the induction machine from the grid at newlinevariable voltage and frequency corresponding to optimum rotational speed in order to newlinemaintain maximum power extraction from the wind. newlineIn the present work, cosine wave firing control technique is used to generate the firing newlinepulse of the cycloconverter. The firing instants of cycloconverter can be adjusted by newlinevarying the relative magnitude of sinusoidal reference waves to cosine waves, which vary newlinethe firing angle continuously to produce cycloconverter output voltages close to sine newlinewaves. The magnitude, phase and frequency of cycloconverter output voltages are newlinelinearly controlled by the magnitude, phase and frequency of the sinusoidal reference newlinewaves.