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Basic Operation Principles and Electrical Conversion Systems of Wind Turbines H. Polinder, S.W.H. de Haan, J.G. Slootweg, M.R. Dubois Norpie, 14-16 June 2004 July 7, 2015 1 EPP Electrical Power Processing 1 Introduction Environmental concern July 7, 2015 2 EPP Electrical Power Processing 1 Structure 1 2 3 4 5 Introduction Basic relations Often used generator systems Alternatives and trends Conclusions July 7, 2015 3 EPP Electrical Power Processing 2 Basic relations: power coefficient 1 1 3 P air C p Ar vw air C p ( , )r 2vw3 2 2 July 7, 2015 4 EPP Electrical Power Processing 2 Power curves 1 P airC p ( , )r 2vw3 2 July 7, 2015 5 EPP Electrical Power Processing 2 Stall principle Increasing wind speed > increasing angle of attack Blade design so that at rated power the airflow becomes loose from the blade > eddy’s July 7, 2015 6 EPP Electrical Power Processing Power , probability & power density 2 Energy yield 0 5 10 15 20 Winds peed [m/s] July 7, 2015 7 EPP Electrical Power Processing 2 Rated speed Rated speed limited because of noise production by blade tip speed: N rated 60 vtrated 2 r July 7, 2015 1 Prated 8 EPP Electrical Power Processing 2 Generator dimensions P T rs F 2rs2ls Fd 2Vr Fd Fd 25 50 kN/m2 For direct drive: 3/ 2 Prated Vr 2 Fd July 7, 2015 9 EPP Electrical Power Processing 3 Generator systems Constant speed •Squirrel cage induction generator •Stall principle Variable speed with gear •Doubly-fed induction generator •Pitch Variable speed direct drive •Synchronous generator •Pitch July 7, 2015 10 EPP Electrical Power Processing 3 Generator system with gear example July 7, 2015 11 EPP Electrical Power Processing 3 Direct drive generator example July 7, 2015 12 EPP Electrical Power Processing 3 Output power of generator systems July 7, 2015 13 EPP Electrical Power Processing 3 Comparison of generator systems CS DFIG DD Cost, size and weight + +/- - 50/60 Hz grid frequency - - + Audible noise of blades - + + Energy yield + + + + +/- + + - Reliability and Brushes maintenance Gearbox Mechanical loads Complexity + + + - -(PM+) + + - Power Quality ‘Flicker’ V&f control Harmonics + + + - + + - Grid faults fault currents restoring voltage + +/- +/+/- +/+ July 7, 2015 Variable speed Gearbox Generator Converter 14 EPP Electrical Power Processing 4 Alternatives • Squirrel cage induction generator with converter • + Brushless generator • + Cheaper (IG) generator • + Grid fault ride through capabilities • - More expensive converter • - More converter losses • Brushless synchronous generator with converter • + More efficient generator July 7, 2015 15 EPP Electrical Power Processing 4 Trends in geared generator systems • Towards variable speed • Hardly development in doubly-fed induction generators • Development in converter performance during grid faults • Brushless synchronous generator? July 7, 2015 16 EPP Electrical Power Processing 4 Trends in direct drive systems Unique machines: • low speed • high torque • limited efficiency • Induced voltage • Resistive voltage drop > Interesting research field! July 7, 2015 Eind B v 4V/m ER Cu J 0.1V/m 17 EPP Electrical Power Processing 4 Trends in direct drive systems • From electrical to permanentmagnet excitation • lower losses • significant reduction in active mass • reduction of magnet cost • Not from radial to axial flux • axial flux may be smaller • but axial flux is heavier and more expensive July 7, 2015 R wo R li R lo 18 EPP Electrical Power Processing 4 Trends in direct-drive: transverse flux? flux N coil S S N N S N magnet July 7, 2015 19 EPP Electrical Power Processing 4 Trends in trends in voltage levels • • • • • The current voltage level is mostly 400 - 700 - 1000 V Windformer ABB Zephyros: 3 - 4 kV NEG-Micon (VTDI) stator: 6 kV Expected: trend to several kV to limit losses July 7, 2015 20 EPP Electrical Power Processing 5 Conclusions • Trend towards variable speed • Geared and direct-drive turbines will remain both • Radial flux permanent-magnet machine most suitable for direct-drive • Rise of voltage level up to several kV for large turbines July 7, 2015 21 EPP Electrical Power Processing