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 December 4, 2018

1 Introduction Environmental concern December 4, 2018

1 Structure Introduction Basic relations Often used generator systems Alternatives and trends Conclusions December 4, 2018

2 Basic relations: power coefficient December 4, 2018

2 Power curves December 4, 2018

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 December 4, 2018

2 Energy yield December 4, 2018

2 Rated speed Rated speed limited because of noise production by blade tip speed: December 4, 2018

2 Generator dimensions For direct drive: December 4, 2018

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 December 4, 2018

3 Generator system with gear example December 4, 2018

3 Direct drive generator example December 4, 2018

3 Output power of generator systems December 4, 2018

3 Comparison of generator systems CS DFIG DD Cost, size and weight + +/- - 50/60 Hz grid frequency Audible noise of blades Energy yield Variable speed Gearbox Generator Converter Reliability and maintenance Brushes Mechanical loads Complexity -(PM+) Power Quality ‘Flicker’ V&f control Harmonics Grid faults fault currents restoring voltage 3 Comparison of generator systems December 4, 2018

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 December 4, 2018

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? December 4, 2018

4 Trends in direct drive systems Unique machines: low speed high torque limited efficiency Induced voltage Resistive voltage drop > Interesting research field! December 4, 2018

4 Trends in direct drive systems From electrical to permanent-magnet 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 December 4, 2018

4 Trends in direct-drive: transverse flux? December 4, 2018

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 December 4, 2018

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 December 4, 2018