Converteam: St. Mouty, A. Mirzaïan FEMTO-ST: A. Berthon, D. Depernet, Ch. Espanet, F. Gustin

Permanent Magnet Design Solutions for Wind Turbine applications Converteam: St. Mouty, A. Mirzaïan FEMTO-ST: A. Berthon, D. Depernet, Ch. Espanet, F. Gustin

Outlines 1. Description of high power electrical conversion systems 2. Why Direct Drive Permanent Magnet Generator (DDPMG)? 3. Possible improvements of DDPMG 4. Designs comparison 5. Conclusion Direct Drive PMG WIND TURBINE 2

Double Fed Induction Generator (DFIG) Most popular high power wind turbine A gearbox is used to increase speed of the generator in order to keep a good efficiency of the machine and to improve compactness The stator of the machine is directly supplied by the grid Electronic converter is designed for ~30% of the power enables to modify rotor characteristics and work with variable speed Direct Drive PMG WIND TURBINE 4

Double Fed Induction Generator (DFIG) Advantages : Cost (small electronic converter, and no high cost materials) Drawbacks : Gearbox Number of elements in the system Limitation of power (5MW with 3 stages gearbox) Wind speed range Picture from Nodex Direct Drive PMG WIND TURBINE 5

Synchronous generator with wound rotor (SGWR) Generator is directly coupled with the turbine A first converter enables to convert stator electrical power, it is designed for 100% of the generator power A second low power converter supply a rotating transformer, a diode bridge is used to supply the rotor with a DC current The rotor can also be supplied by brushes Direct Drive PMG WIND TURBINE 6

Synchronous generator with wound rotor (SGWR) Advantages: No gearbox Drawbacks: Number of elements to supply the rotor Cost Rotor losses Weight Enercon Direct Drive PMG WIND TURBINE 7

Direct Drive Permanent Magnet Generator (DDPMG) Generator is directly coupled with the turbine A converter is used to convert stator electrical power, it is designed for 100% of the generator power Thanks to the permanent magnets the rotor doesn t need to be supplied Advantages: No gearbox High efficiency Drawback: Zephyros Cost of rare earth permanent magnets Direct Drive PMG WIND TURBINE 8

Hybrid Gearbox with low ratio is used to reduce the torque and the permanent magnets weight A converter is used to convert stator electrical power, it is design for 100% of generator power Thanks to the permanent magnets the rotor has not to be supplied Advantages: compromise between cost and efficiency Drawbacks: all drawbacks of permanent magnet and DFIG Multibrid Direct Drive PMG WIND TURBINE 9

Drawbacks of Gearbox With ameliorations made in the last decade, gearbox is not the main cause of downtime in the conversion chains, but its risk of failure remains high Time to repair in case of gearbox failure is long and it is not an easy operation DFIG is not able to produce power on a same speed range than Direct drive turbine Pod weight is larger for a DFIG than for a DDPMG (GE) For all the topologies, lubrication of the bearing need also a periodic maintenance (at least change of oil) Direct Drive PMG WIND TURBINE 11

Permanent Magnets vs Wound Rotor Machines with wounded rotor exists in industry app since one century For direct drives app, supplying the rotor with a dc current could be difficult due to the rotor dimensions A converter is necessary to fed and control the rotor DC current Rotor weight increases because pole s height is approximately 3 times bigger for wounded rotor than permanent magnet The PM permeability is close to the one of the air air gap is bigger with PM rotor and leakage are rather increased Flux weakening with permanent magnet is more difficult than wound rotor it can be difficult to control the over-speeds Direct Drive PMG WIND TURBINE 12

Example of Comparison D. J. Bang, H. Polinder, G. Shrestha, and J. A. Ferreira: Promising direct-drive generator system for large wind turbines. EPE Journal, Vol. 18, No. 3, pp. 7-13, 2008. Active parts: iron sheets + PM + copper Non active parts: structural Gearbox: risk of failure but also a large source of losses Direct Drive enables to maximize the annual energy yield thanks to a bigger wind speed range Structural cost is the first reason of cost gap between Hybrid and DDPMG Direct Drive PMG WIND TURBINE 13

Aim of study Performances point of view: PMSG is the best technology for a wind turbine app The big drawback is its COST Cost of the machine is directly linked to the torque density of the machines: higher is the torque density, less will be the machine weight Aim of Converteam/FEMTO studies is to reduce the weight of the machine and so the cost By keeping the control and the principle of the actual permanent magnet machines: a reduction of the weight will lead to an increase of the losses to find machines improvments to mitigate this limitation 30 Active Parts Weight (T) 25 20 15 10 5 0 90 92 94 96 98 Efficiency (%) Direct Drive PMG WIND TURBINE 15

Possible solutions To increase torque density, modifications on the design and on the supply can be made : To reduce copper with concentrated and fractional windings To modify the conventional EMF and I waveform and to adapt the machine control To use others flux distribution: axial, transverse To modify the rotor location according to the rotor binding To increase the number of phases Outer rotor/ Inner rotor Radial vs axial flux distribution Direct Drive PMG WIND TURBINE 16

Change of back EMF and current waveforms To use harmonics of back-emf and currents in order to produce a higher torque Low frequency machine iron losses < copper losses The reduction of copper losses even if iron losses increase can enable a reduction of the generator weight With leakage between magnet Direct Drive PMG WIND TURBINE 17

Drawbacks: ripple and cogging In order to obtain a back emf waveform close to a square wave, some geometry modifications can be used: To use winding with one slot per pole and per phase To use a magnet arc close to the pole arc These designs choice can lead to important torque ripples 1.0 Torque (p.u.) 0.5 0.0 0 60 120 180 240 300 360 Electrical Angle (Degree) To increase the phase s number enable to increase back emf harmonics (so torque density) and to reduce the ripples Direct Drive PMG WIND TURBINE 18

Specifications to achieve designs comparisons (extract from Polinder paper) Power Speed Poles pairs / frequency Outer stator diameter 3 MW 15 rpm 80 / 20Hz < 5 m Efficiency > 94.5 % Air gap Active material weight > 5 mm < 24 T Direct Drive PMG WIND TURBINE 20

Inner versus Outer rotor Parameter Inner rotor Outer rotor Inner diameter (m) 4.72 4.78 Outer diameter (m) 4.98 5.06 Air gap diameter (m) 4.78 5 Air gap length (mm) 7 7 Magnet weight (T) 1.3 1.3 Iron weight (T) 11.6 11 Copper weight (T) 5.4 5.2 Total active parts weight (T) 18.3 17.5 Efficiency (%) 94.53 94.72 Torque ripple (%) 37 38.9 Direct Drive PMG WIND TURBINE 21

Number of phases: 7 phases versus 3 phases Parameter 7 phases 3 phases Inner diameter (m) 4.8 4.78 Outer diameter (m) 5.07 5.06 Air gap diameter (m) 5 5 Air gap length (mm) 7 7 Magnet weight (T) 1.3 1.3 Iron weight (T) 12.1 11 Copper weight (T) 3.8 5.2 Total active parts weight (T) 17.2 17.5 Efficiency (%) 94.72 94.72 Torque ripple (%) 4.5 38.9 Direct Drive PMG WIND TURBINE 22

Synthesis, Conclusion For High Power Wind Power application: Direct Drive Permanent Magnet = High Efficiency and High Reliability BUT Expensive solution due to magnet and structural cost Improvements Air gap waveforms Phase s number Outer rotor topology Increase of torque density Increase of attractiveness Reduction of pod weight Reduction of cost To keep high efficiency Nevertheless, cost and Chinese monopole on rare earth magnet could be an important drawback regarding the development of this type of generator CONVERTEAM thinks that this risk is under control and has decided to develop solution with permanent magnets Direct Drive PMG WIND TURBINE 23

Outlooks 1 Design of a machine with no end windings: the stator winding is realized with bar (as rotor squirrel cage of induction machine without short-circuit ring) The number of phases is high: possibly each bar is a phase But the use of very high current electronic switches limit the implementation of this type of solution for the moment 2 Superconductor materials are also studied to increase the flux density Depending of the magnets price this could become competitive But the work in order to industrialize such a solution is huge Direct Drive PMG WIND TURBINE 24

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