Zongxi Fang – firstname.lastname@example.org 2010/2/11
Summary of the mini project – Permanent magnet machines topology used in wind power generation Zongxi Fang
Wind energy is one of the world’s fastest growing renewable energy sources. It is important to investigate various topologies and designconcepts of generator both used in real wind power market and laboratory experiments. In real wind power market, three types of wind power system for large wind turbines exit. The first type is fixed-speed wind power with a multi-stage gearbox and a traditional squirrel-cage induction generator (SCIG), directly connected to the grid. The second one is a variable speed wind system using a multi-stagegearbox and a doubly fed induction generator (DFIG), with a power less rating of 30% of the generator capacity passing the power electronic converter from the rotor winding. The same as SCIG, the stator winding of the DFIG is directly connected to the grid. The third type is also a variable speed wind turbine, which is famous as a gearless or direct-drive system. Usually it contains a low-speedhigh-torque synchronous permanent magnet (PM) synchronous generator and a full-scale power electronic converter, which is called direct-drive PMSG system. Normally, a machine with a permanent magnet for exiting source can be called the PM machines. But as for a wind turbine generator, the output voltage of the machine usually must be sinusoidal. The types of PM machines studied in this paper focuson traditional PM generators that can be divided into radial-flux and axial-flux machines, according the flux direction in the air gap.
2. Radial-flux PM wind generators
Basically, there are two types of radial-flux PM wind generators in real application as shown in Fig. 1.
Copper Air gap
Figure 1 Inner and outer rotor radial-flux PM generators In a fact, only the two topologies are applied in MW wind system. There are a lot of experiences and studies in the inner rotor PMSGs. They also show a good electrical operation performance and better cooling effect for stator. As for the outer rotor radial flux PMSG, they are easily directly coupled between wind turbineand generators mechanically .This topology has a better cooling effect for rotor and magnets, but difficult cooling for stator, while most of wind generator loss is on this side. For dual stator and IPMSG, illustrated in Fig. 2, they are just used in small power wind system, from 1kw-50kw. The dual stator structure has higher torque capability and the IPM topology is good for resistingdemagnetization. The IPM topology is widely used in industrial application. But for a large power system, its large volume and the performance is easily influenced by load change.
Figure 2 Dual stator and IPM radial-flux PM generators Double stator and single side axial flux PMSG are widely used in small wind power
system, and the structure can be seen in Fig. 3. Doublestator axial flux PMSG has a higher torque capability and the performance is superior to the single side one. Besides, the topology is too complicated to useful for a large wind generator system. The sing side axial-flux PM generator has simple construction and is easy for mass production. But when it comes to large power and large blade radius, mechanical dynamic balance is a serious problem.Double stator axial flux PMSG
Single side axial flux PMSG
Figure 3 Double stator and single side axial flux wind PMSG Lastly, two problems of PMSG used in wind power are introduced. First is the inherent cogging torque due to magnet materials naturally attractive force. This kind of torque is bad for operation, especially stopping wind turbine starting and making noise and vibration in...