Permanent Magnet Generator Design Solutions for Wind Turbines

Transcription

1 Permanent Magnet Generator Design Solutions for Wind Turbines Stéhane MOUTY, Abdollah MIZAIAN, Frederic GUSTIN, Alain BETHON, Daniel DEPENET 3 and Christohe ESPANET Converteam SAS, France University of Franche Comte, Femto-ST Institute, France 3 University of Technology of Belfort Montbeliard, Femto-ST Institute, France Tel: +33(0) stehane.mouty-ext@converteam.com Toics: Wind Power. Introduction Converteam is a worldwide secialist in ower conversion and in offshore solutions. Wind energy needs ower converters for energy conversion and transort. Converteam has realized wind turbine generators and their converters for several years. The comany has decided to roose only direct drive solutions for future orders and is develoing new concets of ermanent magnet (PM) generators in collaboration with the Energy deartment of FEMTO-ST Institute. In order to be cometitive in the market and to resect the environmental constraints, the international standards have to be resected and economic solutions are researched. Thus, the aim is to obtain a machine which has a low cost and a high efficiency. The full ower converter, which is necessary to connect the machine to the grid, needs to be otimized too. Firstly, a descrition of the common suly chains met in high ower wind turbine alications is give Secondly, advantages and drawbacks of the direct drive ermanent magnet generators will be detailed in comarison with the other toologies. Then, the study will focus on the ermanent magnet generators and imrovements to increase their interest. A comarison between various ermanent magnet generator designs focusing on active arts weight and efficiency is roosed.. High Power Wind Turbine Conversion Chains In order to otimise energy roduction, the use of variable seed generators is required for large wind turbine, i.e. ower roduction above MW. Four main toologies are met in the market for high ower wind turbine systems. The nominal generator seed is different according to the considered toologies. To kee a good efficiency, an induction machine will be used at high seed (>500 rm). To obtain this seed, the only way is to use a gearbox, the turbine seed is around 5 rm for high ower wind alication, so that some gearbox s stage are necessary. Nevertheless with a three stages gearbox the ower is limited at 5 MW. The main suliers are: Vestas, Gamesa, GE Wind, Nordex, Alstom wind.. Synchronous Generator with Wound otor (SGW) With the exerience in high ower machines (oil, hydro or nuclear) where synchronous generator with wound rotor (or direct current machine in old installations) is used, it is not surrising to find this tye of technology in wind turbine generator. This toology is met for a direct drive alication (without gearbox). In this case, the working seed is the same that the turbine seed; thus at a given ower the torque is more imortant than one obtained with the DFIG. Machines will have a big diameter and a short length; due to their form these machines may be called torque or ring generator. This toology leads to exensive active arts and lots of auxiliaries are necessary to create the rotate field as we can see Fig.. Two toologies of excitation exist: firstly with a brush to commutator contact, secondly with a rotating transformer and a diode bridge. This second solution is referred because there are none mechanical contacts between rotated and fixed arts. To suly the rotating transformer, converters are necessary but their ower is lower than the generator one... Doubly Fed Induction Generator (DFIG) The most oular system is induction machine (see Fig.). There are two reasons to exlain this interest, firstly induction machine cost is lower than synchronous machine and secondly ower electronics converter doesn t need to convert all the ower but only around thirty er cent. These converters are used to modify the rotor roerties and allow variable seed behaviour around the rated oint. Figure : Nacelle scheme of a DFIG and descrition of the ower chai

2 Figure : Power chain of the DDW and icture of a machine. Figure 3: Nacelle scheme of DDPMG and descrition of it ower conversion chai Figure 4: Picture of the multibrid generator and scheme of the ower chai To work with variable seed, a full ower converter is necessary to convert the electrical ower rovided by the stator of the machine. With such a ower chain, the machine is able to roduce energy for almost all wind seeds. The main sulier is Enerco. 3. Permanent Magnet Generator In order to increase ower density and to simlify field creation of the direct drive generator, a solution is to use ermanent magnets instead of wound rotor. In this case the ower chain is the same, as shown in Fig. 3. The main suliers are GE and Siemens.. 4. Hybrid Concet A hybrid concet, with ermanent magnet machine and a gearbox having a low transformation ratio, leads to rotational seeds between 00 and 300 rm and enables the reduction of the generator s size. The aim is to have a smaller generator than the direct drive otions and a better efficiency than DFIG. The suly converter is designed for the full ower (see Fig. 4). The main sulier is Multibrid, electrical active arts are realized by Converteam. 3. Direct Drive Permanent Magnet Generator (DDPMG) Some studies can be found in the literature concerning comarison between the different wind turbine toologies. For the four revious cases we can recall the works of H. Polinder. In [], a comarison of cost and losses are give The results are summarized in Fig. 5. Figure 5: Comarison of wind turbine toologies in terms of cost and energy 3.. Magnets rather than Wound otor The fact that no suly is necessary to create the rotate field is the main advantage of the ermanent magnets. Most of the time, ermanent magnets synchronous machines are called brushless to highlight this asect. With magnets the rotor weight and rotor losses are reduced. In industry, most of ermanent magnet machines have their magnets mounted at the surface of the rotor because there is an exerience background, a mounting rocess which is under control and seems to be the easiest solutio But, when magnets are mounted at the surface of the rotor, defluxing workings are more difficult to obtain because oles are not salient. These modes can be interesting when over seeds or over loads haen in wind turbine alications. 3.. Direct Drive rather than Gearbox Even if gearbox is not the main cause of downtime, risk of failures is still imortant []. The strongest constraint is the necessary time for the relacement of the gearbox when it is out of order. And this time may be long, esecially in offshore alications. Losses for a ermanent magnet generator are less imortant than for an induction one and machine is able to work on a larger wind seed range. Moreover, choosing ermanent magnet generator, instead of DFIG, enables the reduction of the entire nacelle weight [3], limits the number of elements in the conversion chain and so reduces the risk of failures.

3 3. 3. Consequence Focusing on the technology, solution with ermanent magnets seems to be the best, because it leads to lower losses, lower total weight, lower risk of failures, more roduced energy. Therefore, direct drive ermanent magnet generator is the toology which has been selected by the comany to be suggested to the customers. The main ermanent magnets drawback is their cost, which is linked to the increase of the demand of rare earth magnets. Some risks must be taken in consideration too: demagnetisation, short circuit torque, bonding with other magnetic arts during the manufacturing, the use of non-magnetic tools. Active Parts Weight (T) Efficiency (%) Figure 6: Evolution of the weight in function of efficiency. 4. Imrovements In site of its technology benefits, we see that the cost of ermanent magnet direct drive active arts is high. Thus, even if rules are not the same for active arts than for global turbine (i.e. to minimize the active arts cost doesn t necessary lead to minimize the wind turbine cost), we are going to roose some ways to otimise the active arts weight of the generator. For the active arts design of a wind turbine generator, the main oints which have to be taken in consideration are: ower density otimization and losses reductio These two asects do not match because the weight increases when the losses decrease, see Fig. 6, then a comromise must be found. Some solutions can increase the ower density of the surface mounted ermanent magnet machine and/or limit the losses: To reduce the coer with a concentrated winding and a fractional number of slot er ole and er hase [4]; To modify the magnet s form and adat the currents waveforms [5]; To have an axial flux machine toologies instead of a radial[6]; To use an outer rotor toology instead of an inner rotor one, if the diameter is constrained; To increase the hases number. As some cases are ever been described in the literature we do not focus on these solutions. In the rest of the aer only influence of the hase number and the location of the rotor comare to the stator are resented. Nevertheless, in order to clarify the understanding it is necessary to describe the choice of the air ga flux density and currents waveforms. 4.. Air ga flux density In ermanent magnet machines, two shaes can be considered for the air ga flux density waveform (Fig. 7) and the back electromotive forces (back-emf) have similar shaes. For each of these cases, in order to obtain a constant torque, the current waveform in the slots will be different: the current will be sinusoidal when the flux density is sinusoidal whereas the current will be traezoidal (nearly rectangular) when the flux density is traezoidal. Figure 7: Different air ga flux density: Sinusoidal and Traezoidal. As the waveforms are similar to the classical alternative machines, when the waves are sinusoidal, the machine is called BLAC (brushless AC). In the others case, as the rincile is closed to the brushed DC machine, these machines are called BLDC (brushless DC). Two control strategies can be used with rectangular currents to kee the torque constant [7]. In the first case, each hase is oen circuited during a art of the eriod. With 3 hases, a hase must be sulied with a constant current during 0 electrical degrees, the switches command signals are adated, as shown in Fig. 8. In the second case, the hases are always connected so that they are fed with a constant current during 80 degrees as shown in Fig.8. In order to have an air ga flux density as close as a square wave we need to have magnet width close to the ole itch and a number of slots er ole and er hase equals to one. In this case, the air-ga flux density shae is given Fig. 9. (c) Figure 8: Machine converter scheme, 0-degree control and (c) 80-degree control.

4 Figure 9: Air ga flux density when the magnet width is equal to ole shoe and its rms value (dotted lines). To obtain a constant torque, the EMF waveforms should be constant during 80 electrical degrees, which is only a theoretical ossibility. Indeed, due to flux leakages between two magnets, the flux density (and as a consequence the back EMFs) cannot be constant under the ole transitio Because of this, erformances, in case of a 80-Degree, are not so interesting and it should be corrected. 4.. Influence of Phase Number To make a comarison between various machines for equivalent losses, the back-emf eak values must be the same to have the same iron losses, and the current MS values must be the same to have same coer losses. A comarison between the cases reviously described is resented in Table. eal gain is obtained when the difference between back-emf eak and MS value is taken into account, this the correction mentioned in the revious sectio A comarison at equivalent losses can be interreted as a comarison at equivalent active arts weight. Indeed, iron and coer volume are reserved and variations of magnet volume don t have a big influence on the total weight. In detail, to kee the same EMF eak value, in all cases, some geometric arameters can be reserved, as magnet thickness, air ga length and rotor diameter but the magnet width can vary. For the currents, with the same MS value, kee the current density leads to a constant coer volume. As a conclusion, BLDC machines have a better ower density than the BLAC. A drawback of BLDC motor is the difficulty to obtain a constant torque. With a 0-degrees converter it is necessary that the currents are erfectly rectangular and in hase with the back EMFs which must be constant on 0 electric degrees. With 80-degrees this becomes nearly unfeasible, as exlained below. The back EMFs are calculated by using finite elements simulations (FLUXD) and they are lotted in Fig 0 for the examle of a three-hased machine.. Torque is determined considering the currents as erfectly rectangular on 80 electric degrees; the waveforms are given in Fig. for 3 hases. We study now what it haens when the number of hases (i.e. the number of slots er ole in this case) increases. The winding scheme will have to be adated according to the hase number (see Fig.). For a hase number of 7 the back-emf is given in Fig. 3 and the torque in Fig. 4. Case Current Peak/MS EMF Peak/MS Power Power Theoretical gain Table : Comarison of the equations BLDC BLDC BLAC degrees degrees With q hases (q being odd) q q q 3q 3q E P I MS 3 E P IMS With 3 hases q ( q ) E P I MS qe PIMS 6 E P I MS 3 E P IMS.5.4 eal gain.5.3 Voltage (.u.) Torque (.u.) Electrical Angle (Degree) Figure 0: Back-EMFs in the 3 hases machine Electrical Angle (Degree) Figure : Torque with 3 hases Figure : Winding connection under a ole airs when the number of slot by ole and by hase is for 3 hases and for 7 hases

5 .5.0 Voltage (.u.) Figure 3: Back-EMFs in the 7 hases machine Torque (.u.) Figure 4: Torque with 7 hases It aears that the modification of the hase number has an influence on the torque rile which does not seem to be negligible. On the other hand, when the number of hases rises, its influence becomes small as shown in Fig. 5. The torque rile is characterized using (). ΓMAX ΓMIN Γ = () ΓMEAN Moreover the hase number can be limited by the teeth size, which can become too small and unfeasible when the ole number is high. 4.3 otor Location Once again in the aim of increasing the torque density, two structures of surface mounted ermanent magnet machine can be studied. These structures are given in Fig. 6. The difference between these two structures concerns the rotating art (rotor) which is laced with regard to the fixed art (stator). In the first case corresonding to the outer rotor toology, the stator is laced in the center of the rotor. For the inner rotor toology, the rotor is in the center. A comarison between these solutions is made in the next aragrah. Torque ile (%) Phase Number Figure 5: Evolution of the torque rile with the hase number Figure 6: Scheme on a ole air of outer rotor toology (left) and inner rotor toology (right) Table : Secification Power 3 MW Seed 5 rm Pole air 80 Outer stator diameter <5 m Efficiency >94.5 % Air ga >5 mm Active material weight <4 T 5. Comarison of ermanent magnet generators designs Two machines resecting the secifications given in Table are designed. The secifications corresond to a wind turbine alication as one described in [8]. Previous results show that with a 80-degree converter and a traezoidal flux density, the machine will have the best ower density, so that this toology will be used for the designs. To obtain the slightest heavy machine a minimization of the weight is made. To achieve this otimization, an analytical model is necessary to estimate the back-emfs for this tye of machine. This model is described below. 5.. Analytical rediction of back-emf adial comonent of air ga flux density can be determined with () and using (3)-(5) for inner rotor machines. These equations can also be adated to analyze flux density of a machine with outer rotor as in [9]. n. B( r, θ ) = n ( a r + b r )cos( n θ ) () with: a = 3 b = c = 3 + n odd ( + ) ( + ) ( ). c (3) + ( ). c π sin K a B rc ( ( ) ).. π 3 where is the radius at the bottom of ermanent magnets, is the radius at the to of ermanent magnets, 3 is the stator interior radius, K a is the factor between magnet arc and ole shoe and B rc is the corrected remanent flux density exressed in (6) B rc Br µ (4) (5) + µ = (6)

6 B r is the remanent flux density of the ermanent magnet and µ is the relative ermeability of ermanent magnet. It is then ossible to redict the flux density in a slotless machine, as in Fig. 7. But to take into account the slot effect it is necessary to correct the flux density with the relative air ga ermeance, as in (7) B G ( r, θ ) = B( r, θ ) λ( r, θ ) (7) where the relative ermeance λ is given by (8) and (9): π3 0.8wS β ( + cos θ ) for 0 θ 0.8wS 3 λ ( r, θ ) = (8) 0.8wS π3 for θ 3 N S with β = (9) w + S 4 g + h M ( + υ ) µ Exression of υ is given [0], w S reresent the slot oening width, N S is the slot number, g is the air ga length and h M is the magnet thickness. The flux density waveforms taking into account the slot effects are given in Fig. 8. The back-emf rms value is obtained with (0) E = B L Ω G (0) MS G MS where L is the length of active arts, G is the air ga radius and Ω is the angular seed in rad/s. Table 3 gives a comarison between the flux densities obtained with D finite-element simulations and the analytical model. Flux density (T) Figure 7: Air ga flux density without slot effects: Comarison between Finite-element (cross) and analytical Table 3: Comarison between FE and analytical calculations Method FE Analytical Error (%) B MS (T) With 3 hases B G MS (T) E MS (V) With 7 hases B G MS (T) E MS (V) Flux density (T) Flux density (T) Figure 8: Air ga flux density: Comarison between Finite-element (cross) and analytical when the hase number is 3, and 7 Even if an error is made on the MS flux density value when the slots are taking into account, this method gives a good estimation of the back-emf. 5.. Designs comarison The stator external diameter is limited by the size of the imregnating tank, the characteristics of the two designed machines are resented in Table 4. The torque rile is always calculated assuming that the currents are erfectly rectangular. Considering the design constraints of these machines, it aears that the active arts total weight decreases when the rotor is laced outer of the stator. This can be exlained by the fact that the air ga diameter can be bigger when the rotor is out. Therefore only this case will be considered. As it was foreseeable, amlitude of torque rile is high; a design with 7 hases is made in order to limit the torque rile. The main arameters of this design are given in Table 5. Table 4: Comarison between the two designs Parameter Inner rotor Outer rotor Inner diameter (m) Outer diameter (m) Air ga diameter (m) Air ga length (mm) 7 7 Magnet weight (T).3.3 Iron weight (T).6 Coer weight (T) Total active arts weight (T) Efficiency (%) Torque rile (%)

7 Table 5: Design with 7 hases and an outer rotor Phase number 7 Inner diameter (m) 4.8 Outer diameter (m) 5.07 Air ga diameter (m) 5 Air ga (mm) 7 Magnet weight (T).3 Iron weight (T). Coer weight (T) 3.8 Total active arts weight (T) 7. Efficiency (%) 94.7 Torque rile (%) 4.5 By keeing the same efficiency, increasing the hase number leads to a decrease of the torque rile; it also aears that the weight decreases. Therefore this increase seems to corresond to an increase of the machine ower density too. All designs made are quite under the secified weight, which can be found in [8], for the same efficiency. In the aer mentioned, the generator is a conventional BLAC 3 hases machine. Difference on the weights agrees with the ower gain given in Table, nevertheless this ga must be weighted by the fact that the diameter, the airga are different. 6. Comments When the number of hases increases, using a 80- degrees or a (q-)/q*80-degrees converter leads to fulfill the machine erformances. But without a 80- degrees converter, torque rile amlitude becomes greater. In fact, without other imrovement, a 0- degrees converter ermits to limit the torque rile with 3 hases but if the number of hases is greater, a 80 degrees converter is referable as shown in Fig Conclusion For a high ower wind turbine alication, direct drive ermanent magnet generator seems to be the best technology: lower weight, lower losses and less failure risks. Nevertheless, it has some drawbacks, generator cost and size. Few imrovements can be done concerning the size. In fact, less the seed is, higher the diameter should be; thus some restrictions can aear as the transort caabilities, but solutions are ossible to increase the ower density and limit the weight, and so the cost, of the active arts. In the aim of increasing the machine ower density, some solutions are roosed, and two oints have been investigated. The first is the hase number; indeed in some cases the choice of the hase number can bring some advantages. But, this modification leads to increase the difficulty of the machine manufacturing and control. To achieve the control, models with fictitious machine could be use as roosed in []. The second oint deals with the rotor osition resect to the stator. When the overall volume is limited, the use of an outer rotor is a solution to reduce the weight of the machine s active arts. But, the imact on the non active arts has to be studied carefully. Torque ile (% ) Phase number Figure 9: Evolution of the rile torque with the hase number with a 80-degree converter (dotted line) and a (q-)/q*80-degree converter. Other solutions can be found in the literature, with multi generators [] or where magnets are not mounted at rotor surface [3]. Because of the manufacturing constraints, which are given reviously, these solutions are not studied by the comany. On the other hand, for the future wind turbine generators, Converteam develos a High Temerature Suerconductors (HTS) machine [4]. eferences: [] D. J. Bang, H. Polinder, G. Shrestha, and J. A. Ferreira: Promising direct-drive generator system for large wind turbines. EPE Journal, Vol. 8, No. 3,. 7-3, 008. [] J. Puigcorde and A. De-Baumont: Wind turbine gearbox reliability, 00, renewable energy world, htt:// cle/00/06/wind-turbine-gearbox-reliability [3] P. Fairley: Wind turbines shed their gears both siemens and GE bet on direct-drive generators, 00, Technology ublished by MIT review, htt:// e/ [4] A. M. El-efaie: Fractionnal-slot concentratedwinding synchronous ermanent magnet machines: oortunities and challenges, IEEE, Transactions on industrial electronics, vol. 57, no, 07-, 00. [5] T. M. Jahns and W. L. Soong: Pulsating torque minimization techniques for ermanent magnet AC motor drives-a review, IEEE, Transactions on industrial electronics, vol. 43, no, 996. [6] N. Balkan Sirnsir, H. Biilent Ertm: A comarison of torque caability of axial flux and radial flux tye of brushless DC (BLDC) drives for wide seed range alications, IEEE, International conference on Power Electronics and Drives Systems, PEDS 99, 999.

8 [7] H. Qiang, N. Samoylenko and J. Jatskevich, Comarison of brushless DC motor drives with 80/0-degree inverter systems, IEEE Canadian conference on electrical and comuter engineering, Vancouver, 007. [8] H. Polinder, F.F.A. Pijl, G-J. Vilder and P. Tavner, Comarison of direct-drive and geared generator concets for wind turbines, IEEE, Transactions on energy conversion, vol., , 006. [9] C. Esanet, Modélisation et concetion otimale de moteurs sans balais à structure inverse alication au moteur-roue, PhD Dissertation, University of Franche Comte, 999. [0] Z.Q. Zhu and D. Howe, Instantaneous magnetic field distribution in brushless ermanent magnet DC motors, Part III: effect of stator slotting, IEEE, Transactions on magnetics, vol. 9, no, 993. [] F. Scuiller, Déveloement d outils de concetion de machine olyhasées à aimants utilisant l aroche multimachine, PhD Dissertation, ENSAM, 006. []The Liberty.5 MW Wind Turbine: Clier Design: htt:// f [3] J. Zhang, Z. Chen and M. Cheng: Design and comarison of a novel stator interior ermanent magnet generator for direct-drive wind turbines, IET, enew, Power Gener. vol I, no 4, 03-0, 007. [4] C. Lewis and J. Muller: A Direct Drive Wind Turbine HTS Generator, Power Engineering Society General Meeting, IEEE, 007.