Jacek F. GIERAS University of Technology and Life Sciences, Bydgoszcz, oland M Generator for Novel Architecture of Wind Turbines Abstract. The paper deals with a peranent agnet (M) generator for novel architecture of wind turbines. The proposed wind turbine consists of a nuber of integrated lower-poer individual generators. The individual generators can be arranged in a rectangular atrix, hexagon or in a circle. The paper discusses sizing procedure for this type of generators, electroagnetic calculations including the finite eleent ethod (FEM) siulation, perforance characteristics and advantages of this type of wind generator. As an exaple, a M brushless generator with concentrated non-overlap stator coils has been considered. Streszczenie. Artykul dotyczy pradnicy bezszczotkowej o agnesach trwalych do nowej architektury turbin wiatrowych. roponowana turbina wiatrowa sklada sie z pewnej liczby zintegrowanych jednostek o niejszej ocy. oszczegolne pradnice oga byc zestawione w atryce prostokatna, hexagon lub wpisane w okrag. W artykule oowiono procedure wyiarowania tego typu pradnic, obliczenia elektroagnetyczne, w ty rowniez za pooca etody eleentow skonczonych (MES), charakterystyki pracy oraz zalety. Jako przyklad, zostala rozwazona pradnica bezszczotkowa o agnesach trwalych z uzwojenie stojana o cewkach o paraetrach skupionych i rozpietosci jednej podzialki zlobkowej. Keywords: wind turbine, novel architecture, peranent agnet generator, electroagnetic calculations, perforance characteristics. Słowa kluczowe: turbine wiatrowa, nowa architektura, pradnica o agnesach trwalych, obliczenia elektroagnetycznne, paraetry. Introduction Wind turbine generators convert wind energy into electrical energy with the aid of echanical rotation of a bladed rotor hub. In the past, ost wind turbines have utilized heavy gearboxes to convert slow, powerful rotor hub rotation into uch faster rotation of a driveshaft connected echanically to the electrical generator. Gearboxes typically require regular aintenance (lubrication), and account for substantial energy losses, thus reducing the overall efficiency of the wind turbine syste. Many newer wind turbines instead utilize direct-drive generators [4] that eschew gearboxes in favour of a large-diaeter generator rotor attached directly to the turbine rotor hub. To achieve the necessary rotor linear speeds, directdrive generator rotors ay be several eters in diaeter. Although direct-drive generators avoid any of the challenges associated with gearbox-driven generators, the extreely large diaeter of ost direct-drive generator rotors adds significant weight and cost to direct-drive wind turbines [1,4]. In addition, the air gap of a large diaeter generator ust typically be increased to allow for proportionally greater radial translation due to rotor hub deflection. Increased air gap deteriorates the generator perforance. Most direct drive generators include heavy support structures designed to iniize the rotor hub deflection. This additional ass contributes significantly to the total aterial, production, and assebly costs of the wind turbine. The advantages of distributed generation fro renewable power sources (such as abient wind) have long been recognized. Despite soe of the challenges associated with power distribution, control and delivery, any successful recent studies have deonstrated the viability of the approach [4,5,6]. This work deonstrates the technical viability of the new concept of distributed wind energy power generation by integration of saller units into large wind turbine syste. Technical approach The proposed distributed wind generator can consist of any nuber of integrated saller individual generators (Fig. 1). Fig. a shows 19 integrated generators arranged hexagonally and Fig. 3b shows also 19 generators arranged circularly. The individual generator units can also be assebled to for a rectangle with rows and n coluns creating a atrix of n generators. The power of a single integrated generator can typically range fro 0 to 50 kw. Using, say, 0 generator units, the total power of the windill is 0.4 to 5.0 MW. For exaple, an electric generator rated at 5 kw has the stator core outer diaeter of about 0.45, and the stack length of about 0.3. Other power ratings, sizes and nubers of individual generator units are, of course, also possible. Fig.1. Construction of a single integrated odule of wind turbine generator: 1 propeller, propeller hub, 3 ri, 4 bushing, 5 electric generator with built-in rectifier, 6 terinal board. Fig.. Novel architecture of electric generators showing an arrangeent of 19 units (odules): (a) hexagonal; (b) circular. Sizing procedure The axiu value of the Betz liit [5] for wind turbines is c p 0.593. A ore practical value including frictional losses is c p = 0.4. For a total power of a wind turbine t = 500 kw, the nuber of individual generator N t = 19, electric efficiency (generator and power electronics converter) η el = 0.885, Betz liit c p = 0.4, incoing air wind speed v 15 /s,
rotational speed of the rotor n r = 13 rp, air density, ρ air = 1. kg/ 3, and nuber of blades n b = 4, the electric output power el, echanical (shaft) power, rotor swept area, A, and diaeter d of rotor blades of an individual generator are, respectively: 500 = 19 6.315 = = 9.735 0.885 t (1) = 6.316 kw el = N t el () = kw η el (3) 9735 A = = = 36.11 3 3 0.5c ρ v 0.5 0.4 1. 13 p air A π 36.11 π (4) d = = = 6.78 The rotational speed of the rotor n r = 13/60 =. rev/s and the tip speed of the rotor (5) v tip d 6.78 = π nr = π. = 36.86 s The rotor tip speed is well below the 80 /s echanical integrity liit [5]. Also, the tip speed ratio v tip 36.86 15 (6) TSR = = 3. 14 = v is very close to the optial tip speed ratio [5], i.e., 4π n 4π 4 (7) λ = = = 3.14 TSR opt b The inner diaeter D 1in of the stator of the electric generator and its stack length L are linked with the electric output power el with the following equation [,3]: π 1 (8) 3 el = nrkw 1D1in LJ axbga pk fill( k y 1) ηel cosφ 6 ε Assuing the stator stack inner diaeter D 1in = 0.3544, stator stack length L = 0.31, EMF-to-voltage ratio ε = 1.08, winding factor k w1 = 0.945, axiu current density in the stator winding J ax =.3 10 6 A/, air gap agnetic flux density B g = 0.7 T, nuber of the stator winding parallel paths a p =, stator slot fill factor k fill = 0.444, stator yoke diaeter-to-inner diaeter k y =1.93, generator efficiency η g = 0.934 and power factor cosϕ = 0.95, the electric power calculated on the basis of eqn (8) is el = 6.4 kw, alost the sae as that given by eqn (1). The results given by eqns (1) to (8) can then be used in the electroagnetic calculations of the electric generator. Electroagnetic design eranent agnet (M) brushless generator has been selected as the ost efficient and copact electrical achine [1]. The electroagnetic calculations have been perfored using both analytical ethod and the finite eleent ethod (FEM) analysis. Table 1 shows the geoetry and diensions of the agnetic circuit, Table shows the stator winding paraeters and Table 3 shows the rated (noinal) paraeters of the M brushless generator for novel architecture of the wind turbine. The stator and rotor core have been stacked with M19 silicon steel lainations. The rotor field excitation syste consists of 60 pieces (p = 60) of Vacody 510 NdFeB Ms (Vacuuschelze, Hanau, Gerany) with reanent agnetic flux density B r = 1.38 T and coercivity H c = 955 ka/ at 0 o C abient teperature. The cost of Ms is significant and estiated as approxiately US$3170 (US$10 per 1 kg of sintered NdFeB). Table 1. Geoetry, agnetic circuit diensions and ass of active coponents. Nuber of stator slots 54 Nuber of poles 60 Stator outer diaeter, 468.0 Stator inner diaeter, 354.4 Rotor outer diaeter, 35.0 Rotor inner diaeter 30.0 Radial thickness of air gap (clearance), 1. Length of the stator and rotor stack, 310.0 Stator slot depth, 5.0 Stator slot opening,.0 Width of stator tooth, 8.0 Radial thickness of stator yoke, 4.8 Radial thickness of rotor yoke, 6.0 M radial thickness, 10.0 Circuferential width of M, 17.4 Mass of copper winding, kg 53.73 Mass of stator core, kg 7.85 Mass of rotor core, kg 14.5 Mass of Ms, kg 6.4 Total ass of active coponents, kg 167.3 Rotor oent of inertia excluding shaft, kg 0.3787 Table. Stator winding paraeters Nuber of phases 3 Connection Wye Nuber of turns per coil 18 Nuber of turns per phase 16 Nuber of layers in slots Nuber of strands in hand 18 Nuber of parallel paths Coil span (throw) 1 slot Winding factor for fundaental haronic 0.945 Conductor AWG 0 Conductor diaeter, 0.818 Stator slot fill factor 0.4439 Resistance per phase at 0 o C, Ω 0.0995 Resistance per phase at 100 o C (hot achine), Ω 0.1308 d-axis synchronous inductance, H 5.0686 q-axis synchronous inductance, H 5.503 Table 3. Rated (noinal) paraeters of M brushless generator. AC line-to-line voltage, V 400 Rotational speed, rp 13 Frequency of stator (arature) current, Hz 66 Load angle δ, elec. degree 5 Shaft torque, N 113.8 Shaft echanical power, kw 9. Electrical output power, kw 7.3 Electroagnetic (air gap) power, kw 8.0 Generator efficiency, % 93.44 Solid state converter efficiency, % 94.71 ower factor cosφ 0.9 Stator d-axis current, A 1.3 Stator q-axis current, A 4.86 Stator current, A rs 4.88 Stator current density. A/.3 EMF constant, Vs/rad 38.193 In the design calculation, the following teperatures have been assued: abient teperature = 0 o C, winding teperature = 100 o C and M teperature = 60 o C. The stator winding has been designed as fractional slot per pole per phase winding called also non-overlap concentrated coil winding. For this type of winding [1] N gcd( N c = (9) 1 c, p) k
where N c = 54 = total nuber of coils in three-phase winding, p = 60 = nuber of poles, k = 1,,3,.., and gcd is the greatest coon divisor of N c and p. For this achine gcd (54,60) = 6 and k 1 = 9, so the stator winding is feasible. The stator overhangs are very short, i.e., the total axial length of the stator with end turns is approxiately 36. The ean length of turn is 666. The 3D iage of the generator is shown in Fig. 3 and the stator winding diagra in Fig 4. Fig. 5. Magnetic flux lines and agnetic flux density distribution in a 10-pole segent of the generator as obtained fro the D FEM Fig. 3. Coputer generated 3D iage of the M brushless generator. Only active coponents, i.e., the stator core, rotor core and Ms have been shown. erforance characteristics The steady-state perforance characteristics as obtained fro the electroagnetic calculations are shown in Figs 6 to 11. Fig.6. Output electrical power and rs current versus speed at constant load angle δ = 5 o and line-to-line voltage 400 V Fig. 4. Three-phase winding consisting of non-overlap coils with one slot coil span (54 slots, 3/10 slots per pole per phase). To reduce the winding losses and increase the efficiency, a low value of the stator current density.3 A/ has been chosen (Table 3). The electric efficiency η el = 0.885 is the product of the generator efficiency and solid state converter efficiency (Table 3). The agnetic flux lines and agnetic flux density distribution in the cross section of the generator as obtained fro the D FEM is shown in Fig. 5. The open-circuit air gap noral coponent of the agnetic flux density averaged over one pole pitch is 0.758 T. The sae agnetic flux density obtained fro the D FEM is 0.803 T. The agnetic flux density in the stator teeth does not exceed.1 T, while the agnetic flux density in the stator yoke is about 1.8 T. Fig. 7. Stator winding current density versus speed at constant load load angle δ = 5 o and line-to-line voltage 400 V
Fig. 11. EMFs induced in arature windings at 400 V terinal voltage and rated load. Fig. 8. Efficiency and power factor cosϕ versus speed at constant load load angle δ = 5 o and line-to-line voltage 400 V Fig. 1. Coparison of noral coponents of air gap agnetic flux density distribution obtained fro analytical calculations and D FEM siulation. Fig. 9. Shaft torque versus speed at constant load load angle δ = 5 o and line-to-line voltage 400 V The generator can deliver the rated (noinal) power when its rotational speed is iniu 110 rp (Fig. 6). If the speed is below 90 rp the generator should be disconnected or operate with reduced load, because at speed lower than 90 rp the current density takes high values (Fig. 7). The efficiency exceeds 90% when the rotational speed is greater than 90 rp (Fig. 8). The higher the rotational speed, the lower propeller torque is required (Fig. 9). The total losses (stator winding, core and windage losses) are about kw at rated load. Fig. 13. Coparison of shaft torque versus load angle calculated analytically and with the aid of D FEM (13 rp, 400 V). Analytical calculations have been verified with the D FEM (Figs 1 and 13). The shaft torque versus load angle δ obtained fro analytical calculations is slightly higher than that obtained fro the D FEM. Fig. 10. Arature winding phase currents at 400 V terinal voltage and rated load Arature winding phase currents for generator operating without rectifier are sinusoidal (Fig. 10). EMFs induced in arature windings are balanced, but contain the 5 th and 7 th haronics (Fig. 11). Electrical connection and rectifier operation An exeplary electrical connection of distributed wind generators to power distribution network is shown in Fig. 13. Individuals generators (odules) are integrated with propellers and solid state rectifiers. To avoid probles with synchronization, the DC output terinals of rectifiers are connected in parallel on a DC bus bars. The AC distribution network can be either a 3-phase or single-phase network. To keep constant output frequency, the solid state inverter 5 (Fig. 14) ust be a variable voltage variable frequency (VVVF) inverter.
Fig. 17. Line-to-line voltage at arature winding terinals and lineto-line EMF at rectifier operation (18.8 kw DC output power, 400 V DC, 47. A rs rectifier current). Fig. 14. Electrical connection of distributed wind generator to the electric power distribution network: 1,,3, n plurality of wind electric generators integrated with propellers and rectifiers, 4 DC bus bars, 5 VVVF solid state inverter, 6 3-phase power distribution network. Fig. 15. hase currents at rectifier operation (18.8 kw DC output power, 400 V DC, 47. A rs rectifier current). ` Fig. 16. Rectifier current 47. A rs at 18.8 kw DC output power, 400 V DC. hase current, rectifier current and voltage wavefors at rectifier operation are plotted in Figs. 14, 15, and 16. All diagras refer to a single generator unit integrated with a solid state passive rectifier. Conclusions An innovative concept of a novel wind power generation syste has been deonstrated. This syste consists of a certain nuber of saller, identically-sized individual wind generator units, each based on the M brushless achine technology. The distributed odular design allows to anufacture only one coon size, one rating generator unit and then to assebly a wind generation syste of any required electrical output power. In addition, unlike conventional single-rotor wind generators, there is an increased reliability and redundancy of the proposed wind generation syste, i.e., if one or ore units fail, the wind generator still will be operating and generating electricity. Saller generator units produce lower acoustic eissions due to their lower individual noise level. The brushless M generators allow sooth operation allowing proper ridethrough control during interittent power transients. Finally, the odular design of the individual wind generators leads to reduced anufacturing costs due to the unification of coon parts. REFERENCES [1] Gieras, J. F., eranent Magnet Motor Technology: Design and Applications, 3rd edition, Taylor & Francis, Boca Raton London New York, 01. [] Gieras, J.G., New Applications of Synchronous Generators, rzeglad Elektrotechniczny (Electrical Review), vol. 88, No 9a, 01, 150-157. [3] Gieras, J.F., Multiegawatt Synchronous Generators for Airborne Applications: a Review, IEMDC 013, Chicago, IL, USA, 013, 653-660. [4] olinder, H., van der ijl, F. F. A., de Vilder, G.- J., Tavner,. J., Coparison of Direct-Drive and Geared Generator Concepts for Wind Turbines, IEEE Transactions on Energy Conversion, Vol. 1, No. 3, Sep. 006, 75-733, [5] Ragheb, M., Ragheb, A. M., Wind Turbines Theory The Betz Equation and Optial Rotor Tip Sped Ratio, Chpt. in Fundaental and Advanced Topics in Wind ower, Ed. by Rupp Carriveau, InTech, Jul. 5 011, DOI: 10.577/731, 19-38. [6] Sobczyk, T.J, Mazgaj, W., Szular, Z, W egiel, T., Energy Conversion in Sall Water lants with Variable Speed M Generator, Archives of Elec. Eng., vol. 60, No, 011, 159-168. Author: rof. Jacek F. Gieras, hd, DSc, IEEE Fellow, University of Technology and Life Sciences, Departent of Electrical Engineering, Al. S. Kaliskiego 7, 85-796 Bydgoszcz, oland, E- ail: jacek.gieras@utp.edu.pl