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Design and optimisation of direct-driven PM variable-flux synchronous generators for directly grid-connected slip-synchronous wind-turbines

Amuhaya, Lilian Livutse (2017-12)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: This dissertation presents design, optimisation and prototype tests of radial-flux, low-speed, permanent magnet (PM) and copper excited, synchronous generators with a new topology. The designed generators are for use in direct-drive directly grid-connected slip-synchronous wind-turbine systems. For direct-drive systems, (PM) generators deployment is more attractive due to the convenience of high-energy permanent magnet types for example NdFeB.Thus, replacing conventional PM synchronous generators with PM variable flux synchronous generators (PM VFSGs) has recently gained great interest. PM VFSG ( have some rotor windings, which result in a flux variation capability and therefore they give better performance in grid compliance than purely PM excited SGs. In addition, they offer better efficiency and reduced mass when compared to purely electrically excited synchronous generators (EESGs). Using analytical and FEM analysis, the PM VFSG design with non-overlap concentrated windings was carried out applying three optimisation algorithms, i.e. modified method of feasible direction (MMFD), non-sorted genetic algorithm (NSGA II) and particle swarm optimisation (PSO). Using simplified equivalent magnetic circuit and PM properties, an initial iterative preliminary design for the proposed generator was carried out. The rotor field coil analysis was performed using FE analysis in MagNet solver to numerically evaluate four generator designs from a benchmarked surface-PM VFSG calculated analytically. The aim of the electromagnetic analysis is to satisfy a grid compliance design, where flux variation is necessary. The rotor design values changes depending on the amount of magneto-motive force (MMF) required, supplied by magnets and rotor field coils. From the analysed rotor designs, it was found that the rotor slot area is a key constraining factor. Another key outcome of the study is that the location of the PMs on/in the rotor tooth influences the flux linked to the stator . The rotor design corresponding to the lowest rotor copper losses was adopted. The optimisation procedure uses d􀀀q analysis at steady state and no-load, to investigate the size, efficiency and active mass of 13 kW, 100 kW and 1 MW generators with a base winding of 34/36. The buried-PM topology is found to achieve the least mass and highest efficiency. The outer diameters of the optimised generators have marginal difference with other conventional systems. Compared with other direct drive generator systems, the proposed generator has a larger air-gap diameter and heavier rotor but with comparable total active mass. The prototype generators were constructed using rare-earth permanent magnets NdFeB- 48H with a remanent flux density of 1.41 T. The rotor and stator lamination cores were laser cut but can be punched in mass production. Open-slots with straight teeth were adopted for the stator with double-layer non-overlap concentrated windings. The short end-winding characteristic of the winding topology enhanced efficiency and lower active mass. Rectangular blocks of permanent magnets with flux in radial direction are used. The surface-PM VFSG topology has the PMs segmented and glued on the rotor tooth surface. On the other hand, the buried-PM VFSG design has full rectangular PMs inserted into the rotor cores. The constructed prototype generator was tested as a three-phase fixed-lowspeed generator with varied load conditions under grid compliance. Good correlation in the theoretical prediction and the experimental results were obtained.

AFRIKAANSE OPSOMMING: Hierdie proefskrif bied ontwerp-, optimalisasie en prototipe toetse van radiale flux, lae spoed, permanente magneet (PM) en koper opgewekte, sinkroniese kragopwekkers met ’n nuwe topologie. Die ontwerpte kragopwekkers is vir gebruik in direkte dryfkrag-verbindings glipsinchroniese windturbinesisteme. Vir direkte dryfstelsels is die implementering van kragopwekkers aantrekliker as gevolg van die gemak van hoë-energie-permanente magneet tipes, byvoorbeeld NdFeB. By die vervanging van konvensionele PM-sinkroniese kragopwekkers met PM-veranderlike flux -sinchroniese kragopwekkers (PM VFSGs) het onlangs groot vertoning gekry. belang. PM VFSG (het ’n paar rotor windinge, wat ’n flux- variasievermoë tot gevolg het en gevolglik gee hulle beter prestasie in rooster-nakoming as suiwer PMopgewonde SG’s. Daarbenewens bied hulle beter doeltreffendheid en verminderde massa in vergelyking met suiwer elektries opgewekte sinkroniese kragopwekkers (EESG’s ). Met behulp van analitiese en FEM-analise is die PM VFSG-ontwerp met nie-oorvleuelgekonsentreerde windings uitgevoer met behulp van drie optimaliserings algoritmes, dws gewysigde metode van uitvoerbare rigting (MMFD), nie-gesorteer genetiese algoritme (NSGA II) en partikel swerm optimalisering (PSO) . Deur gebruik te maak van vereenvoudigde ekwivalente magnetiese stroombaan- en PM-eienskappe, is ’n aanvanklike iteratiewe voorlopige ontwerp vir die voorgestelde generator uitgevoer. Die rotor voedselanalise is uitgevoer met behulp van FE-analise in MagNet-solver om vier generatorontwerpe numeries te evalueer van ’n gekontroleerde oppervlak-PM VFSG wat analities bereken is. Die doel van die elektromagnetiese analise is om ’n rooster nakomingsontwerp te bevredig, waar flux-variasie nodig is. Die rotor ontwerpwaardes verander, afhangende van die hoeveelheid magneto-motiewe krag (MMF) wat benodig word, voorsien deur magnete en rotor veldspoele. Uit die ontleed rotor ontwerpe is gevind dat die rotor gleuf area ’n sleutelbeperkende faktor is. Nog ’n belangrike uitkoms van die studie is dat die ligging van die PM’s op / in die rotor tand die vloei wat aan die stator gekoppel is, beïnvloed. Die rotor ontwerp wat ooreenstem met die laagste rotor koper verliese is aangeneem. Die optimaliserings prosedure gebruik d-q- analise teen bestendige toestand en geen vrag, om die grootte, doeltreffendheid en aktiewe massa van 13 kW, 100 kW en 1 MW kragopwekkers met ’n basiswinding van 34/36 te ondersoek. Die begrawe-PM-topologie is gevind om die minste massa en hoogste doeltreffendheid te behaal. Die buitenste diameters van die geoptimaliseerde kragopwekkers het marginale verskil met ander konvensionele stelsels. In vergelyking met ander direkte dryfgeneratorstelsels het die voorgestelde kragopwekker ’n groter luggaping deursnee en swaarder rotor, maar met vergelykbare totale aktiewe massa. Die prototipe kragopwekkers is gebou met behulp van seldsame permanente magnete NdFeB-48H met ’n remanent vloeistofdigtheid van 1,41 T. Die rotor- en stator laminerings kern was laser gesny, maar kan in massaproduksie gepons word. Oop gleuwe met reguit tande is aangeneem vir die stator met dubbel laags nie-oorvleuel gekonsentreerde windings. Die kort einde-kronkelende eienskap van die kronkel topologie het doeltreffendheid en laer aktiewe massa verbeter. Reghoekige blokke van permanente magnete met vloed in radiale rigting word gebruik. Die oppervlak-PM VFSG topologie het die PM’s gesegmenteer en vasgeplak op die rotor tand oppervlak. Aan die ander kant het die begraafde PM VFSGontwerp volle reghoekige PM’s in die rotor kern. Die vervaardigde prototipe kragopwekker is getoets as ’n drie-fase vaste lae spoed generator met gevarieerde laai voorwaardes onder rooster nakoming. Goeie korrelasie in die teoretiese voorspelling en die eksperimentele resultate is verkry.

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