Doctoral Degrees (Electrical and Electronic Engineering)

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by Author "Amuhaya, Lilian Livutse"

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    Design and optimisation of direct-driven PM variable-flux synchronous generators for directly grid-connected slip-synchronous wind-turbines
    (Stellenbosch : Stellenbosch University, 2017-12) Amuhaya, Lilian Livutse; Kamper, M. J.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    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.