Model predictive control of a grid-connected converter With LCL-filter

Geldenhuys, Joanie Michellene Claudette (2018-03)

Thesis (MScEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Improved efficiency of power conversion is high priority when it comes to renewable energy applications. Model predictive control (MPC) can be used to optimise the switching pattern applied in the converter to reduce switching losses. During this research the suitability of a direct MPC strategy with long horisons is evaluated for current control of a three-phase two-level grid-tied converter with LCL-filter. A cost function was formulated to include two weighted control objectives, namely reducing the reference tracking error and switching frequency. The grid voltage was incorporated into the state-space model as an additional input vector. Therefore the optimisation approach, used to transform the cost-function minimisation problem towards the integer least-squares (ILS) problem, had to be reworked to consider this additional element, hereby extending on previous work done on the integer quadratic programming formulation for long horisons. A sphere decoding algorithm was incorporated to reduce the computational burden. The current controller shows fast transient response and good reference tracking of the fundamental 50 Hz component. The developed strategy complies to the grid-side current harmonic limits set out in the South African grid code at high switching frequencies, but is unable to comply to the even and high-order harmonic limits at low switching frequencies.

AFRIKAANSE OPSOMMING: Verbeterde doeltreffendheid van kragomsetting is ’n hoë prioriteit wat betref hernubare energie toepassings. Model voorspellende beheer (MVB) kan gebruik word om die skakelpatroon wat in die omsetter toegepas is te optimeer om skakelverliese te verminder. Gedurende hierdie navorsing is die geskiktheid van direkte MVB met lang horisonne ge-evalueer vir die beheer van ’n drie-fase, twee-vlak, netwerkgekoppelde omsetter met ’n LCL-filter. ’n Kostefunksie is geformuleer om twee beheermikpunte, naamlik die vermindering van die stroomvolgfout en skakelfrekwensie in te sluit. Die netwerkspanning is by die toestandsruimte model as ’n bykomende intreevektor inkorporeer. Hierdie bykomende element het gemaak dat die optimeringsbenadering wat gebruik is om die kostefunksie minimeringsprobleem tot die kleinste vierkant probleem te transformer, gewysig moes word. Dit is ’n voortsetting van vorige werk oor heeltallige kwadratiese programmeringsformulering vir lang horison toepassings. ’n Algoritme vir ’n sfeer dekodeerder is geïmplementeer om die berekeningslas te verminder. Die stroombeheerder wys vinnige oorgang en bied akkurate stoomvolging van die 50Hz fundamentele frekwensie komponent. Die ontwikkelde strategie voldoen teen hoë skakelfrekwensies aan netwerkstroom harmoniekbeperkings soos in die Suid-Afrikaanse netwerkkode uiteengesit, maar kan nie teen lae frekwensies voldoen aan die ewe en hoë orde harmoniekbeperkinge nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103723
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