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Practical implementation of long-horizon direct model predictive control

Dorfling, Martinus David (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The use of model predictive control in power electronics has increased significantly in recent years. More specifically, the so-called direct model predictive control methods are primarily considered for power electronic converters due to their switching nature. In direct control methods the output of the controller directly manipulates the converter inputs, which are restricted to integers, without the use of a modulator. However, predominantly, only a short horizon of one prediction step is considered. This can be attributed to two reasons. Firstly, it has been previously regarded that longer horizons do not provide any performance benefits in power electronics. Secondly, the computational burden associated with prediction horizon increases exponentially, discouraging practical consideration. Recently it was shown that the stigma that longer horizons do not provide performance benefits is false, and that long horizons do indeed increase the harmonic performance of a converter. In fact, if the prediction horizon is long enough, model predictive control can compete with the highly regarded optimised pulse patterns in terms of harmonic distortion. Furthermore, it was shown that the optimization problem of direct model predictive control with long horizons can be reformulated as an integer least-squares. A branch-and-bound method, known as sphere decoding, can solve the reformulated optimization problem in a time-efficient manner, enabling practical considerations. The primary contribution of this thesis is the practical implementation of long-horizon direct model predictive control. A detailed description of the implementation of the controller within a field programmable gate-array is given. It is shown that, for almost 90% of the cases, only 8:4 μs are required to calculate the optimal inputs for a three-phase neutral-point-clamped inverter when using a prediction horizon of 5 with a sampling interval of 25 μs. Continuing on the practical implementation of long-horizon direct model predictive control, experimental results are captured and analysed for prediction horizons one to five. The claim that longer horizons do provide a performance increase is validated through experimental results. A decrease of roughly 8:5% in total total harmonic distortion at a switching frequency of 250 Hz is achieved when adopting a prediction horizon of five instead of one. The secondary contribution of this thesis is the proposal of a method to selectively suppress selected harmonics. The formulation of the method is explained, and simulations are used to verify the suppression of harmonics.

AFRIKAANSE OPSOMMING: Die toepassing van modelvoorspellendebeheer vir drywsingselektronika het aansienlik toegeneem in die afgelope paar jaar. Die sogenaamde direkte voorspellende beheer tegnieke is veral van toepassing tot die veld van drywingselektronika as gevolg van die skakelnatuur van die toerusting. ’n Modulator is afwesig wanneer direkte beheer metodes gebruik word, omdat die beheersein direk aan die intree van die omsetter gekoppel word. Die beheersein is daarom beperk tot heelgetalle. Oor die algemeen word ’n kort voorspellings horison van een gebruik. Daar is hoofsaaklik twee redes hiervoor. Eerstens, in die verlede was daar verneem dat langer horison geen voordelige bydra tot die optrede van ’n drywsingelektroniese toestel in hou nie. Tweedens, berekeninge wat verband hou met die voorspellings horison verhoog eksponensieel en ontmoedig daarom die gebruik daarvan. Dit was redelik onlangs bewys dat langer horisonne wel ’n beduidende positiewe bydrae tot die werking van drywingselektroniese toestelle maak. Indien die horison lank genoeg is, kan voorspellende beheer redelik goed kompeteer met die hoog aangeskrewe optimalepulspatrone. Verder was dit bewys dat die direkte-voorspellendebeheer optimeringsprobleem herformuleer kan word as ’n heelgetal-kwadratiese optimeringsprobleem. ’n Metode wat bekend staan as sfeerdekodering kan gebruik word om die herformuleerde optimeringsprobleem effektief op te los, wat die praktiese gebruik daarvan bemoedig. Die primêre bydrae van die tesis is die praktiese implementering van direkte modelvoorspellendebeheer met lang horisonne. ’n Gedetailleerde beskrywing om die beheerder binne ’n veldprogrammeerbare hekskikking te implementeer word aangebied. Dit word bewys dat in byna 90% van die gevalle slegs 8:4 μs benodig word om die optimale intree te bereken vir ’n driefase neutralepuntgeklampte omsetter, wanneer ’n voorspellings horison van vyf gebruik word met ’n monsterperiode van 25 μs. Verder word praktiese resultate aangebied vir voorspellendehorisonne vanaf een tot vyf. Dit word bewys dat langer horisonne verbetering rakende die harmoniese gedrag van drywingselektroniese toestelle teweeg bring. ’n Afname van 8:5% in stroomharmoniesedistorsie teen ’n skakelfrekwensie van 250 Hz word waargeneem, wanneer ’n voorspellendehorison van vyf in plaas van een gebruik word. Die sekondêre bydrae van die tesis is die bekendstelling van ’n metode om harmonieke selektief te onderdruk. Die formulering van die metode word verduidelik en simulasies word uitgevoer om te bevestig dat harmoieke wel onderdruk word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103578
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