Model predictive control of a medium-voltage grid-connected converter with LC filter using optimal pulse patterns with relaxed symmetry

Abstract

ENGLISH ABSTRACT: Medium-voltage converters are operated at very low switching frequencies. To minimise the resulting harmonic distortions, offline computed optimised pulse patterns are the modulation method of choice. The optimal switching angles, at which a converter phase switches, are obtained by solving a constrained optimisation problem. To simplify the analysis and computation, quarter- and half-wave symmetry of the switching function is typically imposed. Moreover, for three-level converters, the switch positions are required to be non-negative in the positive half-wave of the fundamental period. The impact of these restrictions on the harmonic performance of the pulse patterns is investigated in the first part of this thesis. By relaxing the symmetry restrictions and by allowing also negative switch positions in the positive half-wave of the fundamental period, the current distortions can be reduced by up to one third compared to the traditional problem formulation. Commonly used linear control techniques require zero current ripple at regularly-spaced sampling instants. For optimised pulse patterns, in general, the current ripple is never zero in all three phases. Moreover, the switching angles may be discontinuous when varying the modulation index. To address these issues, controllers based on trajectory tracking were developed, most notably model predictive pulse pattern control. By treating the fundamental and harmonic components as one single quantity, it achieves superior disturbance rejection and very short settling times during transients. However, only a single (vector valued) variable is controlled with this method, such as the stator flux linkage of a machine or the converter current of a grid-connected converter. To address higher-order systems, such as converters with LC filters, an additional damping loop is required limiting its performance during transients. In the second part of this thesis, a generalised model predictive pulse pattern controller is proposed. It regulates multiple controlled variables along their nominal trajectories and does not require an additional active damping loop. Nearly optimal steady-state performance and fast transient response times are achieved in simulations.

AFRIKAANSE OPSOMMING: Mediumspanning omsetters word bedryf teen baie lae skakelfrekwensies. Aflynberekende geoptimeerde pulspatrone is die modulasie tegniek van keuse om die gevolglike harmoniese distorsie te verminder. Die optimale vuurhoeke, waarteen die omsetter skakel, word verkry deur ’n optimeringsprobleem met beperkings op te los. Om beide die analiese en berekeninge te vereenvoudig, word kwartgolf en halfgolf simmetrie tipies toegepas. Drievlak omsetters vereis dat die skakelposisies nie-negatief moet wees tydens die positiewe halfsiklus van die fundamentele periode. Die gevolg van hierdie beperkings op die harmoniese optrede van die pulspatroon word ondersoek in die eerste deel van hierdie tesis. Deur die kwartgolf simmetrie te ontspan en die negatiewe skakelposisies toe te laat tydens die positiewe halfsiklus, kan die stroomharmoniese distorsies verminder word met tot ’n derde in vergelyking met die tradisionele formulering. Algemene lineêre beheertegnieke benodig nul stroomriffel by die die monsteroomlikke. Oor die algemeen is die stroomriffel nooit nul nie in al drie fases wanneer geoptimeerde pulsepatron gebruik word. Verder kan die vuurhoeke diskontinu word wanneer die modulasieindeks verander word. Om hierdie probleme aan te spreek, was beheerders gebasseer op trajekvolging ontwerp. Modelvoorspellendepulspatroonbeheer is die mees merkwaardigste van hierdie beheertegnieke. Deur die fundamentele komponent en riffel as ’n enkele kwantiteit te hanteer, bereik die laasgenoemde beheer tegniek uitstekende steuringsverwerping and baie kort vestigingstye tydens oordragsverskynsels. Slegs ’n enkele veranderlike (vektor waarde) word beheer met hierdie metode, soos byvoorbeeld die stator vloedkoppeling van ’n masjien of the omsetterstroom van ’n kragnetwerkgekoppelde omsetter. Hoë-orde stelsels, soos byvoorbeeld omsetters met LC-filters, vereis ’n addisionele dempingslus wat hul optrede tydens oorgangsverskynsels belemmer. ’n Veralgemeende modelvoorspellendebeheerder pulspatroon strategie word voorgestel in die tweede deel van hierdie tesis. Dit reguleer verskeie beheerde veranderlikes al langs hul onderskeie nominale trajekte en vereis nie die gebruik van ’n addisionele aktiewe dempingslus nie. Beinaoptimale bestendigde toestand optrede asook vinnige oorgangs-optrede word bereik in simulasies.