Long-Horizon direct model predictive control of an active capacitor for ripple energy compensation in single-phase DC-to-AC converters

Chingwena, Macyln (2021-03)

Thesis (MEng)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: AbstractSingle-phase dc-to-ac converters generate power on the ac side that pulsates at twice the gridfrequency. Inherently, the pulsating power is transferred from the ac side to the dc side andgenerates second-order harmonic currents that flow through the dc bus, also referred to as theripple current. This occurs when power flows either from the ac side to the dc side or whenpower flows from the dc side to the ac side. In this thesis, we assume power flow from the dcside to the ac side.Suppose a battery powers a single-phase dc-to-ac converter that is connected to eitherthe grid or a load. The generated ripple current will unavoidably flow through the battery.Although ideally the current flowing through a battery should be constant, that is nearlyimpossible. Generally, the ripple current passing through the battery should be limited to10 %of the nominal battery current.Usually, a dc-link capacitor is used to reduce the ripple current, and aluminium electrolyticcapacitors are often used due to their availability in large capacitance values. However, theyhave a short lifespan and bulk size, which leads to reliability issues. This creates a trade-offbetween reducing either the ripple current or the capacitance requirements.The concept of using an energy storage circuit for ripple energy compensation and, at thesame time, reducing the capacitance requirements has been proposed. However, a controlproblem is formulated when using this method of ripple energy compensation. The ripplecurrent needs to be diverted away from the battery to the energy storage circuit. In previousyears, classical control strategies were used to address the control problem. Nonetheless, theclosed-loop performance of these controllers still presents challenges. The main contribution of this thesis is on using model predictive control to compensatefor ripple energy, with a dc-to-dc boost converter as an energy storage circuit. Since modelpredictive control has only recently been adopted in power electronics, it still bears a stigmathat longer prediction horizons do not offer performance benefits. In this thesis, the imple-mentation of long-horizon direct model predictive control for a boost converter is given ingreat detail. By using the branch-and-bound technique and the move blocking strategy, theoptimization problem is solved efficiently, enabling practical considerations.Through simulations, the efficacy of the control strategy is verified. For horizons lessthan three, the system did not reach steady-state operation, validating the need for longerprediction horizons. It is shown that, for a prediction horizon of ten, the ripple current isreduced to2.4 %and2.8 %of the nominal battery current, for a grid-connected system and astand-alone system, respectively. At the same time, the capacitance requirements are reducedby over95 %for both systems.The controller is implemented within a field-programmable gate array, and through ahardware-in-the-loop simulation of a stand-alone system, the practical feasibility of the con-troller is verified. It is shown that the ripple current is reduced to roughly3.2 %of the nominalbattery current when using a prediction horizon of seven.

AFRIKAANSE OPSOMMING: Enkelfase DS-na-WS omsetters genereer drywing aan die WS kant wat puls teen twee maaldie kragnetwerk se frekwensie. Die pulserende drywing word inherent oorgedra vanaf dieWS kant van die omsetter na die DS kant. Hierdie proses genereer tweede-orde harmoniekestrome, bekend as die rimpelstroom, wat vloei deur die DS bus. Dit vind plaas wanneer diedrywing vloei vanaf die WS-na-DS kant of andersom.Gestel dat ‘n battery ‘n enkelfase DS-na-WS omsetter aandryf wat gekoppel is aan óf diekragnetwerk óf ‘n las. Die rimpelstroom wat gegenereer word sal onvermydelik deur diebattery vloei. Alhoewel die stroom wat deur ‘n battery vloei konstant behoort te wees, isdit byna onmoontlik in praktyk. Die rimpelstroom wat deur die battery vloei moet tipiesbeperk word tot10 %van die nominale batterystroom.Normaalweg sal ‘n DS-skakel kapasitor gebruik word om die rimpelstroom te beperk.Aluminium elektrolietese kapasitore word dikwels vir hierdie toepassing gebruik aangesiendit beskikbaar is in hoë kapasitansie waardes. Hierdie kapasitore het egter ‘n kort lewensduuren ‘n ongemaklike grootte wat lei tot onbetroubaarheid. Die gevolg is ‘n kompromie tussendie vermindering van die rimpelstroom en die kapasitansievereistes.Die konsep van ‘n energie-bergende stroombaan word voorgestel wat rimpel-energie kanonderdruk sowel as die kapasitansie vereisters verminder. Daar word egter ‘n beheer-probleemgeskep met so ‘n voorstel van rimpel-energie onderdrukking. Die rimpelstroom moet weggeleiword vanaf die battery na die energie-bergende stroombaan. Voorheen was klassieke be-heertegnieke gebruik om hierdie beheer-probleem aan te spreek. Die geslotelus optrede vanhierdie beheerstelsels bied egter steeds uitdagings. Die hoof bydrae van hierdie tesis is om voorspellende beheer te gebruik om te kompenseervir die rimpel-energie, terwyl ‘n DS-na-DS opkapper gebruik word as die energie-bergendestroombaan. Aangesien voorspellende beheer eers onlangs aangepas is vir elektronika, dra ditsteeds die stigma dat langer horisonne nie ‘n beduidende voordeel bied vir die uittree-optredenie. In hierdie tesis word die implentering van ‘n lang-horison voorspellende beheerder indetail weergegee. Deur gebruik te maak van die tak-en-gebonde tegniek sowel as die beweeg-bokkeerstrategie, kan die optimeringsprobleem doeltreffend opgelos word. Dit lei tot dieoorweging van praktiese implenterings.Die doeltreffendheid van die beheerstrategie word bevestig deur simulasies. Vir horisonnekorter as drie het die stelsel nie bestendigdetoestand werking bereik nie, wat die vereiste vanlanger horisonne aandui. Dit word gewys dat die rimpelstroom verminder word vir die nom-inale batterystroom vir ‘n horison van tien tot2.4 %vir ‘n kragnetwerk-gekoppelde stelsel entot2.8 %vir ‘n alleenstaande stelsel. Op dieselfde tyd word die kapasitansie vereistes vermin-der tot95 %vir beide stelsels.Die beheerder word geïmplementeer op ‘n veld-programmeerbare hekskikking (FPGA).Die praktiese uitvoerbaarheid van die beheerder word bevestig deur middel van ‘n hardeware-in-die-lus simulasie van die alleenstaande stelsel. Dit word gewys dat die rimpelstroom vermin-der word tot3.2 %van die nominale batterystroom met ‘n voorspellingshorison van sewe.

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