Browsing by Author "Chingwena, Macyln"

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    Long-Horizon direct model predictive control of an active capacitor for ripple energy compensation in single-phase DC-to-AC converters
    (Stellenbosch : Stellenbosch University, 2021-03) Chingwena, Macyln; Mouton, Toit; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    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.