Browsing by Author "Hinkkanen, Marko, Prof., Department of Electrical Engineering and Automation, Aalto University, Finland"

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Digital Control in Grid Converters: Methods and Comparative Analyses
(2022) Pirsto, Ville
School of Electrical Engineering | Doctoral dissertation (article-based)
Due to the ongoing transition towards fossil-free energy production by means of renewable energy resources, grid converters that act as power-electronic interfaces between the renewable energy sources and the electric grid are deployed in unprecedented numbers. This paradigm change has introduced a host of challenges related to the control of grid converters. The control system is a quintessential part of the grid converter, and in modern converters it is invariably implemented by using a digital processor. This thesis presents a collection of methods and comparative analyses applicable to digital control of grid converters. Throughout the thesis, state-feedback control framework is focused on due to its several desirable properties. The topics of this thesis can be divided into three categories: converter modeling, identification, and control design. Related to converter modeling, an intersample admittance modeling method is presented which takes into account the discrete-time nature of the converter control system. The presented method improves the modeling accuracy over conventional methods and eliminates the need for approximations used in the existing state-of-the-art methods. Regarding identification, focus is on developing computationally efficient methods that can be executed in real time. Two such methods are developed: one for identifying the parameters of an LCL filter and the other for identifying unbalanced grid impedance. These two methods can be embedded to a control system of a pulse-width-modulation-based converter in a plug-in manner. On the topic of control design, comparative analyses of different state-feedback current controllers are carried out to examine their properties as well as reveal connections between the different controllers. In particular, mathematical equivalence of the integrator-based and disturbance-observer-based state-feedback current controllers is shown. Moreover, an enhanced direct pole-placement design for weak-grid tolerant state-feedback positive- and negative-sequence current control is proposed which remains stable in grids ranging from strong to very weak. Related to current limiting in converters operating as voltage sources, a multifunctional cascade control structure is proposed which renders the current reference limiting methods previously restricted to cascade controllers equally available to direct AC-voltage controllers. All the results presented in this thesis are experimentally evaluated using a 12.5-kVA 50-Hz three-phase grid converter system. The results find applications in industry, for example, in wind turbine and solar photovoltaic systems as well as in active front-ends of motor drives.