Control of MMC in HVDC Applications
Authors
Timofejevs, Artjoms ; Perez, Daniel Enrique Gamboa
Term
4. term
Education
Publication year
2013
Submitted on
2013-05-30
Pages
90
Abstract
This thesis investigates control of the modular multilevel converter (MMC) for transformerless HVDC transmission, focusing on operation under asymmetric grid faults. Motivated by grid-code requirements for fault ride-through, grid support, and decoupling of healthy and faulty AC systems, the work examines how the MMC’s inner dynamics are affected and can be managed. Both average and switching MMC models are developed, arm currents and submodule dynamics are analyzed, and a complete HVDC control stack (PLL, current/voltage loops, and active/reactive power control) is implemented. The effect of symmetrical-component control—zero- and negative-sequence current control—during unbalanced conditions is demonstrated in simulated case studies, including elimination of negative-sequence currents and mitigation of active and reactive power oscillations. A modified circulating current suppression controller is proposed to eliminate DC-link voltage ripple, and a converter current limitation strategy is derived; its impact on overall HVDC performance is assessed through simulations. The results provide guidance on how MMC-HVDC can meet grid codes, remain connected during faults, and control internal current and energy oscillations.
Dette speciale undersøger styringen af den modulære multilevel-konverter (MMC) i transformerfri HVDC-transmission med fokus på drift under asymmetriske netfejl. Motiveret af netkoder for fault ride-through, netstøtte og krav om at afkoble en rask AC-side fra en fejlramt, analyseres hvordan MMC’ens indre dynamik påvirkes og kan kontrolleres. Der udvikles både gennemsnits- og koblingsmodeller af MMC’en, armstrømme og delmoduldynamik analyseres, og et komplet HVDC-styringslag med PLL, strøm- og spændingssløjfer samt aktiv/reaktiv effektregulering implementeres. Effekten af styring af symmetriske komponenter (nul- og negativsekvensstrømme) under ubalancerede forhold demonstreres gennem simulerede cases, herunder eliminering af negativsekvensstrømme og dæmpning af aktive og reaktive effektoscillationer. En modificeret cirkulerende-strømsundertrykkelsesregulator foreslås med henblik på at eliminere spændingsbølger i DC-linket, og en strategi for konverterens strømbegrænsning afledes; dens indvirkning på HVDC-anlæggets ydeevne vurderes via simulationer. Arbejdet giver retningslinjer for, hvordan MMC-HVDC kan opfylde netkoder, forblive tilsluttet under fejl og samtidig kontrollere interne strøm- og energisvingninger.
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