Fault Tolerant Hierarchical Control Strategy for Modular Multilevel Converter in HVDC Application
Author
Burlacu, Paul Dan
Term
4. term
Education
Publication year
2014
Submitted on
2014-05-27
Pages
95
Abstract
Modulære multilevel-konvertere (MMC) er effektelektronik, der bruges i højspændings-jævnstrøm (HVDC) eltransmission. Fordi de består af mange ens undermoduler, er de lette at skalere og kan levere en ren udgang med lav harmonisk forvrængning. Kontrollen af en MMC er dog kompleks, og kommercielle løsninger bygger ofte på en enkelt, central controller. Dette speciale udvikler en hierarkisk kontrolstrategi, hvor ansvaret fordeles på flere niveauer. Med et EtherCAT-kommunikationsnet (et industrielt realtidsnetværk) udvikles en metode til at synkronisere de bærebølger, der styrer undermodulernes kobling, og en resamplet faseforskudt pulsbredde-modulationsalgoritme (PWM) implementeres for forskellige MMC-konfigurationer. For at øge pålideligheden foreslås desuden en algoritme, der rekonfigurerer systemet ved fejl i et undermodul, så driften kan fortsætte med mindst mulig afbrydelse. Endelig undersøges gennemsnits- og balanceringskontrol gennem simuleringer for at holde vigtige størrelser ensartede på tværs af undermoduler.
Modular Multilevel Converters (MMC) are power-electronic converters used in high-voltage direct current (HVDC) transmission. Built from many identical sub-modules, they are easy to scale and can produce a clean output with low harmonic distortion. However, MMCs are challenging to control, and commercial systems often rely on a single, centralized controller. This thesis develops a hierarchical control strategy that distributes responsibilities across several levels. Using an EtherCAT communication network (a real-time industrial fieldbus), it introduces a method to synchronize the carrier waves that coordinate sub-module switching and implements a resampled phase-shifted pulse-width modulation (PWM) algorithm for different MMC configurations. To improve reliability, the work also proposes an algorithm that reconfigures the system if a sub-module fails, allowing operation to continue with minimal disruption. Finally, averaging and balancing control—mechanisms that keep key quantities consistent across sub-modules—are studied through simulations.
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