Control of Multilevel HVDC based on Modular Multilevel Converter (MMC) during Faults: None
Translated title
Control of Multilevel HVDC based on Modular Multilevel Converter (MMC) during Faults
Author
Rizadis, Dimitrios
Term
4. term
Education
Publication year
2017
Submitted on
2017-10-06
Pages
90
Abstract
Modulære multilevel-konvertere (MMC'er) bruges i stigende grad i systemer som højspændings-jævnstrøm (HVDC). Denne afhandling forbedrer MMC-styring i situationer med fejl i AC-nettet. Målet er at opfylde netkrav om reaktiv strøminjektion—strøm, der hjælper med at støtte spændingen—uden at overskride konverterens fysiske grænser. Vi udvikler og analyserer tre injektionsstrategier: Positive Sequence Injection; Mixed Sequence Injection med balanceret effekt; og Mixed Sequence Injection med netoverensstemmelse. Strategierne styrker den positive sekvens af spændingen (den balancerede del af AC-bølgen), mindsker den negative sekvens, og reducerer rippel i den aktive effekt. Derudover udvikles en strømbegrænsningsmetode, som prioriterer reaktiv strøminjektion for både positiv og blandet sekvens. Teknikkerne valideres i både simuleringer og eksperimentelle forsøg, og resultaterne sammenlignes og diskuteres.
Modular Multilevel Converters (MMCs) are increasingly used in systems such as high-voltage direct current (HVDC). This thesis improves MMC control for situations where the AC grid experiences faults. The aim is to meet grid-code requirements to inject reactive current—current that helps support voltage—without exceeding the converter’s physical limits. We design and study three injection strategies: Positive Sequence Injection; Mixed Sequence Injection with Balanced Power; and Mixed Sequence Injection with Grid Compliance. These strategies strengthen the positive-sequence voltage (the balanced, healthy part of the AC waveform), reduce negative-sequence components, and minimize ripple in active power. We also develop a current-limiting method that prioritizes reactive current injection for both positive-sequence and mixed-sequence operation. The techniques are validated through simulations and experimental tests, and the results are compared and discussed.
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