Impedance Analysis and Stability Assessment of Modular Multilevel Converters
Time: Mon 2024-12-16 10.00
Location: H1, Teknikringen 33, Stockholm
Language: English
Subject area: Electrical Engineering
Doctoral student: Mehrdad Nahalparvari , Elkraftteknik, Power Electronics
Opponent: Associate Professor Jef Beerten, KU Leuven, Leuven, Belgium
Supervisor: Associate professor Staffan Norrga, Elkraftteknik; Professor Hans-Peter Nee, Elkraftteknik
QC 20241120
Abstract
Owing to their scalability, modular design, and high efficiency, modular multilevel converters (MMCs) are considered the state-of-the-art topology in high voltage dc (HVDC) and flexible ac transmission (FACT) systems. Ensuring converter- and system-level stability is crucial to facilitate the large-scale integration of these converters into future power grids.
Similar to other power electronics systems, the stability of MMC interfaced dc and ac systems can be assessed via the impedance-based stability criterion, which requires detailed representation or measurement of the MMC terminal impedances. The main objective of this thesis is thus, to model the dc- and ac-side impedances of the MMCs taking into account various control system implementations. To this end, the impact of different control schemes and parameters on the converter impedances are thoroughly investigated, resulting in models that serve as tools for analyzing potential undesirable interactions between converter control dynamics and the system to which the converter is connected.
The thesis also focuses on developing impedance models in situations where parts of the control system are concealed for intellectual property protection. By combining frequency-domain system identification and harmonic linearization, these black-boxed control system components are integrated into the impedance model. This approach enables the analysis of the impact of outer-loop control settings on converter stability.
Finally, the thesis assesses the stability of several case studies in which MMCs are interfaced to dc or ac systems. Consequently, active damping solutions are proposed to mitigate harmonic resonances arising from the interaction of the converter and the dc or ac systems. Theoretical analyses are substantiated through time-domain simulations and laboratory experiments.
Key contributions include the development of impedance models under various control schemes and a method for estimating dc-side impedance in MMC systems with black-boxed control. The findings provide insights into impedance shaping, stability challenges, and effective damping strategies in MMC-based systems.