Online Condition Monitoring, Adaptive Control, and Novel Submodule Designs in Cascaded H-Bridge and Modular Multilevel Converters
Time: Fri 2025-02-07 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Language: English
Subject area: Electrical Engineering
Doctoral student: Mohsen Asoodar , Elkraftteknik
Opponent: Professor Remus Teodorescu, Aalborg University, Aalborg, Denmark
Supervisor: Professor Hans-Peter Nee, Elkraftteknik
QC 20250122
Abstract
Reliable operation of flexible AC transmission systems (FACTS) and high voltage direct current (HVDC) systems is necessary for stable operation of modern power grids. Converters used in FACTS and HVDC applications that are designed with high reliability constraints may still experience stoppages as a result of component failures. This may be a result of process variations in component manufacturing that can lead to over-stressing and failure of strategic components. Consequently, online estimation of parameters that provide information about the state-of-health (SoH) of critical components in these converters is crucial. This approach can help avoid unplanned stoppages and increase their availability.
Cascaded H-bridge converters (CHBCs) and modular multilevel converters (MMCs) are currently considered the state-of-the-art solutions for high-voltage and high-power conversion in both AC and DC applications. This thesis provides various solutions for online condition monitoring of submodule capacitors and power semiconductors, which are among the most critical components of CHBCs and MMCs. Moreover, novel methods are proposed to decouple the effect of temperature on health-indicating parameters. The proposed condition monitoring methods are broadly categorized into time-domain and frequency-domain estimation techniques. Both methods are thoroughly analyzed and the advantages and disadvantages of each method are explained. Furthermore, the robustness of the proposed solutions are shown under various load conditions, and in the presence of measurement uncertainties such as noise.
The thesis also presents adaptive online methods to reduce the voltage stress on components estimated to be health-degraded. Stress reduction is achieved through modified modulation schemes, where the capacitor voltage of health-degraded submodules is reduced. The proposed adaptive control is shown to have a minor effect on the quality of the generated output current.
Finally, the thesis proposes novel semiconductor modules that comprise series-connected and parallel-connected semiconductor devices. The proposed solutions are intended to simplify the protection system and potentially provide fault-ride-though functionality in the event of single device failures in the module. The main challenge in the proposed design is the voltage balancing of series-connected devices. Two different active snubber circuits for overvoltage protection of each device are proposed, where either an active closed-loop-controlled design or a sensorless self-triggered design can be used to protect the series-connected devices against overvoltages.
The different solutions presented in this thesis have been validated through a variety of approaches, including analytical methods, time-domain simulations, experimental testing, or a combination of these.