Review of Health Monitoring Techniques for Capacitors in Modular Multilevel Converters

Modular multilevel converters (MMCs) have been considered as one of the most promising topologies in medium- and high-voltage applications. Capacitors are heavily used for submodules energy storage in MMCs. Due to the vulnerability and large quantity, the failure of capacitors has been recognized as one of the major challenges that threaten the healthy operation and reliability of MMCs. The health monitoring techniques for capacitors in MMCs have witnessed an upward trend over the past decade. This article illustrates the degradation and impedance characteristics of capacitors, and classifies and summarizes the state of the art of capacitor condition monitoring (CM) studies for MMCs. First, the classification of existing CM techniques for capacitors is presented. Afterward, a detailed review of different categories of capacitor CM methods in MMCs is conducted with the emphasis on their principles, implementations, and features. Temperature-related capacitor parameters' calibration is then illustrated. Next, a comprehensive summary and assessments of these methods are presented. Finally, some future research trends are predicted.