Health Monitoring Scheme for Submodule Capacitors in Modular Multilevel Converter Utilizing Capacitor Voltage Fluctuations

The modular multilevel converters (MMCs) have emerged as one of the promising topologies for medium/high power applications. The aluminum electrolytic capacitors (AECs) are usually employed in the MMC as floating capacitors due to their high volumetric efficiency and low price. The AECs are reported as one of the most fragile components in the converter, which gradually deteriorates over the time due to vaporization of the electrolyte. As a result, its capacitance decreases and equivalent series resistance (ESR) increases with ageing, which can result in an increase of submodule (SM) capacitor voltage ripple, power loss and could damage the operation of the MMC with prolonged use of the aged capacitors. Therefore, health monitoring of SM capacitors is essential to enhance the reliability of the MMC by predictive maintenance. This paper presents a new health monitoring technique for SM capacitors in the MMC utilizing inherent SM capacitor voltage fluctuations. The capacitance is estimated by second-harmonic impedance, which is evaluated using the ripple in capacitor voltage and current. The proposed method utilizes the available measurement used for the converter control and can be easily implemented in the same converter controller. The proposed scheme is verified for five-level MMC through simulation results in PLECS software.