Quasi-Online Low-Frequency Impedance Monitoring Scheme for Submodule Capacitors in Modular Multilevel Converters

Despite being the most fragile component, aluminum electrolytic capacitors (AECs) are preferred in modular multilevel converters (MMC) due to their high volumetric efficiency and low price. Due to the ageing of AECs, its capacitance decreases and equivalent series resistance (ESR) increases. This can result in an increased voltage ripple, power loss, and could damage the operation of the MMC. Therefore, condition monitoring of submodule (SM) capacitors in an MMC is indispensable to enhance system reliability. This paper presents a new condition monitoring technique for SM capacitors based on the measurement of its low-frequency impedance. The low-frequency impedance is evaluated based on the information of the second harmonic component in the SM capacitor voltage and current. In this scheme, a low-frequency voltage signal is injected in the control loop of the MMC with available hardware and controller for converter control. The proposed scheme has been validated through PLECS simulations on a three-phase half-bridge submodule (HB-SM) based MMC. This technique is also verified experimentally on a single-phase MMC with a four HB-SMs per arm.