An Online Condition Monitoring Method for Series-Connected Capacitors Using Auxiliary Discharging Network

Capacitors are one of the most vulnerable components in power converters. Compared with other types of capacitors, aluminum electrolytic capacitors (AECs) have a shorter lifetime. Due to their low voltage rating, they are generally series-connected, especially in high-voltage dc-link. To meet the total capacitance requirement, more capacitors are needed, resulting in poorer reliability. In this article, an online condition monitoring method is proposed for series-connected AECs. A simple and low-cost auxiliary discharging network is used without interrupting the normal operation of the converter. A linear relationship between the capacitance and the discharging time is identified. Thus, current sensor or high sampling frequency is not needed. Only the voltage response is measured to accurately estimate the capacitance. The effectiveness of the proposed method is validated both by simulation and experimental results. The results illustrate that the error of the estimated capacitance is less than 2%. In addition, a self-powered plug-and-play module is further designed to directly indicate the failure condition.