Accelerated Life Testing Method of Metallized Film Capacitors for Inverter Applications

Abstract

DC-link capacitors are one kind of reliability-critical component in traction inverters. As such, it is essential to evaluate the degradation characteristics of dc-link capacitors. Accelerated life testing is an effective and standard method to evaluate the capacitors’ lifetime and degradation mechanisms. Therefore, an accelerated life testing method for dc-link capacitors in three-phase inverter applications is proposed in this article. In order to reproduce the same encountered stress, exactly reflecting the real condition for the capacitor under test, a three-phase inverter with the capacitor under test and a three-phase balanced inductor load are used as the evaluation circuit. Due to the employment of the inductor load, the power consumption of the inverter can be reduced, while the full-scale current rating can be maintained so that the capacitor under test is subjected to the practical ripple current stress. In order to estimate a capacitor’s capacitance with high accuracy, an effective and direct condition monitoring method, which is based on the proposed evaluation circuit, is also proposed in this article. By injecting an ac voltage into the dc link, the ac current and voltage components in the dc-link capacitor are generated. Then, the capacitance mathematical model can be developed by analyzing the injected voltage component and the resultant current component. As the proposed method only requires estimating the parameters from the input voltage and current, there is no need to install additional capacitor current sensors, which are usually difficult to install. Moreover, the amount of monitored and processed data can be reduced. Thus, the proposed method is easy to implement for accelerated life testing in practice. In addition, experiments are carried out to verify the analysis and effectiveness of the proposed solution.