Supervisor:
Huai Wang

Co-Supervisor:
N/A

Assessment Committee:
Yanbo Wang(Chair)
Sandeep Anand, Indian Institute of Technology (IIT) Bombay
Thomas Ebel, South Denmark University

Moderator:
Yanbo Wang

Abstract:

In power electronics applications, capacitors exhibit one of the highest failure rates within converter systems. Once the capacitor breaks down, the power electronic converters could not continually operate because of the limited topology, resulting in the loss of power and the shutdown of the drive systems, and even the safety accident in severe cases. More realistic testing (i.e., application-oriented) and a better understanding of failure mechanisms and failure modes are key to build-in capacitor reliability in power converter products through design. For availability-critical or safety-critical applications, condition and health monitoring is another reliability engineering tool to reduce unscheduled failure through operation management (i.e., maintenance). This study investigates application-oriented stress emulation methods for capacitors, failure analysis based on accelerated aging tests, and non-intrusive condition monitoring techniques, contributing to the converters safe and economical operation.

Firstly, this study introduces a new test approach to emulate the stress condition experienced by both DC and AC capacitors. Unlike existing methods, the proposed approach provides conditions to test high-power electrical stress (measured in kV and kA). This is unique in its capacity to provide the highest levels of voltage and current ever publicly reported for capacitor testing. Additionally, the method offers robust control of testing conditions despite capacitor degradation and mismatch, emulating various electrical stress levels based on the actual mission profile.

Secondly, based on the proposed emulation method of the realistic stress of capacitors, an analysis of aging impact factors and failure mechanisms of AC power filtering film capacitors in high-power converter systems is conducted. Using the results of approximately 3,500 hours of accelerated aging testing, electrochemical corrosion is identified as the dominant factor in the aging process. The aging and failure mechanism of this type of capacitor is summarized based on the change in capacitance value, temperature analysis, and microscopic observation.

Thirdly, a capacitor condition monitoring method for high-order LC circuits during power-OFF state is proposed. This method involves a portable battery-powered unit that injects signals and uses circuit modeling to determine LC circuit parameters. It is suitable for applications like wind energy, solar power, and transportation systems that require regular maintenance or frequent switching between power states. Compared to traditional impedance analyzers, our monitoring unit offers significant cost-effectiveness and eliminates the need for component disassembly. Unlike conventional capacitor monitoring techniques that operate during power-ON, the proposed method is non-intrusive, requiring no additional hardware or software elements that could disrupt converter systems.

Lastly, the study introduces a novel monitoring indicator for the aluminum electrolytic capacitor based on strain sensing. The indicator does not require an electrical interface to the converter and just needs to be affixed to the top of the capacitor, therefore it has a low risk level compared to techniques in the existing literature. Strain sensing-based monitoring methods also have the advantages of a wide measurement range and high resistance to interference. In addition, the specific point of the pressure relief vent opening can serve as a new failure criterion for capacitors. Compared to the existing failure criterion of capacitance and ESR, the failure criterion of the proposed health indicator achieves smaller variances with respect to the actual time-to-failure.

The above study has been theoretically derived and experimentally verified. The contributions have been presented through a few journal papers and conference papers.