Online Condition Monitoring of DC-Link Capacitor for AC/DC/AC PWM Converter

This article proposes a condition monitoring scheme for the dc-link capacitors used in ac/dc/ac pulsewidth-modulation converters based on the signal injection, which can estimate capacitance in real time when both the rectifier and the inverter are in normal operation. First, a low-frequency signal is injected into the dc-link voltage, and its effects on the total harmonic distortion (THD) of the input current are analyzed. Accordingly, a suitable injection frequency is selected to keep the THD of input current within limits prescribed by the IEEE standard. Then, a novel calculation method of capacitor current without any dc-link current sensor is given, where the converter system is still in normal operation and different inverter operations are both taken into consideration. Finally, capacitance estimation based on the slide discrete Fourier transform is introduced. At the end of this article, experiments have been done to validate the correctness of the conclusion and effectiveness of the proposed scheme.