Wavelet-based estimation method for online condition monitoring of dc-link capacitors of distributed energy resources

This paper proposes a novel capacitor condition monitoring for dc-link of power converters employed in distributed energy resources systems. It follows the active methodology, in which an intermittent noncharacteristic signal is injected into the dc-link to estimate the equivalent series resistance (ESR) and capacitance (C) of dc-link capacitors. A real-time stationary discrete wavelet packet transform (RT-SDWPT) performs the capacitor parameter estimations. In this scheme, an interharmonic is used as a noncharacteristic signal to prevent the influence of the system’s inherent harmonic components, mitigating estimation errors. Moreover, the interharmonic frequency is chosen in the region influenced by the ESR and C parameters to ensure capacitor degradation condition. Besides, to mitigate quality issues, the interharmonic’s amplitude, duration, and the time interval between two signal injections follow the recommendations defined by standards. Furthermore, a parameter deviation detection scheme is employed to adaptively adjust the time interval between two estimations, improving failure detection. Experimental results obtained from a microgrid laboratory setup demonstrate the effectiveness and feasibility of the proposed method.