High-gain adaptive observer for floating voltages estimation and capacitor aging monitoring in multicell converters

Abstract

In multicell converters, capacitors play a critical role in energy management and voltage stabilization of each cell. They ensure balanced regulation of floating voltages and minimize energy losses during switch commutations. However, capacitor aging leads to increased dielectric losses, equivalent series resistance (ESR), and energy dissipation, which degrade the performance of the converter and increase the risk of failure. Consequently, effective monitoring of capacitor aging is essential. Moreover, in such converters, estimating the voltages across flying capacitors is crucial for the synthesis of most control laws. To address these dual issues, this paper proposes a high-gain adaptive observer capable of simultaneously estimating the parameters and state variables of multicell converters. This approach enables real-time monitoring of capacitor aging while estimating floating voltages. Novel hybrid dynamic models of the converter are introduced, incorporating equivalent series resistance and equivalent capacitance as parameters to be estimated to account for capacitor aging. The observer is designed to estimate the voltages of flying capacitors, the load resistance, and the dielectric loss resistance due to aging. Its design relies on a thorough analysis of the observability of states and parameters, which depends on the discrete states of the converter, and its stability is demonstrated using the Lyapunov approach. Numerical simulations performed on a three-cell converter demonstrate the effectiveness of the proposed observer in estimating the voltages of flying capacitors and simultaneously monitoring their health.