Abstract
A cell capacitor is one of the most fragile components in a modular multilevel converter (MMC). Capacitance decreases and equivalent series resistance (ESR) increases, as the capacitor deteriorates. This paper proposes an online monitoring method for both the capacitance and ESR of the cell capacitors in an MMC without any additional components or operation. The proposed method measures the impedance of the cell capacitor not only at a low frequency but also at a switching sideband-frequency of the cell without current sensors for capacitor current. The high-frequency components around the switching frequency improve the estimation accuracy compared with the previous methods that measure only impedance at low frequencies. The experimental results confirm that the proposed method estimates the capacitance and ESR with errors of 3.1% and 8.8%, respectively.
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