An Online Condition Monitoring Method for DC-Link Capacitors of Three-Level NPC Inverters Based on Charge–Discharge Profile

This article proposes an online condition monitoring (CM) method for the DC-link capacitors of three-level neutral-point clamped (NPC) inverters. A decrease in capacitance typically indicates the degradation of DC-link capacitors. The proposed method estimates the capacitance from the charge–discharge profile required for the upper and lower DC-link capacitor voltages to reach a specified difference. This voltage difference is actively generated by injecting an offset voltage into the reference voltages. However, the voltage difference between upper and lower DC-link capacitors distorts the output currents. To address this issue, a compensation strategy is proposed to eliminate the negative effect on output currents during the monitoring process. The proposed CM method is implemented under normal operating conditions without requiring additional hardware, with a mean error of less than 1%. The feasibility and effectiveness of the proposed method are confirmed through simulations and experimental results.