Open-Circuit Fault Diagnosis in Single-Phase Cascaded H-Bridge Multilevel Converter With Switching Duty Ratio-Based Estimated Grid Current

This paper proposes an open-circuit fault (OCF) diagnosis of single-phase cascaded H-bridge multilevel converter (CHB) with switching duty ratio-based estimated grid current. In general, the current is estimated based on the sampled values. This means that the switching state of each switch used for current estimation is represented as binary, either off (0) or on (1). However, when pulse width modulation (PWM) is used, the switching states can vary within the sampling period, leading to inaccuracies in both current estimation and fault diagnosis. To address this issue, the proposed method estimates the grid current by calculating the switching duty ratio of each switch during the sampling period, derived from carriers and voltage references. The fault switch is determined in three steps using only the grid current. First, the OCF is detected using the grid current in dq-axis synchronous reference frame. Subsequently, the fault switch pair is determined through the sign and magnitude of the residual between the estimated and sampled grid currents. Finally, the fault switch location is identified by comparing the magnitude of the residual with the reversely calculated residuals through the product of the dc-link voltages and the switching duty ratios. The validity of the proposed method is verified through the experimental results.