Voltage Level Compensation Method for Post-Fault Operation of Modular Multilevel Converter with Integrated Battery

This paper presents a voltage compensation algorithm as an addition to the existing improved sorting algorithm for post-fault operation of a modular multilevel converter with integrated batteries, aimed at electric vehicle applications. The work focuses on improving the performance of the sorting algorithm that allows the converter to continue operating without degradation after one transistor fault, by using the faulted module in half-bridge mode while preserving access to its battery. However, the existing sorting algorithm has a limitation during continuous high-power operation, where the faulted module cannot discharge sufficiently. This results in a voltage imbalance between the modules and distortion of the output current waveform. To address this issue, a voltage level compensation algorithm is proposed, which adjusts the module operational limits and the reference signal amplitude based on the measured module voltages. The method compensates the positive and negative half-periods of the output waveform independently, since different modules are active in each half-period during fault conditions. The simulation and experimental results demonstrate that the proposed algorithm compensates the output current successfully, even when the module voltages differ significantly. An FFT analysis confirmed the elimination of the DC offset and the reduction of the low-frequency harmonics, resulting in a total harmonic distortion comparable to normal operating conditions.