Analysis of Walking Cell PWM Strategy in Multilevel Frequency Converter in Fault Condition

摘要

Cascaded multilevel frequency converter contains low-voltage cells in each leg. Bypassing the failed cells ensures the uninterruptable operation of a frequency converter. However, it can lead to a unbalance phase-to-phase voltages. A special control algorithm is required to operate under faulty conditions. Most approaches based on the phase-shift pulse-width modulation and level-shift pulse-width modulation are considered in this paper. However, these techniques have limitations, such as time delayed operation which limits current loop response time when reference voltage changes and in case of the cell failure. This paper proposes a modulation strategy which solves these problems. It implements pulse-width modulation in a single cell with sequential cell switching and can rapidly adapt to the failure of any cell without impact to the quality of the output voltage. The operation of the proposed pulse-width technique under faulty conditions was verified using Simulink Model.