Keeping a Constant Level of Losses in the Low-Voltage Cells of a High-Voltage Multilevel Frequency Converter in Emergency Modes of Its Operation

Abstract

High-voltage adjustable-frequency drives require special inverters for their operation. A cascaded multilevel frequency converter constructed on the basis of a multi-winding transformer and a set of low-voltage cells has received a widespread use. The main advantage of this converter topology are a low harmonic distortion level of the output voltage and current waveforms, as well as high reliability owing to the possibility to exclude a failed cell from operation without the need to shut down the drive. For equalizing the losses among the converter’s power cells, a pulse-width modulation (PWM) algorithm with a “walking” cell has been developed. Owing to a series connection of the cells, the effective PWM frequency is increased in proportion to the number of cells. However, if one cell fails, the switching frequency of the cells remained in operation increases in comparison with the switching frequency of phase cells when all of them are in the healthy state. This results in a growth of switching losses in the phase with one of its cells taken out from operation. The article discusses a control algorithm with which the nominal level of losses is kept in the cells of all phases under emergency operation conditions. The proposed algorithm has been verified on a model, in which a uniform distribution of losses and high quality of the output voltage have been obtained.