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Abstract

For multilevel frequency converters FC [1], one of the critical modes is the operating mode when the power cell fails in one of the phases of the inverter. Bypassing of the inverter power cell in this case is dangerous due to the possibility of occurrence of voltage distortion in the phases and, consequentially, the stop of the variable-frequency electric drive due to the current imbalance in the FC load [2]. A flowchart of the FC control system CS is provided, indicating its control signals and correction of the FC output voltage. It is proposed to determine the correction factor of the FC control system, which, in case of the cell failure in the phase of the FC inverter, allows generation of a control signal at the CS output that provides the maximum possible amplitudes of the line voltages while maintaining their interphase symmetry to ensure stable operation of the drive in such mode. The equations determining the CS correction factor in case of a defect in the FC inverter power cell are presented. The analysis of the vector diagram of the output voltage of the FC operating with the bypassed cell in phase, as well as with application of the proposed CS correction, is given on the example of a high-voltage FC with a multilevel inverter.

Keywords

Multilevel frequency converter, inverter, control system, correction factor, mathematical model, vector diagram of inverter voltage, current imbalance.