Modified staircase modulation with low input current distortion for multicell converters

Cascaded H bridge multilevel inverters are traditionally controlled using phase shifted PWM. This method allows an even power distribution among the different cells of the converter, which enables input current harmonic cancellations due to the isolation multipulse transformer and rectifier system. For high power applications a selective harmonic elimination based method, also known as staircase modulation, has been proposed. This method is devised to reduce switching losses to a minimum, improving the efficiency of the converter. The switching angles are calculated to eliminate low order harmonics at the output voltage. However, this method does not provide even power distribution among the cells, avoiding the input current harmonic cancellation. This work proposes a redistribution of the switching angles between the cells, maintaining the overall switching pattern of the staircase modulation. The methods maintains the harmonic elimination characteristic at the output voltage, while improving the power distribution and consequently the input current harmonic performance. Mathematical analysis and simulation results are presented to support the strategy.