Self-capacitor voltage balancing method for optimally hybrid modulated cascaded H-bridge D-STATCOM

A hybrid modulation (HM) for cascaded H-bridge converters is suggested which is a fusion of selective harmonic elimination (SHE) and carrier based bipolar pulse width modulation. It differs from its counterparts in the sense that only one triangular carrier for all H-bridges is used as opposed to multi-triangle waveform set and only one level of capacitor voltage and single type of semiconductor are considered. HM eliminates calculating and storing offline delay angles for each modulation index as in case of SHE. A novel self-voltage balancing mechanism is also proposed for HM which is based on strategically swapping redundant switching states. This mechanism is free from voltage sorting, current measurement, extra circuitry, or additional controllers. The comparison with phase and level shift pulse width modulation schemes reveal that the HM has 15% more DC voltage utilisation, and lower harmonic content for wide interval of modulation index. Moreover, HM has low semiconductor losses, as low as 0.55%. The tests on 1500 kVAR distribution static synchronous compensator shows that HM generates only 4% voltage ripple at the cost of few increase in average switching frequency. Voltage and reactive power balancing in each phase are also established with high power efficiency.