New algorithmic modulation strategy for cascaded H-bridge inverters with IGCT burst capability monitoring and excellent performance

The paper presents a new modulation strategy for cascaded H-bridge inverters which allows easy integration of device specific safety features like IGCT burst capability monitoring. It is compared to PWM using a simulation model of an unusual configuration for a 10 kV medium voltage inverter using just three cascaded cells per phase. The general structure of the cascaded H-bridge inverter and key data of the simulation model, which includes thermal characteristics, is described. The pulse width modulator is explained including details particular to cascaded H-bridge inverters. The new modulation strategy is outlined briefly, more detail being used to show how burst capability monitoring for IGCTs is implemented. In the performance comparisons, which were done using simulation models in which only the used modulators differ, efficiency, total harmonic distortion, loss and temperature distribution among cells and the share of output power contributed by each cell were analyzed. The results reveal improved partial load efficiency due to reduced switching frequency for the new modulator. Harmonic distortion is much improved; particularly at elevated frequencies up to 200 Hz and at low power. Power distribution is quite uniform although the algorithm does not contain any measures to achieve balancing of equal DC-link voltage cells because it was designed for asymmetrical converter configurations.