Control of an hybrid multilevel inverter for current waveform improvement

The use of multilevel converters is widespread in medium voltage applications due to their inherent voltage sharing among the devices. Also, the multilevel offers advantages regarding the waveform quality due to the multiple steps of the output voltage waveform. This advantage is specially true for cascaded H-bridge inverters that can produce a large number of levels due to their modular structure. Nevertheless, this advantage comes at the cost of multiple dc-links that are normally supplied by independent rectifiers coupled though a multipulse input transformer. This multiple dc sources complicates the implementation of converters with high number of levels. The alternative of using the lower voltage cells with floating dc-links to supply only reactive power, compensating for voltage distortion, has been proposed in literature. In this work, this idea is applied to an neutral point clamped (NPC) inverter operating at low switching frequency. Here, the analogy between the floating dc-link converters and series active filters is used to develop a novel control strategy for these dc-links and the three phase output voltage.