Modular Two-Stage DC-Three Phase Converter

The two level voltage source power converter is the industrial workhorse at lower power levels for dc bus voltages up to a few hundred volts. As the power throughput increases and the dc bus voltage exceeds beyond a kV, the three level neutral point power converters become an attractive and viable solution. The three level dc link topology accommodates the use of semiconductor devices rated at half the total dc bus voltage. Furthermore, the adoption of the three level power converter allows an easier management of second order problems such as high dv/dt, bearing currents, common mode EMI, etc. in comparison with conventional two level converters. While their application in medium voltage motor drives has been quite mature, they are becoming increasingly attractive for the solar photovoltaic applications as well. In spite of the relative maturity of the approach, the converter suffers from a lack of modularity in its physical realization. The interconnection of semiconductor switches and diodes in each phase leg is complex and requires careful design of bus-work. The advent of wide-band-gap switches with nanosecond switching exacerbates the problem of parasitic inductances in the commutation paths. The balancing of voltages across the two series connected capacitor banks that form the dc bus remains a persistent control problem with endless diversity of solutions. In this paper the conventional three level converter is modularized to overcome these issues. The approach allows the use of standard two-level half-bridge building blocks to realize three level power converters. Analysis of topology and modulation, along with experimental results and comparison with conventional approaches are presented.