Cascade industrial VSI gives medium voltage - a case study

In this paper, the modularity concept applied to medium-voltage adjustable speed drives (MV-ASD) is addressed. First, the single-phase cascaded (SC) VSI that uses series connection of IGBT H-bridge modules with isolated DC-buses is presented. Next, a novel triphase cascaded (TC) VSI that uses three IGBT triphase inverter modules along with an output transformer to obtain a 3 p.u. multilevel output voltage is introduced. The system yields a high-quality multistep voltage with up to 4 levels and low dv/dt, balanced operation of the inverter modules that supply each a third of the motor kVA. The concept of using TC-VSI is further extended to a new modular motor-modular inverter (MM-MI) system where the MV motor winding connections are reconnected into several three-phase groups, either six-lead or twelve-lead connection according to the voltage height, each powered by a standard triphase IGBT inverter module. Thus a high fault-tolerance is being achieved and the output transformer requirement is eliminated. A staggered space vector modulation (SSVM) technique applicable to triphase cascaded VSI topologies is also demonstrated. Both TC-VSI and MM-MI computer simulations and experimental tests demonstrate the feasibility of the systems. (4 pages)