A Three-Dimensional Space Vector Modulation Scheme for Three-Level Three-wired Neutral Point Clamped Converters

Three-level converters have been used for dc-ac conversion when the desired ac voltages are balanced. To deal with the unbalanced situations, three-level four-leg converters have been proposed. The four-leg converters use an extra leg to conduct any neutral current in a three-phase four-wire system. More switching devices in the four-leg converters result in increased cost and possibly increased switching losses compared with the three-leg converter. In this paper, a 3-D space vector modulation scheme for three-level three-leg is proposed to deal with unbalanced situations. This scheme uses existing topology and does not require any hardware modification. It is also suitable in applications where neutral is not required such as STATCOM, UPFC etc. Simulation and practical results have confirmed that the proposed 3-D space-vector scheme has better low-order harmonic performance than traditional 2-D scheme under unbalanced conditions and less switching transients in the voltage balancing capacitors in the 3-level inverters. These potential advantages could enable cost and size reduction in the site implementation of passive filters and prolong the lifetime of the inverter equipment