H^∞ control of the neutral point in four-wire three-phase DC-AC converters

Inverters used as the interface for a distributed generator in a three-phase four-wire system sometimes operate with a large neutral current because of unbalanced loads and single-phase (possibly nonlinear) loads. Voltage balance within the dc-link of the inverter is important for proper operation of the inverter, and the neutral current is a significant disturbance to this. It is preferable to use fast acting control rather than dissipative balancing or large-valued capacitors. This paper develops a linear model of an actively balanced split dc-link and applies H^∞ control design to provide high-bandwidth robust control. The approach is developed for a conventional two-level inverter, but it remains valid (without change) for a three-level neutral-point clamped inverter. The controller achieves very small deviations of the neutral point (better than 0.5 in 800 V) from the midpoint of the dc source despite the large neutral current (32 A_RMS). The controller design is described and verified first in a PSCAD simulation and second in experimental testing of a 30-kW 415-V (line) inverter.