Design Issues of Natural Reference Frame Current Regulators with Application to Four-leg Converters

Three-phase, four leg voltage source converters by virtue of the fourth leg connected to the neutral points of the wye-connected loads effectively handle the zero sequence currents and voltages occasioned by load impedance imbalances and/or nonlinearity of loads. This paper outlines a generalized carrier-based pulse-width modulation scheme for this converter which with a natural reference frame control scheme directly controlling the actual load currents ensure current regulation irrespectively of the nature of the load. The controller structure and design are discussed explicitly with attention paid to considerations leading to the selection of the controller parameters yielding satisfactory location of system poles and zeros. The proposed modulation and control schemes are effectively validated by computer simulation and experimental results for the converter operation in the linear and overmodulation regions