Decoupled control of a four-leg inverter via a new 4x4 transformation matrix

Four-Leg (3-phase 4-wire) inverters are developed to power unbalanced/nonli near three-phase loads. A unique 4 x 4 decoupling transformation matrix is used that enables direct transformation between the four degree-of-freedom (DOF) leg-modulation space of the inverter and its corresponding 3-DOF outp ut-voltage space. This is analogous to the well-known 3 x 3 "abc-qd " trans formation developed for the three-leg inverter. Details of this new 4 x 4 " Quad" transform are provided, along with a depiction of the voltage-vectors produced. Advanced synchronous-frame control techniques are applied with t his 4-to-3 "abcn-qdo" transform to create a UPS-style inverter with sinewav e output. Experimental results for an 8.6 kVA prototype inverter are presen ted.