Tri-state current source inverter with improved dynamic performance

Traditional dc-ac current source inverter (CSI) has a right-half-plane (RHP) zero in its control-to-output transfer function. This RHP zero causes the inverter output to fall before rising when a step increase in command reference is required (commonly known as non-minimum-phase effect). To achieve a better dynamic response, this paper proposes the design of a tri-state CSI using only an additional semiconductor switch for introducing unique free-wheeling states to the traditional six active and three null states of a CSI. With the free-wheeling states inserted appropriately within the inverter state sequence, the inductive boosting and discharging intervals can be decoupled, allowing the RHP zero to be eliminated with only minor circuit modifications (high level control schemes like predictive and multi-loop voltage/current control remain unchanged). The designed inverter can be controlled using an appropriately formulated digital pulse-width modulation algorithm, which can conveniently be implemented using a digital signal processor with an on-chip carrier-based modulator and an external digital programmable logic device. The resulting inverter is tested by simulation and experimentally using a laboratory prototype for demonstrating its improved dynamic performance with no commutation difficulties introduced.