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Current-mode boundary controller with reduced number of current sensors for a three-phase inverter

Cascaded boundary-deadbeat controller has been proven to be effective in controlling single-phase grid-connected inverter with LCL output filter. Such architecture mitigates filter resonance and offers good stability under stiff and weak grid conditions. However, its merits are offset by requiring many sensors and high-precision intra-cycle information of the circuit variables to dictate the states of the switches. Moreover, the system will also be in variable switching frequency operation, due to coupling circuit variables amongst phases, in controlling three-phase three-wire inverter. This paper presents a boundary controller that utilizes second-order switching surface to track directly the output current of the three-phase three-wire grid-connected inverter with an LCL filter. By applying the 60-degree discontinuous pulsewidth modulation scheme for a fictitious decoupled dual-buck structure in each operation sector, two separate sets of switching criteria with reduced number of current sensors, fixed frequency operation and recovered intra-cycle information of circuit variables for dictating the states of the switches of two half-bridge legs are formulated. A 3kW, 127V/50Hz prototype is built to validate the effectiveness of the proposed control method.