Direct Current Tracking Using Boundary Control with Second-Order Switching Surface for Three-Phase Three-Wire Grid-Connected Inverter

Cascaded boundary-deadbeat controller can ensure system stability of single-phase grid-connected inverter operated under different grid conditions. However, it will require using numerous sensors and may experience performance degradation caused by unbalanced filter parameters in controlling a three-phase three-wire inverter. Furthermore, the system is in variable frequency operation. A boundary controller using second-order switching surface with direct current tracking capability, reduced number of current sensors, and fixed frequency operation of three-phase three-wire grid-connected inverter with inductive-capacitive-inductive (LCL) filter is presented. By applying the 60° discontinuous pulsewidth modulation scheme for a fictitious decoupled dual-buck structure in each operation sector, two separate sets of switching criteria for dictating the states of the switches of two half-bridge legs are formulated. Such technique can avoid dealing with the challenges caused by the interactions among three independent current regulators. A switching table with null voltage vectors excluded is designed to dictate the switching actions. Sensitivities of the system transfer characteristics to the parametric variations are investigated. A 3 kW prototype has been built and evaluated under stiff- and weak-grid conditions. The experimental results are favorably compared with theoretical predictions.