Abstract
This thesis presents design and implementation of a three-phase four-wire inverter with LLCL output filter. To achieve higher applicability in the emerging development of dc micro-grid and renewable energy systems, the inverter is equipped with multiple functions, including grid-connection, rectification, dc-link voltage regulation and active power filtering (APF). In general, control strategies used in most multi-function systems tend to be complex, where each operation mode requires its own way of control. However, the division-summation (D-∑) digital control adopted in this thesis is able to overcome the above problem. With one common duty-ratio expression, various operations can be realized with different current commands, which greatly reduces the control complexity. Furthermore, with system parameters such as grid voltage, dc-link voltage, filter inductance and switching period included in the duty-ratio expression, the inverter is capable of operating normally despite of parameter variation. Being the first to apply the D-∑ digital control to the inverter with an LLCL filter, this thesis also proposes several solutions on problems that could occur in hardware implementation; besides, parameter design procedure is also addressed and can be adapted to applications with different specifications. Finally, each operation mode has been tested with a prototype built in lab. Measured results show consistency with simulation, and most importantly, output current can comply with the total harmonic distortion requirement stated in “IEEE Std 519™-2014”.