Abstract
This thesis focuses on studying digital controls of bi-directional three-phase four-wire converters. Division-summation (D-Σ) digital control and one-sixth line-cycle regulation approach (OSLCRA) for a high power bi-directional three-phase four-wire converter are adopted. The converter has four kinds of operation modes: rectification mode, grid-connection mode, dc-link voltage regulation mode and ac voltage regulation mode. In the grid-connection mode, power is transformed into ac and injected into the utility grid. In the rectification mode, power from the utility grid is transformed into dc to supply dc loads and replenish dc-link capacitors. Under the grid-connection mode and the rectification mode, the converter is considered as a current source. In ac regulation mode, the power is transformed into ac to supply ac loads, and the converter is considered as a voltage source served as an ac emergency source. The D-Σ digital control is based on a current variation direct digital control, summarizing the filter inductance-current variations over one switching cycle to derive control law, which can release the limitation of abc to dq frame transformation. Besides, it takes inductance variation into consideration, and the core size can be reduced significantly and efficiency can be improved. This thesis presents design and derivation of the controller for converter. And then, description of program flowcharts and considerations of hardware implementation are presented. Finally, the converter operation modes are verified with simulated and experimental results. There are two major contributions of this research: analyzing and comparing control approaches for bi-directional three-phase four-wire converters, and applying the D-Σ digital control to a 100 kVA and presenting considerations of hardware implementation during experimental process to overcome the problems caused by non-ideal characteristics of the components.