Novel VSI Synchronization Unit With Increased Robustness: Detailed Analysis and Validation

In conventionally synchronized voltage source inverters (VSIs) that are connected to the main grid, through a transmission line, the influence of some critical state and parameter variations play a key role on the system response. Filter and line parameter variations, voltage deviations at the point of common coupling (PCC) and the positive feedback effect induced by the structure of the phase locked loop (PLL), may lead to undesired transients or even instabilities. Hence, in this paper, a novel synchronization mechanism is proposed, that instead of the PLL-based synchronization, provides an adequate solution to the aforementioned problems. The synchronization unit (SU) dynamics are analytically investigated for a VSI that is connected to the PCC, directly or through a transmission line. A detailed stability analysis and tuning process is presented providing an insight on how to improve the SU transient performance; the impact of filter and line dynamics on the response is investigated while a rigorous analysis shows that the proposed mechanism is robust against unplanned islanding. To establish the superior dynamic response of the proposed SU under filter/line parameter and voltage variations, simulation and experimental results are provided and are directly compared to other conventional PLL-based designs.