Control of Voltage Source Converter Based Multi Terminal DC and Hybrid AC/DC Systems

Abstract

The journey of power systems started with the development of the dc technology pioneered by Thomas Edison in the late 19th century. Meanwhile, Nicola Tesla led the investigation of ac technology, which soon surmounted the dc paradigm in the "War of Currents", driven by the urge to favor higher-efficiency systems. For about a century, ac was the preferred choice in all sections of power systems, including generation, transmission, and distribution. However, with recent advances in power electronic technology, such as Voltage Source Converters (VSC), the conversion between ac and dc has become more practical, leading to the development of dc and hybrid ac/dc grids. These grids have been formed in both transmission and distribution systems. Specifically, on the transmission side, the advancement of the Multi-Terminal High Voltage Direct Current (MT-HVDC) grids enabled the formation of dc grids covering large geographical areas and integrating bulk renewable energy sources such as offshore wind. On the distribution side, on the other hand, the development of Multi-Terminal Direct Current MicroGrids (MT-DCmicroGrid) became an attractive solution for interconnecting increasingly popular dc generation sources with dc loads.

The control of the converters interfacing with those dc grids is one of the challenges for the future expansion of such systems and is the subject matter of the current Ph.D. thesis. The conventional droop control of VSCs has been the widely accepted solution for both MT-HVDC and MT-DCmicroGrid applications as it allows several converters to simultaneously regulate the dc grid voltage and share power imbalance in the system. However, droop-controllers have several application-specific challenges. This thesis investigates those challenges and proposes new control structures to overcome them. Specifically, the effect of the converters’ control on the regulation of dc network-related parameters, that is, on dc voltage regulation and ratio-based power-sharing between converters, is investigated for both MT-HVDC systems and MT-DCmicroGrid and new control approaches are proposed. Furthermore, the effect of the converters’ control on the interaction of MT-HVDC systems with neighboring ac grids is investigated, and advanced solutions are developed for enabling enhanced mutual frequency support of the ac systems interconnected through MT-HVDC systems. The developed strategies are applicable to general structure grids, do not require additional hardware and can be seamlessly integrated into the existing control solutions. The developed control strategies aim to assist systems operators in enhancing the performance of hybrid ac/dc systems. Comprehensive time-domain and modal analysis are conducted to compare the proposed strategies with relevant studies available in the recent literature, validating the advantage of the developed methods.