Candidate: Aram Kirakosyan
Title: Control of Voltage Source Converter Based Multi Terminal DC and Hybrid AC/DC Systems
Date: November 3, 2020
Time: 11:00 AM
Place: REMOTE ATTENDANCE
Supervisor(s): El-Saadany, Ehab (Adjunct) - Salama, Magdy
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, the conversion between ac and dc has become more practical, leading to the potential for development of dc and hybrid ac/dc grids. These grids have been formed in both transmission and distribution systems. Specifically, on the transmission side, High Voltage DC (HVDC) systems are now a preferable alternative to their ac counterparts for transmitting bulk power over long distances, encouraging the conversion of the generated ac power to dc. Among the two popular HVDC technologies, Voltage Source Converter (VSC) based HVDC systems have recently risen to be preferred over the Line Commuted Converter (LCC) based HVDC systems in several applications. An extension of VSC based HVDC systems are the Multi Terminal HVDC (MT-HVDC) grids, which enable the formation of dc grids covering large geographical areas. On the distribution side, on the other hand, the portion of dc generation has increased due to advancements in renewable energy sources such as solar power and fuel cell technologies, as well as in dc storage devices such as batteries. What is more, modern appliances such as elevators, computers, mobiles, light-emitting diode (LED) lighting and electric cars increase the portion of dc technology in customer loads. The advancement of VSC based Multi Terminal DC MicroGrids (MT-DCµGrids), therefore, facilitates the integration of dc energy sources with the mentioned dc load technologies. Thus, the formation of dc grids in both transmission and distribution systems and their integration with the existing ac systems has gained considerable attention in recent years.
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 this thesis. The conventional droop control of VSCs has been the widely accepted solution for both MT-HVDC and MT-DC MT-DCµGrid 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-DCµGrids 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. 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.