Control Techniques for VSC-Based Multi-Terminal HVDC Transmission System to Enhance the DC Voltage and Frequency Regulation of Hybrid AC/DC Power Grid

Abstract

A high voltage direct current (HVDC) transmission system is the solution for integrating offshore wind farms to ac grids, transmitting power to a long distance, and interconnecting asynchronous ac grids and power systems of islands because of its technical advantages compared to its counterpart. Particularly, voltage source converters (VSCs)-based HVDC transmission system is becoming preferable since it has numerous benefits over the line commutated converters (LCCs)-based HVDC system. Most of the existing operating HVDC transmission systems consist of point-to-point HVDC links. However, in the last decades, the research focus is shifted toward a multi-terminal HVDC dc grid that interconnects multiple VSCs to create a more reliable and flexible HVDC system. Although there are very few operating MTDC systems, it is anticipated that the future power system will consist of more MTDC systems. Therefore, enhanced control techniques are required to make the operation of the hybrid ac/dc power grid stable and secure.

DC voltage regulation is an essential requirement for the stable operation of the MTDC system. In the literature, there are different approaches for enhancing the dc voltage regulation by designing the droop gains of the dc voltage droop control. However, the existing techniques focus on improving the dc voltage response at steady-state. Few works study control techniques to obtain decent transient dc voltage response though the presented schemes aren’t effective enough to hold the transient dc voltage deviation within the maximum possible range. As a result, a novel dc voltage regulation strategy based on direct power control is proposed in this thesis to improve the dc voltage dynamic response. Also, a simplified model of the MTDC system is developed to investigate the impact of the active power controller on the dc voltage dynamics.

Furthermore, because of the decoupling effect of the HVDC system, the inertia of the hybrid ac/dc system will be reduced as the interconnection of ac grids through the MTDC system increases. Therefore, MTDC systems are required to participate in frequency support to make the hybrid power system secure. In the literature, there are different frequency support mechanisms. However, there is a need for an enhanced strategy to address different scenarios. In the existing frequency supports techniques, only enhancement of the frequency of the ac grids has given attention. However, the dc voltage is impacted by the frequency support. In this thesis, frequency support strategies that can achieve both enhanced dc voltage and frequency response are proposed. An adaptive droop gain scheme based on the loading of the converters is developed to tackle the limitations of the existing fixed droop gain mechanisms. Also, in most of the mechanisms, the same approach is used to minimize the frequency deviation of ac grids for the case of ac and dc side disturbances. However, the ac and dc side disturbances don’t have the same effect on the frequency deviation of the interconnected ac grids. Therefore, a frequency support mechanism with different approaches for the two scenarios is proposed. Moreover, the developed frequency support strategies are enhanced further to mitigate the impact of dc grid and VSCs power losses on their performance.

Date of Award	Dec 2022
Original language	American English
Supervisor	Mohamed El Moursi (Supervisor)