CAREER: Hybrid Multiterminal DC Grids for Renewable Energy Integration

In this project, a novel Hybrid multiterminal DC (MTDC) grid technology is proposed that can be a potential game-changer in the integration of renewable energy, particularly wind energy from both the offshore and onshore locations. Voltage Source Converter (VSC)-based high voltage DC (HVDC) is preferred over the Line Commutated Converter (LCC) technology in integrating offshore wind energy, whereas onshore wind farms with a few gigawatts of capacity rely on LCC-HVDC in transmitting power over long distances. Point-to-point HVDC links, i.e. HVDC transmission systems with only two converter stations and a DC transmission line that connect wind farms to the AC grid can suffer from issues including curtailment of wind power, poor reliability, and instability of the AC-DC system following huge loss of infeed due to a single-point failure. To address these issues, a Hybrid MTDC grid with multiple LCC and VSC stations that will act as the backbone of the power transmission corridor for evacuating wind energy into the surrounding AC grids is proposed. Although much research attention has been focused on the VSC-based MTDC grid in the recent past, hardly any literature exists on the proposed Hybrid MTDC grid that addresses integration issues of onshore wind farms with LCC-HVDC. For example, fundamental insight is yet to be developed to comprehend the control interactions that determine the frequency in such systems where wind farms are connected to an LCC-HVDC terminal in a 'weak' AC grid. Moreover, there are complex operational challenges in such grids like power sharing issues following converter outage and problems due to the MTDC grid acting as a firewall and, thereby, decoupling the frequency support that is naturally available in traditional AC systems. To address these challenges, transformative ideas of system modeling, autonomous power sharing control, and frequency support strategy in Hybrid MTDC grids have been proposed that will substantially increase renewable penetration without compromising system reliability. In absence of any school in the nation that offers courses on HVDC and without a US manufacturer, this project is expected to contribute to the US efforts in this field. Graduate students working on this project will visit the Manitoba HVDC Research Center on a bi-annual basis to gain international research exposure. This program will promote teaching, training and learning of HVDC in the graduate program and renewable energy integration in the undergraduate program. To cultivate interest of K-12 students in power and energy systems, this project will conduct two STEM workshops in the West Fargo School district and a summer camp at NDSU, each year in coordination with the NDSU Engineering Outreach Office.

This project will develop a transformative approach that will establish a novel dynamic modeling philosophy in a frequency-dependent synchronous framework for Hybrid MTDC grids that interconnect inverter-interfaced offshore and onshore wind farms to the surrounding AC systems. Using this framework, a fundamental insight on the interaction among the weak AC system with low inertia and the controls of the LCC-HVDC terminal and the wind farm will be developed through an eigenvalue sensitivity-based approach. A novel adaptive autonomous control strategy and a novel emulative frequency support scheme of the surrounding AC systems from the offshore and onshore wind farms will also be pursued in the project.