CAREER: Hybrid Multiterminal DC Grids for Renewable Energy Integration

在该项目中，提出了一种新型的混合多端直流（MTDC）电网技术，该技术可以成为可再生能源整合的潜在游戏规则改变者，特别是海上和陆上位置的风能。基于电压源换流器（VSC）的高压直流（HVDC）在集成海上风能方面优于线路换向换流器（LCC）技术，而具有几千兆瓦容量的陆上风电场则依赖LCC-HVDC进行长距离输电。点对点HVDC链路，即仅具有两个换流站和将风电场连接到AC电网的DC传输线的HVDC传输系统，可能遭受包括风力功率的削减、差的可靠性以及在由于单点故障导致的巨大馈入损失之后的AC-DC系统的不稳定性的问题。为了解决这些问题，提出了一种具有多个LCC和VSC站的混合MTDC电网，该混合MTDC电网将作为输电走廊的骨干，用于将风能疏散到周围的AC电网中。虽然在最近的过去，很多研究的注意力都集中在基于VSC的MTDC电网上，但几乎没有任何文献存在于所提出的混合MTDC电网上，该混合MTDC电网解决了陆上风电场与LCC-HVDC的集成问题。例如，基本的洞察力还有待开发，以理解控制交互，确定在这样的系统中的频率，其中风电场连接到“弱”AC电网中的LCC-HVDC终端。此外，在这样的电网中存在复杂的操作挑战，如转换器断电后的功率共享问题以及由于MTDC电网充当防火墙而引起的问题，从而解耦在传统AC系统中自然可用的频率支持。为了应对这些挑战，已经提出了混合MTDC电网中的系统建模、自主功率共享控制和频率支持策略的变革思想，这些思想将在不影响系统可靠性的情况下大幅提高可再生能源的渗透率。在美国没有任何学校提供高压直流输电课程，也没有美国制造商的情况下，该项目有望为美国在这一领域的努力做出贡献。从事该项目的研究生将每年两次访问马尼托巴HVDC研究中心，以获得国际研究机会。该计划将促进研究生课程中HVDC的教学，培训和学习以及本科课程中的可再生能源整合。为了培养K-12学生对电力和能源系统的兴趣，该项目将在西法戈学区举办两个STEM研讨会，并在NDSU举办一个夏令营，该项目将开发一种变革性的方法，该方法将在与逆变器互连的混合MTDC电网的频率相关同步框架中建立一种新的动态建模理念，将海上和陆上风电场连接到周围的交流系统。使用这个框架，一个基本的洞察力弱交流系统与低惯性和控制的LCC-HVDC终端和风电场之间的相互作用将开发通过基于特征值灵敏度的方法。该项目还将采用一种新的自适应自主控制策略和一种新的海上和陆上风电场周围交流系统的仿真频率支持方案。

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.