CAREER: Hybrid Multiterminal DC Grids for Renewable Energy Integration

在这个项目中，提出了一种新型的混合多终端直流（MTDC）电网技术，它可以成为可再生能源整合的潜在游戏规则改变者，特别是来自海上和陆上的风能。在整合海上风能方面，基于电压源变换器（VSC）的高压直流（HVDC）比线路换向变换器（LCC）技术更受青睐，而具有几兆瓦容量的陆上风电场则依赖于LCC-HVDC进行长距离输电。点对点高压直流链路，即只有两个换流站和一条连接风电场和交流电网的直流输电线路的高压直流输电系统，可能会遇到风力削减、可靠性差以及由于单点故障导致的大量输入损失导致交直流系统不稳定等问题。为了解决这些问题，提出了一个混合MTDC电网，其中包含多个LCC和VSC站，这些站将作为输电走廊的骨干，将风能疏散到周围的交流电网中。尽管近年来很多研究都集中在基于vsc的MTDC电网上，但几乎没有文献报道提出的混合MTDC电网，该电网解决了陆上风电场与LCC-HVDC的整合问题。例如，在风力发电场连接到“弱”交流电网的lc - hvdc终端的系统中，控制相互作用决定频率的基本见解尚未得到发展。此外，在这样的电网中还存在复杂的操作挑战，例如变频器停机后的电力共享问题，以及由于MTDC电网充当防火墙而导致的问题，从而解耦了传统交流系统中自然可用的频率支持。为了应对这些挑战，在混合MTDC电网中提出了系统建模、自主电力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，以及本科课程中可再生能源整合的教学和学习。为了培养K-12学生对电力和能源系统的兴趣，该项目将与NDSU工程外联办公室协调，每年在West Fargo学区举办两次STEM研讨会，并在NDSU举办夏令营。该项目将开发一种变革性的方法，在频率相关的同步框架中建立一种新的动态建模哲学，用于将逆变器接口的海上和陆上风电场与周围的交流系统互连的混合MTDC电网。利用这一框架，将通过基于特征值灵敏度的方法，对具有低惯性的弱交流系统与LCC-HVDC终端和风电场控制之间的相互作用进行基本了解。该项目还将研究一种新的自适应自主控制策略和一种新的模拟频率支持方案，用于来自海上和陆上风电场的周围交流系统。

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.