特高压直流分层接入系统强度评估方法及应用研究

Ultra high voltage direct current (UHVDC) systems under the hierarchical connection mode have promising prospects for engineering applications, due to their advantages of guiding the reasonable distribution of power flow, improving the economic efficiency of power grids, etc. A decreasing system strength means a reducing voltage support capability of AC power grids referred at the converter buses, which will consequently increase the risk of static voltage instability. Thus, proposing the accurate evaluation method of system strength, and then using it in establishing the explicit judging criterion for strong, weak, very weak systems as well as analytical method for static voltage stability assessment, are important guarantees for the safe and stable operation of the concerned systems. However, it is difficult to meet these requirements due to many limitations in existing research works. Facing these issues, this project will study the strength evaluation method of UHVDC systems under the hierarchical connection mode and its applications, aiming to provide an important theoretical and engineering reference for the safe and stable operation. Firstly, the accurate evaluation method of system strength will be established after the development of the evaluation method for the complicated interactions between high and low end converter. Secondly, the explicit judging criterion for strong, weak, very weak systems will be proposed using the description method of two-dimensional strength, based on the analysis of the complicated DC power-current coupling characteristics of high and low end converters. Thirdly, the analytical method for static voltage stability assessment with the ability to adapt diversified DC control modes will be proposed based on the system strength, through the obtainment of analytical relationship between the static voltage stability and system strength.

特高压直流分层接入系统具有引导潮流合理分布、提升电网经济效益等优点，因而工程应用前景较好。当系统强度较低时，交流电网对换流母线的电压支撑能力将会不足，从而增加静态电压失稳风险。因此，提出系统强度的准确评估方法，并将之应用于交流电网强弱明确判断标准和静态电压稳定性解析分析法的建立，是系统安稳运行的重要保障。然而，现有研究受限于诸多因素而难以满足上述需求。基于此，本项目拟开展特高压直流分层接入系统强度评估方法及应用研究，旨在为保障系统的安稳运行提供重要的理论和工程依据。具体内容如下：1）通过提出高低压端逆变站之间复杂相互作用的评估方法，建立系统强度的准确评估方法；2）在分析高低压端逆变站直流功率-电流复杂耦合特性的基础上，提出基于二维强度的受端交流电网强弱明确判断标准；3）通过建立静态电压稳定性与系统强度之间的解析关系，基于系统强度提出适应多样化直流控制方式的静态电压稳定性解析分析法。

特高压直流分层接入系统具有引导潮流合理分布、提升电网经济效益等优点，因而工程应用前景较好。当系统强度较低时，交流电网对换流母线的电压支撑能力将会不足，从而增加静态电压失稳风险。因此，提出系统强度的准确评估方法，并将之应用于交流电网强弱明确判断标准和静态电压稳定性解析分析法的建立，是系统安稳运行的重要保障。然而，现有研究受限于诸多因素而难以满足上述需求。本项目围绕“双碳”背景下保障特高压直流分层接入系统安全性和稳定性的重大需求，开展了强度评估方法及应用的基础理论和关键技术研究工作，取得的主要成果如下：1）通过建立特高压直流分层接入系统在“交流侧并联-直流侧串联”复杂耦合背景下的统一潮流分析模型，提出了基于分层馈入电压相互作用因子指标的高低压端逆变站之间相互作用解析评估方法，发展了基于分层馈入交互有效短路比指标的系统强度准确评估方法，揭示了不同系统参数、运行变量、拓扑结构及控制方式对系统强度的影响规律。2）通过建立高低压端逆变站的直流功率-电流耦合特性分析方法，挖掘了换流器最大可送功率点与额定运行点、工作极限点分别重合时的分层馈入交互有效短路比取值特征，提出了利用临界强度识别极弱电网/弱电网以及基于边界强度区分弱电网/强电网的新方法，实现了基于二维强度的特高压直流分层接入系统受端交流电网强弱明确判断标准。3）通过揭示高低压端逆变站直流有功-电压、无功-电压耦合特性与系统静态电压稳定性之间的相关性，建立了静态电压稳定性与分层馈入交互有效短路比、直流功率-电压耦合特性之间的解析关系，基于系统强度提出了适应多样化直流控制方式的特高压直流分层接入系统静态电压稳定性解析分析法，阐明了不同系统参数、运行变量、拓扑结构及控制方式对静态电压稳定性的影响机制。