基于三相串联MMC的直流输电线路中间落点控制策略研究

To increase the flexibility of traditional high voltage direct current transmission (HVDC), this project proposes a novel HVDC tap based on the three-phase series-connected modular multilevel converter (SC-MMC). However, there exist the following issues to be solved: the unbalance of dc voltages under ac faults of this tap, the control strategy of SC-MMC tap connected to weak ac grid and the coordination strategy between the tap and main HVDC converters. This project starts to investigate the operation principle and dynamic behavior of SC-MMC tap, and takes advantage of the ac power regulation to balance the dc voltages under ac faults, and then proposed a set of control and protection strategy of SC-MMC tap under ac faults. Furthermore, this project extends the research to the external grid that SC-MMC tap connects with: at ac side, the synchronous generator emulation control for SC-MMC tap is proposed to increase the support to the weak ac grid; at dc side, the interaction between the tap and main converter is first investigated and the control strategy especially for the traditional HVDC with tap is designed to minimize the interference that the tap could cause to the main converter, and the simulation to validate the proposed control strategy is performed on the scenario based on the real traditional HVDC project. Finally, this project aims to propose the HVDC tap theory based on SC-MMC, which will provide theoretical support for its application in prospective multi-terminal HVDC projects in China.

为增加点对点的传统直流输电的灵活性，本项目提出了一种基于三相串联模块化多电平换流器(MMC)的直流输电线路中间落点，但是该落点在交流故障下存在相间直流电压不平衡问题，落点会对主换流站的运行产生干扰，其接入的弱交流电网还容易发生频率波动和低频振荡。为此，本项目从落点本身的运行机制和动态特征入手，利用交流侧有功功率控制实现落点交流故障下的相间直流电压平衡，提出落点在交流故障下能够安全稳定运行的控制保护策略；然后拓展到落点与外部电网的相互影响机理及协调控制问题，在交流侧，研究适用于三相串联MMC落点的模拟同步发电机控制方法，加强落点对接入交流电网的支撑；在直流侧，分析落点与主换流站的相互影响机理，研究带有中间落点的传统直流系统的控制方式，减小落点对主换流站的干扰，并以实际工程为蓝本对所提控制策略进行仿真验证。本项目将为三相串联MMC落点的实际应用奠定理论基础，相关理论可推广至多端直流输电领域。

为增加点对点的传统直流输电的灵活性，本项目提出了一种基于三相串联模块化多电平换流器(MMC)的直流输电线路中间落点，但是该落点在交流故障下存在相间直流电压不平衡问题，落点会对主换流站的运行产生干扰，其接入的弱交流电网还容易发生频率波动和低频振荡。为此，本项目研究了：MMC落点在交流电网故障下的控制保护策略、MMC落点的虚拟同步控制策略和MMC落点与主换流站的协调控制策略。在研究过程中，根据国内外MMC的发展趋势和应用中出现的新问题，对以上研究内容进行了扩展和深化，研究了：采用不同模块结构时MMC落点的高效仿真模型、用于稳定分析的MMC落点降阶小信号模型和含MMC落点的交直流动态潮流计算方法。项目建立了一套基于相间功率平衡的三相串联MMC通用控制策略，实现了MMC落点在交流故障下的安全稳定运行、接入弱电网时的虚拟同步控制以及与主换流站的协调控制，为其实际应用了提供了理论基础。项目完成了预定的研究目标。扩展和深化后的研究内容产生的研究成果适用于更加广义的MMC落点，为解决多端混合直流输电升级为直流电网时所涉及的稳定分析与控制、仿真建模和潮流计算三大问题提供了基本思路。