0 导语

快速可靠的直流故障保护方案是架空柔直电网在发展过程中所面临的一大技术挑战。本文提出了一种基于模域分析的架空柔直电网故障保护方案。利用相模变换对正负极线路进行解耦，为单极接地故障分析提供理论支撑。在此基础上，利用模域分量构成故障检测和故障选极判据，从而提高保护算法的速动性与可靠性。敬请关注本期推荐。

An Improved DC fault Protection Algorithm for MMC HVDC Grids based on Modal Domain Analysis

Saizhao Yang, Wang Xiang, Li Rui, Xiaojun Lu, Wenping Zuo, Jinyu Wen,

期刊：IEEE Journal of Emerging and Selected Topics in Power Electronics.

Abstract/Highlight  

To detect the DC faults for MMC based DC grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis and sensitive to fault resistance and noise disturbance. To overcome these shortcomings, this paper proposes an improved DC fault protection algorithm using the modal domain approach for the MMC based overhead DC grids, which decouples interaction between positive and negative poles and mitigates the strong frequency-dependency of the characteristic impedance in phase-domain. The DC fault equivalent circuits are established in modal domain and the fault characteristics during the initial stage are analyzed. Based on the modal domain analysis, the line-mode reactor voltage which combines fault characteristics of negative and positive reactor voltages, is employed to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles, is employed to select the faulty pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, AC faults and DCCB operation, which are validated and evaluated by simulations in PSCAD/EMTDC.

1 项目背景

直流电网惯性小，故障电流上升速度快，幅值大，需快速可靠的故障保护技术进行故障隔离。以张北工程为例，故障电流要在5-6ms内进行开断。目前混合式高压直流断路器的动作时间为3ms，这要求保护出口时间为2-3ms，给故障保护的速动性与选择性提出了较高的要求。

现有的故障保护方案存在高阻故障不灵敏，单极接地故障无法分析，保护功能不完善的问题，因此急需一个能解决上述问题保护方案。

2 论文所解决的问题及意义

为解决现有保护方案中存在的“高阻故障不灵敏，单极接地故障无法分析，保护功能不完善”的问题，提出了一种基于模域分析的故障保护方案。利用相模变换对正负极线路进行解耦，为单极接地故障分析提供理论支撑。在此基础上，利用一模电抗器电压进行故障检测；利用零模电抗器电压进行故障选极。该方案具有较高的抗高阻和抗噪声能力，且不受故障类型，运行方式，交流故障以及断路器动作的影响。

3 论文重点内容

1）架空柔直电网的模域等效电路

考虑架空线路正负极间的耦合，并忽略了线路的分布电容参数，可以得到如图1所示的架空线路等效模型。从健全部分向故障线路看进去，忽略电阻影响，可以得到简化模型。进一步对正负极进行解耦，可以得到模域下的等效电路。图2为一模架空柔直电网等效电路；图3为零模架空柔直电网等效电路。

图1  忽略线路分布电容的架空柔直电网的等效电路

图2  一模架空柔直电网等效电路

图3  零模架空柔直电网等效电路

2）单极接地故障下的电抗器电压分析

在上述等效电路的基础上，根据故障的边界条件，可以得到单极接地故障下的复合网络，如图4所示。根据图4的复合网络可以得到一模电抗器电压的表达式与零模电抗器电压表达式。对比区外故障的等效电路图以及表达式，可以得出以下结论：区内故障下，一模电抗器电压大，因此可以构造以下检测判据：

对比不同类型的故障。发现极对极故障下，零模电抗器电压为零；正极接地故障，零模电抗器电压为正；负极接地故障下，零模电抗器电压为负；因此可构造以下故障类型选择判据：

图4  单极接地故障下复合网络

3）完整的故障保护方案

图5展示了完整的故障保护方案。包括故障启动判据，故障选极判据以及故障检测判据。如，当线路直流电压变化率大于整定值时，表明故障发生，故障启动元件动作；若零模电抗器电压大于某个正值，且一模电抗器电压大于整定值时，表明发生了区内的正极接地故障。其余的情况可类比这个逻辑。（这里对电抗器电压进行了积分，是为了消除噪声）

图5 完整的故障保护方案

Conclusion

A fast and selective protection scheme based on modal domain analysis for overhead MMC based DC grids is proposed in this paper, which decouples the dependency of positive and negative poles of overhead lines in phase domain and improves the capability of the faulted pole selection and endurance to fault resistance.

1) The PTG and PTP fault analysis under modal domain are conducted respectively, which provides the theoretical basis for the design of protection algorithm.

2) The derivative of DC line voltage (dVdc/dt) is adopted as the start-up element. Then, the zero-mode reactor voltage (∫VL12_0) is employed to select the faulty pole. The line-mode reactor voltage (∫VL12_0) is employed to identify the internal and external faults.

3) The proposed method can identify faults with fault resistance as high as 200Ω and it is immune to noise with 10dB. By only using local measurements, the proposed protection scheme is fast, no more than 1.1ms. In addition, the method is not affected by DCCB interruptions, operating conditions and AC faults. Quantities of simulation results demonstrate that the smaller current-limiting reactor will reduce the sensitivity to fault resistance and the protection can work well when the CLR is larger than 100mH.

4) In comparison with similar schemes, such as RVCR, RVOFP and ROCOV, the proposed method has advantages on the capability of the faulted pole selection, the sensitivity to high-resistance faults and the robustness to noise respectively.

作者简介：

Saizhao Yang obtained his B.E. degree in electrical engineering from Huazhong University of Science and Technology (HUST), China, in 2018. He is currently pursuing his PhD degree at HUST. His research interests include dc fault protection of MMC-HVDC and dc grids.

Wang Xiang (S’16-M’17) received his B.Eng. and PhD degrees both in electrical engineering from Huazhong University of Science and Technology (HUST), China in 2012 and 2017 respectively. He was a visiting student at the University of Aberdeen and the University of Strathclyde in 2014 and 2016 respectively. He was a research fellow at HUST in 2017 and 2018. Currently, he is a research associate with the University of Strathclyde. His main research interests include MMC-HVDC, high power dc/dc converters and dc grids.

Rui Li received the M.S. and Ph.D degrees in electrical engineering from Harbin Institute of Technology, Harbin, China, in 2008 and 2013, respectively. He is a researcher with University of Strathclyde in Glasgow, UK, since 2013. His research interests include HVDC transmision system, grid integration of renewable power, power electronic converters, and energy conversion.

Xiaojun Lu (M’19) received his B.Eng and Ph.D degree both in electrical engineering from Huazhong University of Science and Technology (HUST), Wuhan, China in 2013 and 2018, respectively. He is currently a post-doctor at HUST. His main research interests include modeling and stability analysis of VSC/MMC-HVDC and DC grids.

Wenping Zuo received the B.S. degree and Ph.D degree in electrical engineering from Huazhong University of Science and Technology (HUST), Wuhan, China, in 2009 and 2017, respectively. Currently he is a Post-Doctoral Research Fellow with HUST. His research interests include DC grid key equipment, energy storage, and renewable energy integration.

Jinyu Wen (M’10) received his B.Eng. and Ph.D. degrees all in electrical engineering from Huazhong University of Science and Technology (HUST), Wuhan, China, in 1992 and 1998, respectively. He was a visiting student from 1996 to 1997 and research fellow from 2002 to 2003 all at the University of Liverpool, UK, and a senior visiting researcher at the University of Texas at Arlington, USA in 2010. From 1998 to 2002 he was a director engineer in XJ Electric Co. Ltd. in China. In 2003 he joined the HUST and now is a Professor at HUST. His current research interests include renewable energy integration, energy storage application, DC grid, and power system operation and control.

期刊简介：

IEEE Journal of Emerging and Selected Topics in Power Electronics，二区SCI，影响因子为：5.9。收稿范围包括电力电子领域的选定主题和新兴技术，包括用于固态能量转换的组件、系统和工艺以及在节能和效率方面的应用。其中，固态能量转换包括用于移动、无线和其他应用的电子能量转换系统，以及与能量转换、存储和电能与非电能转换之间的接口相关的能量转换系统，包括功率转换器和驱动器；在节能和效率方面的应用包括能源的工业转型、制造业运营、本地电网连接、使用和管理、本地发电和热电联产。