考虑三次谐波调制的柔直系统交流侧单相接地保护研究

Research on single-phase grounding fault protection on the AC side of VSC-HVDC systems considering third harmonic modulation

为了简单可靠地实现柔直系统阀交流侧单相接地保护,针对阀交流侧中性点经阻抗接地的柔直系统,分析了交流侧的零序基波分量与直流侧的共模分量,推导了金属性接地故障下故障端与非故障端的零序基波电压、电流解析式。针对采用三次谐波注入调制策略的柔直系统,分析了三次谐波调制引起的纵向三次谐波电压相位特征,推导了三次谐波电压、电流的表达式,提出了利用中性点零序基波电压和三次谐波电压来判断接地故障及其位置的保护判据,并给出了相应的整定计算方法。不同工况下的仿真结果表明,当保护安装端发生接地故障时保护可以正确动作,当其他端发生接地故障时保护不会误动作。

To achieve simple and reliable single-phase grounding fault protection on the AC side of VSC-HVDC sys-tems,this paper focuses on systems where the neutral point of the converter valve′s AC side is impedance-grounded.The zero-sequence fundamental component on the AC side and the common-mode component on the DC side are analyzed,and analytical expressions for the zero-sequence fundamental voltage and current at both the faulted and non-faulted terminals during a metallic grounding fault are derived.For VSC-HVDC systems utilizing third harmonic injection modulation,the phase characteristics of the longitudinal third harmonic voltage induced by this modulation are examined,and expressions for the third harmonic voltage and current are derived.Based on this analysis,pro-tection criteria using the zero-sequence fundamental voltage and third harmonic voltage at neutral point are proposed to detect grounding faults and determine their location,along with the corresponding setting calculation method.Simulation results under various operating conditions show that the protection mechanism correctly activates when a grounding fault occurs at the protection installation terminal and does not operate erroneously when faults occur at other terminals.