混合直流输电系统存在边界元件不统一、故障响应特性不一致、高阻故障影响难解决和近端故障识别准确率低等问题，降低了保护方案的可靠性。为此，分析了混合直流输电系统的区域折射系数相位特征，并据此提出了一种适合混合边界的单端保护方案。首先，建立混合直流输电系统模型，解析不同故障类型的行波传递特征；随后，划分混合直流输电系统故障区域，分别推导区域的折射系数表达式及其相频特征；最后，提出基于特定频率折射系数的单端保护方案并进行性能测试。测试结果表明，所提保护方案具备传统保护方案的快速性外，具备更好的耐高阻故障、噪声干扰等能力。

The hybrid DC transmission system has problems such as inconsistent boundary components, inconsistent fault response characteristics, difficult resolution of high resistance fault effects, and low accuracy in identifying near end faults, which reduces the reliability of protection schemes. Therefore, the phase characteristics of the regional refractive index of the hybrid DC transmission system were analyzed for the first time, and a single ended protection scheme suitable for hybrid boundaries was proposed based on this. Firstly, establish a hybrid DC transmission system model and analyze the traveling wave transmission characteristics of different fault types. Subsequently, the fault areas of the hybrid DC transmission system were divided, and the refractive index expressions and phase frequency characteristics of the areas were derived separately. Finally, a single ended protection scheme based on a specific frequency refractive index is proposed and its performance is tested. The test results show that the proposed protection scheme not only has the speed of traditional protection schemes, but also has better resistance to high impedance faults, noise interference, and other abilities.