Combinations of both overhead and under-ground HVDC line segments frequently occur. 高压直流架空线路和地下线路分段的组合也常采用。
It is difficult to obtain the dynamic models of converters, overlong overhead HVDC lines and passive filters in audio frequency. 然而，由于运行工况及气候的影响，难以求取换流器及长距离高压架空线路在音频带内的动态数学模型。
