说明：  对高压直流输电系统的主要元件、相关元件的PSCAD模型、高压直流输电系统运行与控制 、高压直流输电系统的PSCAD仿真进行说明。
(The main components of HVDC system, the PSCAD model of the related components, the operation and control of HVDC transmission system, and the PSCAD simulation of HVDC transmission system are explained.)