Ultra-high voltage direct current（UHVDC） transmission system has the characteristics of high voltage level

large capacity

long transmission distance

and flexible control. Continuous security operation of large number of ±800 k V DC transmission projects signifies technical maturity and large-scale application prospect. Perfect solution to the influence of UHVDC transmission system on environment and power grid security can improve economy of the project and reliability of power grid operation. Based on the characteristics of UHVDC technology all over the world

critical problems and measures of UHVDC transmission system in planning and design period are described

including site selection

environmental impact

technology scheme

security and stability.The study reveals that

with the rapid development of power electronic technology

the voltage level and power capacity of UHVDC transmission system will be further enhanced

and application of new converter technology will also possibly become reality.