Flexible DC Power Transmission

HIMALAYAL - SHANGHAI - CHINA

The flexible DC transmission also called HVDC Light or HVDC Plus is a new kind of DC transmission technology developed in the 1990s.  

The flexible DC transmission technology is based on source converter and PWM technology. It is characterized by fast response, good control, flexible operation mode. In addition, phase failure will not appear and the communication is not necessary between converter stations. The flexible DC transmission technology can be applicable to renewable energy on-grid, distributed power generation on-grid and islet power supply.    

The first flexible DC project - Wind & electric field flexible DC transmission project in Shanghai Nanhui District was put into operation in July 2011. The DC voltage is ±30kV and capacity of converter station is 18MW.      

Based on the flexible DC transmission technology, a multi-terminal flexible DC transmission system is further established, which can carry clean energies from many sites such as wind energy, solar energy to load centers of many cities through long channels.  



In 2014, the SGCC set up the 5-terminal flexible DC transmission project in the city of Zhoushan, Zhejiang province. The ±200kV DC voltage is applied; four converter stations are built.  

The SGCC is planning to establish the pilot project of renewable energy±500kV flexible DC power grid (pilot project for short) in the city of Zhangjiakou. The 4-terminal circular flexible DC power grid is expected to run in 2019.     

The wiring of pilot project system uses the two-polarity method. Both positive polarity and negative polarity can run dependently, amounting to two independent circular grids. If one polarity breakdowns, the other one can turn the power of fault polarity via the polarity control system if the flow ability allows. The flexible DC circular grid has many advantages, including high reliability, high flexibility and good expansibility.