Yunnan Luxi - Back-to-Back Converter Station

The Yunnan Luxi HVDC project is a large back-to-back HVDC interconnection located in Luoping County, Qujing district, Yunnan province, China. It is a hybrid HVDC system combining both Line Commutated Converter (LCC) and Voltage Source Converter (VSC) technologies. The project consists of one VSC-HVDC unit and one conventional LCC HVDC unit operating in parallel. The total capacity of the project is 2000 MW, with the VSC unit rated at 1000 MW, ±350 kV DC, and the LCC unit at 1000 MW (2 x 500 MW poles).

Technical Specifications

The LCC HVDC classic system comprises 12-pulse converter units in each pole, six 400 MVA converter transformers, two sets of 160 MVar reactive power compensators on the rectifier side, and three sets of 110 MVar reactive power compensators on the inverter side. The VSC unit is rated at 1000 MW ±300 MVar. The converter station is connected to four 500 kV transmission lines on one terminal and three 500 kV lines on the other side connecting to the main grid.

Project Timeline and Significance

The first stage (VSC unit) was completed in 2016, followed by the second stage (LCC unit) in June 2017. The project enables asynchronous interconnection between Yunnan Power Grid and the main grid of China Southern Power Grid (CSG). It also facilitates the transfer of hydroelectric power from southwest China to main load centers in the south, estimated at over 3000 TWh per year. The hybrid configuration was chosen due to the weak AC system in Yunnan province, which did not allow for a purely LCC-based solution.

Technical Achievements and Operation

At the time of commissioning, the Yunnan Luxi HVDC project was the world's first converter station to coordinate flexible DC (VSC) and conventional DC (LCC) units. It featured the highest flexible DC voltage level (±350 kV) and the largest flexible DC capacity (1000 MW ±300 MVar). The VSC and LCC units are independent but can cooperate with each other. The VSC unit provides fast response in real and reactive power control, beneficial for the weak AC system in Yunnan, while the LCC unit is capable of transmitting bulk power efficiently. This hybrid HVDC project combines the strengths of both VSC and LCC systems to meet the specific requirements of the Yunnan power grid and its interconnection with the larger China Southern Power Grid.