In order to overcome shortages of low reliability

high insulation level and difficulties in design and transportation for large capacity transformers

bipolar connection are adopted in voltage source converter based high voltage direct current(VSC-HVDC)projects

which is firstly introduced in ±320 kV/1 000 MW Xiamen VSC-HVDC project. Taking Xiamen project as an example

the study on system design for large-capacity VSC-HVDC project with bipolar connection is presented. Firstly

the single line diagram is optimized for multiple operation modes

in order to improve system reliability. Secondly

as the margin for main equipment is generally small in large capacity VSC-HVDC project

main circuit parameters are calculated accurately so that small-capacity insulated gate bipolar transistor(IGBT)can support large-capacity power transfer. Finally

the transient characteristics of VSC-HVDC systems with both kinds of connections are compared. The conclusion is that the transient overvoltage decreases while the transient current increases. The safety of IGBT under overcurrent is guaranteed by adopting the accurate analytical transient current calculation method and the optimized layout design. The practical work is done to show the feasibility and effectivity of the proposed design method.