Study of AC/DC coupling for Three Gorges to Changzhou HVDC system

The new solution for calculation AC/DC coupling issue in the same ROW was proposed and validated by EMTDC. The brief curve of I_ac-in-dc versus D_ac-to-dc for per coupling length of AC line was obtained by the proposed method (I_ac-in-dc and D_ac-to-dc here mean the fundamental frequency current in DC line and the distance between AC and DC lines respectively). Based on this curve, the value of I_ac-in-dc can be easily estimated for different AC/DC coupling schemes especially in the cases of the AC and the DC lines being built simultaneously, or the DC line being built earlier than the AC line. The relationship between the I_ac-in-dc and the DC bias current in converter transformer valve side with the actual HVDC control system is also investigated