Power flow control in DC transmission grids using mechanical and semiconductor based DC/DC devices

DC power flow can be controlled though N-1 different DC lines/cables in a DC grid having N individual AC/DC converters. A large meshed DC grid can have more DC lines, and since we can not control power in all lines/cables, an overload may result. In this paper, two methods are proposed for DC power flow control through DC lines/cables. The first method involves using extra resistors embedded in series with DC lines/cables and switched by mechanical or semiconductor based DC circuit breakers. The other method includes installing DC/DC converter at one of the DC line/cable ends with the purpose of DC power flow control. It is shown that the first approach suits better for a short transmission line/cable, while the second one is proposed for a long transmission line/cable with high rated current and higher voltage drop. PSCAD simulation results are presented for a half bridge bidirectional DC chopper employed for DC power flow control between two large 370kV DC grids.