Study of capacity-increase for simultaneous AC-DC transmission based on double circuit AC transmission line

The maximum transmission power of high voltage long distance transmission is mainly subjected to the static stability. Through analyzing the relationship between the transmission power and power angle, we find that using the fast control of DC components to respond to load disturbances can improve the stability margin of transmission system and expand the transmission capacity indirectly. In addition, the relationship between the effective short circuit ratio and stability of both sides of the AC system has also been discussed. It is found that there is no need to change the existing current circuit structure if DC components satisfy certain superposition rules. Consequently, the investment and operating cost have been drastically saved by the superposition of AC-DC transmission. Finally, Matlab simulations verify the the feasibility and validity of the scheme.