With the increase of urban electric load

improving the transmission capacity of existing city network has become extremely urgent. In order to solve this problem

a compact transmission scheme for increasing capacity is proposed

in which the sending and receiving ends connect to the original three transmission lines through half-bridge modular multilevel converters and inter-pole current transfer switches. Secondly

the current modulation strategy is modified to serve this topology and the ability of power increases is calculated. Thirdly

the steady-state operation mechanism and the system start-up procedure are analyzed

while the start-up strategy is designed. Finally

through the simulation based on PSCAD/EMTDC

the scheme is verified in terms of start-up and steady-state operation. The results show that the scheme proposed has good performance in increasing capacity while contributing to stable and reliable operation. This work is supported by State Grid Corporation of China (No. SGRI-DL-71-14-007).