Abstract: Due to the lack of sufficient right-of-way in constructing a new transmission system, more and more high voltage direct current（HVDC） circuits are built onto the existing corridor, alongside the high voltage alternating current（HVAC） transmission lines. One problem of such an application is steady state induction of fundamental frequency voltages and consequent circulating currents into the DC circuit. The line-commutated converter-modular multi-level converter（LCC-MMC） hybrid transmission system is a promising way to transmit bulk power over long distance with overhead lines, which has many advantages over the conventional HVDC topology. In the stage of planning a new hybrid system, comprehensive studies need to be carried out on the possible coupling effect from the nearby AC lines. In this paper, the theoretical formula of MMC DC bias under fundamental frequency circulating current was first analyzed, which shows refined consistency to the simulation result. The parameters of the coupled system on DC Bias were then studied, leading to a discussion about the compromise between the separation distance and parallel distance. A suggestive relationship between separation distance and parallel distance was put forward to instruct the future engineering projects.