Tri-pole high-voltage direct current (HVDC) is proposed as a new transmission technique to improve the AC line transmission capacity. This paper presents the basic principles of tri-pole HVDC and designs the power balance controller to make power stable. Based on the platform of PSCAD/EMTDC

a tri-pole HVDC simulation model is built. The operation performance of the system in the steady state is analyzed. Besides

a cooperation strategy for the control and protection under emergency outage of single converter station and under fault occurring in DC transmission line is fully elaborated. The system can be switched over to bi-pole HVDC under fault to maintain continued stable power transmission. The simulation results show that the proposed control strategy is correct and effective and the tri-pole HVDC is a practical way to enlarge AC transmission capacity.