A comprehensive methodology for transient studies of HVDC lines for interconnected AC power systems is proposed as a useful tool for its design and operation. This methodology is based on frequency dependent modelling of HVDC transmission lines and a full modelling of their associated equipment in order to do transient simulations. The model is useful to carry studies of transient overvoltages produced by energization and de-energization, faults in the DC link and AC near systems, lightning and faults at both sides AC and DC. This model is useful for HVDC interconnections between systems without AC interconnections. This methodology is used to model a Colombia-Panama HVDC system interconnection selected as the case of study. Based on the obtained model, different transient phenomena were simulated: line switching transients, transients due to fault conditions and due to lightning surges. The results are also presented in this paper.