Asymmetrical design of VSC-based HVDC transmission lines

The advent of VSC-based overhead dc transmission allows reversal of power direction by reversing current flow rather than voltage polarity. This, in turn, allows the poles of HVdc line to be designed independently, taking advantage of polarity-based differences in insulation strength, radio and audible noise generation, and ground-level electric field perception. Doing so will generally allow a negative voltage from 10% to 20% higher than the positive voltage, resulting in an increase in MW rating of approximately 5% and 10% respectively. The economic value of that gain is shown to be substantially greater than the incremental cost of tower and converter terminal asymmetry.