Feasibility study and design of hybrid AC/DC high power transmission considering unbalanced line impedances

Considering stability, the loadability of long line ac transmission system is typically far below its thermal limitation. To partially address this problem, a hybrid ac/dc approach is investigated in this paper by superposing dc current on ac currents along existing ac lines, and no changes of insulators, towers and conductors are required. First, detailed performance comparison between line commutated converter (LCC) and voltage source converter (VSC) is carried out, which reveals that LCC based bipolar hybrid ac/dc is more suitable in this application due to the controllability of dc fault current on overhead lines. Based on the selected topology, the design methodology of zigzag transformer for dc power injection is proposed, mainly focusing on avoiding dc saturation of magnetic core when ac line impedances are unbalanced. A scaled down prototype is built to demonstrate this design approach and experimental results verify the feasibility of hybrid scheme. Finally, compared to the scheme of converting ac line to pure dc, LCC based hybrid ac/dc can be a much cheaper solution to upgrade HVAC power transfer capacity by 2 times around.