Operating unbalance in long-distance transmission

Unbalance in steady-state operation is of general concern in power transmission, but it can be especially emphasised when transmission is over long distances, owing to the increasing influence of line parameter asymmetries on operating conditions as transmission-line lengths increase. Specifically in the context of long-distance transmission, the paper first develops procedures for evaluating the dependence of operating unbalance on the precise sets of connections when conductor transpositions are made at discrete points along the length of a transmission line. These are included in detailed phase-variable models for transmission-line sections, and on them is based an overall form of Newton-Raphson solution. The methods are applied to a representative long-distance transmission interconnection operating at 220kV and having two separated points of conductor transposition. The inherent operating unbalance in the interconnection is evaluated for a range of power-transfer conditions, and the extent to which the unbalance can be lowered by discrete transposition is quantified. The interaction of load unbalance and transmission-line parameter asymmetry is investigated, and load distributions are identified which lead to the greatest increase and the greatest reduction in unbalance in transmission. For an optimal choice of conductor transposition from a total of 35 possible sets of connections involving the two transposition points, the paper then investigates the further lowering of operating unbalance which saturated-reactor forms of shunt compensation at selected locations can additionally offer. In total, the paper provides a comprehensive investigation of operating unbalance in long-distance transmission from which several conclusions are drawn.