Circuit representation of voltage unbalance emission due to line asymmetry

This paper provides a circuit approach to the phenomenon of voltage unbalance emission due to line asymmetries in three-phase power systems. Indeed, it is well known that under the assumption of symmetrical phases, power systems are effectively analyzed through the symmetrical component transformation leading to uncoupled sequence circuits. In practical power systems, however, the assumption of symmetrical phases is not always met for several reasons including untransposed overhead transmission lines. From a circuit viewpoint, when the symmetrical component transformation is applied, such asymmetry results in coupled sequence circuits. As a consequence, a voltage on the negative-sequence circuit arises, leading to voltage unbalance emission. In this paper, a rigorous circuit representation of coupling between positive- and negative-sequence circuits is derived. The proposed equivalent circuit allows the analytical derivation of voltage unbalance emission as a function of line asymmetry. The derived analytical model is validated trough numerical simulation of a radial network including an untransposed line.