Summary: In power systems, overhead lines in which at least two circuits with different levels of rated voltage are led on a common supporting structure, are appearing more and more often. Such solutions also appear in the National Power System. The development of multi-circuit lines, including multi-voltage lines, is a source of new technical challenges affecting the legal and safe operation of the entire power system. The article presents an overview of multi-circuit multi-voltage overhead lines in the world and discusses the method of determining the mathematical model these lines. The determination of the asymmetry factors allowed analyzing the influence of the geometric unbalance of the line on the correct operation of the power system. In the last part, the influence of the use of simplified symmetrical models for lines with various degrees of geometric asymmetry on the correctness of determination of short-circuit currents was considered.