**Background:** High voltage overhead transmission line parameters – series impedances and shunt admittances – depend on many different factors such as the conductor geometry and physical properties (including the earth return path conditions). The influence of a certain characteristic on the line parameters is not always evident. A dedicated study is therefore required to thoroughly examine the impact of various conductor characteristics on the positive and zero sequence line parameters. The need for such a study is driven by the increasing attraction of researchers all over the world to the online estimation of line and transformer parameters based on synchronized phasor measurements. Line parameter estimation algorithms described in most of the works on this topic (mainly published abroad) are built around an optimization problem solved directly for the line impedances and admittances. This results in a large vector of unknowns even for a three-phase single circuit line. Moreover, these unknowns are regarded by the optimization algorithm as totally independent of each other, which makes the problem more ill-conditioned along with decreasing the computational efficiency. Taking these issues into consideration, it appears reasonable to find out which of the many conductor characteristics have a non-negligible influence on the line impedances and admittances, and then try to reformulate the optimization problem.

**Materials and methods:** A model of a real 500 kV double-circuit line was taken for numerical experiments carried out in MATLAB. Eleven different scenarios were considered, each focusing on a certain factor affecting the line impedance and admittance matrices. In each case, one of the parameters was varied (or a few closely related parameters like the average phase conductor and ground wire heights) while all the others remained equal to their chosen default values. Care was taken when conducting the simulations to avoid considering unrealistic values that the varied parameters would not take on in real-field conditions.

**Results:** A number of experiments have been carried out to identify a group of factors that have a non-negligible impact on the line parameters. These factors are as follows: the average phase conductor heights affected by the sag; the average ground wire heights; the soil conductivity; the conductor DC conductivities, and the ground wire permeability. The influence of all the other uncertainties (such as those in the conductor radii, the horizontal layout of the conductors, and the ground permittivity) can essentially be neglected.

**Conclusions:** The obtained results enable us to try out a completely different approach to overhead line parameter estimation based on synchronized phasor measurements, which should hopefully put us a step closer to incorporating a line parameter tracking algorithm into monitoring software tools utilized in electric power systems (successful practical implementations are likely absent). Correct information about the overhead line parameters could be used in a number of applications such as power system state estimation, impedance-based fault location on transmission lines, distance protection settings correction, and a more accurate monitoring of the loss of power transmitted through overhead lines, to name a few.

**Key words:** high voltage overhead transmission line, line parameters, zero sequence parameters, positive sequence parameters, admittance, synchronized phasor measurements, impedance.