Power quality (PQ) is not only becoming an important factor for proper operation and control of the power system (PS), but its continuous monitoring represents a basis for PS efficient exploiting. The power quality of only a small number of nodes in a power system is monitored. Estimation of certain PQ indicators in transmission and distribution system is a very important factor for the correct PS control and exploitation. By using reliable and fast algorithms, it is possible to conduct an real-time estimation of certain PQ indicators. This paper deals with the voltage harmonics estimation in the part of network without PQ monitoring. The overhead lines of the same voltage level are considered. Input data are: (1) harmonic content in the voltage and current waveforms at the places of electrical energy exchange, (2) length of transmission lines, (3) network topology, (4) data about conductors (5) type of tower head. An objective function is formed by using: (1) the node-voltage method (2) Kron reduction of matrices (3) by PS modelling in the system of phase values and (4) by performing analysis in frequency domain. The equivalent geometry of tower head and the soil specific resistivity are determined by applying an evaluation method, genetic algorithm (GA). A global matrix [Y] is determined by the node-voltage method on the basis of obtained parameters, and harmonics in the part of network without PQ monitoring are then determined by applying Kron reduction of matrices. The problem is solved in the system of phase values and in the frequency domain. The estimated harmonic values and a full control over the part of the network where the harmonic estimation is performed are obtained as the final results. The conclusions in this paper indicate that the implemented algorithm is fast, efficient and applicable in the real power system.