**Abstract**

Computation of a single-phase short circuit mode of an effectively grounded network faces a number of difficulties. One of them is the need of calculation of mutual inductance between conducting elements of an overhead transmission line (OHTL). Current guidelines refer to the symmetrical components method for mutual inductance calculation, but their formulas of OHTL complex impedance are good only for barrel OHTL. In 110 kV industrial electric power supply system OHTL are relatively short and therefore are nontransposed. The authors introduce a combined equivalent scheme, which makes it possible to simulate OHTL with phase coordinates method and the rest network with symmetrical components. Such an approach is preferment for introduction of unsymmetrical position of wires and ropes on the line tower. The paper gives OHTL mathematical model and a comparison of line impedance calculated by different ways. Also the computation of a single-phase short circuit currents is conducted in the context of a real object – iron-and-steel industry enterprise electric power system. The computation is conducted not only for the short circuit on the bas bars, but also for an OHTL short circuit. Elaborated OHTL mathematical model is applied in KATRAN software.

**Keywords**

Electric power system, overhead transmission line, mutual inductance, short circuit, unbalanced mode, phase coordinates, simulation, KATRAN, symmetrical components, mode.

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