Induced Voltage Calculation in Electric Traction Systems: Simplified Methods, Screening Factors, and Accuracy

In modern electric traction systems, the calculation of induced voltage on internal and external conductors is always required for both interference and people and equipment safety. The International Telegraph and Telephone Consultative Committee (CCITT) method is based on simplified relationships, which are integrated by suitable screening factors (representing and hiding the traction system complexity). Focus is on the determination of these factors and the accuracy, which are evaluated with simulation and experimental data. The considered systems are rail return, return conductor, and autotransformer (AT) 2 x 25-kV traction systems with victim cables of various lengths; the influence of the most relevant electric parameters (soil resistivity and conductance to earth of the return circuit conductors) is considered. The results confirm the relevance of the conductance to earth and of the position of the current injection point. The induced voltage is approximately proportional to the length, with a slight increase for shorter victim cables. The calculated AT screening factor is always smaller than the values suggested by the standards by a factor of 2 in the worst case. For the influence of separation of the inducing and victim circuits, the adopted methods agree in finding a critical distance around 6-8 m; at larger distances, CCITT values are slightly optimistic. Measured results confirm that the CCITT coefficients are all conservative by a factor of 2-4 (except the AT coefficient for total length exposure), whereas the multiconductor transmission line (MTL) results agree with a worst-case margin of 13%.