A new analysis of the influence from external EMI sources on multiple underground cables

A theoretical analysis is presented for currents along, and voltages between, sheath and core conductors induced by external sources of electromagnetic interference (EMI) in multiple-conductor underground cables. The mutual coupling among the cables of either infinite length or finite length is taken into account. The terminal effects from the boundary conditions of finitely long cables and the effect of grounding on the coupling are analyzed. The analysis is based on the model presented previously by the author (see J. Electrot., No. 1, 1989). Using this model, the general solutions to currents and potentials along the cable sheaths are first derived, and then the current in each core conductor, and the voltage between the sheath and core conductor are determined. For finitely long cables, the above model is valid if ordinary boundary conditions, i.e. the arbitrary terminal impedance between the sheath and the earth, are taken into account. These so-called terminal effects can either strengthen or restrain the influence from external EMI sources depending on their phase superposition. It is shown that the terminal effect may become less important when the cable lengths get longer. Thus, the results for infinitely long cables can be achieved when the cables are sufficiently long. Examples are analyzed for some special EMI sources such as conductive and current energizations. Satisfactory solutions to these influences are derived