Basic Principles Concerning the Experimental Evaluation of the Frequency-Dependent Parameters of Shielded and Unshielded Three-Phase Symmetric Cables

The high-frequency domain characterization of overhead three-phase cables is a critical issue as far as power line carrier communications is concerned. In many instances a detailed knowledge of the electric and magnetic parameters of the cable is not available. Even if manufacturer cable-data are available, strong proximity effects among cable conductors do inhibit the utilization of closed-form expressions for the determination of the cable constitutive parameters, namely, the entries of the per-unit-length R, L, G , and C matrices. However, the determination of such parameters can always be made experimentally. In this paper, new measurement principles based on the cable excitation by independent propagation modes are presented and discussed. In the case of symmetric cables, the eigenvectors associated to the propagation modes are frequency independent, however, the propagation constants and surge impedances are not. The entries of the frequency-dependent R, L, G , and C matrices are retrieved from open and short-circuit cable measurements involving information on the modal propagation constants and modal surge impedances. For exemplification purposes, experimental results concerning a shielded three-phase cable for low voltage applications are provided. The results in this paper do not apply to buried cables where ground return phenomena would need to be accounted.