Traveling-Wave Voltages in Cables

The problem of lightning protection of cables is more active at the present time than it has been in the past. This is du-e largely to the fact that cables of higher voltage connected directly to overhead lines are becoming very common; cables in the form of short sections are especially liable to need protection. The cost of the cable, the importance of the line from the operating standpoint, and the cost of protective equipment all enter into the picture. In the present paper the factors which determine whether or not a cable will require protective equipment are discussed, and new and simplified formulas, representing with considerable precision any of the standard coordinating waves are developed. These formulas can be used for open-ended cable lines, or cable sections in transmission lines. Where cables terminate in station apparatus, the assumption of an open-circuited cable end may be safely used, since it always gives a calculated value of voltage that is somewhat above the true value. The effect of cable length and surge impedance and of line surge impedance are illustrated in detail. Tentative data on the impulse strength of cable insulation are also included.