Abridgment of study of the effect of short lengths of cable on traveling waves

Abstract

Cathode ray oscillographic tests were made of traveling waves passing from an overhead line into short lengths of cable. Single-phase cable lengths of 500 and 1000 ft. were used. Tests were made with a wide variety of conditions, such as cable at end of line, cable between two sections of line, etc. The tests demonstrated that a short length of cable does not act as an effective protective device when connected between station and incoming line. When the incoming wave on the incoming line has a long flat top, several times the cable, length, the crest of the wave passing the cable is decreased only a few per cent by the presence of the cable, although the wave front is sloped off if it was originally very steep. When the wavelength is approximately the same as the cable length, the cable can reduce the transient voltage to less than half of the value without the cable. Recent measurements on transmission lines show that waves do occur with approximately flat tops which are several thousand feet in length. Therefore the cables do not permit the omission of lightning arresters which are means of reducing the potential of overvoltage surges. This confirms the theoretical calculations and the practise of using lightning arresters with cables. The resistance of the ground connection of the cable sheath was found to have important effects. With the cable sheath at the end grounded through 28 ohms, a potential of 82 kv. was measured at the sheath and 105 kv. at the cable conductor. The protective action of lightning arresters was demonstrated by using a gap and various values of series resistance. The velocity of propagation in the cable was found to be about 61 per cent of the velocity of light. The surge impedance of the cable was determined by several methods. Calculations based on the measured propagation velocity and measured capacitance gave a value of about 50 ohms. Some of the other methods gave values of about 100 ohms.