The Effect of Corona on Coupling Factors Between Ground Wires and Phase Conductors

Measurements of coupling factors between ground wires and phase conductors, as a function of ground-wire voltage, have been made for a sufficient number of cases to determine them for any of the standard transmission-line configurations. It was found that the effect of corona can be expressed in terms of an effective corona ground-wire radius to be used in the conventional coupling-factor equations. Curves are also presented for the quick determination of coupling factors for all standard line configurations. A photographic study of the visual characteristics of surge corona was made for a comparison with the effective radius and with 60-cycle corona. Actual measurements were made for potentials up to about 2,000 kv, but the data were extrapolated to 6,000 kv with what is considered to be reasonably good accuracy. Coupling factors for positive polarity were found to be considerably higher than those for negative polarity and both higher than values previously calculated from purely theoretical considerations. For a potential of 1,000 kv the actual coupling is 1.2 to 1.5 and 1.4 to 1.8 times as great, for negative and positive polarity respectively, as the values obtained by assuming no corona. At 2,000 kv the corresponding figures are 1.4 to 2 and 1.6 to 2.7 respectively, and at 4,000 kv 1.7 to 2.9 and 2 to 4. Ground-wire size was found to be unimportant above about 200 kv and, therefore, need not be considered.