Conductor surface voltage gradients on bipolar HVDC transmission lines

The method of successive images is used to obtain accurate values of maximum surface voltage gradients for practical HVdc transmission lines. The results are presented in the form of graphs of gradient factors as a function of the principal line parameters. The gradient factor when multiplied by the line to ground voltage yields the maximum voltage gradient at the conductor surface. The range of values of parameters chosen covers transmission lines of voltages up to about ± 1200 kV. These graphs are also applicable for determining the center phase gradients on ac lines and therefore should be useful in making comparative studies between ac and dc systems. The results include an analysis of the sensitivity of the conductor surface voltage gradient to variations: in spacing between positive and negative conductors; in height; and in subconductor spacing. An optimum subconductor spacing is found for different numbers of conductors in the bundle as well as for different conductor sizes. However, it is shown that the gradient values are insensitive to fairly large variations in subconductor spacing about an optimum value. Finally, the accurate results presented are compared wlth those obtained for both dc and ac lines using some of the methods found in the literature.