7. Corona effect on DC lines

For the general characteristics of direct-current links, please refer to article D 4760 High-voltage direct-current power transmission. With regard to the particular problem of the corona effect of DC lines, it is sufficient to know that the operating voltages of these links range from ± 100 kV, for the oldest links, to ± 1,000 kV, for the most recent projects. DC lines can be either monopolar (positive or negative) with ground return, or bipolar. As with alternating current, as operating voltage increases, conductor bundles are used to reduce the surface field.