Investigations of Ground Current Phenomena Associated with HV DC Transmission

A brief outline is presented of major considerations when the earth is proposed for use as a return conductor on high-voltage direct-current transmission systems. Approximate values are given of earth potential gradients near an electrode, as determined in 1963 near Tracy switchyard by relatively low magnitude test currents and small spacings of test electrodes. Similar, more precise values are listed from large-scale tests conducted in March 1964 between Tracy switchyard and a point west of Red Bluff, Calif., with larger current flows and an electrode spacing of approximately 175 miles. Disturbances of a significant level are indicated as being confined to very small areas surrounding each electrode. Details of test equipment, transmission line facilities, permanent and temporary ground mats which were used as electrodes, and locations of various oil, gas, and water lines in close proximity to the electrodes are discussed. Design, installation, and general operation of a deep well electrode adjacent to the Tracy switchyard ground mat is described. The shielding effects of separation of ac and dc grounding points, as illustrated by specific test data and confirmed in laboratory experiments, is presented. Some possibilities for use of such shielding in limiting adverse flow of direct current in the neutral circuits of high-voltage ac systems are indicated. Additional tests to obtain further data for use in final designs are suggested.