Abstract
High voltage direct current transmission lines exist in various parts of the world. There are several in the United States. While designed to carry all the current in the wires, in case of emergency or if it is necessary to take one conductor out of service, earth may be used as a conductor. The earth current may be accumulated on, conducted by and discharged from underground metallic structures that cross the voltage gradient created by the current. This paper describes ground current operation tests made in June 1992 on two HVdc lines that run from Minnesota to North Dakota as a follow up to initial tests in 1976 on one line and in 1978 for the other. Results showed that stray current interference was not a major problem. Effects of the stray current interference were mitigated by distance of the structures from the earth electrodes, adequate cathodic protection and use of automatic potential controlled mitigation rectifiers. The tests showed the importance of close cooperation between electric companies and operators of underground facilities. They also showed that should earth be used as a conductor, special mitigative measures will be necessary.
