Magnetic Fields

A magnetic field is produced when electric current flows in a wire. The static magnetic field from the SOO Green underground HVDC line, 3 feet above the ground surface and 20 feet away from the lines is less than the magnetic field a foot away from most home electric appliances. And it’s significantly lower than the magnetic field at the same distance from a traditional overhead HVAC transmission line.

As additional background, current flowing through wiring and appliances inside a home also creates magnetic fields. The magnetic field produced by electrical appliances decreases very rapidly away from the appliances. The magnetic field is described in units of micro-tesla (µT) or milli-gauss (mG). There are major differences between the magnetic field environments of high voltage DC (HVDC) transmission lines like the ones that will be utilized on the SOO Green project and high voltage AC (HVAC) transmission lines. AC transmission lines have three sets of wires. The wires on an AC line are continuously changing between positive and negative (120 times a second in the US). Bipolar DC lines, however, have only two sets of wires, called negative and positive poles, like a battery.  DC lines are also able to operate with one conductor (either positive or negative) in what is called monopolar operation. In this case, the earth supplies the return current path. The SOO Green HVDC transmission line is bipolar.

Typical magnetic field levels one foot away from home electrical appliances range from 3 mG for a refrigerator to 37 mG for a microwave oven. Magnetic fields average 30 mG, 65 feet from a 500-kV AC transmission line, at the edge of the right-of-way[1]. The static magnetic field from the SOO Green underground HVDC line, 3 feet above the ground surface and 20 feet away from the lines, is about 30 mG. Trees and most building material do not reduce the strength of magnetic fields.

The Earth has a static natural magnetic field that is caused by currents of electricity that flow in the molten core. These currents are hundreds of miles wide and flow at thousands of miles per hour as the earth rotates. The powerful magnetic field passes out through the core of the earth, passes through the crust and enters space. The earth’s static magnetic field has a value of approximately 500 mG at the surface. The natural magnetic field is weakest near the equator and almost twice as strong near the poles. The magnetic fields from a DC transmission line can either add to or subtract from the earth’s horizontal and vertical magnetic fields, depending on the direction that current is flowing on the line.