Voltage on any wire produces an electric field in the area surrounding the wire. In the SOO Green project, the HVDC lines are buried underground, eliminating the potential to generate and disperse air ions from the electric field to people and objects. Also, unlike uninsulated wires used in overhead HVDC and HVAC transmission, wires used in the SOO Green underground DC transmission project are insulated and shielded. This contains the electrical field within the cable and prevents release to the environment outside.
As additional background, the electric field from a power line gets weaker farther away from the line. There are major differences between the electric field environments of high voltage DC (HVDC) transmission lines that will be utilized on the SOO Green project and high voltage AC (HVAC). Electrical fields from DC lines are static, like the fields produced by the earth and by batteries. Electrical fields produced by AC lines tend to stay close to the wires, because they are alternately attracted and repelled as voltage alternates between positive and negative. In overhead HVDC transmission, charged particles around the wires (called air ions) can be dispersed by wind. Trees and other obstructions, like soil and building materials, also greatly reduce the strength of power-line electric fields. Electrical wiring and appliances inside a home also create electric fields. Electric fields are measured by the difference in voltage (volts, V) between two points of a specified distance (feet or m).
Typical electric field levels one foot away from home electrical appliances range from 4 V/m for a kitchen range to 60 V/m for a refrigerator. Electric-field strength 65 feet from a 500-kV AC transmission line, at the edge of right-of-way, is 2000-3000 V/m. Under a 500-kV DC overhead transmission line, the static electric field can reach 30 kV/m or higher. With the SOO Green underground HVDC project using shielded cables, the electric fields are contained inside the lines and not radiated into the environment, like they are with traditional transmission lines and home electric appliances.