The Neptune project provides up to 660 MW of electric power from the PJM system to the LIPA grid on Long Island via a 500-kilovolt (kV), direct current (DC) cable. The DC cable extends between two converter stations, one in Sayreville, New Jersey, and one on Duffy Avenue in the community of New Cassel in the Town of North Hempstead. The Sayreville converter station takes alternating current (AC) power from the PJM system and converts it to DC power, while the Duffy Avenue station converts DC power back to AC for use on the LIPA system. The DC cable runs approximately 50 miles under the Raritan River in New Jersey and the Atlantic Ocean, and an additional 15 miles buried alongside the Wantagh Parkway. The project interconnects to PJM in Sayreville at a nearby First Energy substation, and interconnects to the LIPA system at the Newbridge Road substation in Levittown.
High Voltage Direct Current (HVDC) technology used in Neptune has also been used in scores of applications worldwide for many decades. HVDC has long been the preferred technology for moving bulk power over long distances. The global technology company Siemens provided the HVDC equipment for Neptune, as it has for more than 20 HVDC projects throughout the world since 1977.