The Neptune project includes no overhead transmission lines. All transmission cables are buried either under water (approximately 50 miles) or underground (approximately 15 miles).
The Neptune cable route includes a primary DC cable and two relatively short lengths of AC cable. All cables were manufactured by and installed under the direction of Prysmian Group. The DC cable actually consists of three cables: a main high-voltage cable approximately five inches in diameter that carries up to 660 MW of electricity at 500 kV; a medium-voltage “return” cable necessary for carrying current in a DC system; and a fiber-optic cable for system control.
Underwater, the three cables were bundled and buried four-to-six feet under the river and sea bed using a ship and equipment specially designed and outfitted for this purpose. On land, the cables are buried three-to-four feet below ground in separate conduits using conventional trenching and horizontal directional drill (HDD) methods.
In addition, the Neptune system includes a 345-kV AC cable extending about 1.5 miles from the Duffy Avenue converter station to the LIPA Newbridge Road substation; and a 230-kV AC cable extending from the Sayreville, NJ converter station for about half a mile to the First Energy substation.