The cable was designed to transmit electricity generated by the Quang Ninh power plant to the national power system.
The Quang Ninh 500kV substation and the upgraded Thuong Tin 500kV substation were also launched into use.
The new cable, more than 150km in length, runs through five cities and provinces and crosses three major rivers, according to the Management Board of Northern Power Works.
The board, under the National Power Transmission Company, said more than VND1.8 trillion (US$100 million) was invested in the 500kV Quang Ninh-Thuong Tin power cable, which was part of the 2001-10 Viet Nam Power Masterplan.
The project, using funds from the World Bank, was expected to meet the increasing demands for electricity in the north, create a strong connection between different parts of the power network and reduce power loss in the transmission system.