An underwater grid project consisting of four 500-kilovolt alternating current submarine cables linking Hainan island and the mainland's other regions is set to undergo tests by the end of May as the island further diversifies its power transmission infrastructure.

The move comes close on the heels of policymakers charting plans to make the island's free trade zone more attractive to global investors.

Officials from EHV Power Transmission Co, the project's undertaker and a subsidiary of China Southern Power Grid, said CSG Main Grid and the Hainan Power Grid Interconnection Project II will connect Hainan's grid with the mainland's power grid in a two-way direction upon its completion.

"As a four-cable grid complementary to (the) current three-cable grid built as part of the CSG Main Grid and Hainan Power Grid Interconnection Project I, the Project II is a double check to guarantee Hainan's power supply if the island's generation plants are affected by severe weather conditions like a storm," said Chen Jun, director general at Guangzhou Branch's Haikou Subbranch of EHV Power Transmission Co.

In 2005, Hainan was struck by typhoon Damrey. Hainan's five power generators were shut down, causing significant losses and huge pressure on grid companies to bolster supplies on the southern Chinese island.

The Project II came 10 years after the completion of the CSG Main Grid and Hainan Power Grid Interconnection Project I, which consisted of three 500kV AC submarine cables, then the world's second largest underwater project.

"In the past when there were no such projects, severe weather conditions usually led to a massive power shutdown, causing unmeasurable losses and huge pressure to grid companies as power had to be transmitted from other regions through grids of limited capacity," Chen explained.

Project II will increase Hainan grid's power transmission capacity from 600,000 kilowatts to 1.2 million kilowatts. The enlarged capacity will play a major role in quick power supply to hospitals, factories, schools - facilities where a massive power shutdown could lead to injuries and great losses.

"It will also further help with Hainan's clean energy consumption," said Chen.

Once Project II is completed, Chen added it will make full use of Hainan's two nuclear power generator sets built under the Changjiang program jointly initiated by China National Nuclear Corp and China Huaneng Group Co Ltd.

As Project I only undertook limited power transmission, the two generator sets currently operate at 85 percent of their total capacity.

"That means Hainan will see over 1 billion kilowatt-hours of clean energy newly generated per year after the completion of Project II," Chen said.

The amount of newly generated nuclear power will provide strong support to Hainan's plan to promote new energy vehicles, as the province earlier announced it will stop selling fossil-fuel cars by 2030, according to Huang Xianqiu, project deputy manager at Hainan Networking Project Department under EHV Power Transmission Co.