With advanced onshore, offshore and aerial tools combined, the world's first five-terminal flexible DC transmission project — the Zhoushan ±200 kilovolt five-terminal flexible DC project — underwent its first comprehensive inspection since commissioning this month in Zhoushan, Zhejiang province.

The inspection marks the largest maintenance operation in the 10-year history of the Zhoushan flexible DC project. At the Zhoudai station, the hub of the five converter terminals, workers have conducted thorough inspections on submodules, water cooling systems, wall bushings, transformers and other electrical equipment, encompassing eight work areas and over 210 maintenance and testing tasks, representing the most extensive maintenance effort since the project's operation, according to the State Grid Zhoushan Power Supply Co, the project's operator.

Utilizing intelligent equipment and advanced technology for land, sea, and air space, the inspection has reduced required maintenance time from the usual 15 days to merely 9 days, significantly improving work efficiency.

For example, for the first time in flexible DC maintenance, an X-ray inspection device was used to check the integrity of the water electrodes inside the cooling system on a valve tower. For aerial inspection, intelligent inspection drones were deployed to inspect the status of high-altitude equipment inside valve halls. Additionally, an underwater robot was also utilized to examine the wear and tear of submarine cables in intertidal zones, and assess the status of the cables' outer insulation protection layer.

Upon completion, the inspection will significantly enhance the reliability of the power supply network for the northern islands of Zhoushan, and provide a maintenance model for other multi-terminal flexible DC projects in China, said Zhoushan Power Supply Co.

The Zhoushan flexible DC project is the world's first and currently the largest flexible DC transmission project with five terminals. Starting operation in 2014, the project includes five converter stations — Zhouding, Zhoudai, Zhouqu, Zhouyang and Zhousi — connected by 283 kilometers of DC submarine cables, forming an interconnected power grid among the Zhoushan Islands.

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