On the Gansu-Zhejiang ultra-high-voltage DC transmission project, the northern tower of the Yangtze crossing has completed top assembly. Conductors are strung between towers facing each other across the river; river crossings rank among the technically hardest sections of any such line.

The line forms part of the west-to-east power transmission program, carrying wind, solar and thermal generation from the northwest to the eastern seaboard. It is intended to firm up supply in the electricity-hungry Yangtze Delta.

Building across a great river

River spans require unusually tall pylons so shipping is not obstructed, with clearance calculated to allow for conductor sag. Foundations must account for changing water levels and soft ground, and devices to damp conductor vibration are fitted on windy stretches.

Contractors say conductor stringing over the water must be timed around navigation controls and weather, leaving the schedule heavily dependent on the season.

Where grids are heading

China's long-distance transmission is changing character as renewables take a larger share. The more generation swings with the weather, the more valuable stable long-haul DC links — and the system operations that balance them — become.

Neighboring countries face comparable problems. Korea is debating transmission expansion between east-coast generation and capital-area demand; Japan is strengthening interregional interconnectors.