Zhongheng HVDC completes first annual maintenance

The ±800-kilovolt Zhongning Converter Station resumed power transmission on June 12, following its first comprehensive annual maintenance.

The 12-day maintenance work eliminated any equipment hazards and completed the first autonomous overhaul of Ningxia's ultra-high voltage (UHV) valve cooling equipment, ensuring the stable operation of the "Ningxia to Hunan" new energy transmission corridor.

The Zhongning Converter Station is a core terminal for the Zhongheng high-voltage direct current (HVDC) line, which primarily transmits solar and wind energy from the desert and wasteland regions. Officially operational since October 2025, the line has been running at full capacity, accumulating numerous equipment defects along the way. This marked the first in-depth system overhaul since it was commissioned.

The process faced four major challenges: the absence of a mature initial maintenance plan, cross-disciplinary operations, high-precision standards for equipment calibration, and complex multi-unit coordination.

The operation involved 15 units, 592 personnel, and 48 special vehicles. A staggered shutdown and peak-shift construction model was employed, completing the first power-down operation in seven hours and processing all work permits within one hour.

The work included 798 routine checks, seven special inspections, six hazard mitigations, and 193 equipment defect resolutions, with 4,000 switch operations executed flawlessly, achieving a successful power restoration without any violations, delays, residual defects, or safety incidents.

State Grid Ningxia UHV Co coordinated the entire maintenance process through a closed-loop management approach divided into preparation, execution, and acceptance phases.

Preparations included personnel, material, and planning readiness, with dedicated command, technical, and safety teams established. During execution, an intelligent security system was deployed, and work zones were controlled, addressing 299 power-off defects and rectifying 83 percent of existing issues within five days.

The acceptance phase involved a three-tier review by teams, specialists, and dedicated groups, conducting comprehensive performance tests on core equipment like valve groups and main transformers, with any non-conforming procedures immediately rectified.

This maintenance effort established a standardized operational protocol for Zhongheng HVDC's initial inspection. The operations will focus on summarizing experiences in collaborative construction, autonomous maintenance, hazard management, and safety control.

The goal is to enhance equipment monitoring, fault prediction, and emergency response processes, ensuring the long-term stable operation of UHV lines, consistent transmission of Ningxia's renewable energy, and support for cross-province energy distribution and new power system construction.