On August 21, 2014, State Grid Corporation of China (SGCC) released the Catalogue of New Technologies of State Grid Corporation of China (2014 Edition) and the Catalogue of Key Promoted New Technologies of State Grid Corporation of China (2014 Edition), clarifying the overall requirements for the promotion and application of new technologies. Among them, solid-insulated ring main units (RMUs) were once again identified as a key focus of new technology promotion in the coming years.
In fact, as early as August 2011, when SGCC issued the First Batch of Catalogues of Key Promoted New Technologies of State Grid Corporation of China, the technology of solid-insulated RMUs had already been included in the category of new technologies for promotion and application. Given the development trend of stricter requirements for technical reliability and environmental protection, solid-insulated RMUs are trending toward fully replacing sulfur hexafluoride (SF₆) gas-insulated RMUs.
According to the plan in the new catalogues, during 2014–2016, the usage of solid-insulated RMUs in SGCC's new construction or renovation projects accounted for 15%–20% of the total new additions. During 2017–2019, this proportion increased to 25%–35% of the total new additions in such projects. While the application ratio of solid-insulated RMUs is rising year by year, issues such as partial discharge control, resolution of "false breakpoints", and epoxy resin recycling have emerged as challenges for the industry.
"For metal-enclosed switchgear (including gas-insulated switchgear) with voltage levels of 40.5 kV and below, the adoption of vacuum as the arc-extinguishing medium has become an established fact. Products using solid insulating materials and non-SF₆ gases as insulating media are being rapidly promoted and applied," stated Wang Chengyu, a senior engineer from China Electric Power Research Institute (CEPRI). Environmentally friendly switchgear rarely uses or avoids media with significant environmental impacts (such as SF₆ gas), exerting minimal environmental influence throughout the entire product lifecycle and boasting strong environmental adaptability.
All primary live parts of solid-insulated RMUs are enclosed, producing no inhalable particulate matter, toxic gases, or greenhouse gases. They are free from potential hazards such as gas leakage, explosion, and internal arcing, and feature high insulation performance, strong environmental tolerance, compact size, large capacity, high overall strength, flexible combination, and maintenance-free operation. However, for China's power distribution network, solid-insulated RMUs are a new product, and many issues remain unresolved.
Relevant statistics show that in the first half of 2013, SGCC's total tender volume for medium-voltage RMUs reached 85,000 functional units, among which solid-insulated RMUs accounted for only 2,500 units, representing a mere 3% of the total tender volume.
The persistently high price of solid-insulated RMUs and the environmental challenges posed by epoxy resin recycling cannot be ignored.
As a new product, users lack an understanding of the operational performance, operation and maintenance costs of solid-insulated RMUs during use, particularly their impacts on the safety and reliability of power grid operation. Furthermore, users have little first-hand experience with their new characteristics and advantages.
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