Below grade transmission
There are two kinds of underground CGIT installations: buried and open or covered trenches.
Buried installations offer the advantage of lower installation costs in open areas. Trenches can be used when crossing roads or installing in areas where water tables are close to ground level. Either way, below-grade installations are more aesthetically pleasing, eliminate above ground congestion and reduce right-of-way requirements.
Buried CGIT bus installations offer the advantage of reduced installation costs by eliminating civil foundations and trench fabrications. These installations are particularly useful in areas where space is limited or limited access to the system is required.
Applications include buried transmission lines in urban or congested areas and connection circuits within power plants where space is at a premium. Although below grade conditions are rarely ideal for a transmission system, CGIT’s buried system utilizes an extruded polyethylene coating on the enclosure to prevent corrosion. Adjacent sections are welded together in the field and the completed joints are wrapped in polyethylene tape to prevent corrosion.
Open or Covered Trench Installation
Road crossings, water table considerations or power plant and substation access are the main considerations when choosing a trenched installation. This option allows for undisturbed road routing as well as access to the system and ease of installation and maintenance.
The Midway Sunset Cogeneration Project is a perfect example of the advantages of trenched transmission: the site layout consisted of three 242 kV circuits—with each circuit crossing the path of the plant’s main access road. The solution called for the installation of bus in trenches, allowing for undisturbed road routing and accessibly to the trench’s access panels for easy installation of the bus between the GIS and connecting transformers.