under ground transmission line


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INTRODUCTION: This overview contains information about electric transmission lines which are installed underground, rather than overhead on poles or towers. Underground cables have different technical requirements than overhead lines and have different environmental impacts. Due to their different physical, environmental, and construction needs, underground transmission generally costs more and may be more complicated to construct than overhead lines .

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HERE WE WILL DISCUSS : v Types of Underground Electric Transmission Cables v Ancillary Facilities v Construction and Operation Considerations v Costs v Repairs

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Types of Underground Electric Transmission Cables: The common types of underground cable construction also include : v High-pressure, fluid-filled pipe (HPFF) v High-pressure, gas-filled pipe (HPGF) v Self-contained fluid-filled (SCFF) v Solid cable, cross-linked polyethylene (XLPE)

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High-Pressure, Fluid-Filled Pipe-Type Cable: 1. HPFF or HPGF Pipe-Type Cross Section: Welded Externally Coated Steel Pipe Pressurized Gas or Fluid (usually nitrogen or synthetic oil at 200 psi Segmented Copper Conductor Paper Insulation Metallic Shield

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2. High-Pressure, Gas-Filled Pipe-Type Cable: 3. Self-Contained, Fluid-Filled Pipe-Type: 4. Solid Cable, Cross-Linked Polyethylen:e Self-contained cables are laid either directly onto the bottom or into trenches. In crossing bodies of water, the ordinary plastic or lead covered cable is usually protected with a wrapping of tarred jute, and armored with galvanized-steel wire.

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5. XLPE Cables with Different Voltages: Underground XLPE cables left to right: 345 kV, 138 kV, 69 kV, and distribution

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XLPE Cable Cross-Section Cross-linked Polyethylene Insulation Segmental Copper Conductor and Shield Outer Coverings

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Ancillary Facilities : 1.Vaults : 345 kV XLPE project – Cement vault visible with two chimneys extending up to be level with the future road surface. 138 kV XLPE project – Bottom half of pre-constructed vault positioned in trench.

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2.Transition Structures: insulators Pole height range 60’-100’ pole heads Raiser height range 30’-40’

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3.Pressurizing Sources: Construction of Underground Transmission Installation of an underground transmission cable generally involves the following sequence of events: 1 ) ROW clearing, 2) trenching/blasting, 3) laying and/or welding pipe, 4) duct bank and vault installation, 5) backfilling, 6) cable installation, 7) adding fluids or gas, and 8) site restoration. Many of these activities are conducted simultaneously so as to minimize the interference with street traffic.

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Figures shows a typical installation sequence in a city street.

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Examples of Trench Construction:

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Costs: Cable Repairs: V One cable repair needed per year for every 833 miles of cable. V One splice repair needed per year for every 2,439 miles of cable. V One termination repair needed per year for every 359 miles of cable A typical new 69 kV overhead single-circuit transmission line costs approximately $285,000 per mile as opposed to $1.5 million per mile for a new 69 kV underground line (without the terminals). A new 138 kV overhead line costs approximately $390,000 per mile as opposed to $2 million per mile for underground (without the terminals).

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