This document discusses underground electric transmission cables as an alternative to overhead power lines. It describes the main types of underground cables, including fluid-filled pipe cables, gas-filled pipe cables, and solid cross-linked polyethylene cables. Ancillary facilities like vaults and transition structures that are needed are also outlined. The document details the construction process and costs considerations for underground transmission, noting it is generally more expensive than overhead lines. Maintenance and repair procedures are also briefly addressed.

1. Presented
BY
SUROJIT MAHATO
0531Ex091032

3. 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.

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

5. 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)

6. 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

7. 2. High-Pressure, Gas-Filled Pipe-Type Cable: 4.Solid Cable, Cross-Linked Polyethylene:
3.Self-Contained, Fluid-Filled Pipe-Type:
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.

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

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

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

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

12. 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.

13. Figures shows a typical installation sequence in a city street.

14. Examples of Trench Construction:

15. Costs:
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).
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