Page 42 - Energize April 2021

TECHNICAL

Compact AC transmission lines

provide solutions to servitude

problems and environmental impact

by Mike Rycroft, Now Media

Limitations on existing servitudes and difficulties in obtaining new servitudes are necessitating upgrade of
existing routes to higher power ratings, and the design of transmission systems with smaller footprints.
Existing tower designs and insulator equipment limit the extent to which this can be done, but new
equipment and practices resulting in compact designs, which allow higher ratings and a smaller footprint
and environmental impact, offer a solution to the problem.

he increased number of renewable energy sites in the network increase involves applying appropriate modifications to the
has increased the number of transmission lines, as each site configuration of existing overhead lines (OHL). There are two ways in
Trequires connection to the grid. Obtaining servitudes for these which the capacity of an OHL can be increased:
lines is proving difficult, and it has become necessary to reduce the • Increase the current carried by the OHL
impact of these lines on the environment. • Increase the voltage of the OHL
One of the most common methods is to reduce the size of
the transmission line corridor by using compact transmission line The current may be increased if high-temperature low-sag
technology (CTx). A further issue is the visual pollution associated conductors (HTLS) are used to replace existing conductors. This
with transmission lines. CTx technology offers the ability to blend in will give the same safety clearance at a higher current rating. This
better with the surroundings and can reduce the objectionable visual process has been used successfully to upgrade numerous OHL
impact of transmission lines. New transmission lines are designed routes worldwide, including several in South Africa.
to take up far less lateral space by utilising modern materials and Increasing the voltage provides a far greater challenge, as the
altering tower geometries. physical configuration of the route must be changed to allow for
increased safety clearances. This is sometimes possible by utilising
The main restriction on overhead lines is safety clearance, between the experience acquired for HVAC lines and permitting reduced
the conductors, the conductors and ground, and the conductors safety margins in designing clearances. A far more promising
and the supporting structure (tower or pole). In addition, aesthetic method is the use of compact OHL technology, which changes the
considerations require a reduced visual impact, both in terms of clearances needed for the same voltage rating and allows a higher
width and height of towers. The general approach to capacity voltage rating with the same clearances.
Compact AC technology may be applied to an existing tower
structure, or a complete new compact route may be constructed
within the existing servitude. OHL transmission routes using
conventional lattice towers and compact technology offer both
reduced height and width of towers.

Composite insulators (CI)
The composite insulator is the key component of the compact CTx.
The structure of a typical composite insulator is shown in Figure 2.

Figure 1: Compact 400 kV line compared with conventional 400kV
line (Pfisterer) Figure 2: Composite insulator construction (Pfisterer)

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