Page 38 - Impiantistica Italiana

and dynamic stability etc., but they do not increase HTLS ones, and the same has been done for 145
the inherent transmission capacity of a line with its and 245 kV lines.
specific conductors.
2.1.2 Voltage upgrading of an AC
2.1 Upgrading of existing Right of line
Way (RoW) corridors
Figure 3 shows the study performed on a 245 kV
Many alternatives exist, among others: Italian double circuit line equipped with single ø 31
• use of new HTLS (High Temperature Low Sag) mm ACSR conductor per phase. In the back is the
conductors, which allow a 1.5-2 times power original AC tower with 3 arms on the left side of
transfer increase; tower body and 3 on the right side. On the exi-
• voltage upgrading of an AC line maximizing sting tower body and foundations, substituting 6
the use of present towers / conductors with arms with “horizontal V” insulator assemblies (
application of horizontal V insulators; composed by one horizontal post type insulator to
• transformation of an existing AC line to a DC withstand both compression and tension loads and
one, making maximum use of conductors and one inclined conventional string for tension loads)
towers with up to around 3-5 times transfer resulted in a 420 kV circuit with 2 conductors per
Fig. 3 - Study performed on capacity increase; phase. Both the mechanical stresses to the tower
a 245 kV Italian double cir- • substituting in the same RoW an existing AC and foundations and the electrical effects (RIV, co-
cuit line equipped with single line with a compact AC one at higher voltage rona, electric and magnetic fields etc.) have been
ø 31 mm ACSR conductor or with a DC one. deeply investigated and solved.
per phase The power transfer increase resulted of 1.7 times.
Some examples are reported here below. The role of the insulator assemblies is essential.

2.1.1 Use of new HTLS conductors 2.1.3 Transformation of an AC line
Different types of HTLS conductors are now availa- to a DC one
ble on the market and because of their high costs
initially they have been mainly used in a reduced Studies have been performed since the 90’s [4] to
number of spans. Their great advantage is that of utilize as much as possible the components of an
no need for the modifications of towers, founda- existing line (conductors, towers, foundations) in
tions and insulators with the appropriate choice of order to increase transmissible power in a HVDC
conductor diameter and weight. Adequate fittings mode. Clearly, the AC insulators must be substi-
must be provided. The decrease in their price al- tuted by special DC ones; the type and numbers of
lows the applications to rather long lines on the DC insulators depend on contamination conditions
basis of cost (conductors and re-stringing) and be- and affect the maximum DC voltage level applica-
nefit (value of increased power) analyses. ble in the specific case. The type of tower of the
In Italy a 40 km long, 400 kV line has been comple- existing AC lines which is best for transformation to
ted with the change of the original conductors with obtain great increases in transmissible power is the

Fig. 4 - Possible transfor-
mations of Italian 245 kV
AC double circuit line to an
HVDC one

36 Impiantistica Italiana - Marzo-Aprile 2018