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Figure 3 - Terna submarine cables

HVDC ADVANTAGES
HVDC transmission becomes interesting on very long interconnections. The breakeven distance
at which the AC solution becomes technically infeasible, or where it is feasible but economically
not convenient, depends on many factors and it is consequently hard to define a specific figure.
Generally, such figure falls within the 300 to 500 km range for overhead lines and within the 40 to
100 km range for cable lines. Such ranges should be considered as very general for an order of
magnitude only.
The main technical limitation of AC interties is the management of reactive power to be
compensated both at light loads and full load conditions. This requires shunt inductive reactors
and series capacitors for the light and full load compensation respectively. There are a number
of AC overhead lines longer than 1.000 km completed in the last century with intermediate
compensation stations [1]. Such solutions are no longer used today since the reduction in HVDC
converter station costs makes HVDC transmission more attractive. Furthermore, the ability of
a HVDC intertie to regulate independently the power on the DC link gives an opportunity to
modulate the power on bordering AC lines as well. HVDC solutions remain the only option for very
long submarine links. In fact, as mentioned above, the cost reduction of HVDC converter stations
has generated an increase in submarine HVDC interties. Mass Impregnated Non-Draining (MIND)
cables, although the pioneer technology in the cable industry since early 60s, still represents
today a valid and reliable solution for HVDC transmission, especially for very deep waters with no
rival technology. In the last 20 years the extruded HVDC cable technology came on the market
becoming immediately attractive for land applications and for shallow water installations. In these
same years the extruded solution has reached the MIND cable in terms of voltage rating and we
will see the first application in the 500 kV range in the upcoming years [2].
As far as converter station technology is concerned, the new Voltage Source Converter (VSC)
solution has contributed to draw attention on HVDC transmission. In fact, VSC became an
alternative to the par excellence Line Commutated Converter (LCC) technology at the beginning
of this millennium. VSC, with its larger flexibility and its intrinsic capacity to regulate the AC voltage
on the interconnected points with the AC grid, has immediately attracted many Transmission
System Operators (TSO) and in the last 20 years many HVDC interties using the VSC technology
have been completed all over the world. In spite of the increasing number of installations, VSC
technology must still be considered as under development since many versions have been
developed over these last 20 years, and further versions of the current ones are expected to
come. On the other hand, LCC technology remains more reliable, cheaper and much less onerous
from a maintenance point of view. Furthermore, the combination of the “classic” well established
LCC solution with a synchronous condenser and specific schemes of operation makes this “old
school” technology an equivalent solution to VSC technology, as demonstrated by Terna in the
upcoming refurbishment of the SACOI intertie [3,4].

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