Future of Wind Energy - Wind Energy Systems for Electric Power Generation - page 166

8.1.3.1 AC Connection

Considering a high-voltage AC connection it is mainly the voltage level of transmis-
sion which has to be decided. The problem with AC transmission is due to the capac-
itive cable impedance; consequently the current contains a reactive current compo-
nent which gives rise to losses. A limiting cable length may be defined where, under
idle running conditions, the magnitude of the (mainly capacitive) current amounts
to the permissible cable current, without any active power being transmitted. At the
same time a considerable voltage increase is observed at the receiving end.
Using the cable model illustrated in Fig. 8.2, the graph shows in a calculated
example the normalized current at no-load I nl (substantially a capacitive current)
and the permissible active current I act , referred to rated current I r , versus normalized
cable length. The limiting cable length l lim is normally between 90 and 120 km. In
practice inductive compensation devices are employed.

Fig. 8.2 Cable Π -model and limiting length

8.1.3.2 DC Connection

High-voltage DC connections are known in the classical form, where the stations
on both ends of the cable are phase-controlled inverters using thyristors. Experience
with this concept has been collected since first instalment 50 years ago, connecting
Gotland with the mainland of Sweden. The modern version of HVDC transmission
is using PWM-controlled inverters with IGBT semiconductors. Among other ad-
vantages this solution requires less space for the stations and allows power factor
adjustment. Relating to short-circuit power the offshore and onshore AC sides are
decoupled.
When comparing DC to AC transmissions, the AC terminal cost is less, while
the line cost per cable length is less for the DC connection. Consequently AC con-
nections will be of advantage for shorter distances, while the installation of DC
lines is preferable above a certain distance. Figure 8.3 illustrates principle curves of
investment cost over distance, where a rough estimate of the point of equal cost will
be between 100 and 130 km. This is also where the efficiency of the DC transmis-
sion becomes better than for AC.