Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 333

tigue of the wind energy converter in the downwind flow of the main wind direc-
tion. Therefore, on average larger distance factors are assumed for offshore wind
park designs than for onshore wind parks. Usually, they do not have the same
space restriction requirements either.
Grid connection. There are two options for connecting the converters of a wind
park to the power grid:
− connection via a direct current bus or
− direct grid connection by a common alternate current bus.
For the first alternative, synchronous generators operated at a variable speed in
conjunction with a direct current intermediate circuit as well as line-commutated
inverters are applied. In comparison to other concepts, major circuit feedback and
stability problems are disadvantageous. Furthermore, direct current lacks zero-
crossing which may generate stable electric arcs in case of malfunctions. Because
of these disadvantages this type of grid connection is rarely applied.
Synchronous or asynchronous generators with direct grid connection may also
use the same three-phase electrical grid. According to the respective nominal ca-
pacity, current is fed into the medium voltage grid via one or several transformers.
If the low voltage cables are sufficiently dimensioned, a common low voltage bus
may also be applied. Otherwise, the generators must be connected via separate
transformers. This has become common practice. Currently, a wind converter is
normally provided with its own transformer, for instance, feeding into a 20 kV
cable that represents the actual grid connection.
Due to the long distances to be overcome, grid connection of offshore wind
parks is more difficult. The main deciding parameters are the distance from the
shoreline and the installed capacity. For instance, cables laid on the seabed must
be designed and laid to sufficiently withstand deterioration, for example by draw
nets. This is why cables are either buried or washed into the seabed.
The individual converters of an offshore wind park may either be connected
with each other by medium voltage alternate current connections, as for onshore
wind parks, or by medium voltage direct current connections. In the latter case
every wind energy converter needs to be provided with its own controlled rectifier
system. To transmit electrical energy to the on-shore grid connection, besides con-
ventional alternate current transmission also high voltage alternate current or high
voltage direct current transmissions are possible; additionally, the respective
substations are required. According to the respective wind park capacity and the
transmission voltage level, one or several underwater cables must be laid. High
voltage direct current transmission is advantageous as it allows the transmission of
high capacities over long distances without any idle power compensation installa-
tions as required for alternate current transmission. However, high voltage direct
current transmission requires additional facilities for rectifying and inverting cur-
rent and voltage. There are two different rectifying options: conventional Thyris-
tor technology and Insulated Gate Bipolar Transistor (IGBT) technology with
pulse width modulation (PWM) which also provides its own alternate current grid