Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 320

of revolutions reduces the dynamic stress exerted on the converter. However,
the direct current intermediate circuit incurs in additional costs and increases
electrical losses. Grid connection via direct current intermediate circuits is
common practice for medium to large converters, preferably in conjunction
with synchronous generators.
For older converters equipped with direct current intermediate circuits, fre-
quently inverted rectifiers have been used, which in some cases produced con-
siderable overtones. They may interfere with the operation of other equipments
in weak power grids. However, recent investigations in terms of power semi-
conductors have led to the use of inverted rectifiers, which supply current with
relatively little distortion and are also partly suitable for idle power provision
(e.g. IGBT inverted rectifiers with pulse-width modulation (PWM)).
Provided that the required network specifications are strictly adhered to,
wind power converters may also be indirectly connected to the grid via direct
converters.
Wind power converters can either be connected to the grid as individual convert-
ers or in the form of a wind park. The connection grid interference caused by the
converter or the wind park has to be determined at the respective grid connection
point. Short-term power fluctuations, sensitively perceived as flickering lights by
the human eye, but also continuous voltage changes and possible overtones need
to be considered. Fluctuations may be measured by the ratio of converter power to
grid short-circuit power at the grid connection point. If certain values are ex-
ceeded, connection is only possible at a connection point of superior grid short-
circuit power (e.g. at the bus bar of a substation), in order not to interfere with
other consumers connected to the grid.
The main components of a grid connection are the electric coupling of the wind
power converter or the wind park to the transformer, a transformer (if required)
including a distributing substation equipped with medium voltage substation as
well as a medium voltage connecting line up to the grid connection point.
The design of every wind power converter including control and protection
equipment has to rule out all potential damage to the grid due to a failure of the
wind power converter (such as power failure or short-circuit). Complete discon-
nection also has to be ensured for the safe performance of operation and mainte-
nance.
Losses occur when power generated by wind energy converters is fed into the
grid. They are mainly caused inside the transformer when power is converted into
heat. However, they are comparatively low and at the most amount to a few per-
cent.
System aspects of offshore installation. The available technologies need to be
adapted to suit the changed framework conditions of offshore operation, when
compared to onshore installation. Technologies may also be optimised accord-
ingly.