Offshore Wind Technology and Status - Structural Integrity of Offshore Wind Turbines - page 21

turbine spacing. Variations in water depth present a siting obstacle that
often requires a customized approach to individual substructure design
to ensure that each turbine's structural vibration modes will not resonate
with turbine rotational and blade-passing frequencies (IEC 2005; Dolan
et al. 2009).
The power output from all the turbines in the wind farm is collected
at a central electric service platform (ESP). The wind farm's electric
power distribution system consists of each turbine's power electronics,
the turbine step-up transformer and distribution wires, the ESP, the
cables to shore, and the shore-based interconnection system. In U.S.
projects, the cable-to-shore, shore-based interconnect, and ESP system
usually are the responsibility of the developer. In some European coun-
tries such as Germany, the state-run utility is responsible for the power
after it reaches the substation.
Power is delivered from the generator and power electronics of each
turbine at voltages ranging from 480 to 690 V and is then increased via
the turbine transformers (which can be cooled with dry air or liquid) to
a distribution voltage of about 34 kV. The distribution system collects
the power from each turbine at the ESP, which serves as a common elec-
trical collection point for all the turbines in the array and as a substation
where the turbine outputs are combined and brought into phase. Power
is transmitted from the ESP through a number of buried high-voltage
subsea cables that run to the shore-based interconnection point. For
smaller arrays or projects closer to shore, the power can be injected at an
onshore substation at the distribution voltage, and an offshore ESP is not
needed. For larger projects, the voltage is stepped up at the ESP to about
138 kV for transmission to a land-based substation, where it connects to
the onshore grid. The onshore grid may itself have to be reinforced with
higher-voltage circuits to accommodate very large or multiple offshore
projects (Green et al. 2007).
An ESP substation for a 400-MW wind plant requires multiple trans-
formers, each containing about 10,000 gallons of circulated dielectric
cooling oil, which are mounted on a sealed containment compartment
to prevent leakage into the environment (Musial and Ram 2010, Section 2,
10-22). In addition, each containment compartment is mounted to a
secondary containment storage tank capable of capturing 100 percent of
the oil should all four transformers leak.