Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 385

using redundant sensors and back-up systems, also to be incorporated in the design
phase. This will ensure high availability and increased energy capture. It will offer
component cost benefi ts through reduced fatigue load and the ability to mitigate or
avoid some current design driving loads.

5.4 Drivetrain and electrical system

Improving overall system effi ciency is a key goal of any offshore development
effort. To obtain the lowest COE, losses must be reduced wherever possible. Large
diameter, gearless generator technologies such as so-called “direct-drive” which
uses permanent magnets generator are investigated by many companies as one
solution for reducing losses and maintenance cost related to the traditional plan-
etary gearbox-based system. Design trade-offs could be carried out for optimizing
the cost and effi ciency of drive train and the converter based on the generator side
and grid-side converters location in the tower. Factors such as weight and cable
loss should be considered.
The electrical system is the critical interface between the drive train and the util-
ity network. It enables both the optimized drive-train design and the harvesting
and transfer of power to the utility in accordance with grid regulations. An opti-
mized electrical system design will result in higher system effi ciency and lower
system cost. Both of which affect the COE in a positive manner.
The following issues related to optimizing the design of a multi-megawatt to
5-7 MW offshore wind turbine need to be addressed:

• Design trade-offs for optimizing cost and effi ciency of the drive train and

converter: The power converter enables the electro-mechanical drive train to
operate at the optimum power factor and deliver maximum output power
through peak-power-point tracking. New circuit topologies and control, con-
verter packaging and thermal management need to be explored to reduce cost
and losses of the power converter as well as the generator. The power convert-
er's physical location and associated electromagnetics that may include trans-
formers in the tower will be considered as a design factor to minimize cost and
losses in the cables as well as reduction of weight in the nacelle.
Electrical interconnection of wind turbines in a wind power plant: Studies need

•

to focus on optimizing the electrical interface between turbines in an offshore
wind power plant. The feasibility of the medium voltage (MV) DC interconnect
system, based on cost and dynamic stability, need to be studied.
Transmission of power from offshore to onshore: Voltage level and frequency •

are factors which impact cost of cabling and converter interface at the sending
and receiving ends of electricity. MV or high voltage DC transmission is an
enabling technology for such bulk power transfers. Optimum voltage levels
as well as converter topologies need to be achieved to minimize the overall
systems cost.
Protection of electrical system: Electrical system protection from grid-faults and •

component failures is critical to system design from availability and reliability