66 kV Cable arrays for Offshore Wind Farms

Introduction

Offshore Wind Farms, with EPR insulation.
The avoidance of an extruded lead screen yields a much lighter and cost effective cable
design while providing a substantially better fatigue performance.
The qualified "wet-design" is not a new concept for EPR insulated HV cables, with several
systems throughout the world being successfully in operation since more than 40 years up
to 72.5 kV class.


Approach

Offshore windfarm market is considering the application of 66 kV for the array system. Submarine cables are all over the world and it is worthy to check on other cable design possibilities to provide a cost effective and reliable design. Wind farm developers should become more outward-looking and more open to accept the MV designs for this HV application. Therefore, it is crucial to demonstrate that the cable arrays design for 66kV is reliable enough for this application.

Main body of abstract

The offshore wind farm industry is facing significant challenges to reduce installation and
operation costs in recent years. In 2008, a collaborative R&D program between the Carbon
Trust and nine major offshore wind developers was created, called Offshore Wind
Accelerator (OWA). This joint industry project is aimed to reduce the cost of future "Round
3" offshore wind farms in the United Kingdom by 10%.
Current wind farms standard layout consists of an "inter-array" network that collects power
from many wind mills at 33 kV typical nominal voltage. Power is then transported, generally
at higher voltage, by an AC or DC system from offshore substations to the onshore one.
Moving the cable array system from 33 kV to 66 kV has been identified as a potential key
step to provide significant cost reduction per transmitted megawatt since it reduces cable
cost per unit power while number of offshore substations can also be decreased.
The paper deals with the qualification of a 66 kV wet-design cable system suitable for
Offshore Wind Farms, with EPR insulation.
33 kV array cables in use at the time of writing are mainly "semi-wet" design with extruded
insulation surrounded by a metal foil and a PE sheath. A step up to 66 kV would require an
extruded lead screen in case of unfilled XLPE insulation while this is not necessary in case of
EPR insulation which can be operated in wet environment.
The avoidance of an extruded lead screen would yield a much lighter and cost effective
cable design while providing a substantially better fatigue performance which is crucial for
dynamic cables.


Conclusion

High voltage (66 kV) electric systems are currently being considered for the power distribution within the offshore wind farms. Only the common designs for medium voltage submarine cables represent a feasible solution for this new concept, since the common designs for HV submarine cables would result in forbidden cable array’s cost. The selection of the proper insulation materials together with the appropriate testing regime, are two of the key points for successfully implement the step up voltage for these new systems.


Learning objectives
The application of the standard designs for HV submarine cables is not a cost efficient solution for the current offshore wind farms market evolution. Consequently MV submarine cable designs should be considered for the new 66 kV offshore systems. All the existing cable technologies and designs must be reconsidered in order to provide a reliable and cost effective solution for this new market.