Offshore wind farms – transmission cables


The European wind power industry is increasingly turning to the offshore wind resource, and the United States will draw on the Europeans' experience as we begin to plan offshore wind farms. Short of generating hydrogen, or otherwise using or storing the energy offshore (see for instance, Altmann 2001), it must be conducted to the on-shore load centers by submarine cables.

Offshore transmission has proved to be challenging and costly in Europe, and will present additional challenges in the US because of the lack of domestic manufacturers of high-voltage, high-capacity submarine cable, and lack of equipment for and experience in installing this type of cable.

Submarine transmission cables are common in the US for other applications, but this experience has a limited applicability to wind farms.

The offshore gas and drilling industry uses lower power levels and low (under 10 kV) to medium voltage (10-100 kV), whereas the trend in offshore wind power is toward high voltage transmission. A number of medium and high voltage transmission cables have been installed in the US to power islands but submarine transmission from generation offers different problems than transmission to a load.

For instance, windfarms usually have high reactive current demands, since most wind turbines employ induction generators. This can cause resonance with the capacitance of the cables. Economies of scale are driving up the size of offshore windfarms. Larger farms will both allow and demand more sophisticated electrical transmission systems, as wind power makes a greater impact on the onshore electrical grid. As power electronics are being developed, we may expect to see them play a greater role in offshore windfarm transmission and distribution designs, including the introduction of high voltage direct current (HVDC) transmission.

The following is a brief introduction to cable types and components as it pertains to offshore wind installations.

Insulation

Three types of cable insulation are in common use for submarine transmission for long distances (at least several kilometers.) While insulation construction and thickness vary based on voltage, all three types discussed here are used for both medium and high voltages. Insulation is characterized by their insulation material, their construction, and whether the dielectric (i.e. insulation) is lapped or extruded.

Low-pressure oil-filled (LPOF), or fluid-filled (LPFF) cables, insulated with fluid-impregnated paper, have historically been the most commonly used cables in the US for submarine AC transmission. The insulation is impregnated with synthetic oil whose pressure is typically maintained by pumping stations on either end. The pressurized fluid prevents voids from forming in the insulation when the conductor expands and contracts as the loading changes. The auxiliary pressurizing equipment represents a significant portion of the system cost. LPFF cables run the risk of fluid leakage, which is an environmental hazard.

Fluid-filled cables can be made up to about 50 km (30 mi.) in length. They are rarely used for DC applications, which are generally longer than practical for pressurizing. While LPFF cables are widely installed worldwide, the cost of the auxiliary equipment, the environmental risks, and the development of lower-cost alternatives with lower losses, have all contributed to the reduced use of LPFF cables in recent years.

Similar in construction are the solid, mass-impregnated paper-insulated cables, which are traditionally used for HVDC transmission. The lapped paper insulation is impregnated with a high-viscosity fluid and these cables do not have the LPOF cable's risk of leakage.

Extruded insulation is replacing lapped installation as the favored options in many applications. Cross-linked polyethylene (XLPE, also called PEX) is lower cost than LPOF of a similar rating and has lower capacitance, leading to lower losses for AC applications. XLPE can be manufactured in longer lengths than LPFF (Gilbertson 2000.)

Until recently XLPE was not an option for DC transmission, since it broke down quickly in the presence of a DC current, but recent improvements allow its use for DC as well. Figure 1 shows an example of an XLPE cable.




Another extruded insulation used in submarine cables is ethylene propylene rubber (EPR), which has similar properties to XLPE at lower voltages, but at 69 kV and above, has higher capacitance (Gilbertson, 2000). High-voltage submarine XLPE cable is not manufactured in the North or South America. LPOF cables are manufactured here but are not available in the sizes and lengths that will be required for an economically sized offshore wind farm. Currently any offshore windfarm in the US (or anywhere else in the Western Hemisphere) will have to import cables from Europe or Japan.

With cables that may weigh more than 75 kg/m (50 lbm/ft), the transportation costs will be a significant portion of the cost of the cable.


Conductors

The conductor in medium and high-voltage cables is copper, or less commonly aluminum, which has a lower current carrying capacity (ampacity) and so requires a greater diameter. Ampacity increases proportionally with the cross sectional area, which can range up to about 2000 mm2 (3 in2, i.e. 50 mm (2 in) in diameter) before the cable becomes unwieldy and the bending radius is too great. Large cables may have a bending radius as large as 6 m (20 ft).

The design amperage is a function not only of the voltage and the power to be carried, but also the cable length, insulation type, laying formation, burial depth, soil type, and electrical losses. Gilbertson (2000) offers a thorough technical reference on these subjects. The issues of length and losses are discussed in more detail below.


Number of Conductors

When possible in AC-cable applications, all three phases are bundled into one "three-core" cable. A three-core cable reduces cable and laying costs. It also produces weaker electromagnetic fields outside the cable and has lower induced current losses than three single core cables laid separately. As the load requirements and conductor diameter rise, however, a three-core cable becomes unwieldy and no longer feasible.

One advantage of single-core cables is that it is easier and cheaper to run a spare, fourth wire. Another advantage is that longer lengths can be made without splices or joints. Figure 2 shows a three-core cable.

Screening

A semiconductive screening layer, of paper or extruded polymer, is placed around the conductor to smooth the electric field and avoid concentrations of electrical stress, and also to assure a complete bond of the insulation to the conductor.



Figure shows screening on a single-core cable, and Figure 3 shows a three-core cable with screening on both the individual conductors and the three-core bundle.

Sheathing

Outside the screening of all the conductors is a metallic sheathing, which plays several roles. It helps to ground the cable as a whole and carries fault current if the cable is damaged. It also creates a moisture barrier. In AC cables, current will be induced in this sheath, leading to circulating sheath losses; various sheath-grounding schemes have been developed to reduce circulating currents that arise in the sheath.

Unlike other cable types, EPR insulation does not require a metal sheath.




Armor

An overall jacket and then armoring complete the construction. Corrosion protection will be applied to the armor; this may include a biocide to inhibit destruction by marine creatures such as marine borers that are present in Southeast US waters, and have recently been reported in the Northeast (Fox Islands, 2001).

Fiber optic cables for communications and control can be bundled into the cables. Note the bundled fiber optic line in Figure 2. Table 1 summarizes the current availability and limitations of AC & DC cables.
SOURCE: Transmission Options For Offshore Wind Farms In The United States