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, 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 semi conductive 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 2: Three-core cable (Nexans) |
Figure
1 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.
Table 1: Capacities of high voltage cable (Häusler, ABB, 2002)
System | AC 3 single-core cables | DC bipolar operation, 2 cables | |||
Cable insulation type | XLPE polymer | LPOF: Oil- filled paper | LPOF: Oil- filled paper | Mass imp. Paper | XLPE polymer |
Maximum Voltage | 400 kV | 500 kV | 600 kV | 500 kV | 150 kV |
Maximum Power | 1200 MVA* | 1500 MVA* | 2400 MW | 2000 MW | 500 MW |
Max. length, km (mi.) | 100 (62) | 60 (37) | 80 (50) | Unlimited | Unlimited |
* Losses may be excessive at these powers
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.
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