Submarine cables are on average longer than land cables since they are often proposed for cross-border trades and for the integration of off-shore wind farms. Long HVAC submarine cables require shunt reactors to compensate for their charging capacity. When the length of a cable line is further increased, it becomes necessary to install shunt reactors at multiple points along the cable length in order to avoid the reduction of transmission capacity for active power or temporary over voltages. The reactive power compensation incurs an additional cost for the cable project especially when it requires the installation of shunt reactors in the sea. Even though HVDC submarine cables need converter stations, the overall project cost becomes lower for the HVDC option at some cable line length.
HVAC Submarine Cables
XLPE cables are the most preferred option as HVAC submarine cables for the same reasons as land cables. HVAC submarine cables have a layer of armor outside the XLPE land cables as shown in Figure “HVAC submarine cable. Courtesy of VISCAS Corporation”. The armor helps submarine cables to endure the tensile force they experience during installation. In addition, it can help to avoid a cable failure when a submarine cable is damaged in the sea by a third party.
|HVAC submarine cable. Courtesy of VISCAS Corporation|
The most common type of armor is steel wire. As is the case with the copper wire sheath, it is modeled as a solid conductor when building a cable model for EMT studies. The resistivity of steel is modified so that the modeled armor has identical resistance to the actual steel wire armor.
Some HVAC submarine cable has three-phase cables enclosed together in armor. In this case, the submarine cable is often modeled as a pipe-type cable, considering the armor as a pipe of the pipe-type cable.
HVDC Submarine Cables
Until recently, XLPE cables could not be adopted in a HVDC cable project with LCC–HVDC (line-commutated converter HVDC) technology. The space charge was formed in the main insulation for XLPE cables, and it could lead to an insulation breakdown when the voltage polarity was reversed. The reversal of the voltage polarity occurs with LCC–HVDC when the power flow is reversed. Therefore, SCOF cables have been selected for HVDC cable projects with LCC–HVDC.
The most common type of SCOF cables for HVDC submarine cables is MI (mass impregnated) cables. MI cables are used since the insulating oil cannot be supplied at cable terminations. Figure “MI cable. Courtesy of Nexans” shows the construction of MI cables. Unlike SCOF land cables, it is still common to use Kraft paper as the insulating material. However, it is expected that PPLP will soon become more common because of its better performance as an insulating and dielectric material.
|MI cable. Courtesy of Nexans|
An XLPE cable was first adopted in a HVDC cable project with LCC–HVDC in 2012 for the Hokkaido–Honshu HVDC link in Japan. The 250 kV XLPE cable has overcome the problem caused by the voltage polarity reversal by adding nanoparticles to the insulating material. It is expected that the successful operation of this cable will, in future, lead to the increased application of XLPE cables to HVDC cable projects with LCC–HVDC.
Other than the Hokkaido–Honshu HVDC link, XLPE cables are selected for HVDC cable projects with VSC–HVDC (voltage-source converter HVDC) technology. The VSC–HVDC reverses the power flow by the reversal of the current flow. The reversal of the voltage polarity does not occur with VSC–HVDC, which enables the selection of XLPE cables.