East Anglia ONE
EMF Assessment
rated between 320kV (1407A current load and 2400mm2 cross sectional area) and 600kV (1667A and 3300mm2). HVDC cables currently in use are restricted to 320kV and are most likely to be used at EAONE, but 500kV designs are planned for deployment in the near future, and, due to the rate of technological change in this field, it is thought there is a possibility that such cables or slightly larger may be available for the EAONE project. Bipole systems transmit current along two, separate cables in opposite directions (one with positive polarity and the other with negative polarity), thereby completing the circuit. This contrasts with a monopole system, whereby current is transmitted through a cable with a single conductor core and the return current is directed through the external environment (earth/sea) between two sea electrodes (an anode and a cathode). The latter have been associated with deleterious environmental effects due to strong electric fields and the generation of pollution products through electrolysis (Poleo et al 2001) and so will not be considered for EAONE. The temporary use of sea electrodes during cable maintenance cannot, however, be ruled out (see Section 4.5).
Optical fibre units will run alongside the power cables (composite design or separately), but their contribution to EMF is expected to be negligible in comparison to the power cables, and are therefore not considered further.
Whilst the development of a three-core, 420kV AC cable is thought to be possible in the near future, this design is not considered here, as it is considered unlikely to be available within the time frame of the EAONE project. The same applies to DC gas insulated line (GIL) cabling (thought to enable up to 6300A current load at 500kV) and super-conducting cables (thought to enable transmission of 5000MW across 10 to 15km), which are currently limited to onshore trials, and not likely to be commercially available for at least a decade.
4. Electromagnetic field generation
Submarine power cables generate magnetic fields owing to the electric current flowing along the cables. The magnitude of the magnetic fields produced is directly dependent upon the amount of current flow. The design of the cables, including lead sheathing and armoured cores, prevents the propagation of electric (E) fields into the surrounding environment; however, these materials are permeable to magnetic (B) fields, which therefore emanate into the surrounding environment, effectively unimpeded. The B field attenuates with both horizontal and vertical distance from the cable conductor.
Three-core AC cables transmit three current flows that fluctuate between positive and negative polarity. The B fields generated by these cables are therefore constantly changing. In turn, the motion of these B fields through the surrounding seawater continuously induces varying electric (iE) fields (CMACS 2003; Gill et al. 2009).
In contrast, the B field generated by bipole, DC cables is static and thus varying iE fields will not be induced in the same way as AC cables. However, localised, static iE fields may be induced as seawater (tidal flow) or other conductors such as marine organisms pass through the HVDC cable’s B field.
J3184 EAONE v2 5
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