East Anglia ONE 4.2.2. Predictions
Table 5 Table 6 list the magnitude of B and iE fields that might be expected for the different DC cables under consideration for the EAONE project, both at the cable surface, and at varying distances from the cable (assuming no burial). Estimations have been based upon existing information available where possible (Section 4.2), but for cable designs for which no data currently exist, more general approximations have been suggested. For estimations of iE field, a conservative estimate of the maximum tidal flow at the seabed (1.2m/s) has been used, based upon 50% of the maximum tidal flow at the surface depicted on the relevant Admiralty chart.
In general, as would be expected, B fields vary with the voltage of the cable design, with higher rated cables generating stronger fields (although the relationship is not linear). However, the table demonstrates the rapid attenuation of the fields with increasing distance from the cables. Attenuation occurs significantly more quickly around bundled cables compared to those separated by 50m, owing to the cancellation effect of two similar fields of opposite polarity aligned in opposite directions (see Section 3.1.2). B fields are therefore markedly stronger with increasing distance from separated cables when compared to bundled cable fields at similar distances. iE fields induced by seawater moving through the B field (tidal flow) follow similar patterns. iE fields induced by organisms passing through the B fields will be dependent upon both the magnitude of the B field and the organisms’ speed of movement, and whether they are moving with or against the tide. A fast moving fish swimming with a strong tide, for example, will induce a stronger iE field than one swimming more slowly or against the tide. It is therefore possible that electric fields induced by fast-moving organisms could be stronger than those estimated for tidal flow in Table 6, but by how much is uncertain owing to difficulties in knowing swimming speed of marine fauna.
Again, the depth to which the cables are buried will affect how strong the fields at the seabed are with shallower depths resulting in stronger fields and vice versa. Where surface-laid cables are covered by rock-dumping or other methods, B field propagation will not be hindered. However iE field induction is likely to be dampened due to the reduction in tidal flow past the cable, and prevention of marine fauna from swimming through the strongest B fields. Equally, any organisms inhabiting interstitial spaces will not be able to move as rapidly, and the iE fields generated by their movement will also be dampened.
With relatively few data to base predictions upon, estimates of EMF magnitude in Table 5 & Table 6 are tentative. Figures for 500kV are the best supported (based upon the unpublished work CMACS has recently been involved with and BritNed calculations, the latter of which involved cables of slightly less voltage). Interpolations of EMF likely to be generated by 320kV cables have been included by comparing information from 450kV and 500kV cables with HVDCLight 150kV cables, but it must be stressed that the estimates are tentative, especially owing to uncertainty of similarity between HVDCLight and standard HVDC technology. Owing to this uncertainty, extrapolation of 600kV EMF was not possible, due to the unsuitability of using estimates for 300kV EMF on scatter plots. It has therefore simply been suggested that EMF likely to be generated by 600kV cables would be expected to be slightly stronger than those generated by 500kV cables. The worst-
J3184 EAONE v2 13
EMF Assessment
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