Titel: A Baseline assessment of electromagnetic fields generated by offshore windfarm cables – Final report – Report EMF 01-01-2002 66
Författare: CMACS Centre for Marine and Coastal studies, Centre for intelligent monitoring systems, Applied ecology researchgroup (All university of Liverpool)
Utgivare: COWRIE
Årtal: 2003
Ämnesord: Maritimt liv
Rapport (Pdf)
Sammanfattning: An Investigation of EMF Generated by Subsea Windfarm Power Cables
EXECUTIVE SUMMARY COWRIE identified as priority research the issue of electromagnetic fields (EMF) generated by offshore windfarm power cables and their possible effect on organisms that are sensitive to these fields. A consortium, lead by CMACS, was contracted to carry out a Stage 1 investigation to investigate the following:
· The likely EMF emitted from a subsea power cable.
· A suggested method to measure EMF in the field, which could be applied by windfarm developers or in future projects.
· Guidance on mitigation measures to reduce EMF.
· Consideration of the results for the next stage of investigation into the effects of EMF on electrosensitive species.
An assessment of existing publications (both hard and electronic) and direct communications regarding EMF emitted by undersea power cables suggested that the current state of knowledge is too variable and inconclusive to make an informed assessment of any possible environmental impact of EMF in the range of values likely to be detected by organisms sensitive to electric and magnetic fields.
Therefore modelling and direct measurement of the electric and magnetic field components of EMF was undertaken.
An Alternating Current (AC) Conduction Field Solver model and Eddy Current Field Solver model were used within the ‘Maxwell 2D’ software program which allows the simulation of electromagnetic and electrostatic fields in structures with uniform cross-sections by solving Maxwell’s equations using the finite-element method. The modelling was based on EMF generated by a 132kV XLPE threephase submarine cable designed by Pirelli with an AC current of 350 amps buried at a depth of 1m. The results of the model simulations showed that a cable with perfect shielding i.e. where conductor sheathes are grounded, does not generate an electric field (E-field) directly. However, a magnetic field (B-field) is generated in the local environment by the alternating current in the cable. This in turn, generates an induced E-field close to the cable within the range detectable by electro-sensitive fish species. Simulations with non-perfect shielding, i.e. where there is poor grounding of sheathes, showed that there is a leakage E-field, but it is smaller than the induced E-fields and unlikely to be additive.
An Investigation of EMF Generated by Subsea Windfarm Power Cables CMACS/University of Liverpool & Econnect/J2733/Final_v1/07-2003 4
A method is provided for calculating the induced E-field around a low frequency AC power cable due to the emanating B-field. This method requires in situ measurement of the B-field generated by an operational cable.
To consider mitigation, the models simulated changes in permeability of the power cable armour and conductivity of the cable sheath and armour. The model predicted that as the permeability of the armour increased the resultant EMF strength outside the cable decreased. The model also showed that a non-linear relationship exists between electromagnetic field strength adjacent to the cable and permeability of the armouring material. This indicated that using materials with very high permeability values for armouring of submarine power cables could help to reduce the EMF generated to below the lowest known level that electroreceptive elasmobranchs can detect. As the conductivity of the armour was increased the model showed that the resultant EMF strength outside the cable decreased.
A linear relationship was found between electromagnetic field strength and the conductivity of the materials used in the cable. These results indicate that a reduction in the strength of the electromagnetic fields induced by a three-phase 132kV XLPE submarine cable can be achieved through the application of materials with high conductivity and high permeability. These results provide useful information for consideration during the design and manufacture of
submarine cables with reduced EMF emissions. Burial was shown to be ineffective n ‘dampening’ the B-field, however cable burial to a depth of at least 1m is likely to provide some mitigation for the possible impacts of the strongest B-field and induced E-fields (that exist within millimetres of the cable) on sensitive fish species, owing to the physical barrier of the substratum.
An additional mitigation consideration is the use of substations to convert the voltage from 33kV to 132kV would reduce the current carried by a cable and would therefore reduce the induced E-fields by a factor of four. This could be used to add to mitigation of the EMF effects of sending power to the shore but probably has practical and economic limitations.
In terms of the potential significance of the modelled results to electrosensitive fish the following conclusions were made:
· EMF emitted by an industry standard three-core power cable will induce E-fields.
· In the case modelled, this resulted in a predicted E-field of approximately 91μV/m (=0.9 μV/cm) in seawater above a cable buried to 1m. This level of E-field is on the boundary of E-field emissions that are expected to attract and those that repel elasmobranchs (the most widespread electrosensitive fish group of UK coastal waters).
· In addition, the induced E-fields calculated from the B-fields measured in situ were also within the lower range of detection by elasmobranchs.
An Investigation of EMF Generated by Subsea Windfarm Power Cables CMACS/University of Liverpool & Econnect/J2733/Final_v1/07-2003 5
· The options for mitigation using either changes in permeability or conductivity indicate that the induced E-field can be effectively reduced. However, unless highly specialised materials and manufacturing process are used with high permeability values, the E-field will still remain within the lower range of detection of elasmobranchs. Hence any reduction in E-field emission using existing materials could minimise the potential for an avoidance reaction by a fish if it encountered the field but may still result in an attraction response.
· Another important consideration is the relationship between the amount of cable, either buried or unburied, producing induced E-fields and the available habitat of an electrosensitive species.
· There is also a need to determine if the power cable operating frequency (50Hz) and associated sub-harmonic frequencies have any effect on the EMFs that are detectable by UK electrosensitive fishes.
Finally, a number of further studies are recommended to direct future research to fully understand the interaction of the induced E-fields from subsea power cables with electrosensitive fish and any implications of the B-fields for organisms that rely on a magnetic sense.