Environmental Engineering Reference
In-Depth Information
Large field
Small field
Large current
Small current
FIGURE 7.12
The strength of the magnetic field around a wire carrying a current depends
on the amount of current.
The EMFs associated with new marine and hydrokinetic energy designs have not
been quantified; however, there is considerable experience with submarine electri-
cal transmission cables, with some predictions and measurements of their associated
electrical and magnetic fields. For example, the Wave Energy Technology (WET)
generator in Hawaii will be housed in a canister buoy and connected to shore by a
1190-m-long, 6.5-cm-diameter electrical cable. The cable is designed for three-phase
AC transmission, can carry up to 250 kW, and has multiple layers of insulation and
armoring to contain the electrical current. Depending on current flow (amperage),
at 1 m from the cable, the magnetic field strength was predicted to range from 0.1
to 0.8 A/m and the magnetic flux density would range from 0.16 to 1.0 µT. The esti-
mated strength of the electric field at the surface of the cable (apparently the iE) would
range from 1.5 to 10.5 mV/m. The electric field strength, magnetic field strength, and
magnetic flux density would all decrease exponentially with distance from the cable.
The Centre for Marine and Coastal Studies (CMACS, 2003) surveyed cable man-
ufacturers and independent investigators to compile estimates of the magnitudes of
E, B, and iE fields. Most agreed that the E field can be completely contained within
the cable by insulation. Estimates of the B field strength ranged from 0 (by one man-
ufacturer) to 1.7 and 0.61 µT at distances of 0 and 2.5 m from the cable, respectively.
By comparison, the Earth's geomagnetic field strength ranges from approximately
20 to 75 µT (Bochert and Zettler, 2006). In another study cited by CMACS (2003),
a 150-kV cable carrying a current of 600 A generated an induced electric field (iE)
of more than 1 mV/m at a distance of 4 m from the cable; the field extended for
approximately 100 m before dissipating. Lower voltage/amperage cables generated
similarly large iE fields near the cable, but the fields dissipated much more rapidly
with distance.
For short-distance undersea transmission of electricity, three-phase AC power
cables are most common; HVDCs are used for longer distance, high-power applica-
tions (Ohman et al., 2007). In AC cables the voltage and current alternate sinusoi-
dally at a given frequency (50 or 60 Hz); therefore, the E and B fields are also time
varying. That is, like AC current, the magnetic field induced by a three-phase AC
current has a cycling polarity, which is not like the natural geomagnetic fields. On
the other hand, the E and B fields produced by a direct-current cable (e.g., HVDC)
are static. Because the magnetic fields induced by DC and AC cables are different,
they are likely to be perceived differently by aquatic organisms.
