11-05-2012, 03:17 PM
Underground Hvdc Transmission
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Land Use:
An HVDC cable uses significantly less land than an overhead HVAC line. The right-of-way
for a 400 kV OHL Can be a 60 m wide strip where no buildings/high trees are allowed
whereas an underground DC cable needs at most a 4 m wide inspection road on top of it. For
AC OHL the amount of land required for a Transmission is 2,400 hectares (1 hectare =
10,000 m2). However only 160 hectares are required forDC cable (< 6 percent).
Audible Noise:
Restrictions on land use stretch beyond the immediate right-of-way. Audible noise from
transmission line corona – most noticeable when conductors are wet in foggy weather
conditions – might restrict buildings close to OHL. The width of this “noise corridor”
depends on local noise ordinance as well as on the design and voltage of the line. Noise
objections from neighbors make it more difficult to obtain permits. An underground DC cable
naturally has no audible noise emission.
Underground Hvdc Transmission Seminar Report
Electromagnetic Field:
Magnetic and electrical fields can also restrict the use of land close to an OHL. In several
countries a precautionary policy vis-à-vis magnetic fields is in force. The Swedish National
Electrical Safety Board and the Dutch Ministry of Housing and Environment both suggest a
0.4 T safety Level for 50 Hz magnetic fields from transmission lines. This level corresponds
to field levels normally encountered in city environments today. In contrast to an AC line, the
field for a DC cable is static (non-radiant). Applying the same precautionary policy as for AC
would not call for the provision of any “EMF corridor” around an underground DC cable.
The field immediately above the cable is far less than the earth’s natural magnetic field.
Right-Of-Way as a Loss of CO2 Sinks:
Growing forests are considered CO2 sinks because trees convert carbon dioxide from the
atmosphere into carbon stored in the form of wood and organic soil matter. A forest can
absorb 9.2 tons of CO2 per hectare per year. Building a 400 km, 400 kV overhead
transmission line through an area that is 75 percent forest represents a loss of a carbon sink of
16,780 tons of CO2 per year. HVDC Light technology was introduced in 1997 with a small
test installation of 3 MW. Since then both cables and converters have progressed dramatically
In both size and performance.