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High Voltage Direct Current (HVDC)

Page 8: Overhead Lines (OHL)

Low Impact

Overhead lines (OHL) for HVDC are very low impact, producing much less radiation than AC lines and allowing for agriculture and other plants under the lines.

Figure 1: Steel transmission tower for a bipolar 2375 km Rio Maderia 3.1 GW ±600 kV EHVDC line in Brazil.

Figure 2: Transmission towers for the bipolar 2077 km Xingu-Estreito 4 GW ±800 kV UHVDC line in Brazil. Multiple sets of AC towers and much more wire would have been required to transport this much electricity with HVAC instead of HVDC.

“there is no field inside the body as if the body is protected by a ‘Faraday Cage’, possible leakage currents are negligible. The effect of the electrostatic induction only occurs by entering and escaping the static electrical field, and thus the field is not able to produce permanent body currents.”
— 
Nadine May, “Eco-balance of a Solar Electricity Transmission from North Africa to Europe”, Technical University of Braunschweig, 2005, p. 44–45.
“It is best for the proposed transmission servitude route to follow the existing road networks and settled corridors through the five countries… The land footprint must be kept to an absolute minimum so as to allow for continued land use such as undisturbed natural ground or commercial farming etc. and presence of the line has no effect on normal wild life movements.”
— 
Thomas J. Hammons, Pathmanathan Naidoo, “Africa – Integrated gas and electricity transmission planning in power generation and HVDC engineering in harnessing large-scale hydroelectric sites for interconnected regional power systems”, Energy Systems, (2010) 1: 79–112, p. 102.
“the experimental fact that large machines with rubber tires (such as combine harvesters, automobiles, and some others) are not electrically charged to levels dangerous for a human when the machines are standing under HVDC overhead lines should be considered a significant result of the investigation. The electrical resistance in the tires of these machines (at about 10 MOhm) turns out to be enough to prevent the accumulation of a dangerous charge (via charge leakage) even when the machine is standing on dry asphalt. In the case of HVAC overhead lines, inducted capacitive currents on large machines may be lethal in some cases.”
— 
L. A. Koshcheev, “Environmental Characteristics of HVDC Overhead Transmission Lines”, Third Workshop on Power Grid Interconnection in Northeast Asia, Vladivostok, Russia, St-Petersburg, High Voltage Direct Current Power Transmission Research Institute, 2003, p. 3.
Figure 3: Agricultural machinery near bipolar HVDC line in Canada. Three-tier power lines in the distance are AC.

Transmission Towers
Figure 4: HVDC transmission towers may be guyed (left) or self supporting without guy wires (right). [Manitoba Hydro]

Towers with guy wires are called tangent towers and can only be used on sections of transmission lines that do not change direction (referred to as tangent lines).

Figure 5: Bipolar UHVDC tangent tower in Brazil (Xingu-Estreito 4 GW ±800 kV).

Figure 6: Top of a bipolar tangent tower for an Itaipu 800 km 3.1 GW ±600 kV EHVDC line in Brazil. Shield (ground) wires at the top of both sides of the tower are for lightning protection.

Figure 7: Tower at converter station for a bipolar Itaipu 3.1 GW ±600 kV EHVDC line in Brazil. DC switch yard is behind the tower.

New Designs

It is possible to design transmission towers with more aesthetics. For example, poteaux roseau o HVAC transmission towers could provide ideas for designing UHVDC towers.

Figure 8: Poteaux roseau HVAC tower in France. [JNourtier]

Figure 9: Poteaux roseau construction. [Mimram]

Figure 10: Poteaux roseau erected. [Mimram]

Poteaux roseau structure is twin-tube, with multiple tubing forming overall vertical tubes in the lower segments (left photo), like reed bundles (“roseau”). Structurally, this is like saguaro cactus skeletons:

Figure 11: Saguaro cactus skeleton. Live saguaros in left background. [Postdlf]

Figure 12: Twin saguaro skeletons. [Levine]

Transmission towers may be single tube posts:

Figure 13: Tubular HVAC transmission towers (behind bridge). [JNourtier]

Figure 14: Tubular HVAC transmission towers, curving to hold shield (ground) wires above AC conductors for lightning protection. [RTE]

For standard bipole HVDC (±500 kV), compaction of towers may be possible, by placing towers closer together to reduce line sag, and reducing metal sructure between the positive and negative poles (conductors). Positioning conductor bundles closer together on each tower reduces right-of-way (ROW) land requirements.

Figure 15: Possible bipole HVDC tower design, with shield (ground) wire above each pole insulator. [Bystrup Arkitekter]

References

 1.  Baljit Singh, Gagandeep Sharma, “Power upgrading of Transmission Line by converting EHVAC into EHVDC”, International Journal for Science and Emerging Technologies with Latest Trends, 4(1): 20-24 (2012).

(513 K)

 2.  M. Salimi, I. Barthold, D. Woodford, A. Gole, “Prospects for Compaction of HVDC Transmission Lines”, 2016.

(541 K)

 3.  Commission Nationale du Debat Public, “Audit des alternatives a la realisation d′une ligne a tres haute tension entre le Cotentin et le Maine”, 2006 (in French).

(6 MB)

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