High Voltage Direct Current (HVDC)

High voltage direct current (HVDC) transmits electricity much more efficiently than conventional alternating current (AC), emitting less radiation, with less line losses, using less wires. HVDC transmission towers require less space, less materials to construct, and are less expensive.

Figure 1.  Legacy (U.S.) transmission towers and right-of-way (ROW) required to transmit 6 GW of electricity (courtesy of NREL). AC required three large towers with 600 feet ROW. DC required either two medium towers with 180 feet ROW each, or a taller tower with 270 feet ROW. Newer HVDC towers require less ROW.

Narrower right-of-way (ROW) allows HVDC lines to be installed along roads and railroad tracks.

Figure 2.  HVDC transmission towers. [ Manitoba Hydro, 2 MB ]

HVDC transmission towers may be guyed (Figure 2, left) or self supporting without guy wires (Figure 2, right). 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).

HVDC transmission towers can be taller so that no land needs to be cleared underneath.

“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.”
Hammons and Naidoo, p. 102

AC requires multiples of 3 conductor bundles (wires), with total failure of all three conductors if any of the three fails.

HVDC only requires two conductors (not three), and can operate at half power if one of the two conductors fails, using the remaining conductor with ground return.

“single pole operation of dc transmission systems is possible for extended period, while in ac transmission, single phase operation (or any unbalanced operation) is not feasible for more than a second”
Sood, p. 9

Ground (earth) return can be a ring of electrodes (Figure 3), or wide shallow fill, or a deep bore hole one meter (1 m) wide, or ocean electrodes, etc.

Figure 3.  Ring of ground electrodes, 300 m radius, for 500 kV HVDC (Manitoba).

Figure 4.  Circuit diagram for Figure 3. [ Alberta Energy, 2.5 MB ]

Contents of This Report

Page 1 : 
Page 2 : 
Page 3 : 
Introduction (this page)
AC and DC
Pacific DC Intertie

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 1.  Jos Arrillaga, High Voltage Direct Current Transmission 2nd ed., The Institution of Electrical Engineers (UK) 1998. [ WorldCat ]

 2.  Vijay K. Sood, HVDC and FACTS Controllers: Applications of Static Converters in Power Systems, Springer 2004. [ WorldCat ]

 3.  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. [ Abstract ]

 4.  Christof Humpert, “Long distance transmission systems for the future electricity supply – Analysis of possibilities and restrictions”, Energy, 48 (2012) 278–283. [ Abstract ]

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