Single-wire transmission line

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A single-wire transmission line (or single wire method) is a method of supplying electrical power through a single wired electrical conductor.

Contents

[edit] History

In 1729, the English physicist Stephen Grey noticed the phenomenon of electrical conductivity. The essence of this phenomenon consists in the fact that electricity may be transmitted from one body to another along a conductor and that the electrical charge is distributed over the surface of the conductor. At the end of the 19th century, Nikola Tesla demonstrated that by using an electrical network tuned to resonance and using, what at the time would be called, "high frequency" and today would be low frequency, a single wire was necessary for power systems, with no need for a wired return conductor (using the Earth instead). With the apparatus set up in this fashion, the circuit is completed through the earth itself. Tesla called it the "transmission of electrical energy through one wire without return". [1] Tesla stated in 1901,

"Some ten years ago, I recognized the fact that to convey electric currents to a distance it was not at all necessary to employ a return wire, but that any amount of energy might be transmitted by using a single wire. I illustrated this principle by numerous experiments, which, at that time, excited considerable attention among scientific men." [2]

In the spring of 1891, Tesla gave a demonstrations with various machines before the American Institute of Electrical Engineers at Columbia College. His lecture exhibited this feature, the chief import exhibited that all kinds of devices could be operated through a single wire without a wired return. The one-wire transmission system was protected in 1897 by U.S. Patent 0593138 , "Electrical Transformer".

Later, Lloyd Mandeno would developed a single wire system (emphasising the Earth return) in New Zealand around 1925 for rural electrification in a "Single wire earth return method". George Goubau around the 1950s would apply this technology to microwave transmissions.

[edit] Single-wire lines

This electrical technology relates particularly to a method for the continuous transformation of electrical energy with its subsequent transmission from an initial source (transformer) to a consuming device, and also to an apparatus for the implementation of this method of transformation and the supplying of power to electrical devices through a transmission line. It does not form a closed circuit in the strict sense as it consists of a single conducting wire (there is not a wired return, but the Earth can be used as a return). Since both ends of the transmission line are "open" to the surrounding space (known as being grounded), electrons can flow out of one end of the line while a corresponding flow of electrons enters the other end. In this fashion, the voltage that is electromagnetically conducted along the line can cause current to flow through the surrounding transmission medium, completing an electrical circuit through what appears to be, at first glance, an open circuit.

Single-wire transmission lines provide a method and associated apparatus for supplying power to an electrical devices, including generation and subsequent transmission thereof to a receiving device via a transmission line. Single-wire methods being characterised by the transformation of the electrical energy which is generated into the energy of oscillation of a field of free electrical charges such as the displacement current or longitudinal wave of an electrical field, the density of which charges varies in time, and the transmission of the energy via a transmission line comprising a single-wire transmission line and, where necessary, its transformation into the electromagnetic energy of conduction currents.

[edit] Single wire earth return

Single wire earth return is a single-wire transmission line for supplying single-phase electrical power to remote areas at low cost. It is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps, and light rail.

[edit] Application at HVDC

Many HVDC systems using submarine power cables are (or were until their expansion to bipolar schemes) single wire earth return systems. In order to avoid electrochemical corrosion, the ground electrodes of such systems are situated apart from the converter stations and not in the proximity of the transmission cable. The electrodes can be situated in the sea or on land. As cathode bare copper wires in the sea or on land, as anode graphite rods digged in the ground or titanium grids in the sea are used. In order to avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at the surface of the electrodes may be only small and therefore large electrodes are required. The advantage of such chemes is saving money for a second conductor, because the saltwater is an excellent conductor. Some ecologists claim bad influences of electrochemical reactions, but they do not occur on very large underwater electrodes.

[edit] Examples of HVDC systems with single wire earth return

[edit] Electric trains

Nearly all electric trains run by DC or single phase AC use a monopolar overhead wire or a single third rail and the rails as ground return.

[edit] Goubau lines

The Goubau line, or G-line for short, is a type of single wire transmission line intended for use at UHF and microwave wavelengths. [3] The line itself consists of a single conductor coated with dielectric material. Coupling to and from the G-line is done with conical metal "launchers" or "catchers," with their narrow ends connected for example to the shield of coaxial feed line, and with the transmission line passing through a hole in the conical tips.

G-lines behave more as waveguides than as circuitry. The G-line functions by slowing the propagation velocity of EM waves below the free-space velocity, causing the wavefronts to slightly bend inwards towards the conductor, which keeps the waves entrained. Bends of large radius are tolerated, but too sharp a bend in the single wire will cause the line to radiate and lose energy into space. In theory the dielectric coating is a requirement, it slows the wave and focuses it along the wire. But some users note that in practice the finite conductivity of metals may produce a similar effect, and a bare G-line can entrain a propagating wave.

Note that the Goubau-Line are not exclusively coupled to 3 dimensionnal horn antennas, and can be used at other frequencies besides UHF and Microwave. Waves can be "launched" from planar structures like tapered Coplanar Waveguides (CPW) at much higher frequencies such as the Terahertz Band. [4]The dimension of the single metallic conductor is then typically 1µm.

[edit] See also

[edit] Patents

[edit] References and notes

Citations
  1. ^ "Why did Tesla make his coil in the first place? What was it that he was trying to accomplish? Other than just the fun of making one, do they have any practical purposes?", tfcbooks.com.
  2. ^ Nikola Tesla, "Talking with the Planets (1901)". Collier's Weekly, February 19, 1901, page 4-5
  3. ^ Geog Goubau, "Surface waves and their Application to Transmission Lines," Journal of Applied Physics, Volume 21, Nov. (1950)
  4. ^ T. Akalin, A. Treizebré and B. Bocquet, “High Resolution Biosensor based on Surface Wave Transmission Lines at THz Frequencies”, 35th European Microwave Conf., 3-7 Oct. 2005, Paris, France

[edit] Further reading

  • Toby Grotz, "Wireless Transmission of Power, An Attempt To Verify Nikola Tesla's 1899 Colorado Springs Expriments, Results Of Research And Experimentation". Proceedings of the 26th IECEC Conference, vol. 4 (1991).
  • G. Trinkaus, "Tesla--The Lost Inventions". Vantage Press (1988).
  • N. Tesla, "The True Wireless". Electrical Experimenter (May 1919).
  • J.J. O'Neill, "Prodical Genius, The life of Nikola Tesla" (Neville Spearman 1968) pp. 70-73, 128-133.
  • John O'Neill, "Electrical Prometheus" (History of Technology ("Molodaya Gvardiya") 1959).
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