Highlights Of The Transmission Line Development History. Looking For Efficient Means Of Insulation

Power transmission line

At the end of the nineteenth century, when the first industrial systems transmitting energy over long distances were built, there were developed a large number of innovative technological solutions. In particular, the erection of the Lauffen - Frankfurt transmission line helped to gain invaluable experience of transmitting electric current via overhead lines made of uninsulated wires installed on wooden support structures.

First industrial insulators

The first overhead transmission lines were designed to light streets and buildings. They ran across rooftops and were made of metal wire fixed to wooden poles. However, it was soon discovered that, when it was raining, the wet poles cased current leakage, and the longer the line was, the more the current leaked, and the weaker the signal became. This brought about the necessity to insulate the wires where they were fixed to the poles.



The first industrial Clark’s insulators appeared in the 1840s and were utilized for telegraph and telephone lines. The insulators were made of glazed clay and were shaped in a way that kept moisture away from the junction with the wire. To be more specific, the insulator base was fixed to a horizontal wooden bar, which was screwed to a pole. A tin lid covering the upper part of the base provided extra protection from rain.

First industrial Clark’s insulators

As overhead telephone and telegraph lines got longer, the industry requirements for insulators became more demanding. In 1848 Siemens introduced the design of the first bell-shaped insulator. Such an insulator was made of porcelain, burnt clay or glass and got widespread after the 1850s in Europe and America.

Double-petticoat insulator

Bell-shaped Siemens’ insulator

In 1856 Clark modified the Siemens’ insulator. The inner surface of the insulator had a double-
petticoat feature, which prevented air circulation and thus kept away moisture and dust. In the late nineteenth century such insulators became very popular in Europe and were used for the first overhead transmission lines.

Search for the most appropriate conductive material

The wires of the first overhead telegraph lines were made of metal galvanized wire, the cross-section of which depended on both electrical characteristics (current strengths and the line resistance) and mechanical parameters (line installation conditions and the distance between poles). Later, metal wire was replaced by pure copper wire, which is much more conductive and therefore can have a smaller cross-section; moreover, it is not susceptible to corrosion. The drawback of using a copper wire was its considerably lower mechanical strength and ability to stretch. In addition, even at that time it attracted those interested in scrap metal.

The use of aluminium wires facilitated a noticeable progress in the prolification of overhead transmission lines as it helped to considerably decrease the weight of wire and, therefore, the load on the line supports. The falling cost of aluminium eventually made it the main material used for transmission lines.

First transmission towers

Another important experience gained during the erection of first transmission lines was that in the technology of installing supports and wires. This was the time when various fixing mechanisms were invented for mounting wires on supports; also, the industry developed means for the splicing of wires and fixing them to insulators.

Means for splicing of wires and fixing them to insulators

The first supports for overhead transmission lines, like those for telecommunication lines, were wooden poles impregnated with blue copperas solution or tar oil. To impregnate wooden poles with tar oil, they were first dried and then placed in a big container with tar oil. The container was then hermetically closed. After that, powerful pumps were used to evacuate air, and the pressure was increased. Tar oil filled the pores of the wood, which prevented the reproduction of microorganisms causing wood rotting. In addition, in towns overhead transmission lines were installed with the use of metal supports, which were less bulky and more elegant.

Braiding machines for wire insulation

First steps in wire insulation

In cases when the wires of a transmission line, as opposed to going overhead, touched a solid body (like ground, building walls, etc.), it was necessary to cover them with insulating sheath made of non-conducting material that would prevent current leakage. Naturally, that brought the cost of wires up, but was the only way of providing electrical insulation. It turned out to be technically challenging to protect wire from moisture while ensuring the required mechanical strength and keeping the insulation thin.

The easiest way of insulation was winding an insulating thread of cotton, jute, silk or thrum around wires. The first winding machines for wire insulation were designed in Berlin. Afterwards, insulation braiding was made from strips, which were applied in several layers when the voltage load was high.

Search for reliable insulation material

Machine for double braiding

At first, insulating threads and strips were mostly impregnated with bee and mineral wax. The latter had excellent insulating properties, but crumbled easily, which caused the deterioration of insulating properties due to mechanical impact, however light it was. An insulation of high quality was made using gutta-percha, coagulated fluid of the trees growing in Singapore and on the island of Borneo. When heated, gutta-percha became flexible and could be easily used in pressing machines that created insulating sheath.

Gutta-percha insulation was moisture-resistant and maintained its insulating properties for a long time. However, it was crucial that the gutta-percha material was of the highest quality, as even a small amount of impurities negatively affected the insulating properties of this material.

Insulation of equally high quality was made of caoutchouc. Unlike gutta-percha, caoutchouc was vulcanized, that is mixed with sulphur while being heated. Such insulation was more resistant to air oxidation, moisture or heating compared to untreated caoutchouc.

Insulation made of gutta-percha or caoutchouc had good insulating properties while its integrity was maintained. Unfortunately, after some time in the open air the insulation gradually dried-up and became fragile; in the water it was destroyed by animals. This made the industry return to using lead as the main means of insulation from moisture when building, for instance, long-distance water conduits.

Pressing machine for gutta-percha insulation

In the beginning, to apply lead sheath onto braided wires, they were put through 50-60 m long lead pipes; then, these pipes were brazed together. However, moisture that often remained between the wire and its lead sheath destructed the internal insulation over time.

To produce the first power cables of urban electricity distribution systems, the Swiss electrical power engineers developed a method of covering wires and power cables with insulation similar to gutta-percha, that is by pressing. This approach was beneficial in that it not only ensured good contact of the lead sheath with the wire, but also made soldering of separate sheath sections unnecessary.

Further developments: protecting cables from the elements and switching to AC generators

To increase reliability and prevent moisture from penetrating through high voltage cable lead sheath, which sometimes had microscopic pores due to impurities, cables were often covered with double lead sheath. To achieve protection from mechanical damage, lead sheath was frequently covered with armour, made of metal wires or strips wound around it. This made cables much more resistant to mechanical impact.

The need to constantly increase the length and capacity of the transmission lines made electrical power engineers look for new ways to overcome the disadvantages of transmitting energy in the form of direct current. Alternators (AC generators) and induction motors (AC electric motors) were much more efficient and reliable than DC generators.

First single-core cables: a) – Dover-Calais cable 1851; b) – first transatlantic cable 1857; c) – second transatlantic cable 1865; d) – underground cable between Berlin and Halle 1876

To conclude, the beginning of the twentieth century marked the more and more widespread usage of alternating current for transmitting and distributing electric energy, a trend that gained even more popularity with the invention of suspension insulator for AC transmission lines in Germany in 1912.

Source (Russian language): http://energetika.in.ua

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