Publication: Western Electrician
Chicago, IL, United States
The Recent Italian High-tension
BY FRANK C. PERKINS.
Some experiments of unusual importance were recently made by Messrs. Pirelli, the Italian electrical engineers, with pressures ranging from 160,000 volts to 320,000 volts.
In these experiments a 200-horsepower transformer was utilized, arranged as shown in the accompanying diagram, Fig. 1. The primary was wound for 160 volts and fed directly from the city service main. There are two similar groups of coils in the secondary (S), arranged to be connected in parallel or in series, and thereby obtaining potentials of 160,000 volts or 320,000 volts, as desired.
A potential of 200,000 volts was measured directly by means of an electric static voltmeter (V), connected as shown in Fig. 1, while for higher potentials the voltmeter was connected to one of the terminals of the transformer as well as to the middle point of it, thereby measuring half of the total voltage at the terminals of the secondary.
The Italian engineers arranged two drums (B1) and (B2), each of which was wound with a length of high-tension cable of 120 meters. A section of this cable, full size, is given in Fig. 4. Various layers of different dielectrics are used for insulation, partly vulcanized rubber and partly paper, the whole sheathed in lead.
To one of the terminals of the secondary of the transformer, which was connected in parallel, as noted in Fig. 1, one of the ends of each of these two coils (a) (b) was connected, while the other two ends (c) and (d) were connected to a number of vacuum tubes in series, forming the words, "150,000 volt."
In series with the vacuum tubes a small condenser was connected, formed by a disk of ebonite (E) separating two movable copper disks (D1) and (D2), the distance betwen the armatures being regulated from 15 to 20 centimeters in order to adjust the current in the tubes and obtain good brilliancy. As the impressed voltage was too high for the tubes, the condenser became necessary, and the cables formed a part of an electric line working at a pressure of 150,000 volts steadily.
A number of pieces of this cable about 20 feet in length were tested to the point of breakdown, the ends being especially prepared to obviate surface discharges, as shown in the accompanying drawing, Fig. 2. By means of a suitable resistance put in the primary the tension could be gradually increased until a pressure of from 200,000 to 210,000 volts was applied between the lead sheathing and the conductor, when the cable broke down. The diameter of the conductor was 18 millimeters and the total diameter was 48 millimeters over the insulation, making a thickness of 15 millimeters for the dielectric, the experiment showing a very high rigidity. Referring to Fig. 2, (C) is the conductor, (I) and (R) insulation and covering, and (P) a porcelain insulator. The insulation (R) is a resinous material, the lead covering (T) of the cable entering the lower block, of this insulating material at (B). This special precaution in the insulation in the end of the cable was necessary in order to obviate surface discharges in these high-tension experiments. The conductor is absolutely cylindrical, the copper strands being sheathed with lead.
Fig. 5 shows a section of the cable tested by Messrs. Pirelli which was recently laid across Lake Garda. This cable was laid for the transmission of power at a pressure of 13,000 volts from Ponalle to Rovereto, about 6,000 kilowatts being employed, three