A million-volt testing set
A. B. Hendricks · Journal of the American Institute of Electrical Engineers · 1922
The item of the greatest moment in this work is the fact that for the first time in the history of the world the extremely high power potential of a million and a half volts (over 2,000,000 volts peak value) was reached and an electric spark discharge was repeatedly caused to take place between sharp electrodes spaced fourteen feet apart. No damage was done to the equipment. This marks a remarkable accomplishment. A pertinent question is — what of it? The modern physicist and chemist who are trying to realize the ancient alchemist's dream of transforming one element into another may be able to approach one step nearer realization by the availability of this new intensity in high potentials. In this high potential is available a new means of speeding up the tiny disrupting projectiles, the electrons, which blast the elements into fragments. These fragments are actually other elements of lower atomic weight. The art of X-ray production may also be benefited. These factors which deal with the ultimate nature and characteristics of matter are less in the mind of the developmental engineer than certain practical results immediately realizable. Three transformers are ready to produce three-phase currents for a possible transmission at a million volts. The engineers of the two big California transmission systems are just reaching the new high mark in practical transmission of power at 220,000 volts. While no projects at a million volts are being contemplated at present, who can say with the ever-advancing accelerations in progress how soon such a step may become advisable. At any rate with the equipment available there will be found an answer to the question — If not, why not? The new million-volt, three-phase installation will be valuable in magnifying the possible troubles which may arise in the 220,000-volt practical installation — that is to say, such troubles as may arise from the electric pressures now being pushed above the known field of operation. Information thus collected can be turned immediately to the advancement and economics of the applied art. Also the testing set will have its uses in the near future for various researches in insulation, ionization, spark discharges, lightning, high frequencies, designs of apparatus, and so on. A million and a half volts is not by any means the upper limit of high electric pressure immediately attainable. The author in producing this pressure attained a million volts effective from one line to ground and, therefore, by simply duplicating this set two million volts effective (2,800,000 volts peak value) will be available, all in due time. A million and a half volts produces a discharge of the same general appearance as at lower voltages at 60 cycles. The illustration, Fig. 22 shows all the typical effects, namely (a) the corona or brush discharge at the high tension terminal which precedes the spark discharge; (b) the initial spark discharge at full voltage, through the air; the spark manifests itself in the photograph as a brilliant white streak; and (c) the successive arc discharges at low voltage in places where air currents are strong are pictured in the photograph as lace work. Each successive peak of dynamic current at 60 cycles finds a new path and the space between these threads of light does not show because the current is low and the intensity of the arc insufficient to affect the photographic plate. Incidentally the brush discharge is much stronger in the next illustration, Fig. 23. More time was given to record it on the photograph. Odd Spark Discharges. In one test at a million volts effective value, (or 1.4 million volts at the peak of the generator wave) from the transformer terminal to ground and a spark gap of nine feet, the discharge preferred to find a new path 18 feet long (2.7 million volts instantaneous value of potential). The exact nature of this phenomenon is obscure. It is evidently associated with high frequency. It is just as surprising in its occurence as it would be to drop a heavy weight nine feet above earth and observe it move off in a path 18 feet long before reaching the earth. No photographs of this discharge are shown. Another accidental discharge took place from the cap of the main terminal to an iron fence in the background, Fig. 19. The points are marked “A” and “a” and the arc is shown nearly end-on in Fig. 18. While the designs of these transformers are familiar to the transformer designer with certain elements amplified to meet the conditions of higher voltages, there are nevertheless unique features. One of these features is the use of 30 miles of paper-covered aluminum strip, instead of copper, in the high tension winding of each of the 500 kv-a. transformers. This aluminum strip was chosen not only to give desirable mechanical strength to withstand the frequent short-circuit strains to which a testing transformer is subjected, but also to give a better distribution of the internal static strains. The relatively large dimensions of the aluminum strip give the high voltage winding about ten times the kilovolt-ampere capacity of the low voltage winding. Incidentally the mechanical and electrostatic requirements make it impractical to build a small transformer for these high potentials. Another unique feature results from the combination of very high voltage and the above mentioned strip winding. It is a familiar fact that the exciting current of the usual transformer lags behind the impressed wave of electric pressure. In these test transformers it does not. The exciting current is actually in advance of the wave of impressed pressure. The high voltage windings act on the current as would the capacitance of idle transmission wires. The High Voltage Bushing and Terminal. Three notable features are the large metal cap, the large insulating tube and the heavy aluminum sleeve which supports the bushing on the cover of the iron case of the transformer. The data are given in Fig. 12. Without reviewing the specialists' details of this new work, it is of interest to stand off from the technical side and take a glance at the nature of the accomplishment without details. The favorite method of using analogies will be resorted to. The Analogy of a Bridge to a Transformer Design. Bring to mind the spanning of any river by a bridge made up of numerous strips of steel in triangular connection at their ends. No single strip has to withstand the total weight passing over the bridge. The stresses are distributed among the steel members. Likewise in a transformer there is a distribution of insulation along the thirty miles length of wire in the coils. At no spot does this insulation. have to withstand the entire electric pressure of a million volts. Like the mechanical pressure in the struts of the bridge, the electric pressure is distributed all along the insulation.