Apparatus used for testing condenser type of insulation for high-tension terminals to 225,000 volts Auditorium of the Engineers' Building, New York, April 9, 1909 American Institute of Electrical Engineers, Copyright 1909. By A 1. E. E. CONDENSER TYPE OF INSULATION FOR HIGH-TENSION TERMINALS BY A. B. REYNDERS The manufacture of apparatus for pressures of 88,000 volts. or more resolves itself into the proper selection and arrangement of insulating materials. Of all the problems involved in this selection and arrangement, that of insulating the terminal wires where they pass through the case presents the greatest difficulty. When the terminal wires are carried through a metal cover the danger from breakdown is increased, yet the tendency of the latest designs is toward the use of metal tanks and covers. Even with present voltages the greatest difficulty lies in bringing out the terminal wires through the case. may be termed "brute force" has characterized the proportioning of high-tension terminals up to the present time, as almost every class of insulating material in nearly every possible shape has been used to surround the conductor as it passes through the top of the case. The top itself has often been made of insulating material such as wood, simply to assist in insulating the lead. What One convenient way of analyzing the terminal problem is to consider it as a short piece of cable. The failure of such a terminal can occur in two ways; namely, by a puncture through the insulating material separating the inside conductor from the outside conductor or support, or by "creeping" over the surface from the end of the inside conductor to the end of the outside support. Jona in an article on "Insulating Materials in High-Tension Cables" read before the International Congress at St. Louis in 1904, shows how the distribution of stress varies throughout Volts. the dielectric, being inversely proportional to the radius. Furthermore, he shows that by grading the material according to its specific inductive capacity, it is possible to make the stresses nearly uniform throughout the dielectric. Fig. 1 shows graphically the distribution of potential in an insulated cable for a homogeneous material. The ideal condi- . tion is obtained when the distribution lies in a straight line. By the introduction of layers of different materials with FIG. 1-Cable with homogeneous insulation: distribution of potential through dielectric. FIG. 2-Cable with graded insulation; distribution of potential through dielectric. specific inductive capacities decreasing from the center outward, Jona obtains the result approximately as shown in Fig. 2, line A C. The curve A B is the same as in Fig. 1. By this method failure due to puncture in a properly designed cable is practically eliminated, but if the use of a piece of such a cable as the outlet lead from a transformer or a circuit-breaker is attempted, the greatest difficulty would be from breakdowns over the surface by "creepage" from the support to the ends. A method that takes care of both puncture and creepage has been proposed by Ryan, Smith, Nagel, and others. This method consists in dividing the dielectric by means of metal plates into a series of condensers of fixed capacities; for it is a known fact that if a difference of potential is impressed across a number of condensers connected in series, each will take its share of the stress in inverse proportion to its capacity. This principle has been utilized in constructing a terminal consisting of a small center rod or tube just large enough to carry the current, the rod being surrounded by alternative concentric cylinders of insulation and metal having the ends tapered in steps as shown. in Fig. 3. The distribution of stress through the dielectric owing to the metal layers has been changed from a curve as shown in Fig. 1 to a straight line as shown in Fig. 2. Furthermore, the ends of the metal layers fix the distribution of voltage over the surface, which thus can be kept within safe limits. This design may be designated as a condenser type of terminal. Before considering the mechanical design of this type of insulated terminal, it would be well to dwell on a few fundamental facts that will limit the design. It is obvious that in order to obtain the maximum efficiency from the insulating material every part of it should be subjected to a stress proportional to its strength. Further, if the dielectric material is homogeneous throughout, every particle should be strained the same as every other particle. This means that in a series of condensers employing a homogeneous dielectric throughout, each one should have the same capacity and the same thickness as every other one. In order to obtain this desirable result various difficulties must be overcome. It is the purpose of the following to show how this can be done. The formula for the capacity of two concentric cylinders is: capacity in electrostatic units specific inductive capacity of the dielectric. length in centimeters of the conducting cylinders. radius in centimeters of the inside and the outside conducting cylinders, respectively. A glance at this formula shows that the capacity may be varied by changing the thickness between the conducting cylinders, the length of the conducting cylinders, or the specific inductive capacity of the dielectric. It is now very generally known that the dielectric strength of ordinary solid insulating material does not increase proportionally as the thickness is increased. For this reason it is evident that varying the distance between the cylinders in order to obtain equal capacities will cause some layers to be thicker than others, and hence there is a waste of material. FIG. 4-Condenser type of terminal. Layers of insulation of equal thickness. Shaded portions show equal capacities throughout. Full lines show equal capacities on inside and outside layers and capacities, increasing towards center of insulation thickness. The second method of varying the capacity-by changing the length of the conducting cylinder-is determined approximately, when equal capacities in all condensers are desired, by making the surface area, or the product of the length by the diameter, equal for all conducting cylinders. When the diameters of adjacent layers are nearly equal-a condition approached as the diameters increase-and the thickness of insulation remains constant, the ends of the layers come very close together, and failure is liable to occur by creepage. In other words, the most economical design for creepage is to make equal steps with an equa difference of potential between the ends of each step. This cannot be accomplished by varying the areas alone, as shown by reference to Fig. 4. The shaded portion shows the shape which the ends must have for equal areas, while the full straight lines show the shape the ends must have if equal steps between layers are obtained. The best compromise that can |