The writer presented a paper in 1919 on the three-conductor cable, giving experimental results for assisting in the calculation of quantities such as capacity, insulation resistance, dielectric loss, temperature rise, etc., and also data on the calculation of stresses at various points in the dielectric. Most of the data were based on cable models composed of metallic electrodes surrounded by conducting electrolyte. In the present paper the old data are amplified both in regard to geometric properties and stresses of three-conductor cables, not only in regard to the range of sizes covered, but especially, inasmuch as the present paper includes cables with sector-shaped conductors. The cable models used are entirely different, being in the form of tinfoil sheets. Curves are given by means of which stresses can be calculated in various important parts of the cross-section of three-conductor cables with either round or sector-shaped conductors.

Dielectric Field in an Electric Power Cable-II

The insulation characteristics of different types of single- and three-conductor cable, determined largely by dielectric energy loss measurements, are discussed in detail. Particular attention is given to paper insulated cable, both new cable and cable that has been in service being considered. Varnished cambric and different grades of rubber insulation, as well as cable compounds are also dealt with briefly.

Insulation Characteristics of High-Voltage Cables

Two theories of electrical failures in cables, at test voltages, are presented; one, known as the maximum stress theory, is based on critical potential gradients, and the other, known as the ballast theory, is based on critical current densities. Data collected by the Insulated Conductors Committee of the AIEE are evaluated in relation to these theories.

Electric Stresses in Cables

If the dielectric of a single-conductor concentric cable is homogeneous, the voltage gradient at any diameter x is given by d v/d x = 0.868 V/x log 10 /D/d where d v/d x is the voltage gradient or dielectric stress, V the voltage between conductor and sheath, D the diameter over the dielectric and d the diameter over the conductor. A complete discussion of the above formula is followed by considerable experimental data and curves accumulated from many breakdown tests. Results of tests on cables with large ratios of dielectric diameter to conductor diameter are included and a modification of the above theoretical formula is discussed. The modified formula is checked by tests on a special cable which was constructed for this purpose. A new relation between the rupturing gradient at the surface of the conductor and the ratio D/d is suggested and curvnes of experimental data given. Breakdown tests on three-conductor cables are included and the calculated rupturing stresses compared with those for single-conductor ductor cables. Special cables were constructed so that measurements could be made of voltages between layers of insulation. From data obtained from these tests, curves are given showing the change in potential gradient as the internal heat of the cable is increased. Curves are given showing the effect of a change of temperature on the dielectric strenrenth, the stresses and the factor of safety of cables. A complete description is given of the low-capacitance electrostatic voltmeter used in the temperature-potential-gradient tests.

Potential Gradient In Cables Discussion of the Logarithmic Formula, Its Modification and Effect of Internal Heat

AFTER reading with considerable interest, the two papers presented at the annual convention of the Institute, dealing with the dielectric strength of cables1, the writer analyzed the data presented in each paper from an entirely different viewpoint and also the data given in each paper according to the views of the other paper.

The Apparent Dielectric Strength of Cables

In this country rubber compound, paper, and cambric are generally used for cable insulation. From the formula S = 0.868 V/d log10 D/d, the stress at any point in a homogeneous insulation may be determined. The minimum stress and the maximum allowable voltage occur when the conductor is 10/27 of the sheath diameter. The present irrational practise of testing cables should be standardized to conform to this formula or a modification of it. Over-stressing of the insulation is accompanied by a change of insulation resistance and electrostatic capacity. No one factor of safety is applicable to every cable system, but one must consider the conditions of operation as well. In testing the voltage may be applied: (1) by submersion; (2) between the conductor and metallic sheath; (3) between wires. The submersion test is the most severe. A sine wave is desirable for testing purposes, but rarely occurs in a commercial generator under these severe conditions of load. Reactance cannot always be used successfully to reduce the volt-ampere load on the generator. With a distorted wave an a-c. voltmeter gives only a poor indication of the maximum voltage. The writers have devised an instrument based on the oscillograph principle, with which the maximum voltage may be determined, regardless of wave form.

W. I. Middleton

Papers

Voltage testing of cables

Potential Gradient In Cables Discussion of the Logarithmic Formula, Its Modification and Effect of Internal Heat