H. C. Stewart

Hot-Spot Temperatures in Dry-Type Transformer Windings

With the increasing use of dry-type transformers, attention has been focused upon the fact that the difference between the average temperature rise by resistance and the hottest-spot temperature rise is much greater than in liquid-immersed transformers. In this paper there are given the results of an extended series of thermal tests on a variety of dry-type transformer coils. Data are presented in regard to the effect of different heights of coil stacks, different configurations, and, in addition, some essential precautions in making temperature measurements are pointed out. Hot-spot temperatures were investigated in coils cooled by natural draft and by forced-air circulation at different velocities for both directed flow and random flow. Sufficient data are given to determine the ratio between the average temperature rise of the windings and the hottest-spot temperature rise. Based upon this relationship, the need is shown for review and modification of the conventional hot-spot allowances for dry-type transformers.

Aging Evaluation of Dry-Type Transformer Insulating Systems [includes discussion]

The aging performance of dry-type transformer insulating systems is shown to be dependent not only on the materials used but to a large extent on the gas medium in which they are operated. This gas medium may retard or accelerate aging because of its chemical reaction with the base materials and their impregnants. Data are presented which show the effect of gas mediums such as air and nitrogen. The classification of insulations by chemical content with the designations A, B, and H are shown to be no longer significant; the assignment of a temperature limit to an insulating system is to be preferred. Also, the aging effect of periodically introducing specified quantities of dry air or high humidity air into insulating systems operated in a nitrogen atmosphere is shown to have a very minor influence on insulation aging. For the first time, extensive data are presented on silicone insulations subjected to thermal aging and periodically tested at high dielectric stresses. Previous data on thermal aging of silicones have been limited to tests on motors which for the most part operate at low dielectric stresses.

Aging of Class-B Insulating Material in Nitrogen

New studies have been made of the aging of Class-B insulating material in nitrogen, and it has been found that the insulations follow the same behavior rules in nitrogen as they do in air. However, the tests indicate Class-B material in nitrogen can be used in temperatures where the more expensive Class-H insulations were formerly used.

Impulse-Failure-Detection Methods as Applied to Distribution Transformers

Investigation of several methods for the detection of impulse failures in distribution transformers has strengthened our belief in the inadequacy of the methods described in American Standard C57.2. All of the methods are discussed in detail, and the most hopeful one is comnpared with the present method of analyzing the voltage wave and making the physical observations as outlined in the Standards. Data are presented on transformers utilizing present and proposed methods.

Production impulse testing of distribution transformers

Impulse testing, when used as a quality control tool in the manufacturing of distribution transformers, functions as an invisible inspector and acts as a final check on manufacturing operations. It also minimizes the number of transformer failures upon initial connection to the power circuits.

Aging of dry-type transformer insulating systems

THE METHOD CHOSEN for evaluation of insulation aging should be representative of operating conditions, applicable to a large range of materials, and readily interpretable in terms of expected life of apparatus. A dielectric strength criterion meets these requirements for dry-type insulating systems. In utilizing this criterion the initial strength of each insulation system is determined at the temperature being investigated. Duplicate samples of the insulating systems are then thermally aged at this temperature and periodic, 60-cycle, 1-minute dielectric strength tests are applied at a value of one-half of the initial hot-dielectric strength until failure occurs. Plotting the hours of thermal aging to reduce the dielectric strength to one-half of its initial value against the operating temperature for a range of temperatures gives the thermal aging characteristics for the insulating system.

Aging of dry-type transformer insulation

THERE IS a fundamental difference in the life of insulation containing organic material operated in an insulating liquid as contrasted to the same insulation operated in a gas medium. When operated in an insulating liquid, the liquid tends to replace the material deteriorated and so maintains electrical strength. When operated at a high temperature in a gas medium, as the deterioration progresses, the gas replacing the varnish has a much lower dielectric strength. The base material may or may not deteriorate depending on whether it is organic or inorganic.

Distribution-transformer voltage stresses

INFREQUENT REPORTS of severe mechanical damage to small transformers as a result of nearby lightning strokes have led to the conclusion that the cause of the damage was the passage of heavy lightning currents in opposing directions in the two halves of the low-voltage windings. These cases have been rare but are of considerable interest as they have led to the consideration of the effects of the more frequent lightning currents of smaller magnitude flowing in distribution-transformer low-voltage windings.