3-phase Unit Research Articles

The design of welder power supplies incorporating self-oscillating high-frequency transformer inverters

Different aspects of the design of power electronic inverters for low mass welding applications are described. High frequency (15 kHz) is used at power levels up to 10 kW, and emphasis is placed on the minimisation of losses in the ferrite-core transformer and power switches to obtain an efficient and compact system. Attention is given to a very simple low-loss turn-off snubber ideally suited for the application. Two welding systems which were developed and tested are described. These were a 220 V single-phase and a 380 V 3-phase unit. Design considerations and practical results are also presented. It is concluded that such inverters have great potential for application in this field.

Design features of a 240 MVA 275/132 kV site-assembled autotransformer

After a brief review of the conflict between increasing unit sizes and transport-weight limitations and the various methods of overcoming this, the paper outlines the design philosophy underlying the principle of subdivision and reassembly on site and discusses the reasons for adopting the particular method described in the paper and applied to a pilot exercise. Vertical subdivision of the complete unit between phases was adopted, and the various problems encountered in the design, construction and assembly are discussed and illustrated with particular reference to the means adopted for ensuring mechanical stability and accurate realignment of the sections. As adequate dryness of the windings during transport and reassembly is essential, the paper describes the methods used for sealing the windings during transport and for creating suitable conditions of atmospheric dryness and cleanliness during reassembly on an open plinth on site. The steps taken were deemed sufficiently safe to obviate the need for special high-voltage tests on the insulation after reassembly, and this judgment was proved to be correct by the successful commissioning of the unit. The principle so successfully demonstrated on a pilot exercise can be applied to much larger units, and it can also be of great value in those countries where the present transport-weight limitations impose severe restrictions on the maximum size of a 3-phase unit that can be installed on certain sites.

Advantages of thre-phase spot-film radiography.

Most radiologists give considerable thought to the selection of a spot-film device. Their choice is usually based on mechanical convenience and mobility. There is evidence, however, that many other factors involved in spot-film radiography receive only slight consideration. This paper will deal with the possibilities of improving spot-film quality by utilizing 3-phase generators and controls. The usual fluoroscopic-radiographic unit in use in the United States is powered by a single-phase generator. Design of the control and a comparative lack of efficiency in the generators used in the single-phase systems present several handicaps in producing spot-films of highest quality and sharpness of detail. No choice of focal-spot size is permitted for making a spot-film. Fluoroscopy is done on the small focal spot, and spot-filming is permitted on the large focal spot only. In most cases the large focal spot is 2 mm. in diameter. Focal Spot To demonstrate the effect of focal-spot size on sharpness of detail, three radiographs of wire mesh laminated in plastic were made, one with a 2-mm. focal spot (Fig. 1, A), one with a 1-mm. focal spot (Fig. 1, B), and a third with a O.6-mm. focal spot (Fig. 1, C). Enlargement technic was used to simulate the increased part to film distances which are usually encountered in making spot-films. All other factors influencing detail were constant. A comparison of the three films demonstrates the marked blurring that results with large focal spots even though the object is stationary while being filmed. Versatility of Controls The controls used with most single-phase apparatus do not permit exploiting the fluoroscopic x-ray tube even to its capacity for single-phase operation. The rigidity of the milliamperage, time, and peak kilovolt relationships imposed by control design frequently preclude loading the tube at or near its maximum. Most 3-phase apparatus, on the other hand, permits the selection of any milliamperage for spot-film radiography within the range of the x-ray tube capacity for a given time and kilovolt setting. In contrast to the single-phase units, this flexibility is available for use with either the large or the small focal spot with the 3-phase unit; in other words, one has a wide choice of milliamperage stations and a choice of two focal spots in spot-filming as compared to a single milliamperage station and one focal spot with the customary single-phase units. “Efficiency” of X-ray Production and Tube Load Another important advantage of 3-phase equipment is the higher load that may be applied on the x-ray tube. In the spot-film timing ranges, the manufacturers' tube rating charts show up to 50 per cent greater loadability with 3-phase equipment as compared to single-phase.

Are transformer stabilizing windings necessary?

FOR MANY YEARS it has been common practice to include a Δ-connected stabilizing winding in Y-Y-connected transformers and Y-connected autotrans-formers. This applies to 3-phase units of all types of core construction as well as 3-phase banks of single-phase units. The practice has been followed so closely for such a long time that it is generally taken for granted that the stabilizing winding is a necessary part of all Y-connected transformers. In many cases it is; but it is not necessary or even desirable to include a stabilizing winding in all cases regardless of type of core construction and system characteristics.

Are Stabilizing Windings Necessary in All Y-Connected Transformers?

For many years it has been common practice to include a delta-connected stabilizing winding in Y-Y-connected transformers and Y-connected autotransformers. This applies to 3-phase units of all types of core construction, as well as to 3-phase banks of single-phase units. The practice has been followed so closely for so many years that it is generally taken for granted that the stabilizing winding is a necessary part of such transformers. In many cases it is; but is it necessary or even desirable to include a stabilizing winding in all cases, regardless of system characteristics and conditions? It is the purpose of this paper to present a comprehensive examination of the elements which determine the answer or answers to this question, in the light of modern power system operating conditions and operating practice.

Contributing Factors in Determining the Ratings of Power Transformers for Use on the Systems of The Hydro-Electric Power Commission of Ontario

Transformers like most other commodities change in style from time to time as the years pass. It is the intent of this paper to delve back 40 years into the history of the Hydro-Electric Power Commission of Ontario and, in regard to transformers, compare the mode of thinking then with that of the present time, and to follow the changes that have occurred in four decades and the governing factors that have influenced these changes. Today a transformer can be considered to be one of the most reliable pieces of equipment in operation. It has been developed partially from operating experience and partially to satisfy conditions and circumstances that presented themselves. Tappings in a transformer can be extremely hazardous and therefore should be kept to a minimum. The indoor transformer has given place to the outdoor. Water cooling has been largely superseded by self-cooling, then forced-air cooling, finally augmented with forced-oil. Single-phase transformers once used exclusively are not now preferred to 3-phase units. Capacities have been stepped up from 750-kva to 100,000-kva units. Autotransformers are used to advantage whenever they can be applied. Both manually operated off-load and automatically controlled under-load tap changing equipment has been developed. Refinements have been made to temperature indicating devices Current transformers used for relaying and control are now economically located on the bushings within the transformer tanks. Interchangeability of the various manufacturers' bushings has become a reality and is of great advantage to the operating companies.

Recent progress in large transformers

It is not the intention of the author to present a treatise on transformer design, but rather to describe in general the progress made in recent years and to give examples illustrating the present stage of the art. Separate papers might well be written on several subjects touched upon in this paper, and the author hopes that engineers whose experience enables theni to do so may be willing to prepare such papers.The necessity for transformers of large output and high voltage raised considerable problems in regard to transport, but these have been overcome and the largest units contemplated can be transported by road or rail. The relative advantages and economic uses of 3-phase units and banks of single- phase units are discussed, and it is shown that, while in America the latter arrangement is still popular, in this country and on the Continent 3-phase units are now almost standard. The author also shows that the core type of construction is becoming the most usual for both single-phase and 3-phase units, but for certain special applications the shell design may be preferable. The subject of cooling is discussed in some detail, and it is shown that natural cooling, with or without auxiliary air blast, is now supplanting older water-cooling arrangements. Reference is made to the “Inertaire” transformer, and an interesting protective scheme known as the Buchholz device is described. It is essential that transformers shall withstand transient surge conditions, and the author describes the “non-resonating” type, which is claimed to be surge-proof, but he suggests that transformers built with inter-turn insulation according to a proposed new British Standard Specification will be quite satisfactory in service, and the design will be cheaper and less complicated than for a “non-resonating” transformer. Problems connected with short-circuit conditions in a star/star-connected transformer with tertiary winding are touched upon; examples are also given of transformers having three or even four windings for useful power supply. As regards on-load tap-changing, the author shows that the parallel-winding scheme has become almost obsolete in favour of the midpoint auto-transformer arrangement. A description is given of a booster transformer arranged to give simultaneous control of both voltage and phase angle, as is now necessary for the most efficient operation of large power networks. In view of the growing demand for current-limiting reactors. the author describes a type of oil-immersed reactor which is becoming popular and is displacing the older concrete type of construction.In order to present a picture of the stage now reached in transformer construction, the author has collected technical data concerning a number of outstanding transformers built in recent years by manufacturers throughout the world.