US2288201A - High tension transformer - Google Patents

Description

June 30, 1942. A, MEYERHANS 2,238,201

HIGH TENSION TRANSFORMER Filed June 24, 1939 2 Sheets-Sheet l Filed June 24, 1959 2 Sheets-Sheet 2 Patented June 30, 1942 HIGH TENSION TRANSFORMER August Meyerlians, Rieden, near Baden, Switserland, assignor to Aktlengesellschaft Brown, Boveri & Cle, Baden, Switzerland, s Joint-stock company --Application June 24,

The present invention relates to high tension transformers and particularly to improved insulating means therefor.

In high tension transformers, the insulation for the high tension winding consists ordinarily of one or more insulating cylinders arranged between the high and the low voltage windings. In connection with these insulating cylinders, additional end insulation members are employed that shield the high voltage winding from the yoke. In such transformers, in an attempt to provide increased insulating strength, suitably placed oil layers that are comparatively thick are employed on the one hand between the low voltage winding and the insulating cylinder and between the high voltage winding and the insulating cylinder, and on the other hand also at the ends of the insulation.

One known transformer of this type employs a number of insulating collars, each of which is thrust over the end of a special insulating cylinder that is separated from adjacent concentrically arranged insulating cylinders by generously dimensioned oil layers. The collars, separated by thick layers of oil, are likewise secured at certain distances from each other so that the dimension of the whole insulation including the oil layers at the end is greater in axial direction than the dimension of the insulation between the high and low voltage windings in the radial direction. A modification of these transformers likewise provides for several concentric insulating cylinders continuing to the yoke, these being separated from each other and from the high and low voltage windings by a small number of thick oil layers, the collar-like end insulation, however, being secured only to the high voltage winding.

Another constructional form that permits somewhat smaller dimensions of the insulating layers makes use of only a massive insulating cylinder between the high and low voltage windings, that is likewise separated from the windings by oil layers, the thickness of which is comparable with the thickness of the insulating cylinder or exceeds it. At its end, the insulating cylinder is stepped back progressively to provide shoulders forming seats for disks projecting radially beyond the high voltage winding and being spaced by large oil layers in the direction of the axis.

In order to attain suiiicient insulating strength by means of these known measures, generous dimensions of all insulating parts and especially 1939, Serial No. 281,028

June 28, 1988 3 Claims. 175-456) of the end insulation parts are necessary if leakage around or breakdown through the insulation is to be prevented. Attempts have been made to make the spacing of the insulations smaller, with a given operating voltage and given strength of insulation, but these resulted in endangering safe operation. From these experiences it was determined that it is important above all to avoid not only leakage paths, but also, at the same time, those arrangements that favor the appearance of breakdown.

A plurality of insulating cylinders separated from each other with thick intermediate oil layers are absolutely useless in this respect. If for any reason, for instance upon bridging because of impurities, a partial breakdown through one of the oil layers occurs, the increasing elec-'- trical fleld at discharge point goes over into a leakage discharge along the insulating cylinder and complete breakdown of the whole insulation can be caused thereby. The conditions are similar when only one massive insulating cylinder is rovided between the high and low voltage windings, which, however, is separated from at least one of the windings by a thick layer of oil, which due to its slight. dielectrical constants as well known absorbs a greater portion of the difference of voltage than the solid insulating material. Upon a partial breakdown through such a thick layer of oil, the strength of electrical field from the discharge point at the insulating cylinder is so great that the occurrence and the increase of a leakage discharge cannot be suppressed even by generously dimensioned end insulation. Furthermore, there is likewise the danger of a direct breakdown through the insulation cylinder, when a partial breakdown has occurred at one oil layer, that has absorbed a considerable portion of the whole voltage.

The use of,end flanges provided with disk seats and applied on the insulating cylinder likewise has great drawbacks. Experience has shown that it is not possible to fit the seat of the and flange so exactly to the insulating cylinder that even with the shaft part stepped back leakage paths are prevented along the contact surfaces between the end flanges and the insulating cylinder, which places are especially endangered chiefly upon partial breakdown in adjacent thick oil layers. Regarding the end flanges applied only on the high voltage winding, the same drawbacks are to be mentioned, and these could be prevented heretofore only by very generous dimensions.

According to the invention, a very high insulation strength against creeping discharges and breakdown is attained without it being necessa to emplo insulating parts of unduly great dimensionsi There is a considerable saving in space and weight thereby obtained, especially for large transformers, since the weight of the iron core can be reduced with smaller spacing. The size of the transformer vessel and the volume of oil likewise may be consequently much smaller. With large transformers, the saving in space and weight produced by the insulating arrangement according to the invention, under certain conditions, makes railway transportation of the assembled transformer possible for the first time, since due to the reduced dimensions of the parts, its shape and overall dimensions will be within the clearance limits of the railway.

In a high voltage transformer with an insulating cylinder arranged between the high and low voltage windings this reduction in size according to the invention, is attained by having the insulating cylinder rest against the windings with practically no space between and extending at least at one of its ends, in expanded relation, into a flange-shaped end screening the upper voltage winding from the yoke, the dimension of the flanged end in the direction of the axis amounting to a multiple of the radial thickness of the part of the insulating cylinder located between the windings.

Other objects and advantages of the invention will be apparent from the following detailed description with reference to the accompanying drawings which illustrate preferred constructional embodiments of the invention and in which:

Fig. 1 is a semi-diagrammatic view in axial section of one'end of a transformer embodying the invention;

Fig. 2 is a horizontal section on the line 2-2 of Fig. 1;

Fig. 3 is a semi-diagrammatic view in axialsection showing a modified construction;

Fig. 4 is a horizontal section on the line 4-4 of Fig. 3;

Fig. "5 is a semi-diagrammatic view in axial section showing one end of a further modified form of the invention;

Fig. 6 is a horizontal section on the line 6-6 of Fig. 5; and

Fig. '7 is an enlarged fragmentary view of a portion of the transformer shown in Fig. 5.

In the example shown in Fig. 1, the low voltage winding I is arranged directly on the transformer core 3 with an insulating layer, 2 in between. Between the low voltage winding l and the high voltage winding 4 is located an insulatin'g cylinder 5 which extends past the end of the winding and is formed with a multi-flanged end structure 8 which screens the high voltage winding from the yoke 6. The multi-flanged end structure 8, formed by bent back portions of the insulating cylinder preferably does not rest directly on the high voltage winding but rather on an interposed protecting ring I which ring may consist of insulating material provided with a metallized surface or may be made entirely of metal. The protective ring 1 is slotted in the usual way so that it does not act as a shortcircuit winding. This protective ring acts on the one hand for controlling potential and on the other hand for assuring firm seating of the flange-shaped end structure 8 of the insulating high voltage winding 4. The flange-shaped end 8 of the insulating cylinder 5 comprises a plurality of circular flanges or layers 0 which are spread apart. The spreading apart of the flanges 9 can be accomplished by bending these layers in wave-shape such as by corrugating the same. However, the use of inserted segment shaped intermediate pieces l0, spacing the layers 9 from each other, has proved better. These intermediate pieces l0, that consist preferably of the same insulating material as the body of the cylinder 5, are separated from each other in the direction of the circumference as shown in Fig. 2. This is done in such a manner that the slots H between the ends of adjacent pieces ID in the different layers are not directly above each other but are staggered so that the slots in one layer are covered over in the direction of the axis by the intermediate pieces l0 located in adjacent layers above or below the same as will be apparent from Fig. 1. The slots II are provided for liberation of gas from the insulating cylinder and are likewise intended for facilitating the penetration of liquid insulating material into the flanged end structure 8. For increasing the length of leakage paths, the flange layers 9 of the insulating cylinder are made to proiectra- .dially beyond the inserted intermediate spacing pieces I ll. In order to lessen the danger of breakdown through the end, it has proven to be sufflcient to have the thickness or height of the flange-like end portion 8 at least equal to double the thickness of the cylindrical'part of the insulating cylinder between the high and low voltage windings. The thickness of the intermediate pieces Ill, moreover, is preferably selected so that the flange-shaped layers 9 of the insulating cylinder take up all the space between the end of the high voltage winding and the transformer yoke 6, or between the protective ring I and the yoke.

The type of construction of the insulating cylinder described allows a considerable dielectric discharge of the insulation volume found in the cylinder and thereby a uniform pressing of the space l2, that appears desirable chiefly in oil transformers. A conducting covering l3, slotted in the direction of the axis, is applied on the back or inner face of the end portion 8 facing away from the high voltage winding and this layer I3 is connected electrically to the yoke 6. By this means, the entire course of the field at the ends of the insulating cylinder is transferred into the flange portion 8. Due to the resulting satisfactory field distribution that, with a sufficiently great number of intermediate pieces Ill, decreases uniformly towards the ends of the flange portion 8, a high insulation strength is attained since not only is there a sufficiently long leakage path at the end but also, at the same time, th-field strength has sunk down below the critical value for creeping discharges.

In the modified form of the invention shown in Figs. 3 and 4, slotted metal rings l5 insulated from each other or coated with an insulating layer M are inserted in the space l2 for distribution of potential, instead of the conducting layer I3 of Fig. 1, whereby a saving of liquid insulating material is possible without aifecting the insulation strength in the space l2. This construction also provides a generous, well-controlled insulation between the end of the low voltage winding l and the yoke 6, which is desirable when a relatively high potential is developed in the low voltage winding.

Cooling passages, not shown, can be provided in the low voltage winding l or between the the cylinder.

same and the insulating layer 2, or between the rings [5 and the insulating layer 2.

The spaced flanges 9 of the end structure 8 of the insulating cylinder in both forms of the invention thus far described are formed as an integral part of the cylinder. In producing these flanges, the individual layers of asmooth wound paper cylinder are slit in the axial direction at spaced intervals to form tabs or segments which are bent outwardly at right angles to the axis of The slits in adjacent layers are staggered so that the tabs formed from one layer 'bridge across and cover the space between tabs of adjacent layers so that when a plurality of layers oi tabs or segments are glued together, the continuous flanges 9 are formed. After each flange 9 is formed of the desired number of layers of tabs, a course of intermediate segments I is glued in place thereon and the operation is repeated to produce the next flange until the entire end formation is built up. As is obvious, the flange next to the end of the high voltage winding is formed first from the outermost layers of the cylinder and the other flanges are formed progressively from inner layers, the top layer of the uppermost flange being the innermost layer of the cylinder.

It is readily seen that an insulation of this kind meets all of the requirements mentioned above and possesses great resistance to creeping discharges, since partial breakdown through the small oil layers still present are entirely without danger, and at the same time dangerous leakage paths no longer exist.

Transformers made as described, however, still possess a few technical disadvantages from the standpoint of manufacture and maintenance. In most transformers, both ends of the high voltage windings must be provided with flange-like end insulations. In such cases, the spreading of the insulating cylinder to form the flanges must be done after the high voltage winding is completely mounted in place on the insulating cylinder. The spreading of the insulating cylinder itself, i. e., the manufacture of the flange-like end insulation structure as described above, is, moreover, comparatively time consuming, since the wound paper cylinder must be slottedin a staggered manner in layers in the direction of the axis at its ends, after which, when the winding is pushed on, the resulting flaps are bent over and glued together and to the inserted intermediate pieces at the ends, these latter determining the spreading apart of the insulating cylinder. When the insulation is completed therefore, any damaged parts of the winding can no longer be replaced without destroying the end insulation.

This drawback can be avoided without affecting the electrical properties of the transformer by providing instead of the continuous insulating cylinder spread out into the flange-like end insulation, an insulation in which the multi-flanged end insulation structure is formed as a separate unit with a plurality of depending portions which fit into a great number of circumferential grooves in the end of the insulating cylinder. The grooves in the cylinder are arranged in a row in the radial direction and are of such axial depth and radial width as to form such long leakage paths between the upper and lower voltages'and such thin oil layers at the end of the insulating cylinder and in the end insulation, that the whole arrangement is practically equivalent in the electrical sense to an insulating cylinder spread out into the unitary flange-like end structure previously described. A structure of this kind is practical because of the fact that oil layers are not harmful with respect to the partial jumps when a very great number of such oil 1 layers are used, these having a considerably higher specific resistance to jumping through due to their exceedingly slight thickness, and also because of the fact that by providing sufllciently deep and numerous grooves, such long leakage paths are formed between the insulating cylinder and the end insulation that they are without importance in practice.

This modified form of the invention is shown in Figs. 5, 6 and 7. In the constructional example shown in these figures, the high voltage winding 2| and the low voltage winding 22 rest directly against the insulating cylinder 23. The axial dimension 24 of the end insulation structure which screens the high voltage winding 2| from the yoke 25 is in the same manner a multiple of the radial thickness 28 of the insulating cylinder 23. The space 21 between the yoke 25 and the low voltage winding 22 is preferably filled up with one or more slotted metal rings 28 coated with insulating material 28a, these, in

' connection with the protective ring 29 at the end of the high voltage winding, likewise slotted and covered with insulation 29a, causing a uniform diminishing of the electrical stress in the bentdown portion of the end structure and thereby effectively preventing the progress of creeping discharges.

The insulating cylinder 23 is formed with a number of circumferential grooves 30 in its end, into which the mating portion of the end insulation unit can be thrust from above. The leakage path running between the high and low voltage windings can be made the length desired in practice by great depth of these grooves and by provision of a large number of grooves, so that all possibility of leakage is eliminated. The radial thickness of the grooves 30 can also be made so slight that the specific loading capacity of such oil-filled grooves is considerablygreater than that of a thick oil layer. In practice, the desired higher loading capacity is attained by making the grooves with a thickness amounting to about 1 mm. or less.

The production of such narrow and deep grooves at the end of the insulating cylinder is best accomplished by winding the cylinder from two insulating strips 3| and 32 of different width. Two strips of equal width, however, could be used and the strip 32 could be set back in the direction of the axis an amount equal to the depth of the grooves 30, in relation to the strip 3! and the two wound together. In order to obtain the most nearly uniform circular cross-section for the insulating cylinder in the winding up of the two strips the incident edges in the circumferential direction of the wind are displaced in relation to each other so that only one step of the thickness of one strip is the result, this being reduced by subsequent working in certain cases.

This type of manufacture is possible when the insulating cylinder is to be provided at both ends or only at one end with a flanged end insulation structure. The strips are made of a good insulating material, strips formed of press board or the like preferably being used. The end insulation can likewise be made as an independent unit and subsequently be thrust into the grooves of the insulating cylinder.

If an easy subsequent adjustment of the end insulation in the direction of the axis or a simple removal of the same is desired, the structure described in the following has been found advantageous, this being desirable in addition because of facility of manufacture. is built up of strips bent down at an angle to provide flange sections 33 and depending arms 43 which are inserted in a row into the grooves 30 of the insulating cylinder 23 as shown in Fig. 6. The bent over flange parts 33 of the strips inserted in each of the grooves 30 form a flange shielding the high voltage winding from the yoke. In Fig. 5, seven such flanges are shown in section. Between these flanges, further insulation is provided by annular intermediate insulating pieces 35 arranged so that the desired axial dimension 24 of the end insulation structure is obtained.

the strips 33 the same width, thickness and length of depending arms 43. In order to make The end insulation possible a simple adjustment of the end insulation in the direction of the axis it is only necessary to take care that the length of the arms 43 to be inserted in.the grooves is chosen in comparison with the depth of the grooves so that the flange portions 33 carried by the arms 43 located in the grooves with the greatest diameter leave free suflicient space for movement in the direc-' tion of the axis. The subsequent end pressure on the high voltage winding or the removal of certain parts of the same is then easy to accomplish. The thickness of the strips 43 is preferably somewhat less than the thickness of the two strips 3| and 32 of which the insulating cylinder 3 is wound. The strips 3| and 32, for example;

each consists of press board having a thickness of 0.5 mm. while the strips 43 that may likewise be of press board, are made only about 0.4 mm. thick.

The high and low voltage windings and the protective rings 28 and 29 appertaining to the same are constructed and arranged to provide for potential gradient control at the ends 01. the windings in such a way that the electrical field in the region of the grooves in the insulating cylinder traverses the same as nearly perpendicularly as possible. Partial discharges through the oil layers in the grooves and between the bent down portions 33 of the end insulation are without danger in practice due to the high speciflc resistance of the thin oil layers and the great number thereof, the development of discharge paths along the parts of the end insulation being prevented by the constant decrease in field and along the grooves 30 by the inserted arm portions 43 of the strips. The described form of insulation is therefore equivalent to a great extent to an insulating cylinder spread out into the end insulation itself but is more convenient astregards manufacture and subsequent manipula ion.

While preferred embodiments of the invention have been shown and described by way of illustration, it will be understood that further modigfications and changes in the details of construction may be resorted to without departing from the spirit of the invention within the scope of the thickness concentric with and disposed in the gap between the windings in close contact with the surface of each and completely filling the gap therebetween, said insulating cylinder being formed with an end portion extending beyond the ends of said windings in the direction of said yoke, said extending end portion of the insulating cylinder having its laminations separated and spread out progressively from a point adjacent the end of the high voltage winding to the end of the insulating cylinderto form a plurality of integral axially spaced annular flange elements extending radially from the body of said cylinder over the end of the high voltage winding for shielding the same from the yoke, the over-all dimension across said flanges in the axial direction being at least twice the wall thickness of the laminated cylinder in the radial direction.

2. In a high tension transformer comprising a central core and an end yoke and. concentric high and low voltage windings on said central core terminating short of said end yoke and pro.- viding an annular gap therebetween, a laminated insulating cylinder of substantially constant wall thickness concentric with and disposed in the gap between the windings in close contact with the surface of each and completely filling the gap therebetween, said insulating cylinder being formed with an integral end portion continuous with the body of the same and extending beyond the ends of said windings in the direction of said yoke, said extending end portion of the insulating cylinder having its laminations separated and spread out progressively from a point adjacent the end of the high voltage winding to the end of the insulating cylinder to form a plurality of axially spaced annular flange elements integral with and extending radially from the body of said cylinder over the end of the high voltage winding for shielding the same from the yoke, the over-all dimension across said flanges in the axial direction being at least twice the wall thickness of the laminated cylinder in the radial direction.

3. A high tenison transformer according to claim 1, and a plurality of insulating ring segments disposed between adjacent flange elements axially spacing the same to provide for penetration of liquid insulating material between the respective flanges.

AUGUST "MEYERHANS.

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