US2337916A

US2337916A - High voltage transformer

Info

Publication number: US2337916A
Application number: US334914A
Authority: US
Inventors: Meyerhans August; Scharli Rudolf
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1939-05-15
Filing date: 1940-05-13
Publication date: 1943-12-28
Anticipated expiration: 1960-12-28

Description

Dec.'28, 1943. A. MEYERHANS ETAL .meer

HIGH VOLTAGE TRANSFORMER Filed May 15, 1940 AM/A750 PAPER Patented Dec. 28, 1943 UNlTED Sl TES idii'llfgltl'l' OFFICE HIGH VOLTAGE TRANSFORMER .application May 13, 1940, Serial No. 334,914 ln Germany May 15, 1939 (Cl. 17E-362) l0 Claims.

The construction of oil-immersed power transformers for high voltages, with rigid insulation of high value insulating material between the high and low Voltage winding in the form of a cylinder against which the active parts abut with practically no intermediate spaces, is accompanied by special dilculties as regards the insulation. It is not only a question of preventing flashov'ers between parts having different potentials, but also the electric eld which forms at the points where the insulating cylinder passes between windings and/or magnetic core parts at diiferent potentials must be influenced in such a manner that the field strength decreases uniformly in the direction of the transformer casing or yoke and that a maximum allowable longitudinal voltage gradient in the remaining cil layers between the rigid insulating parts is not exceeded. For convenience of description, the term active parts will be employed in the following specification and the claims to identify the electrically conductive and the magnetic elements (windings and core) of the transformer between which, as is well known, substantial differences' in potential are established. y

In addition to overcoming these difficulties it is also the object of the invention to reduce the overall dimensions of the transformer and to achieve a saving in space, weight and production costs, when compared with the designs hitherto used where the insulation consists of layers cil and walls of rigid insulating material.

The invention relates more particularly to the insulation cf high Voltage transformers of the type shown in the patents of August Meyerhans l Nos. 2.279.239 April 7, i942, and 2,288,201 June 30, 1942.

The invention thus concerns a transformer for high voltages with rigid insulation of a high value insulating material between the high and low voltage winding in the form of a cylinder against which the active parts abut withpractically no intermediate spaces, whereby the aforementioned difliculties are overcome and the desired is achieved. According to the invention the insulatN ing cylinder is made to extend beyond the end surfaces of the'active parts of the'transfoi vr in an axial direction in order to prevent dashovers between parts having different potentials, and furthermore electrodes are provided on these end surfaces on both sides of the insulating cylinder which at least partlyoverlap the cylindrical surfaces of the active parts facing the insulating cylinder'and the end surfaces of these parts, in-

sulation inserts of rigid insulating material being also provided which fill the spaces formed between the insulating cylinder and the electrodes and also overlap the active parts, so that the distribution of the electric field forming at the points where the insulating cylinder emerges from the active parts is such that in the gaps between the insulation inserts and the insulating cylinder a maximum allowable longitudinal voltage gradient is not exceeded.

Several constructional examples cf the invention are illustrated diagrammatically in longitudinal section in Figs. 1-3 of the accompanying drawing. Fig. 1 snows the insulation between parts of the iron core, and between the core parts and the transformer casing, whilst Figs. 2 and 3 show the insulation between the high and low voltage winding and between the latter and the yoke of the iron core.

In all the figures, l indicates the insulating cylinder made of high quality paper layers disposed between the high voltage winding 2 which surrounds the central core part il and the concentric low voltage winding 3, the active parts (coils, iron) abutting against the cylinder practically without any clearance. The insulating cylinder l extends beyond the end surfaces of the windings of the transformer in an axial direction towards the yoke 4, 4a and the transformer casing, which latter is not shown in the drawing. Electrodes l, t are arranged on the end surfaces of the outer Aactive parts of the transformer on both sides of the insulating cylinder l, these electrodes overlapping at least part of the cylindrical surfaces of the active parts facing the insulating cylinder, insulation inserts l, 3 of rigid stratified insulating material (paper) being provided to nl] the free spaces between the insulating cylinder l and the electrodes 5, B and thus also overlapping the active parts. The axially elongated insulating cylinder i and the insulation inserts l, simplify the building of the transformer and also the insertion and removal of the coils, but necessitate gaps between these'parts which are filled with oil. The electrodes 5, Si must therefore be given such a shape that the distribution of the electric field which forms at the points where the insulating cylinder emerges from active parts is such that in the gaps between the insulation inserts and the cylinder a maximum allow able longitudinal voltage gradient -is not exceeded. By this means the voltage'drop in the gap both in the longitudinal and transverse direction is reduced to a value which lies below the critical limit for flash-overs in oil.

In the constructional examples shown in Figs. 1 and 2 the

electrodes

5, 6 are drawn into the active parts of the transformer and arranged to form in close contact with the insulating cylinder l screens for the end surfaces of the active parts. The

electrodes

5, 6 have a conical part, as shown in the drawing, the distance of which from the point of contact with the insulating cylinder increases only very gradually, and connected therewith is a iiat part which overlaps the end surfaces of the active parts of the transformer.

Fig. l illustrates the application of electrodes il, 6 for controlling the field and the voltage gradient at the ends of the insulating cylinder i and also of insulation inserts l, 8 for insulatn ing parts of the iron core, namely the yoke ii, from each other and from the transformer casing which is not shown in the drawing. The high voltage winding 2 is surrounded by the insulating cylinder l. The yoke comprises a number ci separate parts, namely the high voltage part resting on the core 4' and the part fla which forms the yoke plate and has a low voltage or earth potential. Between the yoke and the low voltage winding 3 there is a metallic spacing ring 9 which is provided with a covering of insulation. rIhe yoke parts a are stepped towards their end surfaces and together with the electrodes 5, G have such a form that the longitudinal gradient in the gap between the insulating cylinder i and the insulation inserts l, ii does not exceed a definite maximum value so that creeping discharges and oil hash-overs can no longer occur. The insulation inserts between the cylinder and the electrodes are in the form of angle rings one limb of which abuts against the insulating cylinder and is tapered so as to penetrate as far as possible between the stepped yoke parts or the electrodes and the insulating cylinder, whilst the other limb of each ring covers the yoke parts and is so dimensioned that the field forming at the yoke parts causes the insulation. to be uniformly relieved, The angle rings are preferably made of stratied insulating material (paper), the direction of the layers being arranged at least approximately perpendicular to the direction oi the electric eld.

Fig. 2 illustrates the application of the electrodes 5, for controlling the i'ield and the voltage gradient and also the use of insulation inserts l, ii for insulating the high and

low voltage windings

2 and 3 respectively from each other and from the yoke li. In this case the low voltage winding 3 is surrounded by the insulating cylinder l. The end surfaces of the windings are stepped to conform with the shape of the electrodes. The construction of the insulating cylinder, electrodes and insulation inserts is identical with that shown in Fig. 1 and their eiect is also the same. The

electrodes

5, 6 are preferably pro-vided with a rounded edge orbead it at the ends which overlap the

windings

2, 3 in order to prevent corona dischages. In order to supplement the insulation an optional nurnber of insulating rings ii of high quality insulating material can be provided which are mutually supported by spacers l2, these latter also supporting the rings Il against the insulation inserts 1, S and the yoke li respectively. Similar insulating elements I3 can also be provided between the

windings

2, 3 and the electrodes l, 3. The high voltage Winding 2 and the low voltage winding 3 are provided with axial longitudinal cooling ducts.

Instead of employing special electrodes ashown in the constructional examples of Figs. and 2 it is of course possible to construct th end surfaces of the active transformer parte s that they act as electrodes for reducing electric eld which forms at the points wher the insulating cylinder emerges from the active parts. It is also possible to use as electrodes the protective rings which are often provided on the active parts, these rings being either of metal or insulating material with a metallized surface. If such electrodes are in conjunction with the insulation inserts according to the constructional examples illustrated in Figs. l and 2, it is preferable to insert additional insulation consisting of layers of insulating material between the electrodes and the angular insulation inserts` Fig. 3 shows a constructional example where this last mentioned arrangement is employed. The insulating cylinder l is again located be tween the high voltage winding 2 and the low voltage winding 3. The electrodes are formed by protective rings of insulating material with metallized surfaces Hl, l5 arranged on the end surfaces of the windings and by the

windings

2, 3 which are stepped towards the end surfaces. Additional insulation elernents iii, il are disposed between the electrodes Hl, l and the insulation inserts l, 3. The insulation inserts l, il in this case do not penetrate to the point where the stepping of the

windings

2, 3 commences, but the additional insulation elements i6, l'l extend in an axial direction into the windings and are stepped in such a manner that the ends of these stepped layers can be bent around these coil surfaces which are opposite to t1 e end surface of the winding. Care must, however,v be taken that the longitudinal cooling ducts for the windings are kept free. The additional insulation elements i5, il lying between the electrodes lil, i5 and the insulation inserts 1, il can overlap the electrodes radially or their edges can be bent over the electrodes,

as shown in Fig. 3.

The effect of the arrangement shown in Fig. 3 is the same as that described in connection with the constructions illustrated in Figs. l 2. The object of bending the stepped layers ci the additional insulation elements i5, il around the coils of the windings is to prevent the occurrence of partial discharges from the coils. In order to increase the leakage path between the sides of the insulating cylinder l and the insulation inserts l, 8 respectively, he surface area can be increased by undulating, folding or roughening. It is also possible to taper the ends of the insulating cylinder i which extend .beyond the

windings

2, 3 along the length which projects into the additional insulation elements The various arrangements of the insulation and the diiferent forms of electrodes illustrated in Figs. 1 3 can be interchanged, or combined as desired, in order to insulate the active parts (windings, iron) of a transformer from cach other or from other parts. For instance the high voltage winding or the iron part can be insulated as shown in Fig. 2 and the low voltage winding as shown in Fig. 3, or vice versa.

Having new particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, We declare that what we claim is:

1. In a transformer for high voltages comprising active parts including concentric high and low tension windings and core members, an insulating cylinder disposed between and substantially abutting said windings, said insulating cylinder extending axially beyond the end surfaces of the windings, annular metallic electrode members disposed externally and internally of said cylinder adjacent the projecting end thereof, said electrode members each having a. skirt portion extending along the surface of said cylinder between the same and the adjacent portions of the active parts and having a flared portion extending from said skirt portion away from the surface of said cylinder in a curved path and overlapping the end surfaces of said portions of the active parts, annular insulating inserts fitting externally and internally the projecting end of said cylinder and each having a tapered depending skirt portion complemental to the shape of the respective electrode members and filling the space between the same and the respective surfaces of said insulating cylinder, said insulating inserts each having a flange portion substantially coextensive with the flared portion of said electro-des and also overlapping the end surfaces of said portions of the active parts of the transformer.

2. A transformer as claimed in claim 1, characterised by the feature that the electrodes extend at least partly into the active parts of the transformer and are constructed so as to lie in close contact with the insulating cylinder and to form screens for the end surfaces of the active parts.

3. A transformer as claimed in claim l, characterised by the use electrodes which control the field and the voltage gradient at the ends of the insulating cylinder and also insulation inserts in the space between the insulating cylinder and the electrodes for insulating the high voltage winding from the low voltage winding and these latter from the yoke.

4. A transformer as claimed in claim 1, characterised by the feature that the insulation inserts,l in the intermediate space between the insulating cylinder and the electrodes are in the form of an angle ring of which the limb abutting against the insulating cylinder is tapered.

5. A transformer as claimed in claim 1, characterised by the feature that the insulation inserts in the intermediate space between the insulating cylinder and the electrodes are each in the form of an angle ring made of stratified insulating material, the direction of the insulating layers being at least approximately perpendicular to the direction of the electric field.

6. A transformer as claimed in claim 1, characterised by the feature that the electrodes are in the form of protective rings made of metal or metallized insulating material.

7. A transformer as claimed in claim 1, characterised by the feature that an additional insulation of stratified insulating material is disposed between the electrodes and the angle-shaped insulation inserts.

8. A transformer as claimed in Claim 1, characterised by the feature that an additional insulation of stratified insulating material is disposed between the electrodes and the insulation inserts and that the layers of the additional insulation are stepped down in the axial direction of the active parts, the ends of the layers being bent round the coil surfaces opposite to the end surfaces.

9. A transformer as claimed in claim 1, characterised by the feature that the surface paths facing each other on the insulation inserts and the insulating cylinder are enlarged for the leakage currents.

l0. A transformer as claimed in claim l, characterised by the feature that the end surfaces of the active parts of the transformer immediately adjacent the electrodes are stepped in a radial direction to correspond with the shape of the electrodes.

AUGUST MEYERHANS.

RUDOLF SCHRLI.