‘sept- 10, 1946- H. HARTMANNY 2,407,420 HIGH VOLTAGE TRANSFORMER Filed Sept. 9, 1943 X’, 9 Patented Sept-1Q, 1943 2,467,420 UNITED STATES PATENT OFFICE 2,407,420 HIGH-VOLTAGE TRANSFORMER Hans Hartmann, Baden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland Application September 9, 1943, Serial No. 501,696 In Switzerland August 4, 1942 '7 Claims. (C1. 175--356) 2 1 In plants where very high voltages occur it is preferable to use voltage instrument transformers present not be ?xed with regard to the allowable breakdown voltage but was determined by the leakage discharge voltage and this therefore ne which are connected between a single phase and cessitated a much stronger insulation. earth. Several suitable arrangements for this The difficulties described in connection with a, purpose are known Where the voltage of the high Cl voltage Winding increases steadily from Zero up to the high voltage potential. If the active part of such a transformer is located in an insulating transformer with a closed U-shaped core and a U-shaped insulation cover closely ?tted to the high and low voltage winding are avoided ac cording to the present invention by every kind casing containing a liquid or gaesous insulating medium, the surface of this part has a potential 10 of edge effect being prevented by means of a controlled potential distribution, so that it is distribution controlled by the winding. possible to dimension the insulation solely out It is also known that inside such a transformer of consideration for its breakdown strength. the potential gradient between the windings can constructional examples of the invention. are be taken up by a U-shaped insulating cover which encloses the iron yoke on the high voltage illustrated in the acompanying drawing in which side and is open on the earth side, whereby the Fig. 1 is a vertical sectional view of a high volt potential increases continuously from the open age transformer embodying the invention; and Fig. 2 is a fragmentary vertical section through an upper portion of the core and insulating cov ends of the cover up to the high voltage end of ' the high voltage winding. This insulating cover can be located on a conductive support, for in 20 ering of a modi?cation. In Fig. 1 of the drawing, the U~shaped iron stance on the iron core which is also U-shaped. core a is encased in a correspondingly U-shaped With such transformers it is also known that it is sleeve of insulation a’ and is closed by the yoke an advantage if the windings lie in close contact 12. On both core limbs there is the cylindrical with the insulating material. Furthermore, it low voltage winding 0. The insulating cover d is is also known to cover the low voltage inner side a winding of fabric or paper type such as em of the insulating cover, particularly over the sec ployed in cable constructions, being wound di tion which encloses the bent yoke and is free of rectly upon the winding 0 and extending without windings, with a metal layer. interruption over the yoke arch of the iron core, With these known constructions of high volt age instrument transformers it is, however, not 30 the insulating cover being open only at both the ends which abut against the lower yoke b. yet possible to make the best possible use of the The iron core and also the beginning of the high break-down value Of the solid insulation be high voltage winding e at the lower end are at tween the low and high Voltage. If there are oil earth potential. The winding e is a cylindrical gaps between both windings or between one wind ing and the insulating cover, the insulating ; winding composed of disc coils, the potential of this winding increasing from the bottom to the strength of these gaps is much lower than that top where the highest potential prevails, this of the paper insulation; nevertheless these oil being connected to the high voltage terminal 1‘. gaps are subjected to a higher ?eld strength than In order to control the potential at the bend of the paper insulation. Furthermore the insula tion problem is also considerably complicated 40 the insulating cover where there is no winding and in order to avoid edge effects the insulating by the instability of the electrical ?eld caused tube 11 is provided not only on the inside but by the winding edge at the high voltage end of also on the outside of this bend with conductive the high voltage winding cylinder. Because even layers g, h, the outer layer g being maintained if the oil gaps are avoided due to the windings being in close contact with the insulating cover ' at high voltage potential by means of a con ductive ‘connection whilst the inner layer It is corona and leakage discharges occur at this edge at earth potential. The transformer rests on a which are very di?icult to control and cannot be base plate is and is enclosed in a casing of ce prevented by merely providing the inside of the ramic insulating material 2'. The hollow space in insulating cover with a metal layer. The usual arrangement of rounded protective rings on the 50 side this casing can be ?lled with insulating liquid or gas. winding edges can increase the initial voltage The conductive layer it can of course cover at which corona occurs but does not, however, prevent dangerous leakage discharges. The dis the entire inner surface of the insulating cover tance and the insulation strength between both 07.. The layers can consist of metal or some con voltage carrying parts could therefore up to the 55 ductive material, for instance the layers 9', h’ of 3 Fig‘. 2 comprise varnish, mixed with metallic powder. They can for instance be made in such a manner that at least for the bent part of the cover it a conductive band is employed for the innermost and outermost layer, this band being either a metal band, a metallized band or a band which is impregnated or painted with a conduc tive material. The layers can also be wound 4 wound upon the legs of said core, a tubular in sulating cover closely ?tting said secondary sec tions and the portion of the core between the same, a high voltage primary winding on the legs of said core and closely ?tting said insulating cover, said primary winding comprising a plural ity of serially connected disc coils, with a low voltage terminal adjacent an end of a leg of the core and a high voltage terminal adjacent The layers 10 the bend of the U-shaped core, and means sup on or into the insulating cover 12 or pressed on, sprayed or painted on to the cover. g, h can also be made so as to allow liquid or gas to pass through, for instance in the form of pressing edge eliects at the high voltage ends wire netting and the like. It is furthermore also possible to make the layer on the outside (high ing layers at and closely adjacent the portion of the insulating cover enclosing the bend of the of said windings, said means comprising conduct voltage side) in the form of a bare or covered 15 core and not covered by the primary. winding which in the latter case also serves as the ?rst turn of the high voltage winding. With the arrangement according to the‘inven 2. A high voltage transformer as recited in claim 1 wherein the outer conducting layer is electrically connected to the adjacent high po tion all parts of the insulating cover are stressed tential end of the primary winding and the perpendicularly to the wall and solely as regards 20 inner conducting layer is grounded. breakdown. The U-shaped form of the iron core 3. A high voltage transformer as recited in and cable-like insulating cover d surrounding the claim 1 wherein said conducting layers are me core enables the ‘potential distribution along its tallic. surface to be arranged in such a manner that 4. A high voltage transformer as recited in the voltage increases from zero value at the lower 25 claim 1 wherein said conducting layers comprise open ends of the cover to a maximum value at varnish containing conductive particles on the the closed upper end. Since the outer surface of the insulating cover d has high voltage potential at its upper end, the core a which is at earth inner and outer surfaces of said insulating cover. 5. A high voltage transformer as‘ recited in claim 1 wherein said conducting layers are fluid potential is completely screened from the upper 30 permeable layers of conductive material at the metal cap which closes the insulating casing z‘ inner and outer surfaces of said insulating cover. and carries the high voltage terminal f. 6. A high voltage transformer as recited in A further advantage of the arrangement de claim 1 wherein said potential gradient means scribed is that due to the short distance between comprises a conductive winding at a surface of the windings the short-circuit voltage becomes 35 the 'bend portion of said tubular insulating cover so small that even with an iron core having a and connected electrically to the adjacent ter comparatively small cross-sectional area a very minals of the primary transformer windings on large measuring power can be obtained. At the the respective legs of the core. '7. A high voltage transformer as recited in 40 claim 1 wherein said outer conducting layer com tained as regards the voltage. prises a wire wound upon the bend portion of I claim: said tubular insulating cover and connected elec 1. A high voltage transformer comprising a trically to the adjacent terminals of the primary grounded U-shaped core closed by a yoke, a low windings. same time the radial dimensions become reduced despite the large degree of ‘safety which is ob voltage secondary winding including sections HANS HARTMANN.