US3315197A

US3315197A - Transformer coil having improved short circuit strength

Info

Publication number: US3315197A
Application number: US422793A
Authority: US
Inventors: Robert W Russell
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1964-12-31
Filing date: 1964-12-31
Publication date: 1967-04-18
Anticipated expiration: 1984-04-18

Description

April 18, 1967 R. w. RUSSELL TRANSFORMER COIL HAVING IMPROVED SHORT CIRCUIT STRENGTH Filed Dec. 31, 1964 INVENTOR. Robert ZflRz/ssel BY United States Patent 3,315,197 TRANSFORMER COIL HAVING IMPROVED SHORT CIRCUIT STRENGTH Robert W. Russell, New Concord, Ohio, assignor to McGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware Filed Dec. 31, 1964, Ser. No. 422,793

4 Claims. (Cl. 336-84) This invention relates to coils for high voltage electrical transformers and, in particular, to such coils having taps in either winding.

The current in the secondary winding of an electrical transformer flows in a direction around the magnetic core opposite to the direction of the current in the primary winding, and the two windings therefore repel each other when the transformer is loaded. The mechanical forces tending to separate the two windings become very high during severe overloads or short circuits on the transformer. Taps in either winding will displace the magnetic centers of the two windings and unbalance the magnetic flux distribution so that the leakage flux has a substantial radial component, and consequently a large axial force exists tending to displace the two windings relative to each other in an axial direction.

It is an object of the invention to provide a transformer coil construction having taps in one of the windings which can withstand substantially greater short circuit current without deformation than prior art coils.

It is a further object of the invention to provide a distribution transformer coil having novel shielding means which permits it to withstand, without deformation, higher magnitude short circuit current than prior art coils.

These and other objects of the invention will be more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a partial cross sectional view through a core type transformer embodying the invention; and

FIG. 2 is a schematic cross sectionalview through a coil of a shell type transformer embodying the invention.

Referring to FIG. lof the drawing, a core type distribution transformer may include a generally rectangular magnetic core of fiatwise bent magnetic strip steel laminations 11 and havinga pair of straight winding legs 12 (only one of which is shown in the drawing) each of which is surrounded by a coil 14 comprising a secondary winding section 15 and a primary winding section 16 radially outward therefrom. Alternatively the primary winding section can be radially inward from the secondary winding section. One of the

winding sections

15 or 16 in each of the coils 14 is provided with taps to permit changing the turn ratio of the transformer, and as shown in the drawing the taps are in the primary Winding section 16. The secondary winding section 15 may be of conventional construction and comprise a number of concentric layers of conductor tu rns such as helical turns of wire or turns of conductive strip wound on an insulating support sleeve 18 surrounding core winding leg 12. An annular insulating barrier 19 is disposed between the secondary winding section 15 and the primary winding section 16.

Primary winding section 16 may comprise a plurality of

concentric layers

20, 21, 22, 23 and 24 of helical turns of wire 28 surrounding insulating barrier 19 with interlayer sheet insulation 29 disposed between adjacent layers of wire turns. Tap leads T1 and T2 may be connected to taps in the radially inward layer adjacent the start of primary winding section 16.

Tapping out part of the primary winding section 16 displaces the magnetic centers of the primary and second- 3,315,197 Patented Apr. 18, 1967

ary winding sections

16 and 15 so that the magnetic flux distribution is non-symmetrical and a substantial component of leakage flux exists in a radial direction and causes a large axial component of force tending to displace the primary and

secondary winding sections

16 and 15 relative to each other. In accordance with the invention, an annular shield 34 of suitable conductive material such as copper is disposed within the primary winding section 16 and preferably immediately adjacent the tap sections which cause the dissymmetry of the magnetic flux. As shown in the drawing, the shield 34 is disposed. between the first and

second layers

20 and 21 of wire turns of primary winding section 16. Shield 34 is of greater radial thickness than is suitable for mere electrostatic shielding and preferably is of suflicient thickness in a radial direction to cause the flow of eddy currents of substantial magnitude when subjected to the radial component of the leakage flux resulting from tapping out part of the primary winding section 16. The conductive shield 34 extends for the entire axial length of the transformer coil 14, and the generation of eddy currents in shield 34 confines, or contains, the magnetic flux generated by the winding beneath shield 34 for the entire length of the coil 14 so that no appreciable component of the leakage flux flows in a radial direction within the coil. The eddy currents link the magnetic flux which causes them and are in such a direction as to oppose the change in the magnetic field and tend to set up a magnetic field which opposes the increase in flux density, thereby turning aside the leakage flux tending to pass through shield 34 and suppressing the radial component of the coil leakage flux. Consequently, the axial component of the mechanical force tending to displace primary and

secondary winding sections

16 and 15 relative to each other is materially reduced. Short circuit tests on 50 kva. distribution transformers embodying the invention reveal that the ability of coil 14 to withstand short circuit without deformation is increased from a magnitude of approximately 40 times rated current without shield 34 to approximately fifty'five times rated current when shield 34 is present. Further, by suppressing the leakage flux, shield 34 improves the magnetic linkage between primary and

secondary winding sections

16 and 15 and improves the reactance characteristics of the windings.

The degree of correction, or suppression, of the leakage flux distribution is a function of the thickness of shield 34, and the thickness of shield 34 utilized in a specific winding is dependent upon the degree of unbalance of leakage flux created by that winding and the degree of correction desired. The thickness of shield 34 was varied in the range of from 3 to 10 mils in experimental 50 kva. core type transformers embodying the invention, and it was found that a shield 34 having a thickness of .005 inch was adequate to substantially improve the short circuit strength of the coils.

FIG. 2 schematically illustrates a cross section through a coil 40 surrounding a magnetic core 11' of a shell type distribution transformer wherein low

voltage winding sections

41 and 42 are disposed radially inward and radially outward respectively from a primary winding 44 with tubular insulating barriers 45 therebetween in what is termed a low-high-low arrangement. Low

voltage winding sections

41 and 42 may be of conventional construction and comprise a plurality of conductor turns such as conductive strip or concentric layers of helical turns of wire 43. Primary winding 44 may comprise nine concentric layers 46-54 of wire turns 55 with taps adjacent the middle of the winding connected to tap leads T3, T1 and T2 in the fourth layer 49 and to tap leads T6, T5 T4 in the fifth layer 50. The tap leads are connected to stationary contacts (not shown) of a suitable tape changer in conventional manner.

Tubular shields

56 and 57 of suitable conductive material such as copper are disposed radially inward and radially outward from, but preferably immediately adjacent to, the

layers

49 and 50 having the taps which cause the non-symmetry of the magnetic flux distribution, and

shields

56 and 57 are preferably located between the third and

fourth layers

48 and 49 and between the fifth and

sixth layers

50 and 51 respectively. The generation of eddy currents in

shields

56 and 57 corrects, or suppresses, the radial component of the leakage flux generated by the helical turns in the fourth and

fifth layers

49 and 50, in the same manner as explained for shield 34 of the embodiment of FIG. 1, and confines the leakage flux principally between

shields

56 and 57 so that the axial component of the mechanical force tending to displace the primary winding 44 and the

secondary winding sections

41 and 42 relative to each other is materially reduced.

As shown in the drawing, the shield 34 of the embodiment of FIG. 1 (or the

shields

56 and 57 of the embodiment of FIG. 2) may be passive and electrically connected to only one wire turn of the adjacent layer 21 and separated by sheet insulation 29 from adjacent layers of wire turns, but in alternative embodiments the shield may constitute an active current-carrying turn of the winding.

While only a few embodiments of the invention have been illustrated and described, many modifications and variations thereof will be readily apparent to those skilled in the art, and consequently it is intended in the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.

I claim:

1. In combination with an electrical transformer coil having concentric primary and secondary windings each of which comprises a plurality of layers of conductor turns, at least one layer of one of said windings constructed and connected to generate magnetic leakage flux having a substantial radial component, an annular conductive shield disposed within said one winding adjacent said one layer and being of sufficient radial thickness to cause the circulation of eddy currents of considerable magnitude in said shield due to said radial component of said leakage flux, the radial thickness of said shield being in excess of that required for electrostatic shielding, whereby said shield suppresses leakage flux in a radial direction through said winding and improves the ability of said coil to withstand short circuit strength.

2. In combination with an electrical transformer coil having concentric primary and secondary windings one of which comprises a plurality of layers of conductor turns and being provided with taps in one of said layers, whereby the magnetic centers of said primary and secondary windings are displaced when part of said one winding is'tapped out, and annular conductive shield means disposed within said one winding adjacent said one layer having said taps and being of sulficient radial thickness to cause the circulation of eddy current of considerable magnitude therein due to the radial component of the leakage flux from said one winding, whereby said shield suppresses the radial component of the leakage flux in said coil and improves the ability of said coil to withstand short circuit current.

3. In the combination of claim 2 wherein said shield means includes annular conductive members disposed radially inward and radially outward from said layer provided with taps.

4. In combination with an electrical transformer coil having concentric primary and secondary windings one of which comprises a plurality of concentric layers of helical turns of wire and being provided with taps in one of said layers, whereby the magnetic centers of said primary and secondary windings are displaced when part of said one winding is tapped out, an annular conductive shield disposed within said one winding adjacent said one layer having said taps and being of sufiicient radial thickness to cause the circulation of eddy currents of considerable magnitude therein due to the radial component of the leakage flux from said one winding, whereby said shield suppresses leakage flux in a radial direction through said coil and improves the ability of said coil to withstand short circuit current.

References Cited by the Examiner UNITED STATES PATENTS 2,995,685 8/1961 Lord 336-84 X 3,028,539 4/1962 Wright 33684 X FOREIGN PATENTS 235,394- 8/1964 Austria.

LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA, Assistant Examiner.