US3113281A

US3113281A - Tapped transformer winding

Info

Publication number: US3113281A
Application number: US73748A
Authority: US
Inventors: Ralph E Ayers
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1960-12-05
Filing date: 1960-12-05
Publication date: 1963-12-03
Anticipated expiration: 1980-12-03

Description

R. E. AYERs 3,113,281

TAPPED TRANsFoRMER WINDING 4 Sheets-Sheet 1 .No LOAD TAPS Dec. 3, 1963 File'nec. 5, 1960 Dec. 3,

vFiled Dec. 5, 1960 R. E. AYERS TAPPED TRANSFORMER WINDING 4Ysneets-sneet 2 Dec. 3, 1963 R. E. AYERs 3,113,281

TAPPED TRANSFORMER WINDING Filed Dec. 5, 1960 4 Sheets-Sheet 3 Dec. 3, 1963 R. E. AYERs TAPPED TRANSFORMER WINDING 4 Sheets-Sheet 4 Filed Dec. 5, 1960 b Jy Hfs MPa/75g.

United States Patent O 3,113,28i TAPPED TRANSFRMER WlNlNrG Ralph E. Ayers, Rome, Ga., assigner to General Eiectric Company, a corporation of New York Filled Dec. 5, i960, Ser. No. 73,748 6 Claims. (til. 336-150) This invention relates to electrical apparatus and more in particular to an improved winding yarrangement for regulating the output of transformers.

Many types of electrical apparatus, such as transformers and the like, have a certain amount of flux, called leakage flux, which does not link all the turns of every winding. The leakage iiux gives rise to a leakage reactance, which is a measure of the series inductance of the apparatus winding and therefore is of importance when balancing a transformer with a load or another transformer. The leakage reactance of an untapped power transformer is substantially constant throughout its operating range because the leakage flux travels primarily in a path comprising an unsaturable material (eg. air). Such untapped power transformers can therefore be considered as hat reactance devices. However, it is generally desir-able to provide the windings of power transformers with regulating taps in order to provide a range of vol-tages that can be obtained from the apparatus. These tapped power transformers often display a severe change in transtormer reactance when tap connections are changed. This can be explained in part by the introduction of a leakage flux path perpendicular to the axis of the transformer core when turns are added or removed along the length of the winding by changing taps. There is also a change in the leakage flux which links less than all of the turns of the winding when taps are changed, and a change in the volts per turn in the windings.

One method of reducing the variations of leakage fiux in a tapped transformer is to utilize the innermost winding adjacent the core for the tapped winding. This arrangement is effective because the innermost winding normally has the least amount of ux which does not link all of the turns and thereforeis of minimum leakage reactance. insulation considerations normally dictate that the outer winding of a power transformer be the high voltage winding, and consequently prior art tap changing transformers generally utilized taps on the innermost or low voltage winding to regulate output. -lowever, when the power handling capacity of the transformer is very large, the current in the low voltage winding becomes too great for it to be practical to switch taps under load. Consequently, the load tap changing connections were placed on the high voltage winding, and the high voltage winding was placed in a radially outside position for insulation purposes. This configuration resulted in the worst possibie arrangement as far as controlling variations in leakage reactance is concerned.

Accordingly, it is 4an object of my invention to provide an improved electrical apparatus.

Another object of my invention is to provide an improved transformer,

A further object of my invention is to provide a winding arrangement for a tapped power transformer that minimizes reactance variations when taps are changed under load.

VOther objects and advantages of the invention wiil become apparent from an examination of the specification, drawing, and claims which follow.

According to one aspect of my invention, fluctuations in reactance of electrical apparatus having a tapped winding can be reduced by splitting a high voltage coil into two serially connected sections and placing one section adjacent a ground plane, such asa magnetic core, radially inwardly of a low voltage winding. The other high voltlll ICC

age coil is located radially outwardly of the low voltage winding, and the load tap changing taps are placed in one of the high voltage coils.

ln the drawing:

FlGURE l is a schematic representation, partially in a cross-section, of a transformer in accord with the teachings of my invention.

FIGURE 2 is a cross-sectional view of a transformer having the winding arrangement indicated in FIG. l.

yFlGURE 3 is an equivalent circuit for a modification of a transformer 4winding in accord with my invention.

FGURE 4 is a schematic representation, laid out as a developed surface, iof the outer sec-tion of the high voltage winding in the modification of FG. 3.

FlGURE 5 is a graph showing the improvement in reactance variations achieved 1oy the practice of my invention.

My invention will now be explained by reference to the drawing. In FIGURES l and 2 a schematically illustrated transformer is seen to comprise a core it) surrounded by winding coils il, d2, 4and 13. The high voltage winding is formed from a hrst coil section il. and a second coil section l2. The low voltage winding coil i3 is sandwiched in between the first and second coil sections. The first high voltage coil 11 is located in the radially innermost position adjacent the core i0, which will be a ground plane. The first coil section 11 may contain the majority of turns in the high voltage winding. The coil 1i may be divided into two portions 23 and 29 having an equal number of turns. The port-ion 23 may be connected by a lead 14 to a conventional switching reactor i5. The portion 29 may he connected by 4a lead in to a reversing switch i7, the operation of which will be explained in paragraphs that follow. The portions 23 and 29 each may comprise a plurality of disc or pancake-type coils serially connected together. Leads 27 from the high voltage coil 11i. may be connected to line and/or to other similar windings in conventional 4polyphase arrangements.

The outer 'nigh voltage coil i2 may be the regulating section of the winding. The coil i2 has taps Si) which :can be connected under load in a buck or boost fashion to decrease or increase the number of active turns in the high voltage windings. The coil 12 may be formed by placing a plurality of insulated conductor strands 2t) adjacent each other and spirally winding the adjacent strands in an axial layer along the core 19. The exact number of adjacent conductor strands 2G in any specific coil l2 will be dictated by the power capacity of the apparatus :and the range of regulation desired. This has been indicated by lnumbering the adjacent strands from i to N in the drawing. The specitic number of times each strand is wrapped around the `core will be determined by the number of turns necessary to produce the desired. regulation. The various strands 2t) may be connested in series as indicated in FIG. l by connecting the lower end of strand to the upper end of strand 2, and so on until each strand is connected to the ner/.t higher numbered strand. However, it will be appreciated by those skilled in the art that the strands may be connected to obtain transpositions of various types in order to achieve various electrical effects, such as impulse distribution.

in order to achieve the maximum range of regulation from this type of winding arrangement, the strand i may be connected at tap G to a contact 25 and the last strand N at the opposite end of the winding may have its tap R connected to a Contact 26. The reversing switch i7 is placed between the

contacts

25 and 26 so that the turns in the wniding section l2 may be connected in either buck or boost fashion to the turns in the winding section ii. A terminal 27' may be provided for connecting the switching reactor to the reversing switch 17 in order to eliminate the winding section 12 when desired.

When a transformer is of the type connected in a stepdown fashion between a high voltage transmission line and a low voltage distributing line, it is desirable to employ no-load taps 31 to enable the transformer to be connected to diderent transmission lines operating in a range of different voltages. l have discovered that the 'no-load taps can be interwound with the load tap changing taps of a transformer in accord with the teachings of my invention. This can be accomplished because standards set up by the power industry specify that the rio-load taps be 21/2% of the high Voltage rated connection voltage of a transformer, and the load tap changing taps be 11/4% of the high voltage rated connection Voltage. Thus there is a 2 to 1 ratio between the number of conductor turns necessary to produce the specified voltage in the no-load taps and the turns required in the load taps. This enables the strands 2.0 in a high voltage coil 12 to be employed as load tap changing taps 3@ by having a tap connection at each strand end. The strands 2@ can also be employed as no-load taps 31 by having a tap connection at alternate strand ends.

The above-described type of interwinding arrangement of no-load taps and load taps has been illustrated in FIG- URES 3 and 4. FIGURE 3 shows an equivalent circuit for a transformer in accord with my invention having load taps 3h and no-load taps 31 which may be inter- Wound in the outer high voltage coil 12, in the manner shown in FiG. 4. The no-load taps 31 may be connected in series between one winding portion 29 and the reversing switch 17. Any conventional switching means 32 may be employed for connecting to the desired no-load tap.

An outer regulating winding section 12. that can be connected to a winding 11 in the manner shown in FIG. 3 .is illustrated as a developed surface in FG. 4. The winding section 12 may be formed, for example, by placing a 'plurality of strands 2t) of insulated conductor adjacent each other and winding them spirally into a coil that extends axially along a core (not shown in FlG. 4). Each strand is wrapped around the core the same number of times, and this number is determined by the degree of regulation desired and the power capacity of the apparatus. The load tap changing taps 311 are obtained from 'terminals G, H, I, K, L, M, O, P, and Q at each strand end, and a terminal R at the end of the lowermost strand. The various strands may be connected back to each other in the manner indicated in FIG. 1 with like lettered strands ends being connected to each other. The strands forming the loadtap changing taps 3@ have been numbered 1 9 in order to correspond to the numbers used to designate the strands in FG. 1. The no-load taps 31 are obtained from terminals as indicated at E, C, B, D, A, and F, and strand ends having like letters would be electrically connected to each other. The strands forming the no-load taps 31 have been numbered 11d-117. It will be ap- -parent from FIG. 4 that each of the no-load taps 31 can produce a voltage variation twice as great as each load tap 3th because each no-load tap is connected to twice as many turns.

It has been found that the space factor of the apparatus can be increased if the load tap changing taps 31B have the ends of their strands Ztl starting on one side of the circumference of the winding 12 and the no-load taps 31 have their strand ends starting on the opposite side of the winding. This produces a saving in space because the bulky connections which must be brought out from the winding are located on opposite sides thereof and thus do not pile up on one another on the same side of the `wind-ing. This arrangement has been indicated in FIG. 4, since it is apparent that the load taps 39 and the noload taps 31 would be approximately 180 apart if the winding were circular rather than a developed surface as illustrated.

lt will be apparent to those .skilled in the art that a tapped winding as illustrated in FIGURE 4 employs substantialiy the entire axial length of the core for active ampere-turns. The result is. that unbalances in ampereturns are minimized because any gaps that exist in the regulating winding when turns are not employed are distributed throughout the entire length of the winding rather than being grouped together as adjacent turns. Thus, the effect of the inactive' turns on ampere-turn unbalance is less than in winding arrangements where the inactive turns are adjacent to each other and thus produce socalied holes in the winding.

FTGURE 5 is a graph showing the desirable affect of my invention on reactance fluctuations in commercial transform rs. The curves l and il in the graph represent the reactance of transformers at various tap positions and are plotted in terms of the tap position reactance as a percentage of the reactance of the transformer at its rated connection voltage, the later reactance being taken as The curve 1 is an average of the reactance of five, three-phase, class 15 kv. primary connection voltage transformers having a rating of 2000 kva., constructed in the manner practiced by the prior art. The curve Il is the reactance of one, three-phase, class 15 kv. primary connection voltage transformer having a rating of 2000 lava., made in accord with the teachings of my invention. 1t should be noted that the prior art transformers had an average reactance fluctuation of about 20% from the maximum buoi; position (161s) to the maximum boost position (16K). However, the transformer in accord with my invention showed reactance variations of less than plus or minus 5% between these two extreme positions.

The following theory is believed to be an explanation for the desirable reduction in reactance variation of the transformer constructed in accord with rny invention. Reactance variations are believed to be caused by changes in leakage flux resulting from an unbalance in ampereturns, and also from changes in the number of volts per turn produced by the apparatus. In a voltage stepdown arrangement, when the output of the low voltage winding is raised by a change in -tap positions, high voltage iwinding turns are bucked. ln this situation the low voltage winding 13 and the tapped section 12 of the high voltage winding act together in opposition to the other section 11 of the high voltage winding because their ampereturns are in the same direction. This tends to raise the reactance between the high and low voltage windings. At the same time, the increased volts per turn of the high voltage and low voltage winding tends :to decrease the reactance. Thus, these two effects tend to work against each other to minimize reactance variations. On the other hand, when the output of the low voltage winding is lowered by a change in taps, the high voltage winding turns are boosted and the windings are in the so-called form-S design. This means that the inner and

outer sections

11 and 12 of the high voltage winding have their ampere-turns in the same direction and are opposed Ito the ampere-turns of the low voltage |winding. This arrangement tends to lower the reactance between the high and low voltage windings. At the same time, the decreased volts per turn of the high and low voltage windings tends to increase reactance. It is thus apparent that my novel winding arrangement produces phenomena that tend to oppose each other so far as the reactance produced is concerned. Consequently, reactance fluctuations caused by tap changes under load are kept to a minimum.

1t will be understood, of course, that while `the forms of the invention herein shown and described constitute the preferred embodiments thereof, it is no-t intended herein to illustrate all the possible equivalent 4forms or ramifications of `the invention. lt will also be understood that the words used are words o-f description rather than of limitation, and that various changes may be made Without departing from the spirit or scope of the invention herein disclosed, land it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What I claim as new and desi-re to secure by Letters Patent of the United States is:

1. In a transformer, the combination comprising a magnetic core, a plurality of windings around said core, said windings comprising la pair of serially-connected high voltage coils and a low voltage coil, one of said high voltage coils being located adjacent said core, said low voltage coil surrounding said one high voltage coil radially outwardly thereof, the other of said high voltage coils surrounding said low voltage coil nadially outwardly thereof, and the radially outermost of said high voltage coils being tapped at a plurality of locations to provide terminals for changing the output of said transformer under load.

2. In a transformer, the combination comprising a magnetic core, a plurality of windings around said core, said windings comprising a pair of serially-connected high voltage coils and a low voltage coil, one of said high voltage coils being located adjacent said core, said low voltage coil surrounding said one high voltage coil radially outwardly thereof, the other of said high voltage coils surrounding said low voltage coil radially outwardly thereof, the radially outermost of said high voltage coils being tapped at a plurality of locations to provide terminals vfor changing the output of said transformer under load, and said youtermost high voltage coil comprising a plurality of axially adjacen-t insulated conductor strands spirally wound in an axial direction so as to form la layer of interleaved lturns.

3. A power transformer having a rated connection voltage, said transforme-r comprising a magnetic core having a leg, a plurality of windings around said core leg, said windings comprising a pair of serially-connected high voltage coils and a low voltage coil, one of said high volt age coils surrounding said core leg, said low voltage coil surrounding said one high voltage coil radially outwardly thereof, the other of said high voltage coils surrounding said low voltage coil radially outwardly thereof, the radially outermost high voltage coil being tapped at a plurality of locations to provide terminals for changing the output of said transformer under load, said outermost high voltage coil comprising a plurality of axially adjacent insulated conductor strands spirally wound in an axial direction so as to form a layer of interleaved turns, and each of said strands being wound to form 1a number of turns surrounding said core leg that produces a voltage of approximately 114% of the rated connection voltage of said transformer.

4. A power transformer, having a rated connection voltage, said transformer comprising a magnetic core having a leg, a plurality of windings around said core leg, said windings comprising a pair of serially-connected high voltage coils and a low voltage coil, one of said high voltage coils surrounding said core leg, said low voltage coil surrounding said one high voltage coil radially outwardly thereof, the other of said high voltage coils surrounding said low voltage coil radially outwardly thereof, the radially outermost of said high voltage coils being tapped at Ia plurality of locations to provide terminals for changing the output of said transformer under load, said outermost high voltage coil comprising a plurality of axially adjacent insulated conductor strands spirally Wound in an axial direction so as to form a layer of interleaved turns, each of said strands :being wound to yform a number of turns around said core leg that produces a voltage of approximately 111% of the rated connection voltage of said transformer, and some of said strands being serially connected to form pairs of strands having a number off turns between tap connections sufficient to produce 2.1/2 of the rated connection voltage of said transforme-r, the last mentioned tap connections providing no-load taps for said transformer.

5. A transformer as recited in claim 4 in which the terminals of said no-load taps are on the circumference of said outermost high voltage coil on the opposite side thereof Ifrom the terminals of the `load. tap changing taps.

6. In a transformer, a high voltage winding containing load tap changing taps and no-load taps, said Winding comprising a plurality of axially adjacent strands of insulated conductor material -spirally coiled in an axial direction to form a layer of turns, each strand being coiled the same number of times, ends of strands at one end of said winding being connected to ends of strands at the opposite end of said winding to produce series connected turns, and tap connections for said no-load taps bein-g connected between twice as many series connected strands as said load tap changing taps, whereby there is a 2 to 1 ratio between the number of turns between load tap changing taps and the number of turns between no-load taps.

References Cited in the tile of this patent UNITED STATES PATENTS 1,761,732 Kochling June 3, 1930 2,710,947 Gaston lune 14, 1955 2,757,347 Pozaryski July 31, 1956 2,840,790l Vogel June 24, 1958 2,922,132 Kreuzer Ian. 19, 1960

Claims

1. IN A TRANSFORMER, THE COMBINATION COMPRISING A MAGNETIC CORE, A PLURALITY OF WINDINGS AROUND SAID CORE, SAID WINDINGS COMPRISING A PAIR OF SERIALLY-CONNECTED HIGH VOLTAGE COILS AND A LOW VOLTAGE COIL, ONE OF SAID HIGH VOLTAGE COILS BEING LOCATED ADJACENT SAID CORE, SAID LOW VOLTAGE COIL SURROUNDING SAID ONE HIGH VOLTAGE COIL RADIALLY OUTWARDLY THEREOF, THE OTHER OF SAID HIGH VOLTAGE COILS SURROUNDING SAID LOW VOLTAGE COIL RADIALLY OUTWARDLY THEREOF,