US3593243A - Electrical induction apparatus - Google Patents

Electrical induction apparatus Download PDF

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US3593243A
US3593243A US840090A US3593243DA US3593243A US 3593243 A US3593243 A US 3593243A US 840090 A US840090 A US 840090A US 3593243D A US3593243D A US 3593243DA US 3593243 A US3593243 A US 3593243A
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magnetic
core
voltage
segments
coils
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US840090A
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John George Trump
Brian Skillicorn
Bryon Lee Johnson
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HIGH VOLTAGE POWER CORP
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HIGH VOLTAGE POWER CORP
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F30/00Fixed transformers not covered by group H01F19/00
    • H01F30/06Fixed transformers not covered by group H01F19/00 characterised by the structure
    • H01F30/12Two-phase, three-phase or polyphase transformers
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00Details of transformers or inductances, in general
    • H01F27/28Coils; Windings; Conductive connections
    • H01F27/32Insulating of coils, windings, or parts thereof
    • H01F27/324Insulation between coil and core, between different winding sections, around the coil; Other insulation structures
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00Details of transformers or inductances, in general
    • H01F27/33Arrangements for noise damping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00Details of transformers or inductances, in general
    • H01F27/34Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
    • H01F27/343Preventing or reducing surge voltages; oscillations
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00Details of transformers or inductances, in general
    • H01F27/34Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
    • H01F27/346Preventing or reducing leakage fields
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F30/00Fixed transformers not covered by group H01F19/00
    • H01F30/02Auto-transformers

Abstract

An electrical induction apparatus, adaptable for use as a transformer or a reactor which contains a magnetic circuit and an electrical circuit. The active portions of the magnetic circuit are formed of electrically isolated segments, each segment being electrically connected to a distinct portion of the electrical circuit such that when a voltage is impressed on the electrical circuit a systematic and uniform progression of voltage is imposed on the magnetic circuit. The electrical isolation appearing between each segment of the magnetic circuit is coated with a material adapted to establish electric field boundaries between the segments and prevent the creation of high electrical stress points while also preventing excessive eddy current loses.

Description

United States Patent 1 3,593,243
[721 Invent rs J ha wrgd u p 2,909,742 10/1959 Lamberton 330 212 h st r; 3,151,304 10 1904 Miller 336/60X BrilnSklllicormTopsflemBryonLee 3,173,115 3/1965 Peuronn 336/70X Johnson,Chelrnsiord,nl|of.Mass. 3,274,526 9/1966 Emanuelsonm, 336/212X 1 1 rm- 540,090 3,292,127 12/1966 Dormaicrm 336/84 1 1 filled d 1 9 3: 2,945,913 7/1960 Conangla 174/73 Meme 1 FOREIGN PATENTS 73 A a v1 P l l fi' gra g 315,372 1930 GreatBritain 336/84 390500 1933 GreatBritainm. 336/84 on 11-1-1111 111 .N. 1
cams" 732,231 2/1943 Germany 336/184 $67,641, July 25, 1966, now abandoned.
[S4] ELIXZTRICAL INDUCTION APPARATUS 12 Claims, 26 Drawing Fig [52] U.S.Cl 1 336/70, 336/60,336I84,336/184,336/212,336/219 [51] lnt.Cl HHOIHS/(M, H011 15/14 [50] FieldoISearch i. 336/212, 219, 69, 70, 84, I97, 184, 185, 180, 60, 210; 174/73,3S.3
[56] Reierences Cited UNITED STATES PATENTS 1,942,575 1/1934 Shapiro 174/35(.3)X 2,456,941 12/1948 Hodnette........,............. 336/210):
Primary ExaminerThomas J Kozma Attorneys-Francis J. Thornton and lrwin A. Shaw ABSTRACT: An electrical induction apparatus, adaptable for use as a transformer or a reactor which contains a magnetic circuit and an electrical circuit. The active portions of the magnetic circuit are formed of electrically isolated segments, each segment being electrically connected to a distinct portion of the electrical circuit such that when a voltage is impressed on the electrical circuit a systematic and uniform progression of voltage is imposed on the magnetic circuit. The electrical isolation appearing between each segment of the magnetic circuit is coated with a material adapted to establish electric field boundaries between the segments and prevent the creation of high electrical stress points while also preventing excessive eddy current loses.
PATENTEUJuLmsn 3,593 243 sum 1 0F 7 2? h w an I i I FIG. I PRIOR {\RT FIG. 2
PATENTED JUL] 319m sum 2 BF 7 3.243
FIG. l2
PATENTEI] JUL 1 3 I971 SHEET U BF 7 PRIOR ART PRIOR ART PATENTEI] JUU 3B7:
SHEET 7 BF 7 COI TAGE (OUTSIDE TO INSIDE) COIL VOLTAG (INSIDE TO OUTSIDE) ELECTRICAL INDUCTION APPARATUS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our present pending application Ser. No. 567,64l filed on July 25, I966, now abandoned.
BACKGROUND OF THE INVENTION This invention relates generally to devices having timevarying magnetic flux and, more particularly, to transfonners and reactors which use an insulated core.
Today, the pressing requirement for more and cheaper electrical power faces growing technical and esthetic problems including the national desire to maintain the attractiveness of populated areas. To meet this continuing need for more electric power without adding more generating and transmission systems within cities and suburban areas, electric utilities are now building power plants in remote regions close to the source of either large amounts of hydropower or large coal deposits. This power can be transmitted to load centers most economically by overhead transmission lines. However, because of increased population density and pressure to preserve the esthetic and economic values of the countryside, transmission rights-of-way are increasingly difl'rcult to obtain. The utilities are thus compelled to increase several-fold the power transmitting capacity of their existing lines, and to plan on still further increases in power in the future. For these and other reasons, the electric power industry is rapidly converting to extra high voltages (EHV) for the transmission of electric power having line-to-line voltages in excess of 34S lrv. Already 500 kv. systems have recently been built, and plans for 750 ltv. systems have been announced. Such high voltages permit the transfer of larger blocks of power over extensive geographic areas. EHV interconnections will also be used to even out peak demands over large region: and to improve the reliability of the total system. Indeed, the trend toward higher voltages is fundamental to meeting the predictable power needs of the next two decades.
Although there are important technologic and economic reasons for using EHV, serious difficulties have been encountered in designing reliable terminal and line equipment for use at these high-voltage levels. The simple extension of prior art concepts to EHV equipment is proving inadequate and new concepts in power handling equipment at EHV are clearly required. Especially needed are novel transformers and reactors capable of reliable insulating performance at these extra high voltages and characterized by more efficient utilization of their materials and volume.
In its simplest form, a transformer consists of two conducting coils having high mutual inductance. The primary winding is that coil which receives electric power and the secondary winding is that coil which delivers the power induced therein by currents flowing through the primary winding. In normal practice these coils are wound on a core of magnetic material. In EHV transformers, the necessity of increasing the insulation between the high-voltage windings and the grounded core adversely affects the operating characteristics, the cost, and the insulation reliability of the apparatus.
Additionally, it is essential that transformers built for EHV duty be designed to avoid or withstand the greatly increased forces associated with short circuits, voltage impulses, switching surges and the like.
Attempts were made to solve these problems with prior art transformer designs by increasing the required insulation and the strike distances. One idea, for example, was to use a transformer or reactor core of several sections wherein each section was potentially graded by the surrounding winding and insulated one from the other by sandwiching insulation between the core sections. It was intended that the common insulation between the cores and the windings provide an exact linear voltage distribution along the winding column while the voltage on the insulation increases from the edge towards the center and is constant beneath the core. In this manner, a good capacitive division of pulse voltages and a relative sensitiveness of the apparatus against overvoltages is achieved. Furthermore, metal foils were placed on both sides of the insulation layer in order to reduce the capacitive effect between the windings. Although this apparatus combination is suitable at lower voltages, at the extra high voltage levels contemplated by the present invention, the intended result could not be achieved because the creation of severe electrical stress at the corners of the core sections would subsequently cause a breakdown in the insulation. Even by providing an insulation layer thick enough to withstand the extra high voltage, the reluctance of the magnetic path would be so altered as to defeat all the benefits derived by separating the core segments in the first place. Moreover, although the metal foil on both sides of the insulation layer would tend to enchance the capacitive effect between the windings, it would also deteriorate the unit inasmuch as it fails to prevent the excessive eddy current losses and to distribute the electrostatic potential uniformly across the surface of the insulation. The resultant heating would soon cause thermal failure of the unit. Suffice it to say that conventional extensions of prior art apparatus have been unacceptable in high voltage applications and leave serious unanswered problems in the control of the electric field distribution, the tendency toward excessive bulk and uncertain insulation integrity, and proper heat and noise dissipation.
The dilemma remained without a solution until the present invention was conceived which not only solves these problems but also provides a novel EHV transfonner of high electrical and spatial (volumetric) efficiency. Moreover, the design can easily be extended to handle even higher voltages than are presently contemplated.
Large AC magnetic core transformers, as known to the prior art, are highly efficient and practical power transforming devices whose availability has made possible the modern AC power system. However, when a conventional design is operated at the extra high voltage contemplated by this invention, the voltage insulation problem which can be readily managed at low voltages becomes difficult and capable of culminating in catastrophic breakdown.
Similarly, a reactor for electric power systems is primarily a high-voltage-high-power inductance coil used primarily to constitute a high lagging power factor load. For the most part such devices comprise a coil and a magnetic circuit so related as to exhibit high reactance with low resistance. Reactors are usually used as shunt reactors on long lines to supply or compensate for line charging current. With the advent of EHV, shunt reactors carry an increased importance. For example, in EHV systems, leading currents can cause excessive voltages at the end of a long, lightly-loaded line. Unless prevented, these excessive voltages can create instabilities and subsequent failure in the terminal apparatus. Shunt reactors connected as required on the line end would have the desirable effect of preventing these instabilities and failures.
The reactors known to the prior art generally were either of the socalled shell design or the gripped-core design. The shell design reactor consists of an air-core coil having a magnetic shell surrounding it, while the gripped design comprised a modification of this which included an iron core within the coil which was intercepted by segments of stiff nonmagnetic material.
In the shell design, the coils of large diameter and radial buildup are subjected to a high leakage flux with resultant severe eddy current loss. In the gapped-core design, the lower reluctance of the magnetic circuit generally results in lower winding losses but the voltage insulation between winding and core is rendered far more difficult. Saturation of core iron must be avoided both to avoid losses and to insure constant inductance over the entire operating voltage range.
Thus, until the present invention was conceived, the design of EHV reactors was progressively more difficult and uncertain as to insulating strength, reliability, freedom from corona and radio noise. No clearly adequate solution for these problems was discernable, especially when operating at their high voltage limits.
SUMMARY OF THE INVENTION Application of the principles of the present invention provides not only excellent distribution and control of the normal AC operating potentials but also excellent impulse voltage dis tribution. The invention leads to relatively compact windings with fewer turns and with relatively low losses due to leakage flux. Other features of the present invention tend to reduce both acoustical and magnetostrictive noise.
The principles of this invention also contribute to the avoidance of saturation of elements of the magnetic core and thus make possible a constant reactance over the full voltage range and the nonintroduction of harmonic components in the reactor-current.
Additionally, low radio frequency noise levels can be achieved because the electrical stress distribution is properly controlled by the present invention and all parts of the electrified system can be held below corona onset levels. Moreover, the present design permits the return portion of the magnetic circuit to be at maintained ground potential, thus simplifying the insulation and physical support problem.
These and other advantages are accomplished in the present invention while simultaneously reducing the overall size of the required unit and its cost. All of these benefits are realized in both transformer and reactors constructed under the present invention by utilizing an insulating core concept in which the active portions of the magnetic circuit are formed of electrically isolated segments, each electrically connected to its sur rounding coil, to provide systematic and uniform progression of imposed voltage on both the active portions of the magnetic core and its associated electric circuitry.
DESCRIPTION OF THE DRAWINGS The invention will be best understood and appreciated from a perusal of the following description taken in conjunction with the figures wherein:
FIG. I shows in diagrammatic section view a conventional transformer.
FIG. 2 shows a diagrammatical section view of a transformer built in accordance with the principles of the present invention.
FIG. 3 shows in detail one segment of the insulated core.
FIG. 4 shows in detail a lamination of the core of FIG. 3.
FIG. 5 shows detail of the arrangement of the transformer coils and cores together with the insulating discs.
FIG. 6A shows in section an insulating disc suitable for use in the invention.
FIG. 6B shows in section an insulating disc suitable for use in the invention.
FIG. 7 shows the invention used as a three phase transformer.
FIG. 8 shows in section, one type of prior art reactor.
FIG. 9 shows in section, a different type of prior art reactor.
FIG. 10 shows a cutaway view of a reactor built in accordance with the principles of the present invention.
FIG. II shows a partially broken away view of the operational element of FIG. Ill.
FIG. I2 is a view of the device of FIG. 10 taken along the lines 12-12.
FIG. [3 is a detail view of the operation element of FIG. I2 taken along the lines 13-13.
FIG. 14 shows further additional detail of the reactor of FIG. 10.
FIG. 15 shows an additional view of the operational element of FIG. 10.
FIG. 16 shows in schematic form wiring connection suitable for use in the invention.
FIG. 17 shows the detail interconnection of the pattern shown in FIG. 16.
FIG. 18 shows in schematic form, a different wiring conneclion suitable for use in the invention.
FIG. 19 shows the detail interconnection of the wiring pattern of FIG. 8.
FIG. 20 shows a possible modification of the equipotential hoops used in the invention.
FIG. 21 illustrates a possible improvement that can be used with the present invention.
FIG. 22 shows a cutaway view of another reactor built in accordance with the principles of the present invention.
FIG. 23a is a detailed view of three core elements of the reactor of FIG. 22 with the surrounding coils.
FIG. 23b shows graphically the potential decrease through the coils surrounding each core element.
FIG. 24 shows another geometric form of insulation layer which could advantageously be used in the reactor of FIG. 22.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning first to FIG. I which shows in section a conventional transformer, it can be seen that such a transfonner basically comprises a magnetic circuit 20 and a pair of current carrying coils 22 and 23 contained in a housing 24. In such conventional transformers the magnetic circuit 20 usually consists of a closed laminated core of hollow rectangular shape in which transverse nonmagnetic gaps are carefully avoided.
Such transfonners operate as follows. A time-varying magnetizing current in the primary coil 22, connected to a source of AC power, produces a synchronously changing magnetic flux in the magnetic circuit which in turn induces an electromagnetic force in the secondary coil 23. Load current and power delivered by coil 23 is matched by the input to primary coil 22 of a corresponding input current and power plus associated losses. In order to minimize PR losses the coils should be wound close to the magnetic circuit. However, the electrical conductivity and great mass of the core requires that the magnetic circuit be at ground potential; thus adequate voltage insulation is required between the core and the coils. As the operating voltages and transient overvoltages of this type of apparatus are increased, the required insulation must also be reliably increased. The coils are spaced progressively further from the core with an attendant increase in winding losses. Insulation barriers are introduced to control field distribution and the migration of space charge and electrified particulates. The insulation distances must be increased more rapidly than the rated voltage. These factors cause the physical size of both coils and core to increase thus adding to the size, weight and losses of the unit.
The present invention restores the insulation harmony between core and coil by maintaining them at close to the same potential at all times irrespective of the voltage rating of the apparatus. This is done by separating the active, or winding-bearing portion of the magnetic circuit into core elements, mounting them in a stack or column with each core element electrically insulated from its neighbor by an adequate but relatively thin layer of high quality dielectric. Each of these insulated magnetic elements has in close proximity around it a proportional share of the total winding, with the midpoint or some other point in this local winding being electrically connected to its associated core section and firmly establishing its potential at all times. In this way the stack of insulated cores follows closely the potential distribution of the associated total winding and the electrical incompatibility of winding and core which characterizes conventional transformer and reactor designs is almost totally avoided.
One embodiment of the present invention will now be described in conjunction with FIG. 2. which shows a stepdown single-phase, autotransformer, constituting one phase of a three-phase wire-connected system, capable of handling extra high voltages and built in accordance with the principles of the present invention. In this figure the magnetic circuit 30 comprises a pair of magnetic returns 3| and 32 which couple a pair of segmented legs 33 and 34 formed of a stack of magnetic core sections 35 and 38 electrically isolated from one another by insulating discs 36.
Surrounding each leg is a pair of current carrying windings. The autotransformer shown consists of four series windings 4t, 42, 43 and 44 and four common windings 45, 46, 47 and 48. Series windings 4i and 42 and common windings 45 and 46 are mounted on segmented leg 34 while the remaining windings are mounted on the other segmented leg 33. Each winding is composed of a plurality of current carrying coils 37 which surround a core section 35 and are electrically connected thereto.
A central core 38, without a coil, my be provided in each segmented leg to separate the series windings and to serve as a means of introducing the high voltage to the series windings. High voltage is supplied to the four parallel series windings and to cores 38 by high-voltage lead 39 which is connected to a suitable AC power source (not shown).
The output highvoltage tap 40 is connected to the junction of the series windings and the common windings. The other lead from each common winding is, in turn, connected to the closest magnetic return 31 or 32, and to ground.
Turning now to FIGS. 3 to 5 the details of one form of con struction will be described. In this embodiment each core section is made up from a multiplicity of rectangular silicon-steel laminations 50 formed with a plurality of holes 5i whereby the entire series of laminations making up the core section may be united together. To avoid magnetic saturation at the core section edges and to improve the electric field distribution the narrow ends 52 and 53 of each lamination may be formed with a Rogowski shaped profile. After the stampings 50 are assembled, the corners of each core segment may be chamfered and the edges 55 and 56 ground to assume a profile similar to ends 52 and 53. Alternatively the core sections 36 could be formed from a single strip of material spirally wound. The actual dimensions of each core section are dependent on the power to be handled and can readily be determined by one skilled in the art.
Each coil, as shown in FIG. 5, is wound close to its respective core segment.
The coils can be formed in two parts 57 and 58 from insulated conducting strips spirally wound. In this embodiment coil half 57 is wound in one direction while coil half 58 is wound in the other direction. The two halves are then electri cally connected to each other and to the adjacent core section by an appropriate conductor 59. Adjacent coils are joined by a suitable connector 60 to form the total winding.
An insulating disc 36 is provided between adjacent coil-core pairs so as to electrically isolate and insulate each coil-core pair from the coil-core pair adjacent to it. interposed between each coil half is a second insulating disc 61 which has affixed thereto a support 62 for maintaining an equipotential ring 63 around the coils. Both the discs and the support 62 can be made of any suitable insulating material. Examples of suitable materials are laminated resin-impregnated paper and crosslinked polyethylene. To prevent irregularities in the coils from creating high electrical stresses across the discs 6] and mechanical damage to the discs 61, a semifiexible coil spacer 65 can profitably be fitted around each coil pair 57 and 58.
As indicated previously most of the difficulties encountered by the prior art in attempting to produce reliable EHV transformers centered around the necessity of separating and insulating the high-voltage winding from the grounded iron core through which the magnetic working flux flows.
The segmenting and electrical isolation of each core section from adjoining sections solves the voltage insulation problem because each core section and its surrounding coil are at the same potential and no conflict in insulation strength exists between them. Thus the need of large spacings and heavy in sulation between the coils and the cores is avoided. However, introduction of insulation into the gap tends to increase the reluctance of the magnetic circuit which tends to increase the requirement for high magnetizing current and power and simultaneously permits increased leakage of the magnetic flux. Increased leakage flux results in increased reactance drop under load.
The reactance drop can be reduced by providing on each leg two complete windings in parallel so that for a given total power output per leg the load current per winding is halved. With this arrangement, as shown in FIG. 2 the highest voltage level is reached at the midpoint of each leg and only a very small portion of the entire magnetic circuit is at this high potential. Although this arrangement approximately doubles the active height of each transformer leg, our studies show that the overall core utilization is actually improved because a larger proportion of the core is actively used for winding pur' poses. Moreover, each magnetic return is now at ground potential and can be designed and supported more economically and effectively. The insulated core and winding construction defined by this invention has further important advantages. By this series capacitance system better division of surge potential is provided across the core and winding columns. This improved surge division occurs because the insulated, segmented coil-core arrangement constitutes a series chain of high capacitance of nearly equal value. It is practical for the first time to design this series capacitance system so that transient overvoltages will be distributed with nearly exact uniformity along the entire stack. It is therefore expected that higher BIL (Basis Insulation Level) and increased insulation integrity can be achieved under the use of the invention.
By controlling to greater uniformity the normal and transient electrical stresses, the apparatus can be designed for subcorona operation with the elimination of radio noise. It is to be noted that preferably each coil, core and insulator is mechanically and physically identical in shape and function to every other coil, core and insulator. The repeated fabrication of identical subunits is expected to further reduce costs and improve quality.
Preferably the insulator 36 will be molded in the form of a disc with antitracking configurations of its outer periphery. Two such designs are shown in section of FIGS. 6A and 6B. The disc of FIG. 6A comprises a planar disc of insulating material 70 which is molded with a flared periphery 71 and coated on both sides with a thin conductive layer of intermediate resistivity material 72. Typically this coating 72 will have a resistance of between 5,000 to 50,000 ohms per square, thus preventing excessive eddy currents in this coating. A beading 73 is then molded over the flare 71 and the extremities of the coating 72.
The em odiment shown in FIG. 68 also comprises a planar disc of insulating material. In this configuration however, the flare 76, around its periphery, is provided with a plurality of corrugations 78 such that the surface of the flare presents a zigzag pattern and thereby increases the electrical path length between opposite sides of the disc. This disc is also coated with high resistivity material 72.
This smooth conductive coating 72 in intimate contact with the solid dielectric establishes the electric field boundary and thus prevents irregularities in the laminations of the cores from creating points of high electrical stress. The conductive coating is designed to distribute the electrostatic potential uniformly across the entire surface of the insulator 36 with controlled reduction of electric stress at the edges.
In certain circumstances it would be desirable that the insulator 36 be made in two parts. One part would be a central disc having a configuration as shown in FIG. 6A or 6B. The core segments would abut this part. The other portion would comprise a ring concentric with this disc which would isolate the coils from one another. This ring would also have a sectional configuration such as shown in FIG. 6A or 6B. This splitting of the insulator 36 would have the beneficial result of permitting the cores and the coils to move independently of one another thus providing a more flexible design.
It should now become obvious to one skilled in the art that this arrangement will also adapt itself for use with three-phase circuits. In such three-phase applications the structure might well appear as shown in FIG. 7. In this application three segmented core legs 80, 81 and B2 are provided. EAch leg carries the necessary number of coils and is magnetically interconnected to the other two leg top and bottom delta shaped magnetic returns 83.
The present invention can also be advantageously used as a reactor.
Shunt reactor needs are determined by the length of the transmission line, its loading, and the general problem of var control. They are also quite effective in helping to limit transient overvoltages. On many EHV systems, generator reserve requirements or pool reserve requirements will be such as to cause the lines to be on standby service much of the time. in such cases, shunt reactors will be essential to control system voltages.
The daily loading cycle will also affect the placement of this reactive compensation. Even during full load, many lines will require permanently connected EHV reactors. At light loads, additional reactive compensation will be needed as the terminal voltages rises.
The better prior art reactor designs all utilize a so-called shell or frame yoke. The basic elements of such a prior art reactor are shown in FIG. 8. A coil 86 is made in the form ofa hollow cylinder surrounded by a laminated frame yoke 87. This coil is provided with a high-voltage lead 84 at one end thereof and with a low'voltate lead 85 at the other end thereof. Since good design dictates that the yoke 87 be at ground potential it is necessary that the high-voltage end of the coil be adequately insulated from the yoke 87 both at he end and the side. Simultaneously, this spacing must be kept small if the flux lines are to be maintained in the preferred orientation; that is, parallel to the coil axis. Such requirements generally impede the use of this design for extra high voltages. Furthermore, in designing reactors, it is found that the rated volt-amperes (El) are proportional to the product B'V where:
B is the magnetic flux density in the coil V is the volume of the magnetic field.
It is obvious that for a compact design it is preferable to sue large values of magnetic flux density, 3.
The air core, shell type reactor of FIG. 8 precludes the use of high values of the magnetic flux density because of the high reluctance of the air core of the windings. Consequently the volume, V, is large and the flux density, B, is relatively low. Also, the coil is physically large and is exposed to practically the full value of magnetic field in some areas. These factors result in a reactor which will have high resistive and eddy-current losses in the coil.
In an attempt to avoid these problems, the gapped-core shell type and the gapped-core core type reactors were tried. The gapped-core shell type variation is illustrated in FIG. 9 and consisted of the addition of an interrupted iron core 88 inserted in the center of the coil 86. This core 88 is magnetically coupled to the yoke 87. Pieces of a vary stiff nonmagnetic filler material 89 are inserted in the core interruptions to maintain the core segments 79 in spaced relationship. This design permits the use of values of B approaching those that will cause the iron to saturate. Hence, the volume, V, is smaller. This design does not solve the high voltage insulation problem between the coil and yoke nor does it achieve good distribution of transient overvoltages. As a result the overall unit becomes inordinately long when designed for EHV duty. This increase in length of course adds considerable size and weight to the unit.
The present invention specifically avoids the insulation problem especially when the device is used for EHV duty and while operating at high values of}? which permits a reduction in the weight and physical size of the reactorv The consequent reduction in size not only reduces the cost but also reduces electrical losses and the inherent magnetostrictive noise. The efliciency is such that electrical losses are only about one-half of those in conventional reactors. The novel design employed in utilizing the present invention further permits the application of powerful compressive forces in the direction of the induced magnetic field to insure the mechanical stability of the assembly and to reduce acoustical noise.
Additionally the invention reduces the magnetic leakage flux while providing improved and uniform voltage distribution of undersired surges or impulses, thereby eliminating local areas of high voltage stress.
Broadly speaking these and other advantages and features are achieved in a reactor by providing a pair of parallel windings around a magnetic circuit comprising a pair of magnetic returns coupled by insulating magnetic core legs, and electrically and progressively coupling the windings to the insulated core legs to provide systematic and controlled distribution of the impressed voltage along the legs.
A reactor built in accordance with the present invention is shown as a cut away view in FIG. 10. This reactor comprises a housing 90, of generally cylindrical form, mounted by legs 94 on a pad 95. Passing through the top of tank to the interior thereof is a high-voltage bushing 91 and a low-voltage bushing 92. These bushings may be of a conventional condenser type and are matched mechanically, thermally and electrically to the operating element 97 contained within the housing 90. Affixed to the sides of tank 90 and having passageways connecting with the interior thereof are a plurality of radially extending hollow core radiators 93. A suitable insulating fluid 96 is provided within the tank 90 in sufficient amount to cover element 97 and to circulate by convection through radiators 93. These convection currents are established in the fluid by heating of element 97 when power is applied thereto; they may be further assisted by forced fluid pumping. in addition the tank is provided with the usual associated equipment (not shown) normally found on reactors. This equipment includes thermometers, alarm circuits, pressure relief devices, entrance and inspection ports, drain valves and the like.
One novel aspect of any reactor built in accordance with the present invention resides primarily in the operating element 97 which is shown in greater detail in FIGS. 1 I, l2, l3, l4 and 15.
This element 97 basically comprises a magnetic circuit as shown in FIGS. 11, l2, l3, 14 which is composed of a pair of laminated magnetic returns I00 and [01 coupled together by a pair of insulated core legs 102 and 103. Each insulated core leg comprises a plurality of core segments I04 electrically isolated from one another by discs 106 and spacers 107. Each core segment is built up of strips as discussed in conjunction with FIGS. 3 and 4. Each core segment 104 used in this reactor may have Rogowski shaped profiles on all edges to prevent saturation in the cores and to permit operation of the device at optimum flux density.
Each core 104, except for the central core "MA on each leg, is surrounded by a current-carrying coil 108. These coils are electrically connected to each other and to core segment which they surround. Each coil-core pair is electrically isolated from one another by discs I06 and spacers I07. These decks and spacers further aid in positioning the cores and coils in spatial relationship. These decks and spacers may be made of any suitable insulating material such as laminated epoxyimpregnated paper products or pressboard. Each deck 106 may, in turn, be surrounded by an equipotcntial hoop which when properly electrically connected to the coils and cores on either side thereof will assist in the distribution of surge or impulse voltage across the element 97.
The entire assembly is held together, in compression, by a plurality of tension members such as tie bolts 98 which pass through suitable brackets 99 affixed on each magnetic return 100, 101. In order to equalize the compressive forces applied to the assembly by these bolts 98 a dummy coil 11] made up of suitable insulating material may be provided around the central core 104A. In most embodiments this central core 104A and the dummy coil tll can be advantageously eliminated. Additionally equalizing may be provided by making the spacers 107 of a resilient material capable of permitting some lateral movement between each coil and its respective core.
When the assembly 97 is placed in the housing 90 such that the magnetic circuit is parallel to the base of the tank the most 0I3l spacers 107 are perpendicular thereto and a free vertical path 118 is provided between these spacers 107. This path is such as to permit free flow of the insulating fluid 96. Passage of the fluid along these paths 118 cools and insulates the coils and cores. Because of unavoidable losses in the assembly heating of the assembly occurs. This heat is transferred to the fluid 96 by conduction. When the fluid in path 118 becomes sufficiently warm convection currents will be set up in the fluid such that it rises along paths 118 to the top of radiators 93, downward through the radiators as it cools and into the tank again at the bottom. This establishment of these convection currents cool the assembly and keep it at a predeterminable temperature.
It has been discovered however that with the intense electric field encountered when the unit is used with extra high voltages that long conducting hydrocarbon chains can be created in the fluid. The existence of such chains is deleterious to the equipment and can cause electrical breakdown between portions of the assembly and the tank 90. To prevent this, insulating baffles 125 of pressboard or other material, are provided around the exterior of the assembly as shown in FIG. 15.
As noted previously each coil 108 is electrically connected not only to each adjacent coil but also to its respective core 103. Since these connections can be series, series parallel or parallel a brief discussion should now be given in conjunction with FIGS. 16, 17, 18, 19 and 20. FIGS. 16 and 17 show the coils 108 on each deck electrically coupled to adjacent coils so as to provide two parallel windings on each insulating core leg to result in a total of four parallel windings. As shown, the high voltage is introduced to the central point of the combination via lead 109. By introducing the high voltage into the center of the assembly, interconnecting each coil to its adjacent core as shown in H6. 17 while insulating each core seg ment from the next segment in the stack, the frame yoke may be eliminated since the magnetic flux is confined to the magnetic returns 100 and 101 and the insulating core legs 102 and 103. By eliminating the yoke and by connecting the coils to the cores the voltage insulation problem existing in the prior art between the winding and the yoke is also eliminated. The low-voltage power is extracted via lead 105. We have found that a high-voltage reactor built in accordance with the present invention can be lighter in weight and considerably smaller in overall size than a conventional reactor of the same voltage and half the power rating. Even more important, its insulation reliability is inherently higher.
FIGS. 18 and 19 show the coils 105 on each deck serially connected. This wiring arrangement is not preferred however because it produces only two parallel windings for the entire unit and under transient impulse voltage conditions does not have desirable characteristics.
In either case the difficulties associated with prior art devices when used for high-voltage duty are avoided and a progressive, systematic, and preferably uniform distribution of voltage is achieved across each insulating core leg from the midpoint thereof to each magnetic return. This systematic voltage distribution is also achieved under surge conditions due to the excellent voltage division accorded by the large intercore capacities.
It should of course be understood that in either event the winding sense of coils on each disc must be such as to cause the magnetic field to travel in a closed loop as indicated by arrows 114.
FIG. 20 shows a modification of the coils 108, their interconnection and the deck. Here the coil 108 surrounding each core segment 104 is divided into two halves. One-half 119 is wound in one direction and the other half 120 wound in the opposite direction. The core 104 is then connected to the center point between the two coil halves. It is, of course, necessary that each coil half be insulated from the other half. Additionally the deck 106 can be a laminated structure built up of two sheets of suitable insulating material 130, and 131 having a conducting grid I16 sandwiched therebetwcen. This grid 116 is configured to minimize eddy currents. Such a sandwiched disc could be used with each of the configurations described above including the transformer. Insertion of this conduction grid 116 acts to capacitively couple each core segment to its adjacent core segments to further improve the voltage surge and impulse response of the unit. This view illustrates still a further modification that could be utilized in any of the above-described devices. In this modification, the equipotential hoop is replaced with an encompassing hemispherical ring formed of an insulating material having a conducting coating deposited thereon.
Returning momentarily to FIG. 15, additional features of the baffles will be discussed. These baffles 125 are shown shaped to approximately conform to the electric equipotential field lines existing between equipotential hoops of like voltage on each insulating core leg. To assure that any generated conducting hydrocarbon chains are broken the baffles are filled with a suitable number of randomly placed foils 126. To provide adequate flow, a multiplicity of orifices 127 are provided in each baffle. This combination of orifice, foils and baffles creates a great deal of turbulence in the fluid and this prevents the formation of deleterious conducting chains.
it should be noted that a single large opening 124 is provided for the high-voltage lead 109.
FIG. 15 also illustrates a modification that may be found desirable. This modification comprises the addition of compression springs 128 on the end of each tie rod 98 to assure that a constant tension is applied across the insulating core legs at all times. A further modification, not shown, is the additional cylindrical insulation over the tension rods 98.
It should now be obvious to those skilled in the art that the invention not only provides an improved reactor capable of EHV duty but does so with significant savings in weight and cost. The present invention thus permits the design of a reactor of significantly smaller size since the full operating voltage is applied so mat each portion of the winding bears only its proportional part of the total voltage under both normal and transient conditions. Furthermore, by connecting each segment of the insulated core to the windings the need for extensive insulation between the coils and the core segments is eliminated. This coupling of core segment with surrounding coils uniformly varies the voltage down each leg such that the potential gradient is constant down each leg thus making maximum use of the leg length for insulating purposes.
Since core segment, deck structure and coils are identical the unit lends itself to mass production methods and economy of manufacture.
Since the deck material provided the function of electrical insulation as well as mechanical support there is a large value of interdeck capacitance which contributes to the uniformity of the voltage distribution along each leg under impulse conditions. This diminishes the impact of high-voltage transient stress in the unit. Utilization of an insulating core provides additional advantages in that it provides accurate control of the reactor inductance. Additionally, this concept, by permitting uniform application of the magnetizing ampere turns over the whole of the insulating cores, reduces magnetic leakage flux.
The described mechanical configuration provides advantages in that the unit, when mounted horizontally, can be cooled by natural convection currents while simultaneously applying large compressive forces to the unit thereby reducing both acoustical noise and magnetostrictive noise.
It should be obvious that other modifications and adaptations can now be made to described reactor. For example, the reactor could be adapted to three-phase operation by using three legs each of which has a high-voltage input lead in the center.
Still further as shown in FIG. 21, the shaped core 104 can be encased in a molded solid dielectric 117 which is shaped in the form of a spool. The coils are then wound on the spool and the spools stacked to form the insulating core legs of the unit. If desired the conductive grid 116 of FIG. 20 could be inserted between each spool and connected to an equipotential hoop surrounding the spool interface.
Referring now to FIGS. 22, 230, a preferred embodiment of an insulated core reactor built in accordance with the principles of the present invention is shown and designated generally by the numeral 180. This reactor comprises a magnetic circuit which is developed through end yokes 182 and 184, and legs I83 and 185 each of which comprise a plurality of core segments such as 186. Each core segment I86 is surrounded by coils 189. Separation of the core segments is achieved through insulating layer I9]. The entire assembly is enclosed in a tank 193 which holds the end yokes I82 and I84 and the legs 183 and I85 in compression. Connection is made to the reactor by a high-voltage terminal 195 containing a high-voltage line 220. For ease of movement, the reactor I80 is mounted on a skid 197 which in itself may exert some compressive force. In FIG. 230 three core segments 188, I90 and I92 are shown in detail. Each core segment such as I90 is surrounded by a set of four coils I94, 196, I98 and 200 electrically connected such that the coils I94 and I96, and 198 and 200 each form a parallel set by connections 202, 204, 206 and 208. The resulting parallel coil combinations I94, 196 and 198, 200 are then connected in series by connector 210 which also provides a connection to the core segment 190. By this parallel arrangement no potential difference exists between corresponding turns in the space 212 between cores 194 and 196 and space 213 between cores I98 and 200. Accordingly, oil or any other suitable cooling medium can be readily circulated throughout the plurality of spaces such as 2I2 and 213 between parallel core sets without any risk of insulation failure in these areas.
Unlike the previously described induction apparatus, no dummy center core is required. Moreover, as shown in FIG. 22, the core segments 186 are of an even number and thus the high-voltage line 220 is made between the two center core sections 188 and 190. Thus as previously described, the voltage would then decrease from the high-voltage connection in both directions to the grounded end yokes 182 and 184. It should be noted that the end yokes I82 and 184 are extended over the coils in order to carry the total magnetic flux existing in the coil winding area. This feature has the desirable effect of keeping the magnetic field from curving at the end coils and thus reduces significantly any current loss which would otherwise be present. In addition, the end yokes themselves can now be used for clamping the coil assemblies together as well as minimizing vibration and eliminating extra clamping devices.
For purposes of explanation, it may be assumed that the voltage drop across each set of four coils is equal to V=V,V, where V, the potential at connector 202 V the potential at connector 206 and thus the voltage drop across any parallel set would be equal to (Ia-v.12) FIG. 23b shows in graph form the voltage level decrease corresponding to the coil sets 194, 196 and 198, 200, The voltage decrease through the parallel coil set I94, 196 decreases from a maximum value of V at the outer extremity to a value of (V,V,/2) at the inner extremity which is likewise the constant potential at which the core segment 190 is maintained. From the inner extremity of the parallel coil set I98, 200 to the outer extremity the voltage continues to decrease from a value of F t/ to V,. As shown in FIG. 230, the outer extremity of coil set I98, 200 is then connected to the outer extremity of the next coil set 216, 218 through connector 209 and the same voltage decrease then takes place throughout the remaining coil sections and core segments until eventually the respective grounded yoke 184 is reached. Similarly, the high-voltage line 220 is connected to the coils surrounding core segment 188 and the voltage distribution decreases similarly to its respec tive grounded end yoke 182.
Since the voltage decrease throughout each parallel set of coils is equal to the difference in potential between the inner extremity of parallel coils set I98, 200 and parallel coil set 216, 218 is V;- V,. Likewise, the potential difference between the outer extremities of parallel coil sets 194, 196 and parallel coil set I98, 200 is also equal to V,-V These potential gaps are necessarily maintained by separating insulation layers which will inhibit breakdown and assure this potential difference at all times. Accordingly, insulation 19! is provided between adjacent core segments which extend beyond that of the coil extremities. Similarly, insulation 224 is interposed between coil sets in the form of a washerdisc.
The insulation I91 is comprised of two layers 226, 228 each of which has applied on the portion of the surface abutting the respective core segment a conducting layer 230 which has an area substantially equivalent to that of the surface area of the respective abutting core segment. This conducting layer 230 takes the form of a conducting grid and has applied thereon a protective coating 232 which is in intimate contact with both respective conducting grids 230 and the respective core segments and acts to prevent shorting of the grid pattern with the respective conducting core segments. Suffice it to say that without such a protective coating the laminations of a core segment would be shorted and cause a severe eddy current loss. This conducting grid 230 nearest to core segment 190 is electrically connected to connector 210 and core segment I such that the potential of both at all times is equivalent.
Another conducting grid 234 is applied between the two insulation layers 226, 228. This conducting grid extends substantially over the entire surface area of the insulation layers 226, 228 such that it not only covers the corresponding area between core segments but also extends out and under the respective core sets to the insulation edges where electrical contact thereto can be made such as with the high-voltage line 220. This conducting grid between insulation layers is electrically connected by connector 209 to the outer extremities of the parallel coil set on the other side of the insulation 191 and is at a potential equal to half of the potential difference between adjacent core segments. Thus each dual insulation layer 191 acts as a series capacitor and must withstand a potential difference of V/2.
Under surge or transient conditions when a sudden voltage increase appears on the high voltage line 220. the increase in potential is quickly spread throughout the conducting grids 234. Inasmuch as the capacitance of the coils is significantly greater than that existing between adjacent core sections, the potential takes the path of least resistance and will distribute itself capacitively throughout the core sections to the grounded end yokes rather than distributing itself through the surrounding coils with the necessarily attendant damage that would follow due to the inability of the voltage to distribute itself evenly in a sufficient time interval.
The insulation layers with applied grids which separate various core segments and core sections may take other forms than a flat sheet. In FIG. 24, insulation layer 221 is shown which could advantageously be used in the reactor of FIG. 22. The core segment 186 would fit within the center depressed area 223 and the coil sets 189 would be adjacent to the outer extensions 225. The curving up 227 of the insulation layer in the center eliminates areas of intensification of the electric field. The actual shape of the curve can be calculated and ideally would be an expotential curve of infinite length, however, for practical purposes this can be approximated by an arc of a circle of infinite length. This configuration is substantially superior to a parallel or fiat insulation layer which would normally have a high electric field intensification at the core segment edges. This type of field intensification is well known in classical studies of electrostatic field configurations at electrode edges and comers. Continual field intensification will cause the surrounding insulating medium to become ionized and will eventually lead to an accumulation of surface charge on the insulation layer in the vicinity of the core segment edges which would eventually cause a failure or puncture of the insulation layer.
The grid pattern which is applied to the surface of the insulation layer need not be restricted to any particular geometry form and any number of grid patterns could be employed to produce the same result. As previously stated, the function of the conducting grid is simply to provide an electrostatic plane under surge or transient conditions. It is essential that the grid distribute evenly the electrical stress throughout the grid surface area and that no voids occur which would initiate a breakdown. Accordingly, the conductivity of the grid must be sufficiently low to permit charge distribution across the equipotential plane and get the high enough to avoid high eddy currents in the high intensity varying magnetic fields. The resistance of the conducting grid must have a resistivity such that the time constant is adequate for surge distribution while at the same time being able to withstand a high temperature environment of up to 120C. as well as being nondeteriorating in an oil surrounding. The conducting grid has advantageously been applied to the insulation layer by electrolytic deposition but printed circuits, painting or other equivalent methods may be employed.
Again, where the word coil is used in this description of the invention, each coil can be composed of a plurality of smaller coils, e.g. pancake coils, with or without their conductors transposed in some predetermined sequence to further reduce the losses caused by eddy currents flowing in the conductors.
Having now described a specific embodiment of the present invention it is desired that it be limited only by the following claims.
We claim:
1. An electrical induction apparatus for connection to a high-voltage AC power system comprising a plurality of magnetic legs, each of said legs having a first and a second end, a magnetic return interconnecting at least one end of said legs, at least one of said plurality of legs being encompassed by at least one coil means for connecting at least one coil between a high-voltage AC power system and ground, means for placing a portion of each leg at a potential related to the potential of the high-voltage AC power system, means for maintaining at least one of said returns at ground potential, at least one of said legs being comprised of a plurality of magnetic segments electrically insulated from each other by insulation means, each of said magnetic segments being electrically coupled to the interior winding of a respective coil to distribute any voltage developed across said winding, a portion of said insulation means being subjected to both a resultant electric field and active magnetic field that may be created in said apparatus, first voltage surge distribution means positioned on those portions of the insulation means abutting magnetic segments and electrically coupled to respective magnetic segments, second voltage surge distribution means positioned within said insulation means in a plane parallel to said insulation means and extending throughout said second voltage surge distribution being electrically coupled to the exterior winding of the immediate coils on either side of said insulation means, voltage surge distribution means cooperating with said insulation means to create a dual series capacitance between any two magnetic segments and said first and second voltage surge distribution means having low eddy current losses and distributing any created electrostatic potential uniformly across said insulation means with controlled reduction of electric stress.
2. The apparatus of claim I wherein each magnetic segment is surrounded by at least one set of parallel connected coils and wherein each of said sets is connected in series, said coil sets being electrically connected at their interior winding to the respective magnetic segment.
3: The apparatus of claim 2 wherein said insulation means comprises sheet material and bar spacers having open spaces therebetween.
4. The apparatus of claim I wherein said insulation means comprises insulators adapted to conform to said segments.
5. The apparatus of claim I wherein said insulation means has electrically lengthened edges to increase the flashover, surface creepage strength.
6. The apparatus of claim 1 wherein the voltage surge distribution means com rises semiconductin layers.
7. The apparatus claim 1 wherein sai magnetic segments are chamfered to approximate a Roqowski profile such that intensification of magnetic gradients at the edges thereof is substantially avoided.
8. The apparatus of claim 1 wherein there is provided on one of said legs two windings one of said windings having an emf induced therein when the other of said windings is coupled to said high-voltage AC system.
9. The apparatus of claim 1 wherein a portion of said winding is connected between one line of the power consuming portion of the AC power system and ground.
10. The apparatus of claim 1 wherein each of said magnetic returns extends at least to cover the planar area of said coils.
11. The apparatus of claim 1 wherein there is further provided means for electrically insulating said coils independently of the means isolating said segments to provide lateral freedom of movement between said segments and said coils.
12. The apparatus of claim 1 wherein said coils are supported by insulating means physically independent of the insulating means supporting said segments.
.n-nut n--s

Claims (12)

1. An electrical induction apparatus for connection to a highvoltage AC power system comprising a plurality of magnetic legs, each of said legs having a first and a second end, a magnetic return interconnecting at least one end of said legs, at least one of said plurality of legs being encompassed by at least one coil means for connecting at least one coil between a highvoltage AC power system and ground, means for placing a portion of each leg at a potential related to the potential of the highvoltage AC power system, means for maintaining at least one of said returns at ground potential, at least one of said legs being comprised of a plurality of magnetic segments electrically insulated from each other by insulation means, each of said magnetic segments being electrically coupled to the interior winding of a respective coil to distribute any voltage developed across said winding, a portion of said insulation means being subjected to both a resultant electric field and active magnetic field that may be created in said apparatus, first voltage surge distribution means positioned on those portions of the insulation means abutting magnetic segments and electrically coupled to respective magnetic segments, second voltage surge distribution means positioned within said insulation means in a plane parallel to said insulation means and extending throughout said second voltage surge distribution being electrically coupled to the exterior winding of the immediate coils on either side of said insulation means, voltage surge distribution means cooperating with said insulation means to create a dual series capacitance between any two magnetic segments and said first and second voltage surge distribution means having low eddy current losses and distributing any created electrostatic potential uniformly across said insulation means with controlled reduction of electric stress.
2. The apparatus of claim 1 wherein each magnetic segment is surrounded by at least one set of parallel connected coils and wherein each of said sets is connected in series, said coil sets being electrically connected at their interior winding to the respective magnetic segment.
3. The apparatus of claim 2 wherein said insulation means comprises sheet material and bar spacers having open spaces therebetween.
4. The apparatus of claim 1 wherein said insulation means comprises insulators adapted to conform to said segments.
5. The apparatus of claim 1 wherein said insulation means has electrically lengthened edges to increase the flashover, surface creepage strength.
6. The apparatus of claim 1 wherein the voltage surge distribution means comprises semiconducting layers.
7. The apparatus of claim 1 wherein said magnetic segments are chamfered to approximate a Roqowski profile such that intensification of magnetic gradients at the edges thereof is substantially avoided.
8. The apparatus of claim 1 wherein there is provided on one of said legs two windings one of said windings having an emf induced therein when the other of said windings is coupled to said high-voltage AC system.
9. The apparatus of claim 1 wherein a portion of said winding is connected between one line of the power consuming portion of the AC power system and ground.
10. The apparatus of claim 1 wherein each of said magnetic returns extends at least to cover the planar area of said coils.
11. The apparatus of claim 1 wherein there is further provided means for electrically insulating said coils independently of the means isolating said segments to provide lateral freedom of movement between said segments and said coils.
12. The apparatus of claim 1 wherein said coils are supported by insulating means physically independent of the insulating means supporting said segments.
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DE2002192A1 (en) 1970-12-23
DE2002192B2 (en) 1972-05-18

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