United States Patent Trump et al.
ELECTROMAGNETIC INDUCTION APPARATUS Inventors: John George Trump, Winchester; Brian Skillicorn, Topsfield; Bryon Lee Johnson, Westboro, all of Mass.
High Voltage Power Corporation, Wesboro, Mass.
Filed: July 12, 1971 Appl. No.: 161,833
Related US. Application Data Continuation-in-part of Ser. No. 840,090, June 2, 1969, Pat. No. 3,593,243, which is a continuation-in-part of Ser. No. 567,641, July 25, 1966, abandoned.
References Cited UNITED STATES-PATENTS 12/1948 Hodnetle ..336/2l0 X 10/1959 Lambenton ..336/212 10/1964 Miller ..336/60 X 3/1965 Deuron ..336/70 X 51 Aug. 15, 1972 Primary Examiner-Thomas J. Kozma Attorney- Robert B. Russell et a1.
[ 5 7 ABSTRACT Electromagnetic induction apparatus for high-voltage, high-power operation, using a compact interrupted magnetic circuit whose materials and geometry are so designed as to confine magnetic flux to the compact geometry associated with the magnetic circuit, to confine operating electric currents to windings which cooperate in forming the magnetic circuit, and to distribute both normal and transient voltages quickly and evenly along both the winding and the magnetic circuit so that the electric field strength of the system is maintained high. The apparatus may be applied to either the transformer or the reactor function, but in either event the purpose and advantages of the invention generally relate to so-called large power operation (above ten megavolt amperes) at voltages in the EHV and UI-IV levels.
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- SHEET 7 [IF 8 con. VOLTAGE /(OUTSIDE TO INSIDE) v 300 J F/G 26 COIL. VOLTAGE I (lNSIDE TO OUTSIDE [95 E: 220
- /9/ 30/ F/G. 23b ,86 ,89 m ,90 w 3 1 ELECTROMAGNETIC INDUCTION APPARATUS CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our application Ser. No. 840,090, now US. Pat. No. 3,593,243, filed on June 2, 1969 which in turn was a continuation-in-part of our then-pending application Ser. No. 567,641 filed on July 25, 1966, now abandoned.
BACKGROUND OF THE INVENTION This invention relates generally to devices having time-varying magnetic flux and, more particularly, to transformers and reactors which use an insulated core.
As electrical devices have developed, different techniques have evolved depending upon the magnitude of the various parameters involved. For example, high voltage d.c. equipment entails strong electric fields and high power a-c equipment has come to entail strong magnetic fields. Specialized techniques have been worked out for handling these strong electric fields in high voltage d.c. equipment: for example, megavolt accelerator apparatus employ hollow, rounded high voltage terminals and equipotential planes for controlling the electric field and shaping it uniformly. On the other hand, different specialized techniques have been worked out for handling the strong magnetic fields of high power a.c. equipment: for example, ferromagnetic material is used to form magnetic circuits in which elaborate steps are taken to minimize reluctance and eddy currents. However, the need for these specialized techniques exists only for certain ranges of certain parameters. For example, conventional household appliances do not require special techniques for shaping electric fields, and high-voltage electrostatic accelerators do not require ferromagnetic material.
Conventional electric power techniques have evolved in a similar manner. Initial efforts involved do. and the requirements of average household equipment, such as lighting, led to voltages of the order of volts. With the development of a.c. and transmission of electric power over greater distances at higher voltages to reduce losses in transmission, apparatus capable of handling voltages of the order of 10 volts were developed, and related insulating and magnetic circuit techniques were devised. Such techniques'were limited to their own range, however, and the more recent interest in even higher voltages for power transmission has triggered a need for fundamentally new approaches and has generated new generic names: EHV (which stands for extra high voltage and includes the range 345-765 kilovolts in overhead systems and 230 kilovolts and up in underground systems) and UHV (which stands for ultra-high voltage and includes the voltages above 765 kilovolts now contemplated for overhead transmission). At the extra-high voltage range, problems not before faced must be solved: The Ferranti effect becomes a problem in transmission lines, the strong magnetic fields required by transformers must overlap the strong electric fields which must be controlled at these high voltages, and other new problems arise.
The basic approach to the design of apparatus for coping with strong electric and magnetic fields is disclosed in the insulating-core patents of Van de Graaff.
The present invention is an improvement and refinement of that basic approach in which insulating-core principles are applied to electromagnetic induction apparatus for use with EHV and UHV transmission lines for the absorption and storage of large quantities of electric charge associated with the capacitance of long transmission lines, or in the transfer of power from one voltage level to another in such large megavoltampere amounts.
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 surburban 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 densi ty and pressure to preserve the esthetic and economic valuesof the countryside, transmission rights-of-way are increasingly difficult to obtain. The utilities are thus compelled to increase several-fold the power transmitting capacity of their existing right of ways, 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-toline voltages in excess of 345 KV. Already SOOKV systems are in service, and more recently 765 KV systems have been energized. Such high voltages permit the transfer of larger blocks of power over extensive geographic areas. EHV interconnections are also used to even out demands over large regions 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 decade.
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 difficult 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 correspondingly 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 high power 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 increases in the insulation dimensions.
Conventional prior art transformer design utilized a magnetic circuit formed by an iron core which was designed for minimum reluctance in order that the magnetic circuit might indeed function as a circuit in which magnetic flux is confined as much as possible. Hence gaps of non-ferromagnetic material were avoided, and the magnetic circuit was at a common potential, generally ground.
Exceptions to this design principle are found in applications which involve the transmission of only signal and instrumentation amounts of power. For example, high voltage measurement transformers need only to create in the secondary an accurate signal. Indeed absorption of power from the circuit being measured is to be minimized. One specific design of measurement transformer utilized essentially an air-core choke-coil, and then obtained a signal by means of a small secondary coil linked to the primary (choke) coil by a short length of iron core. The original idea has been attributed to Biermanns, and various embodiments were built by AEG. Some of the later embodiments were built by AEG. Some of the later embodiments inserted additional iron core members in the primary (choke) coil, and publications concerning this show how this may be done without impairing the accuracy of the device. However, none of the embodiments were designed for the transmission of high power (one publication indicates upper limits of 40VA in one design and 100 VA in another design) and none of the embodiments were designed for EHV (The highest voltage indicated is 220 KV). Representative publications. concerning this family of equipments are as follows: Elektrotechnische Zeitschrift 1931 Heft l2 (19 March 1931) pp. 378-379 Elektrotechnische Zeitschrift 58 .lahrg. Heft 8 (25 February 1937) pp. 203-205 Stromund Spannungswandler by Walter (2nd ed. 1944)PP-98, 106-109 German Pat. No. 732,281 (1943) Swiss Pat. No. 199,897 (1937) German Pat. No. 592,876 1934) The first serious proposal for using insulating magnetic cores in power devices was made by Van de Graaff and the present invention relates to improvements in the application to various a-c power devices of the basic insulating-core principles disclosed by Van de Graaff in, for example, the following US. Pat. Nos.:
Suffice it to say that conventional extensions of prior art apparatus have encountered serious difficulties in high voltage power applications particularly in the control of the electrical field distribution and the tendency toward unmanagable size and losses.
These difficulties are managed in a superior way by the present invention which provides a novel EHV transformer of high electrical and spatial (volumetric) efficiency. Moreover, the concept of this invention can easily be extended to handle even the higher Ul-IV levels that are presently contemplated.
Large a-c magnetic core transformers, as known to the prior art, are highly efiicient and practical power transforming devices whose availability has made possible the modern a-c 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 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 compensate for line charging current. With the advent of EHV, shunt reactors carry an increased importance. For example, in EHV systems, leading currents due to line capacitance can cause excessive voltages at the end of a long, lightly-loaded line. Unless prevented, these excessive voltages known as the Ferranti affect, 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 so-called shell design or the gapped-core design. The shell design reactor consists of an air-core coil having a magnetic shell surrounding it, while the gapped-core design comprised a modification of this which included an iron core within the coil which was intercepted by segments of stiff non-magnetic material.
In the shell design, the coils of large diameter and radial build-up 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 grounded core is rendered far more difficult. Saturation of core iron must be avoided both to reduce 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 discemable, 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 a-c operating potentials but also excellent impulse voltage distribution. The invention leads to relatively compact windings with shorter total conductor length and with correspondingly lower losses, and to substantial reductions in the required magnetic materials with still further loss reduction.
The principles of this invention also contribute to the avoidance of saturation of elements of the magnetic I present invention while simultaneously reducing the overall size of the required unit and its cost. These benefits are realized in both transformer and reactors constructed under the present invention by utilizing the insulating core concept in which the active portions of the magnetic circuit are formed of electrically isolated segments, each electrically connected to its surrounding 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. 1 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 disks.
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. 11 shows a partially broken away view of the operational element of FIG. 10.
FIG. 12 is a view of the device of FIG. 10 taken along the lines 12-12.
FIG. 13 is a detail view of the operation element of FIG. 12 taken along the lines l3l3.
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 connection suitable for use in the invention.
FIG. 19 shows the detail interconnection of the wiring pattern on FIG. 18.
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, in which a thin, planar high-voltage terminal is employed.
FIG. 23a is a detailed view of several core elements of the reactor of FIG. 22 in the vicinity of the thin, planar high-Voltage terminal thereof, including the various coils associated with these core elements.
FIG. 23b shows graphically the potential decrease through the coils surrounding each core element.
FIG. 24 is a plan view, partly broken away, of the high voltage terminal of FIGS. 22 and 23a.
FIGS. 25a, 25b, 25c and 25d are details of the high voltage terminal of FIG. 24 showing certain steps in the formation thereof.
FIG. 26 is a view, similar to that of FIG. 3, showing a core element of a reactor so constructed as to render it an equipotential member while suppressing eddy currents.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning first to FIG. 1 which shows in a section a conventional transformer, it can been seen that such a transformer 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 non-magnetic gaps are carefully avoided.
Such transformers 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 R 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 over-voltages of this type of apparatus are increased, the required insulation must also be reliably increased. The coils are spaced progressive ly further from the core with an attendent increase in winding impedance and losses. Insulation barriers are introduced to control 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 as 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 mid-pointor 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 step-down, single-phase, auto-transformer, constituting one phase of a three-phase Wye-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 31 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 disks 36.
Surrounding each leg is a pair of current carrying windings. The auto-transformer shown consists of four series windings 41, 42, 43 and 44 and four common windings 45, 46, 47 and 48. Series windings 41 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, may 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. However, in a preferred embodiment of the invention, to be described in detail hereinafter, the high voltage is introduced via a thin, planar high voltage terminal. 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 voltage 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 the details of one form of construction 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 51 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 so that the insulating gap increases toward the edge. After the laminations 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. A preferred form of core segment is described in detail hereinafter.
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 electrically 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. Thus the total winding is electrically connected to each of the core elements at respective neighboring portions thereof so that the electric potential of each core element is main tained close to the potential of that portion of the winding nearest to the core element in question.
An insulating spacer 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. The thickness of each of said insulating spacer 36 should be sufficient to support a voltage of at least 2V/n, where V is the operating voltage of the high voltage lead 39 and n is the total number of insulating spacers in each magnetic column or leg 33, 34. Between each coil half may be interposed a second insulating spacer 61 which may have afiixed thereto a support 62 for maintaining an equipotential ring 63 around the coils. Both the spacer and the support 62 can be made of any suitable insulating material. Examples of suitable materials are impregnated paperboard, plastic or inorganic sheets. To prevent irregularities in the coils from creating high electrical stresses across the spacer 61 and mechanical damage, a semi-flexible coil spacer 65 can profitably be fitted around each coil pair 57 and 58.
As indicated previously most of the difficulties encounteredby the prior transformer 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 insulation 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 purposes. 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. It is important to note that each insulating spacer together with its adjacent core and coil constitutes a capacitor. Thus the active leg forms a series chain of capacitors extending from the high voltage terminal by two parallel routes to ground. 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 provides for each current path between high voltage and ground a series chain of high capacitances of nearly equal value. It is practical for the first time to design this series capacitance system so that transient over-voltages will be distributed with nearly exact uniformity along the entire stack. It is therefore expected that high 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 can be 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.
The insulator 36 can be molded in the form of a disc with anti-flashover configurations at its outer periphery. Two such designs are shown in section in 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.
This smooth conductive coating 72 in intimate contact with the solid dielectric establishes the electric field boundary and thus prevents irregularities in the Iaminations 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.
Referring to FIG. 6B, it is also possible to utilize an insulating sheet 311 which has been rendered flat and parallel and which is compressed between the magnetic core elements 312 and between the coils. In this case it has been found desirable to include a sheet 313 of thin plastic and/or paper on each side of the insulating sheet 311 to permit mechanical accomodation of the two abutting surfaces and to reduce the effects of unevenesses. No resistive coating is used in this configuration which is shown in FIG. 6B.
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 abutt 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 in dependently 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 82 are provided. Each leg carries the necessary number of coils and is magnetically interconnected to the other two legs by 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 assist the control of 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 may be needed to limit terminal voltages rises.
One prior art reactor design utilizes a shell yoke. The basic elements of such a prior art reactor are shown in FIG. 8. A coil 86 is made in the form of a hollow cylinder surrounded by a laminated shell yoke 87. This coil may be provided with a high voltage lead 84 at one end thereof and with a lowvoltage 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.
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 3 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 use large values of magnetic flux density, ,8. The air core, shell type reactor of FIG. 8 precludes the use of high values of the magnetic flux density because of eddy current losses in the coil windings. Consequently, the volume, V, is large and the flux density, B, is relatively low. Also, the coil is physically large and many of its turns are exposed to nearly the full value of magnetic field. These factors result in a reactor which can have high resistive and eddy-current losses in the coil.
The gapped-core shell type variation is illustrated in FIG. 9 and consists of the addition of an interrupted grounded iron core 88 inserted in the center of the coil 86. This core 88 is magnetically coupled to the yoke 87. Pieces of stiff non-magnetic 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.
The present invention avoids entirely the insulation problem between the electric and the magnetic circuits. The device operates at high values of B which permits a further reduction in the weight and physical size of the reactor. The consequent reduction in size not only reduces the cost but also reduces electrical losses and the inherent magnetostrictive noise. 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 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, mounted on skids 94 on a pad 95. Passing through the top of tank 90 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 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 I in FIGS. 11,12,113 14 and 15.
This element 97 basically comprises a magnetic circuit as shown in FIGS. l1, l2, l3, 14 which is composed of a pair of laminated magnetic returns 100 and 101 coupled together by a pair of insulated core legs 102 and 103. Each insulated core leg comprises a plurality of core segments 104 electrically isolated from one another by disks 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 is preferably shaped so that the mutually opposed surfaces of adjacent core elements are spaced apart by a distance which is uniform over said surfaces except in the peripheral regions thereof and which, in said peripheral regions, increases towards the exterior. Such shaping insures uniform electric and magnetic field patterns over most of the gap volume while eliminating undesirable field concentrations, electric as well as magnetic, in the peripheral regions.
Each core 104 is surrounded by a current-carrying coil 108. These coils are electrically connected to each other and to the core segment which they surround. Each coil-core pair is electrically isolated from one another by disks 106 and spacers 107. These disks and spacers further aid in positioning the cores and coils in spatial relationship. These disks and spacers may be made of any suitable insulating material such as impregnated paper products, pressboard or inorganic dielectrics. Each disc 106 may, in turn, be surrounded by an equipotential hoop 110 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 111 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 111 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.
Alternatively, the core assemblies and coil assemblies can be made mechanically independant, though clamped between the same magnetic returns, by providing independent dielectric spacers for core and coils.
When the assembly 97 is placed in the housing 90 such that the magnetic circuit is parallel to the base of the tank the 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 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 a predeterminable temperature.
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 not 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 FIG. 17 while insulating each core segment 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 cords 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 power rating. Even more important, its insulation reliability is inherently higher.
FIGS. 18 and 19 show the coils 105 serially connected between legs. This wiring arrangement is not preferred however because it produces only two parallel windings for the entire unit and under transient impulse voltage conditions has less desirable characteristics.
It should of course be understood that in either event the winding sense of coils on each disk must be such as to cause the magnetic field to travel in a closed loop as indicated by arrows 114.
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 mid-point 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 inter-core capacities.
Excellent electric field distribution is obtained, in accordance with the invention, by ensuring that such core segment 104 acts as an equipotential plane" in which the electric charge introduced by surges or otherwise may be distributed rapidly over the plane while suppressing undesirable eddy currents. A suitable construction is shown in FIG. 26, wherein the metal laminations 50 are electrically connected to each other along one edge only, while remaining electrically insulated from one another at all other points, by means of a suitable weld 300 along one side of the core segment 104.
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. 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 116 sandwiched therebetween. This grid 116 is configured to minimize eddy currents. Such a sandwiched disk could be used with each of the configurations described above including the transformer. Insertion of this conduction grid 116 acts to capactively 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 a 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 bafile. 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, now 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 that 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 he 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, insulating spacers and coils are identical the unit lends itself to mass production methods and economy of manufacture.
Since the insulating material provides the function of electrical insulation as well as mechanical support for both cores and coils there is a large value of inter-core and inter-coil 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 convention currents while simultaneously applying large compressive forces to the unit thereby reducing acoustical noise.
It should be obvious that other modifications and adaptions can now be made to the described reactor. For example, the reactor could be adapted to three phase operation by sjing 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. could be inserted between each spool and connected to an equipotential hoop surrounding the spool interface.
Referring now to FIGS. 22, 23a, 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 magnetic columns or legs 183 and 185 each of which comprise a plurality of core segments such as 186. Each core segment 186 is surrounded by coils 189. Separation of the core segments is achieved through insulating layer 191. The entire assembly is enclosed in a tank 193 which holds the end yokes 182 and 184 and the legs 183 and 185. Connection is made to the reactor by a high voltage bushing 195 containing a high voltage line 220. For ease of movement, the reactor 180 is mounted on a skid 197. In FIG. 23a the high voltage terminal 301 and adjacent core segments 188, 190 and 192 are shown in detail. Each core segment such as 190 is surrounded by a set of four coils 194, 196, 198 and 200 electrically connected such that the coils 194 and 196, and 198 and 200 each form a parallel set by connections 202, 204, 206 and 208. The resulting parallel coil combinations 194, 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 between coils 194 and 196 and the space between coils 198 and 200. Ac.- cordingly, oil or any other suitable cooling medium can be readily circulated throughout these spaces between parallel coil sets with little risk of insulation failure in these areas.
Unlike the previously described induction apparatus, no dummy center core is used. Moreover, as shown in FIG. 22, the core segments 186 are of an even number and thus the high voltage terminal 301 is located in a plane 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 182 and 184 are extended over the soils 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 eddy-current loss which would otherwise be present. In addition, the end yokes themselves can not be used for clamping both the coil and core assemblies thereby 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 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 194, 196 decreases from a maximum value of V at the outer extremity to a value of at the inner extremity which is likewise the potential at which the core segment is maintained. From the inner extremity of the parallel coil set 198, 200 to the outer extremity the voltage continues to decrease from a value of to V As shown in FIG. 23a, the outer extremity of coil set 198, 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 terminal 301 is connected to the coils surrounding core segment 188 and the voltage distribution decreases similarly to its respective 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 198, 200 and parallel coil set 216,
18 V2 V1(Or 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.
Referring now to FIGS. 24, 25a, 25b, 25c and 25d, the high voltage terminal 301 may be fabricated in the following manner. Using the same conductor as that used in the coils, namely, a thin copper or aluminum band 302 approximately one-half inch wide and having a thin insulating coating (not shown), a coil is formed in the usual manner except that it is wound together with a filler strip 303 of insulating material so as to space adjacent turns of the coil from one another. After the double-coil has thus been wound, a weld 304 is made radially across the coil so as to short circuit all turns. At the opposite side of the coil a cut 305 is made creating a radial gap. The exposed ends of the windings 302 are then spread apart and the filler strips 303 cut away to a depth of approximately 3 inches on both sides of the gap.'These are then replaced by filler strips 306 having a length sufficient to fill the space to provide a mechanical but insulating joint across the electrical gap. The resultant structure is given rigidity by overwinding the entire coil with insulating tape 307.
The high voltage terminal is electrically connected by connector 209 to the outer extremities of the parallel coil set on either side of the insulation 19lb.
As a consequence of the thin planar structure of the high voltage terminal, its capacitance to the grounded tank is small in comparison to its capacitance to the coil and core capacitance chains.
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 high voltage terminal 301 and thence down the series capacitance chains to the grounded yoke presented by both the coil assemblies and the core assemblies.
The coil pattern which is used in the high voltage terminal 301 need not be restricted to the particular construction described and a number of other construction of this general geometry could be employed to produce the same result. As previously stated, the function of the high voltage terminal is to provide a low surge impedance equipotential plane which is connected to the high voltage input and has low capacitance to ground and which has negligible eddy-current losses and which is intimately coupled, capacitively and conductively, to the core and coil assemblies.
Having thus described the principles of the invention, together with several illustrative embodiments thereof, it is to be understood that, although specific terms are employed, they are used in a generic and descriptive sense, the scope of the invention being set forth in the following claims.
1. Electromagnetic induction apparatus for highvoltage, high-power operation at a voltage V comprising a tank containing insulating fluid and, mounted therein, the following elements:
a. at least two magnetic columns each having a first end plane, a second end plane,'a midplane and a high-voltage terminal in said midplane b. a first magnetic yoke magnetically connecting said first end planes 0. a second magnetic yoke magnetically connecting said second end planes each of said magnetic columns comprising a series of similar core elements separated by a series of n similar electrically insulating layers each of said core elements being composed of a laminate of ferromagnetic strips separated by insulation of a thickness sufficient only to prevent eddy currents,
each of said similar electrically insulating layers having a thickness capable of supporting a voltage of at least 2V/n,
d. at least one winding surrounding each of said magnetic columns and having a midpoint which is connected to said high-voltage terminal, said winding being electrically connected to each of said core elements at respective neighboring portions thereof so that the electric potential of each core element is maintained close to the potential of that portion of the winding nearest to the core element in question, whereby said winding may be and therefore is wound in proximity to said magnetic column and e. high voltage bushing means providing a conductive path between each high-voltage terminal and the region external to said tank.
2. Apparatus according to claim 1, wherein the electric potential of each part of said winding is always in the vicinity of the electric potential of that core element nearest the part in question, whereby said winding may be and therefore is wound closely about said magnetic column.
3. Apparatus according to claim 1, wherein the mutually opposed surfaces of adjacent core elements are spaced apart by a distance which is uniform over said surfaces except in the peripheral regions thereof, and which, in said peripheral regions, increases towards the exterior.
4. Apparatus according to claim 1, wherein adjacent metal strips in each of said laminated core elements are connected by conductive means occupying a negligible portion of the inter-strip gap.
5. Apparatus according to claim 1, wherein said winding constitutes two parallel power circuits for each magnetic column, and said high voltage terminal consists of a conducting member in the midplane of each magnetic column and conductively connected to the midpoint of said winding for the purpose of distributing more uniformly both surge and a-c voltages over the winding and magnetic columns while at the same time suppressing eddy currents in said distributing means.
6. Apparatus according to claim 1, wherein said high voltage terminal is a thin planar conductive, but eddycurrent-suppressing element and therefore has low shunt capacitance to ground and high capacitance to the winding on either side so that the distribution of surge voltages lengthwise along said magnetic columns and their surrounding windings is rendered more uniform because of the cominance of series capacitances along the column with respect to the shunt capacitances to ground.
7. Apparatus according to claim 1, wherein at least one conductive lead is connected to the winding at a point removed from the high voltage end for the purpose of measuring the potential of the high voltage terminal or for transferring electric power at a correspondingly lesser voltage or both.
8. Apparatus according to claim 1, wherein said high voltage terminal comprises a thin planar conductive but eddy-current-suppressing element supported in the midplane of each magnetic colunm and wherein said high voltage terminal is connected to the junction of said high voltage bushing and said midpoint of said winding and therefore possesses a high series capacitance to the portion of the winding and magnetic posed of dielectric material of Youngs modulus in excess of 10 pounds per square inch but are covered on each surface subjected to compressive forces by a negligible thickness of dielectric material of relatively lower Youngs modulus.
12. Apparatus according to claim 4 wherein said laminated core elements constitutes a low impedance equi-potential plane with negligable eddy-current loss under high a-c magnetic flux densities which distributes both operating and transient voltages uniformly over the surfaces of abutting electrically insulating layers.