Abstract:
A high power static electromagnetic device with a flux path, a main winding and one or more regulation windings surrounding portions of the flux path. A control device is coupled to the flux path for selectively admitting the flux therein. In an exemplary embodiment, multiple flux paths are selectively turned on and off for including and excluding the regulation windings from the circuit. The windings may be formed of a magnetically permeable, field-confining insulating cable.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a selectively controllable high power static electromagnetic device, and in particular to a controllable high power transformer, reactor, inductance, or regulator with switchable step function selectively. As used herein the high power devices include those having a rated power ranging from a few hundred kVA up to more than 1000 MVA with a rated voltage ranging from 3-4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher. 
     In the transmission and distribution of electric energy, various known static inductive devices such as transformers, reactors, regulators and the like are used. The purpose of such devices is to allow exchange or control of electric energy in and between two or more electric systems. Such devices belong to an electrical product group known as static inductive devices. Energy transfer is achieved by electromagnetic induction. There are a great number of textbooks, patents and articles which describe the theory, operation and manufacture of such devices and associated systems, and a detailed discussion is not necessary. 
     Conventional electric high voltage control is generally achieved by transformers having one or more windings wound on one or more legs of the transformer core. The windings often include taps making it possible to supply different voltage levels from the transformer. Known power transformers and distribution transformers used in high voltage trunk lines involve tap-changers for the voltage regulation. These are mechanically complicated and are subject to mechanical wear and electrophysical erosion due to discharges between contacts. 
     SUMMARY OF THE INVENTION 
     The invention provides a high power static electromagnetic or induction device with a rated power ranging from a few hundred kVA up to over 1000 MVA with a rated voltage ranging from 3-4 kV and up to very high transmission voltages, such as 400 kV to 800 kV or higher, and which does not entail the disadvantages, problems and limitations which are associated with the prior art power devices. 
     The invention is based on the discovery that selective switchable control of the flux paths in the device enables broad control functions not hereinbefore available. 
     In a particular embodiment the invention comprises a high power static induction device having a flux bearing path, a main winding and a at least one regulation winding in operative relation therewith. A control in operative relationship with the flux bearing region selectively admits or blocks flux therein. The control may be in the form of a switchable conductive ring having one or more turns. At least one of the windings is formed of one or more current-carrying conductors surrounded by a magnetically permeable, electric field confining insulating cover. 
     In a particular exemplary embodiment, the cover comprises a solid insulation surrounded by an outer and an inner potential-equalizing layer being partially conductive or having semiconducting properties. The electric conductor is located within the inner layer. As a result the electric field is confined within the winding. The electric conductor, according to the invention, is arranged so that it has conducting contact with the inner semiconducting layer. As a result no harmful potential differences arise in the boundary layer between the innermost part of the solid insulation and the surrounding inner semiconductor along the length of the conductor. 
     According to an exemplary embodiment of the invention, the device has a flux bearing region and a control in operative relationship therewith for selectively admitting or blocking the flux there through for regulating the device. In a transformer having a plurality of legs or flux paths in the flux bearing region, the flux may be selectively admitted or blocked in each of said plurality of the legs so that various voltage outputs may be achieved. In a reactor, selective control of the flux in the core results in a switchable flux bearing region in the reactor. In a regulator, switchable voltage control is achieved. Depending on the type of control used, regulation may be in discrete steps corresponding to discrete or selective opening or closing of flux paths. 
     The invention employs windings having semiconducting layers which exhibit similar thermal properties to the solid insulation as regards the coefficient of thermal expansion. The semiconducting layers according to the invention may be integrated with the solid insulation so that these layers and the adjoining insulation exhibit similar thermal properties to ensure good contact independently of the variations in temperature which arise in the line at different loads. At temperature gradients the insulating layer and semiconducting layers form a monolithic core for the conduction and defects caused by different temperature expansion in the insulation and the surrounding layers do not arise. 
     The electric load on the material is reduced because the semiconducting layers form equipotential surfaces and the electric field in the insulating part is distributed nearly uniformly over the thickness of the insulation. 
     In particular, the outer semiconducting layer exhibits such electrical properties that potential equalization along the conductor is achieved. The semiconducting layer does not, however, exhibit such conductivity properties that the induced current causes an unwanted thermal load. Further, the conductive properties of the layer are sufficient result in that an equipotential surface. Exemplary thereof, the resistivity, ρ, of the semiconducting layer generally exhibits a minimum value, pmin=1 Ωcm, and a maximum value, pmax=100 kΩcm, and, in addition, the resistance of the semiconducting layer per unit of length in the axial extent, R, of the cable generally exhibits a minimum value R min =50 Ω/m and a maximum value R max =50 MΩ/m. 
     The inner semiconducting layer exhibits sufficient electrical conductivity in order for it to function in a potential-equalizing manner and hence equalizing with respect to the electric field outside the inner layer. In this connection the inner layer has such properties that any irregularities in the surface of the conductor are equalized, and the inner layer forms an equipotential surface with a high surface finish at the boundary layer with the solid insulation. The layer may, as such, be formed with a varying thickness but to ensure an even surface with respect to the conductor and the solid insulation, its thickness is generally between 0.5 and 1 mm. However, the inner layer does not exhibit such a great conductivity that it contributes to induce voltages. Exemplary thereof, for the inner semiconducting layer, thus, Pmin=10 −6  Ωcm, R min =50 μΩ/m and, in a corresponding way, Pmax=100 kΩcm, R max =5 MΩ/m. 
     In an exemplary embodiment, a transformer according to the invention operates as a series element with selectable leakage inductance and thus reactance. Such a transformer is capable of controlling power flow by redistribution of active or reactive effects between networks connected to the primary and secondary. Such a transformer is capable of limiting short circuit currents, and provides for good transient stability. The transformer is also capable of damping power oscillations and providing good voltage stability. 
     The present invention, allows for a flexible AC transmission system with control of the components wherein the power flow can be controlled. In the particular embodiment, the ability to control or regulate power flow is implemented in a component which is normally needed for other purposes. Thus, the invention allows for dual use without significant increase in cost. 
     In accordance with another embodiment of the invention, a reactor may be switchably operable either as a series or shunt element with selectable inductance and thus reactance. There is no need for power electronics in the main power circuit. Accordingly, losses are lower. Further, the control equipment is generally low voltage equipment and thus, simpler and more economical. The arrangement also avoids the problem of harmonics generation. As a shunt element, the reactor can perform fast variable reactive power compensation. As a series element, the reactor is capable of performing power flow control by redistribution of active or reactive effect between lines. The reactor can limit short circuit currents, provide transient stability, damp power oscillations and provide voltage stability. These features are likewise important for flexible AC transmission systems. 
     The drawbacks of prior art voltage regulation are avoided by a switchable voltage regulator according to the invention, wherein the magnetic circuit of the regulator includes at least one regulation leg having a flux bearing region switchable between open and closed states, and by at least one regulation winding wound around said regulation leg, said regulation winding being connected to the main winding. It is also possible to place at least one winding loaded with a variable capacity on at least one magnetic flux path or leg having a zone with reduced permeability across the magnetic flux, to vary the reluctance of the leg by varying the impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings, wherein 
     FIG. 1 shows the electric field distribution around a winding of a conventional a inductive device such as a power transformer or reactor; 
     FIG. 2 shows an embodiment of a winding in the form of a cable in a high power inductive device according to the invention; 
     FIG. 3 shows an embodiment of a power transformer according to the invention; 
     FIG. 3A illustrates a magnetic switch in accordance with the invention; 
     FIG. 3B shows an open and closed flux path corresponding to open and closed magnetic switches; 
     FIG. 3C is a schematic illustration showing various forms of the control circuit  44 ; 
     FIG. 4 is a schematic illustration of a regulation leg portion of the transformer of FIG. 3; 
     FIG. 5 is a schematic illustration of a reactor in accordance with the present invention; 
     FIGS. 6A and 6B are respective, perspective and sectional schematic illustrations of a device in accordance with an embodiment of the present invention; 
     FIGS. 7A and 7B are respective, perspective and sectional schematic illustrations of a device in accordance with another embodiment of the invention; 
     FIGS. 8A and 8B are respective, perspective and sectional schematic illustrations of a device in accordance with yet another embodiment of the invention; and 
     FIG. 9 is a schematic illustration of a three phase transformer according to the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The inventive concept which forms the basis of the present invention is applicable to various static inductive devices including, power transformers, reactors and regulators. As is known, the devices herein categorized may be designed as single-phase and three-phase systems. Such devices include various types of known devices such as boost transformers, auto transformers and the like. Also, air-insulated and oil-insulated, self-cooled, oil cooled, etc., devices are available. Although devices have one or more windings (per phase) and may be designed both with and without an iron core, the description generally shows devices with an iron core having a selectable region of variable high reluctance. 
     The invention further relates more specifically to a controllable inductance wherein the magnetic flux is selectively redistributed among and between different flux paths by affecting the reluctance of at least one of such paths. In a reactor the invention operates as a series or shunt element with a selectable variable inductance. 
     FIG. 1 shows a simplified and fundamental view of the electric field distribution around a winding of a conventional static induction device such as a power transformer/reactor  1 , including a winding  2  and a core  3 . Equipotential lines E show where the electric field has the same magnitude. The lower part of the winding is assumed to be at earth potential. The core  3  has a window  4 . 
     The potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and between each turn and earth. In FIG. 1 the upper part of the winding is subjected to the highest dielectric stress. The design and location of a winding relative to the core are in this way determined substantially by the electric field distribution in the core window  4 . 
     FIG. 2 shows an example of an exemplary cable  5  which may be used in windings which are included in high power inductive devices according to the invention. Such a cable  5  comprises at least one conductor  6  including a number of strands  6 A with a covering  7  surrounding the conductor. The covering includes an inner semiconducting layer  8  disposed around the strands. Outside of this inner semiconducting layer is the main insulation layer  9  of the cable in the form of a solid insulation, and surrounding this solid insulation is an outer semiconducting layer  10 . The cable  5  may be provided with other additional layers for special purposes, for example for preventing too high electric stresses on other regions of the device. The outer layer  10  may be connected to ground G as shown. From the point of view of geometrical dimension, the cables  5  in question will generally have a conductor area which is between about 30 and 3000 mm 2  and an outer cable diameter which is between about 20 and 250 mm. The covering  7  is an integrated structure which is substantially void free, that is, free of air pockets and the like. 
     FIG. 3 shows a high power inductive device in the form of a single phase core type transformer  11  in accordance with the present invention. The transformer  11  comprises a core  12  which is formed with main or outer legs  14 , 16  and short or inner legs  18  and  20 , and respective lower, middle and upper arms  22 ,  24  and  26 . The core  12  may be made of laminated iron sheets having a main or large aperture or window  28  and a plurality of small or regulation windows  30 - 1 ,  30 - 2  and  30 -m, in a regulation region  32  located generally between the middle and upper arms  24  and  26  as shown. In the exemplary embodiment, m=3. 
     In order to form a core type transformer, a primary winding  34  is wrapped around the leg  14 . In a similar manner, a secondary winding  36  may be wrapped concentrically with the primary winding  34  around the leg  14  or on another leg. A regulation winding  37  formed of one or more regulation sub-windings or coils  38 - 1  . . . ,  38 -n in series of the primary winding  34  may be wrapped around the respective inner legs  18  and  20  as shown. 
     Control means in the form of one or more conductive short circuit rings  40 - 1  . . . ,  40 -n may be located as shown. For example, rings  40 - 1 ,  40 - 2  and  40 - 3  surround the middle arm  24  and extend through the windows  28  and  30 - 1 ,  30 - 2  and  30 -m respectively. In the similar manner rings  404 ,  40 - 5  and  40 -n surround the upper arm  26  in the windows  30 - 1 ,  30 - 2  and  30 -m respectively. It should be understood that the suffix  1 ,  2 ,  3 , m and n are used to designate the position of the corresponding element, and are otherwise not used when the position is not relevant to the discussion. 
     In the exemplary embodiment, and as shown in FIG. 3A, ring  40  comprises one or more turns of a conductor  42 , e.g. copper terminated such as switch  44 . When the switch  44  is closed the corresponding ring forms a short circuit. In other embodiments, the control  44  may be an active or passive filter, a reactance or voltage or current supply. FIG. 3 schematically shows alternative arrangements for the control  44 . For example, the control  44  may be in the form of an active filter  44 A, a passive filter  44 B, a pure reactance  44 C or  44 D, a voltage supply  44 E or a current supply  44 F. The control  44  may also include a power source  44 G capable of varying the amplitude frequency and phase of the flux, for example, by superimposing a fixed or variable signal on the loop  40  so that the frequency amplitude and phase of the flux may be varied or modulated. 
     The windings  34 ,  36  and  38  produce the flux φ, which is carried by the core  12  along one or more possible alternative paths as shown by the dotted lines in each of the legs  14 ,  16 ,  18 ,  20  and the arms  22 ,  24  and  26 . In a device  46  shown in FIG. 3B, when any switch  44  of a corresponding ring  40  is open, the corresponding flux path through the leg or arm of the core, as the case may be, surrounded by ring is open. Likewise, when a switch  44  is closed, the flux path through the core, at that point, is blocked. The core  41  in FIG. 5 may have a central leg,  43  with an air gap  45  as shown. As is well known, the air gap  45  has a region of reduced or low permeability relative to the core  41 . It should be further understood that an insert of a low permeability metal may be placed in the air gap  45 . Blocking the lower legs  47 , as shown, redirects the flux into the central leg  43  through the air gap  45 . 
     In accordance with the invention, when the switch  44  is open circuit, the upper core leg  49  exhibits a given relatively low reluctance (high permeability) to the flux fee. However, when the switch  44  is closed, the leg will exhibit high reluctance (low permeability). Thus zones of high and low reluctance are produced which correspond to zones of low and high reluctance respectively. 
     FIG. 4 is a fragmentary portion of the regulation region  32  of the transformer  11  shown in FIG. 3, illustrating in greater detail stepwise magnetic flux regulation according to the invention. In the exemplary embodiment of FIG. 3, the magnetically regulated transformer  11  has the low voltage (LV) winding  34  (N LV  turns), the high voltage (HV) winding  36  (N HV  turns) and the at least one additional regulation (R) winding  37  (N RO  turns) in series with the LV winding  34 . Voltage regulation is then obtained by changing the transformer ratio N HV /(N LV +N R ), where N R  is an effective number of regulation turns. N R  can be varied over some subinterval of [−N R +N R ] by actively linking the main magnetic flux through different parts of the regulation windings. The linking is performed with an arrangement of switchable magnetic rings  40  in the core  12 , each of which should as completely as possible exclude the flux from a selected region of the core, or admit the flux through with a minimum of reluctance. In the regulation winding  37  the separate subcoils  38 - 1  . . . ,  38 -n (n=2) are wound in series through the windows  30 - 1  . . . ,  30 -m (m=3) in the regulation or upper portion  32  of the core  12 . 
     The principle of the invention illustrated in FIG. 4 shows that magnetic switching is achieved with the short circuit rings  40 , which, when switched closed, block the passage of flux through the corresponding sub-coil  38 . Likewise, when opening, the rings  40  admit the flux  4  into the core segment and direct it through or past the subcoils. Depending on the arrangement, flux control occurs in a number of ways, each representing a single noncirculating path through the regulation region  32  and a unique value of N R . In the example of FIG. 4, N R =1-3=−2. The regulation region  32  is dimensioned for maximum flux along any allowed path. Accordingly, the regulation region  32  is at least twice the size of a conventional core without regulation. 
     In accordance with another embodiment of the invention, a reactor  60  is shown in FIG.  5 . The reactor  60  has a main flux path  62  shown as a dotted line surrounding a lower window  63 , and a regulating flux path  64  shown as a dotted line surrounding the upper window  65 . The path  62  and  64  are parallel when the central leg  67  is magnetically closed so that the flux can pass therethrough. However, the path  62  and  63  become a signal single series loop when the leg  67  is magnetically an open circuit. A main winding  66  in the main path  62  is in series with a regulating winding  68  in the regulating path  64 . A magnetic contact switch  70  is in the regulating path  64  as shown. When closed, the magnetic switch  70  blocks the regulating path  64 , and when open the magnetic switch  70  opens the magnetic path. An additional winding  72  which may be connected in parallel or shunt with the main winding  66 , and a magnetic switch  74  may be added to the main path, as shown, so that more complex regulation of the reactor  60  may be provided. 
     FIGS. 6A-6B;  7 A- 7 B; and  8 A- 8 B illustrate the regulation portion  70  of a transformer, reactor or regulator, as the case may be, depending on the application. The regulation winding  72  having N R =4 turns is divided into spatially well separated subcoils  74 - 1  . . . ,  74 -n having N 1  . . . n terms where N 1 =3 and n=1. Regulation is achieved by linking the magnetic flux past or through each such sub-coil  74  to omit, add, or subtract its corresponding number of turns, n i , to the total number of regulation turns, N R . 
     Three regulation winding arrangements of interest can be identified and are named after the first three elements in the sequence of subcoil turn rations: 1:2:4, 1:3:7, and 1:3:9, respectively. The arrangements are restricted to a construction with 2×4 magnetic switches. Each of these arrangements is illustrated in FIGS. 6A-6B;  7 A- 7 B; and  8 A- 8 B respectively as follows. 
     FIGS. 6A-6B illustrate a 1:2:4 arrangement. The winding  72  in the form of a cable discussed above in FIG. 2 is wound around a common axis A pp1  parallel to the direction of the main magnetic flux φ and with one magnetic switch  40 - 1 A in  40  NA inside each sub-coil  74 - 1  in  74 -n and one switch  40 - 1 B in  40  NB outside each coil. The number of turns is doubled for each coil in the sequence, i.e., n i =2 i−1 , i=1,2,3, . . . , n 1 =1,2,3, . . . The magnetic flux can pass through a coil in just one direction. Accordingly, turns can be omitted or added, but not subtracted. The number of switches  40  required is 2m, where m is the number of subcoils, and the number of possible regulation levels in 2 m . FIGS. 2A,  6 A- 6 B show sixteen possible values of N r : 
     
       
         0,1,2,3(=2+1), 4,5(=4+1), . . . , 15(=8+4+2+1). 
       
     
     FIGS. 7A-7B illustrate a 1:3:9 arrangement. The cable is wound around A d alternate legs  90 - 1  . . . ,  90 -n with axes AP, perpendicular to the main magnetic flux direction. Every second leg  50 - 2  . . . ,  50 -(N−1) is left unwound as a bridge between the upper and the lower horizontal part of the core. The number of turns is tripled for each sub-coil  74 - 1  . . . ,  74 -n in the sequence; n i =3 i−1 n 1 . Switches  40 - 1 A,  40 - 1 B . . . ,  40 -NA,  40 -NB are positioned on the sides of each leg so that the flux ma be linked past or in both directions through a sub-coil  38 - 1  . . .  38 -n. The number of switches required is 4m and the number of possible regulation levels is 3 m . FIGS. 7A-7B show an example with nine possible values of N R : 
     
       
         −4(=−3−1), −3, −2(=−3+1), −1, 0, 1, 2(=3−1), 3, 4(=3+1). 
       
     
     FIGS. 8A-8B illustrate a 1:3:7 arrangement. The cable is wound around legs  94 - 1  . . . ,  94 -n with axes AP perpendicular to the main magnetic flux direction. In contrast to the 1:3:9 case above all legs  94 - 1  . . .  94 -n are wound. The number of turns is approximately doubled for each sub-coil  38  in the sequence; n i =(2 i −1)n 1 . Switches  40 - 1  A,  40 - 1 B . . . ,  40 -NA,  40 -NB are positioned on the sides of each leg so that the flux may be linked past or in both directions through sub-coil  5   74 - 1  . . . ,  74 -n, with the restriction than in a sequence of incorporated coils, turns are added with alternating sign. The number of switches required is 2m+2 and the number of possible regulation levels is 2 m+1 1. FIGS. 8A-8B show an example with fifteen possible values of N R : 
     
       
         −7,−6(=−7+1), −5(=−7+3−1), −4(=−7+3), −3−2(=−3+1), −1,0,1,2(=3−1), 3,4(=7−3), 5(=7−3+1), 6(=7−1), 7. 
       
     
     Thus, in accordance with the invention, a selectable static induction device has been provided in which one or more magnetic switches selectively open and close flux paths in the device. It should be understood that in addition to the short circuit rings described, providing a step function like flux response, variable impedances of various kinds may be used. For example, if a variable inductor is used to load a ring  40 , the reluctance varies inversely with the inductance. Thus, high inductive loading will result in a corresponding high flux distribution in the leg. If a variable capacitance is used, reluctance varies directly. If a variable or high resistance is used as a load for the ring  40 , a variable or high flux distribution results in the leg. If the ring is shorted, the effect is as described in that the flux will be blocked. Various combinations of fixed and variable, real and reactive loading may also be provided. In addition, loading or activation may be provided by an active element, for example, an active filter. Such a filter could be programmable. 
     It is also possible to provide a variable power source, e.g., a voltage or current source to produce an input on the ring which is adapted to modulate the flux in the leg. Modulation may be in terms of amplitude, phase and frequency. It is also possible to provide an active filter to load the ring to thereby vary the performance of the ring and thus modulate the device output. 
     FIG. 9 illustrates another embodiment of the invention wherein a three phase transformer  100  of a shell or core type having a main winding  102  and a regulation winding  104  for each phase wrapped on a core  106  is illustrated. The various flux paths are shown in dotted line in the legs  108 ,  110  and  112  and the yokes  114 ,  116  and  118 . According to the invention, a one or more magnetic switches  120  may be employed as hereinabove described. In the exemplary embodiment shown, switches  120  are located in yokes  114  and  116  to control the flux through the regulation windings  104 . The windings may be in series or shunt as may be the flux bearing paths. For example, flux path  130  forms a closed series outer loop and flux path  132  forms a closed series inner loop which is parallel to path  130 . The coils  102  and  104  may be connected in a variety of series or parallel arrangements by appropriate connection of the leads  134  and  136  as is known by those skilled in the art. 
     The magnetic switches  120  surround regions  144  in the core  106  which may be formed of a conductive material or may be formed of a solid insert of material different from the core material having reduced or low magnetic permeability or an air gap. Also, one or more spacers  143  may be provided between the yokes  114  and  116 . Further details of such arrangements may be seen in U.S. patent application Ser. No. 08/980,210 incorporated herein by reference. 
     While there have been provided what are considered to be exemplary embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications therein may be made without departing from the invention, and it is intended in the appended claims to cover such changes and modifications as fall within the true spirit and scope of the invention.