WO2008034271A2

WO2008034271A2 - Encapsulated electrical valve module

Info

Publication number: WO2008034271A2
Application number: PCT/CH2007/000450
Authority: WO
Inventors: Reinhard Joho
Original assignee: Reinhard Joho
Priority date: 2006-09-21
Filing date: 2007-09-14
Publication date: 2008-03-27
Also published as: WO2008034271A3

Abstract

A valve module in encapsulated design (1) consists of a semiconductor series connection (2), which is coaxially enclosed by a metal tube (3), the tube being filled with an insulating fluid (6). The semiconductors (10) are preferentially arranged in stacks. Focused to the region of the axis of the metal tube, one or several parallel semiconductor stacks are grouped to a semiconductor stack arrangement, and they are stabilized in the tube against lateral forces by insulating end spacers (4) and, if needed, by insulating intermediate spacers (5). An array of shielding tubes (7) around the semiconductor stack arrangement equalizes the electrical field. Encapsulated valve modules are used to build up converter bridge circuits and circuit breakers.

Description

ENCAPSULATED ELECTRICAL VALVE MODULE

Technical Field

The present invention relates to the field of electrical high-power converters in combination with the field of encapsulated electrical switchgear, especially with the field of gas-insulated switchgear.

Background Art

It is known to realize high-voltage switchgear in the range of 50 kV to 1000 kV in encapsulated design. In such switchgear all current-carrying conductors are enclosed by full-length metal tubes, which are at ground potential. The interior of the metal tubes is filled with an insulating fluid, whereby the insulating distances in the interior of the tubes can be kept small. The restriction of the electrical field to the interior of the tubes allows a dense arrangement of the tubes. By the encapsulation an inherent robustness against environmental influences is achieved. By cross connections among the metal tubes of all conductors the a.c. component of the magnetic field outside the tubes is strongly reduced. The application is for transmission lines, mechanical switches, transducers, overvoltage arrestors and power transformers.

Encapsulated design is used in switchgear installations of power plants, substations of the grid, and recently also in lines and switchgear of high-voltage d.c. transmission.

High-power converters are in use for the conversion of electrical power from one frequency to another, or to d.c, and vice versa. High-power converters consist essentially of valve modules, which consist of an array of series-connected semiconductors. Due primarily to the voltage- dependent spaces, such installations are arranged in large buildings. To achieve the voltage withstand the valve modules are built up of columns of semiconductors in disc design with inserted cooling elements. The length of such columns is limited, because of the risk of lateral tilting under weight force and electromagnetic forces.

Semiconductor circuit breakers for high power are in principle known, although not yet in commercial application. For high voltages, as for the converter, they are built up of valve modules. That is why the space needed for a semiconductor circuit breaker is also very large.

Disclosure of Invention

The invention has the task to combine in an advantageous task elements of encapsulated switchgear and of high-power converters in a high-voltage circuit arrangement, which is capable to carry high-power currents in a continuous duty. This is realized by carrying out valve modules in encapsulated design, including the connections between the modules. For that purpose, one or several columns of electrically series-connected semiconductors are arranged in longitudinal direction and concentrated to the area of the tube axis, in electrically good conducting, fluid-tight and mechanically stable tubes. The current at the connection on the one end of the tube flows also at the other end of the tube; there is no return way for the current in the tube. In a multi-column arrangement at a given time section there is always only current in one column. The columns are built up of a stacking of semiconductors, preferably of the disc element design, and of cooling elements, which are pressed by a tension arrangement. The mechanical stability problem of the columns is solved by providing insulating spacer means for the column arrangement at least at the ends of the tube, preferentially also inside the tube. It is also possible to stack discrete encapsulated valve modules in a row, and thereby connect them electrically in series.

Encapsulated valve modules are used for voltages in the range 20 kV to 1000 kV and for currents in the range 1000 A to 15000 A. Each column has a series connection of 6 to 250 semiconductors. The invention takes profit of recent progress in the semiconductor technology, which results in smaller snubber circuits, protection circuits and gate units, and parts of them are integrated into the semiconductor. Recent operating experience assesses the reliability of semiconductors in disc design as extremely good, thus in the view of availability making justifiable an encapsulated arrangement with up to several hundred semiconductors. To perfect the availability the known series redundancy of semiconductors is applied.

All tubes of the built-up circuits are preferentially of good-conducting metal and they are electrically connected over the full arrangement by flange connections. Additional electrical cross connections between the tube walls are provided, and if needed, they can connect the tube walls as an electrical network. In this arrangement all a.c. currents and transient currents of the semiconductor columns are mirrored in the walls of the tubes. By this the tube arrangement acts as shield for leaking magnetic a.c. fields. By the resulting coaxial arrangement the electromagnetic lateral forces on the column arrangement are strongly reduced. Likewise the disturbing magnetic field outside the tubes is reduced significantly. The electrical networking of the tubes may be grounded at one or several points of the arrangement, thereby the arrangement is practically free of electrical fields outside the tubes. By all these reasons the arrangement of encapsulated valve modules can be designed very compact and space saving. Because of the encapsulated design it is inherently also suitable for open-air installation. To simplify transport and to accelerate the installation the encapsulated arrangement can fully be integrated in a metallic frame, or the encapsulated arrangement can be subdivided into several electrically linked frames, which frames taking over the electrical cross-linkage between the tubes. The circuit arrangements built up with the encapsulated valve modules are, in single arrangement or in combination: single-phase or multi-phase circuit breakers, including electronic current limiters and d.c. circuit breakers, controlled and uncontrolled converter bridge circuits having d.c. current link or voltage link, controlled direct converters without d.c. link, and controlling elements for parallel and serial reactive power compensators. The circuit arrangements with encapsulated valve modules according to the invention can be combined with all known elements of encapsulated switchgear.

The tubes are filled jointly or individually with insulating fluid. In contrast to known fluid- insulated switchgear the insulating fluid must not extinguish arcs, it thus can exclusively be tuned to optimum dielectric property, and there is no contamination by contact burn-off products. Such insulating fluids are for example natural or synthetic insulation oils, or a gas or a gas mixture at an atmospheric overpressure of 0.2 MPa to 1.0 MPa. The gas mixture may include several components, e.g. the gas mixture can be pure air, it can be nitrogen, and it also involves an enrichment with an electronegative gas, such as SF₆. For individual filling the discshaped or bell-shaped insulating spacers at the module ends take over the separation function for the insulating fluid.

The conductor connection of an encapsulated valve module is usually at the ends of the tube in the extension of the column, thus coaxially to the tube axis. By design reasons a radial exit of the conductor connection may be provided at one or both ends. The corresponding tube end is sealed with a cover.

The removal of the dissipated power of the semiconductors and of the snubber is provided by periodically arranged cooling elements in the columns and the circulation of a cooling fluid. A cooling with circulating insulating fluid is possible, especially with liquid insulating fluids. By reasons of compactness the cooling through circulation of a separate cooling fluid is preferred. At high thermal capacity and low viscosity this has a specifically low electrical conductivity to cope with the potential difference between cooling elements at high voltage and a connection nipple at the grounded tube wall. Such a cooling fluid is preferentially a liquid. The selection of the cooling liquid has to be coordinated with the insulating fluid in the tube and the extreme ambient temperatures. At not too low ambient temperatures e.g. deionized water can be used. In the insulating gas SF₆ or at extreme ambient temperatures e.g. perfluorcarbon may be chosen as a cooling liquid. An insulating gas may continuously diffuse at minimum rate into the cooling liquid circuit. The cooling liquid circuit may thus be provided with a separation device outside the valve tubes. This may involve a recovery of the insulating gas. In order that in no circumstance cooling liquid leaks into the insulating gas space the highest pressure in the cooling liquid circuit can be set lower than the pressure in the insulating gas. The cooling elements are supplied with cooling fluid by manifold pipes of insulating material, which are placed alongside the column arrangement, preferentially in parallel supply. The manifold pipes are at least at one end connected to the supply nipple in the tube wall via a spirally or helically wound insulating hose. With the hereby-achieved multi-turn arrangement the required length for the potential separation is provided in space-saving arrangement.

The support of the column arrangement in the tube is provided by a plurality of insulating spacers against the tube. For the insulating spacer in a converter arrangement, the appearing D. C. voltage components have to be considered. To avoid a drifting away of the distribution of the electrical field in the insulating material, there is applied, in known art, the application of electrically weakly conducting fillers or weakly conducting coatings to the insulating spacers.

The column arrangement is unavoidably associated with protruding edges of mechanical tensioning elements, cooling elements, gate units and snubber. To reduce further the inhomogeneity in the electrical field, and to thereby increase the dielectric strength, the column arrangement is enveloped by an array of separated electrically conducting shielding tubes. These have rounded ends, which serve for field homogenization, mutually and against the metal tube. Each shielding tube covers several semiconductor places, and it is electrically connected to one of the semiconductors it covers. Thereby results a relatively homogenous distribution of the field between shielding tube and metal tube. The shielding tubes may all be of equal length. To support dynamic voltage balance they may also be progressively longer towards the end regions of the column, especially for low-capacity snubbers of the semiconductors. Another approach is an enhanced capacitive coupling between the ends of adjacent shielding tubes.

Another solution of a support makes use of a mechanically reinforced insulating tube, which envelops the column arrangement. The insulating tube has a tuned weak electrical conductivity and is at least at both ends of the column arrangement electrically coupled to the column. It may also be periodically connected with the semiconductors and thus serve for static voltage distribution among the series-connected semiconductors. The insulating tube may be extended axially into the tube cover and be supported there. It thereby serves also as an insulating support of the column arrangement. Without loss of shielding function it may be perforated for circulation of the insulating fluid. The insulating tube may be used in section-wise closed design for a forced circulation of the insulating fluid as a cooling fluid.

For the use in converters normally one encapsulated valve module is applied per branch of the bridge circuit. All busducting of the circuit is as well done in encapsulated design. Encapsulated valve modules of a circuit with current link are equipped with a single column of diodes or thyristors. Bidirectional thyristors allow the integration of antiparallel bridges or make possible circuit branches in direct converters. Encapsulated valve modules having two parallel columns of thyristors of opposite polarity allow integration of branches of antiparallel bridges, or may equip circuit branches of direct converters. In such an arrangement it is possible to merge snubbers of different columns by cross connections between semiconductors. Current limiting chokes may be integrated as further encapsulated elements in series to the encapsulated valve modules. For the circuit with d.c. voltage link the encapsulated modules are equipped with semiconductors of the type GTO, GCT or IGBT. Freewheeling diodes may be arranged as additional columns in the same tube, or may be arranged in separate tubes. The achievable reduction in the loop inductivity is a large advantage; an even larger advantage results when the supporting capacity of the voltage link is integrated in the encapsulation. The tube topology of multi-level circuits follows essentially the circuit, whereas the voltage-stepped supporting capacity is integrated in the tubing. As a side benefit, the metal encapsulation, and if needed, an embedding of the encapsulation in a suited liquid or solid, shields the semiconductors against the destructive cosmic radiation, allowing higher voltage utilization.

The effect of travelling waves inside the essentially coaxial design is insofar mitigated, in that the circuit arrangement works with very short transmission lines, due to its compact design. Therefore as desired, the switching time of the semiconductors is larger than the wave propagation time in the tubing. It is also possible to harmonize the wave resistance inside the encapsulated valve modules with the wave resistance of feeders in order to achieve lowest reflections.

For the application in circuit breakers, a circuit-breaker pole is realized with an encapsulated valve module in a twin-column arrangement of antiparallel thyristors or in a single-column arrangement with bidirectional thyristors. The antiparallel circuit may also be built up of two single-column valve modules, which are interconnected at both ends. In the same or in an additional parallel tube there may be an electrically parallel column of metal-oxide varistors, which serves for damping of switching transients. Varistors may also interconnect in own encapsulation the poles on each side of the circuit breaker. For forced switch-off of the current in the circuit-breaker pole, known resonant circuits are preferentially integrated in the encapsulated arrangement. The resulting low-inductive design shortens, as anticipated, the switch-off process and allows the choice of small energy storage elements in the resonant circuit.

The converters and circuit breakers, built up of encapsulated valve modules, are on input side and output side connected to busducts, which themselves are built in encapsulated design. This allows a design with a minimum of space required. Instead of individual conductor encapsulation, the busducts of input and output can also be realized as encapsulated multi- conductor systems. It is also possible to provide an encapsulated arrangement of a converter or a circuit breaker with terminals of the design of fluid-air bushings, where the high voltage is connected from an overhead line system to the converter or circuit breaker according to the invention. This allows a quick mounting and exchange of complete encapsulated set-ups.

Because of the insulation by the fluid and the inherent cleanliness inside the tubes the disc housings of the semiconductors must not be encapsulated and are not bound to the normally used creeping distances. The use of open housings is possible, to improve the dielectric properties inside the disc elements; it is as well thinkable in extreme approach to use bare silicon wafers with only the associated molybdenum cover discs and a centering device. These technologies result in cost-advantageous semiconductors and short design of the valve modules. They make possible the full utilization of high-blocking semiconductor wafers, e.g. of the silicon-carbide (SiC) generation.

The controllable semiconductors are preferably controlled with light waves. Thus the problem of the separation of the potential and of the electromagnetic interferences is solved in a simple way. Such semiconductors are available, e.g. as direct light-triggered thyristors. For this, an optical waveguide leads to every semiconductor. Alternatively, every controllable semiconductor can be provided with an electro-optical transducer, which converts the light impulse into an electrical control signal for the gate terminal of the semiconductor. A local transducer extracts power from the snubber circuit of the semiconductor and thereby supplies the electro-optical transducer. In such a constellation the optical waveguide can be looped through all the semiconductor places. The optical waveguides are conducted out of the valve module by fluid-tight seals in the tube wall. Alternatively a transducer can be arranged in the fluid space, which supplies the optical waveguides. In this case only the electrical pulse supply and the power supply for the transducer are to be conducted through the housing.

The tube walls conduct the screening currents and therefore are subject to electrical losses, which, thanks to the comparatively large effective cross section of the tube wall, are inherently lower than in the inner conductor. To limit the temperature rise at highest currents or switching frequencies, the outside of the tube walls may be equipped with cooling pipes. These can be e.g. welded on in helical shape. They can be switched hydraulically in the cooling circuit of the column arrangement, e.g. in parallel or in series. The cooling of the tube wall may also be used for the cooling of the semiconductor columns, this preferentially when using a liquid insulating fluid.

When using large components in the region of the column arrangement, particularly for multi- column arrangements, tube elements over the length of the column arrangement may be provided with a larger diameter than at the connecting flange. For adaption, a stepped or conical tube element will be inserted on both ends. To simplify assembling, at least one of the two stepped or conical tube elements may have a flange connection to the tube element with larger diameter.

An active reduction of the d.c. component of the magnetic field outside the tubes of individually encapsulated d.c. busducts of converters can be accomplished by a controlled current source injecting a current of same amount as the conductor current, and of opposite direction to the conductor current, in the tube loop opened by the positive and negative busduct.

The assembling procedure of a long encapsulated valve module with intermediate spacers is as follows: First, the moderately tensioned column arrangement with the intermediate spacers is inserted into the tube. The intermediate spacers center on shoulders in the tube wall. Next, the column arrangement is tensioned finally, without any risk of tilting. For the design with full- length insulating tube, the final tensioning is applied after inserting the column arrangement into the insulating tube.

An optical waveguide to each semiconductor may also be used for sending back information of the condition of the semiconductor. Such refers to e.g. the momentary switching condition, the polarity of blocking voltage, or the temperature. A local transducer collects the measurands and converts them into optical signals.

Brief Description of Drawings

Fig. 1 shows a longitudinal cut through an encapsulated valve module according to the invention. Fig. 2 shows in detail two embodiments of the cooling fluid supply.

Fig. 3 shows in detail the embodiment of the intermediate spacer of the semiconductor column.

Fig. 4 shows a longitudinal cut through an encapsulated valve module according to the invention, making use of tube elements arranged in a row.

Fig. 5 shows the arrangement of a 6-pulse diode bridge making use of the encapsulated valve modules.

Fig. 6 shows the arrangement of a 6-pulse diode bridge with connections to encapsulated multi- conductor systems.

Fig. 7 shows the arrangement of a two-level inverter with integrated voltage link making use of the valve module according to the invention. Fig. 8 shows the arrangement of a bridge circuit with bushings for a connection to an overhead line system.

Fig. 9 shows an arrangement for a circuit-breaker pole making use of two encapsulated valve modules according to the invention. Fig. 10 shows an arrangement for a circuit-breaker pole making use of an encapsulated valve module according to the invention with a twin-column valve arrangement. Fig. 11 shows a longitudinal cut through an encapsulated valve module with radial connections and a full-length insulating tube enveloping the semiconductor column. Fig. 12 shows a section of the circuit arrangement of a valve column making use of direct light- triggered thyristors.

Fig. 13 shows a cross section through the practical design configuration of a valve column making use of direct light-triggered thyristors.

Fig. 14 shows a longitudinal cut through an encapsulated valve module according to the invention with a twin-column arrangement and an enlarged diameter of the metal tube.

Fig. 15 shows an appliance for the reduction of leaking magnetic fields in the d.c. circuit of an encapsulated converter system.

Detailed Description and Preferred Embodiments Fig. 1 shows a longitudinal cut through an encapsulated valve module 1 according to the invention. The valve module includes a semiconductor column 2. The semiconductor column 2 is enclosed by a metal tube 3, whereas the semiconductor column and the metal tube are essentially arranged coaxially. Insulating end spacers 4 on both sides fix the semiconductor column 2 in the metal tube 3. Insulating intermediate spacers 5 along the semiconductor column support the semiconductor column in the metal tube and thus prevent lateral tilting in long semiconductor columns. The interior of the metal tube is filled with an insulating fluid 6, which is electrically well insulating. Thereby the metal tube can enclose the semiconductor column with small diameter. Along its length the semiconductor column is enveloped by an array of separated shielding tubes 7, which allow an additional reduction of the metal tube diameter.

The connection to other current carrying elements in the circuit is provided by plug contacts 8. These plug contacts also absorb relative movements, which are produced by different thermal expansion of semiconductor column and metal tube. The metal tube 9 of an adjacent encapsulated element is connected to the metal tube 3 of the encapsulated valve module in a fluid-tight and well-conducting mode.

The semiconductor column 2 consists of a stacking of semiconductors in disc design 10 and cooling elements 11 and is closed on both ends by column heads 12. In direction of the stacking it is flown by the current I. The cooling elements 11 are of well conducting metal and ensure the current flow. For enabling current flow and heat flow in a sustainable way the column is provided with a tension arrangement. Such a tension arrangement may consist of tension bolts 13 of insulating fiber-reinforced plastic, which are connected with the column heads 12. The elastic elongation of the tension bolts serves for a sustainable pre-tension in the stacking. Such a pre-tension may also be provided by an additional system of springs. Centering pins, which interact between semiconductors, cooling elements and connecting elements, serve for alignment of the stacking.

The insulating end spacers 4 are in fluid-tight connection with the metal tube 3, and they may be connected to the column heads 12 as closed disks or cups. This allows an individual filling of the modules with insulating fluid as early as in manufacturing. The end spacers e.g. consist of fiber-reinforced epoxy resin. For the even distribution of the d.c. component of the electrical field the end spacers may be provided with a conducting material addition or a conducting material coating.

The need of, and the axial coverage density with insulating intermediate spacers 5 conforms to many parameters and may be chosen accordingly. Quantities of influence are e.g. the diameter of the semiconductor, tension force, lateral forces at transport and electromagnetic lateral forces in an electrical failure, and bending natural oscillation modes of the column. The insulating intermediate spacers consist of the same materials as the end spacers 4. The insulating intermediate spacer may sit on a metal spacer 14 in the semiconductor column 2. It is featured with openings for tension bolts, cooling fluid manifold tubes and firing pulse cabling, and for the even distribution of the insulating fluid in the valve module. It is supported radially in the metal tube 3 and electrically contacts the metal tube.

The shielding tubes 7 envelop the semiconductor column over the full length, involving tensioning system, cooling fluid manifold tubes, as well as electrical controls and electrical snubbers of the semiconductors. Each of the shielding tubes is made electrically conductive. A metal cylinder or a plastic tube with a conductive material addition may be used. A shielding tube extends over several semiconductor places, and it is connected by a potential link 15 with one of the covered semiconductors. Two possible designs of the potential link are shown: a contact spring arrangement 15a and a wire connection 15b. The shielding tube is featured with rounded ends 16, which smooth out the electrical field between the shielding tubes 7 and to the metal tube 3. The spacings between the shielding tubes are rated according to the number of tubes and the blocking voltage of the valve module. The shielding tube may be a closed tube or it may be built up of segments.

The metal tube 3 features a wall thickness, which resists the pressure of the insulating fluid. The shielding current in the metal tube is in the height of the conductor current in the module, and it flows with a characteristic penetration depth according to frequency and conductivity. The wall thickness should at least comply with the penetration depth. A suited material is for example aluminium with a wall thickness in the range of 8 mm to 20 mm. The metal tube may be cast or made of plate material, which is rolled and welded. For a connection to adjacent tubes the metal tube is equipped with flanges 17 on both ends. Screw connection 18 and gaskets 19 serve for fluid-tight connection. The screw connection also secures the continuous electrical connection among the metal tubes. By that the shielding effect for leaking magnetic a.c. fields is ensured.

The feeding of the cooling fluid to the cooling elements 11 is done in parallel from a distribution pipe 20 of insulating material, which extends along the column 2, and which is connected to the cooling elements by hoses 21. Hoses 22 and a collector pipe 23 serve for the discharge of the cooling fluid.

The cooling fluid has a weak electrical conductivity and a maximum allowable electrical operating field strength. The supply of the cooling fluid from the connector nipple 24 at ground potential to the distribution pipe 20, and vice versa from the collector pipe 23 to the discharge, is provided by insulating hoses 25, which are spirally wound along the end spacer 4 for achieving the required potential separation distance.

Fig. 2 shows in detail two embodiments of the cooling fluid supply. For even distance between the turns of the insulating hose 25, and thus for optimum field distribution, combs 26 of insulating material may be arranged radially along the end spacer 4. An integration of the insulating hose 25 into the end spacer 4 is also shown.

Fig. 3 shows in detail an embodiment for the contacting of the intermediate spacer 5 in the metal tube 3. A circular beveled metallic shoulder 27 is positioned in the metal tube 3. It is contacted by a contact spring 28, which itself contacts to a shaped metallic potential ring 29. The potential ring 29 closes around the circumference of the insulating intermediate spacer 5. The arrangement allows the absorbing of cycling thermal expansion while keeping sustainable electrical contacting of the intermediate spacer 5 to the potential of the metal tube 3. Circular O- rings 30 on both sides of the contact spring are provided to prevent rubbing-off products spreading into the insulating fluid 6. They also serve for damping of mechanical vibrations produced by current-induced forces. As an alternative the contact spring may be left out and the material of the O-ring is provided with an electrical conductivity.

Fig. 4 shows a longitudinal cut through an encapsulated valve module according to the invention, in which a series connection of a number of tube elements 31 encloses a semiconductor column 2. Intermediate flanges 32 thereby close via insulating intermediate spacers 33. These insulating intermediate spacers are guided radially tight and axially sliding on metal spacers 14. Thereby the cyclic thermal expansion is guaranteed. Details may be carried out similar to Fig. 3. The spaced shielding tubes 7 are designed preferentially as single piece for each tube element 31. The manufacturing of this valve module is carried out preferentially by vertical stacking of the column arrangement, going along with an intermittent stacking of the tube elements and the shielding tubes 7.

High currents in the encapsulated valve module may require cooling of the metal tube 3 with a cooling fluid. For that purpose, e.g. metallic cooling pipes 34 are arranged helically around the tube and connected to the tube by brazing, welding or another mode of connection, or they may be integrated by partly or fully casting into the tube wall. This cooling may be applied to all designs of encapsulated valve modules and may also be applied to pure connecting encapsulated elements.

An exemplary set-up of a 6-pulse converter bridge is shown in Fig. 5. The valve modules are by way of example equipped with series-connected diodes and represent a non-controlled rectifier bridge 35. The indicated number of series-connected semiconductors is exemplarily only, as in all figures. The leads of the three-phase system are, individually encapsulated, connected to the center taps of the bridge 36. The positive busbar 37 and the negative busbar 38 of the d.c. system are, individually encapsulated, connected to the bridge. By a connection of the tubes to ground potential there is no electrical field outside the tubes, which allows close distances between the tubes. Combined with the inherent networking of the tubes in the bridge, heavy connections between the metal tubes, shown as an example by the electrical connections 39a, 39b, 39c serve for a strong reduction of the magnetic a.c. fields outside the tubes. The space- saving arrangement additionally contributes to the reduction of outside magnetic fields. The inherent stable tubing makes the arrangement resistant against electromagnetic forces by short-circuit currents. Since the decay time constant L/R of the compensation currents in the metal tube is larger than the short-circuit time constants, the magnetic fields in a short circuit act circular around the semiconductor column inside the tube, which results, as desired, in low electromagnetic lateral forces on the semiconductor column.

Leads to arrangements with encapsulated valve modules may also be carried out as encapsulated multi-conductor systems. Fig. 6 shows, by way of example, the arrangement of a rectifier bridge 39 with a feeding of the three-phase system in one metal tube 40 and with conducting both d.c. polarities in one metal tube 41.

Fig. 7 shows the arrangement of a 2-level inverter with d.c. voltage link 42. The valve modules 1 are built up of semiconductors capable to be switched off, e.g. of the type GTO, ICT or IGBT. In the valve module a single-column arrangement with integrated freewheeling diodes may be used. Alternatively a multi-column semiconductor arrangement may be used, e.g. one column with GTO's and a parallel column with freewheeling diodes, whereas when applying electrical cross connections between the columns, one common snubber per semiconductor level may be applied. The voltage supporting system in the d.c. link, here in the form of the high-voltage capacity 43, consisting of a series and parallel arrangement of sub-capacities, is partly or fully embedded in the coaxial system. It thus results in a desired way an exactly predictable commutation inductivity, which facilitates the dimensioning. It also results a low commutation inductivity, which reduces switching voltage surges and losses. Accordingly it is possible to realize a 3-level circuit, a 5-level circuit, and so on in an encapsulated design. The required additional diodes are connected in individual tubing to the voltage steps of the d.c. link.

Fig. 8 shows in the example of a bridge circuit 44 an arrangement in encapsulated design with bushings at the connection points. Both the connections 45 of the 3-phase system and the connections 46 of the d.c. system are made to an overhead line system. Such a bridge circuit may be built up as one transport module and may be exchanged very quickly for maintenance or repair work.

A pole of a circuit breaker 47, built up of valve modules according to the invention is shown in Fig. 9. The design consists of two encapsulated valve modules 1 arranged in antiparallel with built-in, unidirectional conducting, controlled semiconductors. Such semiconductors are for example thyristors. By way of example, a third metal tube 48 is connected in parallel, which contains a series arrangement of metal-oxide varistors 49 for the absorption of the switch-off energy. By way of example, a bushing 50 is flanged to both ends, such that the connections 51 of the circuit-breaker pole can be connected to an overhead line system. Three such circuit- breaker poles 47 form a circuit breaker of a three-phase system. Such an arrangement has strongly reduced outer magnetic fields only in case of short and heavy electrical connections between the metal tubes on both ends of the circuit breaker. Fig. 10 shows a variant of a circuit-breaker pole 52 with two antiparallel columns, which are arranged in the same metal tube. Such switching poles may also be arranged in the matrix arrangement of a direct converter, connecting every phase of a first multi-phase system to every phase of a second multi-phase system.

Fig. 11 shows another design solution for the encapsulated valve module 1 according to the invention. The semiconductor column 2 is enveloped by a mechanically reinforced insulating tube 53, the ends of the tube extending over the column 2 and being fixed in covers 54 on both ends of the metal tube 3. The column heads 55, 56 are supported with positive fit by the insulating tube. Longer columns may be mechanically stabilized by intermediate supports 57. An intermediate support 57 centers a cooling element 11 in the insulating tube 53. The inner support may also be on a metal spacer 14, in analogy to Fig. 1. The mechanically reinforced insulating tube consists e.g. of a glass-fiber reinforced epoxy resin. Connecting elements 58, 59 extending sideward, which penetrate the insulating tube, are connected to the column heads 55, 56. The connecting elements lead through side openings 60 in the metal tube. Insulating end spacers 61 hold the connecting elements and seal the valve module. Cyclic expansion of the semiconductor columns may be facilitated by an electrical sliding contact between column head 56 and connecting element 59. In this arrangement the supply of cooling fluid is provided by an insulating hose 62 helically wound along the free end of the insulating tube. By addition of electrically conducting material, the insulating tube 53 is provided with a specific conductivity, and it is electrically coupled to the column heads 55, 56, to the intermediate supports of the column 57, and to the metallic covers 54. The insulating tube featured in such way takes over the function of a continuous shielding electrode around the semiconductor column. The longitudinal conductivity is tuned in such way that in blocking state the electrical losses in the weakly conducting insulating tube are thermally affordable. In an embodiment the insulating tube 53 may be coupled to every cooling element 11 and thus provides a static voltage balance among the semiconductors 10. By example the module base electronics 63 is shown, which generates the light impulses for the semiconductors. It is arranged inside of the valve module and it is supplied from the outside with electrical energy and control signals. The module base electronics supplies the control signals to optical waveguides, which are conducted to the semiconductors 10 in an insulating optical waveguide tube 64.

As a design alternative an axial connection may be used, as shown in Fig. 1. In such a case the insulating tube 53 is extended coaxially to the connection, and it ends with a positive fit and electrically contacting in the insulating end spacer 4.

Fig. 12 shows a section of the circuit arrangement of a valve column making use of series- connected directly light-triggered thyristors 65, similar to an application as disc elements in high-power converters. The firing pulse to the gate 66 is transferred optically by an optical waveguide 67, which is conducted from the module base electronics 63 via the optical waveguide tube 64 to the thyristor. Since the optical waveguides are electrically insulating such a control is especially advantageous for series-connected semiconductors at high voltage level. A surge voltage protection, symbolized by a breakover diode 68, which protects the thyristor against destructive overvoltages, is for modern elements integrated in the thyristor. External snubber elements for the operational surge voltage limitation at each thyristor are the dynamic snubber in the form of a series connection of snubber resistor R_dyn and snubber capacitor C_dyn, and the static snubber in the form of the resistor R_stat-

Fig. 13 shows the cross section through the practical design of a valve column making use of directly light-triggered thyristors according to Fig. 12. The cut is done in the plain between cooling element 11 and thyristor 65, the view goes on the cooling element 11 , which, besides the cooling function, forms a conducting bridge between two thyristors. Cams 69 at the cooling element have holes 70, through which the tension bolts 13 are conducted. The cut tube 71 stands for a shielding tube 7 or an insulating tube 53. The full-length, made weakly conducting insulating tube 53 may, as mentioned, assist for the static voltage division among the thyristors, or even may integrate the function of the resistor R_stat. E.g. a spring-force contacting between cam 69 of each cooling element and the inner surface of the insulating tube satisfies the requirement. The snubber resistor R_dyn is visible, as it is flanged to the side of the cooling element and which discharges its dissipated heat directly in the cooling element. It is connected with two parallel-connected elements of the snubber capacitor Cd_yn- The optical waveguide tube 64 conducts the optical waveguides to the optical gates of the thyristors. A branching-off optical waveguide 67 is shown. It passes on the cooling element in a radial oriented cavity in the thyristor flange and in the center turns in column direction into the thyristor core. Further shown are the pipes for the cooling fluid 20, 23 as well as the hoses 21 , 22, which connect the cooling elements to the cooling circuit.

Fig. 14 shows a longitudinal cut through an encapsulated valve module 1 with a twin-column arrangement of the semiconductors and a tube element 72 with enlarged diameter over the length of the column arrangement. Between the column heads 73, 74 two parallel-oriented semiconductor columns are arranged, here in the form of the antiparallel thyristor columns 75, 76. The axial connections 77 lead to adjacent elements of the encapsulated system having metal tubes of the small diameter. At each end of the module a stepped or conical tube element 78 for the adaption of the diameter is arranged. The insulating end spacers 79 are inserted in between. Longer modules have insulating intermediate spacers 80, which fix the individual columns mutually and against the tube, whereas the intermediate spacer may slide in the metal tube in longitudinal direction, in analogy to Fig. 1. The center part may be buiit up of several tube elements 72 arranged in a row, in analogy to Fig. 4. For equal clamping force each column has a cylinder 81 with a package of belleville washers 82, which are arranged in cavities in one column head 74, whereas a ribbed contact 83 transfers the current. The tension bolts 13 in such a solution may be provided with stops to the column heads, the tension is then secured by the spring package.

There is a possibility to reduce leaking magnetic field outside the metallic tubing in the d.c. circuits of encapsulated converters according to the invention to small residual amounts. An auxiliary current supply actively produces a current in the tubing, which is in opposite direction to the conductor current and which amounts to approximately the conductor current. The principle of function is explained in Fig. 15 on a high-voltage d.c. transmission system with converters 84 and 85 and a connecting d.c. line 86 with individual encapsulation of the conductors. The d.c. line current is stabilized with a smoothing choke L in known art. The choke must not be arranged inside the encapsulated system as shown, however the tube walls of the two connections must be interconnected. The same applies to other series-connected components, as e.g. switches. The electrically closed tube circuit is electrically opened at a given location, with e.g. an insulating disc 87 and insulated bolting. A controllable auxiliary rectifier 88, supplied by an auxiliary system, is connected across the separation in the tube. It feeds a d.c. current l_comp in the tube circuit, which is of same amount as the actual conductor current be and which flows opposite to the conductor current. The actual conductor current ID_C is measured with a current transducer 89 on the a.c. supply and converted in the auxiliary rectifier with known and fixed proportionality into a d.c. current. The circuit not only reduces outside magnetic fields of the encapsulated line 86, but also reduces d.c. components of outside magnetic fields of the encapsulated valve modules 1 of the converters. In that case the electrical connection 39c between the metal tubes in Fig. 5 has to be equipped with a series capacity, to allow the d.c. compensation currents flow over the tubing of the bridge branches. Because of the low resistance of the metal tubing, which usually is lower than the conductor resistance, the required power of the auxiliary rectifier is small when compared to other losses of the d.c. transmission system. Short-circuit currents in the d.c. system do not need to be actively compensated since here the inductive compensation comes in action, because the time constant L/R of the tube system is usually larger than the short-circuit time constant of the transmission system. Overvoltages at the connection of the auxiliary rectifier during short circuits are reduced by a voltage surge arrester, which may e.g. consist of a series connection of diodes.

Claims

1. Electrical valve module for a high-voltage circuit arrangement, built up of series-connected semiconductors in a column arrangement, comprising at least of one column (2), characterized in that the column arrangement is arranged coaxially in the center of an electrically good conducting and fluid-tight tube (3) in encapsulated design, the current flow in the column arrangement substantially passes between the two end regions of the tube, the interior of the tube is filled with an insulating fluid (6), and the column arrangement, at least in the two end regions of the tube, is mechanically supported in the tube via insulating spacer means (4, 5, 33, 53, 79, 80).

2. Encapsulated valve module according to claim 1 , characterized in that the semiconductors are realized in disc design (10), the semiconductors with periodical insertion of cooling elements (11) are stacked to a column, and the column is mechanically pressed by a tensioning device (12, 13) in longitudinal direction.

3. Encapsulated valve module according to claim 2, characterized in that the cooling elements (11) are supplied with electrically low-conducting cooling fluid, especially with a cooling liquid, via at least one insulating manifold pipe for cooling fluid (20), which extends along the column, and via hoses (21 , 22) in parallel supply, and that at least a same insulating collector pipe for cooling fluid (23) is provided for the discharge of cooling fluid from the cooling elements.

4. Encapsulated valve module according to claim 3, characterized in that at least one insulating hose (25, 62) connects a pipe for cooling fluid (20, 23) with a cooling fluid connector (24) in the wall or in the cover of the tube (3), and that the insulating hose is wound inside the tube (3) spirally or helically around the axis of the tube with spaced windings to achieve the required potential separation.

5. Encapsulated valve module according to claim 1 , characterized in that a plurality of tube elements (31) is fluid-tight and electrically connecting arranged in a row, between which insulating spacers (33) of the column arrangement are flanged.

6. Encapsulated valve module according to claim 5, characterized in that over the length of the column arrangement tube elements (72) have a larger diameter than in the connecting regions.

7. Encapsulated valve module according to claim 1 , characterized in that the column arrangement is coaxially enveloped by an array of spaced electrically conducting shielding tubes (7), which for equalized dielectric stress have rounded ends, and every shielding tube is coupled electrically to one component of the column section, which it covers.

8. Encapsulated valve module according to claim 1 , characterized in that the electrical connections are fed through the insulating end spacers (4, 79) on the tube axis.

9. Encapsulated valve module according to claim 1 , characterized in that the column arrangement is enveloped by a full-length, mechanically reinforced insulating tube (53) with tuned electrical conductivity, which insulating tube is at least at the two ends of the column arrangement connected to column heads (55, 56) with a positive fit, and is electrically coupled to these.

10. Encapsulated valve module according to claim 9, characterized in that the insulating tube (53) extends axially to at least one cover (54) of the tube (3) and is supported there, and the electrical connection is led radially through the insulating tube (53) and through an insulating disk (61) in a sideward flange in the tube (3).

11. Encapsulated valve module according to claim 9, characterized in that the insulating tube (53) axially extends up to at least one insulating end spacer in the tube, the insulating tube sits with a positive fit and electrically connecting in the insulating end spacer, and the electrical connection is fed on the tube axis through the insulating end spacer.

12. Encapsulated valve module according to claim 1 , characterized in that the insulating fluid (6) is a liquid insulation means, especially a natural or synthetic insulation oil.

13. Encapsulated valve module according to claim 1 , characterized in that the insulating fluid (6) is a gaseous insulating means, which is especially a gas mixture, and that the gaseous insulating means is at higher pressure than atmospheric pressure.

14. Encapsulated valve module according to claim 1, characterized in that the encapsulated valve modules (1) are parts of a circuit breaker, and each circuit-breaker pole (47) is equipped with two tubes of semiconductors, preferentially thyristors, arranged in parallel and with opposite valve direction.

15. Encapsulated valve module according to claim 1, characterized in that the encapsulated valve modules (1) are parts of a circuit breaker, and each circuit-breaker pole (52) comprises one tube, which contains two columns of semiconductors of opposite valve direction, preferentially thyristors, or one column with bidirectional semiconductors, preferentially bidirectional thyristors.

16. Encapsulated valve module according to claim 14 or 15, characterized in that columns of metal-oxide varistors (49) are arranged electrically parallel to the semiconductor columns, jointly with the semiconductor columns or in a separate tube (48).

17. Encapsulated valve module according to claim 1 , characterized in that encapsulated valve modules (1) are branches of a converter bridge circuit (35, 39, 42, 44), and the bridge circuit is built up with encapsulated busducting.

18. Encapsulated valve module according to claim 17, characterized in that the elements of the d.c. link, especially capacitors (43) of a d.c. voltage link, are integrated in the encapsulated circuit.

19. Encapsulated valve module according to one of the claims 14 to 18, characterized in that the walls of the tubes are cross-linked by electrical cross connections (39) or via an electrically conducting frame.

20. Encapsulated valve module according to claim 1 , characterized in that the controllable semiconductors (65) are controlled via optical waveguides (67), and one or several module base electronics (63) are arranged in the tube wall, which convert incoming control signals into light pulses for the optical waveguides.

21. Method for the reduction of the outer magnetic field in the encapsulated d.c. circuit of a converter bridge circuit making use of encapsulated valve modules, characterized in that a d.c. current of same amount and in opposite direction to the conductor current, is fed into the tube loop of the d.c. circuit (86), said d.c. current being produced by a controlled supply device (88) and fed into the tube loop over an electrical separation (87) in the tube loop.