EP0374062A2 - Verfahren und Vorrichtung zur Trocknung der Papierisolation eines elektrotechnischen Hochspannungsapparates und Mikrowellenenergieapplikationsvorrichtung zu diesem Zweck - Google Patents

Verfahren und Vorrichtung zur Trocknung der Papierisolation eines elektrotechnischen Hochspannungsapparates und Mikrowellenenergieapplikationsvorrichtung zu diesem Zweck Download PDF

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Publication number
EP0374062A2
EP0374062A2 EP89420493A EP89420493A EP0374062A2 EP 0374062 A2 EP0374062 A2 EP 0374062A2 EP 89420493 A EP89420493 A EP 89420493A EP 89420493 A EP89420493 A EP 89420493A EP 0374062 A2 EP0374062 A2 EP 0374062A2
Authority
EP
European Patent Office
Prior art keywords
coaxial
microwave
transmission line
microwaves
water
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP89420493A
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English (en)
French (fr)
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EP0374062A3 (en
EP0374062B1 (de
Inventor
Pierre Gervais
Michel Duval
Marcel Giroux
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hydro Quebec
Original Assignee
Hydro Quebec
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Filing date
Publication date
Application filed by Hydro Quebec filed Critical Hydro Quebec
Priority to AT89420493T priority Critical patent/ATE73921T1/de
Publication of EP0374062A2 publication Critical patent/EP0374062A2/de
Publication of EP0374062A3 publication Critical patent/EP0374062A3/fr
Application granted granted Critical
Publication of EP0374062B1 publication Critical patent/EP0374062B1/de
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/80—Apparatus for specific applications
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B2206/00—Aspects relating to heating by electric, magnetic, or electromagnetic fields covered by group H05B6/00
    • H05B2206/04—Heating using microwaves
    • H05B2206/046—Microwave drying of wood, ink, food, ceramic, sintering of ceramic, clothes, hair

Definitions

  • the present invention relates to a method of drying, by microwave, a multi-layer paper insulator, forming part of a high-voltage electrotechnical device, such as a transformer bushing.
  • the invention also relates to a microwave energy applicator and a dryer containing such an applicator, the latter as well as the dryer being intended for carrying out the method.
  • bushings pass through the metal casing, in order to allow a connection, from the outside, with the winding terminals.
  • One such crossing includes a conductor wrapped in multi-layer paper insulation.
  • the paper insulation, as well as part of the conductor, are housed inside a cylindrical ceramic envelope, hermetically sealed and filled with oil. The paper insulation is therefore moistened with this oil.
  • the main object of the present invention is therefore to provide a quick and inexpensive means of drying the paper insulation of the bushings, of the type mentioned above, by means of microwaves.
  • a method for drying a dielectric insulator, permeable to water and microwaves, forming part of an electrotechnical device comprising an electrical conductor, internal and elongated, wrapped in the aforementioned insulator infiltrated with water
  • the method consists in : mounting said electrotechnical device in a hollow tubular element made of an electrically conductive material, the electric conductor being coaxial with said tubular element, thereby forming a coaxial line for transmission of microwaves, - transmit microwaves through the coaxial transmission line to heat the water infiltrated into the dielectric insulator and transform it into water vapor, - And evacuate the water vapor outside said coaxial transmission line.
  • the invention also provides a microwave energy applicator for applying microwave energy to water infiltrated into a dielectric insulator, permeable to water and microwaves, forming part of the invention.
  • an electrotechnical device comprising an elongated internal electrical conductor, wrapped in the aforementioned insulation, said applicator comprising: - a hollow tubular element made of an electrically conductive material, means for mounting said electrotechnical device in this tubular element, said electrical conductor being coaxial with the tubular element, so as to form a coaxial microwave transmission line capable of propagating the microwaves through the dielectric insulator to apply the microwave energy to the infiltrated water.
  • the present invention further relates to a device for drying a dielectric insulator, permeable to water and microwaves, forming part of an electrotechnical device comprising an elongated internal electrical conductor, wrapped in the aforementioned insulator infiltrated with water, this device comprising: - a hollow tubular element made of an electrically conductive material, means for mounting said electrotechnical device in this tubular element, said electrical conductor being coaxial with said tubular element, so as to form a coaxial line for transmission of microwaves, means for propagating microwaves through the coaxial transmission line, and therefore through the dielectric insulator, to heat said water infiltrated into the dielectric insulator and transform it into water vapor, - And means for discharging water vapor outside said coaxial microwave transmission line.
  • Microwave drying of the insulation of an electrotechnical device is quick and inexpensive, so that it becomes more advantageous to recondition the electrotechnical device by drying its dielectric insulator , rather than replacing it.
  • the two apparatuses for drying a paper insulator each have a microwave energy applicator 1 .
  • the applicator 1 has a first end which is connected to a first microwave window 2 and a second end which is connected to a second microwave window 3 .
  • FIGS. 2A, 2B and 2C The structure of the applicator 1 and of the windows 2 and 3 appears in detail in FIGS. 2A, 2B and 2C .
  • the applicator 1 first comprises a tubular metallic element 4 , electrically conductive, namely a circular cylinder in cross section, in which is mounted the bushing whose insulator is to be dried.
  • the bushing 5 is designed to be used in a high voltage transformer.
  • a crossing 5 comprises a metallic conductor 6 , namely a hollow elongated tube whose ends are threaded externally to allow an electrical connection.
  • the conductor 6 is straight and cylindrical, that is to say circular in cross section. It is, on the other hand, insulated by means of a dielectric insulator 7 of multi-layer paper, enveloping said conductor 6 .
  • a dielectric insulator 7 of multi-layer paper enveloping said conductor 6 .
  • one end of the insulator 7 of multi-layer paper is perpendicular to the conductor 6 , while the other end of the insulator is conical.
  • the bushing 5 has been removed from its ceramic envelope and, as said above, its paper insulation is impregnated with oil, but also contains water which has got there. infiltrated.
  • the bushing 5 is held in position inside the cylinder 4 by means of two annular end supports 8 and 9 , interposed between the paper insulation 7 and the internal surface of the cylinder 4 .
  • the two supports 8 and 9 are made of a dielectric material permeable to microwaves and are provided with passages, such as 9 ′ in FIG. 2B , passing right through each support when the supports 8 and 9 are inserted between the cylinder 4 and the paper insulator 7 of the bushing.
  • the metal cylinder 4 and the conductor 6 form a coaxial microwave transmission line, capable of propagating the microwaves. More precisely, the conductor 6 constitutes the internal conductor of the line, the cylinder 4 , the external conductor of the same line and the paper insulator 7 acts as a dielectric in the coaxial transmission line.
  • the cylinder 4 is provided with an upper and vertical exhaust duct 10 , two lower and vertical exhaust ducts 11 and 12 and as well as two collar end fittings 13 and 14 .
  • the microwave energy applicator 1 comprises, in in addition, two sets of coaxial-to-coaxial transition 15 and 16 provided at their respective ends.
  • Each of the transition assemblies 15 , 16 comprises a tapered waveguide section 17 , 18 (external conductor), of circular cross section and a conical internal conductor 19 , 20 coaxial with the waveguide section 17 , 18 and, also, of circular cross section.
  • the waveguide section 17 is provided with a collar connection 21 at its close end and another collar connection 26 at its remote end, the two connections being conventional.
  • the waveguide section 18 is provided with a flange connection 23 at its close end and with another flange connection 24 at its remote end, the two connections being conventional.
  • the collar fittings 13 and 21 are fixed together to form an airtight seal between them.
  • An airtight seal is also obtained between the collar connections 14 and 23 of the cylinder 4 and of the waveguide section 18 .
  • the production of such seals is well known to those skilled in the art and it is therefore not necessary to specify further.
  • the internal conductor 19 is provided with a pin 19 ′ , at its close end, this pin 19 ′ being inserted under pressure into the corresponding end of the hollow tube 6 .
  • the internal conductor 20 is provided with a pin 20 ′ , at its close end, which pin 20 ′ is also inserted under pressure into the other end of the hollow tube 6 .
  • the internal conductors 19 and 20 are mounted on the hollow tube 6 , which conducts electricity.
  • the microwave window 2 comprises a section 27 of standard 50 ⁇ waveguide, of circular cross section which comprises, at its close end, a standard collar connector 28 , fixed to the collar connector 26 , to make a airtight seal between fittings 26 and 28 .
  • the waveguide section 27 further includes a standard flange connection 29 at its remote end.
  • the microwave window 3 comprises a section 30 of standard circular waveguide of 50 ⁇ having a standard collar connection 31 , at its close end, attached to collar fitting 24 . Also, an airtight seal is formed between the collar fittings 24 and 31 .
  • the waveguide section 30 obviously has a standard collar connection 32 at its remote end.
  • Each of the two microwave windows 2 , 3 also has an internal central conductor 33 , 34 coaxial with the section 27 , 30 of waveguide and of circular and constant cross section.
  • the hollow end, brought closer to each conductor 33 , 34 is forced inside a coaxial hole dug in the end remote from the internal conical conductors 19 , 20 , so that the conductor 33 , 34 is fixed to the conductor 19 , 20 .
  • Each window 2 , 3 comprises, in addition, an annular stop 35 , 36 , made of a dielectric material permeable to microwaves.
  • Each stop 35 , 36 can be slid longitudinally inside the waveguide section 27 , 30 and thus around the internal conductor 33 , 34 .
  • a first O-ring 37 , 38 seals the seal between the stop 35 , 36 and the bore of the waveguide section 27 , 30
  • a second O-ring 39 , 40 seals the seal between the stop 35 , 36 and the internal conductor 33 , 34 .
  • the stop 35 , 36 keeps the conductor 33 , 34 coaxial with the waveguide section 27 , 30 and its position inside the waveguide section is adjusted so as to obtain an optimal reflection coefficient which is as low as possible.
  • the window 2 , the transition assembly 15 , the cylinder 4 , the conductor 6 , the transition assembly 16 and the window 3 constitute, overall, a coaxial microwave transmission line, capable to propagate microwaves whose external conductor is constituted by the coaxial sections of waveguide 27 and 17 , the cylinder 4 and the sections 18 and 30 of waveguide and whose internal conductor is constituted by conductors 33 , 19 , 6 , 20 and 34 aligned on a common axis.
  • the function of the transition assembly 15 is to adapt the impedance of window 2 to the impedance of the coaxial line of transition formed by cylinder 4 and conductor 6 .
  • the internal diameter of the waveguide section 17 gradually increases from the internal diameter of the waveguide section 27 to the internal diameter of the cylinder 4 , while the internal conical conductor 19 sees its external diameter increase gradually, from that of driver 33 to that of driver 6 .
  • the transition assembly 16 likewise adapts the impedance of the coaxial transmission line formed by the cylinder 4 and the conductor 6 to that of the microwave window 3 .
  • the internal diameter of the waveguide section 18 decreases gradually from the internal diameter of the cylinder 4 to the internal diameter of the waveguide section 30 , while the internal conical conductor 20 also sees its external diameter gradually decrease, from that of conductor 6 to that of conductor 34 .
  • Impedance matching using transition assemblies 15 and 16 , prevents the formation of a standing wave in the coaxial microwave transmission line that includes window 2 , transition assembly 15 , cylinder 4 , the conductor 6 , the transition assembly 16 and the window 3 , due to the reflections that would cause the absence of such an adaptation of impedances.
  • a standing wave of this nature has maximums and minimums and would cause uneven heating of the water in the paper insulation.
  • the windows 2 and 3 , the transition assemblies 15 and 16 , the cylinder 4 and the conductor 6 define an airtight annular enclosure.
  • the paper insulation drying apparatus comprises a high power source 41 of microwaves, advantageously constituted by a magnetron generator.
  • the microwaves, coming from the source 41 are transmitted to the window 2 by means of a conventional waveguide-to-coaxial transition device (not illustrated), connected to the standard collar connection 29 of the guide section. wave 27 ( fig. 2A ).
  • the microwaves, coming from the source 41 pass through the window 2 , the applicator 1 and the window 3 and are transmitted to a suitable load 42 through a conventional coaxial-to-waveguide transition device (not illustrated). ) attached to standard fitting 32 ( fig. 2A ).
  • the load 42 being adapted, there is no reflection, since the load then absorbs all the energy of the microwaves which propagate through the windows 2 and 3 and through the applicator 1 , without being absorbed by the water itself. It follows that a standing wave does not form in the applicator 1 , so that the water is heated uniformly throughout the paper insulator 7 , due to a uniform distribution of the electric field in the transmission line.
  • the high power microwave source 41 transmits microwaves to window 2 , through a circulator 45 connected to window 2 , by means of a waveguide-to-coaxial transition device (not shown) which is fixed to the standard flange connection 29 ( fig. 2A ).
  • the microwaves propagate through the window 2 , the applicator 1 and the window 3 and are transmitted to an adjustable short circuit 47 connected to the collar connection 32 ( FIG. 2A ) of the waveguide section 30 , using a conventional coaxial-to-waveguide transition device (not shown).
  • the microwaves which reach the short circuit 47 are reflected and transmitted to a suitable load 46 through the window 3 , the applicator 1 , the window 2 and the circulator 45 .
  • a standing wave with maximums and minimums is therefore produced inside the coaxial line formed by the cylinder 4 and the conductor 6 .
  • the short circuit 47 is displaced to move the minimums and maximums of the standing wave in the applicator 1 , in order to obtain a heating of the water which is uniform in the insulator 7 of paper.
  • the energy of the microwaves, reaching the appropriate charge 46 is absorbed by the latter, without significant reflection of the microwaves.
  • the microwaves which propagate through the coaxial transmission line formed by the cylinder 4 and the conductor 6 , cause the water molecules to oscillate in the paper insulator 7 .
  • the water is thus heated and transformed into water vapor.
  • a vacuum pump 43 sucks the water vapor thus produced through the upper exhaust duct 10 of FIG. 2A , in order to improve and speed up the drying process, by forcing the evacuation of water vapor from the airtight enclosure. It is therefore important that the O-rings 37 , 38 , 39 and 40 and the pairs of collar fittings 26 , 27 ; 13 , 21 ; 14 , 23 and 24 , 31 create airtight seals, in order to obtain an airtight enclosure which is discussed above.
  • the oil which permeates the paper insulator 7 , is also heated by microwaves and by steam, so that its fluidity increases and that it flows by gravity towards an oil trap 44 , by means of two lower exhaust pipes 11 and 12 , this being true for the two embodiments of FIGS. 1 and 3 .
  • the vacuum pump 43 can be replaced by a cryogenic trap using liquid nitrogen ( N2 ) or carbon dioxide ( CO2 ), such a cryogenic trap being, of course, well known in the art.
  • N2 liquid nitrogen
  • CO2 carbon dioxide
  • the paper insulation 7 is dried quickly by microwaves, the drying time being counted in hours rather than days, as is the case for drying in conventional ovens.
  • the drying of the insulator 7 becomes economical and the restoration at low cost of the crossbeams of high voltage transformers is therefore possible.
  • the frequency of the microwaves is important in that their wavelength must be close to the internal diameter of the cylinder 4 , so as to obtain a propagation according to the dominant TEM mode.
  • a cylindrical metallic shield 48 illustrated in broken lines in FIG. 2A , either provided in the paper insulator 7 .
  • two coaxial and parallel transmission lines are formed, the first being constituted by the conductor 6 and the shield 48 , while the second is constituted by the shield 48 and the cylinder 4 .
  • the present invention can also be used to dry the multi-layer paper insulation, but an appropriate adaptation of the impedances must then be made using the transition assemblies 15 and 16 and, optionally, adjusting screws.
  • the present invention can be used to dry the multi-layer paper insulation from high-voltage transformer bushings or any other high-voltage electrotechnical device, provided that they have a central internal conductor around which the paper insulation multilayer is wound and that such a conductor can be used as an internal conductor of a coaxial microwave transmission line.
  • the central conductor of the electrotechnical device need not be straight. In fact, it may be somewhat curved, provided that the tubular member 4 is similarly curved to form the required coaxial microwave transmission line.
  • the central conductor of the device may also exhibit discontinuities, provided that an appropriate impedance matching is carried out.
  • the present invention can also be used for a dielectric insulator other than paper, provided that such an insulator is permeable to water and microwaves.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Drying Of Solid Materials (AREA)
  • Insulating Bodies (AREA)
  • Paper (AREA)
EP89420493A 1988-12-15 1989-12-14 Verfahren und Vorrichtung zur Trocknung der Papierisolation eines elektrotechnischen Hochspannungsapparates und Mikrowellenenergieapplikationsvorrichtung zu diesem Zweck Expired - Lifetime EP0374062B1 (de)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT89420493T ATE73921T1 (de) 1988-12-15 1989-12-14 Verfahren und vorrichtung zur trocknung der papierisolation eines elektrotechnischen hochspannungsapparates und mikrowellenenergieapplikationsvorrichtung zu diesem zweck.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US284732 1988-12-15
US07/284,732 US4889965A (en) 1988-12-15 1988-12-15 Microwave drying of the paper insulation of high voltage electrotechnical equipments

Publications (3)

Publication Number Publication Date
EP0374062A2 true EP0374062A2 (de) 1990-06-20
EP0374062A3 EP0374062A3 (en) 1990-08-08
EP0374062B1 EP0374062B1 (de) 1992-03-18

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP89420493A Expired - Lifetime EP0374062B1 (de) 1988-12-15 1989-12-14 Verfahren und Vorrichtung zur Trocknung der Papierisolation eines elektrotechnischen Hochspannungsapparates und Mikrowellenenergieapplikationsvorrichtung zu diesem Zweck

Country Status (8)

Country Link
US (1) US4889965A (de)
EP (1) EP0374062B1 (de)
JP (1) JPH0610939B2 (de)
AT (1) ATE73921T1 (de)
CA (1) CA1317643C (de)
DE (1) DE68901033D1 (de)
ES (1) ES2030999T3 (de)
GR (1) GR3004093T3 (de)

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Publication number Priority date Publication date Assignee Title
SE9002117L (sv) * 1990-06-14 1991-08-26 Nils Elander Mikrovaagsanordning foer behandling av precessvaetskor
US5262610A (en) * 1991-03-29 1993-11-16 The United States Of America As Represented By The Air Force Low particulate reliability enhanced remote microwave plasma discharge device
GB9309202D0 (en) * 1993-05-05 1993-06-16 Apv Corp Ltd Microwave ovens
US5423260A (en) * 1993-09-22 1995-06-13 Rockwell International Corporation Device for heating a printed web for a printing press
US6244056B1 (en) 1995-09-20 2001-06-12 Sun Microsystems, Inc. Controlled production of ammonia and other gases
US5873258A (en) * 1995-09-20 1999-02-23 Sun Microsystems, Inc Sorption refrigeration appliance
US5916259A (en) 1995-09-20 1999-06-29 Sun Microsystems, Inc. Coaxial waveguide applicator for an electromagnetic wave-activated sorption system
AU707643B2 (en) * 1995-09-20 1999-07-15 Sun Microsystems, Inc. Absorbent pair refrigeration system
US5842356A (en) * 1995-09-20 1998-12-01 Sun Microsystems, Inc. Electromagnetic wave-activated sorption refrigeration system
US5855119A (en) 1995-09-20 1999-01-05 Sun Microsystems, Inc. Method and apparatus for cooling electrical components
US5869817A (en) * 1997-03-06 1999-02-09 General Mills, Inc. Tunable cavity microwave applicator
US7003979B1 (en) 2000-03-13 2006-02-28 Sun Microsystems, Inc. Method and apparatus for making a sorber
US6630654B2 (en) * 2001-10-19 2003-10-07 Personal Chemistry I Uppsala Ab Microwave heating apparatus
WO2007046085A2 (en) * 2005-10-19 2007-04-26 Clear Wave Ltd. Microwave oven window
ES2342958B2 (es) * 2008-09-03 2011-07-04 Emite Ingenieria Slne Analizador de multiples entradas y multiples salidas.
CN111854375A (zh) * 2020-07-27 2020-10-30 北京金辉景新节能科技有限公司 一种真空微波脱水设备

Citations (6)

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DE867670C (de) * 1950-11-30 1953-02-19 Siemens Ag Vorrichtung zur Vakuumbehandlung, insbesondere Trocknung
US2740756A (en) * 1951-04-19 1956-04-03 Albert G Thomas Electrical drying system
DE1278948B (de) * 1965-01-26 1968-09-26 Licentia Gmbh Vakuum-Trocknungsverfahren fuer porige Trocknungsgueter
US3430351A (en) * 1966-06-14 1969-03-04 Asea Ab Method of drying objects such as electrical machines and electrical insulating material
FR2448699A1 (fr) * 1979-02-06 1980-09-05 Ciba Geigy Ag Procede pour secher une bande continue de matiere par energie micro-onde et appareil pour la mise en oeuvre de ce procede
DE3630567A1 (de) * 1986-09-09 1988-03-10 Kurt Dipl Ing Zeppenfeld Zyklon-druckfilter mit mikrowellentrockner

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Publication number Priority date Publication date Assignee Title
US3452176A (en) * 1967-05-24 1969-06-24 Melvin L Levinson Heating a moving conductor by electromagnetic wave irradiation in the microwave region
US3590202A (en) * 1970-02-24 1971-06-29 Bechtel Corp Construction for tuning microwave heating applicator
US3887722A (en) * 1973-08-31 1975-06-03 United Aircraft Corp Method for producing a plurality of filaments in a radio frequency reactor
US4259560A (en) * 1977-09-21 1981-03-31 Rhodes George W Process for drying coal and other conductive materials using microwaves
SE417780B (sv) * 1980-01-22 1981-04-06 Por Microtrans Ab Dielektrisk uppvermningsanordning
FR2584258B1 (fr) * 1985-06-28 1995-04-14 Elf Aquitaine Procede et dispositif pour le traitement thermique d'un element conducteur au moins partiellement constitue d'un materiau conducteur
US4771156A (en) * 1986-10-20 1988-09-13 Micro Dry Incorporated Method and apparatus for heating and drying moist articles

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE867670C (de) * 1950-11-30 1953-02-19 Siemens Ag Vorrichtung zur Vakuumbehandlung, insbesondere Trocknung
US2740756A (en) * 1951-04-19 1956-04-03 Albert G Thomas Electrical drying system
DE1278948B (de) * 1965-01-26 1968-09-26 Licentia Gmbh Vakuum-Trocknungsverfahren fuer porige Trocknungsgueter
US3430351A (en) * 1966-06-14 1969-03-04 Asea Ab Method of drying objects such as electrical machines and electrical insulating material
FR2448699A1 (fr) * 1979-02-06 1980-09-05 Ciba Geigy Ag Procede pour secher une bande continue de matiere par energie micro-onde et appareil pour la mise en oeuvre de ce procede
DE3630567A1 (de) * 1986-09-09 1988-03-10 Kurt Dipl Ing Zeppenfeld Zyklon-druckfilter mit mikrowellentrockner

Also Published As

Publication number Publication date
GR3004093T3 (de) 1993-03-31
DE68901033D1 (de) 1992-04-23
ES2030999T3 (es) 1992-11-16
JPH0610939B2 (ja) 1994-02-09
ATE73921T1 (de) 1992-04-15
JPH02201824A (ja) 1990-08-10
US4889965A (en) 1989-12-26
EP0374062A3 (en) 1990-08-08
CA1317643C (fr) 1993-05-11
EP0374062B1 (de) 1992-03-18

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