US3805140A

US3805140A - Oil-filled ac-dc thyristor convertor

Info

Publication number: US3805140A
Application number: US00346807A
Authority: US
Inventors: E Takahashi; Y Kamata
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1973-04-02
Filing date: 1973-04-02
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16

Abstract

An AC-DC convertor employing a thyristor as a valve which is used in the high tension DC transmission circuit, in which insulated thyristors are multilaid in a metal container filed with insulating oil, whose inside wall is provided with an insulating barrier of solid with higher insulating resistor than that of the insulating oil, the insulating barrier being used mainly for the DC voltage applied between the thyristors and the inner wall of the metal container, thereby the AC-DC convertor being made small-sized.

Description

United States Patent [191 Takahashi et a1.

[ OIL-FILLED AC-DC THYRISTOR CONVERTOR [75] Inventors: Eiki Takahashi; Yuzuru Kamata,

both of Hitachi, Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Apr. 2, 1973 [21] Appl. No.: 346,807

[52] U.S. Cl. 321/8 R, 321/8 C [51] Int. Cl. H02m 7/00 [58] Field of Search 321/8, 8 C

[56] References Cited UNITED STATES PATENTS 3,173,061 3/1965 Storsand 321/8 C 3,234,451 2/1966 Diebold 321/8 C 3,369,166 2/1968 Lake 321/8 C Apr. 16, 1974 3,586,959 6/1971 Eccles et a1 321/8 3,708,740 1/1973 Pierson 321/8 R 3,733,503 5/1973 Potter 321/8 C X Primary Examiner--William M. Shoop, Jr. Attorney, Agent, or FirmCraig and Antonelli [5 7] ABSTRACT An AC-DC convertor employing a thyristor as a valve which is used in the high tension DC transmission circuit, in which insulated thyristors are multilaid in a metal container filed with insulating oil, whose inside wall is provided with an insulating barrier of solid with higher insulating resistor than that of the insulating oil, the insulating barrier being used mainly for the DC- v oltage applied between the thyristors and the inner wall of the meta] container, thereby the AC-DC convertor being made small-sized.

11 Claims, 17 Drawing Figures TENTEU'APR 16 I974 sugar 1 or 5 FFG.

PULSE GENERATOR PRIOR ART 6O PATENTEDAPR 1s m V 3.805140 SHEET 2 0F '5 PATENTEDAPR 16 I974 SHEET 3 [IF 5 m I a F E G. 7

I I I I I I: I I I I I PATENTEDAPR 16 I914 Y Y 33305; 1 10 SHEET 5 BF 5 F is. I6

OIL-FILLED AC-DC THYRISTOR CONVERTOR This invention relates to AC-DC convertor used in a high tension DC transmission circuit, and more particularly to an improvement on a thyristor AC-DC convertor of oil-filled type employing the thyristor as a valve.

The thyristor has been employed as a valve for AC-DC convertor used in a DC transmission circuit by high or extra-high voltage, since the thyristor of high breakdown voltage was developed.

In the conventional thyristor AC-DC convertor, the desired breakdown voltage thereof is obtained in such a manner that a plurality of thyristor trays being mutilaid are placed in air, each of which consists of a plurality of thyristors connected in series, and modules consisting of the resistors and capacitors for equalizing the voltage applied on thyristors.

However, in the AC-DC converter of this kind, there has been a disadvantage in that increased is the desired distance for insulation among the parts thereof and hence large-sized is the construction of the convertor. In addition, such a convertor must be installed in the house for protection against weather. The large-sized convertor necessitates the large-sized house with the result that the cost for building the house is increased.

An oil-filled thyristor AC-DC convertor, to avoid such a disadvantage, is used in which the above convertor is placed in the metal container filled with an insulating oil.

Since the thyristor stack or the module, in such an oil-filled convertor, may be constructed into a stage which in turn is immersed in insulation oil, the external terminal may be pulled out through an insulating bushing fitted to the wall of the metal container. Accordingly, it was expected that the converter might be made small in size, and has no use for the house in particular. However, it was found that such converter has a disadvantage. Since the DC electric field exists between the inner wall of the metal container and the thyristor stage of electrical charging section, impurities contained in the insulating oil are charged. As a result, the charge impurities adhere to the inner wall and the thyristor by attraction existing therebetween. This results in the reduced dielectric breakdown strength.

It is well known that the dielectric breakdown strength of mineral oil used in power transformer is 15 to 30 kV/mm in AC electric field, and is 5 to kV/mm in DC electric field which is about 7% of AC electric fields.

This result is due to the fact that in the AC electric field dielectric breakdown strength depends upon dielectric constant of the medium while in the DC electric field, it depends upon resistivity of the medium, and also due to the reduced dielectric breakdown strength mentioned on the above.

Accordingly, such an oil-filled converter needs an elongation in the distance for insulation between the thyristor stage on the high voltage section and metal container so that enlargement is required in the size of the metal container itself.

Thus, an object of this invention is to provide an oilfilled thyristor convertor in which dielectric breakdown strength is enhanced and the total construction thereof is made small-sized.

Another object of this invention is to provide an oilfilled thyristor convertor having an enhanced dielectric breakdown strength for the surge from the DC transmission line.

A further object of this invention is to provide an oilfilled thyristor convertor capable of being made smallsized in its entire construction by improving the insulation for DC voltage of the thyristor stage on the side thereof connecting to the high voltage line terminal.

To attain the objects mentioned above, in this invention, the inside wall of the metal container is provided with a solid insulating barrier formed of oil-filled solid insulating material which has high dielectric breakdown strength against either AC electric field or DC electric field, for example an insulating paper such as a press board, manila paper, and linter paper, and a design is made so as to apply most DC electric filed on the insulating barrier.

Generally, the metal container includes a plurality of unit containers each of which is provided with a flange member on the upper side and the lower side thereof and unitely combines them with the flanges.

In construction work, the insulating barrier may be I fitted to the inside wall of the metal container after combining the unit container. Also, it may be fitted to the wall of each unit container before combining them. The latter is preferable in construction work, and has a disadvantage that no covering portion with the insulating barrier occurs on the jointed part of the flanges.

Therefore it is an object to provide an oil-filled thyristor converter in which weak insulation occurring on the joint portion of the flanges is eliminated.

It is preferred in the oil-filled thyristor converter that a cooling system is provided to the metal container to remove heat evolved from the thyristors. The metal container with the cooling system has an inlet or outlet to pass insulating oil therethrough. The inlet or outlet portion has become a weak point for insulation.

Thus, other object of this invention is to eliminate incomplete insulation on the inlet or outlet portions.

In case a press board is used as the insulating barrier, it is observed that the dielectric breakdown strength of the insulating barrier is to kV/mm in the DC electric field, and 40 to 60 kV/mm in AC electric field. The former is about 20 times as large as that of mineral oil while the latter is about two times as large as that of mineral oil. As a result, it is possible to shorten the insulating distance to the given voltage of the transmission circuit.

Considering the surge from the transmission line, it is a sort of high frequency voltage having a high rate of change. Thus, most current flow is determined by the dielectric constant of the insulating material. Accordingly, the voltage share to the insulating barrier and to insulating oil at initial stage, is determined by the dielectric constant.

The dielectric constant of oil-filled paper is usually 3.5 to 3.6 while that of insulating oil is about 2.2. The ratio of those is about l/0.64. Suppose that the thickness of the insulating barrier is equal to that (distance) of insulating oil, the ratio of 1/0.64 shows that the voltage share to the insulating barrier at initial stage is relatively larger than that of the insulating oil.

In this invention, a double insulating construction consisting of insulating oil and solid insulating barrier are provided between the thyristor stage and the metal container. DC electric field, in the stationary operation the metal container. Thus, the voltage share to those insulators is determined by the rate of resistivity of insulating oil to that of the insulating barrier. The resistivity of the former is about 10 Qm while that of the latter is about 10 (Im. The former is 1/100 times the latter. Consequently, the insulating barrier mainly bears the applied voltage.

Thus, the solid insulating barrier, in this invention, is attached to the inner wall of the metal container of the oil-filled thyristor convertor. In this arrangement, insulation in the stationary operation may be made mostly by the insulating barrier. Accordingly, an oil space existing between the thyristor and the insulating barrier may be designed without considering the breakdown voltage of DC, only by considering cooling space for the thyristor and the distance required to reduce electrostatic capacitance between the thyristor and ground.

An effective construction insulation may be obtained by such a way that the thickness of the insulating barrier is reduced in stepwise from the thyristor stage on the high voltage side to that on the lower voltage side.

The insulating barrier may be constructed such that a plurality of barriers instead of a single barrier of given thickness are multilaid being inserted ductpieces thereamong so as to form an oilspace.

The thyristor stage is supported in the metal container by insulators such as an epoxy insulator or porcelain insulator. It is desirable, however, that the insulator disposed between the inside wall of the metal container and the opposited thyristor stage is the corrugated one whose trough is filled with the insulating barrier thereby a creep insulating distance is made elongated.

To enhance the insulation of the joint portion of the flange of the metal container and the oil inlet or outlet portion in the cooling system provided in the metal container, the insulating'barrier extending to those portions are multilaid. In this case, an oil path is formed by inserting the ductpieces between the multilaid barrier. The insulating bushing in the metal container on the high voltage side must be reduced in the longitudinal direction to make small the height size of the converter.

I The insulating bushing in the container is radially bent so as to aligne with inner wall of the metal container in parallel thereby a dish shaped cone is formed I n this case a conductive iayi'ii'ay be fitted to the peripheral portion of the corn and to the opposited portion of the insulating barrier so that both the equipotential surfaces thereof may coincide. The portion of the insulating bushing extended into the metal container may be extended downward in the metal container. The thyristor stages may be multilaid from the lower side to the upper side of the container. This arrangement is effective in reduction of height size of the converter.

Other features and advantages of the invention will beapparent from the following description taken in I connection with the accompanying drawings in which:

FIG. 1 shows a schematic diagram of a thyristor AC-DC converter;

FIG. 2 is a longitudinal sectional view of an embodiment of this invention;

FIGS. 3 and 4 are respectively a longitudinal sectional views of another embodiment of the oil-filled thyristor AC-DC converter according to this invention;

FIG. 5 through FIG. 15 are cross sectional views of various embodiment of the insulating barrier which is provided on the inner wall of the metal container according to this invention;

FIG. 16 shows a longitudinal sectional view of the construction to pull out the lead wire by means of the insulating bushing in the oil-filled thyristor AC-DC converter;

FIG. 17 shows longitudinal sectional view of the insulating bushing.

FIG. 1 shows a schematic and block diagram of an AC-DC converter installed of the DC transmission terminal, wherein three

arms

10, 20 and 30 are arranged in parallel, each of which is connected to the terminals 1 and 2 being connected to DC lines on both the ends, and has a plurality of stages 10A-l0F, 20A-20F and 30A-30F connected in series. Each stage is provided with the thyristors connected in series, and a module consisting of resistors and capacitors (not shown) to equalize the voltage to each thyristor. The arms have nodes 3, 4 and 5 connected to the corresponding terminals u, v and w coupling with the delta connected coil windings 41u, 41v, and 41w of the transformer 40. The delta-connected coil windings 42a, 42v and 42w are connected to the DC transmission lines via terminals U.V.W. The thyristor of each stage is concurrently actuated by pulse signals.

Pulse transformers

50A, 50B and 50C supply the pulse signals to the gate G of the thyristors. The primary coil winding 51 of the transformer is coupled to a pulse generator 50 while the secondary coils 52 thereof may be provided at the same number as that of the thyristor, and each of them being connected to the gate and the anode of the thyristor. Note that the light pulse instead of the electrical pulse by the pulse transformer may be employed to actuate the transformer. No detailed explanation will be made about actuating of the thyristor because it is not material to this invention.

According to this invention a group of the thyristors A through C belongs to one arm while the other group of thyristors D through F to'another arm.

In FIG. 2, there is illustrated an embodiment of the oil-filled AC-DC converter in accordance with this invention. A substrate is supported by the corrugated glasses 101 through 104 which are placed on the bottom plate 111 of a metal container 110. Two columns, each of which has a plurality of thyristor stages 121 through in multilayer, are mounted on the substrate 105. Each thyristor stage is mounted on the supporting plate 107 insulated by an insulating pole 106.

Stages

131 and 132 with the dumping capacitor or the dumping resistor accommodated therein is provided on the uppermost supporting plate 107. An insulating bushing which serves to guide the terminal line for high voltage, is fitted with a flange 113 provided on a cover 112 of the metal container, by way of the flange attached to the insulating bushing. An insulating bushing for low voltage terminal is fitted with the flange 114 by way of the flange 151 attached to the cover 112. The conductor 142 passes through the insulating bushing 140 at the center. The space but one occupied by the conductor in the insulating bushing is filled-with insulating material. The lowermost thyristor stage 121 is connected to the conductor extended from the insulating bushing through the wire 161. Reference numeral 144 indicates a shield to restrict the electrical field being placed near the roof of the lead wire 161 to the insulating bushing 140. Connection between thyrister stages is made by lead wires 162 170. The last thyristor stage 130 is connected to the lower terminal 151 of the insulating bushing 150 for low voltage by way of lead wire 171.

The solid insulating barrier 180 is positioned between each of the thyristor stages 121-130 and the inner wall of the metal container 110. The insulating barrier 180 is constructed such that a plurality of insulating boards 181 185 prepared by multilaying the insulating materials such as press board, manila paper, linter paper etc., are piled up through ductpieces 186 of pressboard which is arranged at a proper interval. Connection between each of those insulating boards 181 through 185 and the ductpieces 186 is made into unification by wellknown couping means of insulating material such as adhesives or insulating bolt. The unified insulating barrier 180 is fastened onto the inner wall of the metal container 1 by mean ofadhesives or the insulating bolts which may be of those mentioned supra or other bolts. The insulating barrier 180 is enhanced in insulation because an insulating oil 115 in the metal container 110 fills the space formed of the insulating boards 181 through 185 and the ductpieces 186. In this embodiment, at high voltage section is placed on lower side of the container 110 so that the enhancement in insulation must be made on the high voltage section. In line of this requisition, relatively a number of the insulating boards are fitted on the lower side as shown in FIG. 2. The number of the insulating boards gradually decreases from the lower side to the upper side of the container 110 which is a low voltage section.

Needless to say that if the high voltage section is arranged on the upper side of the container 110, insulation must be enhanced on the upper side.

The insulating barrier 180 placed on the lower side of the container 110, i.e., those positioned under the base plate 105, has a plurality of the insulating glasses 101 through 104 inserted therein. In constructing the corresponding section of the insulating barrier 180, it is necessary to make a hole in plurality for accommondation of the insulating glasses 101 104.

Each insulating glass is corrugated as described above, in particular is of a pole' with the corrugated circumference surface. Insulating boards 187 through 190 being sandwitched by two insulating boards 181 through 185 on the one end, are fitted in the trough of the corrugation on the other end. Such arrangement is made to prevent the reduction of insulation due to the holes made.

In the converter constructed as described above, a DC voltage is applied mainly to the insulating barrier while a little voltage is applied to the insulating oil.

In construction work of the oil-filled thyristor converter, it is preferred to construct the metal container in dividing it into two or more sections.

Also, it is necessary to cool the insulating oil in the container with the cooler for cooling the thyristors.

Referring now to FIGS. 3 and 4 which show embodiments suitable for this construction. A metal 200 is divided into an upper unit 201 and a lower unit 202,

units

201 and 202 being integrally combined by means of

flanges

203 and 204.

A radiator 205 is attached to the metallic container 200 and oil in the metal container communicates with the cooling means through

oil distributing pipes

206 and 207. Bushing pockets 210 and 211 protruding from the metal container 200 are provided for insulating

bushings

208 and 209 to be pulled out. A thyristor stage 212 contained in the metallic container 200 is totally enclosed by insulating oil 213.

In the embodiment of FIG. 3, an insulating barrier 214 is provided substantially uniformly on the entire inner surface of the container 200. The insulating barrier 214 may be divide as shown in FIG. 4 in which the lower end portion of an upper division laps over the upper end portion of the adjacent lower division.

FIG. 5 through FIG. 15 illustrate examples of a partial structure of the insulating barrier, and particularly FIG. 5 through FIG. 8 show insulating structure at the corners of metallic container 220. In FIG. 5, insulating

barriers

223 and 224 are hermetically attached to the side wall 221 and the bottom wall 222, respectively, and another insulating barrier 225 of L-shaped cross section bridges across confronting end portions of the

barriers

223 and 224 to lap theseover and to be secured thereto. In FIG. 6, there are provided a L-shaped insulating barrier 226 at the corner of the container 220 and insulating barriers 228 spaced from the side wall and the bottom wall of the container 220 by ductpieces 227 to lap over the barrier 226.

FIG. 7 shows a multiple layer structure of insulating barriers shown in FIG. 6. In FIG. 8, there are provided an insulating barrier 229 confronting the side wall 221 of the metal container 220 and another insulating barrier 231 confronting the upper wall 230 of the container with one end being bent in the form of L to lap over the insulating barrier 229 confronting the side wall 221. FIGS. 9 and 10 structures suitable for enhancement of insulation near a hand hole 232 and its cover 233. In FIG. 9, on an insulating barrier 234 provided for the cover 233 laps over a peripheral portion 235a of a through hole formed in another insulating barrier 235. In FIG. 10, an open end of the insulating barrier 235 is bent toward the head hole 232. FIG. 11 shows an insulating structure of an oil distributing passage 236 near an oil distributing pipe 206 of the radiator 205, wherein an upper insulating barrier 238 and a lower insulating barrier 239 confront the side wall 237 of the container and the upper barrier 238 laps over the lower barrier 239 through a ductpiece 240, so that an oil-flow passage directing in an arrow direction may be formed in a space between both insulating barriers. FIGS. 12 and 13 illustrate insulating structures near a flange portion A of the metal container 200 shown in FIG. 3. In the FIG. 12, an insulating barrier 242 is provided close to a space 241 established responsive to the flange portion so as to assure the enhancement of insulation. In FIG. 13, an insulating barrier 243 provided for the upper container component 201 extends toward a lower insulating barrier 244 to lap thereover through a suitable sapce. With a structure of FIG. 13, the fabrication process can be simplified since insulating barriers may previously be attached to the

container components

201 and 202. FIGS. 14 and 15 show insulating structure near a bushing pocket 210, wherein an insulating sleeve 245 of L-shaped cross section laps over the opened peripheral portion of an insulating barrier 246 attached to the inner wall of the metal container 200.

In the above embodiment, there is explained an insulating bushing that a cone shapedinsulator is provided on the central conductor.

In this invention, a lower insulatinb body may be made short in length by radially benting the insulator in the form of L. This results in further reduction in the size of the converter.

That is, the upper insulating barrier 256, the side insulating barrier 257, and the lower insulating barrier 258 are provided around the thyristor stage accommodated in the metal container 250. Also, the lower portion of the insulating

bushings

260 and 270 are formed in dish-shape. The insulating bushings serve to guide the input terminal and the output terminal of the thyristor stage to the outside. A detailed construction of the upper insulating bushing 260 is illustrated in FIG. 17. As seen in FIG. 17, the insulating bushing is fixed on the flange 259 which is fitted on the upper wall of the metal container, by a bolt 281 through the flange portion 261a of the sleeve 261. An insulator tube 263 filled with an insulating oil 262 is hermetrically fixed on the flange portion 261b, i.e. the upper portion of the sleeve 261. A central conductor 264 pass through the sleeve 261 and the insulator tube 263. The central conductor is co e dby the insulator which is cone shaped in the insulator tube and is dish-sl'iap a iii the lower portion of the sleeve 261. An insulator 268 is shaped so as to adapt to the dish-shaped portion of the insulator 267. The insulator 268 and the dish-shaped portion of the insulator 267 are disposed in fitting. The insulator 268 is fitted to the central conductor by coupling means 269 such as a bolt while it is jointed to the flange portion 261a of the sleeve 261 by a packing 282. The lower end portion of the central conductor 264 is electrically connected to the upper thyristor stage 255 via a lead wire 283. Reference numeral 284 is a shield to alleviate the electrical field. The peripheral portion of the opening 256a of the insulating barrier 256 takes a form similar to the dish-shaped portion of the insulating bushing 260. The peripheral portion of the opening 256a confronts the lower portion of the insulating bushing 260. Multilaid conductors 285a through 285k and 286a through 286g are embeded in the peripheral portion of the opening 256a and the peripheral portion of the lower end of the insulator 267. The uppermost layers of the conductor 285a and 286a are connected to the metal container 250 and the sleeve 261, via

lead wires

287 and 288, respectively. The sleeve 261 and the metal container are equipotential. The lowermost conductive layer 285k of the upper insulating barrier is connected to the thyrister stage 255 via a lead wire 289 thereby they are equipotential. Such construction makes it possible to form equipotential surface between the lowermost end of the insulating bushing 260 and the upper insulating barrier 256, as shown in a dotted line. As a result, the size of the insulating bushing 260 may be made small with little reduction of dielectric breakdown strength.

What is claimed is:

1. An oil-filled thyristor AC-DC converter comprising: a metal container; a plurality thyristor stages being multilaid through insulating support means and being connected in series each other; insulating oil filled in said container so as to immerse all said thyristor stages; and an insulating bushing for guiding a lead terminal on the high voltage side of all said thyristor stages and a lead terminal on the low voltage side of the same into the outside through the wall of said metal container;

, characterized in that an insulating solid, barrier, which has a dielectric breakdown strength higher than that of said insulating oil in DC electrical field, is disposed on the inner wall confronting said lead terminal on the high voltage side of said thyristor stage so that the voltage between said lead wire on the high voltage side of said thyristor stage and said inner wall of said metal container mey be applied mainly on said insulating barr1er.

2. An oil-filled thyristor AC-DC converter as claimed in claim 1 in which said insulating barrier is disposed on most of all surface of inside wall of said container.

3. An oil-filled thyristor AC-DC convertor as claimed in claim 2, in which said insulating barrier gradually reduces in the insulating thickness from said lead terminal on the high voltage side to said lead terminal on the low voltage side.

4. An oil-filled thyristor AC-DC converter as claimed in claim 1, in which said insulating barrier is fabricated so that a plurality of insulating papers of given thickness are multilaid by using insulating paper so as to form an oil space thereamong.

5. An oil-filled thyristor AC-DC converter as claimed in claim 1, in which said insulating support means is a corrugated insulator, and each trough of thereof is fitted with said insulating barrier.

6. An oil-filled thyristor AC-DC converter comprising: a metal container consisting of a plurality of container units, in which each of said units has a flange protruding outwardly at both the lower end thereof and the upper end thereof, and those flanges are jointed hermetically; a plularity of thyristor stages being multilaid by using insulating support means in said metal container and being connected in series; insulating oil filled in said metal container so as to immerse all of said thyristor stages; an insulating bushing for guiding the lead terminal on the high voltage side of all of said thyristor stages and the lead terminal on the low voltage side of the same to the outside through a cylindrical projection provided to said metal container; characterized in that an insulating solid barrier, which has dielectric breakdown strength higher than that of said insulating oil in DC electrical field, is disposed on the inner wall of each of said container units so that the voltage between said thyristor stages and the inner wall of said metal container may be applied mainly on said insulating barrier.

7. An oil-filled thyristor AC-DC convertor as claimed in claim 6 in which, an insulating barrier enclosing the space opposite to said insulating barrier, is fitted on the inner wall of the joint portion of the flange of said container unit.

8. An oil-filled thyristor AC-DC convertor as claimed in claim 6, in which the lower end of said insulating barrier fitted on said metal container unit which is positioned above the joint portion of said flange is laid on the outer side of the upper end of said insulating barrier fitted on the inner wall of said metal container unit which is positioned below said joint portion of said flange.

9. An oil-filled thyristor AC-DC convertor as claimed in claim 6, in which a L-shaped insulating sleeve in the cross section being fitted on the end portion of the insulating barrier provided on the inner wall said metal container, is provided on the inner wall of said cylindrical projection.

11. An oil-filled thyristor AC-DC convertor as claimed in claim 1, in which the thyristor stage positioned on the high voltage side of said multilaid thyristor stage in said metal container is disposed on the lower portion of said metal container while the thyristor stage on the low voltage side is disposed on the upper portion of said metal container, and said insulating bushing on the high voltage side is suspended in said metal container, the lower end of said bushing confronting the thyristor stage on the high voltage side.

Claims

1. An oil-filled thyristor AC-DC converter comprising: a metal container; a plurality thyristor stages being multilaid through insulating support means and being connected in series each other; insulating oil filled in said container so as to immerse all said thyristor stages; and an insulating bushing for guiding a lead terminal on the high voltage side of all said thyristor stages and a lead terminal on the low voltage side of the same into the outside through the wall of said metal container; characterized in that an insulating solid, barrier, which has a dielectric breakdown strength higher than that of said insulating oil in DC electrical field, is disposed on the inner wall confronting said lead terminal on the high voltage side of said thyristor stage so that the voltage between said lead wire on the high voltage side of said thyristor stage and said inner wall of said metal container mey be applied mainly on said insulating barrier.

10. An oil-filled thyristor AC-DC convertor as claimed in claim 6, in which said insulating bushing has an insulating layer and a conductive layer, those layer being alternatively laid each other about the central conductor those alternative layers of said bushing is radially bent normal to the axis of said central conductor into a dish-shaped corn, and said conductive layer is embeded in the peripheral portion of said insulating barrier which confronts said dish-shaped corn so as to provide a equipotential surface to said conductive layer of said dish-shaped corn.