Self-contained Transformer-rectifier Assembly

Abstract

A self-contained transformer-rectifier assembly comprises a transformer received in a tank filled with oil. A massive heat sink mounted on the tank has fins which project downwardly into the oil. An electrically insulating coolant such as water is circulated in passages formed in the portion of the heat sink which is out of the tank. Solid-state rectifier elements, such as silicon diodes, are carried by the heat sink.

Description

The present invention relates to a self-conntained transformer-rectifier apparatus adapted to convert alternating current to direct current.

Most prior art transformer-rectifier apparatuses comprise two separate units, whereby number of electrical transmissions are necessary and separate cooling systems should be provided for the transformer and the rectifier.

Attempts have been made in the past to design self-contained transformer-rectifier apparatuses in which both the transformer and rectifier are cooled by a common coolant. That coolant consists of the dielectric liquid in which the transformer windings are immersed. For this purpose, the rectifiers are mounted on an electrically conductive heat sink having heat dissipating fins and which is sealed into an opening in a lateral wall of the transformer tank, with the rectifier cells extending outwardly and the finned side of the sink extending inwardly into the tank. The tank is of conventional construction for dissipating the heat transmitted to the dielectric coolant by the windings and from the electrical sink.

Such a construction has a number of drawbacks. The rectifier temperature is inherently higher than the temperature of the dielectric coolant, since heat flow is from the rectifier to the heat sink and from the sink to the coolant. If the rectifiers consist of currently available semiconductor diodes, they do not operate satisfactorily at temperatures beyond 70.degree.C. On the contrary, transformers generally operate at a temperature much higher for the heat exchanging means between the transformer coolant and the outside atmosphere not to represent an excessive investment. As a consequence, there is no possibility to design a self-contained transformer rectifier apparatus of the above-mentioned type in such a way as to associate satisfactory operation thereof and a commercially acceptable cost.

SUMMARY OF THE INVENTION

It is a general object of the invention to provide an improved self-contained transformer-rectifier apparatus.

It is a particular object of the invention to provide a self-contained transformer-rectifier apparatus wherein the transformer and rectifier are cooled by a common liquid coolant circulated through a heat sink carrying the rectifier means.

The above-cited objects as well as others are accomplished by mounting rectifier means on a thermally conductive heat sink provided with conduit means in which a liquid coolant is circulated, said heat sink having an inner portion extending inwardly into a transformer tank through an opening at the upper end thereof, and an outer portion which carries the rectifier means, said heat sink being provided with means, such as fins, for heat transmission from a dielectric liquid received in the tank to the heat sink.

Further objects and advantages of the invention will become apparent from the following description of particular embodiments and which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a front elevation of a transformer-rectifier apparatus which constitutes a first embodiment of the invention.

FIG. 2 is a sectional view along line II--II of FIG. 1, illustrating the heat sink and the fins thereof.

FIG. 3 is an isometric view of the upper portion of the transformer tank, cover and block secured to the cover.

FIG. 4 is a schematic diagram in elevation of the electrical connections between the transformer windings and the rectifier input bars.

FIG. 5 is a schematic diagram of the arrangement of the diodes which constitute the rectifier elements.

FIG. 6, similar to FIG. 3, illustrates a modified embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown a self-contained transformer-rectifier apparatus which will be considered as consisting of a transformer referred to by the reference number 1 and a rectifier referred to as 2.

The transformer is located in a tank 3 filled with a dielectric liquid (generally mineral oil). Since the dielectric liquid is cooled by the rectifier heat sink, as will be shown later, there is no obligation that the tank has properties of substantial heat exchange with the surrounding atmosphere. As a consequence, the tank may be made from metal sheet rather than from a casting provided with outer fins. Suitable plastics, such as polyvinyl chloride, polypropylene, polyester, possibly reinforced by fiber glass, may also be used, particularly if subjected to an aggressive environment in service.

Referring now to FIGS. 1, 4 and 5, the transformer 1 is of the three phase type. It comprises a core 4 consisting of magnetic sheets having three vertical cross-members which can be seen on FIG. 4. The lower and higher arms of the core are clamped between U-shaped beams 5 and 6 secured to each other by bolts (not shown). Each vertical arm of the core carries a primary or high voltage winding 7 and two secondary or low voltage windings 8 and 9. Such an arrangement makes it possible to use a full-wave rectifier arrangement, but a single secondary winding would be sufficient for a half-wawe rectifier arrangement.

In the embodiment of FIG. 1, deflecting plates 10 and 11 are located close to vertical walls of the tank 3 and parallel thereto and to the magnetic core 4. The plates are connected to the tank wall by lugs 12 or any other suitable connecting means. The deflecting plates are intended to guide the natural convection flow of the oil in the tank and to improve the heat exchange efficiency. The deflecting plates are however not essential to proper operation of the system and they will often times be omitted when the electrical power to be carried remains moderate.

The rectifier 2 comprises a mounting block 13 in which there are formed passages 14 for a cooling liquid. If the mounting block 13 consists of a casting of aluminum base light alloy, the passages may consist of copper pipe lengths which are first engaged into holes pierced in the casting and then bulged for satisfactory contact with the casting. The pipe lengths are connected to each other and to outer supply and discharge means for constituting a complete cooling system. In a particular embodiment, the end portions of the lower and higher left hand pipe lengths seen FIG. 1 are bridged by an elbow and the lower and higher right hand pipe lengths are similarly bridged by another elbow (not shown). The end portion of the lower two pipe lengths remote from those shown on FIG. 1 are cnnected by half-bent pipe section. The free ends of the upper two pipe lengths 14 then constitute an inlet and an outlet. For low rates of flow, the inlet may receive tap-water while the outlet may be directed to waste. For higher rates, a closed circuit may be used.

As shown on FIGS. 1 - 3, the fins are located transversely to the length of the block and to the passage means 14. Although this arrangement is preferable in most cases since it tends to direct the flow of dielectric liquid in a direction transverse to the core, and to cause the flow to sweep a maximum lateral surface of the transformer, thereby emproving the cooling action, it should be considered as in no way limitative and the fins may be parallel to the passage means.

The mounting block 13 is of inverted T-shaped cross-section with the passages being formed along the connecting zone between the vertical and transverse arms of the T. The vertical arm has a thicker portion 15 formed with a row of traversing holes directed perpendicularly to the passages 14. Each hole is sized to accept two rectifier elements engaged in opposed relation, in the form of solid-state rectifying diodes 16. Six such diodes (which will generally be silicon diodes) are located on each side of the thicker portion 15. The six diodes constitute three groups of two diodes electrically connected in parallel. A greater or lesser number of diodes in parallel may be provided, depending upon the D.C. output currents that should be delivered by the apparatus. For a relatively low current, six diodes (rather than twelve in the embodiment of FIGS. 1-3) would be satisfactory. For a stronger current, more than two diodes in parallel may be used.

The vertical arm of the T-shaped mounting block comprises a terminal thinner portion 17 which fulfils two functions. First, the vertical surfaces 18 of the portion 17 are smoothed for providing a satisfactory contact with a copper bar 22 which constitutes a D.C. bus bar for the apparatus. The bar may be secured by bolts received in apertures of the bar and in holes 19 of portion 17. Second, the thinner portion 17 may be used for lifting the mounting block and the transformer core which is positively connected thereto, as will be seen later. For lifting, rods may be engaged in the holes 19 after the bolts have been removed, as seen in dash-dot lines on FIG. 3. The terminal hooks of lifting slings may then be engaged under the rods.

The mounting block 13 is provided with a plurality of lower fins integral with the remainder of the block and parallel to each other. The two terminal fins 20 are of increased thickness with respect to the intermediate fins 21, twenty-eight such fins being provided in the illustrated embodiment. The reinforced terminal fins 20 project down to a level lower than the terminal edge of the fins 21 and are formed with apertures intended to receive bolts (not shown). Such bolts secure electrically insulating pads 23, which may be of the insulating material sold under the trade name PERMALI.

The insulating pads 23 are in turn secured by bolts 23a and nuts on the higher beams 6 which clamp the transformer core.

The mounting block 13 projects through an opening of a cover plate 24 of insulating material, such as the material sold under the trade-name PERMALI. In service, the cover is removably secured to the tank 3 of the transformer by bolts (not shown) or other suitable connecting means.

The electrical connections between the transformer 1 and rectifier 2 are illustrated on FIGS. 1, 3, 4 and 5. The three primary windings of the transformer are fed from a three phase A.C. network under a voltage of 220/380 volts, for instance through six electrical lines 25 (FIGS. 1 and 3).

The three secondary windings 8 and the three secondary windings 9 are associated with a full-wave rectifying network of the push-pull type which is illustrated in simplified form on FIG. 5. On FIG. 5, there has been shown a single rectifying element 16 in each branch for simplicity, but it should be understood that that element corresponds to a pair of diodes in paralllel in the embodiment of FIG. 1. The six outputs of the secondary windings which are closer to the horizontal mid-plane of the transformer are connected to a bus output bar 26 of L-shape (see FIG. 4), which consitutes the negative polarity D.C. output bar of the system. The bar 26 projects out of the tank through a slot of suitable size formed in the insulating cover plate 24. The three outputs of the secondary windings 8 more distant from the mid-plane of the transformer are connected by respective electrically conductive strips 27 and sheaths 28 each to a pair of diodes 16 (illustrated in the form of a single rectifier element on FIG. 5). The three outputs of the secondary windings 9 more distant from the mid-plane are similarly connected by electrically conductive strips 29 and sheaths 30 each to a pair of respective diodes 16 located on the side of the block 13 opposed to the side which receives the diodes associated with the windings 8.

Since strong A.C. currents flow in the secondary windings 8 and 9, each such winding preferably consists of a bundle of tightly packed flat lines, each provided with an electrical insulation layer.

The positive D.C. output of the assembly consists of the bus bar 22 securely connected to the mounting block. The bar 22 could as well be secured on a flat upper surface of a mounting block 13 if the latter is deprived of the thinner portion 17. In that case, vertical threaded holes can be formed in the horizontal flat upper surface of the block for receiving screws securing the bus bar 22 under normal operation. After the bar is removed, the holes may temporarily receive lifting rings or hooks when the rectifier and transformer are to be removed from the tank. Such a construction again renders dismantling easy.

For more clarity, the clearances between the strips 29, the strips 27 and the L-shaped bar 26 have been illustrated on FIG. 1 thicker than they are actually. In fact, in actual embodiments the vertical plates 10 and 11 which constitute deflecting members would be closer to the fins 21 than shown on the FIGS. for guiding the thermally induced flow.

A self-contained assembly providing an output D.C. power of 24 kW (8 volts - 300 amperes) was constructed and includes a transformer whose primary windings are designed for being fed from a 380 volt three phase (50 cps or 60 cps) network. The coolant is tap-water circulating at a rate of four litres per minute successively along four passages 14. The temperature differential between the water input and the water output is about 5.degree.C during operation at full power. Water obviously constitutes the liquid easiest to handle and will practically be used each time the D.C. output voltage does not exceed 48 volts. Beyond that voltage an electrically insulating mineral oil or one of the heat transfer liquids sold under the trademark DOWTHERM may be used. Anyway, the arrangement is inherently safe, since the coolant circuit is located entirely out of the transformer tank. In addition, it was experimentally found that the vibrations in the hundred cps range due to operation of the transformer inhibit or reduce scaling in the passages and make it possible to use non-demineralized water.

Advantages of the invention are apparent from the above description and will be briefly summarized only. The oil inventory is much lower than necessary in a conventional transformer, wherein the thermal power evolved by the transformer provided with an air-cooled externally fumed tank (and possibly the rectifier diodes) during operation should be transferred to the atmosphere, through the tank wall. As an example, the oil inventory is about 75 litres in the 24 kW assembly referred to above, while the corresponding inventory in a conventional arrangement is about 1,500 litres. Since cooling is through the mounting block, the diodes are maintained at a temperature lower than that of the transformer oil. This is a definite advantage since the diodes -- and more generally all solid state rectifier elements -- are much more sensitive than the transformer to high temperature. Since the mounting block may easily be manufactured from a single element, for instance by casting, the thermal resistance on the thermal path from the transformer to the coolant passages is very low. Practically an increased thermal power may be transferred to the cooling fluid by a ready modification in design, consisting in increasing the contact surface between the water and the block.

Referring to FIG. 6, there is shown a modified embodiment which differs from that of FIG. 1 as regards the construction of the means for transferring the heat evolved by the transformer from the dielectric liquid to the mounting block of the diodes. For more clarity, the elements of FIG. 6 which correspond to those already illustrated on FIGS. 1 - 5 bear the same reference numerals with a prime mark affixed thereto.

There is shown on FIG. 6 a mounting block 13' which constitutes a heat sink. Four parallel coolant passages are formed in the block 13' parallel to the length thereof and are connected in series relation by bridging pipe sections to constitute a coolant path 14' similar to that in the embodiment of FIG. 1 and having an inlet and outlet for connection to an outside circuit. Two pipe lengths similar to the pipe lengths 14' and parallel thereto are embedded in the block 13'. A U-shaped bridging pipe section 32 connect two corresponding ends of the pipe lengths to each other for constituting a hair pin flow path 31 for circulation of the transformer oil. The distance between the conduits 31 and the lower pair of passages 14' is preferably approximately equal to the distance between the higher pair of passages 14' and diodes 16'.

Pumping means located out the block provides for circulation of oil along the circuit 31. In the embodiment of FIG. 6 the pumping means comprises a pump 33 driven by a motor which may be energized by an electric line which constitutes a derivation of the electrical supply to the primary windings of the transformer. The pump 33 draws oil from the tank of the transformer (not shown) through an inlet pipe 34 at one location or at several locations distributed within the tank in the upper layer of dielectric oil and forces an oil flow into a first passage (the direction of flow being preferably so selected that there is a counter-current flow of the dielectric oil and cooling liquid). The oil flow is returned to the tank by an extension of the return passage pipe. That extension is formed with distributed apertures and constitutes a diffuser 35 which delivers the oil at a relatively lower temperature into the lower portion of the tank. Arrangements other than that illustrated in FIG. 6 may obviously be designed. It is however of advantage to arrange the pump for the flow induced by the pump to be of the same as that of the natural flow due to thermal effects along the transformer core.

Claims

I claim:

1. A self contained transformer-rectifier assembly comprising: a tank having an upper opening; a dielectric and insulating liquid in said tank; a transformer received in said tank and immersed in said dielectric liquid; a massive heat sink of thermally and electrically conductive material mounted on said tank and having a first portion located out of said tank and a second portion extending downwardly into the tank through said opening and into the dielectric liquid; means for circulating a liquid electrically insulating coolant separate and distinct from said dielectric liquid within said first portion of the heat sink; a plurality of rectifier elements mounted on said first portion; and a means for electrically connecting the transformer and rectifier elements.

2. A self contained assembly according to claim 1, wherein the heat sink consists of a mounting block whose underside has a plurality of heat radiating parallel fins integral with the block, said fins dipping into the dielectric liquid.

3. A self contained assembly according to claim 2, wherein said plurality of fins include two reinforced end fins and intermediate fins, said end fins projecting downwardly from the underside of the block by an amount greater than the intermediate fins.

4. A self contained assembly according to claim 3, having dismountable electrically insulating means for connecting said transformer and end fins.

5. A self contained assembly according to claim 4, wherein said first portion is provided with means for receiving a lifting mechanism.

6. A self contained assembly according to claim 1, wherein the tank is of a material having poor heat conduction properties.

7. A self contained transformer-rectifier assembly comprising a tank for receiving a dielectric liquid and having an upper opening; a transformer in said tank; an electrically and thermally conductive heat sink having an upper portion and a lower portion integral with the upper portion; said lower portion being formed with a plurality of parallel heat exchange fins, said heat sink being mounted across the opening with its lower portion extending downwardly into the tank and its upper portion extending outwardly out of the tank; at least one row of solid state rectifying elements mounted on said upper portion and in thermal contact therewith; passage means formed in said upper portion in a direction substantially parallel to said row and transversal to said fins; and means for circulating a liquid coolant separate and distinct from said dielectric liquid along said passage means.

8. A self contained assembly according to claim 7, having two rows of said rectifying elements mounted one on each of two opposed surfaces formed on said outer portion.

9. An assembly according to claim 7, wherein said heat sink is a casting of a metal or alloy of high thermal and electrical conductivity.

10. An assembly according to claim 7, having electrically insulating connecting means securing said transformer and heat sink.

11. An assembly according to claim 7, wherein the tank and heat sink are so constructed and arranged that the major portion at least of the heat evolved by said rectifier elements and transformer is taken by said liquid coolant.

12. An assembly according to claim 7, wherein said tank is of a material such as plastics or reinforced plastics which is resistant to a corrosive environment.

13. A self contained transformer-rectifier assembly comprising a tank, a dielectric and electrically insulating liquid in said tank; a transformer received in said tank and immersed in said dielectric liquid; a solid mounting block of high thermal and electrical conductivity; electrically insulating means for connecting said block to the upper portion of said tank; solid state rectifying elements carried by said block and in thermal contact therewith; electrical connection means for connecting said transformer and rectifying elements; passage means formed in said mounting block; means for circulating a liquid coolant separate and distinct from said dielectric liquid along said passage means for cooling said block; and means for transferring heat from said dielectric liquid to said block.

14. An assembly according to claim 13, wherein said means for transferring heat from said dielectric liquid to said mounting block comprise means for circulating said dielectric liquid along a path a part of which is formed in said block.

15. An assembly according to claim 14, wherein said path is partially limited by fins integral with said block and projecting into the dielectric liquid through an upper opening of said tank.

16. An assembly according to claim 14, wherein said path comprises conduit means extending form said tank, inside the mounting block and back into said tank and pumping means for circulating said dielectric liquid along said path; said conduit means for the dielectric liquid being substantially parallel to the passage means for the coolant in said mounting block.