GB2045012A - High voltage transformer rectifier - Google Patents

Abstract

The invention concerns a modularised form of high voltage power supply comprising a number of sub- assemblies, each of which may for example give an output of 5Kv, connected in series to provide the high voltage d.c. voltage output required. Each sub-assembly comprises a transformer secondary winding 34 connected to a rectifier assembly 36 to provide a d.c. voltage at a pair of output terminals. The sub-assemblies may be physically stacked one upon another and the secondary windings inductively coupled to a common primary circuit 32 connected to an a.c. power supply 40. The output terminals can then be connected in series so that the individual sub-assembly output voltages are added together to provide the required high voltage. The sub-assemblies may be potted in solid dielectric material having a hole to receive the primary circuit 32 and encompassed by a core carrying the secondary winding. The core is toroidal and is made of ferrite or laminated strips of silicon steel. <IMAGE>

Description

SPECIFICATION High voltage transformer rectifier This invention relates to power supplies, and particularly to high-voltage power supplies constructed using a plurality of linked modular sub-assemblies.

Power supplies for electrostatic precipitators use direct current, typically in the range 500 mA to 2 Amperes at up to 70 kilovolts, commonly supplied by a transformer-rectifier unit usually, but not necessarily, powered from a single-phase alternating current supply at one of the standard a.c. distribution voltages. The transformer-rectifier units are often supplied in steel tanks, with porcelain bushings for h.t. terminal(s), as being a standard package for h.v.

transformers; dielectric oil filling is employed for overall compactness, good insulation, corona suppression, and efficient, reliable cooling; and moreover oil is employed because clean air for insulation and cooling is often not available in the environments where electro-static precipitators are employed.

The use of oil in precipitator power supplies introduces certain problems. There are manufacturing problems in ensuring the oil is uncontaminated, e.g. by moisture, during manufacture and installation, and whereas high-voltage a.c. transformers are usually installed in clean environments, precipitator power supplies are more usually in dirty locations, such as outdoors in quarries and lime kilns, where maintainance may not be given, and regular reconditioning or replacement of the oil may be neglected.

Also it is usual to employ semiconductor rectifiers to convert the supplied a.c. power to the direct current necessary in precipitators, thermionic and commutator rectifiers generally now being obsolete.

Economic semi-conductor diodes have a reversevoltage-withstand capability which is only a fraction of the required d.c. output voltage of the power supply, and, in consequence, some form of forced reverse-voltage-sharing is necessary to avoid overstressing individual diodes or sub-groups of diodes.

Afurther point is noteworthy, namely that highvoltage d.c. power supplies are not usually individually susceptible of repetitive, mass-production techniques, and so lose the economies of large-scale production-line methods.

It is with a view to obviating or mitigating at least certain of these problems that the present invention was devised.

According to one aspect of the present invention, there is provided a power supply sub-assembly for use in a high voltage a.c. input-d.c. output power supply comprising an a.c. transformer secondary winding adapted to be inductively coupled to an a.c.

primary winding means, rectifying means having its input connected across the ends of the secondary winding and its output connected across a pair of power supply output terminals.

According to a further aspect of the present invention, there is provided a high-voltage a.c.

input-d.c. output power supply comprising a plurality of power supply sub-assemblies of the type referred to, wherein the secondary windings of all the sub-assembly are inductively coupled to a common a.c. primary winding means.

The primary winding means may be a single conductor or plurality of parallel-connected conductors (such as, for example, the strands of a cable) whereby to form a single-turn-primary step-up transformer-rectifier unit. The sub-supplies may therefore have, as a matter of convenience but not necessity, a construction similar to known a.c. current transformers having added thereto any convenient arrangement of a semiconductor diode or diodes. Preferably each said sub-supply is potted in a solid dielectric, for example epoxy resin of known composition, so as to have a window through which the primary conductor means can pass and also have two conductors protruding from the dielectric, preferably proximate the points of connection to electrically adjacent sub-supplies, upon which, in use, the respective fraction of the high-voltage direct current appears in use.For the same of uniformity of manufacture, each said sub-supply may be identical in essential respects, variations in secondary and output voltage levels with respect to the primary conductor means being accommodated by external dielectric thickness on the sub-supplies and/or suitable insulation thickness grading on the primary conductor means (the sub-supply window dimensions being chosen to accommodate the maximum cross-sectional dimensions of the primary conductor means, including insulation thereon, over which the sub-supplies have to pass).

Any "looseness" in the magnetic coupling of the secondary windings to the primary conductor means, such as would lead to a high on-load voltage regulation, is not necessarily undesirable since, usually, some step must be taken to limit the output current of the power supply when a flash-over occurs, and a high a.c. supply reactance inherently provides current limitation when the supply's output flashes-over or otherwise short circuits. If the inherent reactance is of sufficient magnitude, it may not be necessary to provide a discrete series reactance or other series-connected current-limiting impedance in the primary (or secondary) circuit.

In order that the invention should be more readily understood and put into effect, preferred embodiments of the same will now be described by way of example with reference to the accompanying drawings wherein Figure 1 is an electrical schematic diagram of a circuit proposed in the prior art; Figure 2 is an electrical schematic of the basic circuit of the present invention; Figure 3 is a schematic diagram of an embodiment of the present invention; Figure 4 is a schematic diagram of a modification of the embodiment of Figure 3; Figure 5 is a schematic diagram of a practical realisation of the embodiment of Figure 3, or of the modification of Figure 4; and Figure 6 is a schematic diagram of a modified layout of the embodiment of Figure 3, or of the modification of Figure 4.

Referring first to Figure 1, this schematically illustrates a prior art transformer-rectifier high voltage power supply. Atransformer 10 has a standardvoltage single-phase 50Hz primary winding 12 fed from a.c. supply terminals 14 via a series-connected, inverse-parallel connected pair of thyristors 16 and an inductor 18. The thyristors 16 are phase-angle controlled in known manner. The inductor 18 provides substantially lossless transformer current limitation in the event of the supply's output being short-circuited by a flashover. A voltage step-up secondary winding 20 of the transformer 10 is provided with sufficient turns so as to deliver a suitable high-voltage a.c. output which is rectified by a diode bridge 22.The positive output 24 of the bridge 22 is earthed, leaving the negative voltage terminal 26 of the bridge 22 to assume the design output d.c. voltage in use. To cope with high output voltages of the winding 20, as many rectifier diodes as are suitable are employed in series, preferably in conjunction with a voltage-sharing arrangement (not shown), such as a multitap potentiometric chain of resistors and/or capacitors. The inductor 18, the transformer 10 and the diode bridge 22 are enclosed within a dielectric oil-filled steel tank 28 from which the negative e.h.t. terminal 26 is led via a porcelain bush (not shown).

Points to be noted in the prior art arrangement of Figure 1 are that the secondary winding 20 will require a substantial number of turns, with voltage grading and substantial insulation. Also, the diode bridge 22 requires, usually, a multiplicity of silicon diodes in series with, often, the added complexity and cost of a voltage-sharing arrangement. The tank 28 and its oil filling are an added complication. Such arrangements tend to lead away from mass production techniques to "special" and low volume production techniques with consequent loss of the economy of scale.

The present invention proposes arrangements which tend to obviate or mitigate these problems associated with prior art high-voltage d.c. power supplies.

Referring now to Figure 2, this illustrates a schematic circuit diagram of the basic essentials of a power supply in accordance with the invention. The supply is composed of a plurality of sub-supplies, which are preferably identical but could be mutually different in detail to suit various requirements. Each sub-supply comprises a transformer 30 having a low-voltage primary winding 32 and a mediumvoltage secondary winding 34, producing an output voltage which is only a fraction of the total secondary a.c. voltage of the overall power supply and chosen so as to present no significant winding or insulation problems. The secondary winding 34 feeds a diode rectifier bridge 36 preferably composed of silicon semiconductor rectifier diodes 38 which are economically and readily available in reverse-voltage ratings of many hundred of volts and adequate current ratings.Any of the other known transformerirectifier configurations could be employed at choice, for example, the bi-phase with centre-tapped secondary. Thus each sub-supply receives an a.c. input and delivers a d.c. output. As shown in Figure 2, the a.c. input windings 32 are connected in series to a suitable, preferably regulated, single-phase a.c. supply 40. However, to suit current and voltage ratings, the primary winding 32 could be connected in parallel or series/parallel configurations (not shown) to the supply 40. The transformers 30 and the diode bridges 36 are adaptable to polyphase operation by suitable modifications.

The d.c. outputs of each bridge 36 or other rectifier arrangement are series-connected to build up the chosen power supply d.c. output voltage in a series of steps. Thus different design output voltages may be arrived at simply by altering the number of series-connected sub-supplies 30/46, together with appropriate adjustment of the primary windings of the transformers to be compatible with voltage changes resulting from altering the sub-supply arrangements.

The remaining Figures illustrate, by way of examples, practical modes of realisation of the invention.

Figure 3 schematically illustrates a possible practical example of the supply arrangement of Figure 2.

In Figure 3, the transformer of each sub-supply 30/36 has a toroidal or ring core 42 upon which the secondary winding 34 is wound in the schematically illustrated mode. The cores 42 could be toroids of laminated transformer grade silicon steel, e.g. a coil of strip, or moulded of ferrite. The windings 34 could, as an alternative to the idealised distributed form shown in Figure 3, be would on a bobbin or bobbins, the core 42 being conveniently, but not necessarily, formed as two split "C-cores" together forming a complete magnetic circuit. The whole transformer (minus the primary winding) and preferably also the associated diode bridge 36, could be potted in resin to leave only the d.c. output connections protruding, and with a window through which to thread the primary conductor or conductors (as described below).Resin potting readily provides all the necessary insulation in dry form, to produce self-contained, mechanically robust "units" combinable in suitable numbers to produce the desired electrical output.

The power supply is physically formed from a suitable number of sub-supplies 30l36, by stacking the sub-supplies 30/36 into a tower with the windows vertically aligned and that the d.c. voltages appearing in use increase with increasing height up the tower and rise to a maximum at the top of the tower. A single primary conductor, suitably insulated per se or in conjunction with the thickness of resin and/or possibly in conjunction with a tube or tubes of insulating material (not shown) lining the windows, is threaded through the windows as shown in Figure 3. The fact that the primary winding 32 describes a large loop and is not tightly coupled to the secondary windings 34 is no disadvantage, since a high primary leakage reactance reduces or eliminates the need for additional reactance to limit peack current in the event of flashoverior other short-circuit of the high-voltage d.c. output of the power supply.

The assembly of sub-supplies 30136 into a tower ensures a large creepage distance between the parts of the supply that are at a high voltage (with respect to earth) and the parts that are electrically near earth, and the only locations that have a high voltage stress are between the cores of the upper sub-supplies and the primary winding, the latter being capable of being well separated from the cores and completely encased in adequate insulating material. Apart from the sub-supply to primary winding insulation, which can be dealt with by extra window insulation, as an extra resin thickness and/or an insulation sleeve on the primary winding, each sub-supply 30/36 and the parts thereof, need to be insulated only for the relatively low fraction of the output voltage that each sub-supply 30/36 produces.

Not shown in the supply circuits of Figures 2 or 3, are arrangements for voltage-sharing between the sub-supplies 30/36 which, in practice, might be necessary or desirable. Voltage-sharing arrange ments could conveniently take the form of voltagedependent resistors or other non-linear electrical circuits connected across the rectifiers 38 of each sub-supply 30136, or resistor/capacitor networks, or adjustment of the magnetising currents of the transformers 30 by introducing gaps into the cores and/or by the choice of a suitable core material. The rectifier diodes 38 might be of the controlledavalanche type, thereby themselves assisting the prevention of a damaging mis-sharing of voltage.

An alternative arrangement of primary winding is schematically illustrated in Figure 4, wherein the primary winding 32 has a small but plural number of turns, each turn being in the form of a "U" with separate terminations so that the winding 32 can readily be threaded through the sub-assembly cores 42 during construction of the power supply; the winding 32 then being completed by connecting links as required between the ends of the "U's".

The single-turn primary winding of Figure 3 could conveniently consist of a cable with a high-voltage insulating sheath, or an ordinarily-insulated or uninsulated cable threaded through an insulating tube or tubes. The plural-turn primary winding of Figures 4 could consists of a number of insulated strips laid together, wrapped in an insulating film such as "Kapton" (Registered Trade Mark), and enclosed in weather-proof cast resin. If necessary or desirable, an earthed screen may be interposed between the primary and secondary windings, most simply by enclosing the primary winding in a metal tube or tubes.

A possible mechanical construction of power supply is schematically illustrated in Figure 5. The sub-supplies 30/36 are mounted on the outside of a dust-sealed enclosure 44 of insulating material, for example glass-re-inforced plastics, in the form of a hollow central pillar and integral upper insulating shield. To the pillar of the enclosure 44 is attached a trunk 46 for a cable 48 leading away the high-voltage d.c. supply (with an earthed co-axial current return conductor). The whole assembly is enclosed in a ventilated earthed metal case 50 for safety reasons.

A disposition of sub-supplies 30/36 on the primary winding 32,which maybe foundmoreeconomical of space and materials, is schematically illustrated in Figure 6. The sub-supplies 30/36 are stacked round each of the vertical !imbs of the primary winding 32, with a zig-zag interconnection of the sub-supply outputs.

The sub-supplies for a power supply in accordance with the invention can be made by conventional manufacturing techniques in economic batch sizes and stored for assembly into complete power supplies with hardly more than a primary winding having to be taller made for a given power supply.

Performance, safety, and reliability of a sub-supply can be more easily demonstrated than in respect of a complete and indivisible power supply of the prior art type, and thereby greater customer confidence can more readily be gained.

The power supply of the present invention is particularly intended for energising an electrostatic precipitator arrangement, but is not exclusively limited to such an application. Other possible uses for the power supply of the invention are in experimental physics, e.g. as a charged-particle accelerating supply; in electrical testing of insulation in transformers and cables and such like; and with, if necessary output voltage stabilisation, as the h.t.

supply for an X-ray apparatus.

A power supply of the present invention may be operated by mains a.c. frequencies, i.e. at fifty Hertz or sixty Hertz with appropriately modified dimensions were necessary, or at higher frequencies of serveral hundred Hertz, operating frequencies in kilohertz regions are possible with suitably designed assemblies. Better performance, e.g. improved efficiency, is possible at these higher frequencies as well as more economical construction. In one design of power supply the higher operating frequency is achieved by means of a frequency converter which may also include voltage control means. Such converters may be operated from d.c. supplies or single-phase or multi-phase fifty Hertz or sixty Hertz mains supply.

Claims (20)

1. A power supply sub-assembly for use in a high-voltage a.c. input-d.c. output power supply comprising an a.c. transformer secondary winding adapted to be inductively coupled to an a.c. primary circuit conductor, rectifying means having its input connected across the ends of the secondary winding and its output connected across a pair of power supply output terminals.

2. A sub-assembly according to Claim 1 encased, or potted, in solid dielectric material and having formed therein a window or hole encompassed by a core carrying the secondary winding and arranged to receive therethrough the a.c. primary circuit conductor.

3. A sub-assembly according to either Claim 1 or 2 wherein the secondary winding is wound upon a toroidal or ring core.

4. A sub-assembly according to Claim 3 wherein the toroidal or ring cores comprise laminated strips of transformer grade silicon steel.

5. A sub-assembly according to Claim 3 wherein the toroidal or ring cores are formed of moulded ferrite.

6. A sub-assembly according to any preceding Claim wherein the a.c. transformer secondary winding comprises a winding formed on a supporting bobbin and a magnetic circuit comprising a core formed of at least two mating sections.

7. A sub-assembly according to any preceding Claim adapted for polyphase a.c. operation comprising a plurality of rectifying means each connected to receive an input voltage from a respective one of a plurality of secondary phase windings adapted to be inductively coupled to a primary circuit conductor of respective phase.

8. A high-voltage a.c. input-d.c. output power supply comprising a plurality of power supply sub-assemblies of the type referred to according to any of Claims 1 to 7 wherein the secondary windings of all the sub-assembly are inductively coupled to a common a.c. primary circuit conductor.

9. A high-voltage power supply according to Claim 8 comprising a plurality of sub-assemblies according to any preceding claim dependent on Claim 2 wherein the sub-assemblies are stacked one above another to form a tower structure with the primary circuit windows aligned to receive a common a.c. primary circuit conductor.

10. A high-voltage power supply according to Claim 9 wherein the output terminals of the subassemblies are connected in series to provide a d.c.

output voltage increasing in value from the base of the sub-assembly tower.

11. A high-voltage power supply according to either Claim 9 or 10 wherein the a.c. primary circuit conductor comprises a single turn winding.

12. A high-voltage power supply according to Claim 11 wherein the single turn primary winding comprises a single core cable surrounded by an insulating sheath or tube.

13. A high-voltage power supply according to Claims 9 or 10 wherein the a.c. primary circuit conductor comprises a plural turn winding.

14. A high-voltage power supply according to Claim 13 wherein the plural turn winding comprises a plurality of conducting strips laid together through the superimposed windows of the sub-assembly tower but insulated from each other and connected to form a plurality of superimposed turns.

15. A high voltage power supply according to Claim 13 wherein the a.c. primary circuit conductor comprises a plurality of substantially "U"-shaped mutually insulated conducting members, one limb of each of which extends through the superimposed windows of the sub-assembly tower, the opposite limbs of adjacent members being linked to form a plural-turn winding.

16. A high-voltage polyphase a.c. input-d.c. output power supply comprising a plurality of power supply sub-assemblies according to Claim 7 wherein each secondary phase winding is inductively coupled to a primary circuit conductor of respective phase.

17. A power supply according to any preceding claims wherein the a.c. input voltage is provided by a.c. frequency converter means.

18. A power supply according to any preceding claim wherein the a.c. input voltage is provided by d.c. input voltage to a.c. output voltage.

19. A power supply sub-assembly substantially as described herein with reference to any of Figures 2to6.

20. A high-voltage power supply substantially as described herein with reference to any of Figures 2 to6.