WO1992003834A1

WO1992003834A1 - High voltage isolation transformer

Info

Publication number: WO1992003834A1
Application number: PCT/US1991/005842
Authority: WO
Inventors: Marlin Niles Schuetz; David Archie Vroom
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1990-08-17
Filing date: 1991-08-16
Publication date: 1992-03-05
Anticipated expiration: 1993-02-17

Abstract

An isolation transformer (10) provides high voltage isolation between a primary winding and a secondary winding thereof. The isolation transformer includes a loop core structure (12) formed of a spiral loop of ferromagnetic tape material. The primary winding (20) is helically wound around substantially the entire extent of the loop structure. A primary winding support (30) is provided for supporting the loop core and primary winding. The secondary winding (14) is formed as an annulus over, and spaced away from the loop core structure and primary winding. The secondary winding is positioned relative to the loop core structure so as to lie in a first plane substantially perpendicular to second plane including the loop core structure. A secondary winding solid dielectric support (42) provided for supporting the secondary winding and positioning it with respect to the loop core structure and primary winding.

Description

HIGH VOLTAGE ISOLATION TRANSFORMER

Field of the Invention

The present invention relates to isolation transformers in which primary and secondary windings are adapted to withstand enormous potential differences of up to about one million volts, or more.

Background of the Invention

Particle accelerators are very useful for a number of industrial applications and processes. Such accelerators typically include an electron- emissive filament structure which is powered by low AC voltage in a 2-10 volt range with filament currents being as much as 5-30 amperes. In these accelerators the transmission window is maintained at ground potential, so that the workpiece may be placed very close to the window; and the particle emitter structure is therefore maintained at a very high negative or positive potential relative to the window, so that a beam is formed and directed toward the window relative to the emitter.

Typically, the filament structure is mechanically and electrically at or near the positive or negative high voltage terminal and potential, and it is essential that the filament power be tied to the high voltage potential accelerating the particle beam. Since the emitter structure typically employs hot cathode or direct heater technology, the heater or filament typically operates at low voltage, e.g. 2-10 volts, and at high current, e.g. 5- 30 amperes. Delivery of the heater energy to the filament from the primary power mains requires a step-down transformer which manifests efficient transconductance while providing the requisite potential difference between ground and the very high potential of the filament structure.

Prior approaches have been to construct isolation transformers with toroid cores of ferrite ceramic material which operate at a stepped up AC frequency rate, such as 400 Hz so as to reduce the size and weight needed to deliver the requisite filament power, such as 2 kilowatts. Since 400 Hz AC mains power is not generally available, powerful and expensive converters are required to convert the 50-60 Hz primary power to 400 Hz before application to the toroidal filament transformer.

Another prior approach has been to include a filament transformer within a high voltage power supply, or at the high voltage terminal, and deliver filament current through plural conductors operating at very high DC potential to a filament structure of a particle beam accelerator. While this approach has been used successfully for many years, it is expensive and cumbersome, and requires extremely conservative design in order to overcome the lack of transient protection because of the impedance and consequent energy storage associated with the heavy current conductors required to conduct sufficient filament current at the very high DC potentials employed by the accelerator.

Thus, a hitherto unsolved need has remained for an efficient, reasonably sized filament transformer for particle beam accelerators and the like which manifests efficient transfer of sufficient power to operate the filament structure and which also provides the requisite isolation between the primary and secondary in order to withstand the tremendous potential difference therebetween, which may be on the order of a million volts, or more.

Summary of the Invention with Objects

A general object of the present invention is to provide a high voltage isolation transformer for providing voltage isolated currents in a manner which simplifies and overcomes drawbacks presented with prior art approaches. A more specific object of the present invention is to provide a high voltage isolation transformer for supplying filament current to an electron beam emitter structure operating at extremely high negative potentials. One further specific object of the present invention is to provide an isolation transformer enabling a primary winding to be operated by the primary power mains at DC ground potential and one or more secondaries to be operated at extremely high negative or positive potential, such as 450 kilovolts or more with respect to DC ground.

Yet another specific object of the present invention is to provide a high efficiency DC isolation transformer which includes a primary winding which may be operated conveniently by the AC power mains at a DC ground potential and which includes a secondary for delivering considerable current to a load at a very high DC potential relative to DC ground. Additional secondary windings may be provided for other related circuits.

In accordance with the principles of the present invention, an isolation transformer provides high voltage isolation between a primary winding and a secondary winding thereof. The isolation transformer includes a loop core structure, preferably formed as an annular spiral of ferromagnetic tape material. The primary winding is helically wound around substantially the entire extent of the loop core structure. A primary winding solid dielectric support is provided for supporting the loop core and primary winding. The secondary winding is formed as an annulus over the loop core structure and primary winding, and preferably has a diameter substantially not less than the diameter of the loop core and primary winding. In order to achieve a maximum practical spacing between elements, the secondary winding is substantially equally spaced away from the toroidal core and primary winding, and the secondary winding is positioned relative to the toroidal core structure so as to lie in a first plane substantially perpendicular to a second plane including the loop core structure. A secondary winding solid dielectric support is provided for supporting the secondary winding and positioning it with respect to the loop core structure and primary winding.

In one aspect of the invention the isolation transformer is contained within a pressure vessel containing a gaseous or liquid dielectric preferably under pressure and a graded high voltage mounting location for mounting the secondary winding solid dielectric support.

In a further aspect of the present invention, the ferromagnetic tape material forming the annular core structure comprises hydrogen or vacuum annealed electrolytic iron.

In one more aspect of the present invention the secondary winding solid dielectric support is spaced away from the primary winding solid dielectric support so that there are no ungraded solid dielectric leakage paths.

In a further aspect of the present invention, a corona discharge inhibitor is disposed around the loop core structure and the primary winding. In this aspect of the present invention, the corona discharge inhibitor preferably comprises a grounded metal annulus formed about the loop core structure and the primary winding thereon, the grounded metal annulus being formed electrically to present an open turn to the primary winding.

In a related aspect of the present invention, the metal annulus is formed of metal tape wound over the primary winding and further includes a circumferential groove in the metal tape to render the annulus electrically as an open turn.

In one preferred application, the isolation transformer secondary winding supplies driving current to an electron emitter structure operating at a very high negative DC potential relative to the zero or ground reference DC potential of the primary winding.

These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated by those skilled in the art upon consideration of the following detailed description of a preferred embodiment, presented in conjunction with the accompanying drawings.

Brief Description of the Drawings

In the Drawings:

Fig. 1 is a side view in elevation of a high voltage isolation filament transformer in accordance with the principles of the present invention, with certain portions of the structure broken away to reveal underlying structure.

Fig. 2 is a preferred mounting arrangement for mounting the coils comprising the Fig. 1 filament transformer.

Fig. 3 is a side view in elevation of a coaxial particle beam accelerator showing a mounting arrangement for mounting the Fig. 1 filament transformer in pressure vessels pressurized with a high dielectric insulative gas, such as sulfur hexafluride.

Detailed Description of a Preferred Embodiment

A high voltage isolation transformer 10 in accordance with the principles of the present invention includes a loop core 12 having a primary winding 20 thereon, and a secondary winding 14. The secondary winding 14 lies in a plane normal to and is evenly spaced away from the loop core 12 and primary winding 20, as shown in Figs. 1, 2 and 3.

The core 16 is preferably formed as a toroidal or cylindrical annulus of hydrogen-annealed electrolytic iron tape of a thickness of .030" and a width of 1.75". The tape core 16 is spiral wound to e.g. 31 layers with an inside diameter of approximately 6.5" and with an outside diameter of approximately 8.5". The dimensions of the resultant, generally square cross-sectional area of the generally cylindrical core 16 are approximately 1.75" by 1.75". After the tape has been wound into the annular form shown in Fig. 1, the core 16 is fully annealed by heating to 1500 degrees C. in a gaseous hydrogen atmosphere (H2).

After the core 16 has been wound as shown in Fig. 1 and annealed, the windings of the core 16 are slightly expanded and separated to enable placement of an insulation material between the turns in order to reduce eddy current losses in the core 16 (the expansion of the windings being limited so as not to reduce significantly the results of the annealing process). After the insulation material is in place, the core 16 is then tightened and covered with cutthrough resistant paper tape 18 and fiberglass reinforced tape 19 prior to winding of the primary winding 20.

The primary winding is formed as a helix about the extent of the core 16 by even spacing and distribution of e.g. 500 turns of 18 gage solid copper wire_^coated with a suitable enamel insulator. The turns are evenly distributed throughout the annular extent of the loop core 16 in order to avoid magnetic shunt paths around the secondary.

A fiberglass tape 22 is wrapped directly over the primary winding 20. In order to inhibit corona emissions from either the primary 12 or secondary 14, a metal foil tape 23 is wrapped over the fiberglass tape 22 surrounding the primary winding 20. (The ends 24 of the primary winding 20 are extended outwardly from the fiberglass tape 22 and metal tape 23 to a suitable connection to a primary power source, such as the 110 volt AC mains.)

So that the metal tape 23 does not present a shorted turn to the primary winding 20, a narrow circumferential gap 25 is cut into the tape 23. Alternatively, a machined or spun aluminum housing may be used as a corona inhibitor over the primary. The tape 23 or metal shield, as well as the core 16, are electrically connected to DC ground.

A power source controller (such as an autotransformer, not shownj may be interposed between the mains and the primary connections 24, so that the voltage potential feeding the primary 20 may be varied, thereby to vary the voltage induced in and put out from the secondary 14.

With reference to the dimensions and materials used to form the annular core 16 and primary winding 20, the secondary 14 is preferably formed of 23 two-wire turns of 13 gauge enamel coated solid copper wire wound together on an 8" inside diameter over the primary 12. A two-part mandrel (not shown) provides a suitable winding form for winding the secondary 14 after the primary has been completed. Once the secondary windings are in place and are secured together, the mandrel is loosened and slipped out from underneath the secondary 14. Suitably sized plastic cable ties are used to hold the windings of the secondary 14 together. The secondary coil 14 may alternatively be potted in a suitable potting material and/or fitted with its own surrounding corona discharge attenuator structure.

Referring to Fig. 2, the primary 12 is mounted separately from the mounting of the secondary 14. This arrangement is preferred in order to avoid ungraded solid dielectric mounting paths. Thus, the primary 12 is mounted between a double flange structure 30 comprising two mounting plates 32 and 34, and a base plate 36. Several spaced apart cable ties 38 extend through the plates 32 and 34 to secure the primary 12 firmly in place. Preferably, the plates 32, 34 and 36 are formed of a high dielectric sheet plastic material, such as transparent plastic sheet material (e.g., Plexiglass tm). Mounting holes 40 enable the primary mounting structure 30 to be mounted to an interior wall within a gas-insulation pressure vessel of a particle beam accelerator 49 (Fig. 3).

With reference to Fig. 3, a separate and spaced apart mounting structure 42 is provided for the secondary 14. This structure 42 includes a baseplate 43, top plate 44 and spacer block 46. Cable ties 48 (see Fig. 2) secure the secondary 14 between the baseplate 43 and top plate 44 and evenly position the secondary 14 away from the primary 12. A mounting lug 50 of a voltage graded accelerator tube 56 of the accelerator 49 is used for mounting the secondary mounting structure 42. The plates 43 and 44, and block 46 are also formed of high dielectric strength plastic sheet material, and are glued together by a suitable solvent to form the structure shown in Fig. 2.

The coaxial beam accelerator 49 shown in Fig. 3 makes use of two identical filament transformers 10, and 10'. The transformer 10 of an upper accelerator unit will be described, with the understanding that the same description applies to the transformer 10^' of a lower accelerator unit.

The transformer 10 is mounted inside of a pressure vessel 51 which is charged with and contains a suitable gas or liquid high dielectric insulating material preferably under pressure, such as sulfur hexafluride (SF6) at 80 psi, for example. The primary mounting structure 30 is mounted to the inside of the vessel 51 at ground potential, while the secondary mounting structure 42 is mounted to the lug 50 at the high negative voltage end of a voltage graded accelerator assembly 56. An elongated filament 52 at negative high voltage, supplied from a high voltage connector 53, is heated by the current supplied from the secondary 14 of the transformer 10, and the filament 52 emits electrons which are shaped into a beam 54 which is further shaped and directed by passage through the graded accelerator structure toward a tubular window target area 58 through which a workpiece, such as a strand or filament is pulled.

In this particular accelerator 49, a deflection electromagnet assembly 60 dynamically deflects the electron beam over a sweep field denoted by the branching of the beam 54. A synchronized convergence electromagnet assembly 62 converges the swept electron beam toward the tubular window area 58. A conduit 64 interconnects the upper vessel 51 with a lower pressure vessel 65 containing the transformer 10', and a similarly pressurized gaseous dielectric environment. A vacuum pump 66 evacuates the interior of a vacuum chamber 57 in which the upper and lower beams 54 and 54' are formed.

With pressure of the sulfur hexafluride dielectric gas within the vessel 51 fixed at 80 psi, the transformer 10 easily stands off a potential difference between primary and secondary of 450 kilovolts. By increasing the pressure of the sulfur hexafluoride dielectric gas, the voltage level may be increased to 500 kilovolts or more. Increasing the spacing between the primary and secondary windings enables even higher potential differences to be withstood.

In an unloaded state, but with primary power applied to the primary winding 20, heating of the core 16 is nearly undetectable after having been on over extended periods of time. With full load, the efficiency of the transformer 10 approaches 80 % or better. Most of the losses appear to be resistance losses in the secondary. At full load of 32.2 amperes ( output voltage 3.22 volts into a load resistance of 0.1 ohms), the transformer 10 is easily capable of shedding heat without any additional active cooling apparatus. To those skilled in the art to which the present invention pertains many widely differing embodiments will be suggested by the foregoing without departing from the spirit and scope of the present invention. The descriptions and disclosures herein are intended solely for purposes of illustration and should not be construed as limiting the scope of the present invention which is more particularly pointed out by the following claims.

Claims

What is claimed is:

1. An isolation transformer for providing a predetermined high voltage isolation between a primary winding and a secondary winding thereof, the transformer including: loop core means formed of ferromagnetic material and having a predetermined loop inside dimension, the primary winding being helically wound substantially around the entire extent of the loop core means, the secondary winding being wound over the loop core means and the primary winding and having an inside secondary winding dimension in relation to the loop inside dimension such that the secondary winding is spaced away from the primary winding by a spatial dimension related to the predetermined high voltage isolation, the secondary winding being positioned relative to the loop core means and primary winding so as to lie in a first plane generally perpendicular to a second plane including the loop core means, primary winding support means for supporting the loop core means and primary winding, and secondary winding solid dielectric support means for supporting the secondary winding and positioning it with respect to the loop core means and primary winding.

2. The isolation transformer set forth in claim 1 wherein the loop core means defines a central opening and wherein the secondary winding is formed and positioned within the central opening so as to be spaced away from the primary winding by the spatial dimension related to the predetermined high voltage isolation.

3. The isolation transformer set forth in claim 2 wherein the loop core means is formed to be substantially toroidal and wherein the central opening is a substantially cylindrical space coaxial with the substantially cylindrical loop core means along a central axis, and wherein the secondary winding is formed and positioned to be aligned with the central axis.

4. The isolation transformer set forth in claim 1 further including pressure vessel means for enclosing the isolation transformer, gaseous dielectric means being contained by the pressure vessel means, and a graded high voltage mounting location for mounting the secondary winding solid dielectric support means.

5. The isolation transformer set forth in claim 1 wherein the ferromagnetic material comprises annealed electrolytic iron tape formed as a spiral into the loop core means.

6. The isolation transformer set forth in claim 1 wherein the secondary winding solid dielectric support means is spaced away from the primary winding solid dielectric support means.

7. The isolation transformer set forth in claim 1 further comprising corona discharge inhibitor means disposed around the toroidal core means and the primary winding thereon.

8. The isolation transformer set forth in claim 7 wherein the corona discharge inhibitor means comprises a metal annulus formed about the toroidal core means and the primary winding thereon, the metal annulus being formed electrically to present an open turn to the primary winding.

9. The isolation transformer set forth in claim 8 wherein the metal annulus is formed of metal tape wound over the primary winding and further including a circumferential electrical opening in the metal tape.

1 3 10. The isolation transformer set forth in claim 1 wherein the secondary winding supplies filament current to an electron beam emitter structure operating at a high negative DC potential relative to the DC potential of the primary winding.

11. The isolation transformer set forth in claim 1 wherein the primary winding is adapted to be connected directly to the AC power mains operating at DC ground potential, and wherein the secondary winding is adapted to be connected to a load operating at a very high DC potential relative to ground, in excess of 100,000 volts.

12. The isolation transformer set forth in claim 3 wherein the secondary winding is substantially ring shaped so that an inside open-space dimension thereof is a diameter which is substantially the same as an inside open-space dimension of the substantially toroidal loop core means.