AU610762B2 - Spark protection for high voltage resistors - Google Patents

Description

0. 1; FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1962 COMPLETE SPECIFICATION (Original) FOR OFFICE USE: Class Int. Class e a a a e a> e o a «0 a e a e a e oo a a oa Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: This do2uincnt c( n Laiis W amendments made und r Section.49 and is correct for printing a a.

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a a 0 00 0000 a a Related Art: Name of Applicant: THE AUSTRALIAN NATIONAL UNIVERSITY

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,Address of Applicant: Acton, Australian Capital Territory, 2601 Actual Inventor(s): DAVID CALVIN WEISSER, ALAN KENNETH COOPER, ALISTAIR GLEN MUIRHEAD, HOWARD JOHN WALLACE, JAMES DOUGLAS STEWART, ROBERT BRIAN TURKENTINE.

Address for Service: DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne V 3000, Australia. Complete Specification for the invention entitled: "SPARK PROTECTION FOR HIGH VOLTAGE RESISTORS" The following statement is a full description of this invention.

including the best method of performing it known to l

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2-- Technical Field This invention concerns high voltage resistors. More particularly, it concerns resistors and resistor assemblies for use in high voltage dc applications, which require protection from spark damage when voltage surges occur. It is particularly suitable for use with the supporting columns for the high voltage terminal in an electrostatic accelerator, and a with the accelerator tube of such an accelerator.

I0 However, the use of the present invention is not limited to these applications.

t o 0 Background to the invention Electrostatic accelerators in general, and Pelletron accelerators in particular, are well known. They 15 have been installed in a number of research 04 establishments throughout the world. Currently, Pelletron electrostatic accelerators are manufactured o..oo only by National Electrostatic Corporation (NEC) of Sa Middleton, Wisconsin, USA, and the major manufacturer 0oo0o 20 of Van de Graaf accelerators is High Voltage Engineering Corporation (HVEC), Massachusetts, USA.

The Australian National University has a Pelletron accelerator and because the present invention was developed as a consequence of an appreciation of deficiencies in the voltage grading used in that accelerator, this description of the present invention will tend to emphasise its use in a Pelletron accelerator.

p0 SoEp i 3 In a Pelletron accelerator, a high voltage terminal, which is charged by the movement of charge-carrying pellets mounted in a conveyor type of arrangement, is supported within a large metal high pressure vessel by a plurality of supporting columns. Each supporting column extends from a wall of the pressure :00 vessel (which is at ground potential) to the high a 0 0o voltage terminal and comprises a large number of insulating discs, separated by thinner metallic o 10 discs. The vessel is filled with an insulating gas 00 a sulphur hexafluoride to a pressure of about 6 Ofo atmospheres. To ensure that there is no inadvertent 0 sparking within the vessel while the high voltage terminal is being charged (typically to 14 MV) or is being maintained at its charged level, there must be o an equal electrical stress on each insulator. To Qo0P achieve such an equal electrical stress, it is necessary to provide a uniform voltage gradient along 0o01OO the supporting columns.

000001 The high pressure vessel also contains the tube of Ga the accelerator, which is constructed from a large ,oa, number of annular insulating discs, separated by thinner, annular, metallic discs or flanges. It is normally a requirement that the voltage gradient along the tube of the accelerator should be uniform, although in some cases it is desirable to be able to vary the voltage gradient (for example, to achieve particular beam optics).

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4 Traditionally, the voltage gradient has been controlled by a corona discharge device connected between adjacent metallic discs of the columns and the tube. The corona discharge devices are relatively inexpensive and are robust, but there are some disadvantages associated with the corona voltage grading system.

The major disadvantage of the corona devices is that o o the voltage across the corona point-to-plane gap o 10 depends upon the separation distance, point o sharpness, the presence of foreign material and the o00 insulating gas pressure. When corona point discharge cooo devices have been tested after their installation, 0 using a spacer jig, more than 10 per cent of them have shown variations from the mean, in the voltage drop across the devices, of more than 10 per cent.

0 In non-standard installations, such as between 00 0 o00 o accelerator tube flanges, it has been established o a' that more than 50 per cent of the corona point o0000 20 devices exhibit a variation from the mean voltage 0 drop across the devices of more than 10 per cent.

0 0 0oo a 0 Thus over-stressed gaps in the components of the 0o00o accelerator tube are quite common.

0ooo S° Another major disadvantage in the use of corona devices is that the corona discharge system causes some breakdown of the insulating SF 6 gas, and the breakdown products are corrosive. The resultant chemical attack can be reduced to an acceptable level by efficient chemical scrubbing, but scrubbing incurs a significant capital cost to perform.

Most corona discharge devices in electrostatic accelerators need to be replaced at intervals of about two years. Although the replacement of the components in the accelerator involves a cost of only a few thousand dollars, the accelerator has to be ,o shut down for at least two weeks while the o o° 10 replacement is effected. About 60 per cent of the 0 shut down time is directly assignable to corona point replacement. It has been estimated that the true 0oo cost of replacement of the corona point devices is approximately $110,000 per year.

0000 The deliberate variation from linear voltage gradient along an accelerator tube, mentioned briefly above, o0° is very awkward to effect with corona point discharge 0 0 00 devices.

000000 To overcome the above-noted disadvantages associated o000 0 20 with the use of corona point discharge devices, it has been proposed to use resistors in place of the 0'0060 corona discharge devices. There are no SF 6 breakdown 0000 o products when resistors are used, and varying the 0 a voltage gradient along an accelerator tube is a trivial operation using resistors. In fact, in some of the accelerators where this change has been made, the benefits have far outweighed the initial high cost of installing resistors in place of the corona r b9 0 0 O4 00 0 049 00 o 0 1 08 0 o 0 V 00*V 44a .9 0 0 9 6 point discharge devices. For example, at Daresbury, in the United Kingdom, an electrostatic accelerator incorporating a resistor system has been operated for years without lost time due to resistor failure.

Unfortunately, some problems are experienced when resistors are used in an accelerator. High voltage resistors, by their nature, tend to be long devices.

For example, in the HVEC accelerators, the resistors are 50 cm long and grade about 50 KV per gap between conducting plates in a support column or tube.

Multiple element resistors in both in-line and serpentine configuration have also been used, but they also span about 50 cm. Such long resistors tend to act as aerials, and the induced charge developed 15 by each "aerial" during a major accelerator discharge can substantially increase the electrical stress imposed upon the resistors.

The obvious solution of using shorter (and therefore thicker) resistors is impractical because, to fit into the space available in most electrostatic accelerators, the resistors must fit within a cylindrical housing having an outer diameter of about 16 mm. Marginally thicker resistors can be used in those accelerators having a higher electrode spacing (pitch) of 25 mm.

A recent development, by the French company Vivirad, is to m6unt a pair of resistors on each column or tube electrode, in a side-by-side arrangement, with I l----a~maul~ I I 00 0 o 00 o o 0 00 G" 0 0000 o o 0 00 0 0 00 000 0 00 0000 0 0 0000 o o 0 00 0000 0 0 0000 0 000000 0 0 7 the resistors connected in series by a spring extending between their ends which are remote from the column or tube.

Disclosure of the Present Invention It is an object of the present invention to provide a high voltage resistor assembly having a novel arrangement of components, which is relatively short, is adapted to be assembled on the flanges or metal discs of a supporting column or tube of an electrostatic accelerator, has a minimised aerial effect, incorporates a protective housing which can have an outer diameter as small as 18 mm, and has a resistance value which remains stable and accurate to within a small predetermined percentage of the 15 required resistance of the resistor.

This objective is achieved by providing a resistor which has a single elongate resistive element, one end of which is mounted on a metal clamp which is adapted to be clamped to the edge of a metal disc of 20 the type used in a supporting column or tube of an electrostatic accelerator. The other (free) end of the resistor is unsupported, but carries a tubular capacitive electrode, which provides, with a cylindrical metal housing which is mounted on the clamp coaxially with the resistive element, a spark gap. The free end of the resistor also supports a light-weight connector, into which or on to which a plug can be inserted or affixed to make an external electrical connection to the resistive eleiaent.

8 The resistive element of the resistor should be supplied with an accuracy that is suitable for the application in which the resistor is to be used. In the case of electrostatic accelerators, that accuracy is 2 per cent of the nominal value of the resistor.

The production of resistive elements to this degree of accuracy is known technology. One company in o°o' Britain (Welwyn) for example, makes resistive elements from a resistive film coating a ceramic g o soo 10 former, the film being machined along the middle ~oo Dportion of its length to remove a helical strip of 0 the coating. A metal (usually brass) cylinder is 0000 9ooo pressed into intimate contact with the non-machined ends of the resistive coating, to enable good electrical contact to be made with the ends of the resistive element without distorting the resistive 0Qnq o 0 coating. Other resistor manufacturers use different 0°0 techniques to create the required resistive element and provide cylindrical end caps.

00 0 0000oooo oooo0, 20 Thus, according to the present invention, a resistor for use in high voltage dc applications comprises: 0 0o oooa a metal clamp, adapted to be clamped to the edge 0 portion of a metallic disc or flange used in the high voltage dc application, said metal clamp having a support region of substantially circular cross-section; a cylindrical metal tube that has been force fitted on to the support region of said metal clamp said metal tube extending from the support region away from the clamping portion thereof; '4

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9 an elongate resistive element, each end of which is terminated by a metal cap which fits over the end of the resistive material of the resistive element and makes intimate electrical contact with said resistive material, each metal cap having a centrally mounted, externally threaded projection from the end thereof, the projection ao having an Allen key socket or the like formed centrally therein, one of said metal caps being securely screwed into a receptive threaded o aperture in the centre of the support region of a o said clamp, whereby the resistive element projects from the support region coaxially with said cylindrical tube; a cylindrical capacitive electrode having an outer diameter less than the inner diameter of 00 o o said cylindrical tube mounted on the projection '0o V from the end of the metal cap of the resistive element which is remote from said support <O 20 region; said capacitive electrode being coaxial a o o9 with said resistive element and said cylindrical f tube and defining, with said cylindrical tube, a spark gap for said resistor; and 0o0~o" a connector also mounted on the end of said resistive element which is remote from said support region, said connector being mounted adjacent to said capacitive electrode.

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Embodiments of the present invention willalso encompasses a system ofbe attachement of pairs of resistors, constructed in accordance with the present invention, between adjacent metal discs of an accelerator support column or tube.

o* Embodiments of the present invention will now be described, by way of example only, with reference to o the accompanying drawings.

'o Brief Description of the Drawings 10 Figure 1 is a schematic view of a Pelletron accelerator.

.0 Figure 2 illustrates the construction of support columns and the tube of an electrostatic accelerator, o0 with resistors attached to the metal discs thereof.

0000 0 0 15 Figure 3 is a cross-sectional view through a resistor 0 .o of the present invention.

OO b o o Figure 4 is a perspective sketch of the clamp of the 0' resistor illustrated in Figure 3.

Figure 5 is a perspective sketch (partly schematic) of the cap at each end of the resistive element of the resistor of the present invention.

Figure 6 illustrates a preferred arrangement for connecting resistors to the accelerator tube of an electrostatic accelerator.

11 Detailed Description of the Illustrated Embodiments The Pelletron accelerator shown schematically in Figure 1 has a high voltage terminal 10 supported within a metal pressure vessel 11 by a plurality of columns 12. A tube 15 is mounted within the pressure vessel. The "conveyor" of charged, insulated, metallic pellets, which passes from a charging region 0 to the high voltage terminal 10, is not shown in this 0 9* drawing. The interior of the tube 15 is maintained 09 10 at low pressure. The vessel 11 is filled with too sulphur hexafluoride gas at a pressure of about 6 9000 atmospheres. The operation of this type of accelerator is well known to those skilled in this art.

0000 0 00 0 0 15 As shown in Figure 2, each of the columns 12 0 04 comprises a series of solid insulating discs 21 separated by thin metal discs 22. Each column has 0 several hundred such pairs of insulating and metal Sdiscs. As is well known, the tube 15 of the linear 0oooo 20 accelerator is constructed in the same manner, except that the insulating and metal discs are annular 0 0 discs.

When resistors are used in accordance with the present invention to provide a uniform (or a required) voltage gradient along a supporting column or tube of the accelerator, the resistors are mounted as shown in Figure 2. Each resistor 23 is connected in series with another resistor 24, and the pair of resistors provides the electrical interconnection of I I mara~n.

12 adjacent metal discs 22. The respective connections at the clamps of a pair of resistors to the discs 22 are typically about 1 cm apart in their locations on the circumferences of the adjacent discs. The pairs of resistors are mounted alternately on opposite sides of the column or tube, to minimise the possibility of sparking between adjacent pairs of resistor protective tubes.

When used in connection with an accelerator tube, the resistor pairs may be sequentially staggered around laq the tube, to produce a helical array of resistor pairs, as shown in Figure 6. However, there is substantial flexibility in the way in which the array of resistor pairs may be arranged to bridge the S15 adjacent metal discs of a support column. For 0 Q* example, the resistor assemblies may be mounted cob tangentially to the tube electrodes to maximise the o clearance between the connecting loops (which join o the unclamped ends of the resistors) and the column 20 surfaces.

09 oo The construction of the resistor assembly of the present invention is illustrated in Figure 3. A protective metal tube 31 (usually of stainless steel) is force fit on to the support region 32 of a metal clamp 33 (see also Figure The force fitting of the tube to the clamp causes the circumference of the tube to "stretch" by 0.05 to 0.10 mm. The clamp 33 also has two arms 34 which are spaced apart a distance approximately equal to the thickness of a t -13metal disc 22. The clamp also has a threaded aperture 35 extending transversely through the arms 34, so that a clamping screw can be used to facilitate the clamping of the resistor to the edge of a metal disc 22.

A metal (usually brass) cap 37 is press-fit coaxially, and mechanically strongly, on to each end CO 0 of an elongate resistive element 36, having a 4 circular cross-section. As shown in Figure 5, the 10 closed end of each cylindrical metal cap 37 has a oOo central threaded aperture 51 into which is screwed, and secured in position by an adhesive such as LOC-TITE (trade mark), an externally threaded stud 52. The stud 52 (which may be fabricated from a stainless steel grub screw with its nylon tip 000:9 flattened) is provided with a cavity 53 having a c° hexagonal cross-section, to receive an Allen key.

0 0 o o Grub screws having such Allen key sockets are coo* commercially available. The presence of the Allen 20 key socket enables torque to be applied to the metal cap without influencing the connection between the resistive material of the resistive element and the 0°00 metal cap. Thus, when connecting the resistive element to the clamp 33, an Allen key is inserted through a clearance aperture 40 which passes through the clamp 33 from between the arms 34 to a tapped bore adapted to receive the threaded stud 52. The Allen key is used to tighten the connection of the i wn 14 brass cap 37 to the clamp 33, and thus mount the resistive element on the clamp, in a cantilevered manner, coaxially with the protective tube 31.

At the other end of the resistive element a cylindrical capacitive electrode 39 is securely mounted on the cap 37 using a hexagonal nut 48 which forms part of an RF connector 49. In assembling the o combination of electrode, connector and cap, the hexagonal nut is tightened against the stud 52 of its ro e 00 10 associated cap 37 while the stud is held stationary by an Allen key inserted into the socket 53 through o o an aperture which extends through the connector 49.

0o00 Preferably the capacitive electrode 39 is of titanium, to minimise spark erosion and weight. It 15 provides a spark gap with the protective metal tube «000 2 31, which reduces the electric field, and hence the .00 stress, on the resistive element at the junction of the resistive element with the metal cap. In 0. a* addition, during transient over-voltage conditions, 20 the electrode 39 will couple current into the first few convolutions or turns of the resistive element, e0.0o thus again reducing the electric stress.

o t Connections to the free end of the resistive element are achieved using loops of copper "pigtails" crimped or soldered to gold pla+.ed miniature banana plugs.

The banana plugs fit into the sockets of the RF connectors 49, being either force-fitted into the sockets or soldered into the sockets for good i i __I

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15 electrical contact. Preferably the hexagonal nuts 48 and the connectors 49 are of aluminium, with gold plating.

Assemblies of 800 Megohm (with two per cent tolerance) resistors constructed in accordance with the present invention have been tested in the 14UD Pelletron accelerator of The Australian National o S°o University. Each resistor had the features shown in ao Figure 3. The outer diameter of the protective tube o %10 31 was 25 mm for the resistors used with the support O060 o 9 columns of the accelerator and was 18 mm for b 0 0 resistors used in association with the accelerator 00 -a O tube.

In one test, a set of thirty-four resistors, each having a protective tube 31 of 18 mm outer diameter, was mounted with the resistors in pairs as shown in 0 o Figure 2. Each resistor pair successfully withstood a voltage difference of 73 KV, without breakdown.

There was no resistcr to resistor sparking when the °V .20 accelerator was used with the high voltage electrode V at voltages of from 16.0 to 16.7 MV.

0 Ube o After this test, the assembly of thirty-four resistors was retained in the accelerator while it was used in the normal manner for over a year.

During that use of the accelerator, the high voltage electrode was operated at voltages up to 15.5 MV.

The thirty-four test resistors were removed from the accelerator in March 1989 and the resistance of each resistor was measured at 5KV and at 3 volts. The i" 16 resistance of the resistors were all within the two per cent tolerance that had been specified originally.

A total of 3,044 resistors, constructed in accordance with the present invention, were installed in the Pelletron accelerator at the Australian National *o University, which has subsequently been operated o 0: normally, without any resistor breakdown.

Seoo Bo e o0 o Those skilled in this art will appreciate that a 10 particular form of the invention has been illustrated o.e and described above, and that variations of the illustrated embodiment can be made without departing from the present inventive concept.

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Claims (3)

1. A resistor for use in high voltage dc applications comprising: a metal clamp, adapted to be clamped to the edge portion of a metallic disc or flange used in the high voltage dc application, said metal clamp having a support region of substantially circular cross-section; a cylindrical metal tube that has been force fitted on to the support region of said metal clamp said metal tube extending from the support region away from the clamping portion thereof; an elongate resistive element, each end of which is terminated by a metal cap which fits over the end of the resistive material of the resistive element and makes intimate electrical contact with said resistive material, each metal cap having a centrally mounted, externally threaded projection from the end thereof, the projection having an Allen key socket or the like formed centrally therein, one of said metal caps being securely screwed into a receptive threaded aperture in the centre of the support region of said clamp, whereby the resistive element projects from the support region coaxially with said cylindrical tube; -I S 4 18 a cylindrical capacitive electrode having an outer diameter less than the inner diameter of said cylindrical tube mounted on the projection from the end of the metal cap of the resistive element which is remote from said support region; said capacitive electrode being coaxial with said resistive element and said cylindrical tube and defining, with said cylindrical Sa 0 tube, a spark gap for said resistor; and a connector also mounted on the end of said resistive element which is remote from said .oo support region, said connector being mounted adjacent to said capacitive electrode. 0 2. A resistor as defined in claim i, in which said °0 clamp has a pair of generally flat arms extending parallel to the axis of said support #oo oo region, said arms being separated by slot which has a width substantially equal to the thickness of said disc or flange, one of said arms having oo a threaded aperture extending therethrough in a 0 direction perpendicular to said slot, whereby a clamping screw may be inserted into said threaded aperture in said arm, to clamp said arms to said disc or flange.

3. A resistor as defined in claim 2, including a channel extending through said slot and said support region along the axis of the support 4 ~I .i P

14- 00 a 0000 0 Q 0 00 0 00 S00 0 O.9 a 0 0 000 a 0 o 00 0 0 00 00 0 0 0000 0000 O 0 19 region, for receiving an Allen key to facilitate the screwing of the externally threaded projection of a metal cap of said resistive element into said receptive threaded aperture. 4. A resistor as defined in claim 1, claim 2 or claim 3, in which said connector is formed as part of a nut having a channel therein through which an Allen key can pass, the end of said channel through the nut which is closest to said resistive element being threaded to receive the externally threaded projection of a metal cap of said resistive element. A resistor as defined in claim 4, in which said nut secures said capacitive electrode to its associated metal cap of the resistive element. 6. A first resistor as defined in claim connected in series to a second resistor as defined in claim 5 by a length of a copper "pigtail", each end of said "pigtail" terminating in a respective plug, each plug being connected to a respective connector of one of said first and second resistors. 7. A series connected pair of resistors as defined in claim 6, the clamp of each resistor of the pair being clamped to a respective one of a pair of adjacent metallic discs of an electrostatic accelerator. I 20 8. A resistor for use in high voltage dc applications, substantially as hereinbefore described with reference to Figures 3, 4 and of the accompanying drawings. DATED this fourth day of January 1991 THE AUSTRALIAN NATIONAL UNIVERSITY €0 G 0 00 00 0 By its Patent Attorneys 00 9 oo o DAVIES COLLISON Soo 0 0 o o 0 0 o0 0