NZ524719A - A portable high impedance resistive voltage divider device for measuring high voltages accurately - Google Patents

Abstract

A high impedance voltage divider is disclosed having a hook (3) for electrically connecting the divider to a high voltage conductor such as a live line (4), which is to be measured and tested. Further disclosed is a connecting sleeve (5) and a metal tube (6), both of electrically conductive material, where one end of the metal tube (6) is rigidly connected to the hook (3), and the other end of the metal tube (6) rigidly connected to the sleeve (5). The divider further includes a rigid tube (7) of electrically nonconductive material, which is rigidly connected at one end to the sleeve (5) and the other end to a housing (20) and a handle (12); the tube (7) and handle (12) are coaxial with the tube (6). Tube (7) contains a high-voltage resistance (13) having a first end which is adapted to be electrically connected to the sleeve (6) which is in turn connected to the metal tube (6) and the high voltage conductor. The second end is electrically connected to one end of a low voltage resistance (8) which is connected to earth. Controlling means for controls the electrical stress within the divider.

Description

524719 New Zealand Patent App. 524719 Filed: 13th March 2003 Patents Form No. 5 InteHectual Property Office of NZ MAR 2004 received Patents Act 1953 COMPLETE SPECIFICATION High-voltage Measurement Device We, Canterprise Limited, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, a New Zealand company, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 (to be followed by 1a) Title: High-voltage Measurement Device Technical Field The present invention relates to a high-voltage divider used as a portable or an in-situ device for measuring high voltages under live line conditions, to a high level of accuracy (typically at least +/-1.0% accuracy or better). As used herein, "high-voltage electricity lines" means lines that carry electricity at voltages above the domestic level of voltages, i.e. at distribution or transmission level voltages; typically, in New Zealand, these range from 6.6 kV - 220 kV (kilo volts) for AC (alternating current); DC (direct current) voltages may be higher. The divider of the present invention may be used for either AC or DC lines.

To measure such high voltages, it is necessary effectively to scale down the voltage by a known ratio, so that the voltage can be measured using a conventional low voltage meter.

Typically, the voltage would be scaled down by about 1000: 1. However, such scaling down can introduce errors, and for an accurate reading it is necessary to produce as close as possible to a perfect ratio, i.e. to minimise ratio errors and phase angle errors. Major sources of errors include: (a) capacitive coupling between the divider and other objects at different potentials; including anything at ground or higher voltage potential; (b) electric stress within or about the divider, causing ionization.

As used herein the term "electrical stress" means the voltage gradient, i.e. voltage per unit length.

The voltage divider is subjected to electrical stress because, to function, the divider must be connected between the high-voltage line and an earth connection. Under AC conditions capacitive coupling exists between points along the length of the divider and objects at different potentials, which causes current variation along the length of the divider, which in turn causes errors in the divider output, as measured with a low voltage voltmeter. 1a Background Art Known equipment capable of measuring voltages accurately under the above described conditions is very expensive, very bulky, and is sensitive to being moved. In 5 situ voltage dividers are used, e.g. in substations or in substation switches, for constant monitoring of the voltage. However, for many applications, a voltage divider needs to be portable, to test power transmission lines, often in remote sites.

Currently available portable voltage dividers are not very accurate - typically, about +/-10 3.0% accuracy. Currently available in situ voltage dividers achieve the required degree of accuracy, but only by using relatively heavy, bulky equipment which can suffer from ferro-resonance. The comparatively large size of this equipment means that it is costly in terms of storage space required, is expensive to transport and has a significant power consumption throughout its service life Disclosure of Invention It is therefore an object of the present invention to provide a high-voltage divider which is inexpensive, small and lightweight compared to currently available equipment, but 20 which has a high accuracy.

The present invention provides a high impedance resistive voltage divider, said divider including: - a high-voltage resistance having a first end adapted to be electrically connectable to 25 a source of voltage to be tested or measured and a second end opposite to said first end; - a low voltage resistance having a first end connected in series with said second end of the high-voltage resistance, and a second end connectable to earth; - controlling means for controlling the electrical stress within the divider.

The high-voltage resistance and the low voltage resistance may be formed separately and electrically connected together or may be formed as a single unit.

As used herein, the term "resistance" may mean a single resistance or two or more 35 separate discrete resistors, connected together as discussed below. 2 Preferably, the means for controlling the electrical stress also mitigates the effect of capacitive coupling and comprises an electrically conductive shield which substantially surrounds the high-voltage resistance, but is electrically insulated therefrom; one end of said shield being adapted to be electrically connectable to the same source of voltage as said high-voltage resistance, and the other end of said shield being connectable to earth.

Preferably, said shield comprises a tube formed from a series of segments of electrically conductive material, said segments being spaced apart, and connected together, by a plurality of resistors arranged such that current passing from one end of said shield to the other must pass through each said segment and through each said resistor.

Alternatively, said shield may comprise a tube of semiconductor material or a tube of insulating material supporting a layer of semiconductor paste material.

The high-voltage and low voltage resistances each may be selected from one or more of the group consisting of: a single resistor of known type; two or more resistors of known type connected in series; a plurality of resistors of known type arranged in sets connected in parallel, said sets being connected in series; a tube of semiconductor material selected to provide a predetermined resistance; a rod of semiconductor material selected to provide a predetermined resistance; a rod or tube of insulating material supporting a layer of semiconductor paste material selected to provide a predetermined resistance.

In one embodiment of the invention, the high voltage resistance is selected from the group consisting of: a plurality of resistors of known type connected in series; and a plurality of resistors of known type arranged in sets connected in parallel, said sets being connected in series; and the means for controlling the electrical stress comprises a plurality of discs of electrically conductive material, said discs being spaced along the length of the divider, arranged between, and electrically connected to, said resistors.

Brief Description of the Drawings By way of example only, preferred embodiments of the present invention are 3 described in detail with reference to the accompanying drawings, in which:- Fig. 1 is a schematic side view of the present invention incorporated in a portable high voltage measurement device; Fig. 2 is a section view, part cut away, of a first embodiment of the high voltage divider, Fig. 3 is a plan view of a ring disc shown in Fig 2; Fig. 4 is a section view of the ring disc along line XX shown in Fig. 3.

Fig. 5 is a schematic side view of the second embodiment of the high voltage divider, Fig. 6 is a perspective view of the shield segment shown in Fig 5, Fig. 7 is an end view of the spacer shown in Fig 5, and Fig. 8 is a side view of the spacer shown in Fig 7.

Fig. 9 is a longitudinal section through a third embodiment of the present invention; Fig. 9a is an isometric view of a shield segment of Fig 9; Fig. 10 is a longitudinal section through a fourth embodiment of the present invention; Fig. 11 is a section on line Y-Y of Fig. 10; Fig. 12 is a longitudinal section through a fifth embodiment of the present invention; Fig. 13 is a longitudinal section through a sixth embodiment of the present invention; and Fig. 14 is a section on line Z-Z of Fig. 13.

Best Mode for Carrying out the Invention Referring to Fig.s. 1 - 4, a high-voltage divider 2 includes an electrically conductive 25 hook 3 for electrically connecting the device to a high-voltage conductor such as a live line 4 which is to be measured or tested, a connecting sleeve 5 and a metal tube 6, both of electrically conductive material, with one end of the metal tube 6 rigidly connected to the hook 3, and the other end of the metal tube 6 rigidly connected to the sleeve 5. The divider further includes a rigid tube 7 of electrically nonconductive 30 material, which is rigidly connected at one end to the sleeve 5 and the other end to a housing 20 and a handle 12; the tube 7 and handle 12 are substantially coaxial with the tube 6.

This device is designed to be transportable, and the tube 7 may be designed to be 35 disconnected from the tube 6 for transport, to reduce the overall length of the device. 4 The handle 12 also may be designed to be disconnected from the housing 20 and tube 7, and may be made in one or more sections if additional length is needed. The device can be adapted for mounting in a mechanical support if necessary.

The tube 7 contains a high voltage resistance 13 which extends down the centre of the tube 7. The high-voltage resistance 13 incorporates a series of connected resistors 14, as described below, which are electrically connected in series; one end 13a of the resistance is electrically connected to the sleeve 5; the other end 13b of the resistance 13 is electrically connected to one end of a low voltage resistance 8. The other end of the resistance 8 is connected to earth via a metal housing or screen 20 which is earthed by an earth connection 11. A volt meter 9 of known type is connected across the low voltage resistance 8, referenced to earth, by a shielded cable 21. The voltmeters earth connection is via the metal screen 20.

The meter 9 may be of any suitable known type. Preferably, the meter 9 is electrically isolated from earth to prevent electromagnetically and/or electro statically induced currents affecting the accuracy of readings.

Referring in particular to Fig.s. 2-4, the high-voltage resistance 13 mounted within the tube 7 is made up of a sequence of pairs of resistors 14 arranged in parallel, the sets of paired resistors being connected in series, with an electrically conducting disc 15 positioned between each set and at each end of the resistance 13. Each resistor 14 is electrically connected to the adjacent discs 15 (e.g. by soldering if the discs are of metal). Each set of resistors is arranged at 90° to the preceding and succeeding sets.

The number of resistors used, and their arrangement, may be varied to suit particular applications. The rating of the resistors used, and the number of resistors used, are determined by the design voltage of the device and are selected such that at the rated voltage of the device, the resistors are performing below their rated wattage and voltage.

As shown in greater detail in Fig.s. 3 and 4, each disc 15 is a thin disc 17 with an annular rim 18 which has a radius larger than the thickness of the disc and which provides a relatively large, smoothly rounded surface which is designed to avoid any concentration of field lines around the edge of each disc and so the rounded shape of the edges of the discs 15 assists in reducing the electrical stress between adjacent discs caused by the voltage drop along each resistor 14.

The diameter of each of the discs 15 is significantly greater than the diameter of each resistor 14, so that the effect of the discs 15 is to spread the electrical field about the resistance 13 over a greater area, and thus reduce the electrical stress. The diameter of each disc 15 and the radius of curvature of the annular rim 18 of each disc 15 are determined by the maximum operating voltage across each resistor 14 and by the ionization inception value for the dielectric about the divider at normal operating voltage. The discs 15 also act as a heat sink to help dissipate heat from the resistors 14 and as a mechanical support for the whole of the resistance 13.

A central hole 19 in each disc 15 allows the discs 15 and resistors 14 to be completely encapsulated. Additional spaced holes 19a may also be provided to assist in mounting the resistors 14 on the discs.

The discs 15 may be made of any robust material which conducts electricity and heat effectively and to which the resistors 14 can be electrically connected. One suitable material is brass.

The resistors 14 and discs 15 are encased in an electrically insulating material 16 within the tube 7. The insulating material may be a solid, liquid or gas, or a vacuum. The use of a solid insulating material increases the mechanical strength and stability of the resistance 13. A solid or a liquid insulation material improves the shock absorbing ability of the resistors 14. Suitable solid insulating materials which are electrically insulating and thermally conductive are silicon rubber compounds or epoxy resin compounds. A suitable liquid insulating material is oil; suitable insulating gases air and sulphur hexafluoride.

The use of the insulating material 16 limits the magnitude of currents in paths other than through the resistance 13, and helps to minimise ratio error resulting from unequal currents flowing in the high and low voltage resistances.

The low voltage resistance 8 is connected in series with the high-voltage resistance 13; Fig. 2 shows the low voltage resistance 8 consisting of a single resistor, but two or more resistors may be used if required. The low voltage resistance 8 is tunable to provide ratio adjustment under live conditions. The resistance 8 is enclosed in the 6 earthed metal screen 20 and may be encapsulated in an insulation material 25, which is selected using the criteria discussed above relating to the insulation material 16. The insulation material 25 assists in controlling the effect of leakage of current down the outside of the tube 7; this effect is further controlled by screening the meter 9 by using a shielded cable 21 between the screen 20 and the meter 9.

The above described device is used by connecting up the earth connection 11 and then hooking the hook 3 over the live line 4 to be tested, and reading the test result from the meter 9. If necessary the device is first calibrated by measuring a known voltage and tuning the low voltage resistance 8 as necessary to get the correct reading on the meter 9.

Example 1 given below lists the specifications for a device of the above described type.

EXAMPLE 1 Appropriate specifications, derived from practice, for a device for stand alone voltage measurements could be as follows: AC measurement:: 50 kV rms DC measurement:: +/-70 kV Divider ratio: 10,000:1 Rating of high voltage resistors: 10 kV a.c., 7 kV d.c. ring discs : thickness:1 mm. rounded edges: 3 mm radius overall diameter:26 mm. insulating material: silicon rubber.

The device (2) was overvoltage tested to 120 kV rms for one minute.

A device constructed to the specifications given in Example 1 has been found to be accurate to +/-0.5%, as determined using regression analysis of recorded outputs. It will be appreciated that with appropriate selection and design of the resistors 14 and discs 15, a device 2 can be devised suitable for the accurate testing of high voltage in any particular range of voltages. 7 The above described device gives particularly accurate readings for DC voltages. However, when AC voltages are to be measured, the design has a drawback in that under AC excitation, capacitive coupling of the external electrical fields between the high-voltage resistors objects such as earth/the high-voltage line may induce spurious currents which are not directly derived from the connection of the divider to the voltage source, giving rise to measurement errors. Since the discs 15 are electrically connected to the resistors 14, this error source cannot be corrected for. To overcome this source of error, the designs described with reference to Fig.s 5 -14 are used. It should be noted that the embodiment shown in Fig.s 5 - 14 are suitable for use with either AC or DC voltage.

Referring to Fig.s 5 - 8, in this embodiment, in the high-voltage resistance 13, the discs 15 are omitted and are replaced by an electrically conductive shield 26 which extends around the resistance 13, substantially coaxial with the string of resistors 14 which are connected together in series to form the resistance 13. For simplicity, Fig. 5 shows the resistors 14 assembled as a straight line of resistors connected in series, but, as in the embodiment described with reference to Fig.s 2-4, the resistors 14 may be connected together in a variety of different configurations. In this embodiment, the low voltage resistance 8, meter 9, earth connection 11, and other components not illustrated in Fig. 5 are the same as in the embodiment shown in Fig.s 2-4, and the same reference numerals are used where appropriate. For clarity, the outer tube 7 of electrically nonconductive material is not shown in Fig. 5.

The shield 26 is made of a series of spaced segments 27 (shown individually in Fig. 6) which are electrically and physically connected together in series via shield resistors 28 to form a substantially cylindrical shield 26.

Each segment 27 is an open ended tube with cutouts in its ends 29,30, the cutouts being bent and cropped to form two pairs of diametrically opposed tabs 31. The segments 27 are assembled together with the open ends 29,30 of adjacent segments aligned to form a passage through the shield, along which the high-voltage resistance 13 extends. The resistance 13 does not contact the shield 26.

The segments 27 are spaced apart and connected together by pairs of shield resistors 28, each of which is connected between one tab 31 on one segment 27 and the 8 opposite tab 31 on the adjacent segment 27. Each shield resistor 28 is slightly longer than the space between each corresponding pair of tabs 31, so that the presence of the shield resistor spaces the corresponding segments 27 apart. The shield resistors are secured to the tabs 31 by any suitable means, e.g. soldering. The segment 27 5 also are spaced apart by spacers 32 (Fig.s 7 and 8) of insulating material. Each spacer 32 is a flat plate sized to lie between adjacent segments 27 and formed with a central aperture 33 which allows the high-voltage resistance 13 to extend through the spacer 32.

As in the previous embodiment, the components may be encapsulated by a suitable insulating material (not shown) which provides further mechanical support for the components whilst increasing the dielectric strength of the medium around the high-voltage resistance 13, thus providing the control of the electrical stress.

At the end of the divider opposite to the low voltage resistance 8, both the shield 26 and the high-voltage resistance 14 are connected to the electrically conductive sleeve 5 and metal tube 6. At the earth end, the shield is connected to earth. Thus, in use, the shield 26 is live:- current travels down the shield by passing through consecutive segments 27 via the corresponding shield resistors 28. Because the shield 26 is raised to the same potential as the high-voltage resistance 13, and surrounds the resistance 13. The voltage difference between the high-voltage resistance 13 at any point of its length and the corresponding adjacent portion of the shield, is low or zero, so there is little or no tendency for stray currents to be created along the high-voltage resistance 13. The shield carries the majority of the external electric field coupling current.

Example 2 gives a sample specifications for a device made in accordance with the above described second embodiment.

EXAMPLE 2 Appropriate specifications, derived from practice, for a live line measuring device on a single phase 220 kV a.c. system could be as follows: AC measurement:: 150 kV rms DC measurement: +/-200 kV 9 Divider ratio: 10,000:1 Metallic shields: overall diameter: 30 mm. insulating material: silicon rubber.

The device (2) was overvoltage tested to 400 kV rms for one minute.

Fig. 9 shows a variant of the embodiment described with reference to Fig.s 2 - 4.

This embodiment is designed especially for in situ applications; Fig. 9 shows the whole of the device 40.

The device is contained in a tube 41 of insulating material and includes a high-voltage resistance 42, a shield 43 and a low voltage resistance 44. One end of the tube 41 is closed by a cap 45 of electrically conductive material which is electrically connected to a terminal 46, which is in use connected to the high-voltage line to be measured or tested. The cap 45 is electrically connected to the high-voltage resistance 42 and to the shield 43. The other end of the tube 41 is closed by an earthed plate 47 with earth pins 48. The adjacent portion of the shield 43, and the low voltage resistance 44, both are electrically connected to the plate 47. A voltage meter 49 of known type is connected across the low voltage resistance 44 and referenced to earth.

The high-voltage resistance 42 is depicted as a simple string of resistors 50 connected in series, but in fact may incorporate resistors arranged in any suitable configuration, as discussed with reference to the embodiment shown in Fig.s 2-4.

The shield 43 is assembled from shield segments 51 as shown in Fig. 9a, spacers 52 and shield resistors 53. Each shield segment is made of an electrically conductive material and is in the form of an open ended tube with two diametrically opposed cutouts, one on each end of the tube, each cutout being cropped and in-turned to form a tab 54 to which the spacers 52 and shield resistors 53 are secured, e.g. by soldering. Each spacer 32 is made of electrically insulating material.

The shield segments 51 are assembled to form the shield 43 with their open ends aligned, to form a hollow column through which the high-voltage resistance 42 passes without contacting the shield. The tabs 54 on each shield segment 51 are aligned with the tabs on the preceding and succeeding segments, and the shield resistors 53 are secured between each pair of aligned tabs. The spacers 52 are secured between consecutive aligned resistors 53, to maintain the correct spacing between adjacent shield segments 51 and to strengthen the whole assembly. The completed shield is such that current travelling through the shield must pass from one segment to the next via a resistor, then travel through that shield segment to reach the next resistor and pass into the next segment, and so on down the length of the shield.

The low voltage resistance 44 comprises a disc of electrically insulating material 55 which supports one or more resistors 56. Preferably, two or more resistors 56 are used, one or more of which is connected to a switch 57 which permits the or each corresponding resistor to be switched in or out of circuit, to enable fine tuning of the device.

All of the components within the tube 30 preferably are encapsulated in a thermally conductive but electrically insulating medium, (e.g. a suitable epoxy resin).

The above described device operates in the same manner as the device is described with reference to Fig.s 5-8.

Example 3 gives sample specifications for a device as described with reference to Fig. 9:- EXAMPLE 3 Appropriate specifications, derived from practice, for a live line measuring device on a single phase 7.4kV AC system could be as follows: AC measurement: 4.2kV rms divider ratio: 1000: 1 shields: overall diameter approximately 38 millimetres insulating material: epoxy resin the device was over voltage tested to 22kV for one minute.

Fig.s 10 and 11 show a further embodiment of the invention, in which the high-voltage resistance is formed by a rod 60 of semiconducting material and the low voltage resistance 61 is formed integrally with the high-voltage resistance, by making an electrical connection to the volt meter 62 at a position 63 a short distance from the earth connection 64 on the rod 60. 11 The semiconducting material of which the rod 60 is made is selected from a range of known materials, (e.g. silicon carbide) to provide the required degree of resistance for the selected length and diameter of the rod 60, to form both the high-voltage resistance and the low voltage resistance. The earth connection 64 is connectable to earth 65 in known manner. The volt meter 62 is connected across the low voltage resistance and earth as discussed with reference to the other embodiments.

The shield of the device is formed from a tube 66 of semiconducting material; this may be the same or similar material as is used for the rod 60. The tube 66 is concentric with the rod 60 and is arranged with its longitudinal axis parallel to that of the rod 60. An electrical contact 67 which is adapted to be connected to the high-voltage line or other component whose voltage is to be measured or tested, is secured at the end of the tube 66 and rod 60 opposite to the earth connection 64, with the contact 67 electrically connected to both the rod 60 and tube 66.

Fig. 12 shows a variant of the embodiment of Fig.s 10 and 11:- in this embodiment, the low voltage resistance is formed by a separate resistor 70 which is connected in series with the high-voltage resistance 71, which is formed from a rod of semiconducting material of the same type as described with reference to Fig. 10. The low voltage resistance 70 is connected in series between one end of the high-voltage resistance 71 and the earth connection 64. Except for having a separate resistor as the low voltage resistance, the Fig. 12 embodiment is otherwise identical to the embodiment of Fig.s 10 and 11, and the same reference generals are used where appropriate.

Fig.s 13 and 14 show a further variant of the embodiment of Fig.s 10 and 11. In this embodiment, the low voltage resistance 75 and the high-voltage resistance 76 are formed integrally, and both these resistances and the shield 77 are formed from a layer of semiconducting paste 78,79 respectively, spread over a suitable supporting surface of insulating material 80,81. A range of semiconducting pastes are commercially available (e.g. Resistive composition type 3910, available from Electro Science Laboratories); the paste is applied to the supporting surface in the same manner as the paint, and is then baked to form a hard stable surface. The paste is selected such that, for the volume of paste to be used, the paste provides the required resistance for the high-voltage and low voltage resistances and for the shield respectively. The paste may be applied on the inside or the outside of the supporting 12 tubes.

As in the Fig. 10 embodiment, the low voltage resistance 75 is formed integrally with the high-voltage resistance 76. However, a separate low voltage resistance (as in Fig. 5 12) may be used if preferred. The arrangement of the earth connection and volt meter is the same as for Fig.s 10 and 12, and the same reference numerals are used.

At the end of the high-voltage resistance 76 opposite to the earth connection 64, the high-voltage resistance 76 and the shield 79 both are in electrical contact with the 10 electrically conducting applied 82 which also is electrically connected to a contact 83 to which the high-voltage line to the tested or measured may be connected.

In the embodiments described with reference to Fig.s 10 - 14, the space 84 between the shield and the high and low voltage resistances preferably is filled with a thermally 15 conductive but electrically insulating material such as an epoxy resin. Also, the outer surface of the shield may be protected by an outer insulating tube if required; this is not shown in the drawings for reasons of clarity.

In the embodiments of Fig.s 10 - 14, the high and low voltage resistances and the 20 shield are shown as circular in cross-section, concentric and coaxial. It is believed that this is the most efficient configuration, since it ensures that the spacing between the shield and the resistances remains constant along the length of the resistances, and avoids any concentrations of electric fields. However, other cross-sectional shapes and configurations would be possible. Further, in the Fig. 10-12 embodiments, the 25 resistances 60,71 could be formed as tube rather than as a solid rod.

It will be appreciated that the embodiments of Fig.s 10-14 function in the same manner as the embodiments of Fig.s 5-9.

If necessary, the embodiments of Fig.s 10 to 14 could be made as a series of sections connected together in series; this could be useful if a very long device were required to be portable. Further, the embodiments of Fig.s 10-14 could be modified to use a combination of the resistances made of semiconducting material as described above, and separate resistors, connected in series. The separate resistors may be of any 35 suitable known type, e.g. metal film high-voltage resistors such as the resistors sold under the trademark Phillips VR 68. 13

Claims (15)

1. A high impedance resistive voltage divider, said divider including: - a high-voltage resistance having a first end adapted to be electrically connectable to a source of voltage to be tested or measured and a second end opposite to said first end; - a low voltage resistance having a first end connected in series with said second end of the high-voltage resistance, and a second end connectable to earth; - controlling means for controlling the electrical stress within the divider.
2. The divider as claimed in claim 1, wherein the means for controlling the electrical stress also mitigates the effect of capacitive coupling and comprises an electrically conductive shield which substantially surrounds the high-voltage resistance, but is electrically insulated therefrom; one end of said shield being adapted to be electrically connectable to the same source of voltage as said high-voltage resistance, and the other end of said shield being connectable to earth.
3. The divider as claimed in claim 2, wherein said shield is substantially coaxial with the longitudinal axis of the high-voltage resistance.
4. The divider as claimed in claim 2 or claim 3, wherein said shield comprises a tube formed from a series of segments of electrically conductive material, said segments being spaced apart, and connected together, by a plurality of resistors arranged such that current passing from one end of said shield to the other must pass through each said segment and through each said resistor.
5. The divider as claimed in claim 2 or claim 3, wherein said shield comprises a tube of semiconductor material selected to provide a predetermined resistance.
6. The divider as claimed in claim 2 or claim 3, wherein said shield comprises a tube of insulating material which supports a layer of semiconductor paste material selected to provide a predetermined resistance. 10
7. 7. The divider as claimed in claim 6, wherein said high-voltage resistance and said low voltage resistance each are selected from one or more of the group consisting of: a single resistor of known type; two or more resistors of known type connected in series; a plurality of resistors of known type arranged in sets connected in parallel, said sets being connected in series; a tube of semiconductor material selected to provide a predetermined resistance; a rod of semiconductor material selected to provide a predetermined resistance; a rod or tube of insulating material supporting a layer of semiconductor paste material selected to provide a predetermined resistance.
8. 8. The divider as claimed in claim 7, wherein the high-voltage resistance and the low voltage resistance are separate resistors.
9. 9. The divider as claimed in claim 7, wherein the low voltage resistance is formed is integrally with the high-voltage resistance.
10. 10. The divider as claimed in claim 1, wherein the high-voltage resistance is selected from the group consisting of: a plurality of resistors of known type connected in series; and a plurality of resistors of known type arranged in sets 20 connected in parallel, said sets being connected in series; and wherein the means for controlling the electrical stress comprises a plurality of discs of electrically conductive material, said discs being spaced along the length of the divider, arranged between, and electrically connected to, said resistors. 25
11. 11. The divider as claimed in claim 10, wherein each said disc has its outer circumference formed as a rounded rim having a greater thickness than the remainder of said disc.
12. 12. The divider as claimed in any one of the preceding claims, wherein said divider 30 further includes a volt meter connectable across the low voltage resistance referenced to earth via an earth connection.
13. 13. The divider as claimed in any one of the preceding claims, wherein said divider further includes a tube of electrically insulating material which surrounds the 35 means for controlling the electrical stress. 15
14. 14. The divider as claimed in claim 13, wherein the components of the divider surrounded by said tube are embedded in an electrically insulating material.
15. 15. A high impedance resistive voltage divider substantially as hereinbefore described with reference to and as shown in Fig.s 1 - 4 or Fig.s 5 - 8 or Fig. 9 or Fig.s 10 and 11 or Fig. 12 or Fig.s 13 and 14 of the accompanying drawings. Intelhctuai d °^ce 0f jy|erty 15 HAR 2004 received 16