GB2249666A - Improvements in or relating to capacitive couplers and associated gas insulated switch systems - Google Patents

Abstract

A capacitive coupler (22) is constructed from two electrically conductive tubes (24, 26) separated by a di-electric tube 28, and a central, electrically conductive pin (30). The resulting shape enables the coupler (22) to be fitted to suitable external features of a gas filled switch enclosure (12a) in a one or more phase circuit and thus obviates the need for piercing the enclosure walls for the passage of wiring. The coupler (22) is mounted in dielectric-separated flanges (20a) of the enclosure (12a) by nuts (36, 38) and receives at one end a coaxial cable (34) leading to circuitry for detecting unacceptable discharges within the switch enclosures. A brass screen (40) is provided. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO CAPACITIVE COUPLERS AND ASSOCIATED GAS INSULATED SWITCH SYSTEMS Many electrical systems which operate at high voltages e.g. the national grid, use electrical switch gear which is enclosed in a di-electric gas, usually SF.6.

The switch and busbar enclosures in some such systems are constructed from flanged members, the . flanges of abutting portions thereof being bolted together with a di-electric busbar support sandwiched therebetween.

The function of the switch gear is to enable, direct, or prevent a flow of electricity, as desired. During operation, partial discharges occur in random manner, and are caused by a variety of phenomena which includes dust, insulated electrodes, highly electrically stressed regions, mechanical vibration, transient electrical flow during switching action, and faults.

It is important to monitor the partial discharge activity, so as to detect those discharges the persistence of which is on a scale which is an indication of malfunction, rather than an indication of those discharges which occur for other stated reasons.

It is convenient to detect the potential differences which develop when partial discharges occur, and use signals obtained therefrom to assess the electrical conditions within the enclosures.

Commonly, existing capacitive couplers are positioned within the gas filled enclosures, and their associated wiring passes through the walls of the enclosures for connection to the remainder of the circuit. As, typically, about one hundred such couplers are required in a sixteen circuit sub station, a corresponding number of holes are required in the walls of the gas filled enclosures, in order to connect the couplers to their external circuits, all of which holes are potential gas leak sources, with attendant risk of equipment failure.

Other known capacitive couplers have also been tried.

Such couplers are suitable for mounting on the external surfaces of the enclosures, and so wiring holes are not needed. Apertures do have to be provided however, so as to enable the fitting of di-electric windows, over which the couplers are positioned. Again, one window is provided for each coupler used, and each window need to be sealed, and so creates a risk of leakage. A further drawback manifested itself by way of excessive electrical noise which masked the signals being sought.

The present invention seeks to provide an improved capacitive coupler.

According to the present invention a capacitive coupler comprises a tube having electrically conductive end pieces axially separated by a di-electric centre portion and an electrically conductive pin nestable within the tube over the full length thereof in radially spaced relationship, and so as to bridge the di-electric centre portion, one end of the pin and one end piece surrounding said one end being respectively adapted for connection via wiring to a control console, and means for fixing the coupler in a desired structural location on electrical switch enclosure structure.

The invention will now be described, by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a sub station 3 phase busbar enclosure arrangement including capacitive couplers in accordance with the present invention.

Figure 2 is an enlarged, axial cross-sectional view of a capacitive coupler in a flanged joint of an enclosure of Figure 1.

Figure 3 is a block diagram of circuitry for connecting the capacitive coupler of Figure 2 to a control console.

Referring to Figure 1. A sub station busbar enclosure arrangement is indicated generally by the numeral 10.

The busbars (not shown) are contained, one in each of a plurality of parallel enclosures 12a, 12b and 12c in known manner. Further busbars (not shown) are contained in further enclosures 14a, 14b and 14c, which are supported on ground standing pillars 16 and 18.

The enclosures 14a, 14b and 14c lie in directions normal to that of enclosures 12a, 12b and 12c.

Each of the aforementioned enclosures are constructed from coaxially arranged conduits which are Joined in known manner by bolted flanges e.g. 20a, 20b and 20c. The fastening bolts and their cooperating nuts are not shown.

Each flanged joint e.g. 20a, 20b and 20c has a di-electric material therebetween, for the support of the busbars (not shown) in known manner.

A capacitive coupler 22 is fitted in each of some of the flanges e.g. 20a, 20b and 20c. Both a capacitive coupler 22 and the di-electric support 21 are more clearly seen in Figure 2, to which reference is now made.

Each capacitive coupler 22 is an assembly of a pair of electrically conductive metal (preferably brass) tubes 24 and 26, held in axially spaced relationship by a dielectric spacer tube 28.

An electrically conductive metal pin 30, again, preferably brass, is coaxially nested within the assembly of ferrules 24, 26 and 28, and extends therethrough so as to protrude from one end, and to bridge the di-electric spacer tube 28.

The protruding end of the pin 30 is adapted so as to fit in locking engagement with the metallic core 32 of a standard coaxial cable 34.

The length of the assembly of ferrules 24,26 and 28 is such that when the capacitive coupler 22 is positioned in aligned holes through the flanged joint 20a and the di-electric support 21, so that the di-electric spacer 28 is bounded by the di-electric support 21, the ferrules 24 and 26 protrude from respective sides of the flanged joint 20a.

As thus described, the position of the capacitive coupler 22 with respect to the flanged Joint 20a, is the desired position, and this applies to all the capacitive couplers 22 and their respective flanged joints.

The protruding portions of the ferrules 24 and 26 have external screw threads on which knurled nuts 36 and 38 are screwed into abutment with respective faces of e.g. the flanged joint 20a, thus retaining the coupler 22 in the desired position. The nuts 36 and 38 also hold a brass screen 40 in position, over that part of the flanged joints in which each coupler 22 is located.

Referring back to Figure 1. It is seen that the capacitive couplers 22 are effectively arranged in groups of three, in that a coupler 22 in each of respective flanged Joints 20a, 20b and 20c e.g. are connected via respective coaxial cables 34, 34a and 34b to a common detector circuit 42, which is fixed to a pillar 16. Thus several detectors 42 each receives signals from each of the three phases of the circuitry of the station.

A further positional relationship exists between the flanged joints on each enclosure. Thus e.g. the enclosure 12a has a number of flanged joints 20a, some of which contain a coupler 22 as described hereinbefore.

When a partial discharge occurs within the or any of the enclosures 12a, 12b or 12c, radio frequency (RF) signals, the velocity of which is known, are emitted at the discharge source. Since the velocity is known and the distance between the flanged joints 20a which contain the couplers 22 is known, the development of a formula is enabled, use of which gives an indication of the position of the source of the discharge.

The formula is: 1 =!L (L - 0.3t)m where 1 t distance of discharge source from nearest coupler 22 L = distance between each given coupler 22 and the next coupler 22 in the line t = time in nano seconds m = metres.

The arrangement of the couplers 22 in any given enclosure also enables assessment of the source of origin of a partial discharge, when used in conjunction with a characteristic of the discharge, wherein the waves generated by the discharge reduce in amplitude as they move away from the source. Thus, that or those capacitive couplers 22 in a row of capacitive couplers 22 which develop the highest potential differences, indicate the area in which the partial discharge occurred.

In operation, when partial discharges occur, on reaching a flanged joint in the relevant enclosure, say 12a, the energy so developed travels through the di-electric support 21 and into a coupler 22. There, the energy pulse generates an electrical potential difference across the two end pieces 22 and 24, and this is passed, via the coaxial cable 34 to circuitry which is described later in this specification.

As has been stated hereinbefore, it is important to monitor those partial discharges, the persistence of which is such as to indicate a malfunction. It has been found that such discharges occur in the frequency range 500 to above 1100 MHz whilst partial discharges frequencies in total, range from several hundred to 1500 MHz. Thus the circuitry which connects each capacitive coupler 22 to some read out station, includes means for filtering out those signals developed from discharges, the frequencies of which are in those ranges either side of 800 MHz to 1100 MHz.

Such circuitry is shown in Figure 3, to which reference is now made.

Each detector unit 42 receives voltage inputs from a group of three respective capacitive couplers 22j which inputs enter the detector 42 via coaxial cables 34, 34a and 34b.

The signals pass via signal conditioners 36a, 36b and 36c, which amplify them, to filters 38a, 38b and 38c. The filters remove signals, the frequencies of which do not fall within the bands 800 MHz and 1100 MHz and then pass the remaining signals via outputs 40a, 40b and 40c, to a phase summation device 62 which adds the outputs of the three phases before passing them via an output 64 to a control console (46 in Figure-l) and, at the same time via a line 66 to delay and counter devices 67 and 68.

The delay portion 67 delays entry of pulses into the counter until such time as the output from the summation device 62 has persisted for, say, five seconds, in which case a pulse is issued to the counter 68. If over a period of, say, one minute, ten pulses have issued to the counter 68, it means that a signal is persisting and the counter 68 issues a signal to the output 64 to cause a warning device (not shown) in the console 46 (Figure 1) to give an indication that investigation is needed.

The necessary steps having been taken, a reset device 70 resets the counter to zero, and the system is reactivated.

If the persistence of signals is insufficient to cause the counter 68 to fully count over the stated period, then at the end of that period, the reset device 70 automatically resets the counter 68 to zero, from whatever number it had reached.

A test circuit 72 is included, whereby the counter and cutout system can be checked by the deliberate insertion of appropriate pulses at the input of the circuit. This is necessary, as in the field, the circuit is expected to lie dormant for considerable periods of time.

With regard to the mode of manufacture of the capacitive coupler of the present invention, the pin 30 can be permanently fixed in spaced relationship with the interior of the assembly of tubes 24, 26 and 28. The fixed spacing could be achieved by the use of di-electric, disc or spoked shaped spacers adjacent each end of the pin, which engage the interior surface of the assembly and through which the pin passes in close fitting relationship.

Alternatively, the pin could rely upon its engagement with the coaxial cable and the engagement of the coaxial cable with one of the tubes 24 or 26.

The man skilled in the art having read this specification, will appreciate the benefits to be achieved by its use, namely the total obviation of holes or apertures in the walls of gas filled enclosures, the ease of assembly and fitting of the capacitive coupler 22, and the ability to locate it in places where signal interference is low.

Claims (10)

Claims:

1. A capacitive coupler comprising a tube having electrically conductive end pieces axially separated by a di-electric centre portion and an electrically conductive pin nestable within the tube in radially spaced relationship and so as to bridge the di-electric centre portion, one end of the pin and an end piece surrounding said one end being adapted for connection via wiring to a control console and means for fixing the capacitive coupler in a desired location on electrical switch enclosure structure.

2. A capacitive coupler as claimed in claim 1 wherein the means for fixing the coupler in a desired location comprises screw threads formed on the outer diameters of the end pieces and a pair of nuts which when screwed thereon, clamp the coupler to said structure at said desired location.

3. A capacitive coupler as claimed in claim 1 or claim 2 wherein the end pieces and the pin are manufactured from brass.

4. A capacitive coupler as claimed in any previous claim wherein the pin is permanently fixed within said tube.

5. A capacitive coupler as claimed in any of claims 1 to 3 wherein for operation, the pin is maintained in said spaced relationship within the tube by its mode of fixing to said wiring.

6. A capacitive coupler as claimed in any previous claim wherein said wiring comprises a coaxial cable.

7. A capacitive coupler as claimed in claim 6 wherein said coaxial cable connects the capacitive coupler to a partial discharge detector upstream of the control console.

8. A capacitive coupler substantially as described in this specification and with reference to Figures 1 and 2 of the drawings.

9. An electricity switching station including capacitive coupler means as claimed in any previous claim in this specification.

10. An electricity switching station substantially as described in this specification and with reference to the drawings.