-1 Regulation 3. AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT (ORIGINAL) Name of Applicant: Noja Power Switchgear Pty Ltd Actual Inventors: Dr Brett Watson Address for Service: DAVIES COLLISON CAVE, Patent Attorneys, Level 3, 303 Coronation Drive, Milton 4064, Queensland. Invention Title: "Electrical screening system" Details of Associated Provisional Application No: Australian Provisional Patent Application No. 2008901446, filed on 26 March 2008 The following statement is a full description of this invention, including the best method of performing it known to us: Q:\oper\nS\2009\march\40132255 noja AU It PO filing complete 1l032009.doc - 10/3/09 -2 ELECTRICAL SCREENING SYSTEM Background of the Invention The present invention relates to insulating apparatus for use in electrical apparatus, and in particular to a bushing suitable for use in a switching apparatus such as a recloser. Description of the Prior Art The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. Reclosers have been in use world wide for more than 30 years. Reclosers are medium voltage circuit breakers used in medium voltage applications, such as on overhead power lines and substation applications, which typically operate at between 10-38kV. Reclosers operate to detect over current faults on a line, and then break the current path, to thereby interrupt power supplies. After the over current has passed, the recloser closes again automatically, thereby restoring the current path. This operation is typically performed up to four times, to allow transient faults such as lightning strikes, to be interrupted, thereby preventing damage to the power lines and equipment coupled thereto. Typically the fault is interrupted within 200 milliseconds and 0 supply is restored within 500 milliseconds to several seconds. The protection characteristics can be programmed by the user and today are all based on microprocessor based electronic controls. In general all reclosers include a switching element contained within a housing. Such reclosers typically require the presence of one or more bushings for allowing conductors to 5 extend through the housing. Such bushings are typically formed from an insulating material and may include a screening element for distributing any electrical fields so as to avoid high field concentrations, which can in turn damage the bushing leading to bushing failure.
-3 EP-A-1,017,142 describes a recloser that comprises a vacuum switch having its contacts in the shape of current inputs as well as transmission-type transformers connected to said current inputs. The components are all coated with a polymeric isolation material which is covered by a metallic protection casing attached to the isolation of the current inputs. All conductive parts are isolated so that a loss in the casing tightness has no incidence on the operation capacity of the apparatus. The current inputs are connected to isolated conductors by collet clamps and protected by an atmosphere-resistant resin-based isolation. The lower part of the casing is made of an atmosphere-resistant semi-unbreakable glass of the Lexan type so that the position indicator of the switch remains clearly visible and that the operation of the vacuum switch can be visually monitored. It should be noted that bushings with multiple screens, known as condenser bushings, are known from transmission voltage applications. However, these operate under significantly different voltage ranges to recloser bushings, typically at 110 kV and higher. In this instance, the bushings are formed from layers of insulating paper and insert layers of conducting foil that are wrapped around the conductor being insulated. However, these systems are delicate and complex to manufacture, making them unsuitable for use in medium voltage applications, such as reclosers and the like. Summary of the Present Invention It is an object of the present invention to substantially overcome, or at least ameliorate, one or o more disadvantages of existing arrangements. In a first broad form the present invention provides an insulating apparatus for use in electrical apparatus, the insulating apparatus including: a) a body including an aperture for receiving a conductor; and, b) at least two screening elements mounted in the body. 5 Typically the body is formed from at least one of: a) a polymeric material; b) a plastic; c) a rubber; and, -4 d) a resin. Typically the body is formed from an epoxy resin. Typically the body includes a covering formed from at least one of: a) a polymeric material; b) a plastic; c) a rubber; and, d) a resin. Typically the covering is silicone based. Typically the body includes an inner surface, at least part of the inner surface being electrically conductive and electrically coupled to the conductor. Typically in use, the aperture is arranged such that an air gap is present between at least some of the conductor and at least part of the inner surface. Typically each screening element is substantially cylindrically shaped. Typically each screening element includes an end portion having a greater diameter than at least a central portion of the screening element. Typically each end portion has a flared shape. Typically the body is substantially cylindrically shaped, with the aperture being provided along a body axis. Typically each screening element is substantially cylindrically shaped, and wherein an axis of 0 each screening element is positioned coaxially with respect to the body. Typically the apparatus includes a first screening element, and a second screening element, the second screening element being positioned between the first screening element and the aperture. Typically the first screening element is electrically connected to ground.
-5 Typically the second screening element is electrically isolated. Typically the screening elements are arranged to distribute field lines relatively evenly across an outer surface of the body in use. Typically the screening elements are formed from at least one of: a) a cast metallic material; and, b) a wire mesh. Typically the body and the screening elements are formed from materials having similar thermal expansion properties to thereby reduce differential thermal expansion. Typically at least one screening element is coupled to a sensor for measuring a voltage in the conductor. Typically the apparatus is coupled to a housing in use, and wherein at least one screening element is electrically connected to the housing. Typically the housing is mounted to a support in use. Typically the support is a support pole for supporting the housing above ground level. Typically the bushing is used in medium voltage applications. Typically the apparatus is for use in switching apparatus. Typically the apparatus is a bushing. In a second broad form the present invention provides apparatus for switching electrical currents, the apparatus including: 0 a) a switching device for switching electrical currents; b) a housing containing the switching device; and, c) insulating apparatus for allowing a conductor to extend through the housing to couple the switching device to at least one electricity line, the bushing including: i) a body including an aperture for receiving the conductor; and, 5 ii) at least two screening elements mounted in the body.
oupavn raacm wormOJa rewa awucngarrny LIGO- 3 U IzJZZz>_le, c C=prCSpci -UpaatWGC.c- I JLuIn -6 Brief Description of the Drawings An example of the present invention will now be described with reference to the accompanying drawings, in which: Figure lA shows a schematic perspective view of an example of a switching apparatus; Figure lB shows a schematic plan view of the switching apparatus of Figure lA; Figure 2 shows a schematic cross sectional view of the switching apparatus of Figure IA; Figure 3 shows a schematic cross sectional view of a bushing of the switching apparatus of Figure IA; and, Figure 4 shows a schematic representation of the electrical field lines generated within the bushing of Figure 3. Detailed Description of the Preferred Embodiments An example of a switching apparatus, such as a recloser, will now be described with reference to Figures lA and lB. In this example, the switching apparatus 100 is coupled to a support pole 101 via a bracket 102. The support pole 101 is used to support electrical distribution lines shown generally at 110, 111 which are electrically isolated from the support pole 101 by insulating elements 112. The switching apparatus 100 includes terminals 120, 121 which are connected to the power lines 110, 111 via respective connections 122, 123. In use the switching apparatus 100 can selectively interconnect the terminals 120, 121, which in turn interconnect the distribution 0 lines 110, 111 to allow transmission of electricity. In use, the switching apparatus may be coupled to a controller 130 via a connection 131 allowing operation of the switching apparatus to be controlled and/or to allow information regarding the operation of the switching apparatus 100 to be displayed. Control of the switching apparatus may be performed automatically, and/or in accordance 5 with input commands from an operator. For example, under normal operation the switching apparatus will operate automatically to disconnect the distribution lines 110, 111 should a fault arise, and acting to reconnect the distribution lines 110, 111 once the fault has been -7 corrected. However, additionally or alternatively, an operator may provide input commands via the controller 130, allowing operation of the switching apparatus 100 to be manually controlled. The switching apparatus 100 and the controller 130 are typically connected to ground via connection 132 as shown. One example of the switching apparatus is shown in more detail in Figure 2. In this example, the switching apparatus is formed from a tank 200, containing a housing 210 which is typically formed from an insulating material such as a polymer. The housing includes bushings 211, 212 having terminals 220, 221 mounted therein, and a switch 222, which in one example is a vacuum interrupter, for selectively interconnecting the terminals 220,221. In one example, the switch 222 includes a first arm 223 electrically coupled to the terminal 220 and a second arm 224 electrically coupled to the terminal 221, via an electrical connector 227. The arm 224 is biased in the direction of arrow 230 via a resilient member, such as a biasing spring 225. The arm 224 is also physically coupled via a push connector 226 to an insulating movable member, such as a push rod 228, which is in turn coupled to an actuator 229, such as a magnetic actuator. In use, the actuator 229 selectively urges the push rod 228 in a direction opposite to the arrow 230, which in turn urges the arms 223, 224 into a closed position against the biasing action of the spring 225. In the closed position the arms 223, 224 are in contact, and as a result, the 0 terminals 220, 221 are electrically interconnected. If the actuator 229 is opened, the spring 225 urges the arm 224 in the direction of arrow 230, thereby moving the arms 223, 224 to an open position in which the arms 223, 224 are not in contact, which in turn disconnects the terminals 220, 221. Operation of the actuator can be controlled using suitable control electronics, provided for 5 example, in the controller 130. It will be appreciated that the controller 130 can be connected to suitable sensors, such as current and/or voltage sensors (not shown), which allow faults to be detected. and hence the switch 222 to be controlled.
--s Law, - P C WM.rt - Pu-O .O.,UfJW -8 An example of one of the bushings 211, 212 will now be described with reference to Figure 3. In this example, the bushing includes a body 300 formed from an electrically insulating material, such as a polymeric material, a plastic, a rubber, a resin or other similar material. In one example the body is formed from an epoxy resin. The body may also optionally include an insulating covering 400 (shown in Figure 4) on an outer surface, commonly referred to as a boot. The covering may also be formed from any suitable material, such as a polymeric material, a plastic, a rubber, or a resin, and in one example is formed from silicone. The body 300 has a substantially cylindrical shape defining a body axis 340, which in this example tapers towards each end, so that the diameter of the body 300 decreases away from a central portion. The body 300 includes an aperture 301 for receiving an electrical conductor 302, such as the terminal 220, and at least two screening elements shown generally at 320 and 330. The screening elements are generally formed from a conductive material, such as a wire mesh, pressure die case metallic components, or the like, and act to provide screening between the conductor 302 and the tank 200. In this example, each of the screening elements 320, 330 have a substantially cylindrical shape, and are positioned aligned with the body axis 340, with a first one of the screening elements 330 being positioned radially outwardly of the second screening element 320, which is therefore positioned between the first screening element 330 and the aperture 301. 0 In one example, each of the screening elements 320, 330 has a shaped end portion 321, 331 which has a greater diameter than the remainder of the screening element. The diameter increases towards the end such that the ends of the screening elements 320, 330 have a flared shape. In addition to this, the second screening element 320 is longer than the first screening element 330, so that the screening element 320 extends a greater distance along the body axis 5 340. In one example, the first outer screening element 330 is electrically connected to the tank 200, and or to ground, using a suitable connection 332. As the tank 200 is typically grounded, this results in the outer screening element 330 being held at a zero or ground potential. The inner or second screening element 320 is then typically electrically isolated, so -9 that it maintains at an electrical potential between that of ground and the potential of the conductor 302. Additionally, and/or alternatively, the outer screening element 330 may be coupled to a sensor, such as a sensing resistor, which is used to measure current flow from the outer screening element 330 to ground, which can in turn be used to indicate the presence of and measure the value of a voltage in the conductor 302. In this instance, it will be appreciated that whilst the outer screening element 330 may not be held at zero potential, it does still typically have a low potential. In addition to the use of screens, as outlined above, the aperture 301 can be arranged so as to provide an air gap 303 between the conductor 302 and at least part of an inner surface 304 of the body 300. The inner surface 304 is typically electrically conductive and electrically coupled to the conductor 302 so that the inner surface 304 is at an electrical potential equivalent to that carried by the conductor 302. However, in further embodiments of the invention, the body 300 can be moulded around the conductor 302 so that no air gap is present between the inner surface 304 and the conductor 302. The use of a conductive inner surface 304 can avoid the presence of a potential difference across the air gap 303, which can in turn be detrimental to bushing performance. In one example, the inner surface 304 has a conductive layer painted thereon, although this is not essential, and any manner of making the surface conductive can be used. 0 Maintaining the inner screening element 320 at an intermediate potential, whilst having a greater screen length, assists in evenly distributing electrical field throughout the bushing, this in turn, reduces electrical stresses, increasing reliability and allowing the bushing to carry a higher potential than would otherwise be achievable. An example of the field distribution is shown in Figure 4. 5 The above described configuration can be created relatively easily by positioning the screens within a mould, and then filling the mould with an epoxy material. During this process, the outer screening element 330 can be held in place by the conductive connection that is used to couple the outer screening element 330 to the tank 200. The inner screening element 320 can -10 then be held in position by an insulating clip, or the like. Once the epoxy material has set, the mould can be removed, allowing the inner surface 304 of the body 300 to be coated with an electrically conductive material, and allowing the outer screening element 330 to be coupled to the tank 200, using an appropriate connection. The screen and body materials can be any form of suitable material, but are typically selected to have similar thermal properties. This avoids problems, such as cracking, which can result from differential thermal expansion of the different components within the system. It will be appreciated that the use of multiple screens in this manner helps spread the electrical surface stress evenly over the bushing, thereby reducing the chance of bushing failure. This therefore helps provide long term product reliability in a compact design, whilst providing sufficient protection to operate at up to 38kV. Increasing the radius edges of the screens towards each end also helps reduce internal electrical stresses, reducing the potential for partial discharge and increasing product yield. The use of the aperture 301 eliminates much of the mechanical stress caused by temperature variation through the body 300, and provides for a more robust solution at very low temperatures. This will minimize the potential for internal cracking caused by differential expansion. Internal cracks will lead to premature product failure in a devastating fashion. The screen arrangement can be used as a capacitive pick-up for voltage measurement. The multiple screen arrangement seems to have improved stability over single screen 0 implementations. The above described examples have focussed on the use of the bushing in a recloser. However, this is not essential, and it will be appreciated that the bushing can be used in any medium voltage application, which typically operates at up to 38 kV, and more typically between 10 kV and 38 kV. Suitable applications include any switch gear, such as vacuum 5 switches, vacuum circuit breakers, load break switches, sectionalisers, reclosers or the like, or any other medium voltage apparatus requiring screening.
- 11 Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art should be considered to fall within the spirit and scope that the invention broadly appearing before described.