GB2471925A

GB2471925A - Electrical switchgear

Info

Publication number: GB2471925A
Application number: GB1005085A
Authority: GB
Inventors: John Gerard Casey
Original assignee: LUCY AND CO Ltd W
Current assignee: LUCY AND CO Ltd W
Priority date: 2009-07-15
Filing date: 2010-03-26
Publication date: 2011-01-19
Also published as: GB2471864A; GB0912281D0; GB201005085D0

Abstract

Electrical switchgear can comprise an electrical input and output 102, 112 and a conductive path from the input to the output that includes a vacuum interrupter 10. Vacuum interrupter 10 is enclosed within a space 130 that is filled with a liquid and is in fluid communication with a cylinder 136 in which there is a moveable piston 138. The liquid can be an insulating liquid such as an ester-based oil. The enclosure can have apertures for the input 110 the output (Fig 1, 114) an actuator 116 and a conduit 132 leading to the cylinder. The enclosed space 130 can be evacuated and filled with liquid via drain block 134. Air voids are inhibited and failure of the vacuum interrupter due to a leak is indicated by the piston being drawn toward the bottom of the cylinder, which is detectable using a sensor.

Description

Electrical Switchgear

FIELD OF THE INVENTION

The present invention relates to an item of electrical switchgear.

BACKGROUND ART

Electrical switchgear is provided in an electric power distribution system (or grid) and consists of a combination of electrical disconnects, fuses and/or circuit breakers which are used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done, and to clear downstream faults.

Originally, switchgear was insulated by appropriate air gaps. However, as the voltage and power levels increased, and the size of the equipment decreased, the dielectric strength of air proved to be inadequate and oil-insulated equipment became common with a bank of conductors and switches housed within an open oil bath. As switches opened or closed, any arc that formed would be quenched and extinguished by the oil. Frequently and after switching, the oil would have to be drained from the bath to allow for maintenance of any individual switch, following which the switch contacts would have to be cleaned or replaced, and the oil replenished; this was inconvenient and the presence of large volumes of oil near to electrical arcing was an obvious fire hazard.

Vacuum interruptors were later employed. An example of a vacuum interrupter 10 is shown in figure 1, and has a pair of contacts 12, 14 held in a vacuum 16 which could be brought together to make a contact or separated in order to isolate a circuit. The contacts are mounted within a vacuum flask, usually consisting of a ceramic cylinder 18 with metallic end caps 20, 22 sealed to the ceramic. Conductive rods 24, 26 set in each of the end caps 20, 22 both support a respective contact 12, 14 and allow an electrical connection to be made with it. A set of bellows 28 within one end cap 22 allows the associated rod 26 to move back and forth relative to the end cap 22 and, indeed, the remainder of the interrupter 10, so as to bring the contacts 12, 14 into and out of physical and electrical contact without harming the vacuum 16.

Vacuum interrupters have minimal arcing, as there is esentially nothing to lonise other than the contact material. As a result, simplistically, the arc is extinguished when it is stretched by a very small amount (say 6-8 mm) whilst being controlled by a magnetic field. Vacuum interrupters are therefore frequently used in modern medium-voltage switchgear to 36,000 volts.

As an alternative, an insulating gas with a high dielectric strength can be employed, Sulphur Hexafluoride (SF6) being the most common. SF6 is used in medium-and high-voltage (from 1000V up to 36kV and above) interrupters, switchgear, and other electrical equipment. SF6 gas has a much higher dielectric strength than air or dry nitrogen, making it possible to significantly reduce the size of electrical switchgear and making such switchgear more suitable for certain purposes such as indoor placement. SF6 gas also has arc-quenching properties not found in other gasses. Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment.

In recent times, however, vacuum interrupters are beginning to displace SF6 breakers for a number of reasons. Although most of the decomposition products of SF6 tend to quickly re-form the original gas, some highly toxic by-products are produced during arcing. SF6 is also believed to be a greenhouse gas, the Intergovernmental Panel on Climate Change reporting that it was the most potent greenhouse gas so far evaluated as of 2007, with a global warming potential of 22,800 times that of CO2.

SUMMARY OF THE INVENTION

Vacuum interrupters do present technical challenges, however. The space around the interrupter will be subject to a high electrical stress, and this can give rise to severe potential gradients that ultimately cause electrical discharge and fiashover, resulting in failure of the equipment. This, in turn, causes long-term reliability problems and short-term noise pollution. In addition, it is difficult to inspect a vacuum interrupter (such as during routine maintenance) to confirm that the vacuum still exists.

The present invention therefore provides electrical switchgear comprising an electrical input, an electrical output, and a conductive path from the input to the output that includes a vacuum interrupter, the vacuum interrupter being located within an enclosed space that is filled with a liquid medium and in fluid communication with a cylinder in which a moveable piston is located.

In use, the moveable piston is freely moveable such that changes in the pressure inside the enclosed space result in corresponding movement of the piston. This allows users of the electrical switchgear to immediately ascertain if the vacuum interrupter has failed by noting a large degree of movement in the position of the piston, for example, as noted below.

The electrical switchgear is suitable for use with medium and high voltages. Low-voltage systems are defined in the British standard BS EN 60664-1:2007 as those able to handle a rated voltage up to AC 1000V with rated frequencies up to 30kHz, or a rated voltage up to DC 1500V. The electrical switchgear is therefore able to handle voltages substantially greater than this.

The liquid medium acts as a dielectric and prevents any discharge in air from developing in the gaps and other spaces around the vacuum interrupter.

The small change in volume of the vacuum interrupter as it opens and closes is accommodated by movement of the piston, enabling the arrangement to be formed within an enclosed space. The piston also allows for the thermal expansion and contraction of the liquid medium.

A useful additional benefit of the arrangement is that it reveals when the vacuum in the vacuum interrupter has failed. Over time, it is possible for the vacuum seal to fail, leading to loss of the vacuum and an increased likelihood of flash over or failure to clear resulting in out of control arcing. This is generally quite difficult to detect in normal circumstances, but in the above arrangement will be quite obvious since the liquid will be drawn into the vacuum interrupter and a dramatic volume change will be observed.

The liquid will usually be an insulating liquid, although it need not (strictly) be insulating as a conductive liquid would serve to eliminate air potentials equally well. Other forms of insulation around the body of the vacuum interrupter would then be needed, however. A suitable insulating liquid is an oil, such as an ester-based oil.

This arrangement avoids the difficulty of previous oil-insulated systems since the oil is (in normal use) kept away from both the arc and the open air and is not therefore a fire risk. The oil (or other liquid) will be confined to the enclosed space and many maintenance activities will not require the oil to be drained. Equally, only a small volume of oil will be needed, which (if necessary) can be drained from the enclosed space easily and in a controlled manner.

The enclosed space preferably has apertures for the electrical input, the electrical output, an operating actuator for the vacuum interrupter, and a conduit leading to the cylinder. It is particularly preferred that these are the only apertures, leading to a completely contained system.

The enclosed space will typically be defined or bounded by the walls of a structure. That structure can be located within an outer housing.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows the typical structure of a vacuum interrupter; Figure 2 shows an item of electrical switchgear according to the present invention, in part sectional form; Figure 3 is a schematic graph of the Paschen curve for air, showing the variation of dielectric strength with gaseous pressure; and Figure 4 shows more detail of the region of figure 2 around the vacuum interrupter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows a known and generally conventional vacuum interrupter, and has been described already.

Referring to figure 2, this shows an item of electrical switchgear according to the present invention, which incorporates such a vacuum interrupter 10. The gas pressure inside the vacuum interrupter 10 will typically be at or below around 106 atmospheres, in order to achieve an adequate dielectric strength (see the Paschen curve illustrated in Figure 3). The switchgear 100 has an electrical connection 102 for an incoming or outgoing high voltage electrical supply, typically one phase of a three phase supply. This is brought within the housing 104 of the switch gear 100 via a suitably insulating conduit 106. That conduit 106 forms part of an insulating structure 108 within the switch gear 100 which will be described further later. The insulating structure 108 is attached to the housing 104 and is otherwise supported within the housing 104. The insulating sleeve 106 shields within it a conductive rod 110 which leads from the electrical input 102 to one side of the vacuum interrupter 10 which is housed within the insulating structure 108. At the other end of the vacuum interrupter 10, the remaining electrical terminal 26 is connected to a flexible conductor 112 that leads to an electrical terminal 114. The output 26 is also connected to an insulated operating shaft 116 that is driven by a mechanism 118 to insert or withdraw the electrical output 26 and make or break the vacuum interrupter 10 as was described above in relation to figure 1. The operating shaft 116 extends from a position inside the insulating structure 108, to one outside the insulating structure 108. The mechanism 118 for driving the shaft 116 is therefore also positioned outside the insulating structure.

The electrical terminal 114 leads to a disconnector contact 117. This is driven by a mechanism 121, and is hinged at its point of connection to the electrical contact 114 so that it can take up one of two positions. In the first position shown in figure 2, the switch conductor 117 is in electrical contact with an earth conductor 119, the majority of which is supported in an insulated conduit 120 which is attached to (and supported by) the housing 104 and leads the earth conductor 119 out of the housing to an earth contact 122. This will generally be connected to earth during installation of the electrical switch gear 100.

In a second position 117a, shown dotted in figure 2, the switch conductor 117 makes contact with a conductor 124 to which is connected one bus bar 126 of a set of three-phase bus bars 128. Thus, by placing the switch conductor 117 into the live position 117a and closing the vacuum interrupter 10, the electrical input or output 102 is connected to the electrical bus bar 126.

The structure shown in figure 2 is then duplicated at least twice alongside the structure shown in figure 2, so as to provide a similar supply to the other phases of the three phase output 128.

Figure 4 shows in more detail the region containing the insulating structure 108 and the vacuum interrupter 10. It will be seen that the insulating structure 108 forms a closed containment vessel around the vacuum interrupter 10, interrupted by apertures for the incoming electrical conductor 110, the outgoing conductor 112, and the insulated operating shaft 116. In order to allow the necessary tolerances for assembly, there is a space 130 around the vacuum interrupter 10, which would ordinarily be prone to the creation of air voids. Air voids are undesirable and may result in damaging electrical discharge due to the significant potential difference that will exist across the live parts of the vacuum interrupter 10 and the significantly lower potential of the inner surface of the insulating structure 108. A single further aperture 132 is therefore provided in the insulating structure 108, in the form of a conduit leading (via a 1-junction) to an outlet sealed with a drain plug 134, and a cylinder 136 containing a piston 138.

Prior to use of the switch gear 100 and (typically) during the factory assembly phase the drain plug 134 is removed and the outlet that it previously sealed is used to inject a suitable oil or other liquid into the space 130 around the vacuum interrupter 10. During this period, the piston 138 can be held in a mid position along the cylinder 136, for example by attaching an externally accessible handle to a socket 140 on the rear of the piston 138. Once the void is completely filled then the drain block 134 can be re-inserted, and the piston 138 released to move freely.

Often, complete filling of void 130 can be assisted by first evacuating the space prior to filling with liquid, as this removes the need to exhaust the space during filling and avoids the creation of bubbles, etc. Once the space is filled 130 and the device is put into operation, movement of the various parts of the vacuum interrupter 10 under control of the actuator 118 will cause a small change in the volume of vacuum interrupter 10.

This is accommodated by the surrounding liquid flowing through the conduit 132 into (or out of) the cylinder 136, causing small corresponding movements of the piston 138. The width of the cylinder 136 can be chosen so as to determine the distance moved by the piston 138, and ensure that it remains within the length of the cylinder 136.

Should the vacuum interrupter 10 begin to leak, instead of merely losing its vacuum it will draw in fluid from the surrounding space 130 until the piston 138 is drawn to the bottom of the cylinder 136. Such a gross movement of the piston 138 will be readily apparent from the outside of the device, or will be readily detectable using known sensor apparatus. This will give an immediate and obvious warning of the failure of the vacuum interrupter 10 and the need for remedial maintenance.

The fluid medium filling the space 130 can be chosen so as to provide adequate dielectric strength and prevent the development of air potentials.

Several ester-based insulating oils are known, for example that sold under the Midel� 7131 brand. Other suitable insulating oils are based on vegetable oil derivatives, silicone oils, or mineral oils. Such liquids are known for use as transformer liquids, and are readily available.

As an alternative, the fluid within the space 130 could in fact be conductive. This would entirely eliminate the development of air potentials since no potential difference can exist within a conductive medium. However, it would require some form of sealing around the circumference of the vacuum interrupter 10 in order to prevent a short circuit arising. This might require independent supplies of fluid to each end of the vacuum interrupter 10, or some alternative arrangement.

It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.

Claims

CLAIMS1. Electrical switchgear comprising an electrical input, an electrical output, and a conductive path from the input to the output that includes a vacuum interrupter, the vacuum interrupter being located within an enclosed space that is filled with a liquid medium and in fluid communication with a cylinder in which a moveable piston is located.
2. Electrical switchgear according to claim 1 in which the liquid is insulating.
3 Electrical switchgear according to claim 1 in which the liquid is oil.
4. Electrical switchgear according to claim 3 in which the liquid is an ester-based oil.
5. Electrical switchgear according to any one of the preceding claims in which the enclosed space has apertures for the electrical input, the electrical output, an operating actuator for the vacuum interrupter, and a conduit leading to the cylinder.
6. Electrical switchgear according to claim 5 in which the enclosed space has no other apertures.
7. Electrical switchgear according to any one of the preceding claims, comprising a structure having walls that define the enclosed space therewith in.
8. Electrical switchgear according to claim 7, comprising an outer housing within which at least the structure is located.
9. Electrical switchgear substantially as herein disclosed with reference to and/or as illustrated in the accompanying figures.