GB2114388A - Under oil metal oxide surge arrester - Google Patents

Abstract

An overvoltage surge arrester (38) for protecting a transformer (14, Figure 1) immersed in an insulating fluid (20, Figure 1), preferably oil, in a transformer tank (10, Figure 1), the arrester including one or more metal oxide valve blocks (46, Figure 2) immersed in the insulating fluid. The valve blocks may be enclosed in a perforated or mesh type, see Figure 7, housing (48, Figure 2) to allow for the free flow of insulating fluid around the valve blocks. This allows the high dielectric strength of the oil to reduce corona discharge from any of the valve blocks, reduce the possibility of surface flashover on the blocks and also cools the blocks. A fuse (44 in Figure 1)I may also protect the transformer. <IMAGE>

Description

SPECIFICATION Under oil metal oxide surge arrester Surge arresters have been used under oil for the protection of pad mount or pole top transformers.

The arresters are connected between the high voltage winding and the ground contact. The arresters are sometimes provided with an external ground connection to facilitate over voltage testing of the transformer. The surge arrester when immersed in the oil of a transformer is subjected to a relatively high temperature environment and must be well sealed to prevent ingress of oil. If oil enters a conventional spark gap surge arrester, the oil will ultimately fill the spark gaps in the arrester and increase the spark over to a level of uselessness. Oil which enters a sealed arrester may infiltrate the silicon carbide blocks if they are porous causing a potential problem if the transient heat produced by the passage of surge current through the arrester blocks rises to a temperature sufficient to decompose or carbonize any of the oil.Under any circumstances where the arrester blocks or gaps flash over, the failure of the arrester could lead to a rapid energy release within the transformer tank causing it to bulge, rupture, and/or catch fire.

Many of the above problems can be avoided by using a under oil gapless surge arrester of the type proposed herein. Contrary to current practice, the gapless arrester has non-porous valve blocks housed in a structure which provides for oil entrance and circulation around the valve blocks. Typical valve blocks consisting of zinc oxide with small percentages of other metal oxides are well known in the art as metal oxide varistor elements which have non-linear resistance characteristics providing the function of a conventional gap type arrester. Arresters utilizing valve blocks of this type have no gap element. Since the valve blocks are non-porous no detrimental effects can occur due to the oil immersion.This arrangement of the valve blocks within an open housing uses the higher dielectic constant and the higher dielectic strength of the oil to prevent corona discharge from any of the valve blocks. The arrester can be located in a closed spaced relation to any conductive planes within the transformer tank.

The high dielectic constant and strength of the oil reduces the possibilities of flash over on the surface of the elements should the arrester receive a very high surge current. The oil also acts as a heat transfer medium providing a more rapid cooling of the element blocks with a corresponding temperature stabilization of a block which becomes heated from transient or steady state operating conditions.

Figure 1 is a top section view of a pad mounted type distribution transformer shown schematically with a surge arrester according to the invention connected to the primary winding of the atrans- former.

Figure 2 is an elevation view of the surge arrester partly broken away to show the valve blocks.

Figure 3 is a perspective view of a modified form of a surge arrester according to the invention.

Figure 4 is a perspective view of one type of mount for a surge arrester according to Figure 2.

Figure 5 is a perspective view of bayonet type mountforthe surge arrester to Figure 2.

Figure 6 is another form of mount for a surge arrester of the type shown in Figure 2.

As seen in Figure 1, a distribution transformer of the type contemplated herein generally includes a tank 10 having a cover 12. Atransformer 14 having a primary winding 16 and secondary winding 18 is schematically shown within the tank 10 immersed in insulating fluid 20. The primary winding 16 is connected to a primary bushing 22 by a line 24 and to ground 26 by a line 28.

The secondary winding 18 is connected to secondary bushing 30,31, and 32 by lines 34,35 and 36. A surge arrester 38 according to the invention herein is shown connected to the primary winding 16 buy a line 40 and to ground 26 by a line 42. A fuse 44 may be provided in the line 24.

In accordance with the invention, the surge arrester 38 includes an array of pre-assembled valve blocks 46, positioned within a tubular housing 48 having conductive end caps 50 and 52 at each end.

The valve blocks 46 are formed from metal oxide additives such as zinc oxide to produce an extremely dense and non-porous structure. The array of valve blocks 46 is biased into engagement with the end cap 50 by a means of a conductive spring 54. The spring 54 also providing electrical communication between the end cap 52 and the end valve block 46.

Threaded contacts 54 and 56 can be provided on each end cap 50, 52, respectively, to connect the surge arrester to the primary winding 16 and to the ground.

Insulating fluid 20 such as oil is allowed to circulate through the tubular housing through holes 58 located at various locations in the housing 48. The number of holes can be increased to the extent where the tube becomes a mesh screen if desired. It is also within the contemplation of the invention to provide holes 49 in the end caps 50, 52 as a means for allowing the oil to circulate through the housing particularly if mounted in the manner as shown in Figure 1.

In the modified form of the invention shown in Figure 3, a surge arrester 60 is shown which includes an array of surge arrester blocks 62 interconnected by a conductive cement 64. Stud fittings 66 are provided at each end of the array of valve blocks for connecting the arrester to the primary winding and to ground. In the absence of a housing, the surge arrester 60 should be mounted so that it is spaced from any conductive surfaces to eliminate the possibility of flash over under a high over voltage surge.

Referring to Figure 4, one type of mounting arrangement 65 is shown for surge arrester 38. In this embodiment, a mounting plate 68 is shown supported on a number of spacers 70 and secured thereto by hex nuts 72. The surge arrester is secured to the mounting plates 68 by springs 74 which pass through the holes 76 provided in the plate 68.

In Figure 5, another embodiment of the mounting bracket is shown wherein the surge arrester 38 is mounted within a bayonet type fuse holder 78. The bayonet includes a housing 80 which is mounted on a transformer wall and extends downwardly and inwardly into the oil. The surge arrester is located within the end of the housing in electrical contact with the electric contacts 82 and 84. The surge arrester 38 is mounted on an insulated holder 86 positioned within the housing 80. The housing 80 extends downward into the insulation fluid so that the surge arrester 38 is immersed in oil. The electrical contact between the surge arrester and the primary winding and the ground connection are made through contacts 82 and 84, respectively.

In the embodiment of the mounting arrangement in Figure 6, the surge arrester 38 is shown mounted on a mounting bracket 90 having ventilated spacers 92 at each end. Clips 94 are secured to conductive members 96 which are secured to the spacers 92 with the clips in a position to engage the contact studs 54 and 56 of the surge arrester 38.

In the embodiment of the invention shown in Figure 7 a housing 98 is shown in the form of a mesh of open weave construction. This type of housing will allow for the circulation of oil and also prevent actual contact between the blocks and grounded or metallic objects in the transformer housing.

Claims (13)

1. A submersible surge arrester for protecting electrical equipment immersed in an insulating fluid from over voltage surges, said arrester comprising a valve element formed of one or more valve blocks, a first connector at one end of the valve element for connecting said valve element to system voltage, a second connector connected to the other end of said valve elements for connecting said valve element to ground and means for supporting said valve element under the insulating fluid.

2. An arrester according to Claim 1, wherein said valve blocks are interconnected by conductive cement.

3. An arrester according to Claims 1 or 2 wherein said supporting means includes a housing for enclosing said valve elements, said housing being perforated to allow for the free flow of insulating fluid through the housing and having conductive end caps at each end of the housing.

4. An arrester according to Claims 1,2 or 3 wherein said end caps includes holes to allowforthe flow of oil through the housing.

5. An arrester according to Claims 1,2,3 or 4 including means in said housing for biasing said valve blocks into electrical communication with said end caps.

6. An arrester according to Claim 5 wherein said biasing means comprises a conductive spring for biasing said blocks against the conductive members at the ends of the housing.

7. An arrester according to Claims 1,2,3,4,5 or 6, including a housing in the form of a mesh screen.

8. In a fluid insulated distribution transformer, a surge arrester connected between the primary winding of the transformer and ground, said arrester being immersed in the transformer dielectric fluid, said arrester including a valve element formed of one or more metal oxide valve blocks, and means for supporting said arrester in said transformer.

9. An arrester according to Claim 8 including conductive cement between said valve blocks to form a solid self-supporting structure.

10. An arrester according to Claims 8 or 9 including an insulating housing enclosing said valve element, said housing including an electrically conductive cap at each end, said housing being perforated to allow for circulation of insulating fluid around said valve element.

11. An arrester according to Claims 8,9 or 10, wherein said end caps are perforated to allow for circulation of insulating fluid through said housing.

12. An arrester according to Claims 8,9, 10 or 11 wherein said supporting means comprises a housing in the form of a mesh screen.

13. A submersible surge arrester substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.