Abstract:
Device for the protection from effects of an electric field, of the metal parts of chains of insulators of already mounted and operational high-voltage transmission lines includes:
       a fastening/support plate;   anchoring elements to anchor the plate to the suspension yoke of the conductors of the transmission line, the anchoring elements being adjustably mounted on the plate according to at least two degrees of freedom; and   at least one guard ring, which may be anchored on the plate with fastening elements. The plate has an upturned-U cross-section and its upper surface is formed by geometrically coupling with the lower surface of the suspension yoke. The anchoring elements include a pair of arms articulately, mounted on the plate; such articulation including, for each arm, a pin pivotally, mounted within seats formed in the plate perpendicularly to the longitudinal axis thereof, and a pair of holes running through the pin.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   None. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   None. 
   THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
   None. 
   INCORPORATION BY REFERENCE OF MATERIALS SUBMITTED ON A COMPACT DISC 
   None. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of protection of the chains of insulators of high-voltage transmission lines. 
   2. Description of the Related Art 
   In high-voltage transmission lines, normally suspended to chains of insulators and anchored to tall support poles, the phenomenon of flashovers is known, which may arise particularly in critical environmental conditions—between the suspension means associated with the support pole, and hence grounded, and the transmission line conductors. 
   The corona effect is also known, produced by abnormal increments of the electric field. Such phenomenon is due to high electric loads and manifests itself on the first insulators of the chains, on the line side. 
   This is also the location where, in bundle lines, the supporting yoke of the conductors is connected to the last insulator by means of a metallic component which, despite its relatively small size, undergoes strong tensile stress. 
   In order to prevent the above-mentioned phenomena (flashovers and corona effect) from translating into corrosion of the metal parts next to the conductors, the use of so-called “guard rings” to be applied to the bottom of the chain of insulators has long been known. These guard rings have the function of improving the distribution of the electric load on the insulators, i.e. of reducing electrical field irregularities on the first insulators on the line side. As a result, by reducing the occurrence of electrical phenomena, the corrosive action thereof is reduced and any risk of early structural weakening, possibly leading to failure, is prevented. 
   Guard rings are generally manufactured from a rod or metal tube, bent so as to form a ring surrounding the lower end of the chain of insulators. These guard rings are further equipped with a support allowing the anchoring thereof directly on the spacing yoke. The support is suitably formed, so as to project also towards the insulators of the suspension chain. 
   It was also possible to ascertain that a guard ring must not necessarily fully surround the lower end of the chain of insulators: a protection projecting on the outer side of the line and of a suitable form, also called “racket”, is sufficient. Such protection, formed by a rod or by a tube bent bow-like which does not surround the entire chain of insulators, performs equally well the function of modifying the electric field shape and of moving away the flashover. In any case, whenever reference is made in the present description to a “guard ring”, it is intended to refer in general also to any type of suitably formed “racket-like protection”. 
   Although said phenomena (corona effect and flashovers) are well known as being damaging to chains of insulators, some high-voltage transmission lines so far lack guard rings or similar protections. For such plants the problem then arises of how to achieve protection thereof. 
   It is clear that the currently most readily available solution of this problem is that of detaching the conductors from their respective chain of insulators and of replacing the existing spacing yoke with another one equipped with a guard ring and hence suitably formed to house it. Such a solution, however, is not feasible due to at least 3 reasons:
         in order to be able to replace the existing assembly, the alternative assembly, i.e. spacing yoke and guard ring, cannot be of just any shape: it must be designed so as to interface with the sockets existing on the chain. It is therefore necessary to design and manufacture ad hoc a plurality of alternative assemblies according to the features of the various types of existing chains, with evident significant costs;   the replacement of the existing spacing yoke with an alternative assembly implies the need, as said, to detach the line conductors from their respective chain, with evident, equally high, operating costs;   finally, the detachment operation of the conductors from the chain must necessarily be carried out on a de-energised transmission line. This is generally not acceptable due to the resulting discontinuing of the power supply which, considering that this operation must be repeated on multiple poles, is relatively prolonged.       

   BRIEF DESCRIPTION OF THE INVENTION 
   It is hence the object of the present invention to suggest a device for the protection of the chains of insulators of high-voltage transmission lines, the manufacturing of which is simple and inexpensive, at the same time being very sturdy, and apt to be quickly and easily installed on existing plants, possibly on a live line also. 
   This result is achieved thanks to the features mentioned in claim  1 . 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Further features and advantages of the invention are in any case more evident from the following detailed description of some preferred embodiments, given purely by way of a non-limiting example and illustrated in the accompanying drawings, wherein: 
       FIG. 1  is an elevation view of a device according to the present invention, applied to a triangular supporting yoke, seen from the front; 
       FIG. 2  shows the same arrangement of  FIG. 1 , in a view at 90° to  FIG. 1 , with a spacing yoke shown in an axial cross-section; 
       FIG. 3  shows the detail encircled in A in  FIG. 2 , with a different configuration of the spacing yoke; 
       FIG. 4  is a view similar to that of  FIG. 1 , out with a supporting yoke having a different configuration, intended for a suspension having two inclined chains of insulators, converging in the point of suspension; 
       FIG. 5  is an elevation view similar to that of  FIG. 1 , but referring to the device according to the invention only; 
       FIG. 6  is an end view, partially in cross-section according to line VI-VI of  FIG. 5 , of the device according to the invention only; 
       FIG. 7  is a view similar to that of  FIG. 6 , but referring to a section according to line VII-VII of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIGS. 1 and 2  a type of suspension of the conductors of a so-called double high-voltage transmission line is shown which, in a well-known manner, consists of a single vertical chain  1  of insulators, ending at the bottom in a pin  2  for the attachment of an anchoring saddle  3  of a spacing yoke  4 ; said spacing yoke is generally formed as an isosceles triangle and carries, at the two side ends  4   a  and  4   b , two clamps  5  supporting the two conductors (not shown) of the transmission line. The anchoring of clamps  5  is accomplished by means of pins  5   a ,  5   b  having their axis parallel to that of the conductors, in order to allow a certain lateral oscillation of said conductors. 
   It is assumed that this double arrangement is mounted on a series of supporting poles of a transmission line and that such transmission line has already been tested and is fully operational. 
   The protection device according to the present invention substantially consists of a fastening/support plate  10  (better shown in  FIG. 6 ), of a pair of U-Bolts  11 ,  12  and of at least one protection  13 . 
   More in detail, plate  10  is formed as an elongated body with an upturned-U section, the upper surface  10   a  of which is formed so as to rest on the lower surface of spacing yoke  4  with a substantially geometric coupling; in actual fact, it may be sufficient to provide two parallel ribs  10   h  projecting by a short length from said upper surface  10   a , at a short distance form each other. Thereby, plate  10  may be applied with its upper surface  10   a  against the lower surface of spacing yoke  4  with the certainty that, even when this surface of spacing yoke  4  is rounded off (as in the case of the arrangement of  FIG. 2 ), it is housed and securely retained between the two ribs  10   b.    
   Anchoring means hold plate  10  in tight contact with the lower surface of spacing yoke  4 ; such anchoring means in actual fact consist of the above-mentioned U-Bolts  11  and  12 . Each of these U-Bolts has the shape of a U-bolt with threaded ends formed by anchoring arms  11   a ,  11   b ,  12   a ,  12   b.    
   The ends of these U-bolts are apt to engage with gauged holes which cross diametrically a pair of large-diameter pins  14 . More precisely, these pins  14  go through the vertical walls of plate  10 , in respective seats wherein they are mounted freely pivoting about horizontal axes, such as being perpendicular to the longitudinal axis of plate  10 . 
   The ends of U-bolts  11 ,  12  are, as said, threaded; once they have been introduced into said gauged holes, nuts  15  are threaded thereon, each of which abuts with the surface of respective pin  14 , with the interposition of a support washer  16  and possibly of an elastic washer  17 , for the function better described in the following. 
   Thanks to this construction, the position of the two U-bolts  11 ,  12  is. with respect to plate  10  by way of an articulation system having two degrees of freedom; more precisely, they have the option of pivoting, integrally with pin  14 , about the axis thereof, and the option of sliding along their longitudinal axis, along the above-mentioned gauged hole in the same pin  14 . 
   The guard-ring protection  13  consists—at least in the two embodiments shown—of a pair of rackets  13   a ,  13   b . These protections  13   a ,  13   b  are manufactured by suitably bending, in a well-defined manner, a length of rod or tube, bent circle-wise or ellipse-wise, the ends of which are welded to retain and support brackets  18 . 
   More precisely, it can be seen from  FIGS. 1 and 6  that each of the two brackets, which are found at the two sides of plate  10  and which support protections  13   a  and  13   b , respectively, is U-shaped, with a bottom part  18   a  and two square-bent arms  18   b ,  18   c.    
   For the fastening of rings  13   a ,  13   b  on plate  10 , the bottom part  18   a  of each bracket  18  is fastened and tightened against the side wall of plate  10  by means of bolts  19 —which run across both side walls of plate  10 —and of respective nuts  20 . In order to ease mounting and tightening operations, it is possible to manufacture bolts  19  as stud-bolts, fastened, for example welded, to plate  10 . 
   When the device assembly like the one just described is to be mounted on a suspension  1  of a chain of insulators, without disassembling any part of said suspension, the operator proceeds as follows: 
   a)—he brings plate  10  in contact with the lower surface of spacing yoke  4 , and retains it in this position. (It can be noticed from  FIG. 2  that, when the lower surface of spacing yoke  4  has a rounded off strengthening rib, support is also provided on ribs  10   a , which ensure correct centering and more precise retaining; instead, when said lower surface of spacing yoke  4  is plane, as shown in  FIG. 3 , correct support is ensured also without the cooperation of ribs  10   a ); 
   b)—he brings the two U-bolts  11  and  12 , one by one, to fasten to the upper ribs  4   a  and  4   b  of spacing yoke  4 , and at the same time introduces the lower threaded ends thereof, each into its respective gauged hole, which pin  14  runs through; 
   c)—he engages nuts  15  with said threaded lower ends, downwardly coming out of the holes in pin  14  and screws them up until a light tightening is obtained. In this step of light tightening, the operator will take care to orient U-bolts  11 ,  12  into such a position as to guarantee maximum stability of the assembly under traction. When the two U-bolts  11 ,  12  are mounted in this manner (with light tightening) and the operator has correctly determined the inclination thereof to spacing yoke  4 , it is provided to further screw nuts  15  up to a final tightening; 
   d)—he finally engages bolts  19  through the respective holes in the walls of plate  10  and mounts, on their opposite threaded ends, brackets  18  supporting the protections, which are definitively fastened by the tightening of nuts  20 . 
   Of course, the device according to the invention can be delivered to the user disassembled into all of its components or pre-assembled in a unitary assembly; in this second case, the assembly, however, will have to be apt to be easily disassembled for the final mounting. As a matter of fact, it is clear that the steps from a) to c), which are relatively more delicate due to the need to achieve a precise positioning of plate  10 , can be more easily performed if plate  10  is lightweight, i.e. lacking protections  13 ; the final step d), consisting in the mounting and fastening of said protections  13 , which are remarkably more heavy-weight, is instead performed when plate  10  is stable in its final position. 
   The arrangement illustrated in  FIG. 4  shows how the device according to the invention—still consisting of the same, identical group of already-described elements  10  to  13 —can be used in case of a suspension of the transmission line having a completely different conformation. 
   As a matter of fact, in the case shown precisely in  FIG. 4 , it is a suspension with two chains of insulators  1 A and  1 B arranged diagonally and converging towards a spacing yoke  4 A, which in this case is substantially formed as an isosceles trapezium. The chains  1 A,  1 B of the insulators are fastened at the two angles  21 ,  22  of the upper edge of the trapezium spacing yoke, while clamps  5  supporting the conductors are fastened at the two angles  23 ,  24  of the lower edge of the trapezium spacing yoke. 
   With such a type of suspension, U-bolts  11 ,  12  are mounted substantially vertical (as is clearly shown by  FIG. 4 ) since they can be fastened to the essentially horizontal upper edge of the trapezium yoke. As can be easily understood without any particular additional explanations, the operator acts exactly as described in the previous case, by fastening the anchoring arms  11   a ,  11   b ,  12   a ,  12   b  of the U-bolts  11  and  12  one by one and locking them in position; in this arrangement, on the contrary, the manoeuvre is easier than in the arrangement of  FIG. 1  because U-bolts  11 ,  12  can freely rest on the upper horizontal edge of spacing yoke  4  in any position thereof without falling off. 
   It is intended, however, that the invention must not be considered limited to the device illustrated above, which represents only one of the possible exemplary embodiments, but that different variants are possible, all within the reach of a person skilled in the field. These variants can concern for example the shape of U-bolts  11 ,  12 , each of which might consist of a pair of rods formed as a hook and which may be independently coupled with pin  14 , so as to be able to be fastened and secured to ribs  4 A and  4 B of spacing yoke  4  in different positions, for the purpose of a more precise and secure anchoring. The variants can further concern not only the U-bolts, but also fastening/support plate  10 , for example to adapt it to very specific shapes of supporting yokes, for different transmission lines from the double ones illustrated here. These variants, evidently within the reach of a person skilled in the field, will therefore all have to be considered as falling within the scope of protection of the invention, as defined in the following claims.