Production process for elastomer sleeves for covering electric cable joints and associated sleeve

Abstract: The process calls for extrusion of a multilayer tubular structure with semiconducting innermost layer. In production of the sleeve said inner layer is partially removed leaving only a central portion forming an electrode. In addition the intermediate layer is made with a compound having pre-determined relative dielectric constant and dielectric rigidity.

Description: The present invention relates to a production process for prefabricated elastomer sleeves for covering electric cable joints. Specifically the present invention relates to covering sleeves designed to be applied on joints or terminals of cables in medium- or high voltage electric lines. Because of the high electrical potentials in play (on the order of tens or hundreds of kV) which cause locally very high electric field gradients, there is a risk that the insulating layers might be damaged by electrical discharges, compromising the functionality of the sleeve. As known, in the production of electric cable joints it is provided that at the ends of the cables to be joined the conductor of each cable be locally deprived of the protective and insulating elements covering it in order to be connected with the conductor of the other cable. Then, the elastomer sleeve is first fitted in elastic expansion condition on a cylindrical tubular support body of rigid plastic material engaged around one of the cables before completion of the joint. Subsequently, after completion of the conductor connection, the tubular body is removed from the elastomer sleeve so that the latter can shrink elastically and tighten upon the cables at the joint. Special care must be given to control the distribution of the electrical field in the conductor joint area. Indeed, as mentioned, the high voltage can bring about high electrical field concentrations in the presence of breaks or geometrical irregularities. Said concentrations can result in perforation of the sleeve layers. To control electrical field distribution in the joint area two techniques are employed. The first calls for the use of insulating or field control resins (optionally in the form of self-amalgamating tapes) as fillers of the empty spaces in the conductor joint area. This permits annulling or reducing the irregularities of form of the conductor and a steadier distribution of the electric field lines in the surrounding area. The second technique calls for employment of a tubular conductive shield (commonly termed electrode or deflector) applied inside the sleeve and around the conductors. There is thus created a sort of Faraday cage with constant potential which annuls the effect due to form irregularity of parts inside the electrode. The first technique has the shortcoming of requiring specific machining by the assembler when making the joint. This increases joint making time and introduces a risk factor due to the possibility of manufacturing errors. To realise a sleeve with incorporated electrode, different solutions can be applied. It is possible to manufacture the electrode by moulding and then causing the electrode to adhere to the sleeve. As an alternative it is possible to merely expand the electrode on a support separate from that of the sleeve so as to mount it subsequently on the connection area of the two conductors of the cables to be connected. Or the electrode can be mounted separately on the same support on which is then mounted the sleeve. Or also it is possible to apply the electrode as a layer of semiconductive varnishes. In all cases, it is difficult to ensure precise centring of the electrode in relation to the sleeve and the conductor joint and its correct operation. All these methods also do not permit obtaining the electrode in an industrially valid manner with reduced costs. GB-A-2 046 032 discloses an extruded radially shrinkable sleeve for covering joints and terminations of cables made by radially uniform machining of a multilayer structure comprising an insulating layer (see page 2 lines 80-117) and a field control layer (see page 3 lines 75-81) with stress control properties. US-A-4 822 952 discloses an electrical cable joint in which the insulation structure includes an extruded multi-layered heat shrinkable tube comprising an inner semiconductive layer, an intermediate auxiliary insulation layer and an outer primary insulation layer. Then there is the problem of controlling the distribution of the electrical field in a sleeve of the above type so as to prevent the creation of field concentrations capable of damaging the sleeve. The present invention therefore - in a first aspect - relates to a process for the production of a covering sleeve for a medium-voltage cable joint according to claim 1. In a second aspect, the present invention relates to a covering sleeve for a medium voltage electric cable joint according to claim 3. Further characteristics and advantages of the process and sleeve in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. In the drawings: FIG. 1 shows a schematical cross section of a covering sleeve in accordance with the present invention fitted over two jointed electrical cables before removal of the support which holds them expanded, FIG. 2 shows a perspective view in partial cross section of a tubular structure from which the sleeve of FIG. 1 is made, FIG. 3 shows a front view of a tool employed in the process in accordance with the present invention , FIG. 4 shows schematically an embodiment of the sleeve used in the tests set forth below, FIGS. 5 to 10 show schematically the distribution of a discharge in the sleeves of the examples, FIGS. 11 to 16 show schematically the end form of some samples of semiconductive electrodes. With reference to the figures reference number 1 indicates as a whole an elastic sleeve fitted in elastically expanded condition on a support 3 for protection of the joint between two electric cables 4 and 5, e.g. for medium voltage (10-30kV). The support 3 is e.g. of the two-part type in accordance with European patent application no. EP-A-0541000 of this applicant. The cables 4 and 5 comprise respective conductors 6 and 7 covered by respective insulations 8 and 9. Externally of the insulations are normally provided capacitive shields 10 and 11 of semiconductor material and external protective sheaths 12 and 13. Opposite the junction between the electric cables 4 and 5 all the above said elements of the cables 4 and 5 are removed one after the other as shown in FIG. 1. The dimensions of the uncovered portions of each element are established accurately by standards, e.g. French standard EDF HN 68-S-08 dated June 1987. The bare portions of the conductors 6 and 7 are solidarized together by a metal clamp 14 tightened on the conductors 6 and 7 or by welding or otherwise. The sleeve 1 is provided of elastomer material with particular elastic and electrical characteristics in accordance with the present invention. Specifically said sleeve 1 for employment in the field of medium voltages, e.g. for 24kV lines, provides an outer sheath 17, an external insulation layer 20, an intermediate layer 18 with field control and an internal electrode or deflector 15 The electrode 15 is a portion of an internal layer 16 of semiconductor material present in the finished sleeve 1 only in the middle portion thereof. The electrode 15 which has the purpose of nullifying the electrical field in the area around the clamp 14, after assembly, surrounds the conductors 6, 7 forming a sort of Faraday cage around the clamp 14 so that the free spaces where the insulation is removed do not require fillers. The intermediate layer 18 of the so-called field control type, capable of reducing the electrical field intensity present at the ends of the electrode 15, is made out of elastomer material having a predetermined relative dielectric constant and dielectric rigidity. The ends of said layer 18 in assembly of the sleeve 1 are located accurately in relation to the ends of the shields 10, 11 so that the electrode 15 centred on the clamp 14 can correctly perform its functions. The internal structure of the sleeve 1 comprising the layers 16, 18 and 20 is manufactured by means of simultaneous extrusion of the three layers with achievement of an unbroken multilayer tubular structure 19. A subsequent phase of vulcanisation and winding on a coil allows completion of the processing phases of the multilayer tubular structure 19 in a manner basically like that of a medium/high voltage energy conveyance cable having extruded insulator. Using in the extrusion an appropriate metallic support, e.g. an aluminium tube with internal diameter of 12mm and outside diameter of 16mm (not shown) it is possible to perform the extrusion and subsequent vulcanisation continuously. With particular reference to FIG. 2, at the end of the manufacturing process the multilayer tubular structure 19 comprises in radial succession from the inside towards the outside: a semiconductor layer 16, e.g. 0.5mm thick; a field control layer 18 having a predetermined relative dielectric constant and dielectric rigidity and, e.g., thickness of 2.5mm; an insulating layer 20, e.g. with thickness of 8mm. The inside diameter (Di) of the multilayer tubular structure 19 is thus e.g. 16mm while the nominal outside diameter (De) is 38mm. The dimensions set forth are suitable for providing a sleeve for cables having insulation diameter between 20mm and 32mm. The tubular structure 19 is then cut in lengths each designed to form a single sleeve 1. Naturally the lengths resulting from the cut are appropriately shaped and provided with accessories as set forth below. The tubular structure 19 before being cut can usefully be tested to verify its electrical characteristics thanks to the presence of the continuous semiconductor internal layer 16. For this purpose graphitization of the insulating outer surface of the tubular structure 19 is performed to make said surface electrically conductive. Preferably there is applied simultaneously also a semiconductor textile tape, e.g. based on polyammide cloth, and a metallic shield comprising e.g. one or more copper ribbons 0.07*35mm with 5mm overlap. The electrical testing, in a manner like that used for medium voltage cables having extruded insulator, consists of an alternating current screening test at a voltage of 60kV for a period of 30 minutes followed by measurement of partial discharges: < 10 pC (pico Coulomb) at 24kV, in accordance with standards derived from CEI 20-13 or IEC 502. All the layers of the tubular structure 19 are made out of special compounds (semiconductor, field control, insulating) described e.g. in European patent applications nos. 0379056, 0393495 and 0422567 of this same applicant. In the description given below is indicated a composition of a special compound for the intermediate field control layer 18 especially suited for the purposes of the present invention. To widen the range of diameters of cables on which a single sleeve type can be mounted recourse can be had to a solution described in European patent application no. 0393495. This solution calls for the use of the encircling semiconductor sheath 17 fitted with negative allowance on the 3-layer sleeve made from the tubular structure 19 and preferably obtained also by continuous extrusion and vulcanisation. In the example described the dimensions are Di = 16mm and De = 26mm, and permit to fit cables having a wide range of diameters. In addition the sleeve 1 can be provided with a metallic shield, not shown in the figures, formed e.g. from a tinned copper wire braid and a protective sheath not shown. The process in accordance with the present invention permits making the electrode 15 by mere removal of the excess of the layer 16 lengthwise inside the length of tubular structure 19. For this purpose there is performed a machining process with an appropriate tool allowing removal of the excess of the layer 16 at the ends of the sleeve 1 to obtain an electrode 15 with the desired dimensions. A tool suitable for performing this operation is shown in FIG. 3. The form of said tool is derived from a milling tool. But it is possible to use with comparable results a grinding wheel type tool. The starting thickness of the semiconductor layer 16 from which is obtained the electrode 15 is preferably less than 1 mm, e.g. 0.8mm, in such a manner as not to have considerable variations of the sleeve inside diameter. The border zone between the field control layer 18 and the electrode 15 is clear, i.e. without radiusing. The bevel angle, determined by the cutting angle of the tool, is preferably less than 15°, e.g. 7°, to allow the elastic effect to completely spread the bevelled portion without leaving empty spaces once the sleeve 1 is mounted on the support 3. The above mentioned basically pointed form of the electrode 15 is made possible in accordance with the present invention by the employment of appropriate compounds whose composition is described below. Specifically said compounds allow obtaining the layer radially most inward having pre-set relative dielectric constant and dielectric rigidity capable of supporting the electric gradient at the tip of the electrode. For correct electrical operation of the sleeve 1 it is necessary that some phases of the process in accordance with the present invention be performed with the greatest care and specifically : 1) machining must be performed in such a manner as to completely remove the semiconductor layer over the entire predetermined length so as to obtain a correct 'line of escape', and 2) the intermediate field control layer 18 must have electrical properties such as to permit overcoming all the expected electrical stresses. Specifically, when under these circumstances there is a zone with concentrated electrical gradient at the end of the electrode the severest test to be passed, as is well known to those skilled in the art, becomes the pulsed rigidity test at the highest expected overload temperature. In the embodiment described herein it is sought to overcome with a margin of approximately 20% the pulsed voltage of 150kV, ±10 pulses at 130°C. To achieve this result it became necessary to carry out a series of tests before completing definition of the present invention. Said tests are described below for completeness of exposition. The sleeve used initially comprised: an internal layer with high relative dielectric constant (equal to or higher than 15) having diameter 17mm and thickness 2.5mm such as e.g. that described in European patent application no. 0393495 as tubular layer 4 with reference to FIG. 4, an insulating layer having a thickness of 8mm and diameter 38mm such as e.g. that described as a tubular layer 5 in FIG. 4 of said application, and an encircling semiconductor sheath having at rest the dimensions Di = 16mm and De = 26mm. A formulation example is described in the above mentioned European patent application no. 0393495. First there were performed five tests in which the electrode 15 consisted of a very thin semiconductor layer with thickness 0.1-0.2mm and length 220mm and vulcanised in the centre of the sleeve whose length at rest was 350mm. In these tests the sleeves were mounted on MV cables insulated with XLPE (reticulate polyethylene) with cross section 1*95mm 2 and insulating diameter 23mm. All the joints passed a rapid alternating current test at a voltage of 55kV for 5 minutes. The partial discharge measurement showed that they were < 10pC at 24kV. In addition all the joints passed the pulsed test at surrounding temperature starting at 100kV (±10 pulses) and rising to 25kV at each step up to 150kV (±10 pulses). The same test was repeated at 130°C and the results are set forth in the annexed table 1 and commented below: sleeve no. 1 : pierces at the 3rd pulse with positive polarity at 150kV, sleeve no. 2 : pierces at the 10th pulse with negative polarity at 150kV after withstanding at the 10 pulses with positive polarity, sleeve no. 3 : behaves like no. 2. To appraise the influence of test temperature sleeve no. 4 was brought to piercing at surrounding temperature (again starting at 100kV ±10 pulses). Under these conditions it pierced at 175kV at the 2nd pulse with negative polarity and after withstanding the 10 pulses with positive polarity. Sleeve no. 5 was tested only at 130°C. Under these conditions it pierced at 175kV at the 2nd pulse with positive polarity. FIG. 5 shows schematically the path of the discharge, i.e. piercing P due to the discharge. In all five cases the piercing started at one end or tip of the electrode and affected only the inner layer with high relative dielectric constant. Basically it was observed that the 'tip' effect due to the thin electrode causes piercing of the high relative dielectric constant layer at pulsed voltages on the order of 150kV (thus insufficient). Test temperature has a limited influence on piercing voltage. For further comparison a joint made without an electrode was subjected to a similar test, i.e. filling the empty spaces with a self-amalgamating tape of the NITTO type. At 130°C the joint withstands the test at ±200kV while it pierces at the first 225kV pulse. Further experimental tests were performed on other sleeves in which the ends of the electrode were radiused and no longer with sharp corners. Three assemblies were made (sleeves nos. 6, 7 and 8 of table 1) with sleeves having the electrode with rounded edge with radius R = 0.20mm and overall thickness 0.4mm. Length was identical to that of the previous ones and equal to 220mm at rest. The three sleeves were again mounted on the same type of XLPE insulated cable with cross section of 1*95mm 2 and insulating diameter equal to 23mm. Again in this case the sleeves passed a rapid alternating current test at a voltage of 55kV for 5 minutes and the partial discharges were < 10pC at 24kV. Finally in the pulsed test all three sleeves withstood a voltage of 150kV (±10 pulses) at surrounding temperature and again pierced at 150kV at 130°C with piercing behaviour the same as the above. From these results it is clear that the radiusing made at the end of the electrode with curvature radius R = 0.2mm is insufficient to reduce the 'tip' effect. A series of tests was then carried out to clarify whether the thicker and hence better radiused electrode at the end might permit improvement of pulsed performance. In addition, to simplify performance, recourse was had to electrodes fitted separately from the sleeve on a separate support. Simultaneously it was sought to make clear the effect of the presence of the internal layer with high relative dielectric constant in the sleeve. The results of these further tests are set forth in table 2. In tests nos. 2 and 3 an electrode was used with dimensions Di = 16mm, De = 20mm, curvature radius R equal to 1mm and length at rest 220mm. The cable used for the tests is comparable to the previous ones. In addition in test no. 2 was used a sleeve without the high relative dielectric constant layer while in test no. 3 the sleeve included the layer with high relative dielectric constant. The results are as follows: piercing takes place again at pulsed voltage of 150kV at 130°C. Therefore, even using an electrode with curvature radius R at its ends of 1mm the situation does not improve. Test no. 3 shows that the layer with high relative dielectric constant employed for the sleeve is not able to reduce the 'tip' effect on the electrode and in fact the discharge runs through just this layer. The other two tests nos. 4 and 5 were performed using a thicker electrode as shown in FIG. 4 with a thickness of 5mm instead of 2mm. Such an electrode is better radiused having a curvature radius R = 2.5mm at the end. In addition bevels were added filling the empty spaces around the bevels with a compound 22 with pre-set relative dielectric constant and improved dielectric rigidity. The results obtained are again set forth in table 2 and commented below: Test no. 4 shows a decided improvement over test no. 2. In test no. 5 the discharge took place at the usual 150kV level with pulsing at 130°C and has the usual behaviour of the initial tests. It thus emerged from the entire series of tests that the weak point of the system is not so much linked to the tip effect, which is linked to the form of the electrode end, but more probably to the intrinsic dielectric rigidity of the field control layer adopted for the sleeve and at the value of its relative dielectric constant. Finally it was clear, as identified previously, that the employment of an electrode separate from the sleeve presents risks of erroneous positioning if it is mounted on the same support as the sleeve while it is too costly and impractical to apply it on a separate support from that of the sleeve. In accordance with the present invention the solution of the problem set forth above calls for the employment of a vulcanised electrode adhering perfectly to the other layers of the sleeve and obtained by continuous extrusion with reduced thickness ( 3 / 4 0.8mm with typical thickness of 0.5mm) whose excessive length is removed by a simple machining using also for the field control layer a compound characterised by relative dielectric constant values lower than the conventionally used values and by dielectric rigidity values higher than those of the compounds of known type. Advantageously in accordance with the present invention the field control layer of the sleeve 1 is provided with a compound having a formulation (indicated as FORM. A in the tables) permitting achievement of the pre-set objectives. The composition of the compound is as follows: terpolymer ethylene-propylene-diene, e.g. that known in trade under the name DUTRAL-TER 4046-E3 of ENICHEM SYNTHESIS 100 zinc oxide 5 calcined kaolin surface-treated with trimethoxyethoxy vinylsilane 60 carbon black, e.g. that known in trade under the name SEVACARB MT 80 paraffinic plastifier 40 poly 1.2 dihydro 2.4.4 trimethylkinolin 1.5 stearic acid 1 triallyl cyanurate 1 dicumyl peroxide 3 The characteristics of the compound set forth above after reticulation are as follows: tensile strength 6MPa elongation by % at tensile breakage 600% (module) load at 100% elongation 1.5mpa residual deformation with elongation set at 50% determined in conformity with UNI 7321.74 on solid test specimen at 100°C after 500 hours 25% relative dielectric constant determined in conformity with ASTM Std D 150 8 volume resistivity determined in conformity with ASTM Std D 257 1.2 x 10 11 Ωcm dielectric rigidity determined in conformity with ASTM Std D 149 7kV/mm For comparison purposes the electric characteristics of the compound of known type with field control employed initially were as follows: relative dielectric constant 15 volume resistivity 1.0 x 10 10 Ωcm dielectric rigidity 3kV/mm Three sleeves obtained by extrusion with dimensions and conformation similar to those described in FIG. 1 complete with encircling semiconductor sheath, metallic shield and protective sheath were subjected to the usual series of tests after mounting on the typical MV cable insulated with XLPE with cross section of 1 x 95mm 2 , D insulation 23mm. The results of the tests are summarised in Table 3. The new field control compound (see tests nos. 7, 8 and 9) permits achieving values of pulsed rigidity considerably higher (200kV as against 150kV achievable with the previous compound). It is deemed that the result achieved is attributable to the values of the relative dielectric constant and dielectric rigidity of the field control material. In the materials of the known art said values were equal to approximately 15 and 3 while in the above compound they are equal to approximately 8 and 7 respectively. It is believed that it would be possible to achieve similar results with values of relative dielectric constant between 6 and 12 and with values of dielectric rigidity over 5 depending on to the specific application conditions. Furthermore, the results obtained with tests nos. 7, 8 and 9 indicated that even with a very thin electrode (approximately 0.5mm) from which the extra length has been removed by machining (using the tool shown in FIG. 3) the dielectric rigidity is ensured by the field control layer. The continuous process in accordance with the present invention exhibits doubtless advantages in comparison with conventional methods such as e.g. moulding which is a discontinuous process and also requires a great variety of high-cost moulds to produce the different sleeve sizes. Another important advantage of the present invention is the fact that it is possible to proceed with electrical testing of a complete piece (even 500 to 1000m) wound on a coil instead of individual sleeves as takes place with the moulding technique.

Claims: Process for the production of a coverinq sleeve for a medium-voltage cable joint comprising the following steps: extruding together a multilayer tubular structure (19) comprising an internal semiconductor layer (16), an intermediate field-control layer (18) having a relative dielectric constant between 6 and 12 and a dielectric rigidity greater than 5kV/mm, and an external insulating layer (20), cutting lengths of said tube (19) having each a length corresponding to that of the sleeve (1), removing from each length the internal semiconductor layer (16) leaving only a central portion thereof to provide an electrode (15) of semiconductive material having pointed bevelled shape, shaping each length. Process in accordance with claim 1, wherein the relative dielectric constant of the field-control layer (18) is between 6 and 10. Covering sleeve for a medium voltage electric cable joint comprising: an insulating external layer (20), a conductive shield (15), provided in the central zone of the sleeve (1) as a residual portion of a partially mechanically removed layer (16), the conductive shield (15) having a pointed bevelled shape and characterised by comprising an intermediate field-control layer (18) having a relative dielectric constant between 6 and 12 and a dielectric rigidity greater than 5kV/mm. Covering sleeve in accordance with claim 3, wherein the relative dielectric constant of the field-control layer (18) is between 6 and 10. Covering sleeve in accordance with either claims 3 or 4, in which the field-control layer (18) is provided by an elastomer having the following mix: terpolymer ethylene-propylene-diene 100 zinc oxide 5 calcined kaolin surface-treated with trimethoxyethoxy vinylsilane 60 carbon black 80 paraffinic plastifier 40 poly 1.2 dihydro 2.4.4 trimethylkinolin 1.5 stearic acid 1 triallyl cyanurate 1 dicumyl peroxide 3

Inventors: PORTAS FRANCESCO, VALLAURI UBALDO, PORTAS, FRANCESCO, VALLAURI, UBALDO
Applicants: PIRELLI CAVI E SISTEMI SPA, PIRELLI CAVI E SISTEMI S.P.A.
Application year: 1995
Main classification: H02G15
IPC codes: H01R4, H02G15