Patent Application
20230135919
This application claims the benefit of priority from European Patent Application No. 21 306 531.1, filed on Oct. 29, 2021, the entirety of which is incorporated by reference.
The present invention relates to methods of minimizing the roughness of cable surfaces, in particular the roughness of insulation surfaces.
High voltage cables usually comprise an electric conductor and an insulation system surrounding the electric conductor, the insulation system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer. High voltage can be applied with alternating current (HVAC) or direct current (HVDC).
The quality of cable transitions, such as cable termination assemblies and joints between two HV cables is dependent on the quality of the surface at the interface between the cable and the joint, that is to say the surface of the insulation material. For these transitions, a rough insulation surface has been proven to be detrimental, in that, rough surfaces tend to give rise to low DC breakdown strength and thus poor performance. For AC the same rough surfaces have proved to feature similar strong field enhancement and roughness is thus detrimental for both HVAC and HVDC applications.
This is especially true for cable joints made with pre-molded joints, for example with pre-molded joints of U.S. Pat. No. 6,916,193B2.
As such preparation of the cable ends having an insulation layer surface with low roughness before jointing is important.
Current techniques for preparing the insulation layer of a cable end prior to jointing or preparing a termination assembly include removing the semiconducting layer with a designated tool, sanding, glassing, etc. Typical examples of peeling and stripping tools are the CP series from Hivotec and the AMSI from Alroc.
However, these methods do not provide the low level of roughness that is desirable in joints and termination assemblies.
The object of the present invention is to provide an improved method for obtaining an insulation layer surface wherein the surface roughness is minimized and improved.
The present invention is defined by the appended claims and in the following:
In a first aspect, the invention relates to a method for preparing an uncovered insulation layer surface of an end section of a cable, wherein the cable comprises an electric conductor and an insulation system surrounding the electric conductor, the insulation system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer, the method comprising the steps of:
The side surface may extend from the whole length of the cutting edge. The side surface is arranged to face the cable during use of the cutting tool. The side surface may be in contact with the insulating layer during cutting. Due to the contact between the insulation layer and the side surface during cutting, the side surface may burnish the obtained outer surface of the insulating layer during cutting
In an embodiment, the surface roughness of at least one of the edge and the side surface may be measured using optical profilometry, and the root mean square surface slope Sdqc and or the average roughness Sac values for these surface and edge are quantified.
The skilled person will understand that these measurements have to be realised on a representative sample of the surface.
The obtained Sdqc and/or Sac values may be compared to preselected reference values of root mean square surface slope Sdqrefc and average roughness Sarefc respectively, such that
The reference values Sdqrefc and Saref are selected to be the threshold values of an acceptable (or suitable) surface roughness. The skilled person will understand that these values are application dependent.
The skilled person will understand that a roughness defining parameter is any mathematical algorithm or equation that derives from the measured surface coordinates, yielding a scalar value or curve that can be used in a comparison against a reference value as an acceptance criteria. For example, amplitude parameters such as Sa, Sq, Ssk, Sku, Sp, Sv, Sz, hybrid parameters such as Sdq, Sdr, Sds, Ssc, other parameters listed in ISO 25178-2, Abott-Firestone curves, histograms, calculation of Field Enhancement Factors. In addition, if we here refer to Sa and Sdq as specific example of parameters to qualify the surface roughness, any data derived from the surface height distribution used as a pass or fail parameter may be used.
In an embodiment Sdqrefc may be 0.4. In another embodiment Sdqrefc may be 0.2. In another embodiment Sdqrefc may be 0.1. In another embodiment Sdqrefc may be 0.04. In another embodiment Sdqrefc may be 0.02. In another embodiment Sdqrefc may be 0.01.
In an embodiment Sarefc may be 800 nm. In another embodiment Sarefc may be 600 nm. In another embodiment Sarefc may be 400 nm. In another embodiment Sarefc may be 200 nm. In another embodiment Sarefc may be 80 nm. In another embodiment Sarefc may be 40 nm. In another embodiment Sarefc may be 20 nm. In another embodiment Sarefc may be 10 nm.
The method according to the first aspect of the invention further comprises the step of:
This may be achieved by rotating the edge of the cutting tool around the cable at a set distance to the center of the cable in order to remove a certain width of the outer surfaces. That is, the outer semiconducting layer and optionally a part of the insulation layer is removed from an end section of the cable.
In other words, the method provides a cable having an end section. At the end section, at least the outer semiconducting layer is removed such that the uncovered outer surface of the cable end section is provided by the insulating layer.
This method is especially useful in preparing a cable end section before jointing or before assembling a termination assembly on it. To ensure that no outer semiconducting layer is present on the cable, it is common to also remove part of the insulating layer.
In one embodiment of the inventive method, the outer semiconducting layer and 1% to 10% of the insulation layer, measured in thickness, i.e. radial thickness may be removed in step c. In another embodiment of the inventive method, the outer semiconducting layer and 10% to 90% of the insulation layer may be removed in step c. In another embodiment of the inventive method, the outer semiconducting layer and 30% to 70% of the insulation layer may be removed in step c. In another embodiment of the inventive method, the outer semiconducting layer and 10% to 50% of the insulation layer may be removed in step c.
In another embodiment of the inventive method, the outer semiconducting layer and 1-2 mm of the insulation layer may be removed in step c.
In another embodiment of the inventive method, the method may further comprise the following step, prior to and/or during step c):
In another embodiment, at least the outer semiconductive layer may be cooled to between the glass transition temperature of the outer semiconductive layer and 10, 20, 30 or 40° C. below the glass transition temperature of the outer semiconductive layer.
In another embodiment of the inventive method, the method may further comprise the following step, prior to and/or during step c):
In another embodiment, at least the outer semiconductive layer and the insulating layer may be cooled to between the glass transition temperature of the insulating layer and 10, 20, 30 or 40° C. below the glass transition temperature of the insulating layer.
In another embodiment, the outer surface of the cable, i.e. to the outer semiconductive layer and/or insulating layer may be cooled to between 0° C. and −50° C.
In another embodiment, the outer surface of the cable, i.e. to the outer semiconductive layer and/or insulating layer may be cooled to between −10° C. and −50° C.
In another embodiment, the outer surface of the cable, i.e. to the outer semiconductive layer and/or insulating layer may be cooled to between −20° C. and −40° C.
In another embodiment of the inventive method, the method may further comprise the following step after step c)
The skilled person will understand that this heating is applied so as to reduce the roughness of the uncovered insulation surface, but not so that said uncovered insulation surface will start to deform at the macroscopic scale or burn.
In an embodiment, the uncovered insulation surface obtained in step c) may be heated to or over the melting temperature of the insulating layer.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between the melting temperature of the insulating layer and 20° C. over the melting temperature of the insulation layer.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between 110° C. and 300° C.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between 110° C. and 220° C.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between 120° C. and 200° C.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between 120° C. and 140° C.
In another embodiment, the uncovered insulation surface obtained in step c) may be heated to between 180° C. and 200° C.
In another embodiment of the inventive method, the method may further comprise the following step of
The reference values Sdqref and Saref are selected to be the threshold values of an acceptable (or suitable) surface roughness. The skilled person will understand that these values are application dependant.
The skilled person will understand that these measurements have to be realised on a representative sample of the surface.
The skilled person will understand that a roughness defining parameter is any mathematical algorithm or equation that derives from the measured surface coordinates, yielding a scalar value or curve that can be used in a comparison against a reference value as an acceptance criteria. For example, amplitude parameters such as Sa, Sq, Ssk, Sku, Sp, Sv, Sz, hybrid parameters such as Sdq, Sdr, Sds, Ssc, other parameters listed in ISO 25178-2, Abott-Firestone curves, histograms, calculation of Field Enhancement Factors. In addition, if we here refer to Sa and Sdq as specific example of parameters to qualify the surface roughness, any data derived from the surface height distribution used as a pass or fail parameter may be used.
In an embodiment Sdqref may be equal to 0.4. In another embodiment Sdqref may be 0.2. In another embodiment Sdqref may be 0.1. In another embodiment Sdqref may be 0.04. In another embodiment Sdqref may be 0.02. In another embodiment Sdqref may be 0.01.
In an embodiment Saref may be 800 nm. In another embodiment Saref may be 600 nm. In another embodiment Saref may be 400 nm. In another embodiment Saref may be 200 nm. In another embodiment Saref may be 80 nm. In another embodiment Saref may be 40 nm. In another embodiment Saref may be 20 nm. In another embodiment Saref may be 10 nm
In a second aspect, the invention relates to a method of jointing a first cable and a second cable, the first cable and the second cable comprising an electric conductor and an insulation system surrounding the electric conductor, the insulation system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer, the method comprising the steps of:
In an embodiment of the second aspect of the invention the uncovered insulation layer surface may be prepared at a cable end for both the first and the second cables.
In an embodiment of the second aspect of the invention, the jointing step may be realised using a pre-molded joint.
In a third aspect, the invention relates to a method for preparing a termination assembly on a cable, the cable comprising an electric conductor and an insulation system surrounding the electric conductor, the insulation system comprising an inner semiconducting layer, an insulating layer and an outer semiconducting layer, the method comprising the steps of:
In an embodiment of the third aspect of the invention, the mounting a termination assembly step is realised using a pre-molded termination assembly.
In a fourth aspect, the invention relates to a cable, comprising a cable end, obtainable by the method according to the first aspect of the invention.
In a fifth aspect, the invention relates to a joint between a first and a second cable, obtainable by the method according to the second aspect of the invention.
In a sixth aspect, the invention relates to a termination assembly on a cable, obtainable by the method according to the third aspect of the invention.
In the following description this invention will be further explained by way of exemplary embodiments shown in the drawings:
The quality of cable transitions, such as cable termination assemblies and joints between two HV cables is dependent on the quality of the surface at the interface between the cable and the joint, that is to say the surface of the insulation material. For these transitions, a rough insulation surface has been proven to be detrimental, in that, rough surfaces tend to give rise to low DC breakdown strength and thus poor performance. For AC the same rough surfaces have proved to feature similar strong field enhancement and roughness is thus detrimental for both HVAC and HVDC applications.
In the preparation of a cable transition, that is to say the transition from a cable end to another element, such as in cable termination assemblies and joints, a first electric cable 100 is provided, the first cable 100 comprising an electric conductor 110 and an insulation system 120 surrounding the electric conductor 110, the insulation system 120 comprising an inner semiconducting layer 121, an insulating layer 122 and an outer semiconducting layer 123. Other layers, such as sheathing, water barriers may also be present on the cable. This cable is illustrated in
The end section of the cable 100 may be prepared for having termination assembly mounted over it or to be jointed to a second electric cable 200. The second cable 200 comprising an electric conductor 210 and an insulation system 220 surrounding the electric conductor 210, the insulation system 220 comprising an inner semiconducting layer 221, an insulating layer 222 and an outer semiconducting layer 223. Other layers, such as sheathing, water barriers may also be present on the second cable 200.
The outer layers down to but not including the outer semiconducting layer 123, 223 are removed according to known techniques. This is done on a length of cable that is suitable for jointing, known to the skilled person.
The inventive method of preparing a cable end comprises the steps of:
The cutting tool 400 comprises a blade and may be any cutting tool known to the skilled person. Rotary stripping tools are a typical example of such a tool. Cutting tools 400 comprising a blade at least partially made of steel, tungsten, or ceramic may be advantageous because they are well suited for being polished. Steel blades are preferred. The blade of the cutting tool 400 comprises an edge 401 and a side surface 402, starting from the cutting edge that is intended to be facing the cable during removal of the outer layers of the cable (and not the parts of the cable being removed), thereafter called the contact surface 402, as illustrated in
The cutting tool 400 is prepared by polishing at least the cutting edge 401 and the contact surface 402 of the cutting tool.
Any polishing technique known in the art may be used here. These polishing techniques include both chemical and mechanical treatment such as chemical coating, etching, laser sharpening.
Surface roughness is measured thanks to any known method. Typical measurements are made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical (e.g.: a white light interferometer or laser scanning confocal microscope).
In an exemplary embodiment the blade of the cutting tool 400 is prepared by polishing. The roughness of the edge 401 and of the contact surface 402 of the cutting tool 400 are measured using known methods, and parameters such as the root mean square surface slope Sdqc and or the average roughness Sac values for the edge and of the contact surface are quantified, and:
In an example of the invention Sdqrefc=0.4.
In an example of the invention Sarefc=800 nm.
After being prepared the cutting tool is used in a step:
Step c) is illustrated in
Steps a)-c), by themselves, are sufficient to improve the quality of the surface 125, 225 of the insulation layer 122, 222.
In addition, one or more of the following steps may be undertaken to further reduce the roughness of the surface 125, 225 of the insulation layer 122, 222.
The cooling may for example be realized by using dry-ice or an electrical cooling blanket. The cooling temperature is often between −10° C. to −50° C.
The cooling step will help stiffen the outer surfaces to be removed 122,123,222,223, which will result in a lower roughness on the surface 125, 225 obtained in step c).
By applying heat to the surface obtained in step c), the polymer at the surface will reach their melting temperature and existing irregularities will be further lessened on the uncovered insulation layer surface 125;225 obtained in step c). Heating may be applied using a hot air flow such as with a heat gun.
Surface Texture Parameters
Features in the surface texture can be quantified using roughness parameters, which represent features in a certain measurement domain with a single parameter value, thus facilitating direct comparisons of certain surface types. The average roughness (or arithmetical mean height variation) of a certain 3D surface is quantified through Sa, expressed as:
Sα=∫∫
α
|Z(x,y)|dxdy
where Z(x,y) represents a matrix of 3D coordinates on the surface, with Z expressing the local distance from the mean plane in m. Another relevant parameter, the root mean square (RMS) roughness Sq, is expressed as:
Sq=√{square root over (∫∫α(X(x,y))2dxdy)}
Sa and Sq parameters, in m, are useful for comparing the height in the surface texture. However, two surface types having identical Sa and Sq parameter values can have vastly different textures, as the texture's spacing can be present with different widths. Therefore, a third parameter, the root mean square (RMS) surface slope Sdq, is introduced as:
where A is the area of the measurement domain in m2, Lx and Ly are respectively the x and y length of the investigated domain in m. The Sdq parameter thus quantifies the steepness in the surface texture (i.e. with Sdq=0 for a fully flat surface and Sdq=1 for all gradient components at 45° incline) and is sensitive to both the amplitude and spacing in the surface texture. By using both the average roughness Sa and the root mean square surface slope Sdq parameters in surface type comparisons, a texture sensitive analysis is made covering most surface types.
Surface roughness may be measured by any known method. Typical measurements are made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical (e.g.: a white light interferometer or laser scanning confocal microscope).
This optional step is preferably done in the field, and preferably without damaging (i.e. taking a sample of) the surface obtained in step c) or e).
Portable optical profilometry as described in the co-pending application with title “optical profilometry on HV-cable ends and on samples extracted form HV-cables” is a highly advantageous method for measuring the surface roughness.
The cable end may then be transitioned, e.g. jointed to another cable end or termination, using known methods. The method of preparing a cable end is particularly relevant for transitions involving a physical interface, in particular for method using a pre-molded transition element 300, such as a pre-molded joint or a pre-molded termination assembly.
The pre-molded transition element 300 typically comprises an electric transition element to connect the electric conductor 110;210 to a further conductor element and an insulation system surrounding the electric transition element 310, the insulation system comprising an inner semiconducting layer 321, an insulating layer 322 and an outer semiconducting layer 323.
In a joint, the electric transition element may for example be a ferrule or any other suitable element known in the art.
The skilled person will understand here that a physical interface here refers to an interface between to solid surfaces, for example between a surface of a cable end and a pre-molded joint, whereas a chemical interface refers to a interface between a surface of a cable end and a non-solid surface, for example a melted insulation layer extruded on top of the cable end. Here the physical interface is the interface between the uncovered insulation layer surface 125;225 of the cable 100;200 and the inner surface of the insulating layer 321 of the transition element 300.
The obtained physical interface, between a cable end and a pre-molded joint or a pre-molded termination assembly is illustrated in
An uncovered insulation layer surface (125, 225) of an end section of a cable (100, 200), was prepared using different methods, on at least 3 cables in each case. The surface roughness of the obtained insulation layer was measured in each case using an optical profilometry. Sa and Sdq were derived from these measurements.
A first series of cable ends on different cables was prepared first roughly cutting away the outer semiconducting layer using a peeling tool, followed by stepwise abrasion starting from rougher (lower) down to finer (higher) grit sizes, up to a final step using P600.
Measurements on the first series of cables yielded the following results:
Sa=1010±369 nm
Sdq=0.46±0.3.
In a first example, a series of cable ends on different cables was prepared with the inventive method comprising the steps of:
This second series of cables yielded the following results:
Sa=423±126 nm
Sdq=0.18±0.04.
In a second example, a series of cable end on different cables was prepared with the inventive method comprising the steps of:
This third series of cables yielded the following results:
Sa=120±23 nm
Sdq=0.14±0.03.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21306531.1 | Oct 2021 | WO | international |
