METHOD FOR CONTRASTING INJECTION OF ELECTRICAL CHARGE IN INSULATING LAYER OF HVDC ELECTRICAL APPARATUS, HVDC TRANSMISSION CABLE AND ACCESSORY FOR HVDC CABLE

Technical field

This invention relates to a method for contrasting injection of electrical charge in an insulating layer of a high voltage direct current (HVDC) electrical apparatus, a high voltage direct current (HVDC) transmission cable and to accessories for high voltage direct current (HVDC) cables.

Background art

In HVDC transmission cables or accessories for HVDC cables such as couplings and terminals, it is common practice to use dielectric insulating materials to insulate electrical conductors. Known in the field of electrical energy transportation, therefore, are solid insulators made of polymeric materials such as polyolefins and, in particular, polyethylene (PE). These materials have the advantage of being easy to process by extrusion or moulding. To improve the resistance of these materials to weathering and to dielectric degradation due to the prolonged action of the electric field, treatment technologies have been developed to improve their insulating properties and durability. For example, it is known that PE used for cable insulation can be treated by subjecting it to a cross-linking process to make what is known as XLPE.

In direct current applications and, in particular, in high voltage direct current (HVDC) applications) one of the causes of faults and inefficiency is connected with the phenomenon of space charge accumulation.

It should be noted that the accumulation of electrical charge increases the concentration of electric field at some points of the insulation, thus accelerating the degradation of the polymeric material.

Space charge accumulation may occur by internal generation within the polymeric material itself or by injection of electrical charge carriers from the electrodes. Internal generation within the polymeric material is connected with the chemical reactions that occur in the polymeric material subjected to thermal and electric stresses. The injection of electrical charge, on the other hand, may occur as a result of the high electric fields applied to the contact between the electrode and the polymeric insulating material, leading to the migration of charge to the opposite electrode and trapping within the insulant, thus intensifying the local electric field and accelerating the degradation of the insulating material. This phenomenon is particularly critical at the points where there are breaks in the insulation, in proximity to couplings and HVDC cable terminals, since the space charge can accumulate in considerable quantities at these breaks.

To contrast the injection of electrostatic charge in the electrical insulation, nanometric additives may be added to the polymeric material. For example, according to prior art, nanometric additives are added to polymers such as polypropylene or ethylene vinyl acetate. The purpose of these nanometric additives is to reduce the depth of the traps present in the insulant, allowing greater mobility for the electric charges injected by the electrode and thereby limiting the space charge accumulation within the insulant.

In addition, patent document EP271 1933A1 describes the production of cable insulating material from a composition including polyolefin and, embedded therein, nano particles which include graphene, in order to reduce cable deterioration due to accumulated space charges.

The addition of nano additives or nano particles to the insulating material requires the production of a dedicated mixture and is therefore expensive. Furthermore, the reduction of space charge accumulation is directly correlated with the uniformity of the behaviour of the material: guaranteeing the uniformity of dispersion and distribution of nano charges in the mixture (which is processed by extrusion) without aggregations is quite complex and difficult to check with in-process inspection systems. It is noted that in the case of HVDC cables, the insulant must be uniform along the full length of the cable, which may in some case be several hundred metres for each single size. Localized aggregations of nano charges may give rise to defects, which may in some case be potentially highly critical in terms of the capability of limiting space charge accumulation, thus constituting weak points in the reliability of the HVDC cable. This is one of the main reasons why HVDC cables with fully nanostructured insulation are not so common in the market. Further, the addition of nanoparticles to the polymeric material of the insulating material has negative effects on the rigidity of the insulant itself.

Also known are methods which involve applying nanostructures to the surface of commercial insulating materials or special surface treatments. These include methods of coating with nanoparticles of silver contained in polymeric matrices, as well as methods of treating the surface of the insulant by fluorination. These methods have the disadvantages of limited efficiency and high cost of raw materials.

Further, patent documents CN205609239U and KR20120086072A describe the addition of graphene powder to the semiconductive layer covering the insulant, in order to improve its conductivity; these documents do not, however, tackle the problem of space charge injection and, in practice, the solutions they provide do not appear to be suitable for blocking the space charges because the semiconductive material is processed by extrusion and, therefore, the distribution of graphene particles is inevitably non-uniform. In effect, as already explained, it is known that, during extrusion, the nano charges dispersed in a polymer tend to form aggregates again, leading to a non-uniform distribution which inevitably reduces their effectiveness in preventing space charge injection.

Disclosure of the invention

The aim of this disclosure is to provide a method for contrasting injection of electrical charge in an insulating layer of a high-voltage direct current (HVDC) electrical apparatus to overcome the above mentioned disadvantages of the prior art.

Another aim of this disclosure is to provide an HVDC cable and an accessory for an HVDC cable in which injection of electrical charge in an insulating layer is well contrasted.

These aims are fully achieved by the method, cable and cable accessory of this disclosure, as characterized in the appended claims.

More specifically, the method comprises a step of coating at least one portion of a surface of the high voltage direct current (HVDC) electrical apparatus with a coating layer.

In an embodiment, the surface of the electrical apparatus is a surface of an insulating layer of the electrical apparatus. In an embodiment, the surface of the electrical apparatus is a surface of a semiconductive layer (outer or inner) of the electrical apparatus.

In an embodiment, the coating layer includes graphene. In an embodiment, the coating layer includes a nanomaterial comprising graphene. In an embodiment, the coating layer includes one or more nanomaterials comprising graphene. In an embodiment, the coating layer includes a plurality of nanomaterials comprising graphene. In an embodiment, the coating layer includes a nano graphite. In an embodiment, the coating layer includes a combination of nanomaterials comprising graphene and/or nano graphites.

In an embodiment, the nanomaterial comprises nanoplatelets of graphite and few-layer graphene. In an embodiment, the nanomaterial belongs to the family of 'graphene related materials'. In an embodiment, the nanomaterial has a minimum carbon content of 98%.

In an embodiment, the insulating layer is made of dielectric material. In an embodiment, the insulating layer is made of polymeric dielectric material. In an embodiment, the polymeric dielectric material is a thermoplastic polymer. In an embodiment, the polymeric dielectric material is a thermosetting polymer. In an embodiment, the polymeric dielectric material is an elastomer. In an embodiment, the step of coating comprises a step of preparing a solution. The solution is prepared by mixing a nanomaterial comprising graphene in solid phase with a liquid solvent. In an embodiment, the solution is prepared using ultrasonic immersion tools.

In an embodiment, the nanomaterial (or the one or more materials) comprising graphene in solid phase is in the form of nano-powder. In an embodiment, the nano-powder is made up of graphite nanoplatelets. In an embodiment, the graphite nanoplatelets have a thickness of between 5 and 30 nm. In an embodiment, the graphite nanoplatelets have an average surface size of between 1 and 200 μιη. In an embodiment, the graphite nanoplatelets have a carbon content greater than 98%. In an embodiment, the graphite nanoplatelets have a bulk density of between 0.01 g/cm ^A 3 and 0.05 g/cm ^A 3. In an embodiment, few-layer graphene (FLG) is added to the nanoplatelets. In an embodiment, the FLG have a thickness of between 0.2 and 5 nm. In an embodiment, the FLG have an average surface size of between 1 and 30 μιη. In an embodiment, the FLG have a carbon content greater than 95%. In an embodiment, the FLG have a bulk density of between 0.1 g/cm ^A 3 and 0.9 g/cm ^A 3.

In an embodiment, the liquid solvent has high volatility.

In an embodiment, the liquid solvent has high compatibility with the nano charges comprising graphene (that is, high wettability) so as to form a suspension which is uniform and stable (that is, whose viscosity and decantation values are stable over time).

In an embodiment, the liquid solvent is a polar solvent. In an embodiment, the liquid solvent is acetone. In an embodiment, the liquid solvent is ethyl acetate. In an embodiment, the liquid solvent is ethanol. In an embodiment, the liquid solvent is tetrahydrofuran. In an embodiment, the liquid solvent is water.

Thanks to its properties, the solvent interacts with the surface on which it is applied (which, in an embodiment, is the surface of the material of the insulating layer) in such a way that the nanomaterial adheres to that surface. In an embodiment, the ratio by weight in the solution between the nanomaterial comprising graphene in solid phase and the liquid solvent is between 1 % and 4%. In an embodiment, the ratio by weight in the solution between the nanomaterial comprising graphene in solid phase and the liquid solvent is between 1 .5% and 2.5%. Preferably, the ratio by weight in the solution between the nanomaterial comprising graphene in solid phase and the liquid solvent is between 1 % and 3%. Preferably, the ratio by weight in the solution between the nanomaterial comprising graphene in solid phase and the liquid solvent is 2%.

In an embodiment, the solution has a viscosity of between 50 cP and 1000 cP.

In an embodiment, preparing the solution includes blending the solution. Blending has a disaggregating and deagglomerating effect. In an embodiment, blending is carried out with ultrasonic immersion blending systems. In an embodiment, the ultrasonic immersion blending systems have a wavelength of between 30 micron and 80 micron. In an embodiment, the ultrasonic immersion blending systems have a power rating greater than 500 Watts.

In an embodiment, blending is carried out with high-shear blenders. In an embodiment, the high-shear blenders comprise a rotor and a stator. In an embodiment, the stator is perforated so as to guarantee dispersion of the nanoparticles. In an embodiment, the rotor has a rotation speed of between 1000 and 15000 rpm.

In an embodiment, blending is carried out with a combination of ultrasonic immersion blending systems and high-shear blenders.

In an embodiment, the solution is prepared at a constant temperature, preferably between 10°C and 30°C.

In an embodiment, preparing the solution includes adding a polymeric binder. The polymeric binder is compatible with the polymeric dielectric material of the insulating layer and, at the same time, is soluble in the solvent. In an embodiment, the dielectric material is a polyurethane. In an embodiment, the dielectric material is a vinyl copolymer. In an embodiment, the dielectric material is an epoxy.

In an embodiment, the step of coating comprises a step of depositing the solution on the surface of the electrical apparatus. In an embodiment, the solution is deposited on the insulating layer. In an embodiment, the solution is deposited on the semiconductive layer (outer and inner). In an embodiment, the step of depositing the solution on the insulating layer is accomplished by spraying the solution in nebulized form. In an embodiment, the step of depositing the solution on the insulating layer is accomplished by smearing. During the step of depositing, the solution is deposited on at least one portion of a surface of the insulating (or semiconductive) layer to produce the coating layer; thus, deposition has the effect of forming a coating layer which adheres to a portion of the surface of the insulating (or semiconductive) layer. In an embodiment, spraying or smearing is coupled with continuous or discontinuous surface rubbing processes. In an embodiment, rubbing is carried out with "rubbing process" technology. In an embodiment, rubbing is carried out with "polishing process" technology. The processes of rubbing the solution on the surface consist in applying shear stresses which can exfoliate the graphite nanoplatelets to reduce them to just a few nanometres in thickness. That way, the presence of few layer graphene is increased. This creates van der Waals interactions between the graphene and the surface, thereby improving adhesion of the solution to the surface.

In an embodiment, the method comprises a step of desiccating the solution. During the step of desiccating the solution, the solvent evaporates. The step of desiccating the solution is carried out after the step of depositing the solution. In an embodiment, desiccation is carried out in air at ambient temperature. In an embodiment, desiccation is carried out in an oven at high temperature. High temperature means approximately between 50°C and 70°C. The step of desiccating requires a desiccating time which depends on the solution, the desiccating method (ambient temperature or high temperature) and the thickness of the coating layer. In an embodiment, the desiccating time is a few minutes. In an embodiment, the desiccating time is in the order of half an hour. In an embodiment, the desiccating time is in the order of one hour.

In an embodiment, the method comprises a step of polishing the coating layer. Polishing is accomplished by a mechanical action. The mechanical action comprises applying shear stresses to the coating layer applied on the substrate (which may be, for example, the insulating layer). In an embodiment, polishing is carried out by rotary brush systems. In an embodiment, polishing is carried out by coated, counter-rotating cylinders. In an embodiment, the rotary brushes or the counter-rotating cylinders have a rotation speed of between 30 and 500 rpm.

Thanks to the mechanical polishing (or smearing) action of the coating layer, the nanoparticles including graphene are distributed and aligned on the surface in such a way as to create films of graphene oriented tangentially relative to the cable. In effect, as is known, graphene is an anisotropic material which has high electrical conductivity within the film but which blocks the charges in the direction perpendicular to the film (that is from one film to another). Thanks to this alignment, the electrical conductivity at the interface surface which the coating is deposited on is maximized on the surface and minimized transversely to the surface.

Further, the polishing action has the effect of exfoliating and homogenizing the nanometric material (preferably nanoplatelets of graphite and few-layer graphene). In an embodiment, polishing results in a coating layer of nanometric thickness (the thickness is defined perpendicularly to the surface the coating is applied on). In an embodiment, after polishing, the nanoparticles are distributed and aligned parallel to the surface the coating is applied on, reaching a micrometric length.

The graphene creates deep traps which retain the electrons. Polishing is important because it gives the films the tangential orientation so that the space charges are retained by the graphene or, at most, can move in a direction tangential to the layer of the cable the coating is applied on (for example, the insulating layer) but are blocked in the direction radial to the cable. Thus, the charges cannot pass through the coating layer and cannot therefore penetrate the insulant.

Furthermore, the mechanical polishing action creates van der Waals interactions which facilitate effective and durable adhesion of the nanoparticles to the surface of the cable the coating is applied on.

In an embodiment, polishing creates a uniform coating layer.

In one embodiment, deposition of the solution and polishing create an uninterrupted coating layer on the surface. In another embodiment, deposition of the solution and polishing create a broken coating layer, localized in zones which act as traps for the electrical charge injected at the interface.

Polishing requires a polishing time. In an embodiment, the polishing time is less than 15 seconds. In an embodiment, polishing is carried out during evaporation of the solvent. In an embodiment, polishing is carried out in two or more stages while the solvent evaporates.

The coating layer has a coating thickness. The coating thickness is chosen on the basis of the values of resistivity and transmittance required of the coating layer. In an embodiment, the steps of depositing the solution, desiccating the solution and polishing are repeated cyclically in order to obtain a coating layer with the required values of resistivity and transmittance.

Adding the polymeric binder to the solution, if necessary, allows increasing the quantity of nanomaterials comprising graphene which is deposited, thus obtaining a thicker coating. Adding the polymeric binder, if necessary, also allows improving the adhesion of the coating layer to the substrate; that is, to the insulating layer of polymeric material. The better adhesion of the coating layer to the substrate improves resistance to wear and chemical agents.

In an embodiment, the coating layer is applied on a portion of the insulating layer of the electrical apparatus. In an embodiment, the coating is applied on an interface between the insulating layer and the semiconductive material (on the outside of the insulant or on the inside of the insulant). This disclosure also provides a transmission cable. In an embodiment, the cable comprises a coating applied with the method according to this disclosure.

In an embodiment, the electrical apparatus is a high voltage direct current (HVDC) power cable.

In an embodiment, the transmission cable comprises an inner conductor. The inner conductor acts as a high-voltage electrode. The inner conductor has its own surface.

In an embodiment, the transmission cable comprises an inner semiconductor. The inner semiconductor surrounds the inner conductor. In an embodiment, the inner semiconductor is made of polymeric dielectric material and conductive particles (carbon black) dispersed in the dielectric material. In an embodiment, the inner semiconductor is made of a thermoplastic polymer and conductive particles (carbon black) dispersed in the thermoplastic polymer. In an embodiment, the inner semiconductor is made of a thermosetting resin and conductive particles (carbon black) dispersed in the thermosetting resin. In an embodiment, the inner semiconductor is made of an elastomer and conductive particles (carbon black) dispersed in the elastomer.

In an embodiment, the function of the inner semiconductor is to uniformize the electric field in proximity to the inner conductor, thereby avoiding unwanted concentrations of the electric field.

In an embodiment, the transmission cable comprises an insulating layer made of dielectric material.

In an embodiment, the insulating layer surrounds the inner conductor. In an embodiment, the insulating layer surrounds the inner semiconductor. In an embodiment, the inner semiconductor has an inner surface, in contact with the inner conductor, and an outer surface, in contact with the insulating layer.

In an embodiment, the dielectric material of the insulation is a cross-linked polyethylene (XLPE). In an embodiment, the dielectric material of the insulation is a polypropylene (PP). In an embodiment, the dielectric material of the insulation is a copolymer of ethylene and vinyl acetate (EVA). In an embodiment, the dielectric material of the insulation is a copolymer of ethylene and polypropylene (EPR). In an embodiment, the dielectric material of the insulation is a silicone rubber.

In an embodiment, the dielectric material of the insulation is a polymeric dielectric material. In an embodiment, the dielectric material of the insulation is a thermoplastic polymer. In an embodiment, the dielectric material of the insulation is a thermosetting polymer. In an embodiment, the dielectric material of the insulation is an elastomeric polymer. In an embodiment, the dielectric material of the insulation is a polymeric dielectric material.

In an embodiment, the transmission cable comprises an outer semiconductor. The outer semiconductor surrounds the insulating layer. In an embodiment, the outer semiconductor has an inner surface, in contact with the insulating layer. In an embodiment, the outer semiconductor has an outer surface, in contact with the inner surface.

In an embodiment, the outer semiconductor is made of a thermoplastic polymer and conductive particles (carbon black) dispersed in the thermoplastic polymer. In an embodiment, the outer semiconductor is made of a thermosetting resin and conductive particles (carbon black) dispersed in the thermosetting resin. In an embodiment, the outer semiconductor is made of an elastomer and conductive particles (carbon black) dispersed in the elastomer.

In an embodiment, the transmission cable comprises an insulating layer screen (or outer conductor). In an embodiment, the insulating layer screen (or outer conductor) surrounds the insulating layer.

In an embodiment, the insulating layer screen (or outer conductor) surrounds the outer semiconductor. In an embodiment, the function of the inner semiconductor is to uniformize the electric field in proximity to the outer conductor, thereby avoiding unwanted concentrations of the electric field. In an embodiment, the transmission cable comprises a sheath. The sheath surrounds the screen of the insulating layer.

In an embodiment, the transmission cable comprises a coating applied to at least one portion of a surface of the cable. In an embodiment, the coating is applied using the method provided by this disclosure.

In an embodiment, the coating includes graphene. In an embodiment, the coating includes a nanomaterial comprising graphene. In an embodiment, the coating includes one or more nanomaterials comprising graphene. In an embodiment, the coating comprises a coating layer applied to at least one portion of the surface of the inner conductor.

In an embodiment, the coating includes a coating layer applied to at least one portion of the inner surface of the inner semiconductor.

In an embodiment, the coating comprises a coating layer applied to at least one portion of the outer surface of the inner semiconductor.

In an embodiment, the insulating layer has an inner surface facing the inner conductor (or the inner semiconductor). In an embodiment, the insulating layer has an outer surface, opposite to the inner surface.

In an embodiment, the coating comprises a coating layer applied to at least one portion of the outer surface of the insulating layer. In an embodiment, the coating comprises a coating layer applied to at least one portion of the inner surface of the insulating layer.

In an embodiment, the coating includes a coating layer applied to at least one portion of the inner surface of the outer semiconductor.

In an embodiment, the coating comprises a coating layer applied to at least one portion of the outer surface of the outer semiconductor.

In an embodiment, the coating layer is applied on the inside of the outer surface of the insulating layer; more specifically, the coating layer is applied to an outer surface of the inner semiconductor, which is surrounded by the insulating layer. In an embodiment, the coating layer is applied on the outside of the outer surface of the insulating layer; more specifically, the coating layer is applied to the outer surface of the insulating layer or, if the cable comprises the outer semiconductor, the coating layer is applied to an outer surface of the outer semiconductor.

This disclosure also provides an accessory for a high voltage direct current (HVDC) transmission cable. In an embodiment, the accessory comprises a coating applied with the method according to this disclosure.

In an embodiment, the electrical apparatus to which the method according to this disclosure applies is an accessory for a high voltage direct current (HVDC) power cable.

In an embodiment, the accessory is a coupling. The function of the coupling is to connect an end of a cable to an end of another cable. In an embodiment, the accessory is a terminal. The function of the terminal is to connect an end of a cable to a terminal device. The terminal device may be a rectifier. The terminal device may be a voltage converter.

In an embodiment, the accessory comprises a coating including a nanomaterial comprising graphene, applied to at least one portion of a surface of the accessory. In an embodiment, the coating is applied using the method provided by this disclosure.

In an embodiment, the accessory comprises an insulating layer. In an embodiment, the insulant is a silicone rubber. In an embodiment, the insulating layer has a smooth surface. In an embodiment, the insulating layer has a corrugated or finned surface. The corrugated or finned surface makes it more difficult for the charge to travel along the surface. In an embodiment, the accessory comprises a conductive or semiconductive layer. In an embodiment, the conductive or semiconductive layer is at least partly incorporated in the insulating layer. In an embodiment, the coating comprises a coating layer applied at least to an outer surface of the insulating layer.

This disclosure also provides an apparatus for applying a coating to a surface of an insulating layer made of dielectric material of a high -voltage direct current (HVDC) electrical apparatus (or to a portion of the surface) a coating which includes a nanomaterial comprising graphene.

The apparatus comprises a container. The apparatus comprises a solution including a liquid solvent and a nanomaterial comprising graphene in solid phase.

In one embodiment, the apparatus comprises a nebulizing device configured to deliver the solution in nebulized form, to make the coating. In another embodiment, the apparatus comprises a tank (which may coincide with the container) configured to allow the electrical apparatus to be immersed in the solution.

The nebulizing device (which may itself constitute an object of this disclosure) is configured to deliver a solution including graphene in nebulized form.

In an embodiment, the nebulizing device comprises a container. In an embodiment, the nebulizing device comprises a solution, contained in the container. Preferably, the solution comprises a liquid solvent and a nanomaterial including graphene in solid phase.

In an embodiment, the nebulizing device comprises a nebulizing nozzle. The nebulizing device (that is, the nebulizing nozzle) is configured to deliver the solution (including a nanomaterial comprising graphene) in nebulized form to make a coating layer on a surface. In an embodiment, the surface is a surface of an insulating layer of a high voltage direct current (HVDC) power transmission cable. In an embodiment, the surface is a surface of an insulating layer of an accessory for a high voltage direct current (HVDC) power transmission cable.

In an embodiment, the nebulizing device (that is, the nebulizing nozzle) is an "airless" spray system.

In an embodiment, the nebulizing device (that is, the nebulizing nozzle) is an air mix system.

In an embodiment, the nebulizing device is configured to control one or more deposition parameters. The deposition parameters controlled include one or more of the following parameters: flow rate of the liquid phase; flow rate of the dispersion (or solution) delivered by the nozzle; flow rate of the gas phase, i.e. of the air (in the case of air mix systems); pressure of the gas phase delivered by the nozzle (in the case of air mix systems); gram weight (which is preferably controlled by a remote-controlled robotized movement system); mechanical mixing speed of the solution (i.e. of the dispersion) in the container.

Preferably, the flow rate of the liquid phase is between 1 and 1 Ό L/h. Preferably, pressure of the dispersion delivered by the nozzle is between 1 bar and 3 bar. Preferably, the flow rate of the gas phase (in the case of air mix systems) is between 1 and 10 L/h. Preferably, the pressure of the gas phase delivered by the nozzle (in the case of air mix systems) is between 0.7 and 2.7 bar. Preferably, the gram weight is between 50 g/m ² and 5000 g/m ² . Preferably, the mechanical mixing speed of the solution in the container is between 50 and 500 rpm.

Applying the coating including a nanomaterial comprising graphene to a surface produces a structural break. The structural break creates a barrier which contrasts the phenomenon of charge injection. The coating may also have an action which prevents aging of the insulating material.

This disclosure also provides for the use of a nebulizing device to coat a portion of a surface (or the entire surface) of an insulating layer made of dielectric material of a high-voltage direct current (HVDC) electrical apparatus with a coating layer including a nanomaterial comprising graphene. The nebulizing device comprises a solution comprising a liquid solvent and a nanomaterial including graphene in solid phase. In an embodiment, the nebulizing device comprises a container to hold the solution. The nebulizing device comprises a nebulizing nozzle configured to deliver the solution in nebulized form.

This disclosure also provides for the use of a nebulizing device in a method according to one or more aspects of the disclosure. This disclosure also provides a solution comprising a liquid solvent and the nanomaterial including graphene in solid phase.

Brief description of the drawings

This and other features will become more apparent from the following description of a preferred embodiment of the invention, illustrated by way of non-limiting example in the accompanying drawings, in which:

- Figure 1 is a longitudinal cross section of a coupling and two respective high voltage direct current power transmission cables according to this disclosure;

- Figure 2 is a longitudinal cross section of a portion of one of the cables of Figure 1 and a terminal according to this disclosure;

- Figure 3 shows one of the cables of Figure 1 in a transverse cross section;

- Figures 4, 5 and 6 illustrate respective possible embodiments of one of the cables of Figure 1 in a transverse cross section. Detailed description of preferred embodiments of the invention

With reference to the accompanying drawings, the numeral 1 denotes a high voltage direct current power transmission cable.

In an embodiment, the cable 1 comprises an inner conductor 101 . In an embodiment, the inner conductor 101 is made of a metallic material. In an embodiment, the inner conductor 101 is made of copper. The inner conductor 101 is elongate in a longitudinal direction. The inner conductor 101 has a cross section which, in a transverse direction at right angles to the longitudinal direction, is circular. The inner conductor 101 has an outer surface. In an embodiment, the outer surface of the inner conductor 101 is a cylinder having as its axis an axis of the outer surface of the inner conductor 101 . The axis of the outer surface of the inner conductor 1 01 defines an axis A of the cable 1 . The axis A of the cable 1 extends in the longitudinal direction.

In an embodiment, the cable 1 includes an inner semiconductor 102. In an embodiment, the inner semiconductor 102 longitudinally surrounds the inner conductor 101 . In an embodiment, the inner semiconductor 102 is made of a semiconductive material.

In an embodiment, the cable 1 includes an insulating layer 103. In an embodiment, the insulating layer 103 longitudinally surrounds the inner conductor 101 . In an embodiment, the insulating layer 103 surrounds the inner conductor 102. The insulating layer is made of dielectric material. In an embodiment, the dielectric material of the insulating layer is a cross- linked polyethylene (XLPE). In an embodiment, the dielectric material of the insulation is a polypropylene (PP). In an embodiment, the dielectric material of the insulation is a copolymer of ethylene and vinyl acetate (EVA). In an embodiment, the dielectric material of the insulation is a copolymer of ethylene and polypropylene (EPR).

In an embodiment, the cable 1 includes an outer conductor 104. In an embodiment, the outer conductor 104 longitudinally surrounds the insulating layer 103.

In an embodiment (not illustrated), the transmission cable 1 comprises an outer semiconductor. The outer semiconductor is interposed between the insulating layer 103 and the outer conductor 104.

The outer conductor 104 has opposite polarity to the inner conductor 101 . In an embodiment, the outer conductor 104 acts as a screen for the insulating layer 103. In an embodiment, the outer conductor 104 is a semiconductor. The outer conductor has its own surface. In an embodiment, the surface of the outer conductor 104 is cylindrical and its axis coincides with the axis A of the cable 1 . The outer conductor has an inner surface, facing towards the insulating layer 103, and an outer surface, opposite to the inner surface.

In an embodiment, the cable 1 includes a sheath 105. In an embodiment, the sheath 105 longitudinally surrounds the outer conductor 104. In an embodiment, the sheath 105 is made of polypropylene. The sheath 105 acts as a protection for the cable.

In an embodiment, the sheath 105 is in contact with the outer conductor 104. In an embodiment, there is a waterproofing sheath between the outer conductor 104 and the sheath 105. In an embodiment, the waterproofing sheath is made of polyethylene. In an embodiment, there is a metal armour between the outer conductor 104 and the sheath 105. The metal armour acts as a mechanical reinforcement. In an embodiment, there is a semiconductor tape between the outer conductor 104 and the sheath 105. In an embodiment, the insulating layer 103 has an outer surface 103B and an inner surface 103A, opposite to the inner surface 103B. In an embodiment, the outer surface 103B is cylindrical and its axis is the axis A of the cable 1 . In an embodiment, the inner surface 103A is cylindrical and its axis is the axis A of the cable 1 . In an embodiment, the outer surface 103B of the insulating layer 103 is in contact with the screen 104. In an embodiment, the inner surface 103A of the insulating layer 103 encloses the inner conductor 101 . In an embodiment, the inner surface 103A of the insulating layer 103 is in contact with the inner conductor 101 . In an embodiment, the inner surface 103A of the insulating layer 103 is in contact with the inner semiconductor 102, which is in contact with the inner conductor 101 .

In an embodiment, the cable 1 comprises a coating 100. In an embodiment, the coating 100 comprises a coating layer 106.

In an embodiment, the coating layer 106 is applied on the outer surface 103B of the insulating layer 103.

In an embodiment, the coating layer 106 is applied on the inner surface 103A of the insulating layer 103.

In an embodiment, the coating layer 106 is applied to a portion of the surface of the inner conductor 101 .

In an embodiment, the coating layer 106 is applied to a portion of the surface of the outer conductor 104.

In an embodiment, the coating layer 106 is applied to a portion of the inner surface of the outer conductor 104.

In an embodiment, the coating layer 106 is applied on the outer surface of the inner semiconductor 102.

In an embodiment, the coating layer 106 is applied on the outer surface of the outer semiconductor.

In an embodiment, the coating 100 comprises a plurality of coating layers 106. In an embodiment, at least one of the plurality of coating layers 106 is applied on the inner surface 103A of the insulating layer 103. In an embodiment, at least one of the plurality of coating layers 106 is applied on the outer surface 103B of the insulating layer 103. In an embodiment, at least one of the plurality of coating layers 106 is applied on the surface of the inner conductor 101 . In an embodiment, at least one of the plurality of coating layers 106 is applied on the surface of the outer conductor 104. In an embodiment, at least one of the plurality of coating layers 106 is applied on the outer surface (that is, the one facing towards the insulating layer 103) of the inner semiconductor 102.

In an embodiment, at least one of the plurality of coating layers 106 is applied on the outer surface (that is, the one facing towards the outer conductor 104) of the outer semiconductor (if the cable 1 comprises an outer semiconductor).

In an embodiment, the coating 100 includes a nanomaterial comprising graphene. In an embodiment, the coating layer 106 is a nanostructured coating including nanoplatelets of graphite and few-layer graphene.

With reference to the accompanying drawings, the numeral 2 denotes an accessory for a high voltage direct current (HVDC) power transmission cable. In an embodiment, the accessory 2 is a coupling 2A. The coupling 2A connects two cables 1 at respective ends thereof, through pins 201 . In an embodiment, the accessory 2 is a terminal 2B. The terminal 2B connects a cable 1 to a terminal device 3. The terminal 2B connects a cable 1 to a terminal device 3 through pins 201 . The terminal device 3 may be a transformer. In an embodiment, the terminal device 3 is an alternator. The terminal device 3 may be a rectifier. In an embodiment, the terminal device 3 is a rail of a switchboard. In an embodiment, the terminal device 3 is an isolating switch. In an embodiment, the terminal device 3 is an interrupter. The coupling 2A has an inner surface 207A, facing towards the inner conductors 101 of the cables 1 it is connected to, and an outer surface 208A opposite to the inner surface 207A. The terminal 2B has an inner surface 207A, facing towards the inner conductor 101 of the cable 1 it is connected to, and an outer surface 207B opposite to the inner surface 207A.

In an embodiment, the accessory 2 comprises an insulating layer 203. In an embodiment, the coupling 2A comprises an insulating layer 203A. In an embodiment, the terminal 2B comprises an insulating layer 203B.

In an embodiment, the accessory 2 comprises a conductive or semiconductive layer 204. In an embodiment, the coupling 2A comprises a conductive or semiconductive layer 204A. In an embodiment, the terminal 2B comprises a conductive or semiconductive layer 204B.

In an embodiment, the conductive or semiconductive layer 204 of the accessory 2 is at least partly incorporated in the insulating layer 203 of the accessory 2. In an embodiment, the conductive or semiconductive layer 204A of the coupling 2A is at least partly incorporated in the insulating layer 203A of the coupling 2A. In an embodiment, the conductive or semiconductive layer 204B of the terminal 2B is at least partly incorporated in the insulating layer 203B of the terminal 2B.

In an embodiment, the inner surface 207A of the coupling 2A coincides with the inner surface of the insulation 203A.

In an embodiment, the outer surface 208A of the coupling 2A coincides with the outer surface of the insulation 203A.

In an embodiment, the inner surface 207B of the terminal 2B coincides with the inner surface of the insulation 203B.

In an embodiment, the outer surface 208B of the terminal 2B coincides with the outer surface of the insulation 203B.

In an embodiment, the conductive or semiconductive layer 204A of the coupling 2A connects the outer conductors 104 of the two cables 1 which the coupling 2A is connected to in such a way as to provide continuity for the field lines of the electric field generated across the two cables 1 .

In an embodiment, the conductive or semiconductive layer 204B of the terminal 2B is connected to the outer conductor 104 of the cable 1 the terminal 2B is connected to. In an embodiment, the conductive or semiconductive layer 204B of the terminal 2B has a longitudinal cross section which diverges from the outer conductor of the cable 1 towards the terminal device 3, in order to guide the field lines of the electric field generated across the two cables 1 .

In an embodiment, the conductive or semiconductive layer 204A and the insulating layer 203A of the coupling 2A are made of a single material having a conductivity gradient: higher conductivity in proximity to the outer conductors 104 and lower conductivity in proximity to the central part of the coupling 2A, away from the outer conductors 104. In an embodiment, the conductive or semiconductive layer 204B and the insulating layer 203B of the terminal 2B are made of a single material having a conductivity gradient: higher conductivity in proximity to the outer conductor 104 and lower conductivity in proximity to the terminal device 3.

In an embodiment, the accessory 2 includes a coating 200. In an embodiment, the coupling 2A includes a coating 200A. In an embodiment, the terminal 2B includes a coating 200B.

In an embodiment, the coating 200 includes a coating layer 206. In an embodiment, the coating 200A includes a coating layer 206A. In an embodiment, the coating 200B includes a coating layer 206B.

In an embodiment, the coating layer 206 is applied to at least one portion of a surface of the accessory 2.

In an embodiment, the coating layer 206A is applied on the outer surface 208A of the coupling 2A. In an embodiment, the coating layer 206A is applied on the inner surface 207A of the coupling 2A. In an embodiment, the coating layer 206A is applied on the outer surface of the insulant 203A of the coupling 2A. In an embodiment, the coating layer 206A is applied on the inner surface of the insulant 203A of the coupling 2A. In an embodiment, the coating layer 206B is applied on the outer surface of the insulant 203B of the terminal 2B. In an embodiment, the coating layer 206B is applied on the inner surface 207B of the terminal 2B. In an embodiment, the coating layer 206B is applied on the inner surface of the insulant 203B of the terminal 2B.

In an embodiment, the coating layer 206, 206A, 206B includes a nanomaterial comprising graphene. In an embodiment, the coating layer 206, 206A, 206B is a nanostructured coating including nanoplatelets of graphite and few-layer graphene.

In an embodiment, the coating 200 of the accessory 2 comprises a plurality of coating layers 206.

In an embodiment, the coating 200A of the coupling 2A includes a plurality of coating layers 206A. In an embodiment, one of the plurality of coating layers 206A is applied on the outer surface 208A of the coupling 2A. In an embodiment, one of the plurality of coating layers 206A is applied on the inner surface 207A of the coupling 2A.

In an embodiment, the coating 200B of the terminal 2B includes a plurality of coating layers 206B. In an embodiment, one of the plurality of coating layers 206B is applied on the outer surface 208B of the terminal 2B. In an embodiment, one of the plurality of coating layers 206B is applied on the inner surface 207B of the terminal 2B.

In an embodiment, each coating layer 206, 206A, 206B of the plurality of coating layers includes a nanomaterial comprising graphene. In an embodiment, each coating layer 206, 206A, 206B of the plurality of coating layers is a nanostructured coating including nanoplatelets of graphite and few-layer graphene.

This disclosure also covers a method for contrasting injection of electrical charge in an insulating layer made of dielectric material of a high-voltage direct current (HVDC) electrical apparatus.

In an embodiment, the electrical apparatus is a cable 1 . The cable 1 includes an insulating layer 103 and an inner conductor 101 . The insulating layer 103 surrounds the inner conductor 101 . The insulating layer 103 has an inner surface 103A, facing towards the conductor 101 , and an outer surface 103B, opposite to the inner surface 103A.

In an embodiment, the electrical apparatus is an accessory 2. In an embodiment, the accessory 2 comprises an insulating layer 203.

In an embodiment, the accessory 2 is a coupling 2A. The coupling 2A is connected to two cables 1 . The coupling 2A has an inner surface 207A facing towards the inner conductor 101 of the cables 1 the coupling is connected to. The coupling 2A includes an insulating layer 203A. The insulating layer 203A has an inner surface which coincides with the inner surface 207A of the coupling. The insulating layer 203A has an outer surface which coincides with the outer surface 208A of the coupling.

In an embodiment, the accessory 2 is a terminal 2B. The terminal 2B is connected to a cable 1 . The terminal 2B includes an insulating layer 203B. The insulating layer 203B has an inner surface which coincides with the inner surface 207B of the terminal. The insulating layer 203B has an outer surface which coincides with the outer surface 208B of the terminal.

In an embodiment, the method comprises a step of coating at least one portion of a surface of the insulating layer 103 of the cable 1 with a coating layer 106 including a nanomaterial comprising graphene.

In an embodiment, the coating layer 106 is applied on the outer surface 103B of the insulant 103. In an embodiment, the coating layer 106 is applied on the inner surface 103A of the insulant 103.

In an embodiment, the method comprises a step of coating at least one portion of a surface of the inner conductor 101 of the cable 1 with a coating layer 106 including a nanomaterial comprising graphene.

In an embodiment, the cable comprises an outer semiconductor 104. The outer semiconductor 104 surrounds the insulating layer 103. In an embodiment, the method comprises a step of coating at least one portion of a surface of the outer conductor 104 of the cable 1 with a coating layer 106 including a nanomaterial comprising graphene. In an embodiment, the method comprises a step of coating at least one portion of a surface of the insulating layer 203 of the accessory 2 with a coating layer 206 including a nanomaterial comprising graphene. In an embodiment, the method comprises a step of coating at least one portion of a surface of the insulating layer 203A of the coupling 2A with a coating layer 206A including a nanomaterial comprising graphene. In an embodiment, the coating layer 206A is applied on the outer surface of the insulating layer 203A, coinciding with the outer surface 208A of the coupling 2A. In an embodiment, the coating layer 206A is applied on the inner surface of the insulating layer 203, coinciding with the inner surface 207A of the coupling 2A.

In an embodiment, the method comprises a step of coating at least one portion of a surface of the insulating layer 203B of the terminal 2B with a coating layer 206B including a nanomaterial comprising graphene. In an embodiment, the coating layer 206B is applied on the outer surface of the insulating layer 203B, which coincides with the outer surface 208B of the terminal 2B. In an embodiment, the coating layer 206B is applied on the inner surface of the insulating layer 203B, which coincides with the inner surface 207B of the terminal 2B.

In an embodiment, the step of coating comprises a step of preparing a solution. The solution is prepared by mixing one or more nanomaterials comprising graphene in solid phase with a liquid solvent. In an embodiment, the solution is prepared using ultrasonic immersion tools.

In an embodiment, the nanomaterial comprising graphene in solid phase is in the form of nano-powder. In an embodiment, the nano-powder is made up of nanoplatelets of graphite and few-layer graphene.

In an embodiment, the liquid solvent is a polar solvent. In an embodiment, preparing the solution includes adding a polymeric binder.

In an embodiment, the step of coating comprises a step of depositing the solution on the insulating layer 103, 203A, 203B.

In an embodiment, the insulating layer 103, 203A, 203B is made of polymeric dielectric material. In an embodiment, the insulating layer 103, 203A, 203B of the cable 1 is a thermoplastic, polymer: for example EPR. In an embodiment, the insulating layer 103, 203A, 203B of the cable 1 is a thermosetting polymer. In an embodiment, the insulating layer 103, 203A, 203B of the cable 1 is an elastomeric polymer. In an embodiment, the insulating layer 103, 203A, 203B is made of polymeric dielectric material. In an embodiment, the insulating layer 103 of the cable 1 is made of cross- linked polyethylene (XLPE). In an embodiment, the insulating layer 203A of the coupling 2A is made of silicone rubber. In an embodiment, the insulating layer 203B of the terminal 2B is made of silicone rubber.

In an embodiment, the step of depositing the solution on the insulating layer 103, 203A, 203B is accomplished by spraying the solution in nebulized form. In an embodiment, the step of depositing the solution on the insulating layer 103, 203A, 203B is accomplished by smearing. During the step of depositing, the solution is deposited on at least one portion of a surface 103A, 103B, 207A, 208A, 207B, 208B of the insulating layer 103, 203A, 203B to produce the coating layer 106, 206A, 206B.

In an embodiment, the method comprises a step of desiccating the solution. During the step of desiccating the solution, the solvent evaporates. The step of desiccating the solution is carried out after the step of depositing the solution.

In an embodiment, the method comprises a step of polishing the coating layer 106, 206A, 206B. Polishing is accomplished by mechanical action. Provided below are the experimental data showing the effect that applying a coating according to this disclosure has on space charge accumulation. The data shown below in graphs 1 and 2 are obtained by measuring the accumulated space charges in an XLPE insulant of a high voltage cable. The measurements were taken using the pulse-electro-acoustic (PEA) technique.

To obtain the following data, acetone or ethanol was used as the liquid solvent; the nanomaterial used belonged the family of graphene related materials, comprising few-layer graphene and nanoplatelets of graphite with a carbon content greater than 98%; the solution obtained has a content of nanomaterial comprising graphene of between 1 % and 3% by weight and a content of liquid solvent of between 99% and 97% by weight; the solution was deposited on the surface of the XLPE insulant by a nebulizing device as described in this disclosure.

Graph 1 shows (on the y-axis) the total density of space charge (expressed in Coulombs per cubic metre) measured in the XLPE insulant without coating (dark/solid bars) and with coating (light/blank bars) of nanomaterial comprising graphene, at ambient temperature and (on the x-axis) the variations in the intensity of the electric field. The effectiveness of the coating on the reduction of the accumulated space charges is evident.

Electric field fkV/mm}

Graph 1

Graph 2 shows (on the y-axis) the total density of space charge measured in the XLPE insulant without coating (dark/solid bars) and with coating (light/blank bars) of nanomaterial comprising graphene, in an electric field of intensity equal to 38 kV/mm, and (on the x-axis) the variations in the temperature of the insulant.

Graph 2