WO2014029447A1

WO2014029447A1 - A high voltage direct current cable product

Info

Publication number: WO2014029447A1
Application number: PCT/EP2012/066507
Authority: WO
Inventors: Andreas FRIBERG; Anneli JEDENMALM; Gustavo Dominguez
Original assignee: Abb Research Ltd
Priority date: 2012-08-24
Filing date: 2012-08-24
Publication date: 2014-02-27

Abstract

The present invention relates to an HVDC cable product, such as a power cable, a cable accessory or a bushing, where the HVDC cable product comprises an elongate conductor and an insulation system arranged to concentrically surround the conductor. The insulation system comprises an insulation layer formed from a composite material comprising a polymer matrix and filler particles of an aluminosilicate.

Description

A HIGH VOLTAGE DIRECT CURRENT CABLE PRODUCT Technical field

The present invention relates to the field of high voltage direct current technology, and in particular to high voltage direct current cable products such as cables, cable accessories and bushings.

Background

High Voltage Direct Current (HVDC) power cables are used to transfer electrical power from one location to another, and are often buried underground or placed at the bottom of the sea. Unless the cables are appropriately insulated, significant leakage currents will flow in the radial direction of the cables, from the conductor to the surrounding ground/water. Such leakage currents give rise to significant power loss, as well as to heating of the electrical insulation. The heating of the insulation can further increase the leakage current due to the reduction of the resistance with increasing temperature. To avoid power loss and possible thermal runaway, the leakage current should therefore be kept as small as possible.

In order to reduce the leakage current, electrical insulation systems where an insulating material of low DC conductivity is arranged to surround the conductor of an HVDC power cable are widely used. Since the thermal energy released in the cable needs to be allowed to leave the cable, there is a desire to limit the thickness of the insulation system

surrounding the conductor. Moreover, an insulation system of lower thickness will make the power cable easier to handle and transport. Thus, there is a desire to find electrical insulation systems of good insulation properties suitable for HVDC applications.

Summary

A problem to which the present invention relates is how to provide reliable, low loss transfer of electrical power in an HVDC system.

This problem is addressed by an HVDC cable product, such as a power cable, a cable accessory or a bushing, where the HVDC cable product comprises an elongate conductor and an electrical insulation system arranged to concentrically surround the conductor. The insulation system comprises at least one insulation layer formed from a composite material comprising a polymer matrix and filler particles of aluminosilicate. The insulation system may also, if desired, comprise insulation layers formed from other materials.

By this HVDC cable product is achieved that a high voltage can be applied to the HVDC cable product, while maintaining adequate cooling. The aluminosilicate filler particles drastically reduce the conductivity of the composite material as compared to the polymer matrix, and a higher electrical field can thus be applied to the insulation layer.

The aluminosilicate particles can be of tubular structure or, if other structure types are available, the particles can be of other structures such as prismatic, rolled, pseudospherical and platelet forms. The particles can advantageously be nano-sized, in order to obtain a low DC conductivity in the insulation layer. When the particles are of tubular structure, the majority of the aluminosilicate tubes could for example have dimensions such that the outer diameter falls between 5 and 200 nm, and the length falls between 50 and 5000 nm. Examples of suitable aluminosilicates are halloysite and imogolite. The composite material could also comprise particles of different aluminosilicates, such as particles of halloysite and particles of imogolite. The matrix can for example be a polyolefm matrix comprising at least one polyolefm, for example a polyethylene matrix, a polypropylene matrix, a copolymer matrix such as ethylene-propylene or ethylene butadiene etc., or a blend of different polyolefins. The matrix may or may not be cross-linked.

The amount of aluminosilicate filler particles can advantageously lie in the range of 0.1 wt% to 10 wt%. The HVDC cable product can be rated for high DC voltages, for example voltages of 150 kV, or higher. The design of the HVDC cable product in terms of insulation thickness and rated voltage can be such that the radial electric field in the insulation layer is expected to exceed 10 MV/m at the rated voltage of the cable product, and for many applications, such that the radial electric field in the insulation layer is expected to exceed 25 MV/m at the rated voltage.

The insulation system of the HVDC cable product could comprise the insulation layer formed from the composite material of polymer matrix/alumino silicate particles only, or could comprise further insulation layers. The insulation layer(s) formed from the composite material comprising a polymer matrix and filler particles of aluminosilicate can advantageously be thick enough to provide a majority of the resistance between the conductor and the outside of the cable product. If a majority of the resistance originates from insulation layer(s) formed from said composite material, adequate electrical insulation can be provided at high voltages, while the thickness of the insulation system of which the insulation layer forms a part can be low enough to provide adequate cooling.

Further aspects of the invention are set out in the following detailed description and in the accompanying claims.

Brief description of the drawings

Fig. la is a schematic illustration of an HVDC cable having an electrical insulation system including a single insulation layer.

Fig. lb is a schematic illustration of an HVDC cable having an electrical insulation system including multiple insulation layers.

Fig. 2 is a schematic illustration of an example of an aluminosilicate particle, the aluminosilicate particle being of tubular shape.

Fig. 3a illustrates the structure of a monolayer of tubular imogolite.

Fig. 3b illustrates the structure of a monolayer of tubular halloysite.

Fig. 4 is a graph illustrating the conductivity σ of different polyethylene matrix materials, with (filled symbols) and without (empty symbols) additives of tubular halloysite particles, at different electric fields E.

Fig. 5 is a graph illustrating the conductivity σ of a polyethylene matrix material as a function of halloysite filler particle concentration C.

Fig. 6 is a SEM picture of a composite material formed from a polyethylene matrix with 1 wt% of tubular halloysite.

Detailed description

A schematic illustration of an example of an HVDC power cable 100 is shown in Fig. la. The power cable 100 of Fig. la comprises a conductor 105, concentrically surrounded by an electrical insulation system 120 comprising an electrically insulating layer 110, here referred to as an insulation layer 110. Another example of an HVDC power cable 100 is schematically illustrated in Fig. lb, where a conductor 105 is concentrically surrounded by an electrical insulation system 120 comprising a plurality of insulation layers 1 lOi, 1 lOii, 1 lOiii etc. When referring to all or any of the insulation layers of an electrical insulation system 120, the reference numeral 110 will be used. An electrical insulation system 120 could include one, two, three or more insulating layers 110 - the power cable 100 of Fig. la is an example of an electrical insulation system 120 having one insulation layer 110. An HVDC power cable 100 typically includes further sheathing layers, such as

semiconducting layers on the inside and/or outside of the insulation system 120, metal screening layers, protective outer layers, etc. The outside of the power cable 100 is indicated by reference numeral 125, and the thickness t of an insulation layer 110 is indicated in Fig. la.

In order to ensure low levels of leakage currents in the radial direction of the cable 100, which leakage currents can give rise to significant power losses, the insulating properties of the electrical insulation system 120 are important. The requirement of low DC conductivity plays a far greater role in direct current cables than in alternating current cables, in which leakage current losses will generally be much lower than dielectric losses.

The insulating properties of the electrical insulation system 120 depend on the thickness of the insulation system 120, as well as on the DC conductivity of the material(s) forming the HVDC system 120. Generally, there is a desire to limit the thickness of the electrical insulation system 120 surrounding the conductor, in order to allow for efficient cooling of the power cable 100, as well as to facilitate the handling and shipping of the power cable 100. Therefore, for direct current power cables 100 of the higher voltages, the requirement of low DC conductivity of the material(s) forming the electrical insulation system 120 is high.

Examples of materials that are often used in insulation systems for power cables are different polyolefms, for example polyethylene (LLDPE, LDPE, HDPE), polypropylene (PP), ethylene propylene rubber (EPR). Other resin, polymer or rubber materials can also be used.

It has been shown that particles of some inorganic materials, when added to a matrix of an insulating material, can improve the insulating properties of the insulating material. This is true for nanoparticles of for example magnesium oxide (MgO) and silicon dioxide (Si0₂) when added to for example polyethylene. As the interest in HVDC systems rises and the rated voltage of HVDC systems increases, there is a need to find other materials of similar or even better insulating characteristics. Our experiments have shown that by adding particles of alumino silicates to a polymer matrix, the DC conductivity of the polymer is drastically reduced to a level which makes the material suitable for insulation of HVDC cable products.

An interesting example of an aluminosilicate which gives rise to a considerable reduction in the DC conductivity of a polymer is hallo ysite, which is a natural clay. Imogolite is another example of a suitable aluminosilicate.

Halloysite is an 1 : 1 layered clay with the empirical formula of Α1₂8ι₂θ5(ΟΗ)4. Halloysite most often occurs in nanotubes, although other morphological forms are also known, such as prismatic, rolled, pseudospherical and platelet forms. Imogolite is a 2: 1 layered clay with the empirical formula Al₂Si₂053(OH)₄, which also occurs as nanotubes.

An illustration of an example of an aluminosilicate particle 200 is shown in Fig. 2. The aluminosilicate particle 200 of Fig. 2 is of tubular shape, such particle also being referred to as an aluminosilicate tube. The length of the tube 200 is denoted L, the spacing between the layers is denoted s, the inner and outer diameters of a tube are denoted Di and Do, respectively. A particle 200 of a tubular aluminosilicate can have a single layer or more than one layer, i.e. be mono-layered or multi-layered. Illustrations of monolayered tubes 200 of imogolite and halloysite are shown in Figs. 3a and 3b, respectively (see J. Phys. Chem. C 2010, 114, "Structural, Electronic, and Mechanical Properties of Single-Walled Halloysite Nanotube Models "). The position of the included atoms H, O, Al and Si have been indicated in the figures.

Fig. 4 illustrates experimental results of the change in DC conductivity obtained by adding 1 wt% of halloysite to a polyethylene material. The graph shows measurements of the DC conductivity σ plotted versus applied electric field E for three different polyethylene matrix materials, with and without the halloysite filler. The thickness of the samples used in the measurements was 1 mm, and the measurements were taken at 70 °C. In the graph, empty symbols indicate measurements of matrix materials without halloysite, while filled symbols indicate measurements of matrix materials to which 1 wt% of hallo ysite has been added. The three different polyethylene matrix materials can be described as follows: PEl (indicated by diamonds): a commercially available low density polyethylene; XLPE1 (indicated by squares): the commercially available low density polyethylene of PEl, which has been cross-linked with a cross-linking agent of diculym peroxide (DCP) at a concentration of 0.6 wt%; PE2 (indicated by circles): a low density polyethylene wherein the amount of antioxidants is lower than in PEl, the material therefore having a lower DC conductivity than the PEl material before adding the hallo ysite. The hallo ysite used in the experiments of Fig. 4 is a hallo ysite-7A in the form of nano-tubes, where the outer diameter Do is in the order of 50 nm, the inner diameter Di in the order of 15 nm, the spacing s between layers is approximately 0.7 nm, and the length L of the majority of the nano-tubes lies within the range of 0.5-3 μιη.

As can be seen in Fig. 4, the DC conductivity σ of all three materials is drastically reduced by the addition of a hallo ysite filler. At an electric field of 30 MV/m, corresponding to the radial electric field experienced by the insulation layer 110 of a 600 kV cable having a 20 mm insulation layer, the addition of the hallo ysite filler to the PEl material causes a drop in conductivity of more than two orders of magnitude. For the XLPE1 material, the drop in conductivity caused by the addition of halloysite is nearly two orders of magnitude at 30 MV/m, and in the material PE2, the corresponding drop is well over one order of magnitude. The halloysite filler gives rise to a drastic decrease in conductivity for all three matrix materials - also in a polymer such as PE2 where the initial conductivity, before the filler has been added, is already at a very low level. For HVDC cable products, the heat generation due to the leakage current is one limiting mechanism hindering very high electric fields E in the insulation layers 110: There is a risk that thermal breakdown will occur if the temperature increases and the electric field is sufficiently high, since the DC conductivity of the material in the insulation layer increases with temperature. Therefore, when designing an HVDC cable product, a balance has to be considered between insulating properties and thickness t of the HVDC cable product: The thicker the insulation layer, the higher the resistance between the conductor 105 and the outside 125 of the cable, and the lower the amount of heat generated in the insulation layer 110 - on the other hand, the thinner the insulation layer, the more efficient cooling of the insulation layer 110. The resistance of an insulation layer 110 depends on the conductivity of the material forming the insulation layer, as well as on the thickness t of the insulation layer: The resistance increases with decreasing conductivity σ, while it decreases with decreasing thickness t. Hence, the resistance of an insulation layer of thickness t, which is made from a composite material where a polymer matrix comprises filler particles 200 of an aluminosilicate, will be higher than the resistance of an insulation layer 110 having the same thickness t, but which is made from the same polymer matrix to which no filler particles have been added. Thus, compared to an insulation layer comprising no filler particles 200, there is room for either an increase in the rated voltage of the HVDC cable product in which the insulation layer 110 is used, or for a decrease in the thickness t of the insulation layer 110. Both an increase in the rated voltage and a decrease in the thickness t will result in an increase in the electric field E in the insulation layer 110.

In Fig. 4, it can be seen that the DC conductivity σ of the composite material formed from a polyethylene matrix to which hallo ysite has been added increases slightly with increasing electric field E, as it does also for the corresponding matrix material to which no filler particles have been added. The relative increase of σ with electric field is the smallest for the composite material comprising PE2 as the matrix material, which is the composite material that comprises the smallest amounts of impurities. For the XLPEl material of Fig. 4, the DC conductivity σ takes a value of ~30 fS/m at an electric field of 20 kV/mm. For the XLPEl material to which 1 wt% of Hallo ysite has been added, on the other hand, this value of the DC conductivity is not reached until the electric field E is ~ 45 kV/mm. Thus, an HVDC cable product, made from the insulation XLPE with 1 wt% Hallo ysite and which has a voltage/thickness ratio such that the expected electric field is 45 kV/mm, shows the same electrical properties as an HVDC cable product with XLPEl (without any Hallo ysite) with a voltage/thickness ratio such that the expected electric field is 20 kV/mm. This allows for, when halloysite is added to the XLPE, either a reduction of the thickness t of the insulation layer 110, or an increase in the rated voltage of the HVDC cable product. When the thickness t is reduced, the thermal resistance is also reduced, thus allowing for more efficient cooling of the insulation layer.

The ratio U/t, where U is the rated voltage of the power cable 100 and t is the thickness of the insulation layers(s) 110 formed from a composite material having aluminosilicate particles 200 in a polymer matrix, could for example take of value of 10 MV/m or higher, for example 25-35 MV/m, or higher. If more than one such insulation layers 110 are present, the thickness t here represents the total thickness of all insulation layers that are formed from a composite material having alumino silicate particles 200 in a polymer matrix. If the power cable 100 includes only one insulation layer 110, the ratio U/t represents the expected electric field E in the insulation layer when the power cable 105 is in use. If the insulation system 120 of the power cable 105 includes insulation layers 110 of other materials, the electrical field E in the insulation layers(s) formed from the composite material having aluminosilicate particles 200 in a polymer matrix will be lower than U/t, i.e. E<U/t, since part of the voltage drop will occur in the other insulation layers 110, thus allowing for a higher value of U/t when insulation layers of other materials are present. Our studies show that the electric field in the composite material during normal operation could reliably be 50-60 MV/m or higher. Lower electric fields could naturally be applied. We believe that part of the reduction in DC conductivity seen in Fig. 4 is due to impurities present in the matrix material being trapped inside the nano-tubes of the aluminosilicate. Impurities which could be present in the matrix material include anti-oxidants added to protect the polymer during production, as well as by-products from any cross-linking processing. A further effect which contributes to the reduction in conductivity also seems to be present, this effect being present also in composite materials comprising non-tubular forms of aluminosilicate. In Fig. 4, the greatest reduction in conductivity is obtained in material PE1, where no cross-linking has occurred and no cross-linking by-products are present. Fig. 5 is a graph illustrating the dependency of the DC conductivity σ on the filler particle concentration C for a composite material, where the filler particles are the nano-tubes of halloysite-7A described in relation to Fig. 4 and the matrix material is PE1 of Fig. 4, i.e a commercially available polyethylene which has not been cross-linked. The measurements of Fig. 5, which have been taken at an electric field of 30 MV/m, show that the DC conductivity σ decreases drastically with filler particle concentration C at low

concentrations, and that the decrease in conductivity with concentration levels off at about 1 wt% of hallo ysite particles. Noticeable effects on the conductivity can be seen also for small concentrations of filler particles. A suitable concentration of aluminosilicate filler particles typically lies within the range of 0.1 wt% to 10 wt%, with an optimal range of 0.25 wt% to 3 wt%. When the filler particle concentration C increases, the mechanical flexibility of the material decreases and the power cable 100 will be more stiff. Acceptable mechanical properties of the power cable 100 can typically be obtained at least up to 10 wt% of alumino silicate filler particles.

The experiments presented in Fig. 4 and 5 have been made on materials where different forms of polyethylene were used as the matrix. Different kinds of polyethylene may be used, such as LLDPE, LDPE or HDPE. Other polyolefms could also be used as the matrix material, such as polypropylene (PP), ethylene propylene rubber (EPR), or co-polymers, such as ethylene-propylene, ethylene butadiene etc., or blends of different polyolefms.

Other resin, polymer or rubber materials may alternatively be used. The matrix materials suggested above are non-polar materials. However, also polar materials could be used as the matrix material. The insulating matrix material may or may not be cross-linked. Cross-linking could be achieved physically, for example by means of electron beaming, X-ray, or γ-radiation; or achieved chemically. When chemically cross-linked, the cross-linking agent could for example be DCP (cf. Fig. 4), a silane or a peroxide, and the cross-linking agent could for example occur in a concentration within the range of 0.1-2.2 wt%. A cross-linked material is typically more mechanically stable at high temperatures than a material which has not been cross-linked.

The effect of reducing the DC conductivity by means of adding aluminosilicate filler particles to a matrix of a polymer will be stronger if the particles are nano-sized. The size of the tubular aluminosilicate particles 200 could for example be such that for a majority of the particles 200, the outer diameter Do lies within the range of 5-200 nm, the length L lies within the range of 50-5000 nm and the spacing s between layers lies within the range of 7-50 A. The breakdown properties of the material will typically be even better if a majority of the tubular particles have an outer diameter Do within the range of 10-50 nm, and/or a length L within the range of 50-500 nm. Alternatively, the length L of the majority of the particles could advantageously be within the range of 500-3500 nm. For non-tubular forms of aluminosilicate particles, the size distribution could for example be such that a majority of the particles have a size which falls within in the range of 1-2000 nm, where the size refers to length and/or diameter of the particle, depending on particle shape. Breakdown properties of the composite material with non-tubular particles will typically be the best if the size falls within the range of 1-100 nm. The filler particles used in the experiments illustrated in Figs. 4 and 5 were tubular nano-particles of hallo ysite-7A, for which the outer diameter Do was approximately 50 nm and the length L for the majority of the particles was in the range of 500 - 3500 nm. For hallo ysite-7A, the spacing s between layers is 7 A.

The experimental results show that by adding aluminosilicate filler particles to a polymer matrix, the conductivity of the polymer can be drastically reduced, to levels which are lower than what is expected by adding for example MgO. A further advantage of the aluminosilicate fillers as compared to for example MgO is that a suitable dispersion of the filler particles is easier to achieve. Fig. 6 is an electron microscope (SEM) picture of an example of a composite material 600. The composite material 600 of Fig. 6 comprises a matrix 605 of the PEl material, to which 1 wt% of aluminosilicate particles 200 of tubular halloysite has been added. From Fig. 6, it can be seen that the dispersion of the particles 200 is very good. A good particle dispersion ensures uniform properties of the composite material, while a poor dispersion reduces the breakdown strength of the material.

A composite material as described above can for example be produced by mixing polymer pellets with an aluminosilicate powder in a compounder extruder, the aluminosilicate powder comprising aluminosilicate particles of a desired size distribution and the amount of aluminosilicate powder corresponding to the desired concentration C. The resulting composite material can then be used to form an insulation layer 110 in an HVDC cable product by extruding the composite material around a conductor 105 (the conductor 105 possibly already being covered by other sheathing material(s)). Alternatively, polymer pellets and a larger amount of aluminosilicate powder can be mixed in a compounder extruder to form a composite material of a higher aluminosilicate concentration than the one desired in the insulation layer 110, this higher concentration composite material forming a master batch. A suitable amount of this master batch, together with the polymer, can then be added to a compounder extruder upon forming the insulation layer 110 around the conductor 105. Other ways of producing the insulation layer around the HVDC cable product can alternatively be used.

As mentioned above, the cable 100 may include one or more insulation layers 110. An insulation layer 110 formed from a polymer matrix to which aluminosilicate filler particles have been added can be combined, if desired, with one or more insulation layers 110 formed from another material, such as a polymer without any filler particles; a polymer to which filler particles of another composition have been added such as MgO or Si0₂, etc. If desired, insulation layers having filler particles of different alumino silicate compositions can be combined, and/or two or more insulation layers 1 10 with the addition of the same aluminosilicate composition can be used and separated by a different insulation material, etc. If an electrical insulation system 120 comprising two or more insulation layers 110 is used, the insulation layer(s) 110 comprising a polymer matrix with aluminosilicate filler particles could form the main insulation, i.e. represent a majority of resistance between the conductor 105 and the outside 125 of the HVDC cable product, in order to obtain the desired insulation.

The invention has been presented above mainly in relation to insulation of HVDC power cables 100. However, an insulation layer comprising a polymer matrix to which aluminosilicate filler particles have been added could advantageously be used also in other HVDC cable products wherein a conductor 105 is insulated by an insulation system 120, such as in HVDC cable accessories (including e.g. cable joints, cable terminations and cable connectors); in HVDC bushings etc. An HVDC cable product typically includes a single conductor 105, around which one or more insulation layers 110 are concentrically arranged. Such single conductor 105 could be made of a single wire, or of multiple wires.

The invention is applicable to HVDC cable products of all nominal voltages, although the benefits of low conductivity will be more pronounced for devices of nominal voltage of around 150 kV and higher, for example for cables products with a rated voltage in the range of 300 - 800 kV, or higher.

The alumino silicates used in insulation layers of the HVDC cable products according to the invention may be natural products, e.g. found in clay mines, or the alumino silicates could be synthetically produced.

Although various aspects of the invention are set out in the accompanying claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims. One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the following claims.

Claims

1. An HVDC cable product, such as a power cable (100), a cable accessory or a bushing, the HVDC cable product comprising:

an elongate conductor (105); and

an electrical insulation system (120) arranged to concentrically surround the conductor; wherein

the insulation system comprises an insulation layer (110) formed from a composite material (600) comprising a polymer matrix (605) and alummosilicate filler particles (200).

2. The HVDC cable product of claim 1, wherein

the alummosilicate particles are of tubular structure.

3. The HVDC cable product of claim 2, wherein

the majority of the alummosilicate tubes have dimensions such that the outer diameter lies between 5 and 200 nm, and the length lies between 50 and 5000 nm.

4. The HVDC cable product of any one of the above claims, wherein

the composite material comprises a polymer matrix and filler particles of halloysite and/or of imogolite.

5. The HVDC cable product of any one of the above claims, wherein

the amount of alummosilicate filler in the composite material lies within the range of 0.1 wt% to 10 wt%.

6. The HVDC cable product of claim 5, wherein

the amount of alummosilicate filler lies within the range of 0.25 wt% to 3 wt%.

7. The HVDC cable product of any one of the above claims, wherein

the matrix is a polyolefm matrix.

8. The HVDC cable product of any one of the above claims, wherein

the matrix is a cross-linked polymer.

9. The HVDC cable product of any one of the above claims, wherein

the ratio of the rated voltage of the power cable to the total thickness of the insulation layer(s) formed from said composite material is 10 MV/m or higher.

10. The HVDC cable product of claim 9, wherein

the ratio of the rated voltage of the power cable to the total thickness of the insulation layer(s) formed from said composite material is 25 MV/m or higher.

11. The HVDC cable product of any one of the above claims, wherein

the rated voltage of the cable product is 150 kV or higher.

12. The HVDC cable product of any one of the above claims, wherein

the insulation layer(s) formed from the composite material comprising a polymer matrix and filler particles (200) of aluminosilicate provides a majority of the resistance between the conductor and the outside (125) of the cable product.

13. The use of a composite material as an insulation layer of an HVDC cable product such as a cable, a bushing or a cable joint, wherein

the insulation layer is formed from a composite material (600) comprising a polymer matrix (605) and filler particles (200) of aluminosilicate.