AN HVDC DEVICE HAVING AN INSULATION SYSTEM FOR HVDC ELECTRICAL

INSULATION

Technical field

The present invention relates to the field of electrical insulation, and in particular to the electrical insulation in high voltage DC systems.

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 and should therefore be avoided.

Insulation systems where an insulating material of low electrical 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 would make the power cables easier to handle and transport. Thus, there is a desire to find HVDC insulation systems of improved insulation properties. Summary

A problem to which the present invention relates is how to improve the electrical properties of electrical insulation systems for HVDC applications.

This problem is addressed by an HVDC insulation system, the insulation system including at least two insulation layers: at least one insulation layer formed from a first material, and at least one insulation layer formed from a second material, where the conductivity of the second material is higher than the conductivity of the first material. The fraction of the total thickness of the insulation system that is formed by the at least one insulation layer of the first material falls within the range of 0.40 - 0.75. By dimensioning the insulation system so that the fraction of the total thickness that is formed by the material having the lowest conductivity falls within the range of 40% - 75% of the total thickness of the insulation system, the equivalent conductivity of the insulation system will be low, and the breakdown probability will be also be low. Hence, adequate insulation, by which for example adequate reduction in leakage currents is achieved, can be obtained with a thinner insulation system. Easier handling and shipping of an HVDC device in which the insulation system is used is thus facilitated.

A further reduction in the equivalent conductivity of the insulation system will often occur if the thickness fraction, which is formed by the material having the lowest conductivity, falls around 60 %. Furthermore, the breakdown probability goes down as the fraction of the first material increases. Hence, even more favourable electrical properties can be achieved if the fraction of the total thickness which is formed by the first material falls within the range of 0.55 - 0.75.

The first material can for example be a polyolefin containing an inorganic filler for reducing the conductivity, wherein the inorganic filler is chosen from the following groups: metal oxides, clays, carbonates, nitrides, borates, silicon oxide. Examples of such inorganic fillers are zinc oxide, magnesium oxide, silicon dioxide, aluminum oxide, iron oxide, kaolin, carbon black, silicon carbide, barium titanium oxide etc.

When the inorganic filler comprises magnesium oxide, the percentage of MgO in the first material can for example lie within the range of 0.1 wt% - 6 wt%. Within this range, MgO exhibits a negative electric field derivative of the conductivity for some electric fields. By selecting the first material such that the derivative of the conductivity with respect to electric field takes a negative value for at least part of the electric field range to be expected in the insulating layer(s) of said first material when an apparatus, in which the insulation system is arranged, is in use, the equivalent conductivity of the insulation system can be further reduced.

The problem is further addressed by an HVDC insulation system including at least two insulation layers: at least one insulation layer formed from a first material, and at least one second insulation layer formed from a second material, where the conductivity of the second material is higher than the conductivity of the first material. The first material is a material for which the derivative of the conductivity with respect to electric field takes a negative value for at least part of the electric field range to be expected in the insulating layer(s) formed from said first material when an apparatus, in which the insulation system is arranged, is in use.

By designing an insulation system so that the insulation layer(s) of the lowest conductivity exhibits a negative field derivative of the conductivity for an electric field for at least part of the electric field range to be expected in these layer(s), the equivalent conductivity of the insulation system will be low.

The first material could be a polyolefin containing an inorganic filler for reducing the conductivity, wherein the inorganic filler is chosen from the following groups: metal oxides, clays, carbonates, nitrides, borates, silicon oxide. Examples of such inorganic fillers are zinc oxide, magnesium oxide, silicon dioxide, aluminum oxide, iron oxide, kaolin, carbon black, silicon carbide, barium titanium oxide etc.

The insulation systems described above could for example be used in the insulation of a device such as an HVDC power cable, an HVDC cable accessory or an HVDC bushing. Oftentimes, the insulation layers are concentrically arranged to surround the device. In order to reduce the electric field on the outside of (i.e. at the outer interface of) the insulation system, the outermost insulation layer could be formed from a material, the conductivity of which is higher than the conductivity of the first material. In order to reduce the electric field at the inner interface of the insulation system, the innermost insulation layer could similarly be formed from a material, the conductivity of which is higher than the conductivity of the first material.

The insulation systems described above can, if desired, comprise at least one insulation system of at least one further insulating material, where the conductivity of the further material(s) is higher than the conductivity of the first material. The second material could for example be a polyolefin, a resin or a rubber. Any further insulation materials could for example also be a polyolefin, a resin or a rubber.

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

Fig. 1 is a schematic illustration of an HVDC power cable including an embodiment of an insulation system which includes a first and second insulation layer.

Fig. 2 is a graph illustrating results of measurements of the conductivity as a

function of electric field for an XLPE material (crosses) and an XLPE-MgO material (dots).

Fig. 3 is a graph illustrating the equivalent conductivity as a function of XLPE layer thickness for an insulating system comprising an XLPE and an XLPE-MgO layer, for an average electric field of 25 MV/m.

Fig. 4 is a graph illustrating the product of the conductivity and the electric field as a function of electric field for an XLPE material and an XLPE-MgO material. Fig. 5 is a graph illustrating the equivalent conductivity and breakdown probability of a layered XLPE/XLPE-MgO insulation system as a function of

XLPE-MgO fraction.

Fig. 6 is a graph illustrating the electric field distribution in a power cable

comprising a two-layered insulation system where the conductivity of the outer layer is higher than the conductivity of the inner layer.

Fig. 7 is a schematic illustration of a power cable including an embodiment of an insulation system which includes three insulation layers.

Detailed description

A schematic illustration of an example of an HVDC power cable 100 is shown in Fig. 1. The power cable 100 of Fig. 1 comprises a conductor 105, concentrically surrounded by a first insulating layer 1 lOi and a second insulating layer 1 l Oii. An HVDC power cable typically includes only a single conductor 105. The first and second insulation layers together form an HVDC insulation system 120. The thickness of the insulation system 120 is denoted D, the thickness of the first insulation 1 lOi layer is denoted dnoi, while the thickness of the second insulation layer 1 lOii is denoted duou. The diameter of the conductor 105 is denoted Φ ₀ . The total insulation thickness D equals the sum of the thicknesses of the insulation layers, D= dnoi + dnoii. Further insulating layers may be used to form the insulation system 120 of thickness D, in which case D =∑ _x d _iWx . When referring to insulation layers in general, the reference numeral 1 10 will be used. The current I _r in the radial direction of the cable 100 will have the same value through each of the insulation layers in an insulation system 120. Ohm's law then gives:

^noi Viioii , ^ *.

P . R .. ^

^K 110 l " 11011

where V is the voltage across an insulating layer in the radial direction, R is the resistance in the radial direction and the subscripts 1 lOi and 1 lOii refer to different layers of the insulation system 120. Let the area A denote the area of the insulation system 120 through which a leakage current may flow, so that for an insulation system 120 concentrically surrounding a circular conductor 105, the area A denotes the lateral area of the cylinder formed by the insulation system 120. The area A ₁₁₀ then denotes the corresponding area of an insulation layer 110. By making the approximation that the area Α _110ί of insulation layer 1 lOi and the area Anou of the insulation layer 1 lOii are the same, i.e. by assuming a planar geometry, this expression can be re- written as:

Furthermore, the voltage V across the insulation system 120 can be expressed as:

^ ⁼ ^iio t ^iio ; + Etwudiwii (3)

If the insulation system 120 comprises more than two layers, the right hand side of expression (3) would have an additional term for each of such additional insulation layers. Examples of materials that are often used in insulation systems for power cables are different polyolefins, for example polyethylene (PE, LLDPE, LDPE, HDPE),

polypropylene (PP), ethylene propylene rubber (EPR). Other resin, polymer or rubber materials may alternatively be used. The insulating material may or may not be cross- linked.

It has been shown that by adding inorganic filler particles to a matrix of an insulation material, the insulating properties of the material can be improved. Examples of such additives include metal oxides, clays, carbonates, nitrides, borates and silicon oxides, for example zinc oxide, magnesium oxide, silicon dioxide, aluminum oxide, iron oxide, kaolin, carbon black, silicon carbide, barium titanium oxide etc. The effect of reducing the conductivity by means of adding inorganic particle fillers to a matrix of insulating material is generally stronger if the particles are nano-sized. One often used example of an insulating material comprising inorganic fillers is cross-linked polyethylene (XLPE) to which particles of magnesium oxide (MgO) have been added, such material here being referred to as XLPE-MgO.

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, the total thickness of the insulation system 120 should preferably be small. However, the insulating properties of the insulation system 120 depend on the insulation thickness, and a thinner insulation system 120 generally provides poorer insulation properties. Thus, a suitable balance between good insulation properties and good cooling properties of the power cable 100 has to be found.

The conductivity σ of an insulating material generally depends on the applied electric field E, σ=σ(Ε), and the dependency is typically such that the conductivity increases with increasing electric field. However, our experimental studies on XLPE-MgO have shown that for certain electric fields, the derivate of the conductivity with respect to the electric field takes negative values. Hence, for certain electric fields, the conductivity of XLPE- MgO decreases with increasing electric field. In Fig. 2, experimental results for the conductivity σ as a function of applied electric field E is shown for one XLPE and one XLPE-MgO material, the two materials being specified below:

A) the XLPE material is a cross-linked low density polyethylene (XLPE) with a cross- linking agent of diculym peroxide (DCP) at a concentration of 0.6 wt%

B) the XLPE-MgO material comprises XLPE with 0.6% DCP as a matrix, to which 1,5 wt% of MgO has been added, where the average size of the MgO particles were 20 nm. In Fig. 2, results of measurements performed on the XLPE material are indicated by crosses, while results of measurements performed on the XLPE-MgO material are indicated by dots. (Further measurements were made and were used in the fits discussed below, but are not included in the graph). At each value of the electric field E in the range of interest, the conductivity of the XLPE material, GXLPE, is higher than the conductivity of the XLPE-MgO material, axLPE Mgo- Thus, we here refer to the conductivity of the XLPE material GXLPE as being higher than the conductivity of the XLPE-MgO material a uE-Mgo, although cxxLPE-Mgo niay be higher for one field than OXLPE is for another field.

The measurements in Fig. 2 show that the conductivity of the XLPE-MgO material has a negative derivative of the conductivity in the electric field range of around 3-4 10 ⁷ V/m. The experiments were extensively repeated with similar results. This quite surprising result could possibly be explained by the theoretically predicted negative derivative of the conductivity for some materials if hopping conduction is present at high field, see the disclosure in Nguyen and Shklovskii, Solid State Commun.38, 99 (1981), and A V

Nenashev, F Jansson, S D Baranovskii, R Osterbacka, A V Dvurechenskii, F Gebhard, Physical Review B (2008) 78(16-5207). The conductivity of the XLPE material, on the other hand, increases with electric field over the entire electric field range studied, as expected.

In the following, materials exhibiting a negative field derivative of the conductivity will be referred to as negative-E-derivative material. Although we can't fully explain the reasons for the negative field derivative of a LPE-Mgo, we have realized that the property of the negative field derivative over a certain field range can be exploited when designing the relative thickness of different insulation layers 1 10 of an insulation system 120. This will be explained with reference to Figs. 4 and 5, in which properties have been plotted of an insulation system 120 comprising a first insulation layer 1 10 of an XLPE-MgO material as specified under B) above, and a second insulation layer 1 10 of an XLPE material as specified under A) above. Values of GXLPE(E) which have been used to produce the graphs have been obtained by use of a fit of experimental values to the expected relation

GXLPE (E) ⁼ ^o^ ^aE , while values of axLPE-M _g o(E) have been obtained by performing a spline fit of experimental values.

Since the insulating system 120 for which properties are plotted in Figs. 4 and 6 is formed from layers of materials of different conductivities, no value of a homogenous conductivity can be defined for the insulation system 120. However, in the following, the equivalent conductivity∑ will be used when discussing the conductive properties of the insulation system 120. The equivalent conductivity∑ for a planar geometry is defined as where R is the resistance of an area A of the insulation system, and D is the total thickness of the insulation system 120 used in the calculations.

In Fig. 3, the equivalent conductivity of an insulation system 120 formed from the materials XLPE and XLPE-MgO as defined under A) and B) above is plotted as a function of the thickness of the XPLE layer, dxpLE, at a total thickness D of 20 mm and a nominal voltage of 500 kV. Indicated in the graph is also the conductivity of the XLPE material, CT LPE, as well as the conductivity of the XPLE-MgO material, axLPE-Mgo, at the electric field of 25 MV/m, corresponding to the situation if only one insulation material had been present.

As can be seen in Fig. 3, there is a range of dxLPE within which the equivalent conductivity ∑ of the insulation system XLPE/XLPE-MgO will be lower than if the entire insulation system 120 were made up of XLPE-MgO. This can be explained as follows: The voltage is divided over the XLPE and XLPE-MgO layers, respectively, in a manner so that the electric field in the XLPE-MgO layer will be higher than when the entire insulation system 120 is made up of XLPE-MgO (i.e. if while the electric field in the XLPE layer will be lower than if the entire insulation system 120 is made up of XLPE (i.e. if dxLPE = D). The electric field in the XLPE-MgO layer in a layered structure will here be referred to as E _x ^a _L ^y _p ^e _{E Mg0} , the electric field in the XLPE layer in a layered structure will be referred to as E _X ^l _L ^v _PE ^ed , while the electric field in the single layers will be referred to as Εχ ρΕ ^{8 an} d ExLPE-MgO ' respectively. Thus, as can be seen in Fig. 2, the conductivity of the XLPE in a layered insulation system 120 will be lower than if dxLPE = D, at a fixed (nominal) cable voltage, since E _{x PE} ^red <E _XL ⁿ _pE ^e . In the XLPE-MgO-layer, ^ ¹ ^^ ₀ > -,sinqle _r r -sinqle _^ -.layered

ΕχΐΡΕ-Mgo■ ^{For some values of} dxLPE, for which the range of E _{x E} _ _Mg0 - E _XL ^y _PE _ _Mg0 includes at least part of the field range wherein the conductivity a _XhPE _ _Mg0 exhibits a negative conductivity, the conductivity at E _x ^la _L ^v _PE t _Mg0 will be lower than the conductivity at E xLPE-Mgo - Thus, for such values of dxLPE, the following inequality holds: ^σ χΐΡΕ-Μ ₉ ο {ΕχΐΡΕ-Μ ₉ ο ) ^{~ a} xLPE-M _e o{ES%lttgo) ^> °» ^{and the} equivalent conductivity ∑ of the layered insulation system 120 will be lower than if a single layer of XLPE-MgO (or XLPE) were to be used. The concept of different electric fields in different insulation layers 1 10 can be further illustrated by the plot in Fig. 4, wherein the product of the conductivity σ and the electric field E is plotted as a function of electric field E for the respective materials XLPE and XLPE-MgO. As discussed above in relation to expression (2), the product Ε*σ(Ε) will take the same value in all layers 1 10 of the insulation system 120. Thus, depending on the thicknesses of the different layers of the insulation system 120 in relation to the nominal voltage V, the electric fields within the different layers will take different values such that expression (2) will be fulfilled. This is illustrated in Fig. 4 as a dashed line 400, which line indicates the resulting value of the product E∑(E) for a particular insulation system 120 having insulating layers 1 10 of the two materials XLPE and XLPE-MgO, to which a particular voltage V has been applied. Which value of E∑(E) is obtained for a particular insulation system 120 depends on the applied voltage, V. For the voltage V applied to the insulation system 120 of Fig. 4, the resulting electric field in the XLPE material is indicated in the figure as E _XL PE, while the resulting electric field in the XLPE-MgO material is indicated as E _XL pE-M _g o.

In the insulation system 120 illustrated in Fig. 3, the range of dxLPE, for which a reduction is achieved in the equivalent conductivity∑ as compared to the conductivity of a single layer of the low conductivity material XLPE-MgO, can be determined as approximately 5.5 mm - 12.5 mm. Since the total thickness D of the insulation system 120 illustrated in Fig. 3 is 20 mm, i.e. dxLPE+ dxLPE-Mgo = 20 mm, this range can be expressed as if dxLPE corresponds to 25% - 65% of the total thickness D, i.e. ^dx ^ ^PE lies within the range of 0.25 - 0.65. Correspondingly, the range can be expressed as if dxLPE-Mgo corresponds to 35% - 75% of the total thickness D, i.e. ^dxLPE~M9 ° \ _{Q& wl} thin the range of 0.35 - 0.75. This result

' D °

has been obtained from measurements and calculations for a particular total insulation thickness D of 20 mm and cable voltage V of 500 kV, i.e. for an average electric field Ε _Άν of 25 MV/m. However, for other nominal voltages, a similar average electric field would typically be applied, and a similar range of the fraction of XLPE-MgO material would hence be suitable. The average electric field E _av in the insulation system 120 could be for example lie within the range 1 -50 MV/m . In Fig. 5, the equivalent conductivity∑ is plotted as a function of the XLPE-MgO fraction of an insulation system 120, i.e. as a function of ^dxLPE ^ ^M9 ° (continuous line). The graph was obtained from measurements on a two layer insulation system 120 of XLPE and XLPE-MgO as defined under A) and B) above, respectively, wherein the total thickness of the insulation system 120 was 20 mm and the nominal voltage was 500 kV. However, as discussed above, the results shown in Fig. 5 can generally be extrapolated to other combinations of nominal voltage and total insulation thickness D, since the range of suitable total insulation thickness typically scales well with the nominal voltage of the power cable 100.

As can be seen in Fig. 5, a minimum of the equivalent conductivity∑ occurs for the XLPE- MgO layer occupying approximately 3/5 of the insulation.

The conductivity of the XPLE-MgO material σχυΕ-Mgo at the electric field of 25 MV/m, corresponding to the situation if XLPE-MgO material had been the only insulation material within the total thickness D, has been indicated in the graph as a dotted line 500. As can be seen in the graph and expected from Fig. 3, a favourable equivalent conductivity∑ is obtained when ^dxLPE~M9 ° f _a n _{s w} thin the range of 0.35 - 0.75.

D ⁶

When designing the insulation system 120 of a power cable 100, not only a low

conductivity of the insulation system 120 is desired, but also an adequate breakdown strength, in order to avoid electrical breakdown of the insulation system 120 in case of over- voltages being applied to the power cable 100. Therefore, Fig. 5 also shows how the breakdown probability P of the XLPE/XLPE-MgO insulation system 120 varies with the

XLPE-MgO fraction ^dxhP ™° (dashed line).

As can be seen in the graph of Fig. 5, the breakdown probability P exhibits a peak when dxLPE-Mgo is approximately 3/10 D. At this fraction of XLPE-MgO, the electric field across the XLPE-MgO layer will be at a level where the probability of breakdown is

comparatively high. In order to keep the breakdown probability at an acceptable level, the fraction of XLPE-MgO could advantageously be chosen such that ^{dxLPE^M3} ° > 2/5. Thus, a favourable situation will occur when ^{d LFg M9} ° falls within the range of 0.40 - 0.75. This

D ^&

corresponds to the situation when dxi D falls within the range of 0.25 - 0.60.

In order to obtain information on the breakdown probability P, breakdown tests on the XLPE and XLPE-MgO materials were made, the results of which were fitted to a Weibull distribution, F(E, α, β), where a and β are the shape characteristic parameters of the Weibull fit. For the materials A) and B) above, the following values of the parameters were obtained for a reference thickness of 0.1 mm: CIXLPE · 306 kV/mm; PXLPE: 9.44; cixuE-Mgo · 284 kV/mm; PxLPE M _g o: 12.75. The breakdown strength of a material, and hence the parameter a, varies with sample thickness d. As suggested in R. Liu et al. CEIDP 201 1 , 18-521, the variation of a with sample thickness d can be estimated by use of the following expression:

a(d) = ^ (5),

d ^m

where ao is the breakdown strength at the reference thickness and m is a constant, which was determined to 0.222.

The breakdown probability of the insulation system 120 shown in Fig. 5 was then obtained as a sum of the breakdown probabilities of the different insulation layers 110, using expression (6):

P =∑Y ₌₁ F _i (E _i , a _i (d _i )^d (6),

where z-1 to n are indices referring to different insulation layers 110, where E; is the calculated electric field in insulation layer i when insulation layer i takes up a thickness di of the total thickness D. The breakdown strength data of Fig. 5 were also obtained for a total thickness D of 20 mm and a nominal voltage of 500 kV.

When an insulating material of low conductivity is arranged to surround a conductor 105 at high potential, the electric field in the radial direction will be high in the insulating material. Since the area outside of the power cable will typically be grounded, a high difference in electric field will occur at the cable surface, if the insulating material is the outermost layer of the cable. A large jump in the electric field at the interface of two different materials increases the likelihood for electric discharges etc, which can cause problems for example at cable junctions. This is for example discussed in patent application SE0901493. A semiconductor layer is therefore often provided on the outside of the insulating layer, between the insulating layer and ground. This semiconductor layer will inter alia facilitate for any charges at the outside of the insulating material to leave the cable. However, the difference in conductivity between such insulating material and the semiconductor material is typically large, thus giving rise to a large discontinuity in the radial electric field component at the interface between the insulating material and the semiconductor layer. It is hence often desirable to keep the electric field low at the side of the insulation system 120 which faces away from the conductor 105, towards the surrounding, this side being referred to as the outside of the insulation system 120.

In order to reduce the electric field at the outside of the insulation system, the electrical conductivity of the outermost insulating layer of insulation system 120 could

advantageously be higher than the conductivity of the insulating layer which provides the lowest conductivity of the layers forming the insulation system 120. In an insulation system 120 comprising only two layers, this translates into the conductivity of the first

(inner) layer 1 lOi being lower than the conductivity of the second (outermost) layer 1 lOii (as mentioned above, an insulation system 120 may comprise further insulating layers 110). In the following, the material forming the insulating layer(s) 110 of the lowest conductivity will be referred to as the main insulating material. The outermost insulating layer of insulation system 120 will be referred to as the outermost insulating layer 110. In an embodiment wherein the outermost insulation layer is not formed from the main insulating material, any further insulating layers could be located either between the conductor 105 and an insulation layer 110 of the main insulating material, and/or between the outermost insulating layer and an insulation layer 110 of the main insulating material. As mentioned above, an HVDC power cable 100 may include, in addition to the insulating system 120, semiconducting layers and/or screening layers, etc, which have not been shown in Fig. 1 or Fig. 7.

Fig. 6 is a schematic illustration of how the radial component of the electric field, Er, will vary with the distance dr from the centre of the conductor 105 in an HVDC power cable 100 having two insulation layers, where the outermost insulating layer 1 lOii is of higher conductivity than the inner insulation layer 11 Oi, the inner insulating layer being formed from the main insulating material. When in use, the conductor 105 is at the electrical potential corresponding to the voltage of the power transmission. The conductivity of the conductor 105 is high, resulting in an electric field in the conductor 105 being close to zero. The voltage V at different points of the cable cross-section is illustrated in Fig. 6 by a dashed line. Here, it is assumed that the outermost layer 1 lOii is grounded, whereas in many implementations, further layers, such as semi-conducting layers or screening layers, will be located outside of the outermost layer 110.

Since the HVDC insulation system 120 illustrated in Fig. 6 is arranged such that the conductivity of the inner insulation 1 lOi is lower than the conductivity of the outermost insulation layer 11 Oii, the electric field in the inner insulation layer 11 Oi is higher than the electric field in the outermost layer 1 lOii (cf. expression (1)).

The discussion above has been made in terms of an insulation system 120 comprising two insulation layers 110 formed from different materials. However, the effect of a reduction in the equivalent conductivity∑ can also be achieved for insulation systems 120 of three or more insulation layers 110, where two or more different materials can be used to form the different insulation layers 110. In one example of a multilayer insulation system 120, only two different insulation materials are used, so that the two different materials are alternatingly arranged. In fact, the results obtained for the two layer system and illustrated in Figs. 4 and 6 also apply to a multi-layer system of two or more different materials, as long as the fraction of the total thickness D which is formed by the material showing the lowest conductivity falls within the range of 0.40 - 0.75.

The reduction in equivalent conductivity achieved by using a layered insulation system 120 wherein at least one layer comprises a negative-E-derivative material can be achieved regardless of at which position in the insulation system the negative-E-derivative material is located. In some embodiments, however, the negative-E-derivative material will be used as an inner insulation layer 1 10, and the outermost insulation layer will be formed from another material. If the outermost insulation layer is made from a material having a higher conductivity than the negative-E-derivative material, the electric field at the outside of the insulation system 120 will be lower.

An illustration of a three-layered insulation system 120, comprising an innermost layer 1 lOi, an inner (middle) layer 1 lOii and an outermost layer 1 lOiii is shown in Fig. 7. In order to achieve a lower electric field at the outside of the insulation system 120, the middle layer 11 Oii could be of a material having lower conductivity than the materials used in the outer and inner layers. The innermost and outermost layer could for example be formed from the same material. The insulation layer(s) of lowest conductivity could for example be formed from a matrix material to which particles of an inorganic filler has been added. The other insulation layer(s) 110 could then for example be formed from the matrix material of the main insulation material, with or without additives, or from any other suitable insulation material. In one example of the insulation system 120 shown in Fig. 7, the main insulation material is XLPE-MgO and the inner and outer insulation layers are formed from XLPE.

The results presented in Figs. 2-5 above have been obtained from measurements on materials as defined above under A) and B). However, similar results would be obtained on other compositions of the XLPE material and of the XLPE-MgO material. For example, the XLPE could comprise a different crosslinking agent, such as silanes or peroxides; the XLPE could comprise the crosslinking agent DCP in a different concentration, such as within the range of 0.1-2.2 wt%; or it could contain no crosslinking agent at all (the polyethylene then not being cross-linked). Furthermore, the XLPE-MgO could contain MgO of a different concentration, for example in the range of 0.1-6 wt%; the MgO particles could be of a different size, for example within the range of 1-200 nm; and the XLPE matrix of the XLPE-MgO could be varied as described above in relation to the XLPE material. Moreover, the XLPE material and/or the matrix of the XLPE-MgO material could be replaced by another insulating polyolefin, an insulating resins or an insulating rubber, examples of which are given above.

Moreover, similar results could be obtained for other combinations of materials, for example for a material combination where the insulation layer 110 having the lowest conductivity is formed by a matrix material to which inorganic filler particles have been added. Examples of such additives include metal oxides, clays, carbonates, nitrides, borates and silicon oxides, for example zinc oxide, silicon dioxide, aluminum oxide, iron oxide, kaolin, carbon black, silicon carbide, barium titanium oxide etc. The inorganic filler could for example be of a concentration within the range of 0.1-6 wt%. The matrix material, as well as a second and possibly further materials from which at least one insulation layer 110 is formed, could for example be different polyolefins, for example polyethylene (PE, LLDPE, LDPE, HDPE), polypropylene (PP), ethylene propylene rubber (EPR). Other resin, polymer or rubber materials may alternatively be used. The insulating materials may or may not be cross-linked. The layered insulation system 120 has been presented above in relation to insulation of

HVDC power cables. However, an insulation system 120 according to the invention could be applied in any HVDC device wherein a varying electric field across an insulation system 120 can be beneficial, such as in HVDC cable accessories (including e.g. cable joints, cable terminations and cable connectors); in HVDC bushings etc. Such HVDC devices typically include a single conductor, around which the insulation layers of the insulation system 120 are concentrically arranged.

The invention is applicable to HVDC devices of all nominal voltages, although the benefits of low conductivity will be more pronounced for devices of nominal voltage of around 300 kV and higher.

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.