SE1651576A1 - Impregnation fluid for mass-impregnated power cables and method of preventing filtration effect in insulation system by using PPLP tape - Google Patents

Description

15 20 25 2 appropriate tape around the conductor, followed by impregnation ofthe wound tapes using an impregnation fluid.

The tape forming the wound insulation layer is typically made from Kraft paper, and the tape forming the semi-conductive layers typically comprises carbon-black paper and/or metal- laminated paper.

The impregnation fluid typically exhibits decreased viscosity at high temperatures and high viscosity at lower temperatures. This allows the tape-wrapped cable to be impregnated using the impregnation fluid at impregnation temperatures in excess ofthe standard cable operating temperatures, and allows for an operational temperature window whereby the impregnation fluid has a suitable viscosity and the mass-impregnated cable operates suitably.

However, the transmission capacity of today's mass-impregnated cables may be considered to be limited by the viscosity profile of the impregnation fluid used, which is typically T2015.

TZOlS is a high *viscosity insulatšng fluid, sold by HSLR ChemPharm (UK) Ltd., based on a rnšneral oil added tvith about 2% by weight of a high moleculai' weight polyisobuteaie as viscosity increasing agent. Both the high viscosity at lower temperatures and the low viscosity at higher temperatures constitute a limit for increasing the transmitted power.

At the lower service temperatures, cavity formation in the insulation during heating cycling prevents voltage and current from being further increased. The cavities form when the cable is rapidly cooled due to current turn-off. The voltage applied over the insulation will give rise to partial discharges in the cavities that will deteriorate the cable insulation over time. increasing the current will generate a larger temperature drop over the insulation, increasing the extent of the cavity formation during current turn-off. increasing the voltage will generate more partial discharges as the voltage over every individual cavity consequently will increase.

Eventually some of the cavities will extinguish as the impregnation fluid is transported due to the pressure gradient created in the insulation. How fast the cavities extinguish depends on the viscosity of the impregnation fluid. The lower the viscosity of the impregnation fluid, the faster the extinction of the cavity.

At the limiting upper service temperature, axial drainage of oil in a cable with a certain slope prevents the current from being further increased. At the maximum service temperature of 10 15 20 25 3 today's I\/ll cables, the T2015 impregnation fluid may reach the lowest viscosity value permitted without draining the cable enough for a breakdown.

A number of attempts have been made to address these known limitations.

Patent specification US 6,245,426 Bl discloses a mass-impregnated DC cable wherein the dielectric impregnation fluid comprises a polymer and a hydrocarbon-based fluid. The dielectric fluid behaves in such a manner that at temperatures within a first low temperature range from about 0° C to about 100° C, it is in a highly viscous and elastic state, exhibiting a viscosity of about 10 Pa-s to about 100 Pa-s or more. At elevated temperatures within a second higher temperature range from about 80° C to about 150° C, the dielectric fluid is in a low viscosity essentially Newtonian easy flowing state exhibiting a viscosity of 200 mPa-s or less. Over a transition temperature range between the first and second temperature ranges, the dielectric fluid changes between the highly viscous state and the low viscosity state.

Document WO 01/93279 A2 discloses a mass-impregnated electric cable having a dielectric impregnating fluid exhibiting a different viscosity behaviour. Here, the dielectric fluid within the first lower range of operating temperatures, has a viscosity within a range from 1 Pa-s to 200 Pa-s, which is low enough to allow it to flow under the influence of shear stresses in the order typically developing in the radial direction in the cable insulation of a DC cable under operation. At the second higher range of impregnating temperatures, the dielectric fluid has a viscosity within a range from 1 mPa-s to 1 Pa-s. Over a third intermediate transition range of temperatures, the dielectric fluid exhibits a change in the viscosity such that the ratio between the viscosity at the first lower range of temperatures and the viscosity at the second higher range of temperatures for the dielectric impregnating liquid is 5:1 or more. Within a range of temperatures from O °C to 80 °C, the impregnating material exhibits a weak temperature dependency of the viscosity of 4 % per °C or less.

However, despite previous attempts, there remains a need for improved insulation systems for mass-impregnated power cables.

SUMMARY OF THE INVENTION The present disclosure identifies several shortcomings in the prior art mass-impregnated cables and insulation systems. The viscosity profiles obtained by the prior art impregnation 10 15 20 25 4 fluids were not necessarily optimal. Moreover, the known impregnation fluids demonstrated issues with filtration effects, meaning that the viscosity modifiers used could become unevenly distributed in the impregnation fluid, leading to non-uniform viscosity properties within different portions ofthe cable insulation. Furthermore, the viscosity modifiers known in the art were not necessarily hydrocarbon-based, meaning that they potentially could have dielectric properties differing significantly from the properties of the base impregnation fluid. lt is therefore an object ofthe present disclosure to provide an insulation system for mass- impregnated power cables that addresses these known shortcomings, and others. More specifically, it is an object of the present disclosure to provide an insulation system for mass- impregnated power cables, and/or other electrical apparatus, that has an improved viscosity profile, lower propensity for filtration effects, and/or improved dielectric properties.

These objects are achieved by an insulation system for a mass-impregnated power cable according to the appended claims. The insulation system comprises an inner semi-conductive layer, an insulation layer and an outer semi-conductive layer. The insulation layer comprises a porous sublayer impregnated with a gel, said gel comprising one or more mineral oils, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners.

The gel disclosed in the present disclosure exhibits desirable viscosity properties. The viscosity value of the gel is decreased at low temperatures as compared to T2015 fluid. This allows sufficient gel fluidity for extinction of any cavities arising in the insulation system, thus permitting for example larger operating currents, voltages, field strengths and/or temperature drops. At the same time the viscosity is increased at the upper end ofthe conventional temperature operating window as compared to T2015, thus allowing for a larger service temperature interval, such as up to 85 °C or higher, and/or higher voltage ratings, such as 1650 kV or higher.

The gel may also demonstrate a reduced propensity to unevenly distribute the thickeners present, thus allowing for a more uniform and predicable viscosity profile at all locations within the cable insulation.

The insulation layer may comprise at least one porous sublayer and at least one non-porous sublayer. lf present, the non-porous sublayer may be selected from polymers such as PET, 10 15 20 25 5 BOPET, PP, BOPP, PE, LLDPE, LDPE, l\/IDPE, HDPE, XLPE, PVC, polyester, aramid, polyimide, rubber, EPDM, EPR, silicone rubber, and mixtures thereof. This list is not exhaustive and the skilled person recognises that other non-porous materials may be used. For example, the insulation layer may comprise paper-polypropylene laminate (PPLP) tape. The porous and/or non-porous sublayers may be wound, lapped or stacked in such a manner as to provide gaps between abutting layers. These measures, each in isolation or in combination, may allow for a further reduction of filtration effects in the insulation system. Thus, the predictability and uniformity of the viscosity properties ofthe gel may be improved regardless of location in the insulation system.

The porous sublayer may be selected from cellulose paper, kraft paper, foraminated polymer such as PET, BOPET, PP, BOPP, PE, LLDPE, LDPE, l\/IDPE, HDPE, XLPE, PVC, polyester, aramid, polyimide, rubber, EPDM, EPR, silicone rubber, and mixtures thereof. This list is not exhaustive and the skilled person recognises that other porous materials may be used. Thus, a wide range of insulation materials may be used, and the properties of these materials may be tailored to prevent filtration effects.

According to another aspect of the present disclosure, an impregnation gel is disclosed that achieves the objects described above. The gel comprises one or more mineral oils, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners. For example, the gel may comprise, essentially consist of, or consist of one or more mineral oils, a tri-block copolymer thickener, a di-block copolymer thickener and an antioxidant.

The tri-block copolymer thickener may be selected from polystyrene-block-poly(ethylene- ethylene/propylene)-block-polystyrene (SEEPS), polystyrene-block-poly(ethylene/butylene)- block-polystyrene (SEBS), polystyrene-block-poly(ethylene/propylene)-block- polystyrene (SEPS), enhanced rubber segment (ERS) polymers, S-EB/S-S, and mixtures thereof. However, this list is non-exhaustive and the skilled person recognises that other tri-block copolymers may be used.

The di-block copolymer thickener may be polystyrene-block-po|y(ethylene/propylene).

However, this list is non-exhaustive and the skilled person recognises that other di-block copolymers may be used. 10 15 20 25 6 The gel may have a transition temperature up to 100 °C, such as about 80 °C, about 90 °C or about 100 °C. This allows for a higher maximum operating temperature as compared to the commercial T2015 impregnation fluid.

The molecular weight ofthe di-block and tri-block copolymer thickeners may be from about 10 kDa to about 1000 kDa.

The gel may have a viscosity value of 1 Pa-s or higher at a temperature below the transition temperature, and/or a viscosity value of less than 1 Pa-s at a temperature above the transition temperature. This allows for a suitable transition from a higher viscosity state to a lower viscosity state, thus facilitating impregnation of the cable.

According to a further aspect of the present disclosure, a mass-impregnated power cable is disclosed that achieves the objects described above. The mass-impregnated power cable comprises an insulation system as described above.

According to yet another aspect of the present disclosure, a mass-impregnated cable joint is disclosed that achieves the objects described above. The mass-impregnated cable joint comprises an insulation system as described above.

The mass-impregnated power cable or mass-impregnated cable joint may be arranged for use in high-voltage direct-current (HVDC) applications and/or high-voltage alternating current (HVAC) applications, preferably HVDC applications with rated voltages of i650 kV or higher.

According to yet a further aspect of the present disclosure, an insulation system for application in power systems and/or power devices is disclosed that achieves the objects described above. Such systems and devices include power cables, transformers, capacitors, motors, switchgears, HVDC converter stations and HVAC substations. The insulation system comprises at least one or more lapped porous sublayers impregnated with a gel, the said gel comprising one or more mineral oils, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners.

A process is disclosed for manufacturing a mass-impregnated power cable as described above.

The process comprises the following steps: a) Providing sublayers of the insulation layer; 10 15 20 25 7 b) Winding, lapping or stacking the sublayers around a conductor in such a manner as to provide a wound insulation having gaps between adjacent sublayers; c) Providing a gel for impregnation ofthe wound insulation; d) lmpregnating the wound insulation with the gel to provide a mass- impregnated insulation layer; e) Optionally providing a moisture barrier around the mass-impregnated cable; f) Optionally providing a protection layer, such as an armour layer, around the mass-impregnated cable.

Although impregnation of the insulation sublayers is performed subsequent to winding in the process above, as is typically the case, it is also recognised that impregnation of the sublayers may be performed prior to winding, for example by using pre-impregnated tapes.

Moreover, it is recognised that any features applicable to the products described above are equally applicable to the process herein described. Thus, the process may be limited by, for example, the features of the porous and/or non-porous substrates, thickeners, mineral oils and other materials used in the production ofthe power cable.

Further objects, advantages and novel features ofthe present invention will become apparent to one skilled in the art from the following detailed description.

DETAILED DESCRIPTION For a full understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings.

Herein disclosed is a gel impregnation fluid suitable for a mass-impregnated power cable. The new gel is a mixture of mineral oil with a group of copolymers. The gel has an upper service temperature limit up to 100 °C, such as about 80 °C, about 90 °C or about 100 °C. No lower limit is considered. Between 0 °C and the upper service temperature limit, the viscosity has a value between 1 and 50 Pa-s and at a temperature of 120 °C and above the viscosity value is considerably lower than 1 Pa-s. The composition has a possible impregnation temperature range of from about 40 °C to about 180 °C, preferred of from about 110 °C to 130 °C. Between the upper service temperature limit and the lower impregnation temperature limit, there is a 10 15 20 25 30 8 transition from high to low viscosity. The transition temperature is defined as the temperature at which the gel passes from the high viscosity regime to the transitionary viscosity regime. lt can be seen from graphs of viscosity as a function of temperature that at the transition temperature there often is a marked change in dn/dT, i.e. dn/dT is weakly negative in the high viscosity regime and more significantly negative in the transition regime.

Figure 1 illustrates the viscosity as a function of temperature for the gels herein disclosed (line 1) and for the known impregnation fluid T2015 (line 2). lt can be seen that for the known fluid T2015, the viscosity ofthe fluid drops below a threshold level nmin at a threshold temperature Tmaxgzo15. However, for the new gel compositions, the sharper transition between a high viscosity regime and a low viscosity regime allows for a higher upper threshold temperature Tmax,new_ Likewise, regarding the lower threshold of the temperature operational window, it can be seen that at low temperatures the viscosity of T2015 becomes excessively high, meaning that cavities developed in the cable insulation cannot be extinguished. However, for the gel compositions of the present disclosure, the temperature dependence of viscosity within the temperature operating window is much weaker, meaning that there is essentially a much lower lowest temperature threshold, or none whatsoever. Thus, the temperature operating window of the present gel compositions is much enlarged compared to known impregnation fluids such as T2015.

The gel of the present disclosure may comprise, essentially consist of, or consist of a mineral oil, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners. The oil may for example be a hydrotreated naphthenic oil, such as NS100, although further oils are apparent to the skilled person, such as insulating oils, such as Nytro 10XN.

The proposed thickeners and their alternatives are summarized in Table 1, below. The thickeners (copolymers) are a tri-block copolymer and a di-block copolymer consisting of polystyrene-b-poly(ethy|ene/propylene) (Septon SEP 1020). The function of the tri-block copolymer is to create a suitable transition knee temperature, as seen in Figure 1, and that of the di-block copolymer is to increase the stability of the gel composition system.

The tri-block copolymer thickener may comprise, essentially consist of, or consist of polystyrene-b-poly(ethy|ene-ethylene/propylene)-b-polystyrene, for example as sold under the name Septon SEEPS 4099. Alternatives to SEEPS 4099 include, but are not limited to SEEPS 9 4077, SEEPS 4055, SEEPS 4044, SEEPS 4033, SEBS 8006 (polystyrene-b- poly(ethylene/butylene)-b-polystyrene), SEPS 2006 (polystyrene-b-poly(ethy|ene/propylene)- b-polystyrene), Kraton A 1535 (S-EB/S-S polymer), Kraton G1651 (SEBS polymer) and G1641 (ERS polymer).

The di-block copolymer thickener may comprise, essentially consist of, or consist of polystyrene-b-poly(ethylene/propylene), for example as sold under the name Septon SEP 1020. Alternatives to SEP 1020 include, but are not limited to Septon SEP 1001, Kraton G 1701 and 1702 (SEP).

Table 1. Proposed thickeners and their alternatives Proposed Thickeners Alternatives polystyrene-b- poly(ethylene- ethylene/propylene) -b-po lystyrene polystyre ne Composition Commercial Composition Commercial Name Name A tri-block Septon: A tri-block consisting of Septon: copolymer SEEPS 4099 polystyrene-b-poly(ethylene- SEEPS 4077, consisting of (I\/|W;600000) ethylene/propy|ene)-b- SEEPS 4055 polystyrene-b- poly(ethy|ene/butylene)-b- polystyre ne Se pton: SEBS 8006 polystyrene-b- poly(ethylene/propylene)-b- polystyre ne Septon: SEPS 2006 S-EB/S-S polymer Kraton: A 1535 SEBS polymer Kraton: G1651 ERS polymer Kraton: G1641 A di-block copolymer consisting of polystyrene-b- Septon: SEP 1020 (iviwflsoooo) A di-block consisting of polystyrene-b- poly(ethylene/propylene) Septon: SEP 1001 SEP polymer Kraton: G1701 10 15 20 25 10 po|y(ethy|ene/propy lene) The gel compositions may comprise further additives known in the art, such as antioxidants.

The gel compositions may comprise from about 0.1 weight% to about 5 weight% tri-block copolymer thickener, such as from about 1 weight% to about 5 weight% tri-block copolymer thickener, from about 2 weight% to about 5 weight% tri-block copolymer thickener, from about 3 weight% to about 5 weight% tri-block copolymer thickener, from about 4 weight% to about 5 weight% tri-block copolymer thickener, from about 0.1 weight% to about 4 weight% tri-block copolymer thickener, from about 0.1 weight% to about 3 weight% tri-block copolymer thickener, from about 0.1 weight% to about 2 weight% tri-block copolymer thickener, from about 0.1 weight% to about 1 weight% tri-block copolymer thickener.

The gel compositions may comprise from about 0.1 weight% to about 5 weight% di-block copolymer thickener, such as from about 1 weight% to about 5 weight% di-block copolymer thickener, from about 2 weight% to about 5 weight% di-block copolymer thickener, from about 3 weight% to about 5 weight% di-block copolymer thickener, from about 4 weight% to about 5 weight% di-block copolymer thickener, from about 0.1 weight% to about 4 weight% di-block copolymer thickener, from about 0.1 weight% to about 3 weight% di-block copolymer thickener, from about 0.1 weight% to about 2 weight% di-block copolymer thickener, from about 0.1 weight% to about 1 weight% di-block copolymer thickener.

The gel compositions may comprise from about O weight% to about 5 weight% further additives, such as from about 0.1 weight% to about 5 weight% further additives, from about 1 weight% to about 5 weight% further additives, from about 2 weight% to about 5 weight% further additives, from about 3 weight% to about 5 weight% further additives, from about 4 weight% to about 5 weight% further additives, from about 0.1 weight% to about 4 weight% further additives, from about 0.1 weight% to about 3 weight% further additives, from about 0.1 weight% to about 2 weight% further additives, from about 0.1 weight% to about 1 weight% further additives. 10 15 20 25 11 The balance of the gel composition may comprise, essentially consist of or consist of mineral oil. ln order to further improve the stability ofthe insulation system and reduce filtration effects which may locally alter the composition of the gel composition, the tape used in the insulation system may be modified and/or wound in an amended manner. Figure 2 schematically illustrates two paths in which the impregnation fluid (gel) may permeate the solid insulation material 23 (wound tape). The aim is to block the impregnation fluid from penetrating directly through the body of the film (path 22) and thus to force the impregnation fluid to go only through the butt gaps 24 and then along the interfaces between two tapes 25 (path 21).

This may for example be achieved by using a PPLP tape as the insulation tape. PPLP is typically a laminate comprising of a layer polypropylene sandwiched between two layers of Kraft paper.

The PPLP should be made in such a way that the gel is able to propagate through the interfaces between the two layers of PPLPs. Some commercially available PPLPs as received may possess this feature.

A gapped structure may be provided to allow the gel to penetrate between gaps. This may be provided by adapting butt gaps 24 in the wound insulation to provide a suitable size of gap.

The solid insulation material need not necessarily be PPLP. Any suitable material may be used which hinders or prevents direct penetration ofthe gel as shown by path 22, and/or promotes interfacial propagation as shown by path 21.

Examples Gel compositions Three different gel compositions comprising mineral oil NS100, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners, and an antioxidant were manufactured, and their complex viscosity as a function of temperature were tested.

Viscosity was measured by the method of ISO 3219, using a rotational viscometer with defined shear rate. The results are shown in Figure 3. lt can be seen that all three compositions demonstrated a transition from a high viscosity regime to a low viscosity regime at temperatures between about 80 °C and 120 °C. At temperatures lower than about 80 °C the dependence of viscosity on temperature was weak. All three gels exhibited excellent stability. 10 15 12 Reduction of filtration effects lt was proved by a small scale filtration test that the formulated gel was able to penetrate through a sandwiched insulation system consisting of gapped layers of PPLPs (polypropylene laminated paper). The experimental setup is schematically illustrated in Figure 4. Holes were made through the individual layers of PPLPs (6 layers of 170 pm PPLPs). The diameter of the holes had about the same size as that of a typical butt gap in a |\/|| cable insulation. The distance between the holes in two consecutive layers was about the same as the width of a PPLP tape which is to be used in a MI cable. A gel compound was placed on top of the gapped PPLP structure which was supported by a metal mesh from the bottom. The setup was heated to 120 °C and a pressure difference of 2 bar was put over the PPLP layers until the gel compound had passed through the PPLP stack. Size exclusion chromatography analysis shows that the concentrations ofthe copolymers in the impregnated PPLPs and in the filtrated compound were about the same as those in the original compound which indicated that there was no filtration effect in the performed PPLP test.

For comparison, a reference test was made with Kraft papers (6 layers of 100 um papers). The concentrations of the copolymers in the tested Kraft papers were higher than those in the original compound indicating that a filtration effect might exist in the performed test.

Claims (16)

1. 0 15 20 25 30 13

2. CLAIMS lnsulation system for a mass-impregnated power cable, the said insulation system comprising an inner semi-conductive layer, an insulation layer and an outer semi- conductive layer characterísed in that the insulation layer comprises a porous sublayer impregnated with a gel, said gel comprising one or more mineral oils, one or more tri- block copolymer thickeners and one or more di-block copolymer thickeners. lnsulation system according to claim 1 wherein the insulation layer comprises at least one porous sublayer and at least one non-porous sublayer. lnsulation system according to any one of the previous claims, wherein the porous and/or non-porous sublayers are wound, lapped or stacked in such a manner as to provide gaps between abutting layers. lnsulation system according to any one of the previous claims, wherein the porous sublayer is selected from cellulose paper, Kraft paper, foraminated polymer such as

3. PET, BOPET, PP, BOPP, PE, LLDPE, LDPE, l\/IDPE, HDPE, XLPE, PVC, polyester, aramid, polyimide, rubber, EPDM, EPR, silicone rubber, and mixtures thereof.

4. Gel for use in an insulation system of a mass-impregnated power cable, the gel comprising one or more mineral oils, characterísed in that the gel comprises one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners.

5. Gel according to claim 5, wherein the gel essentially consists of one or more mineral oils, a tri-block copolymer thickener, a di-block copolymer thickener and an antioxidant.

6. Gel according to any one of claims 5-6, wherein the tri-block copolymer thickener is selected from polystyrene-block-poly(ethylene-ethylene/propylene)-block-polystyrene

7. (SEEPS), polystyrene-block-poly(ethylene/butylene)-block-polystyrene (SEBS), polystyrene-block-poly(ethylene/propylene)-block- polystyrene (SEPS), enhanced rubber segment (ERS) polymers, S-EB/S-S, and mixtures thereof.

8. Gel according to any one of claims 5-7, wherein the di-block copolymer thickener is polystyrene-bIock-poly(ethylene/propylene).

9. Gel according to any one of claims 5-8, wherein the gel has a transition temperature up to 100 °C. 10 15 20 25 30

10.

11.

12.

13.

14.

15.

16. 14 Gel according to any one of claims 5-9, wherein the molecular weight of the di-block and tri-block copolymer thickeners are from about 10 kDa to about 1000 kDa. Gel according to any one of claims 5-9, wherein the gel has a viscosity value of 1 Pa-s or higher at a temperature below the transition temperature, and wherein the gel has a viscosity value of less than 1 Pa-s at a temperature above the transition temperature. A mass-impregnated power cable characterised in that it comprises an insulation system according to any one of claims 1-4. A mass-impregnated cable joint characterised in that it comprises an insulation system according to any one of claims 1-4. A mass-impregnated power cable or mass-impregnated cable joint according to any one of claims 12-13, wherein the cable or joint is arranged for use in high-voltage direct-current (HVDC) applications and/or high-voltage alternating current (HVAC) applications, preferably HVDC applications with rated voltages of 1650 kV or higher. insulation system for application in power systems and/or power devices such as power cables, transformers, capacitors, motors, switchgears, HVDC converter stations and HVAC substations, characterised that it comprises at least one or more lapped porous sublayers impregnated with a gel, said gel comprising one or more mineral oils, one or more tri-block copolymer thickeners and one or more di-block copolymer thickeners. A process for manufacturing a mass-impregnated power cable, the process comprising the following steps: a) Providing sublayers of the insulation layer; b) Winding, lapping or stacking the sublayers around a conductor in such a manner as to provide a wound insulation having gaps between adjacent sublayers; c) Providing a gel for impregnation ofthe wound insulation; d) lmpregnating the wound insulation with the gel to provide a mass- impregnated insulation layer; e) Optionally providing a moisture barrier around the mass-impregnated cable; f) Optionally providing a protection layer, such as an armour layer, around the mass-impregnated cable.