AN INSULATED ELECTRIC DIRECT CURRENT CABLE

Abstract: The invention relates to a direct current cable having a polymer based insulation system including an extruded low density polyethylene, LDPE, based composition, disposed around a conductor. The direct current cable has excellent properties with regard to space charge accumulation. The direct current cable is useful for direct current transmission and also for direct current transmissions where the option of polarity reversal is possible.

Description: An insulated electric direct current cable The present invention relates to an insulated electric direct current cable having a polymer based insulation system disposed around a conductor. The present invention also relates to the use of the insulated direct current cable for high voltage direct current transmissions. The direct current (DC) technology becomes economical for systems intended for transmissions over long distances as for when the transmission distance typically exceeds the length for which the savings on transmission equipment exceed the cost of the terminal power plant. The DC technology is also advantageous for connecting non- synchromc nets. Furthermore DC technology is commonly used m connection with windmills, etc. In general, the use of DC technology is expanding. An important benefit of DC operation is the virtual elimination of dielectrics losses, thereby offering a considerable gain m efficiency and savings on equipment. The DC leakage current is of such small magnitude that it can be almost ignored m current rating calculations, whereas m AC cables dielectric losses cause a significant reduction in current rating. This is of considerable importance for higher system voltages. Similarly, high capacitance is not a penalty m DC cables . Extruded solid insulation based on a polyethylene, PE, or a cross-linked polyethylene, XLPE, has for almost 40 years been used for AC transmission and distribution cable insulation. Therefore, the possibility of using XLPE and PE for DC cable insulation has been under investigation for many years. WO 99/44207 discloses an electric direct current cable having a polymer based insulation system comprising an extruded, grafted and cross-linked polyethylene, XLPE, based composition, disposed around a conductor and a method for production of such a cable. The PE is a peroxide initiated PE and the XLPE based composition comprises a polar modification m the form of a polar segment comprising a polar co-monomer. The polar co- monomer is a necessary additive m the XLPE composition m order to achieve the desired qualities of the insulation material especially with regard to space charge accumulation. A major problem m the use of polyethylene as insulation material for direct current cable so far, is the need for additives to create proper insulation qualities m the polyethylene especially with regard to conductivity, space charge accumulation and de-charge properties . At the present stage, these qualities have been improved m the polyethylene by adding additives such as maleic acid, polar species, dielectric species and others and/or incorporating monomers. The known DC polyethylene cables are normally manufactured from conventional AC polyethylene and have to be modified with the above- mentioned additives and/or incorporated monomers. It has now surprisingly appeared that it is possible to achieve an insulated direct current cable with low- density polyethylene with excellent qualities without the necessity to add helping aids such as dielectric species, polar species and others. An insulated direct current cable with good qualities with regard to insulation, conductivity and m particular space charge accumulation and which is easy to handle can be achieved at a relatively low cost. Another purpose of the invention is to achieve the same good qualities m the DC cable as m the above-mentioned WO 99/44207, but at lower costs and by less steps in the production line. Especially due to the fact that the known polyethylenes for cables are grafted, which is not the case with the polyethylene used according to the invention. The above-mentioned qualities are achieved with the insulated direct current cable according to the invention as defined in claim 1. With the insulated direct current cable according to the invention is provided a DC cable with excellent qualities, based on the unexpected fact that low-density polyethylene m a substantially homogeneously and substantially pure form has special qualities with respect to direct current insulation. The use of low-density polyethylene as insulation material m DC cables provides an insulated direct current cable as defined m the claims, having significant improved qualities with regard to conductivity and particularly with regard to space charge accumulation and de-charge properties. Furthermore, it is less complicated and less expensive to manufacture the cables, as none or only few additives m small amounts are required for the insulation polyethylene. The insulated electric direct current cable according to the invention can easily be adapted for transmission of 1-72 kV, 72-150 kV, 150-400 kV, 400-600 kV and over 600kV. The invention comprises an insulated electric direct current cable having a polymer based insulation system comprising an extruded low density polyethylene, LDPE, based composition, disposed around a conductor said polyethylene has a density from 0,825 to 0,920 g/cm J , preferably from 0,855 to 0,915 g/cm J and even more preferably from 0,865 to 0,910 g/cm J wherein the space charge accumulation after an initial space charge accumulation after voltage over the cable is interrupted fades to a value below 2 C/m J measured by pulsed electro acoustic method. The low level of space charge accumulation m the cable makes the cable extremely useful m many applications. Especially the cable is useful for DC transmission where the option of polarity reversal is possible. In a preferred embodiment of the insulated direct current cable according to the invention the space charge accumulation fades to a level below 2 C/m J withm 1000 minutes after voltage is cut of. Preferably the space charge accumulation fades to a level below 1 C/m within 1000 minutes, preferably 500 minutes after voltage is cut of. The low levels of space charge accumulation helps to avoid breakdown of the cable during reversal of the direct current. Preferably the insulated direct current cable according to the invention is able to resist reversal of polarity during a polarity voltage test without breakdown. Furthermore an optional breakdown of the cable is normally caused by voltage overload, which will usually cause breakdown of any type of cable. In a preferred embodiment of the insulated direct current cable the breakdown stress level m a polarity voltage test is substantially equal to the breakdown stress level m a DC voltage test, as the breakdown is caused by overload m voltage. In a preferred embodiment of the insulated electric direct current cable according to the invention, the polyethylene is a metallocene catalysed polyethylene. The resulting polyethylene is very pure and homogeneous. The metallocene catalysed polyethylene has been found to be particularly suitable for the insulated electric direct current cables according to the invention In order to achieve sufficient strength of the insulation layer on the insulated electric direct current cable it is preferred that the polyethylene is cross-linked, preferably m a degree of 40-95%, more preferably m a degree of 60-85%. Furthermore, it is preferred that the polyethylene is cross-linked by the use of a radical former. In a preferred embodiment the radical former is a peroxide, preferably dicumyperoxide . It is preferred that the radical former is added in an amount of about 0,2 to 5 % by weight, and more preferably about 1 to 2 % by weight. For the purpose of giving the polyethylene of the insulation layer on the insulated electric direct current cable according to the invention the best possible qualities with respect to durability and stability, it is preferred that the polyethylene comprises an antiscorch agent (anti oxidant, antidegradat ) such as substituted phenols, e.g. 4 , 4' -thiobis ( 6tert-m-crecol ) m an amount of 0,1 to 2% by weight. In order to make further improvements m the electric qualities m the insulated electric direct current cable according to the invention, the polyethylene m a preferred embodiment therefore comprises one or more oils m an amount up to 5% by weight, and preferably from 0,2 to 2% by weight. In a preferred embodiment the oil is dielectric oil selected from the group consisting of mineral and synthetic oils, where the synthetic oils are chosen among polyisobutylene, silicon oils and lower molecular PE waxes . The invention also comprises the use of an insulated direct current cable as described above and which is characterized m claims 1 to 10 for high voltage direct current transmissions. The insulated electric direct current cable according to the invention will now be described m further details with reference to a drawing, m which Fig. 1 shows an insulated electric direct current cable according to the invention, Fig. 2 shows the data for charge density measurement on a cable material according to the invention, Fig. 3 shows the curves for space charge measurements on two cables according to the invention and a traditional cable. As seen m figure 1 the insulated electric direct current cable 1 according to the invention comprises a conductor surrounded by three layers of material. The conductor 2, stranded or solid, of any desired shape and construction, such as stranded multi-wire, solid conductor or segmental built conductor is placed m the centre of the cable 1. An extruded semi-conductmg shield 3 is disposed around and outside the conductor 2. An extruded insulation layer 4 of a PE is disposed around the conductor 2 and semiconducting shield 3. An extruded outer semiconductor 5 is disposed around and outside the insulation layer 4. The three layers of extruded materials can be processed at conventional multi-layer extrusion equipment and cross-linked such as m a Catenary Continuous Vulcanising (CCV) line. Known techniques to obtain a cable from these electrical insulated cores can be used. This can include metallic screens, outer covering sheets, swelling and sealing layers or strips, wires of metal or polymer, as well as use of powder and bitumen products. As for the cable system this invention is also for use m joints of all sorts, including termination and starting joints as well as connections and SF6 joints. Example 1 The space charges were measured on flat samples, produced of a polyethylene used for a DC cables according to the invention. The space charges were measured using the Pulsed Electro Acoustics (PEA) method. The high voltage electrode m the PEA test set-up was a metallic cylinder. All the samples were tested at room temperature, and with constant electric stress. The flat samples were connected to two semi-conductive electrodes, one on each side. The two semi-conductive electrodes had a diameter equal to the diameter of the high voltage electrode. The semi-conductors were common carbon black filled compounds based on acrylic polymer. All samples had a thickness of about 1.6 mm and were tested with an electric field of 20 kV/mm direct voltage (DC) , which resulted m that the total voltage over one sample was approximately 32 kVDC. Figure 2 shows the measurement on a sample made of cross- linked metallocene PE for the DC cables according to the invention. It was a low crystalline PE with a density = 0,885 g/cm J , MFI = 2 g/10 mm, Tm = 80 C, and the cross- linking agent was dicumyl peroxide (dicup) . The resulting curves did not show the peaks indicating space charge, and thereby indicated the good qualities of the material for DC insulation. Example 2 Direct current cable insulation according to the invention was investigated for space charge accumulation by use of the pulsed electro acoustic method (PEA) . Space charge measurements performed by means of the Pulsed Electro Acoustic method are based on measurement of an acoustic wave caused by moving charges m a rapidly changing electrical field. A homogeneously distributed transient field is generated m the planar dielectric by application of a 20ns wide high voltage pulse. The field change causes a sudden electrostatic force simultaneously on all charges m the dielectric and on the electrodes, resulting m a pressure wave travelling through the dielectric, which is measurable outside the electrodes by means of a piezoelectric film. Depending on the sound of speed m the dielectric, the detected pressure profile as detected is a measure for the space charge distribution inside the dielectric. The system is calibrated for charge density m nC/mm J (=C/m J ) . Long-term aging tests can be performed automatically under voltages applied to the dielectric varying m magnitude and polarity. Two types of direct current cable insulation according to the invention, XLPE 01 and XLPE 02, was measured with PEA and compared with conventional AC XLPE insulation. The resulting measurements are seen on the curves m figure 3. It is clear that the space charge accumulation is lower according to the invention and fades to an even lower level than for conventional XLPE cable material. Example 3 60kV DC cables were produced and tested for electrical DC properties. Cable samples of 25 m were tested. A conventional AC XLPE cable was tested for reference. A DC withstand voltage test was performed at 20°C by raising the applied voltage with 30 kV every 30 minutes until breakdown. A polarity reversal DC withstand test was performed by raising the voltage 30 kV and then reversing the polarity instantly (within 30 seconds) and holding the voltage for 30 minutes. Then reversing the voltage, holding the voltage for 30 minutes, then raising the voltage with 30 kV until breakdown. The point of breakdown was observed. It was also observed if the breakdown appeared during reversal or during increase of the voltage. The results of the tests are seen m table 1. Table 1 The cable performance showed a similar DC withstand voltage as for the conventional AC XLPE, but surprisingly the polar reversal test showed even better performance for breakdown and for time of breakdown. The conventional AC XLPE cable did not withstand the reversal as well as the cables according to the invention, while the conventional AC XLPE cables broke down during reversal. The breakdown m the cables according to the invention is caused by high voltage. This important difference is mainly caused by the difference m space charge accumulation and the fact that the space charge fades away, as described m example 2.

Claims: Patent Claims : 1. An insulated electric direct current cable having a polymer based insulation system including an extruded low density polyethylene, LDPE, based composition, disposed around a conductor, said polyethylene having a density from 0,825 to 0,920 g/cm J , preferably from 0,855 to 0,915 g/cm J and even more preferably from 0,865 to 0,910 g/cm J wherein the space charge accumulation after an initial space charge accumulation after voltage over the cable is interrupted fades to a value below 2 C/m J measured by pulsed electro acoustic method. 2. An insulated direct current cable according to claim 1 wherein the space charge accumulation fades to be below 2 C/m J within 1000 minutes after voltage is cut of. 3. An insulated direct current cable according to claim 1 wherein the space charge accumulation fades to be below 1 C/m" withm 1000 minutes, preferably 500 minutes after voltage is cut of. 4. An insulated direct current cable according to claim 1 wherein the direct current cable is able to resist reversal of polarity during a polarity voltage test without breakdown. 5. An insulated direct current cable according to claim 1 wherein the breakdown stress level a polarity voltage test is substantially equal to the breakdown stress level m a DC voltage test. 6. An insulated electric direct current cable according to any of the preceding claims wherein said polyethylene is metallocene catalysed polyethylene. 7. An insulated electric direct current cable according to any of the preceding claims wherein the polyethylene is cross-linked, preferably a degree of 40-95%, more preferably a degree of 60-85%. 8. An insulated electric direct current cable according to claim 7, wherein the polyethylene is cross-linked by the use of a radical former. 9. An insulated electric direct current cable according to claim 8 wherein said radical former preferably is a peroxide, more preferably dicumyperoxide . 10. An insulated electric direct current cable according to claims 8 and 9 wherein said radical former is added m an amount of preferably 0,2 to 5% by weight, more preferably 1 to 2% by weight. 11. An insulated electric direct current cable according to any of the preceding claims wherein the polyethylene comprises an antiscorch agent (anti oxidant, antidegradat ) , preferably m the form of one or more substituted phenols, more preferably the form of 4,4'- thiobis (6tert-m-crecol) , said antiscorch agent preferably being incorporated the polyethylene m an amount of 0, 1 to 2% by weight. 12. An insulated electric direct current cable according to any of the preceding claims wherein the polyethylene comprises one or more oils m an amount up to 5 % by weight, preferably from 0,2 to 2% by weight. 13. An insulated electric direct current cable according to claim 12 wherein the one or more oils are one or more dielectric oils selected from the group consisting of mineral and synthetic oils. 14. An insulated electric direct current cable according to claim 13 wherein the one or more synthetic oils are selected from the group consisting of polyisobutylene, silicon oils and lower molecular PE waxes. 15. Use of an insulated direct current cable according to claims 1 to 14 for high voltage direct current transmissions .

Inventors: HJERRILD JESPER, JENSEN ANDERS, STROEBECH ESBEN, HJERRILD, JESPER, JENSEN, ANDERS, STROEBECH, ESBEN
Applicants: HJERRILD JESPER, JENSEN ANDERS, NKT RES CT AS, STROEBECH ESBEN, HJERRILD, JESPER, JENSEN, ANDERS, NKT RESEARCH CENTER A/S, STROEBECH, ESBEN
Application year: 2001
Main classification: H01B3
IPC codes: H01B3