B EHV XLPE CABLE SYSTEMS UP TO 400 kv - MORE THAN 10 YEARS FIELD EXPERIENCE

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1 21, rue d'artois, F Paris B1-102 Session 2004 CIGRÉ EHV XLPE CABLE SYSTEMS UP TO 400 kv - MORE THAN 10 YEARS FIELD EXPERIENCE W.G. Weissenberg U. Rengel R. Scherer Brugg Kabel AG Elektrizitätswerke Nordostschweizerische des Kanton Zürich Kraftwerke (Switzerland) (Switzerland) (Switzerland) 1 Introduction Throughout the world a growing use of cross-linked polyethylene (XLPE) as insulation material can be recognised. In HV and EHV cables, this trend is supported by the cleanness achieved in the polyethylene material and the improvement in technical experience at the cable manufacturer. A high level of safety and reliability for the accessories has been achieved as a result of prefabrication technology and pre-testing of insulating bodies. In Switzerland, the first 400 kv cables with insulation made of cross-linked polyethylene were installed in the power transmission network at the start of the nineties in the last century. The accessories used for this purpose also included pre-fabricated and pre-tested slip-on components made of silicon. These systems are running smoothly up to date. The article describes the many years of experience that have now been accumulated with EHV cable systems using cable insulation made from cross-linked polyethylene and accessories with silicon insulation. Methods for diagnostic measurements have seen further development in recent years; for this purpose, some of the systems have been retrofitted with sensors to enable applications such as on-site PD measurement for the accessories with nominal voltage, with measuring sensitivity of < 2pC. On the basis of all these positive experiences with EHV cable and accessories in service and in prequalification tests for 420 kv, the article reports on the continuing development of cable design thanks to the optimization of wall thickness and field strengths. Likewise, the article describes experiences with sensors for temperature measurements over the whole cable length. There is a critical discussion of possibilities for the further development of the accessories and their installation. The increased requirements for AC and PD testing for cables and W.G. Weissenberg, Brugg Kabel AG, Klosterzelgstrasse 28, CH-5201 Brugg, [email protected]

2 accessories during routine tests in the factory but also in the cable system after it has been laid are highlighted. 2 Experience with EHV-XLPE cable systems The electrical lifetime of EHV cables with insulation made of cross-linked polyethylene (XLPE) is determined by internal and external influences while operational loads are applied. We are basically familiar with two electrical ageing processes: partial discharge ageing and field ageing /1,2/. PD ageing due to discharge processes in cavities very quickly leads to an electrical breakdown of the XLPE insulation. For this reason, PD measuring technology for the routine testing of EHV- XLPE cables and prefabricated accessories /3/ has undergone especially intensive development, making it into a reliable form of post-production quality control. Developments are in progress all over the world as regards on-site PD testing of cable systems (see Chapter 4). Field ageing is based on spurs and occlusions in the XLPE insulation, and it can only be evaluated by means of a highvoltage test. This is why PE materials of maximum purity and extremely clean manufacture are prerequisites for the production of EHV cable systems. Fig. 1 Production of XLPE cables by Brugg Kabel AG, Switzerland Fig.2 PD-On - Site-Test at UW Bonaduz/Switzerland Lengthy experience /4,5/ of manufacturing HV cables with insulation made of cross-linked polyethylene and of constructing cable systems up to 275 kv provided the best basis for the manufacture of 400 kv XLPE cables (Fig. 1). The first 400 kv cable with insulation made of cross-linked polyethylene (XLPE) and a corrugated copper sheath was produced in At Nordostschweizerische Kraftwerke, we were able to set up the first high-voltage cable system with XLPE insulation in the 380 kv network on the site of the Bonaduz/Grisons substation in spring The cable system initially comprised two outdoor terminations and about 200 m of cable. After five years of operating this cable system in the 380 kv network with no damage, the cable was cut in mid-length, a prefabricated and pre-tested straight joint with silicone elastomer (SiR) insulation was fitted, and operation of the system was resumed.

3 Fig.3 Test set-up for on-site-pd-measurement on 380- kv-joint in UW Bonaduz/Switzerland After a further three years of operating the network without faults, the joint was diagnostically tested with a new broad-band partial discharge measurement system (Figs 2, 3) in This new testing process based on the directional coupler sensor method /6,7,8/ allows reliable recording of possible PD ageing processes in the joint, even in unscreened conditions. We were able to prove that our joint is free from partial discharge after many years of operation at the network voltage, i.e. that no damaging processes which could shorten the lifetime took place in the joint. After the measurement, network operation was continued until today using our cable system. More than 1950 km of cables and 6900 accessories up to 400 kv have been installed in the last 10 years, and they have displayed excellent operating behaviour (Tables 1 and 2). Table 1 Experience in HV- and EHV-XLPEcables Table 2 Experience in prefabricated and pretested accessories 3 Improvements Over recent years, the qualitative properties of the XLPE compounds for EHV cables have been substantially improved. Table 3 Highest permissible concentration (#/kg) of contanimants for selected size classes /16/

4 Fig. 4 Effect of metallic contaminant size on the ac break down strength of minicables and distribution cables The size and number of metallic and non-metallic impurities in the compound have been considerably reduced, so that they have now attained the levels shown in Table 3 /16/. The size and number of the impurities determine the electrical breakdown strength of the XLPE insulation. In the area surrounding the fault location, the field strength increases and locally accelerated field ageing takes place. The influence of the size of these impurities on the a.c. voltage breakdown strength was investigated using model cables /9/ and real high-voltage cables /2/; this is shown in Fig. 4. Due to the volume effect with regard to the breakdown probability, and to the desired failure probability for a cable in the energy distribution network of 1 fault per 50 year in 100 system kilometres, the values determined on model cables with insulation measuring only 1.5 mm thick could not be used as a field strength for the purposes of determining the design. The critical sizes for impurities in the XLPE insulation for EHV cables are shown in relation to the maximum field strength on the conductor in Fig. 5. Fig. 5 Critical size of contaminant in the XLPE-insulation /2/ Fig.6 Log.Number of Particles and Size per 1kg PE with the same Risk of electrical Breakdown Fig. 6 shows the influence of the number and size of the impurities with the same risk for an electrical breakdown. These results show that not only the size, but also the number of impurities per kg exert a strong influence. For these reasons, and in accordance with the high specified design field strengths of >10 kv/mm, insulation material of class EHV Super Clean, with purity levels as shown in Table 3, is used to manufacture EHV-XLPE cables. The proven high purity of the compound has made it possible to use higher design field strengths and to reduce the insulating wall thicknesses (Fig. 7). So that the maximum field strength always remains the same for a given nominal voltage, the insulation wall thicknesses were dimensioned in relation to the conductor cross-section (Fig. 8)

5 Fig. 7 Max. electrical strength in XLPE- Fig. 8 Wall thickness of insulation of 400 kv- XLPE-Cables Cables The manufacture of EHV-XLPE cables using horizontal technology has now been perfected and is reliable. Clean material handling and precise temperature and pressure monitoring during extrusion, dry curing and cooling guarantee a high standard of quality during manufacture. Fig. 9 Assembly of a prefabricated slip-on joint Fig PD-On-Site-Measurement-Systems: Directional Coupling Sensors and a Directly Modulating Discharge Measurement System When installing prefabricated and pre-tested joints and stress cones made of silicone elastomers, use is also made of innovative technologies such as slipping the joints on with the help of a gas flow (Fig. 9), and homogenisation of the cable insulation after removal of the outer semiconductive layer by means of thermal treatment. 4 Tests After successful development and type tests on cable systems, a pre-qualification test was carried out with 400 kv at CESI in Milan. The test loop consists of 120m of cable with an HDPE outer sheath and laminated aluminium foil (1600mm 2 ) and 120m of corrugated copper sheath cable (1600mm 2 ), two outdoor terminations (porcelain and composite insulators), two straight joints and two back-to-back GIS terminations. The test set up is shown in Fig.10. The cables were laid section by section in sand, pipe systems, in a tunnel zone and in the air. An optical fibre was positioned along the test section to monitor the temperature (Fig. 11). The first heating cycles showed that there is better heat dissipation in the cable with the HDPE outer sheath and laminated aluminium foil than in the cable with the corrugated copper sheath. Additional thermal insulation for the cable with the HDPE outer sheath and laminated aluminium foil enables the

6 two cables to heat up to the maximum desired conductor temperature with the same heating current in the conductor. The pre-qualification test was successfully completed in May To measure partial discharge during the test, PD sensors based on the directional coupler sensor method were built into all accessories; good experience had already been gained with these sensors for on-site PD measurements. In addition, another new Directly Modulating Discharge Measurement System /10,11,12/ (which needs no active components in the accessories) was built in for test purposes (Fig. 12). Fig kv prequalification test set-up

7 Fig. 11 Monitoring of temperature A directional coupling sensor having one side terminated into 50 Ω couples energy from the PD pulse directly to an electrooptic modulator. The resulting modulated optical signal can be transmitted over large distance (km) due to the low loss of single-mode fibres. Such a system needs no electrical power at the location of the sensor, can be made very reliable, and can be buried underground. Thus, it is an attractive choice for PD monitoring. /10/. Good experiences have also been gained with on-site diagnostic PD measurements on GIS terminations during operation with a UHF-PD measuring method /13/ and on cross-bonding links in EHV cable systems /14/. 5 Conclusions All these tests and many years of practical experience have shown that solid XLPE and siliconepolymer insulations as used in EHV cables and accessories are characterised by long electrical lifetimes (Figs. 13, 14) /15/. When they are subjected to electrical loads at network voltage, an electrical lifetime of well over 50 years can be expected. However, if the electrical ageing is accelerated by partial discharges, early failures may occur. This is why it is very important for the cables and accessories to be free from partial discharges. The PD measurement as a routine test for cables and prefabricated HV and EHV cable accessories is the first important step towards quality and towards proving long-term electrical stability.

8 As regards electrical tests after the installation of a cable system, the standards do not generally specify a PD measurement as yet. For the post-installation electrical test, if the PD measurement is used to prove that the joints and terminations are free of PD, a lower test value can be used for the a.c. voltage test, thus ruling out the possibility of advance damage to the newly-produced cable joint as a result of the commissioning test. The general technical trend for the assessment of accessories in EHV cable networks is moving towards diagnostic assessment of freedom from PD. A very varied range of methods is nowadays in successful use, and these should be taken into account when Standard IEC is revised. Fig. 13 Life curve of XLPE-insulated HV and EHV cables

9 Fig. 14 Life curve for accessories 6 References /1/ v. Olshausen, R.; Weissenberg W.: The electrical long-term performance of crosslinked polyethylene, 30 WIRE 5/2001. /2/ Weißenberg, W.: Einfluß makroskopischer Fehlstellen auf die elektrische Alterung von Polyethylenkabeln bei Wechselspannungsbelastung. Thesis TU Dresden /3/ Weissenberg W.; Kuschel, M.: Test methods for SiR-accessories used in high voltage cables up to 400 kv, 6 th Jicable, Paris 2003, paper. /4/ Badent, R.; Eisebraun, T.; Hansen, R.; Schwab, A.; Schwing, U.: VPE-Kabelanlage für eine 220- kv-energieableitung, Elektrizitätswirtschaft, Heft 6/1998 /5/ Ritter, L.: Scherer, R.; Laurent, M.; Bautz, R.; Zimmermann, H.: HV cable networks in Switzerland Particularities and Experience, CIGRE session 1994, Paris (1994), Paper /6/ D. Pommerenke, T. Strehl, R. Heinrich, W. Kalkner, F.Schmidt, W.Weißenberg: Discrimination between Internal PD and other Pulses using Directional Coupling Sensors on High Voltage Cable Systems, IEEE Transactions on Dielectrics and Electrical Insulation, Vol.6, No 6, December 99, pp /7/ D. Pommerenke, T. Strehl, W. Kalkner : Directional Coupler Sensor for Partial Discharge Recognition on High Voltage Cable Systems, International Symposium On High Voltage, ISH 1997, Montreal, Canada.

10 /8/ D. Pommerenke, I. Krage, W. Kalkner, E. Lemke, P.Schmiegel: On-site PD measurement on high voltage cable accessories using integrated sensors, International Symposium On High Voltage, ISH 1995, Graz, Austria. /9/ Bostrom, J-O.; Marsden, E.; Hampton, R.N.; Nilsson, U.; Lennartsson, H.: Electrical Stress Enhancement of Contaminants in XLPE Insulation Used for Power Cables, IEEE Electrical Insulation Magazine July/August 2003 Vol. 19, No.4. /10/ D. Pommerenke, K. Masterson : A Novel Concept for Monitoring Partial Discharge on EHV- Cable System Accessories Using no Active Components at the Accessories, Dielectric Materials, Measurements and Applications Conference Publication No. 473, IEE /11/ K.D. Masterson, D.R. Novotny, K.H. Cavecy : Standard antennas designed with electrooptic modulators and optical-fibre linkage, Intense Microwave Pulses IV, H.E. Brand, ed, Proc. SPIE, Vol , pp /12/ A. Donval, E. Toussaere, R. Hierle, J. Zyss : Polarization intensitive electro-optic polymer modulator, J. applied Physics, Vol. 87, No.7, April 2000, pp /13/ Lemke, E.; Elze, H.; Weissenberg W.: Experience in PD diagnosis tests of HV cable terminations in service using the ultra-wide band PD probing, 13 th ISH 2003, Delft, paper. /14/ W. Weissenberg; F. Farid; R. Plath; K. Rethmeier : On-Site PD Detection at Cross-Bonding Links of HV Cable Systems, CIGRE 2004, paper /15/ R. van Olshausen; E.F. Peschke; R.G. Schroth : Development of EHV XLPE Cables: Dimensioning and Test Philosophy, CIGRE 1990, Paper /16 / J-O. Boström; A, Campus; R.N., Hamton; E., Marsden: Reliable HV & EHV XLPE cables, CIGRE 2002, Paper