FP_B.1_BorougePte Ltd _Medium voltage XLPE cable for long and reliable service life

FP_B.1_BorougePte Ltd _Medium voltage XLPE cable for long and reliable service life Medium Voltage XLPE cable for long and reliable service life Hakan Lennartsson, Borouge Pte. Ltd, hakan.lennartsson@borouge.com Abstract Crosslinked polyethylene, XLPE has become the preferred insulation material for medium voltage cables and has been used in cable applications for about 50 years. The first generation of XLPE insulated cables showed early field failures typically after 5 to 15 years in service. The degradation mechanism reported for XLPE cable installed in wet conditions and under electrical stress is commonly referred as water treeing. The established test protocols for wet electrical ageing can be found in specifications such as the AEIC/ICEA and Cenelec, which cover accelerated ageing in wet conditions, typically for a period of one to two years respectively. The testing time can be reduced to approximately four months, by increasing the test frequency to 500 Hz and keep the cable immersed in water, and this fast testing protocol is described in Cenelec HD 620 S2. The major part of medium voltage underground cable installed in Asia use PVC jacketing, while European and American utilities has a preference for linear polyethylene jacketing. The reason for selecting PE jacketing is enhanced mechanical protection combined with improved barrier properties to minimize ingress of moisture into the cable core. Modern underground cable installed and operated according to good industry practice should have an indicated life span of 40 years or more. The purpose of this paper is to share some learning from the first generation medium voltage cable failures and discuss the important characteristics and performance of materials for extruded medium voltage cables, some different options in cable design as well as importance of testing and inspection. Key Word: XLPE Cable, Medium Voltage, Reliability, Long life, Jacketing Water treeing It is not the intention to go deep into the details of the electrochemical mechanism causing water treeing, but understanding of how various factors influence performance and service life can help to mitigate the risk. The water treeing process is driven by electro oxidation, which takes place in the direction of the local electrical field. The water tree growth takes place in amorphous regions in the insulation and contaminants or protrusions are sites where the electrical stress is increased. Polymer chains are broken and a tree structure is formed with water condensation and the defect is growing with time [1]. Major factors influencing water tree growth: Presence of water - water treeing does not seem to take place at relative humidity below 70% [2] [3]. Electrical stress - low voltage insulation does not tend to suffer from water tree failures. High voltage cable design has typically metallic moisture barriers Contaminants and protrusion. These are initiation sites for water treeing, due to increased local electrical field. Presence of ionic specimen will accelerate the tree propagation. Carbon black in screen compounds can be a source of ionic specimen and protrusions and thus it is of importance to test the cable core to confirm compatibility between XLPE and semi-conducting screen. There are two different types of water trees: Bowtie water trees are typically located within the XLPE insulation from contaminant, void or any other defect and has the shape as a bowtie Vented water trees are originating from the semi-conductive screen/insulation interface. Poor smoothness with pips increase the local electrical stress and Vented trees are considered to be more harmful to electrical failures as the can grow much longer than bowtie trees

Figure1. Vented water tree from conductor screen Figure 2. Bow tie tree in insulation Improvements of cable quality and reliability The wire and cable industry together with material suppliers has made a lot of improvements in a number of areas which are contributing to reduce risk of failure and help to improve cable service life. The areas of improvement relate to cable manufacturing, cable design, cable materials and cable testing. Cable manufacturing The first generation of continuous vulcanization (CV) lines used steam for curing and during cooling of the cable the steam could condense inside the XLPE insulation forming micro voids. Halos may be seen particularly in the middle of the insulation and measurements indicated high water content [4]. Introduction of dry curing using nitrogen at elevated pressure has significantly reduced risk of micro void formation in medium voltage cable insulation and has become good industrial standard in the cable industry. The extrusion process has also been optimized by development of triple extrusion, where all layers of the cable core is formed in the extruder head to minimize risk of defects or contamination. Triple extrusion technique combined with smoothness improvements of particularly conductor screen material has significantly improved the interface quality, reducing number of pips and sites for local electrical stress increase. Improvement in material handling at cable producers has progressed and today the practice is to use clean room and closed systems to transfer of XLPE insulation from package to extruder hopper. However there are still influence and differences between different cable manufacturers as shown by a study by researchers at State Grid in China. Five cable manufacturers produced cable core using commercial XLPE grades and the cables was subjected to accelerated water tree test, AWTT, according to DL/T 1070-2007. Three of the test cables come out with good performance while two cables showed performance below expectations [5]. Cable design The first generation of XLPE cable produced in America had very simple design with a semi-conducting tape screen applied helically over the conductor and graphite paint and semi-conductive tape as insulation screen. As mentioned in earlier section this weakness with screens were addressed by introduction of extruded screens and triple extrusion. Furthermore absence of extruded polymeric jacket and underground installation caused high failure rate. The importance of PE jacketing was demonstrated by researchers at Hydro Quebec. They compared water uptake for cable with and without LLDPE jacket. The cable samples were immersed in a water tank and subjected to temperature cycling and water up take in the XLPE core was measured. The cable with LLDPE jacket significantly reduced water ingress into the insulation [6]. These results confirm that proper extruded PE jacketing is the first defense in reducing moisture ingress into the cable core and support improved cable reliability. Water swelling tape in combination with extruded jacket of MDPE and PVC respectively, was studied to establish how it can keep relative humidity controlled. Calculation shows the superiority of MDPE jacketing compared to PVC jacketing [2] [3]. Water blocking design has been developed by applying water swelling design elements. Longitudinal water blocking is used to minimize water penetration along the cable and minimize length of cable damage. Yarn or powder in the conductor to block water penetration along the conductor Water swelling tape outside the cable core to minimize water penetration along the cable core Some cable design use longitudinally applied Aluminum/polyethylene (Al/PE) tape for minimizing radial water diffusion to the cable core. Al/PE has an overlap and is commonly combined with water swelling tape.

Cable materials The first generation of XLPE insulations was based on low density polyethylene, LDPE, typically a film grade with addition of antioxidant and crosslinking agent. However the awareness of cleanliness and methods for industrial inspection of cleanliness were not available. Over the years the technology for making very clean and consistent material has developed and the state of the art in XLPE manufacturing is closed loop technology where the quality and consistency is monitored during the route from ethylene to XLPE compound. The latest development in XLPE insulation is to design special base resin to balance extrusion properties as well as curing characteristics. The development of specially designed polymers and optimized XLPE compounds support improvements in productivity and over all economics of cable manufacturing [7] [8]. Development of XLPE materials designed to suppress the tendency for water tree formation can be made by modification of the characteristics of the polymer. There are two concepts which have been commercially used for more than 30 years: Europe polymer modification of LDPE or sometimes referred as copolymer used to reduce tendency for water tree formation [9] [10]. XLPE copolymer insulation is used with bonded insulation screen Americas - chemical additive to LDPE used to reduce tendency for water tree formation [11]. The XLPE insulation with chemical additive is used in designs with strippable insulation screen Unfortunately, inclusion of polar additives increase risk of electrical losses and this limit use in high voltage application. Semi-conductive screen materials are compounds with controlled conductivity typically based on copolymer, carbon black and additives [12], [13]. The major requirements for good performance are: Smooth interfaces requiring good quality of carbon black and optimized compounding operation Chemical cleanliness particularly referring to level of ions and sulfur content from carbon black The type of carbon back and the compounding process has major impact on the material characteristics, performance and service life in cable. Acetylene black, often referred as super smooth, show the best characteristics with respect to smoothness and chemical cleanliness. Acetylene based screens are still preferred by American utilities, but unfortunately the economics of the manufacturing process makes it a very expensive raw material and the use in conductor screen application is decreasing. Furnace carbon black is today the most commonly used carbon black in medium voltage screen applications, and these are based on selected clean furnace black. The quality of furnace CB varies significantly with respect to chemical cleanliness and some utilities have today requirement for ion content and pip size of screen materials. Studies has also shown the importance of verifying performance of the specific grade as also other additives may have influence on water tree growth and performance [14]. This highlights the need to do qualification for a specific material combination conductor screen/insulation/insulation screen and if any material is changed a new qualification test is recommended. Jacketing is of importance for protection of the cable core from manufacturing, during storage transport and installation and during the entire service life and both PVC and Polyethylene has been used over the years. Polyethylene based jacketing has benefits with mechanical robustness minimizing damages at installation and better moisture barrier properties for underground installation. The development of polyethylene jacketing has focused on reducing tendency to environmental stress cracking, improved process performance and controlled shrinkage behavior and excellent weather resistance of black compounded formulation. It is important to recognize that the excellent properties are achieved with virgin polymer and fully compounded jacketing material. Studies on recycling of materials, suggests that jacketing is one application for recycled material. However research on performance of jacketing materials in China has identified very large variations in quality level due to use of recycled jacketing materials. The defects are ranging from poor stability, voids, low CB content and large content of inorganic matter, all defects which would have significant impact on performance level and most likely also on service life [15], [16]. The development of high quality material solutions and drive for reduced cost has resulted in review and update of medium voltage cable design. Work done by utilities in Europe had focus to harmonize specifications and develop cost efficient cable design based on performance specification and utilizing improved manufacturing technology and improved material solutions[17], [18]. All of the suggested cable design used polyolefin cable jacketing. Also research organizations in South Korea has evaluated jacketing materials for underground power cable jacketing and found that polyethylene has preferred performance versus PVC [19].

Wet electrical cable testing Accelerated tests are important tools to evaluate performance of cable materials. A number of laboratory test methods using small test objectives have over the years been developed to study water treeing resistance of XLPE materials. These tests can give guidance on material performance, but has limitations as the samples are different from the cable case. The evaluation methods discussed are all based on cable core design including conductor screen, insulation layer and insulation screen. The reason to use full design is that there are significant influences from material combination as well as there are also influences from cable manufacturing technology and material handling. Testing of cable specimen support cable engineers with valuable information in cable development as well as these methods support documentation, qualification and cable manufacturing quality control. Model cable testing use a small model cable for wet electrical ageing and was originally developed in Germany early 1990s [23]. The method has been further developed by Borealis to use in development and documentation of material in the innovation process of new insulation system [24]. The cable is a three layer cable core but with significant reduced dimensions and manufacturing is done on a laboratory CCV line equipped with triple extrusion head and using dry curing (Nitrogen). The cable design uses a solid 1.5 mm 2 copper conductor. The insulation system consists of the insulation screen with thickness of 0.70 mm, the XLPE insulation thickness is 1.5 mm and the insulation screen thickness is 0.15 mm. The model cable is cut into 2 meter lengths and the insulation screen is removed at both ends to leave an active length of 0.50 meter. The copper conductor is pulled out (stretched) and replaced by a thinner copper wire to make it possible to introduce water along the copper conductor. Five samples is placed in the water bath with a temperature of 70 C and water reservoirs are mounted on the cable ends to ensure water along the conductor. Ageing at 9 kv and 50 Hz with a conductor temperature of 85 C for 1000 h and summary of ageing parameters is given in table 1. The ageing is followed by AC break down strength testing with linear ramp up of 100 kv/min and the characteristic breakdown strength of the five samples is calculated. Microscopic inspection of 0.50 mm thick slices taken close to the breakdown position and stained with methylene blue gives additional information. The water tree density and length of longest bow tie tree and vented tree in each slice is determined. Working over many years the model cable testing has generated a comprehensive database and been a very helpful tool in material development and documentation of new insulation system. Parameter Value Applied voltage 9 kv Frequency 50 Hz Mean stress 6 kv/mm Water In conductor and outside Conductor temperature 85 C Water bath temperature 70 C Ageing time 1000 h (42 days) Table 1 Model cable test for water treeing, summary of test parameters Full size cable test AEIC/ICEA S94-649 Accelerated Water Tree Test, AWTT, cable core qualification is the preferred test protocol for accelerated wet electrical ageing in North Americas. Similar test protocol has also been established in China and South Korea and gain acceptance to monitor cable quality with respect to water treeing. AWTT is performed on standard AEIC 15 kv cable cores, typically using a 53.5 mm 2 conductor area and 4.45 mm insulation thickness. The cable samples are preconditioned for 14 days with 8 hours of conductor heating, before testing, to remove all crosslinking by-products. The cables are aged at three times the rated voltage to ground, U 0, in water filled tubes. Water is also introduced to the stranded conductor and maintained during the test. Tap water is used both in tube and conductor strands. The cables are load cycled with conductor current, 8 h on and 16 h off. The current loading cycling is adjusted such that the temperature at the insulation screen temperature in water reaches 45 C at the end of the 8 hour heating cycle. The cables are loaded five days on and two days off. AC breakdown is tested on three samples each after cyclic ageing and after 120, 180 and 360 days of ageing in water respectively. Microscopic investigation of slices from the insulation is done to determine frequency and size of water trees.

Parameter AEIC/ ICEA S94-649 Applied voltage 3 U 0 / (26.2 kv) Frequency 60 Hz Mean stress 6 kv/mm Water In conductor and outside Current load cycling 8 hours on & 16 hours off/ 5 days on & 2 days off Water temperature in tube Not controlled Ageing time 360 days AC breakdown testing 120, 180 and 360 days respectively Table 2 Summary of test conditions for AWTT test according to AEIC/ICEA Insulation type Minimum AC breakdown strength kv/mm 120 days AWTT 180 days AWTT 360 days AWTT TR- XLPE 26.0 22.8 15.0 XLPE 11.8 N.A N.A Table 3 Requirement for breakdown test after AWTT test according to AEIC/ICEA European development and Cenelec specifications The association for the electrical industry in Europe, UNIPEDE, collected information about failure statistics of medium voltage cables during 1980s & 1990s and found unexpected premature failures of installed XLPE cables. They recognized that the existing standards and test program was not sufficient for long reliable service life and suggested a long duration test to address the problem with water treeing. Different testing protocols were developed and gained acceptance by utilities over the years. The most accepted long duration qualifications were testing protocols from VDE and UNIPEDE respectively. Both these protocol were used in parallel over more than 20 years and the wet electrical ageing time is two years. The reason to use 2 years was that this ageing time was needed to distinguish between good and bad cable. Accelerating factors: UNIPEDE 3 U 0 at 50 Hz with immersion in water at 30 C for 2 years VDE 4 U 0 at 50 Hz with immersion in water at 50 C for 2 years Comprehensive studies and comparison of data resulted in CENELEC harmonization document between the method released around 2005 and the chosen ageing parameters were 3 U 0 at 50 Hz with immersion in water at 40 C for 2 years. It was recognized that it would be favorable to reduce the testing time and Kema initiated work to use increased frequency for accelerating the ageing of cables and work to verify the new test method was done in CIGRE working group 21-11 under study committee 21 (SC21) [25]. The test conditions chosen were 3 U 0 at 500 HZ with immersion in water at 40 C for 3000 hours. Parameter CENELEC 50 Hz CENELEC 500 Hz Applied voltage 3 U 0 3 U 0 Frequency 50 Hz 500 Hz Mean stress 6 kv/mm 6 kv/mm Water Outside Outside Current load cycling No load cycling No load cycling Water temperature 40 C 40 C Ageing time 360 & 720 days 3000 hours AC breakdown testing 360 & 720 days respectively 3000 hours Table 4. Summary of test conditions for wet electrical testing according to CENELEC at 50 Hz and 500 Hz respectively.

Requirement CENELEC 50 Hz CENELEC 500 Hz Standard requirement All 6 samples > 14 kv/mm All 6 samples > 14 kv/mm Min 4 samples > 18 kv/mm Min 2 samples > 22 kv/mm Min 4 samples > 18 kv/mm Min 2 samples > 22 kv/mm High requirement, Germany All 6 samples >23 kv/mm Alternative 1 Min 4 samples > 29 kv/ mm Min 2 samples > 35 kv/mm High requirement, Germany Alternative 2 All 6 samples >29 kv/mm Table 5. Summary of electrical break down requirement for wet electrical testing according to CENELEC at 50 Hz and 500 Hz respectively. Results Learning of accelerated water tree testing There are different solutions for manufacturing of reliable medium voltage cable and there are many contributing factor in materials as well as in cable manufacturing technology. There are different test protocols for wet electrical ageing which has been accepted as qualification tools and accepted as documentation of enhanced performance. It needs to be recognized that the test result refers to the tested cable core i.e. the combination of material used in the cable core; if a component is changed it is relevant to make a new test or additional testing. Model cable testing Comparison of insulation materials Semi-conductive layer the same in all test - selected furnace black and bonded design AC Breakdown strength (kv/mm) Longest vented water tree (µm) Standard XLPE (homo-polymer) 38.2 1020 Polymer WTR XLPE 52.2 620 Additive WTR XLPE 65.3 375 Model cable testing Comparison of semi-conductive scree Insulation standard XLPE (homo-polymer) and bonded design Selected Furnace CB 38.5 1020 Acetylene 50.8 650 Table 6. Results from Model cable testing in material development comparing different formulations. The results show differences in material performance. It confirms the importance of compatibility of materials and that all materials in the cable core has influence of wet electrical ageing. Material High Productivity XLPE Copolymer modified XLPE Cenelec test 50 Hz 500 Hz 50 HZ 500 Hz Electrical breakdown 42.4 42.5 49.1 53.5 strength kv/mm Vented tree size µm <200 <300 <100 <200 Conductor screen Vented tree size µm 0 0 0 0 Insulation screen Bow tie tree size µm <150 <300 <150 <200 Table 7. Comparison of test results from Cenelec test protocol at 50 Hz and 500 Hz respectively. The results are based on multiple tests. Cables tested have bonded insulation screen and standard semi-conductive screen based on selected furnace black.

High Productivity Figure 3.Comparison of test results for high productivity XLPE with requirement in CENELEC 500 Hz testing. The results confirm very good performance and all results are significantly above the requirements. Figure 4.Results from AWTT testing according to AEIC/ICEA protocol of XLPE with WTR additive. All results are well above the requirement High Productivity XLPE XLPE WTR additive Figure 5.Comparison of test results for high productivity XLPE and formulation using WTR additive at testing in accordance with CENELEC 500 Hz.

The experience and track record show that materials can be ranked according to performance: Low performance Not optimized XLPE grades (standard XLPE and less good cleanliness management) combined with low cost semi-conductive screens or less optimized cable manufacturing. The first generation show water tree growth and problems commonly starts to occur after 10 years of service. Enhanced performance High productivity clean material produced by closed loop technology, (ethylene to compound) and with selected semi-conductive screens and a cable manufactured according to good industrial pass testing according to Cenelec 500 Hz testing with good margins Additive modified XLPE material in combination with selected quality semi-conductive screens pass testing according to AEIC/ICEA and Cenelec with good retained electrical break down strength and low count of water tree Copolymer modified XLPE and bonded selected semi conductive screens pass testing according to Cenelec 50 & 500 Hz with very good retained electrical break down strength and few & short water trees Reliability Cross-linked polyethylene has been used for cable manufacturing since end of the 1960s and during the years a lot of experience has been developed to improve performance, reliability and service life of XLPE cables. However due to the rapid growth of the wire and cable industry, with a lot of new entrants among material suppliers and among cable manufacturers, it has to be recognized that not all XLPE cable in the market has the same performance. Third party testing is an important and independent measure of performance and DNV GL is one of the institutes supporting the power industry with cable testing. Published studies show that a number of cables submitted for test do not meet the requirements in type test [20]. Unfortunately the failure rate indicates that substandard materials or poor manufacturing technology and quality control have been used and it may raise concern about reliability and long tem performance. Failure at testing of components in power systems [20} Failure in % (total number of components tested) Component Medium Voltage Cable 12 % (214) Joint 43% (80) Termination 51 % (88) Table 6 Failure statistics at third party testing presented as CEPSI 2014 [20]. At introduction of early XLPE cable system in Europe there were problems with cable failure. By implementing dedicated test protocol, stringent specifications and monitoring of quality, failure rate has been significantly reduced. Figure 6. Improvements in reliability and reduced cable failures at EON, region Ostbayern [10]

Asset management Electricity demand is increasing with need for new network investments. Utilities are facing increasing pressure from customers and regulators to enhance service reliability while at the same time keep prudent control of cost. The new environment drive the need for good asset management to best use existing distribution network and to operate them in a safe and under optimized conditions. Power utility engineers require cable systems with evident quality, safe and reliable performance and a long service life to minimize the need for repair, maintenance or replacement as this significantly influence the operation cost of the distribution systems. The experience shows that stringent specifications for cable and accessories need to be in place, best practice of installation need to be applied combined with controlled operation and well defined strategies for monitoring, diagnostics and maintenance. One of the pillars in asset management is to understand performance and service life of the cable system and to base purchasing decisions on life cycle cost instead of only looking at initial investment cost [21], [22]. Life cycle cost for medium voltage cable includes cost of investment, cost of operation, cost of maintenance and repair, loss of revenue due to cable failure and recycling or disposal cost. Asset management aims to best utilize investments and to be aware of need for maintenance, replacement and investments. With respect to distribution cables there are a number of considerations to address for asset management and best total cost of the investment [23]: Specifications and procurement to verify that products meet or exceed the accepted industry standard and to follow quality by incoming inspection programs, manufacturers audits or third party testing Installation is a common reason for early problem and it is of importance to manage and give necessary training to develop best practice in installation. Studies have shown that more than 70% of service failures relates to poor installation practices Monitoring and diagnostics of distribution cables in service it is important to understand condition, decide on maintenance and predict remaining life. Analysis of failures can give information on failure mode and the information helps at modeling and prediction. Conclusions and recommendations Extruded XLPE cables are important components in medium voltage distribution systems and there are technology solutions which can work with high reliability over long service life. However performance level of XLPE can vary depending on materials, cable manufacturing and quality assurance and it is important to have ways of monitoring cable quality and performance level. There are different testing protocols developed for testing of water tree performance, which have very comprehensive track records of improving reliability and service performance of cable. In water tree testing both XLPE insulation and the semi-conductive screens have influence on the result i.e. the result is valid for the material combination. It needs also to be recognized that cable manufacturing process has influence on water tree testing. Introduction of polyethylene jacketing has significantly improved cable resistance towards moisture ingress and has become the preferred jacketing material for underground cable in Europe and Americas It is recognized that cost efficiency is of very high importance in utility operations due to regulation and customer expectations. However at purchasing it is important to understand life cycle cost and not only looking at initial investment cost. Cable manufacturing specifications including necessary testing and qualifications and procurement strategy are parts of sound asset management strategy. Other focus areas are development of best installation practice and monitoring of cables in service and proactive maintenance to accommodate long and reliable service. References [1] J.J. Xu, S. Boggs The chemical nature of water treeing; Theories and evidence IEEE Electrical Insulation Magazine September/October 1994 Vol. 10 No. 5 [2] W.S.M. Geurts, R. Ross, M.G.M. Megens, E.F. Stennis Moisture penetration in XLPE and PILC cables Jicable 99 B.1.3 [3] W.S.M. Geurts, E.F. Stennis, C.J.H.M. Poorts, G.J. Meijer Water diffusion through sheaths and its effect on cable constructions Jicable 95 F.3 [4] J. Pelissou, J. Krine, J. Castonguay, S. Haridoss, T.K. Bose, M. Merabet, R. Tobazeon The nature and distribution of water in steam cured XLPE cables Jicable 87 B4.4 [5] M.K. Yan, R.K.Yang, F.G. Miao, L.M. Yang The influence of manufacturing technique to power frequency breakdown characteristics of XLPE power cables Jicable 11 C.2.2 [6] S. Pelissous, S. St Antoine Water penetration in the insulation of medium voltage cables Jicable 99 B.1.5 [7] A. Smedberg, B. Gustafsson, T. Hjertberg What is cross-linked polyethylene? International conference pf solid dielectrics 2004

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