AXALJ-TT: 3-žilni SN kabel z aluminijastim ekranom, izboljšana vodotesnost in pričakovana daljša življenjska doba Karl-Johan Mannerback, Kristoffer Berglund 1 Prysmian Group Sweden * E-pošta: karl-johan.mannerback@prysmiangroup.com Povzetek: Električni ekran SN kabla je kritični del kabla in hkrati ključnega pomena ohranjanja njegove funkcije skozi celotno pričakovano življenjsko dobo kabla tako v normalnih pogojih obratovanja kot tudi v primerih napak. Da bi ohranili varno in zanesljivo delovanje SN / VN kabla, je pomembno, da so kovinski deli kabla (vodnik in ekran) zaščiteni pred izpostavljenostjo vodi, ki vpliva na nevarnost korozije. To se doseže z izbiro primernega, visoko kakovostnega kabla, kot tudi z zagotovitvijo kvalitetne in natančno izvedene montaže kabelskih spojk in zaključkov. Proizvajalec Prysmian Group je v zadnjih letih razvil resnično vodotesno konstrukcijo 3-žilnega SN kabla, ki bo podaljšala pričakovano življenjsko dobo kabla z zmanjšanjem tveganja vdora vode v kabel. Ta dokument pojasnjuje zgradbo kabla skupaj z izvedenim preizkušanjem. Poleg tega bodo predstavljena tudi priporočila za namestitev (spajanje in zaključevanje vodotesnih kablov) in testiranje kabla med in po namestitvi. Ključne besede: SN kabel, izboljšana vodotesnost, popolnoma vodotesen, korozija, ekran kabla AXALJ-TT: 3-core MV cable with AL screen, improved water tightness and expected longer lifetime. Abstract: The electrical screen of a MV cable is a critical part of the cable and it s vital that that the functions is maintained throughout the whole expected life time of the cable both under normal operating conditions as well as during fault conditions. In order to maintain a safe and reliable function of a MV/HV cable it s important that the metallic parts of the cable (conductor and screen) are protected from being exposed to water that will lead to a risk of corrosion. This is achieved by selecting a suitable, high quality cable as well as making sure the installation of cable and connected accessories is made with accuracy and good quality. Prysmian Group has during the last years developed a truly watertight MV 3-core cable design that will prolong the expected life time of the cable by minimizing the risk of water penetrating into the cable. This paper explains the cable design together with performed tests. Furthermore, recommendations for installation (jointing and termination of water tight cables) and testing of cable during and after installation will also be presented. Keywords: MV cable, enhanced water tightness, totally water tight, corrosion, cable screen 1
1 BACKGROUND 13. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV - Maribor 2017 In order to maintain a safe and reliable function of a MV/HV cable it s important that the metallic parts of the cable (conductor and screen) are protected from being exposed to water that will lead to a high risk of corrosion. This is achieved by selecting a suitable, high quality cable as well as making sure the installation of cable and connected accessories is made with accuracy and good quality. It is well known that the normally used radial water blocking for cables, applying a thin plastic-coated Al-tape, is not sufficient to produce a cable that is actually radially water tight. This fact has resulted in various solutions to protect the screen from corrosion, ending up in the use of more noble metals (i.e. Cu) or claimed corrosion protected wires (i.e. copper clad aluminum or polymer coated aluminum.) 2 DESIGN OF AXALJ-TT - WATERTIGHT 3-CORE MV CABLE WITH AL SCREEN The cable, AXALJ-TT, is a longitudinally and radially watertight designed 3-core medium voltage cable. It is suitable for fixed installation in air, ground and water. It was designed as an alternative to the copper screened AXLJ-TT that has been produced by Prysmian Group Sweden (Draka Kabel) for many years. To overcome the difficulties of corrosion, Prysmian Group aimed to create a cable with a proper radial water barrier. The target was to create a radial water barrier similar to that of the since many, many years used AHXAMK-cable in Finland. Since the AHXAMK cable contains only the coated Al-tape as both radial water barrier and screen (no wires) it presents an interesting design in terms of challenges of corrosion. Long extensive studies have shown that the design, using longitudinally applied thicker Al-tape, resists corrosion very well during its life time. Prysmian Group was successful to manufacture a 3-core cable applying such a longitudinally applied coated Altape, thus creating a superior 3-core cable in terms of water tightness. Picture 1 Design of AXALJ-TT A solution where the aluminum wires and aluminum foil/tape have direct electrical contact (i.e. no semi conductive tape or compound between wires and tape) also reduces the risk of discharges during transients (lightning, switching etc.). Semiconducting material can be considered as fully conducting at 50 HZ but as 2
insulation at impulse, causing discharges between the two metals with the risk of having to replace the cable after a strike of lightning 3 CORROSION PROTECTION The longitudinally applied coated Al-tape, together with multiple swellable semi conducting tapes and longitudinally watertight conductor creates a superior 3-core cable in terms of water tightness. The idea is simply to keep the water out of the cable, by avoiding any insertion of water due to capillary effects as found when helically applied Al-tapes are used. Keeping water out of the cable removes the risk of corrosion. 3.1 In case of fault - Over dimensioning of electrical screen With a proper water barrier in place to secure the function of the cable during normal use, the next challenge was how to secure the integrity of the screen if a cable is damaged, i.e. during or after installation in the ground. Of course, any damage where protecting layers/functions are removed is very problematic. When the water barrier is removed, then for sure water will penetrate into the cable. However, by using other water blocking defenses (i.e. water blocking tapes) any water that penetrates into the cable due to a damaged section will be stopped from further spreading along the cable. The section/hole where water is penetrating will be subject to increase risk of corrosion, as is the case with any other cable in the market. This means that if the radial water barrier is removed exactly where one of the screen wires is positioned, there is a risk of corrosion of that wire assuming the environment is such that corrosive environment is created. However, the corrosion will only occur in the location where the radial water barrier was removed. The water cannot penetrate into the cable due to the longitudinal water blocking protection, making only the exposed wire exposed to corrosion. In the cable design, the full electrical screen is considered in the wires. The addition of the Al-tape to the electrical screen is not considered during design (even if allowed according to Cenelec HD 620). Instead, the existence of the Al-tape, in direct electrical contact of the Al-wires, in reality makes the screen much larger than what is specified Thus, even if an Al-wire would break due to corrosion, the integrity of the screen is obtained by the over dimensioning /extra safety resulting from applying the Al-tape in the form done in AXALJ-TT. Picture 1 Radial water barriers of AXALJ-TT 3
3.2 Tests performed 13. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV - Maribor 2017 Research at the Prysmian Group's central R&D unit has shown that aluminum screened MV cables require a completely watertight design to ensure the function of the screen during the cables expected lifetime. The following extended tests has been performed on the AXALJ-TT [1], [2] Water penetration test According to IEC 60502-2 Corrosion test in water The test was performed by creating a hole in the cable jacket, in order to remove the plastic layer up to the aluminum tape, but without damaging the screen. Then the cable was submerged in a water filled tank. A Current of 10 A was continuously circulated in the screen in order to simulate realistic severe conditions. After 1 month of immersion of the sample, the cable was extracted and examined. There was no water penetration found under the aluminum tape. Factory bending test According to IEC 60502-2 sub clause 18.1.3 [4]. The screen and aluminum tape was thoroughly investigated. Partial discharge test Several partial discharge tests have been carried out in accordance with IEC 60840 [5] with the sensitivity being 5 pc or better. The test voltage was set to 2,5 U0 4 INSTALLATION OF AXALJ-TT Poorly made joints presents a high risk. However, these risks tend to present the same risk regardless of cable brand. We strongly advise to use high quality accessories (joints, terminations, connectors), especially when using Al-screened cables. Also, we advise to not interrupt the Al-tape over the joint, i.e. maintain the radial water tightness also over the joint. The fact that AXALJ-TT has a design with bare AL wires means much less risk of damaging the indicudual wire during installation. A covered AL wire needs to be bared before jointing or terminating. If the covered AL wire is scorched during this handling there is an increase risk the AL wire will break. 5 TESTING DURING AND AFTER INSTALLATION In order to minimize risk of degraded radial barriers due to bad installation we recommend that sheath testing and a real time PD testing is done. The first before energizing the cables and the second after energizing. 5.1 Sheath testing The purpose of sheath testing is to make sure the sheath is intact, i.e. not damaged during installation. Recommended procedure and values for sheath testing of MV cables with poyethelen sheath can be found in table one below [6]. 4
5.2 PD testing Table 1: Test voltage and max recommended max leakage currents for MV cables The purpose of PD measurement is to make sure no dangerous PD activities are present in the MV/HV system. More that 80% of disruptive failures in HV/MV assets are related to Partial Discharge (PD) activity [7]. Further, a non neglieble origin to PD activities is due to badly installed joints and terminations. PRY-CAM wireless technology allows PD testing to be performed on energized MV- and HV grids, without the need for a direct connection to what is being tested. This means that measurements can be taken without having to disconnect the part to be tested. PRY-CAM s revolutionary technology allows online, accurate and reliable PD measurements, diagnosis and defect localisation. PRY-CAM PORTABLE is an integrated portable instrument for the automatic acquisition, processing and classifi cation of pulse signals generated by PD phenomena occurring in insulating materials of medium and high-voltage electrical systems and equipment, such as transformers, electrical machines, cables systems and switchgear. With Pry-Cam portable 100% of critical defects originating from PD can be detected on HV and MV systems. REFERENCE [1] Prysmian Group Internal test report - P. Anelli, S. Frigero, D. Martelli»Alternative screens for MV/HV cables Ongoing corrosion / oxidation tests«, TR-R&D-CT-14-003, Milano, 2014 [2] Prysmian Group Internal test report R. Besson, S. Frigero, D. Martelli»Corrosion test on metallic screens upon contact with water MV cable«, TR-R&D-CT-16-003, Milano, 2016. [3] CENELEC HD 620 Part 10 Section M [4] IEC 60502-2 [5] IEC 60840 [6] Elforsk rapport 97:26, Mantelprovning av 12 och 24 kv polymerkablar [7] IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems, 493-1997 (Gold Book) 5