Reliable Power Cable Screen Connections

Several severe and costly service failures have occurred in the last years in the Norwegian distribution network due to local overheating in power cables. The crucial issues are the surprisingly high screen currents and the quality of the metallic ground screen and laminate connections close to cable joints and terminations. Today no national or international standardised qualification test for such connections exist. The root causes for failure are not fully understood. The main objective for the project is to establish a guideline for testing and installation of ground screen connections in power cables to avoid future service breakdowns and to increase the reliability of distribution and transmission cable networks. This project will provide the necessary knowledge to elucidate the routes to failure, and use this information to propose an international qualification test for new equipment. In addition, new software for calculation of resulting screen currents will be developed. This should lead to less future failures providing a more reliable distribution network in the future. Several research tasks are defined within the project and grouped into six work packages (WP): WP1: Failure types and failure mechanisms WP2: Ampacity limit of ground screen connections WP3: Performance of connections during load cycling and overvoltages WP4. Condition assessment WP5: Implementation of qualification tests WP6: Dissemination and publication A steering committee composed of 5 Norwegian power utility companies has been formed. In addition, a reference group made up of several cable and cable accessories manufacturers shall assist the project with competence and recommendations concerning project activities and result application. The project is a co-operation between Norwegian utilities, cable manufacturers and equipment vendors, NTNU, SINTEF Energy Research and REN. WP1 has focused on getting an overview of incidents, methods, standards, experiences, cable designs, screen connection designs and typical loads for power cable screen connections. The results have been gathered in a project memo. The memo describes typical network configurations and loads for Norwegian cable systems. The most common cable designs and screen connection methods are included. Further, the memo includes examples of failures and failure mechanisms in cable screen connections gathered from experiences in the Norwegian grid. The norms and recommendations on the topic are reviewed. The memo serves as a basis for the remaining work packages. WP2 focuses on studying the capacity and qualities of available cable screen connections during constant load in a lab setup. Commercially available ground screen connections and cable designs has been gathered. A test method/protocol for studying the different screen connections is developed. Laboratory tests have shown that the electrical contact between the copper strands and laminate screen should be considered as poor. It is therefore important to connect the strands and laminate in all joints. A software for calculations of screen currents with respect to different cable configurations, dimensions and loads has been developed. This is an online-tool and is available from RENs homepage. In WP5 the project has set a goal of proposing an international qualification test for ground screen connections through publication of a CIRED technical report. An international working group with participants from 10 countries has been established. Two meetings were held in 2017 and 2018. In 2019, three meetings were held. The main topics in the report shall be to 1) assess ground screen currents for different MV system configuration (case studies), 2) to survey different cable and cable accessory designs and connection types, 3) provide an overview of failure types in MV cable system components related to overheating in ground screen connections, and 4) propose a qualification test for power cable ground screen connections. Each of the main topics has a dedicated working/writing group within the CIRED working group, which are responsible for writing and arranging meetings/skype meetings. Due to the complexity of making such a qualification test, it was not completed before the end of the project. There are laboratory tests ongoing at international laboratories which should be completed before the qualification test is completed. The aim is to issue the report in 2020. As part of WP6, the project and results from the work has been presented at REN Teknisk Konferanse 2016 and 2017, REN Metodedager 2017 and CIRED 2017 in Glasgow, and meeting with REN reference group for cable and substations. In June 2019, one article was published at the conference Jicable. Project reports and calculation software is available at RENs homepage.

Dette prosjektet har gitt problemstillingen anerkjennelse og økt kompetansen hos partnere og kabeleiere gjennom prosjektets rapporter, publikasjoner (tidsskrift og fagblad) samt i større fagtekniske fora. Man har sett et høyere fokus på riktig valg av materiell og hvordan man kan unngå flere feilscenarioer. Kabeleiere kan med bakgrunn i dokumentasjonen og programvaren på RENs hjemmeside vite hvilke anlegg hvor det er nødvendig med tiltak for å redusere sannsynligheten for havari. Det er spesielt for store kabeltverrsnitt, med en spesiell kabeltype (TSLF) og stor avstand mellom kablene at overoppheting og havari er mest observert. Når den standardiserte testprosedyren i regi av CIRED er utgitt vil man kunne kjøpe materiell som er testet etter en internasjonal anbefaling. Vi har merket stor etterspørsel etter denne, både nasjonalt og internasjonalt, hos leverandører, entreprenører og kabeleiere.

Several severe and costly service failures have occurred in the last years in the Norwegian distribution network due to local overheating in power cables. The main issue is quality of the metallic ground screen and laminate connections close to accessories and in particular cable joints. Today no national or international standardised qualification test for such connections exist. The root causes for failure are not fully understood. This project will provide the necessary knowledge to elucidate the routes to failure, and use this information to propose an international qualification test for new equipment. In addition, a simple but robust methodology for calculation of resulting screen currents will be developed. This, together with national seminars and courses provided by REN, should lead to less future failures providing a more reliable distribution network in the future. In this work, financial support from the Norwegian Research Council is essential to establish fruitful co-operation between Norwegian utilities, cable manufacturers and equipment vendors, NTNU and SINTEF Energy Research.