The Prospects and Challenges for HVDC Cable Technology in a Smart Grid World
By Thomas Andritsch, Giovanni Mazzanti, and Jérôme Castellon
High voltage direct current (HVDC) cable systems are traditionally the best solution for long-distance submarine transmission, but are not very common on land. However, the improved performance of AC/DC converters, in terms of cost and power throughput, and public concerns for the environmental and visual impact of overhead lines, are making HVDC cable technology more and more appealing. Indeed, it is generally believed that the increasing penetration of HVDC cable transmission will make the world grid smarter, fostering flexibility, reliability, and sustainability by integration of renewables, which are often far away from load centers. This has led to a near exponential growth of HVDC cable lines worldwide in the past two decades.
Among HVDC cable systems, extruded cables with polymeric insulation (typically cross-linked polyethylene, XLPE) are becoming increasingly competitive with “classic” Mass Impregnated Non-Draining (MIND) HVDC cables. Indeed, extruded cables have some key advantages, including: 1) they are much more environmental friendly, since there is no possible leakage of oil; 2) the maximum permissible conductor temperature in normal operation is higher; 3) jointing is much simpler. Thanks to extensive R&D activities, mostly described in IEEE DEIS Journals and Conference Proceedings, several HVDC extruded cable systems at 320 kV-rated voltage are now in service worldwide, including a 400 kV/1000 MW XLPE-insulated HVDC cable system between UK and Belgium – the submarine “Nemo Link” – which is being commissioned. DC extruded cable systems are now commercially available at voltages up to 640 kV and for power above 1 GW. The global smart grid continued acceleration towards even higher performances and utilization of HVDC cable systems can be easily envisaged. While the future looks bright, HVDC cable technology also needs to face a number of challenges:
1) Improved space charge behavior of cable and accessory insulation. It is well known that extruded insulation for cables subjected to high DC voltage is particularly affected by trapped space charge. The quest for higher design temperatures (>70°C) and applied electric fields (>20 kV/mm) calls towards materials with reduced space charge accumulation to minimize their limiting effects of cable and accessory insulation. For this reason, various non-destructive methods for investigating space charge in solid polymeric materials have been developed and are steadily been improved upon over the last decades, enabling space charge measurements on real full-size cable loops during qualification tests for HVDC cable system projects. The IEEE DEIS Technical Committee (TC) on “HVDC Cable Systems (cables, joints and terminations)” has addressed this issue by developing IEEE Std. 1732-2017 entitled “Recommended practice for space charge measurements in HVDC extruded cables for rated voltages up to 550 kV”. Two measurement techniques called PEA (Pulse Electro-Acoustic) and TSM (Thermal Step Method) appear as good candidates for monitoring trapped space charges within the HVDC cable insulations.
2) Developing new smart (better performing and more environmentally-friendly) materials for cable insulation. Today extruded insulation for HVDC cables does not necessarily mean XLPE, since new thermoplastic compounds are being developed, which are fully recyclable (being not cross-linked), and can bear higher operating temperatures and voltages. They also promise improved space charge behavior so as to withstand even voltage polarity reversal. Conventional extruded cables are much more sensitive to the negative effects of the space charge accumulated in DC cable insulation than MIND cables, particularly in the presence of voltage polarity inversion. This makes such new thermoplastic insulation amenable for usage not only with Voltage Source Converters (VSC, those typically used for HVDC extruded cable systems) but also with the well-assessed and more reliable Current Source Converters (CSC, those typically used for HVDC MIND cable systems), which require voltage polarity inversion to change the power flow direction.
3) Developing smart accessories (joints). It is a fact that accessories are the weak point of HV cables. Joints (of both factory and pre-moulded type) are of concern, due to their large number in long HVDC links. Joints feature many sub-components with a number of interfaces between different materials: different insulation materials, semiconducting layers and conductors are in contact. Adverse thermal, chemical and physical phenomena put stress on such a joint, reducing the reliability. Such interfacial stresses affect material selection and electrical, thermal and mechanical design of such components. Striving towards higher performances and utilization of HVDC cable systems means higher applied fields, temperatures and mechanical stresses at critical interfaces within joints.
4) Developing new effective testing methods for accessories. This challenge is a direct consequence of the previous. New and more effective testing methods are needed for developing smarter accessories, especially joints. These techniques include: 1) space charge measurements on full size cable system accessories with high resolution, which are not trivial at all due to inherent difficulties but nevertheless explored by the scientific community (test cell positioning, signal attenuation, etc.); 2) innovative partial discharge (PD) measurement devices based e.g. on VHF/UHF wireless electromagnetic sensors. These devices have a number of advantages for measurements on cable system loops, including accessories: free positioning in the broad cable system loops, high signal to noise ratio, ultra-wide band for an accurate acquisition of the whole PD pulse (thereby enabling noise rejection, identification and separation of PD sources). This is the topic the activity of the IEEE DEIS TC “HVDC cable systems” is presently focusing on.
5) HVDC cable operation within the smart grid. This challenge regards the power systems level view and how the HVDC cable operates within it, involving various smaller challenges. The first is diagnostics and online monitoring of HVDC cable systems to assess the condition of a system in real time, thereby achieving peak performance without compromising its reliability and safety. Innovative online diagnostic methods include monitoring systems based on PD detection, Distributed Temperature Sensing (DTS), Distributed Acoustic Sensing (DAS), etc. Another issue is assessing of unconventional and critical waveforms for HVDC cable systems. Here there is a lot of work in progress for developing on two fronts: effective simulation algorithms for a correct estimation of critical waveforms, and assessment of the relevant, possibly detrimental effects on extruded cables. Current focus is on Temporary Over-Voltages occurring with VSC converters, an activity carried out at present by CIGRÉ JWG B4/B1/C.4.73. A further sub-challenge is the limitation set by space charge on the dynamic behavior of polymeric cables, since – as emphasized above - space charge does limit performance and operational flexibility of polymeric cable systems, since operators do not want to risk damage to a cable due to polarity reversal.
This document describes the challenges that must be addressed in terms of HVDC cable technology in a smart grid world. Whether it concerns the material, the component or the system, the design and the monitoring during operation appear indispensable for their use in a grid with rapidly changing power flow. These aspects are widely covered by many academic and industrial actors and are discussed and valued within the IEEE DEIS TC "HVDC cable systems" and CIGRE working groups. It is clear that an intelligent network must contain smart materials, smart components (cables and accessories) and smart monitoring tools.
Thomas Andritsch (M ’11) was born in Innsbruck, Austria, in 1980. He received the Dipl.-Ing. degree in electrical engineering from Graz University of Technology in 2006 and his PhD in the same field from Delft University of Technology in 2010. Since 2013 he has been a lecturer at the University of Southampton with a research focus on advanced insulation materials for high voltage applications. He has extensive experience with preparation and testing of polymer-based electrical insulation materials, including nanodielectrics, electroactive polymers, and syntactic foams. Andritsch is currently member of the IEEE DEIS AdCom, chair of the UK and Ireland Chapter of the IEEE DEIS and was member of IEEE and CIGRE working groups focusing on polymer-based nanodielectrics.
Giovanni Mazzanti (M’04–SM’15) is Associate Professor of HV Engineering and Power Quality at the University of Bologna, Italy. His research interests are life modeling, reliability and diagnostics of HV insulation, power quality, renewables and human exposure to electro-magnetic fields. Since 2009 he has been a consultant to TERNA (the Italian TSO) in the HVDC and HVAC cable systems area. He is author or coauthor of more than 270 published papers, and coauthor of the book Extruded Cables for High Voltage Direct Current Transmission: Advances in Research and Development, John Wiley-IEEE Press, 2013. He is member of IEEE DEIS and IEEE PES, chairman of the IEEE DEIS Technical Committee/Working Group “HVDC cable systems”, member of the IEEE DEIS Technical Committee “Smart grids” and member of the the CIGRÉ Joint Working Groups B4/B1/C4.73 on “New overvoltage shapes in HVDC cable systems”.
Jérôme Castellon (M’08) received his PhD degree in electronic, optronic and systems from the University of Montpellier (UM), France, in 1997. He has been the technical manager of advanced metrology for electrical engineering "Am2e" company to 2003. Since then, he has been an Associate Professor at UM. Currently, has been working with the Energy and Materials Group "GEM" of the Institute of Electronics and Systems "IES". He has been involved in various research fields related to insulating materials for electrical engineering. His major research activities involve electrical characterization, dielectric phenomena and diagnostic technique developments. He is a member of the IEEE Dielectrics and Electrical Insulation Society, CIGRE SC-D1 and SC-B1, of the French Society of Electrostatics and of the Société des Electriciens et Electroniciens. He is active in the IEEE DEIS TC on “Smart grids”, “Nanodielectrics” and “HVDC Cable Systems”.