The following tutorials will be proposed on Monday, November 8th in 2 rooms at the same time. 



14:00-16:00 ROOM A
T1: Technical conception and design considerations for ensuring high availability of giant HVDC land cable connections

by André by A. Wagner, TenneT TSO, Germany; P. Gruber, 50Hertz Transmission, Germany; J. Brüggmann, Amprion, Germany

Executive summary:  In Germany the transmission grid expansion plans contain the implementation of so-called HVDC onshore corridor connections in order to fulfill the climate targets while shutting down nuclear power plants until 2022. These corridor connections will each have a route length of 300 km to 700 km and a transport capacity of 2 GW. Consequently they are to be considered as the future backbone of the German transmission grid, thus high availably is the driving requirement. With respect to public acceptance of such giant projects, power cables will be the majorly applied power line technology. This leads to the necessity of reconsideration of known technical conceptions and designs for HVDC cable systems.

In this workshop innovative approaches for the new and enhanced challenges with regard to ensuring high availability of giant HVDC land cable connections are addressed. It covers the implementation of auxiliary infrastructure, the use of cable monitoring systems and the discussion on new testing requirements.

Auxiliary infrastructure such as cable segmentation stations, cable-OHL transition stations, intermediate station for fibre-optic based systems and accessible linkboxes represent approaches from a system point of view that serve as enablers for availability optimized operation and maintenance of the HVDC cables. Monitoring systems allowing online cable fault locations or online cable condition assessments represent distinct measures for availability optimized operation which require special preconditions for applicability on super-long cable systems. New test requirements in both the cable production and the installation phase arise from a quality assurance (QA) point a view due to new or enhanced technical challenges such as the huge amount of joints to be assembled or new electrical stress conditions. However, innovative testing approaches need to be sketched not only with respect to their QA purpose but economic aspects and possible impacts on the project progress must also be taken into account.


16:30-18:30 ROOM A
T2: Current stage and perspectives of studies and simulations on TOV affecting HVDC link

by Hani Saad for RTE, France and Markus Saltzer, from NKT, Switzeland

Executive summary: Application of voltage source converters (VSCs) in power systems is rapidly growing due to advantages such as absence of commutation failures, ability of independently controlling active and reactive power, and fast dynamic response. VSC based on Modular Multilevel Converter (MMC) topology has become the most attractive solution, mainly due to their higher performances and lower cost compared to early VSC implementations.
MMC technology does not require the inversion of the voltage polarity when reversing the power flow direction. This has made the utilization of extruded insulation cables (XLPE) easier for DC applications. Since then, the number of extruded insulation cables, used in combination with VSCs, has increased for HVDC power transmission applications. Even if VSC does not require the inversion of the voltage polarity (during normal operation), several events can cause transients on cables that are not covered by standard tests. To fill this gap, an ongoing CIGRE JWG B4/B1/C4.73 investigates surges and extended overvoltage testing for DC cable systems to provide recommendation for upcoming HVDC projects and improve cable testing standards.
Circuit configuration of a converter station can vary depending on project specification and selected manufacturer. Therefore, the impact of MMC station’s transients on DC cable are evaluated during the design phase of each HVDC project.
DC transients (i.e. overvoltage and overcurrent) during converter faults leads to non-linear behavior and, thereby, are difficult to predict using analytical tools. Therefore, EMT simulations are commonly used to compute the overvoltage transients because these provide the most accurate and reliable results.
This workshop explains the typical events that lead to voltage fluctuation on cables connected to VSC converters such as DC faults or internal converters faults. In addition, the impact of converter parameters on the DC transient within the cable is provided an overview on the sensitivity of the system. Finally, technical solutions to limit stresses on cables are proposed and discussed.


14:00-16:00 ROOM B
T3: Scientific challenges and threats in the HVDC transmission systems (cables, accessories, converter devices)
by G. Teyssedre, from Toulouse University, Petru Notingher, IES Montpellier, Seddik BACHA, from G2ELab and Martin Henriksen, from SuperGrid Institute, France

Executive summary: Energy experts agree that the current transmission system must address new requirements for hosting huge quantity of renewables and need to be adapted to meet the new quantity and quality of demand. There are two main visions: enhancing the existing HV AC grid or building a new HVDC grid in parallel to the existing one. Either way, power electronics will play a critical role in the development and operation of the future power grid.

Power electronics devices make it possible to control energy flow through optimal pathways, to interconnect non-synchronized areas, to transfer energy via long distances and create large subsea interconnections. However, these same devices may affect grid stability and protection and devices. The cables are directly affected and must be designed in consequence.

In this context, the stresses driven by the HVDC cable conditions and construction on the dielectric materials constituting cables and accessories will be addressed in this workshop. Besides the questions of controlled over-stresses driven by qualification/type tests and operation with polarity reversals, HVDC cables are submitted to uneven stresses resulting from the interaction with the network. Based on typical HVDC technology, our purpose will be to identify such stresses induced by converters, linked to over-currents or over-voltage during operation or failure, or to ac/dc network coupling, and how far it could impact cable insulation.

On the material side, physical processes and governing rules considering field distribution under DC stress, and space charge effects in synthetic insulations, will be reviewed as well as methods to probe these phenomena. The challenges for materials with going to very high voltage will be addressed, considering both static and dynamic stresses as well as physical phenomena likely to be at play. Some solutions proposed to improve material endurance will be discussed.


16:30-18:30 ROOM B
T4: Two new recommendations for testing Extruded and lapped insulated HVDC & EHVDC cable systems at a rated voltage up to 800 kV
by Stefano Franchi Bononi from Prysmian Group, Italy and Gunnar Evenset, from Power Cable Consulting, Norway

Executive summary: The decision to activate Working Group B1.62 for extruded DC cable systems and B1.66 for lapped DC cable systems stems from the fact that commercially available HVDC systems above 500 kV were emerging with both laminated (even new kind of laminated) and extruded insulations. In fact, at the time of preparing these recommendations there was laboratory experience at voltages up to 800 kV and operating experience already at 400 kV for extruded and 600 kV for laminated insulation; during the preparation of the recommendations, contracts for extruded cable systems with voltage level at 525kV were also awarded. A further increase in voltage level is to be expected and these recommendations will therefore cover voltages up to 800 kV. The test recommendations for lapped insulation published in Electra 189 and Electra No. 218 were already valid for voltage levels up to 800 kV, but a revision of the test criteria was necessary to reflect current and future HVDC system technology and to merge the recommendations into one technical brochure.

Furthermore, the Working Groups had to give clear answers to several questions that emerged with the testing and operational experiences gained in the past 10-20 years on HVDC cable systems.

The large number of projects and the quick development needed by the industry to match the demand of always more performing HVDC systems requires commonly shared and agreed testing method to speed up the time to market of technology solutions. The already well-known Prequalification and Type test protocols were reviewed and integrated when needed with suggestions and improvements. In particular, the heating method, temperature control and thermal test parameters had to be reviewed for extruded insulation.

Extension of qualification test was introduced for extruded insulations, trying to mirror as much as possible what already defined in TB 303 for AC cable systems.  

Prequalification test and Extension of qualification test were also introduced for lapped insulations to give guidance for qualification of new materials or major changes in operating conditions.  

A proposal to standardize qualified voltage levels (nominal and maximum) was introduced, after collecting field experience also in cooperation with SC B4.

More flexibility was introduced allowing for different test methods (like impulse test configuration with both blocking capacitor and sphere gaps method), leveraging on the significant test experiences cumulated by the industry.

The emergence of new grid configurations (point-to-point, radial and meshed) and development in converter technology are introducing new types of overvoltages (commonly referred to as Temporary Overvoltages, TOVs) and the standard switching and lightning impulse wave shapes might not cover typical voltage stresses anymore. Thanks to the interface with JWG B4/B1/C4.62, the new waveshapes were evaluated and new testing methods are now included in these Technical Brochures.

The new manufacturing and testing experience cumulated for systems with extruded insulation by the industry when supplying more than 6000 km of HVDC cables (at present) allowed also to review the routine and sample testing previously outlined in the Technical Brochure 496 and to define more precise criteria, test regimes and test methods. Test recommendations were given about how to routine test cables and accessories, trying to align as much as possible with IEC 62895 if possible, considering the different voltage levels the two documents. An extensive work was done, mainly on sample tests, to introduce new fingerprinting methods and to open the testing to new types of materials which are now being proposed and purchased in the market, such as polypropylene laminated, low cross-linked polyethylene, nano-filled materials and thermoplastic insulations.