Paper about Factory Joints for DC Cable at Jicable '23

Few publications exist and are readily available which treat factory joints in the area of Extra High Voltage DC cable. This paper sheds some light on joints and subsea cable.

Few publications exist and are readily available which treat factory joints in the area of Extra High Voltage DC cable. With an energy cable market developing around green DC alternatives, Maillefer and PeakVolt Professionals AB of Sweden decided to combine their efforts. Their goal was to shed some light on the topic to professionals gathered at Jicable 2023.

The stage was set at the 11th International Conference on Insulated Power Cables held in Lyon during June 2023. The program included a slot for a presentation entitled “Challenges with development and optimization of process parameters in design and production of EHVDC submarine cable flexible factory joints”. For our two-company three-member team, the time had come to put forth their white paper. The abstract follows below.

Abstract highlights higher voltages and longer lengths

"With ever increasing demand for extruded submarine cables of higher DC voltages and longer lengths, having a good understanding of factory joint production methods and its process parameters is of great importance.

It is well-known that process parameters such as time and temperature during curing and degassing impact the conductivity of EHVDC XLPE insulation materials. For cables, strictly following the process settings in combination with sample and routine tests help to obtain desired chemical composition and electrical properties in the insulation system. However, production of factory joints is very different and more complicated than cable, therefore optimization of process parameters and quality control of the final product is more challenging.

In this paper a description of XLPE insulated tape molded factory joints is presented. Most critical moments and related process parameters are highlighted, discussed and a development procedure involving experimental joints, [Finite Element Modeling] simulations and design optimization software is presented."

Promise of FEM simulations in conclusion

According to the stated conclusion, "Some of the key challenges for factory jointing of HVDC cables were presented and discussed in this paper. Besides the normal considerations for HVAC factory joints, extra challenges for HVDC factory joints have to be handled as well.

Due to sensitivity of DC conductivity to chemical composition of the insulation system, the materials and consumable have to be selected carefully. Furthermore, design of tapes, vulcanization and heattreatment procedures can impact the chemical composition and DC conductivity in the insulation therefore such secondary effects should also be considered.

For EHVDC cables with thicker insulation and lower crosslinking degree, necking of the insulation can be a challenge therefore temperature distribution in the system has to be optimized to avoid the necking problem while achieving desirable crosslinking degree.

An example of [an] unsuccessful factory joint is presented with temperature measurements and its analysis to describe how the process settings can be examined and optimized.

Considering the large number of parameters to take into consideration for factory jointing, purely using experimental joints is a slow and cumbersome approach. Instead, a combination [using] experimental joints, advanced finite element modelling as well as simpler modelling tools can be used to accelerate the design procedure and obtain a reliable tool for calculating the design rules and process parameters."

Shedding a light on factory joints

Did the team achieve their goal of revealing details about factory joints for EHVDC cables? Most importantly, how are innovations in cable designs and cable processing progress to meet growing demand in green energy transmission?

Maillefer and Peakvolt exhibited their developments, and in that sense the conference was a success. Maillefer recognizes the authors, Hossein Ghorbani, of Peakvolt Professionals AB, and Mikko Lahti, Timo Mäkelä of Maillefer Oy. More details about their paper and session can be found on the Jicable organization website jicable.org, or requested via contact@jicable.org.