EuroWire July 2023

Technical Article

Breakthrough XLPE insulation for power cables By Roshan Aarons, Mikhail Bogdanov, Edit Berczi and Saurav Sengupta, Dow Inc; and Dr Jaber Dandachi and Samir Fraija, Jeddah Cables

Introduction Key drivers, technical requirements and challenges

However, DCP may also undergo a low level of homolytic cleavage, even at relatively low extrusion temperatures, such that premature low levels of crosslinking can occur during normal extrusion of cable insulation. Such premature crosslinking, referred to as scorch, can result in serious product defects that can affect the long term electrical performance of the cable insulation. Additionally, this propensity for scorch limits the production run length. To manufacture a product that has acceptable quality, the cable manufacturer extrudes the peroxide-containing polyethylene compound at a relatively low temperature and for a limited time before the extruder needs to be stopped and cleaned. Peroxide decomposition by-products mainly include methane, cumyl alcohol and acetophenone. They remain incorporated in the insulation after extrusion and may seriously affect cable performance. Methane is also a substance of high concern due to its flammability, and could lead to explosion during cable jointing or installation. Methane has to be removed or its concentration needs to be reduced to an acceptable level before the cable is moved to the next process. Although there is no specification on the permissible residual methane content in the insulation, many cable producers target to reduce it below 100 ppm. Polar peroxide decomposition by-products (such as acetophenone and cumyl alcohol) may further contribute to higher dielectric losses, so their concentration has to be reduced as well. Removal of peroxide decomposition by-products is achieved through a degassing process in specialised ventilated chambers after the insulation layer has cooled down and solidified. The cable core is heated to 60-70°C and remains in the chamber for a certain duration depending on the cable design and insulation thickness. Generally, the degassing process takes from one week for 110kV cables to several weeks in the case of 500kV cables with higher insulation thickness. The chambers occupy significant space in cable production plants and the process is energy intensive and time consuming. Therefore, it is very valuable to find a way to significantly shorten or eliminate the degassing process, without compromising the cable performance and service life. In response to these industry needs, Dow Inc has developed a novel XLPE insulation – Endurance™ HFDD-4201 – that offers cable manufacturers 80 per cent faster degassing time and improved scorch resistance for longer production campaigns on continuous vulcanisation (CV) lines. Dow Inc partnered with Jeddah Cables, one of the leading high voltage and extra high voltage cable manufacturers in the Middle East, to assess and validate the performance of this next-gen XLPE compound. Cross-linking by-product evaluation Assessment of the initial and residual methane concentration in conventional and novel XLPE insulation was performed using computational prediction models. Analytical methods such as thermo-gravimetrical analysis (TGA) and gas chromatography (GC) were then used to validate the simulation via empirical measurements on high voltage cables.

The power sector is undergoing a global transformation – decarbonisation is a major priority while the cost of renewables has dropped substantially over the last decade. Urbanisation and the upgrade/extension of existing electric grids are two other trends influencing rapid growth in the sector, such that manufacturing efficiency becomes even more critical to cable manufacturers.

2010 2020 2030 2040 2050

6.9

7.7

8.5

9.2

9.7

Population (billion) [1]

Energy demand (quadrillion Btu) [2]

530 625 700 800 900

Electricity consumption (quadrillion Btu) [2]

202 253 299 353 419

Renewables (% of electricity) [2]

20

33

43

48

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[1] UN Department of Economics and Social Affairs [2] US Energy Information Administration, International Energy Outlook 2021 Table 1 : Global megatrends For high voltage transmission cable applications, cross-linked polyethylene (XLPE) with peroxide initiators has become the dominant chemistry. The peroxide of choice has been dicumyl peroxide (DCP), since it enables high productivity in the cable continuous vulcanisation process. The mechanism involves peroxide-initiated cross-linking of long molecular chains of low density polyethylene (LDPE) to form a three-dimensional network. The crosslinking is irreversible and improves the heat resistance of the insulation compound. Thermoplastic LDPE melts at ~105°C, while XLPE remains stable at higher temperatures, thus enabling XLPE insulated cables to function at 90°C operating temperature, 130°C overload conditions and 250°C short circuit temperature. The crosslinking process involves the use of organic peroxides. The reaction initiates in a vulcanisation tube immediately after extru sion, at high temperature and pressure under inert atmosphere. Free radicals are generated in the process by the homolytic decom position of DCP. These abstract hydrogen atoms from the polymer chains whereby stable decomposition by-products and polymer radicals are formed. These polymer radicals combine to produce stable carbon-carbon crosslinks.

Figure 1 : Peroxide crosslinking

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July 2023