PEX COMPOUNDS | WIRE & CABLE
EHV PEX-a process claims green gains
A new PEX-a compound technology aims to deliver new levels of cleanliness and cost effectiveness for the latest generation of HV and EHV power cables. Chris Smith reports
The world is electrifying rapidly. According to International Energy Agency predictions, electric- ity’s current 20% share of total global final energy demand will rise to near 30% by 2030 as Net Zero Scenarios are adopted – an annual average growth rate of 3.5%. This will mean considerable invest- ment in global power grids. PEX-a (peroxide crosslinked XLPE) is a proven option for production of insulation for medium, high and extra high voltage (MV, HV and EHV) AC and DC cables. However, while trusted and established, PEX-a compound production is not a simple process as the peroxide crosslinking agent will decompose (activate) at temperatures above 130°C or so. Ensuring this limit is not exceeded becomes increasingly difficult with cable com- pounds for use at higher voltages, as fine filtration is required to ensure the required purity levels.
Traditional routes The traditional route to production of HV and EHV cable compounds avoids the risk of peroxide decomposition by separating compounding and peroxide addition. It uses a compounding extruder, most commonly a twin-screw machine but also other continuous mixers and kneaders, to incorpo- rate additives such as antioxidants into a base compound. This is then passed through a fine melt filtration system to a pelletiser. The cool pellets are directed to a batch mixer, where they are warmed and surface-coated with peroxide before entering a vertical “soaking” chamber. Maintaining the warm coated pellets in the soaking tower for an extended period of time — more than 10 hours in some cases — allows the peroxide to diffuse from the surface into the pellet without decomposing. The pellets are then cooled, inspected, and packaged. The capacity of the soaking system is matched to the output of the compounding extruder, which
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means towers can be as high as 50m or more for the largest throughputs of more than 3.5 tonnes/h. The system also need to be clinically clean, which means high capital costs, while the cooling and reheating of the pellets adds to energy usage. Over the past few years, cable industry technology firm PM Cable Consulting has developed an alterna- tive compounding technique that is claimed to address these traditional challenges. It has partnered with compound- ing machinery maker Buss to roll out the LSHC (Linear Short Hyper Clean) technology globally. Last year, Spanish petrochemical company Repsol became the first to commit to it, placing a contract with Buss for installation of a 27,000 tonnes/yr plant with a planned start-up in 2024. The front end of the LHSC process is similar to
the traditional process up to the fine melt filter (it uses a Buss Compeo kneader extruder rather than a twin-screw machine). However, from there the process differs. After filtration the molten com- pound is fed into a Post Extrusion Peroxide Addition unit (PEPA), where the temperature is regulated so a liquid peroxide can be metered and incorporated without risk of decomposition. No soaking is required; after passing through the PEPA unit the compound is simply pelletised, dried and classified, 100% inspected and directed to a clean room packaging unit. Temperature control is critical, something the Buss
Compeo kneader compounding extruder is very well suited to, according to Denis Labbé, Managing Director at PM Cable Consulting. “Twin-screw extruders are violent in terms of shear and tempera- ture — temperatures can reach 250°C. This generates carbonyl groups, which are detrimental to the
May 2023 | COMPOUNDING WORLD 27
Main image: The Swiss- developed LSHC PEX-a compound technology is intended for production of high perfor- mance HV and EHV AC and DC power cable insulation
IMAGE: BUSS CORP
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