EuroWire July 2023

Technical Article

Computational degassing prediction model To assess degassing, a diffusion model based on Fick’s law was developed and applied to typical single-core 132kV (17mm insulation thickness) and 220kV (24mm insulation thickness) cables. Conventional XLPE insulation initially contains ~600 ppm of methane, and its concentration gradually decreases as the degassing continues. In the case of 132kV cable, degassing of approximately seven days at 70°C is needed to reduce it below the 100 ppm threshold (a lower CH 4 concentration threshold may be selected for EHV cables). For the thicker (24mm) 220kV cable insulation, it would take around two weeks to reach the same methane threshold. Initial methane concentration in the novel Endurance HFDD-4201 insulation is significantly lower – approximately 150 ppm – which enables much faster degassing. Figure 2 indicates that for 132kV cable insulated with Endurance HFDD-4201, the 100-ppm methane threshold will be achieved after one day of degassing, while for the 220kV cable it would take two days.

The TGA method for by-product analysis provides an indication of the overall by-product content but does not quantify the exact residual methane concentration. Head space gas chromatography (HSGC) Quantitative methane concentration in XLPE insulation can be conveniently determined using the head space gas chroma- tography (HSGC) test method. Principles of this method are documented in the technical brochure TB 850 published by CIGRE in 2021 (Harmonised test for the measurement of residual methane in insulating materials). According to this document, pie-shaped insulation samples (approximately 5g) shall be taken at the cable manufacturer’s site and immediately placed in gas-tight flasks. The samples must be prepared with care to avoid unintended release of methane prior to submitting the samples to the flask, thus affecting the overall measurement. Analysis itself includes heating of the flask at 120°C for eight hours to release methane from the sample into the flask and subsequent injection of the gas into GC apparatus by a syringe. The residual concentration of methane is calculated as the ratio between the amount of methane released in the flask and the original weight of the sample. Samples for GC analysis were collected at the cable manufacturer as produced and after one, two, three and ten days of degassing at 70°C. Results of the GC analysis can be observed in Figure 4 . The cable core produced with conventional XLPE initially contained 400 ppm of methane prior to degassing. Degassing of seven days was required to reduce its concentration below 100 ppm. In contrast, initial methane concentration in the cable core produced with Endurance HFDD-4201 insulation was around 150 ppm, and decreased below 100 ppm after two days of degassing. What does it mean for a cable producer? Since the degassing process is the most time-intensive process in a cable-manufacturing factory, reducing the degassing time represents significant productivity improvements for the cable manufacturer. For EHV cables, this could mean reducing the production cycle time by up to three to four weeks.

Figure 2 : Degassing simulation of 132kV and 220kV XLPE cables

Thermo-gravimetrical analysis (TGA) Thermo-gravimetrical analysis (TGA) is an analytical method widely used in the cable industry for assessment of crosslinking by product concentration. This method is relatively simple, fast and easy to implement. Principles and requirements of TGA degassing assessment are defined in HD 632 S2 (Part 2 – 2.4.15 – Crosslinking by-product concentration). According to this specification, small (20 ±5mg) samples of cable insulation must be taken from the inner, middle or outer part of the insulation. The test pieces are heated to 175°C for 30 minutes in the TGA apparatus with a minimum rate of 50°C per minute. Loss of weight of each sample is recorded over this period. The samples pass the test if the following three criteria are met: • Total weight change after 30 minutes is less than 1.6% • Rate of weight change during the first five minutes is less than 0.18%/min • Rate of weight change during the last 15 minutes is less than 0.015%/min According to HD 632 S2, samples of conventional XLPE and novel Endurance HFDD-4201 were analysed by TGA. The samples were taken from fresh (non-degassed) full-size 132kV cable cores of identical cable design and processing conditions (1,200mm 2 copper Milliken conductor, 17mm insulation thickness). Results of the TGA evaluation are summarised in Figure 3 , where a fresh (non degassed) cable core produced with Endurance HFDD-4201 has met all three criteria while conventional XLPE insulation failed to pass the test, as expected.

Figure 4 : Results of GC analysis

Productivity improvements are generated due to a combination of factors, such as: • Lower energy consumption from degassing chambers • Reduced inventory cost • Improved cycle time and potential capacity increase by de-bottlenecking of the degassing process The reduced degassing energy consumption and the reduced methane emissions from the cable also contribute to CO 2 footprint reduction from cable processing.

Figure 3 : TGA analysis of fresh full-scale cable cores

Dow Inc 2211 HH Dow Way, Midland MI 48674, USA Tel: +1 989 636 1000 Fax: +1 989 832 1456 Website: www.dow.com

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July 2023