Influence of polypropylene content in PPLP for high voltage insulation at cryogenic temperature

Influence of polypropylene content in PPLP for high voltage insulation at cryogenic temperature Luhan Zu 1, Stéphane Holé1*, Nicolas Lallouet2, Christian-Éric Bruzek3 1LPEM (UMR8213), ESPCI Paris – PSL University, Sorbonne University, CNRS, Paris, France 2Nexans France, Calais, France 3ASG Superconductor, Genova, Italy *[email protected] Abstract— This study compares the dielectric performance of polypropylene-laminated paper (PPLP) insulation impregnated with liquid nitrogen in the context of high-power superconducting cables within the SCARLET European project. Samples with polypropylene (PP) contents of 43% and 55% are tested in a representative cable geometry including almost 20% gap density in the insulation. Breakdown voltage is found to increase with hydrostatic pressure. Weibull analysis predicts similar breakdown field strength at 63% probability for both kinds of samples (about 893kV/mm) but a larger scale parameter for 55%-PP-content sample (about 10 versus 7). This better performance for 55% PP content is mitigated by a larger brittleness upon breakdown. Keywords—Voltage breakdown, lapped insulation, cryogenic, super-conducting, cables I. INTRODUCTION Polypropylene laminated paper (PPLP) has been recognized as a high-performance composite insulation material for high-voltage applications, particularly in cryogenic environments. Composed of layers of Kraft paper and biaxially-oriented polypropylene (BOPP) film, PPLP offers a combination of high dielectric strength, low dielectric loss, and good mechanical flexibility [1]. These characteristics have made it the insulation of choice in many cryogenic systems, including high-voltage bushings, superconducting cable terminations, and power feedthroughs [2]. In the context of superconducting power transmission, the insulation must not only withstand strong electric fields at cryogenic temperatures but also tolerate mechanical stresses caused by thermal contraction and cable bending [3]. The European Union’s SCARLET [4] (Superconducting Cables for Sustainable Energy Transition) project aims to develop advanced superconducting cable systems for grid integration of renewable energy. Within this effort, a key challenge is to optimize the cable insulation to ensure reliability and high voltage withstand under realistic operating conditions. While PPLP has proven performance of its internal composition and specifically the proportion of polypropylene versus paper [5], it can significantly influence its dielectric and mechanical behavior. Traditionally, a higher polypropylene content is assumed to enhance electrical insulation due to the superior dielectric properties of Polypropylene (PP) [6] compared to Kraft paper. However, this assumption is largely based on flat-sample tests and does not account for the behavior of PPLP when applied as helical tape in cylindrical geometries, as in real cable structures. The geometric configuration of insulation layers significantly affects both the mechanical stability and dielectric breakdown characteristics of cable insulation under operational conditions [7]. In actual superconducting cables, the layered PPLP tapes are helically wound around a cylindrical conductor. This configuration introduces complex mechanical constraints and stress distribution patterns arising from thermal contraction differences between layers and the conductor core at cryogenic temperatures [8]. Additionally, electrical stress distributions become less uniform due to layer overlaps and butt gaps inherent to the winding process, potentially creating localized weak points that influence dielectric performance [9]. Furthermore, the balance between polypropylene and Kraft paper not only impacts electrical performance but also influences the impregnation quality of cryogenic liquids such as liquid nitrogen [10]. The fiber structure of Kraft paper facilitates better liquid penetration and impregnation, enhancing dielectric uniformity and mechanical performance under operational stresses [11]. Conversely, higher polypropylene content, while intrinsically offering superior dielectric strength, increases the rigidity of the PPLP layers. This rigidity reduces flexibility, which may limit impregnation efficiency and potentially introduce mechanical vulnerabilities, especially under rapid temperature changes or mechanical stress during handling [12]. Previous studies on the dielectric performance of PPLP have been primarily conducted using flat-sheet samples or simplified geometries under controlled electric fields [13-15]. These investigations have generally reported a positive correlation between higher polypropylene content and improved dielectric strength, based on the superior intrinsic electrical properties of polypropylene compared to Kraft paper. However, such configurations may not fully reflect the conditions encountered in practical cable systems, where the insulation is applied as helically lapped layers with specific mechanical and geometric constraints. Factors such as winding-induced stresses, layer interfaces, butt gaps, and localized field enhancement may influence the actual breakdown behavior in ways that are not easily captured in flat-sample tests and also its mechanical performance [16]. The insulation is no longer only PPLP, but a composite structure mixing PPLP at some positions and cryogenic liquid filling the gaps between PPLP strips at other positions. In this study, the influence of polypropylene content on the dielectric strength of two kinds of PPLP material is investigated using a representative cable configuration. Both kinds of PPLP are commercially available in large quantity that makes them convenient for the insulation of high voltage cables. Samples, measurement setup and measurement protocol are described in the next section. Before conclusion, breakdown voltages are analyzed in terms of voltage Author Accepted Manuscript version of the article by Luhan Zu et al. in Journal of Electrostatics, 2025, vol. 138, 104164 https://doi.org/10.1016/j.elstat.2025.104164 For the purpose of Open Access, the authors have applied a Creative Commons Attribution License (CC BY 4.0) to any accepted manuscript version arising. withstanding but also in terms of mechanical degradation upon breakdown. II. MEASUREMENT SYSTEM A. Sample Description Two types of multi-layer PPLP insulation samples are prepared for breakdown testing, differing in their polypropylene content and structural thickness. The first sample (from Tervakoski) contains 55% polypropylene with an average density of 0.95Hg/cm³, while the second sample (from Sumitomo) has 43% polypropylene, with an average density of 0.86Hg/cm³. Due to differences in the manufacturing process, the thickness of a single layer for the 55%-content PPLP is 0.168Hmm, whereas it is 0.125Hmm for the 43%- content PPLP. The thickness tolerance each kind of layer is ±5%. Visually, the higher-polypropylene-content sample exhibits a darker coloration, providing a convenient means of distinguishing the two materials. Fig.H1: Tested samples still enclosed in their protection film: (a) PP ratio 55%; (b) PP ratio 43%. Both samples were fabricated by winding four or five layers of PPLP butt-to-butt over a 70 cm long stainless-steel tube, about 25Hmm in diameter. The overall insulation thickness is thus about 0.65Hmm for both samples (0.625Hmm for 43%-content PPLP and 0.672Hmm for 55%-content PPLP). A custom-designed winding machine at NEXANS [4] is used to reproduce the real cable manufacturing process (but for small sample length), ensuring precise control over critical parameters such as tape tension, winding inclination, butt gaps, and layer recovery. This process guarantees uniformity in the lapped layer thickness and ensures reproducibility across different test samples such as those shown in Fig. 1. As PPLP tapes are 20-mm width and gaps are about 4Hmm, then 4 or 5 layers already give a similar sample gap density in the composite insulation structure. In addition, such a limited number of layers does not require very high voltage source for breakdown voltage experiments. B. Measurement Setup The experiment is conducted in a cryostat equipped with highvoltage feedthroughs and sealed mechanical access ports (see Fig.H2). After placing the sample inside the holder and completing the assembly, the cryostat is fully filled with liquid nitrogen using an external tank, a process that take approximately three hours. The cryostat provides good sealing performance, and internal pressure can be regulated by adjusting the gas outlet flow rate. Upon completion of the tests, the remaining liquid nitrogen is returned to the storage tank through pressure control. The system is then left to warm up to room temperature before opening the cryostat for surface inspection of the sample. Fig.H2: Photography of the setup. The sample holder is designed to enable breakdown measurements at multiple positions along a cylindrical insulation sample. As shown in Fig. 3, the structure includes a fixed cylindrical support that holds the test sample and a movable grounded electrode mounted on three parallel rods. The ground electrode consists of a conductive tube connected to the spark point which enables the persistent conduction after the movement. Springs in the clamping system maintains stable contact during displacement as shown in Fig.H4. The electrode assembly can be controlled by a motor and moved along the sample axis over a length of approximately 50 cm allowing up to 30 different measurement points without breaking measurement conditions. Fig.H3: Photography of the holder mechanism for adjusting the electrical breakdown point using a threaded rod. The breakdown occurs at the intersection between the inner high-voltage electrode embedded in the sample and the external grounded rod. The cylindrical geometry of the electrode allows repeated positioning without causing surface damage to the sample. The resulting electric field distribution at the intersection point remains relatively uniform through the sample thickness. The electric field is however slightly higher close to the electrodes than in the middle, but this variation is limited to approximately 1%, which is not expected to have a significant impact on the overall analysis of the experimental results [16]. Author Accepted Manuscript version of the article by Luhan Zu et al. in Journal of Electrostatics, 2025, vol. 138, 104164 https://doi.org/10.1016/j.elstat.2025.104164 For the purpose of Open Access, the authors have applied a Creative Commons Attribution License (CC BY 4.0) to any accepted manuscript version arising. Fig.H4: Photography of the high-voltage breakdown point contact configuration. A DC high-voltage power supply is connected to the inner conductor at the bottom of the cryostat. The voltage is increased at a constant rate of 200HV/s under continuous monitoring of both voltage and current. A breakdown event is detected when the current exceeds a fixed threshold of 100 µA. This threshold is selected based on the value of the resistor in the ground path. Upon detection, the power supply automatically interrupts the voltage to terminate the event. C. Experimental Method After installation of the sample in the cryostat, liquid nitrogen is introduced automatically by utilizing the pressure difference between the cryostat and the storage tank. A level sensor is used to monitor the filling process and ensures that the sample is fully submerged. This process takes approximately three hours. Breakdown tests are first conducted under a 1 bar pressure condition. The high-voltage supply (model: iseg-HPp-700) is set to begin at 20HkV, and the voltage is ramped up at a constant rate of 200HV/s until breakdown occurrence. The occurrence of breakdown is determined by a preset program within the power supply system. After each breakdown event, the discharge location is shifted by 1.5 cm along the sample, and the procedure is repeated twelve times. Subsequently, the internal pressure is increased to 3 bar, and the same test sequence is repeated. After completing the tests for one sample, the cryostat is emptied, the sample is replaced, and the entire testing process is repeated under identical conditions for the other sample. After all tests completed, the breakdown data are analyzed using Weibull statistical distribution. This method allows for the estimation of the characteristic breakdown field at a 63% probability level, along with the calculation of the shape parameter, which indicates data dispersion. In addition, 95% confidence intervals are determined to assess the reliability and statistical significance of the measured results. This analysis provides a quantitative basis for comparing the dielectric strength of the two PPLP samples and predicting their expected performance under cryogenic high-voltage conditions. III. RESULTS ANALYSIS A. Comparison of Breakdown Fields Strength As samples have not exactly the same overall thickness, results are indicated in electric field units corresponding to voltage over overall insulation thickness. Both PPLP samples are tested under 1 bar and 3 bar hydrostatic pressure conditions, with twelve breakdown points measured for each condition. The original data are in the Appendix. In the case of the 0.55-PP-content sample at 3 bar, all measured breakdown voltages exceeded the upper limit of the power supply (70 kV), and therefore are not included in comparative analysis. However, it is worth noting that under this condition, all points in the Weibull plot would have been above 70HkV/672HµmH=H104.17HkV/mm, thus the scale parameter corresponding to the breakdown strength at 63% probability would also have been above 104.17HkV/mm. Figure 5 shows the Weibull diagram of the measurements as well as the breakdown field estimation at 63% (scale parameter), its variation with probability (shape parameter) and the 95% confidence domain. At 1 bar, the 0.55-PP-content sample (light blue) shows a breakdown field at 63% of 88.9 kV/mm, which is slightly higher than the 85.9 kV/mm observed for the 0.43-PP-content sample (light green) under same pressure. The shape parameter is also larger for 0.55-PPcontent sample indicating a better withstanding even at low breakdown probability. Under 3 bar, the breakdown field of the 0.43-PP-content sample (light red) increases up to 98.1 kV/mm but still remains in the measurement window contrary to 0.55-PP-content sample as already indicated. Fig.H5: Comparison of breakdowns for different PPLP sample under 1 bar and 3 bar The effect of the polypropylene content in each PPLP layer seems to improve the breakdown voltage. When the PP content increased from 43% to 55% (a relative increase of 27.9% in PP content), the breakdown field strength at 1 bar is only improved by 0.5% at 63% breakdown probability. This is not really significant considering estimation errors (see TableHII for precise data). In this condition, it can be assumed that gaps control the breakdown. However, the shape parameter of 0.55-PP-content sample is significantly larger than the one of 0.43-PP-content sample. Therefore, at lower CDF, which is expected by high voltage insulation, the breakdown strength of 0.55-PP-content sample becomes rapidly better than the one of 0.43-PP-content sample. Under 3 bars, the breakdown field strength of the 0.55-PP-content sample is at least 104.17HkV/mm whereas the one of the 0.43PP-content sample is only 102.4HkV/mm. It is recalled that 0.55-PP-content sample did not breakdown under 70HkV at 3Hbar, hence its scale parameter is larger than 104.17HkV/mm. These results suggest that in order to fully utilize the dielectric potential of high-PP-content insulation, increasing the operating pressure is an effective and necessary approach. Author Accepted Manuscript version of the article by Luhan Zu et al. in Journal of Electrostatics, 2025, vol. 138, 104164 https://doi.org/10.1016/j.elstat.2025.104164 For the purpose of Open Access, the authors have applied a Creative Commons Attribution License (CC BY 4.0) to any accepted manuscript version arising. Analysis of Confidence The Weibull slope (shape parameter) reflects the concentration of data and the stability of expected performance. As this is a noise-sensitive parameter, the leaveone-out method was used to assess its stability in the parameter estimation. This method consists of estimating the Weibull parameters with all data but one. This is as if one of the data where not measured. When all parameter estimations are done by removing one of the data each time, one obtains the variability of the estimation. Among the tested configurations, the 0.55-PP-content sample exhibits a significant higher shape parameter, indicating greater consistency in breakdown behavior. This can be attributed to the more uniform nature of the polypropylene layer compared to Kraft paper. TABLEHII: WEIBULL PARAMETERS WITH MIN MAX VARIABILITY CALCULATED BY THE LEAVE-ONE-OUT METYHOD. Parameters 0.55PP 1bar 0.43PP 1bar 0.43PP 3bar Scale (kV/mm) 89.0(-1.98+0.96) 89.4(-4.38+1.86) 102.4(-2.44+1.71) Shape (unitless) 10.1(-0.60+1.18) 6.32(-0.35+1.73) 7.31(0.42+1.00)- A higher PP content contributes to improved overall stability of dielectric performance, making it a key factor influencing the slope. In addition, increased pressure also leads to a noticeable improvement in data consistency, suggesting that operating PPLP insulation under higher pressure conditions can enhance its reliability, especially under extreme environments. B. Analysis of Perforations and Fractures After the tests, inspection reveals that the outermost insulation layer of the 0.55-PP-content sample is fractured, showing clear perforation. The sample disassembly shown in Fig.H6 occurred during the sample warming up. At cryogenic temperatures, the sample is sufficiently stiff due to the freezing to remain in place while the breakdown point is displacing. Indeed, the consistency of the breakdown data does not show any sample deterioration during the test. During warming up, the polypropylene becomes softer, then the disassembly appears upon liquid nitrogen drainage and warming back to room temperature. These observations confirm that electrical breakdown events can reduce the structural integrity of the insulation material due to a higher brittleness of the material at low temperature. Fig.H6: Photography of the 0.55-PP-content sample after the testing procedure and warming up. Upon reassembling the fractured insulation layer, a small breakdown hole is identified at the fracture site (Fig. 7a). An irregular fracture propagates outward from this hole, exhibiting smooth, burr-free edges typical of brittle fracture. At other breakdown locations, clear and large perforations are observed (Fig. 7b), where a small piece of material detached due to brittleness of the PPLP layer, forming big throughholes. The hole edges display evidence of shear stress directed toward adjacent regions, indicating that thermal stresses induced by temperature variation causes these fractures. Fig.H7. Mechanical fracture and arc-induced perforation: (a) brittle fracture observed at spark point on outermost layer (0.55PP); (b) distinct perforation caused by electrical arc (0.55PP); (c) arc mark without mechanical fracture (0.43PP). During the breakdown, nitrogen gas bubbles appear and tend to expand. This constrains PPLP to a certain point after which PPLP explodes. The thick PP layer in the 0.55-PP-content sample is more resistant than the thin PP layer in the 0.43-PPcontent sample. It takes therefore more time for bubbles to break the 0.55-PP-content sample, then bubbles store more energy, and the resulting holes are bigger in the case of the 0.55-PP-content sample (Fig. 7a and 7b) than in 0.43-PPcontent sample (Fig.H7c). There is no evidence of fracture propagation from the perforation toward the edges in 0.43-PPcontent sample. Additionally, no direct material detachment due to brittleness is observed in any of the 0.43-PP-content samples already tested. These observations confirm that the 0.43-PP-content samples possess higher flexibility and better resistance to shear stresses. It is worth noting that only the outer layer breaks in the 0.55PP-content sample suggesting that the absence of outer constraints (the outer PPLP layer has a free interface) increase the material deformation and thus band rupture. As in real cables a copper outer layer is used to cover the PPLP layers to well distribute the ground electrode, the constraint imposed by this copper layer should limit mechanical fracture in PPLP. Obviously, such a copper layer cannot be used for the breakdown tests as it would not be possible to displace the breakdown point. IV. Conclusion This study evaluates the dielectric breakdown performance of polypropylene-laminated paper (PPLP) insulation with two different polypropylene contents, tested in a representative cylindrical geometry under liquid nitrogen conditions. The results confirm that breakdown strength increases with hydrostatic pressure and is influenced by both material composition. While the 55%-PP-content sample demonstrates slightly higher breakdown field values at 1 bar and significantly higher values under 3 bars, its performance advantage is not as large as expected based on PP content alone. This indicates that other factors can come into consideration such as the Kraft paper thickness that offers Author Accepted Manuscript version of the article by Luhan Zu et al. in Journal of Electrostatics, 2025, vol. 138, 104164 https://doi.org/10.1016/j.elstat.2025.104164 For the purpose of Open Access, the authors have applied a Creative Commons Attribution License (CC BY 4.0) to any accepted manuscript version arising. more or less porosity between layers, the bonding quality and interfacial defects, by limiting the effectiveness of high-PP insulation if not properly controlled. The Weibull analysis further revealed that higher PP content contributes to more consistent breakdown behavior, as evidenced by narrower confidence intervals and higher slope values. However, these results are mitigated by a higher brittleness of the higher PP content sample that shows large fractures at breakdown points. These results suggest that to fully benefit from high-PPcontent PPLP, proper control of the winding process, encapsulation of the outer layer and the use of pressurized environments are essential to minimize structural imperfections, limit as much as possible fracture propagation upon breakdown and ensure reliable insulation performance in cryogenic superconducting cable systems. APPENDIX Measured breakdown electric fields (kV/mm) are given in the following table. 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