The value of an insulation formulation depends on where its properties affect the complete cable system and which constraints govern the project.

Thermal Capability and Usable Ampacity

Conventional transmission-class XLPE cable systems are commonly designed around a maximum continuous conductor temperature of approximately 90°C. Permissible conductor temperature is an important input to cable current rating, but it is not the sole determinant of circuit ampacity. Conductor losses, metallic sheath losses, cable geometry, bonding arrangement, installation configuration, ambient conditions, soil or backfill thermal properties, and external thermal resistances all contribute to the achievable current rating.

The distinction is important when evaluating higher-temperature thermoplastic formulations. A higher permissible conductor temperature can provide additional thermal margin, but the resulting ampacity benefit depends on the complete installation. External thermal resistance, adjacent circuit heating, and the temperature limits of accessories or other components can reduce or constrain the additional capacity available from a higher-temperature insulation system.

A comparative Dutch utility study illustrates this point at medium voltage. When Prysmian P-Laser and XLPE cables were compared at the same 90°C conductor limit, the tested P-Laser cable exhibited lower ampacity because of differences in insulation thermal resistivity. The P-Laser cable achieved higher ampacity only when its higher permissible conductor temperature was utilized. Although those results should not be extrapolated directly to high-voltage (HV) or extra-high-voltage (EHV) designs, the engineering lesson is transferable: A polymer temperature rating is not the same as an ampacity result.

HVDC Converter Topology and DC Insulation Duty

HVDC operation introduces insulation stresses that differ from high-voltage alternating current (HVAC) service. Conductivity, temperature gradients, charge transport and space-charge accumulation can affect the electric-field distribution within the insulation under DC electric stress. Space-charge measurements are therefore important tools for evaluating insulation behavior, alongside the electrical and thermal testing used to qualify complete HVDC cable systems.

The converter technology is an important part of that discussion. In a line-commutated converter (LCC) system, reversal of power flow involves reversal of the operating DC voltage polarity on the cable. In a voltage-source converter (VSC) system, power flow can be reversed without changing cable voltage polarity. The qualification duties are therefore not identical.

Polarity reversal is significant because charge accumulated within the insulation before reversal can temporarily reinforce the applied electric field after the voltage changes direction. CIGRE consequently includes polarity-reversal duties in qualification programs intended for LCC operation, whereas VSC qualification does not require normal operating polarity reversal.

This distinction changes how certain material claims should be evaluated. Sumitomo Electric’s DC-XLPE technology, for example, is specifically differentiated by its ability to operate at a 90°C conductor temperature while supporting polarity reversal for LCC systems. Earlier extruded HVDC cable systems were commonly designed at lower conductor temperatures, so 90°C can represent a meaningful thermal increase in the HVDC context even though 90°C is familiar in conventional HVAC XLPE systems.

For an owner considering a VSC interconnector, polarity reversal is not part of the normal operating duty. For an LCC project, it may be critical. The converter topology should therefore be identified before material-level HVDC comparisons are made.

Manufacturing Throughput Versus Project Schedule

Conventional peroxide-cured XLPE manufacturing requires chemical cross-linking followed by degassing to reduce residual cross-linking byproducts. Depending on cable dimensions and processing conditions, this stage can require substantial factory time.

Thermoplastic insulation follows a different production route. Because chemical cross-linking is not required, the associated degassing stage is eliminated. This can increase manufacturing throughput and could improve a manufacturer’s ability to utilize extrusion capacity.

The project implication, however, should not be overstated. Reduced factory processing time is not equivalent to reduced project lead time. For a 115- to 345-kV urban underground circuit, cable manufacturing might be only one component of the critical path. Permitting and civil construction, utility conflicts, trenchless crossings, joint-bay construction, outage windows, shipping and delivery-length logistics, and availability of qualified jointing crews can equal or exceed the schedule influence of insulation processing.

The manufacturing advantage of thermoplastic insulation is therefore most valuable when the cable factory is a project or portfolio constraint. Where civil execution or system outages control the schedule, eliminating several days of factory processing could improve supplier throughput without materially advancing the project energization date.

Reduced-degassing XLPE offers another route to improving factory throughput, making the manufacturing comparison formulation-specific.

End-of-Life Circularity and Market Readiness

The permanently cross-linked molecular structure of XLPE prevents conventional melting and mechanical reprocessing in the same manner as a thermoplastic. Research into alternative recovery, chemical recycling and circular pathways for XLPE cable materials is ongoing, but these approaches should not be confused with a mature closed-loop market for transmission-cable insulation.

PP-based thermoplastic insulation provides a more straightforward technical route to remelting and reprocessing. That is a legitimate material advantage. It does not, however, guarantee that insulation removed from a transmission cable will economically return as new transmission-grade insulation.

Separation economics, contamination, collection volume, material traceability, qualification requirement, and the existence of a buyer for recovered polymer are all part of the actual recycling pathway. At transmission scale, neither thermoplastic nor XLPE systems currently should be assumed to have a universally available closed-loop recovery chain.

For owners seeking a meaningful circularity benefit, the preferred approach is to move beyond a specification statement such as “recyclable insulation.” The procurement strategy should identify the intended end-of-life treatment, responsibilities for material recovery, any manufacturer take-back program, and the expected reuse or recycling pathway. Technical recyclability is a material property; circularity is a supply-chain outcome.

Polypropylene offers attractive electrical and thermomechanical properties for power-cable insulation, but conventional PP can exhibit stiffness that is undesirable for practical cable manufacturing and installation. Development has focused on PP copolymers and blends that retain favorable electrical characteristics while achieving the mechanical behavior needed for cable applications.

One of the most developed proprietary examples is Prysmian’s P-Laser technology, which uses a high-performance thermoplastic elastomer. HPTE should not be treated as a separate generic polymer family from polypropylene; it is a PP-based thermoplastic formulation engineered to combine electrical, mechanical and processing characteristics suitable for power cables.

There is substantial commercial experience with P-Laser at medium voltage. Prysmian has reported tens of thousands of kilometers of medium-voltage deployment, including utility installations in Europe. That service history is significant, but it should not be used as a direct proxy for maturity at transmission voltage. Medium-voltage distribution service and 115- to 345-kV transmission service have different electrical stresses, accessory requirements, installation practices, consequence of failure and owner qualification expectations.

At higher voltage classes, the maturity picture is more specific. HPTE systems have progressed through HVAC development and EHVDC qualification programs, including cable and accessory testing at 525-kV-class HVDC voltage levels. Prysmian also identifies P-Laser systems for 320-kV and 525-kV HVDC projects. While this establishes HPTE as more than a laboratory material, its long-term transmission operating history, particularly in North America, remains materially less extensive than conventional XLPE.

The distinction is important for procurement. “Commercial” at medium voltage does not mean that an owner should assign the same technology-risk weighting to a 345-kV transmission circuit. Maturity should be assessed by voltage class, application, accessory system, installed population, operating duration and geography, rather than by whether the insulation has been commercialized somewhere within a manufacturer’s portfolio.

Temperature capability also requires similar qualification. Prysmian’s EHVDC HPTE system is designed for conductor operation up to 90°C. In an HVAC comparison, that temperature would not inherently exceed the common XLPE design basis. In an HVDC comparison with systems designed around lower conductor temperatures, however, it can represent meaningful additional thermal margin. The true value of HPTE depends on the operating duty against which it is being compared.

A new insulation material without a qualified accessory system is not a viable transmission cable system. Joints and terminations contain interfaces between different insulating materials, field-control components and mechanically assembled surfaces that must remain electrically, thermally and thermomechanically compatible throughout system life.

Prysmian’s 525-kV P-Laser platform includes qualified joint and termination designs and is described by the manufacturer as compatible with its existing HVDC accessory technology. This demonstrates accessory readiness within a qualified proprietary system.

It should not, however, be interpreted as universal accessory interchangeability across suppliers. When evaluating a thermoplastic transmission cable, the owner should determine:

• Which joints and terminations have been qualified with the exact cable construction.
• Whether transition joints to other insulation systems have been demonstrated.
• Who is qualified to install the accessories.
• What alternatives exist if the original supplier is unavailable during future repair or expansion.

Accessory maturity is therefore both a technical criterion and a life-cycle procurement criterion. 

Innovation in cable insulation is not limited to thermoplastics. Manufacturers and material suppliers continue to develop XLPE formulations intended to improve electrical performance and manufacturing efficiency while retaining the established XLPE platform.

Sumitomo’s proprietary DC-XLPE provides an important example. Its insulation incorporates inorganic fillers to modify charge behavior and improve DC characteristics. More recent development has introduced new filler formulations that have undergone CIGRE-recommended type testing and additional long-duration electrical testing. Sumitomo’s qualification for 90°C conductor operation with polarity reversal illustrates how a purpose-built formulation can address a specific HVDC duty.

A second development path focuses on manufacturing rather than DC electrical performance. Dow’s ENDURANCE HFDD-4201 remains a peroxide-cross-linked compound but has been formulated to reduce cross-linking byproducts and substantially shorten degassing time. This addresses part of the manufacturing advantage associated with thermoplastic systems without changing the fundamental insulation platform.

Advanced XLPE illustrates an important point for owners: The established platform is not static. A procurement comparison between “XLPE” and “thermoplastic” can obscure meaningful differences among conventional AC XLPE, purpose-built DC-XLPE, reduced-degassing compounds and other proprietary formulations.

For utilities and system planners, insulation technologies should be compared as complete cable systems rather than solely by base-polymer chemistry.

Figure 1: Technology comparison.

The table in Figure 1 is intentionally qualitative. A specific material formulation may outperform these generalizations in one or more categories. More important, the cable, joints, terminations, installation conditions and required operating duty must be evaluated as a system.

The characteristics compared in Figure 1 may carry different weights, depending on the circuit’s operating duty, installation conditions and project requirements.

HVAC Underground Transmission, 115- to 345-kV: Near-Term Procurement

For a near-term U.S. HVAC transmission project, conventional or advanced XLPE provides a practical starting point for comparing qualified cable systems.

The reasons are practical rather than ideological. XLPE has the deepest domestic operating history, broadest supplier base, mature accessory ecosystem, established Association of Edison Illuminating Companies (AEIC)-based utility specifications, and an experienced installation and jointing workforce. Reduced-degassing XLPE may also provide some manufacturing benefit without requiring the owner to introduce a new insulation platform.

A PP-based thermoplastic alternative deserves serious consideration when the owner has a defined objective that the alternative directly addresses, such as a formal circularity requirement, a specific manufacturing constraint or demonstrated system-level thermal benefit. In that case, the specification should require qualification of the complete cable-and-accessory package and explicitly evaluate supplier concentration, spare strategy, transition joints, installation training and long-term repair support.

The insulation choice is unlikely to “unlock” a constrained urban corridor by itself. If ampacity is governed by duct-bank geometry, native soil, engineered backfill, mutual heating, depth or other external thermal resistance, changing insulation may have limited impact. The civil and thermal design should be established before assigning significant project value to the polymer selection.

HVDC Land Cable, 320- to 525-kV: Converter Topology

For an HVDC project, the first insulation question should be, “What operating duty will the converter impose on the cable?”

For an LCC project that requires polarity reversal, compare qualified cable-and-accessory systems under that operating duty, including the permitted conductor temperature. Purpose-built DC-XLPE systems have specifically targeted polarity reversal at elevated conductor temperature. HPTE systems have also been tested for demanding DC electrical stresses and qualified at EHVDC levels.

For a VSC project, the comparison can place greater emphasis on factors such as conductivity and field stability at operating temperature, allowable conductor temperature, cable losses, accessory packages, factory capacity, project delivery slot, installed reference base, repair strategy, and the owner’s end-of-life requirements.

For either converter technology, the owner should evaluate the cable and accessory system as the qualified product. A proprietary insulation with excellent material data but limited accessory or repair options may represent greater life-cycle risk than a nominally less-advanced polymer with a mature system package.

Thermally Constrained Urban Duct Bank: Model Before Specifying

Projects constrained by available corridor space often create the strongest temptation to specify a “higher-performance” insulation in pursuit of additional ampacity. This is also where system-level analysis matters most.

The first step should be to identify what actually limits the rating. If the critical constraint is conductor temperature and a qualified cable system permits a higher operating temperature, a higher-temperature thermoplastic formulation may justify detailed study. If the governing limitation is an accessory, sheath losses, external thermal resistance, mutual heating, tunnel ambient temperature or another installation parameter, the higher insulation temperature rating may provide little or no additional usable capacity.

Model the circuit first. If conductor temperature is truly the binding constraint, evaluate higher-temperature insulation. If it is not, spend the engineering effort on the constraint that actually limits the circuit.

Evaluation of emerging cable insulation should not result in replacing an XLPE default with a polypropylene default. The more useful outcome is a change in how insulation decisions are made.

For near-term U.S. HVAC transmission projects, XLPE’s field history, qualification maturity, accessory availability and supplier competition make it a practical starting point for evaluation. PP-based thermoplastics warrant consideration when their manufacturing or end-of-life characteristics address a defined project requirement and the owner is prepared to qualify and support the complete proprietary system. For HVDC projects, converter topology and the resulting cable duty should drive the comparison: Polarity-reversal performance matters substantially more for LCC operation than for VSC. For thermally constrained corridors, insulation selection should follow, not precede, the system thermal analysis.

The most consequential change might be in owner specifications. Where project requirements and procurement practices allow, owners should define the performance and qualification requirements of the complete cable system before deciding whether a specification should permit more than one insulation platform. Those requirements may include allowable continuous and emergency conductor temperature, current rating under defined installation conditions, DC conductivity and field stability where applicable, LCC polarity-reversal capability where required, cable-and-accessory qualification, manufacturing constraints, supplier and repair-support expectations, and a defined end-of-life treatment strategy.

Material technology will continue to evolve. The role of the consulting engineer is not to predict which polymer will “win,” but to determine whether a particular innovation changes the governing constraint, reduces project or life-cycle risk, or creates measurable value for the owner. The next step beyond conventional XLPE is not a new default; it is better engineering judgment.