Wanma Macromolecule XLPE WMY-4201 EHV is supplied as a peroxide-curable low-density polyethylene compound for the insulation layer of extruded extra-high-voltage alternating-current land cables. In continuous vulcanization, the compound is co-extruded as the central layer between an inner semiconductive conductor shield and an outer semiconductive insulation shield through a triple crosshead. Barrel-zone setpoints are held within 115–125°C; head melt temperature is not permitted to exceed 130°C because dicumyl peroxide decomposition begins to create prematuration scorch. The dry-cure tube operates under nitrogen at a pressure not less than 10 bar for EHV wall sections to suppress voiding during the removal of peroxide decomposition by-products. Specific batch-to-batch variance data for WMY-4201 EHV as supplied are not published in open literature; the following controls reflect standard EHV XLPE insulation practice and cable manufacturing operational envelopes.

For rated voltages from 220 kV to 500 kV, insulation radial thickness is not fixed by the compound itself but by the cable design’s lightning impulse withstand level and screen field strength under IEC 62067. Typical EHV AC cores therefore use insulation sections with radial thicknesses in the multi-tens-of-millimetres range, which require extrusion line speeds commonly below 1.0–2.5 m/min to maintain adequate cure progression through the wall. At the cable factory, the controlling variables are the operating pressure of the nitrogen tube, the conductor preheat temperature, and the concentricity of the triple-layer wall. Radial thickness tolerance of ±0.5 mm or better is frequently controlled by X-ray diameter gauges installed after the cooling stage, because off-centre insulation alters the dielectric field distribution and creates premature partial discharge at the semicon interface.

Production acceptance for the insulation layer follows type tests under IEC 60840 for cables rated 30 kV to 150 kV and IEC 62067 for cables rated above 150 kV to 500 kV. Partial discharge is measured with a conventional discharge detector under IEC 60270, with production sampling sensitivity commonly set at 5 pC for EHV core lengths. Dielectric loss factor is checked on the core using a Schering bridge, with values validated against the cable manufacturer’s internal control limits rather than a single universal public limit. Hot set testing under IEC 60811-501 at 200°C and 0.2 MPa verifies crosslink density after solvent extraction, with maximum elongation under load and permanent set limits used to reject under-cured sections. Gel content is additionally measured by xylene extraction according to ASTM D2765; EHV XLPE insulation characteristically requires gel content not below 82%, but the precise acceptance window is set by the cable manufacturer’s qualification dossier.

When DC Space Charge Thresholds Extend Degassing Beyond 45 Days

For extruded HVDC land cable insulation, the acceptance philosophy differs from AC applications because the direct-current field distribution is governed by conductivity rather than permittivity. After the same triple-layer dry-cure extrusion, peroxide decomposition by-products such as cumyl alcohol, acetophenone, and methane remain dissolved in the XLPE wall. These species act as charge traps and can contribute to heterocharge accumulation when the cable transitions from no-load to full-load temperature profiles. For insulation walls typical of ±320 kV and ±525 kV extruded HVDC systems, forced-air degassing chambers are operated at 60–70°C with temperature uniformity within ±2 K. Degassing duration is not a fixed parameter; it is terminated only when residual methane concentration in the insulation falls below the cable maker’s internal limit, and for thick walls this can exceed 45 days. The process is therefore a long-cycle batch operation interposed between continuous vulcanization and final cable assembly.

Space charge profiles are measured by the pulsed electroacoustic method on moulded plaques or cable sections under DC fields before and after load cycles. The critical operational boundary is the threshold field at which heterocharge density distorts the internal field beyond the design margin. If degassing is truncated, the effect is not necessarily visible in the factory acceptance test; it appears later as increased apparent conductivity under a temperature gradient or as space-charge accumulation near the semicon interface after polarity reversal in older line-commutated converter systems. The compound must therefore be processed with a degassing schedule linked to wall thickness and residual by-product limits. Published data for this specific WMY-4201 EHV formulation’s space-charge response is limited; procurement specifications generally require type testing under IEC 62895 and the relevant CIGRE recommendations.

End-product configurations are land-based point-to-point HVDC circuits, frequently with 2,500 mm² copper or aluminium conductor section and an impermeable metallic sheath. The insulating layer is not used with water-tree retardant additives because their ionic residues are incompatible with DC charge transport control in EHV service. Compliance for the complete cable system includes load-cycle testing with DC voltage, superimposed lightning and switching impulses, and embedded thermocouple conductor temperature measurement during thermal qualification. The compound is not downgraded to AC service after a failed HVDC qualification; a separate AC qualification must be completed because the observed space-charge behavior has no direct AC analogue.

In submarine HVAC export cable core production, the triple-layer dry-cure construction is qualified for continuous immersion service rather than directly buried duct service. The moisture barrier is the continuous metallic sheath, normally extruded lead or an aluminium laminate with an outer polyethylene jacket; the XLPE insulation itself does not receive a water-tree retardant package at EHV because polar retardant additives raise the dielectric loss factor. The lead sheath wall is applied immediately after core heating, at a thickness determined by fatigue and corrosion calculations under IEC 60287 ampacity and the cable laying depth. The insulation layer is therefore kept free of regrind; no scrap is fed into the insulation extruder on submarine export lines because contaminant ingress during storage or transport creates a direct dielectric defect.

Offshore wind export circuits commonly operate at rated voltages from 66 kV to 220 kV, with some interarray designs entering the EHV range. The insulation is exposed to dynamic bending during lay-vessel handling and to static bending in J-tube or horizontal directional drilling pulls. Processing limitations in the cable factory focus on maintaining concentricity across long continuous lengths, because a single export cable section can exceed 20 km without factory jointing. X-ray diameter gauges and thermal-shrinkage compensation in the cooling tube are used to control radial thickness within ±0.3 mm across the core; thickness variation directly affects bending strain distribution in the oversized insulation. This is a more stringent concentricity requirement than on many land-based cable cores because installation bending radii in submarine export projects are often close to the minimum permitted by the cable design.

Type qualification follows IEC 60840 or IEC 62067 depending on rated voltage, plus additional mechanical preconditioning simulating installation loads. The finished core is not declared compliant until a full high-voltage AC test, partial discharge measurement, and final insulation resistance check are completed on the core before metallic sheathing. In offshore export service, the use of the compound is therefore inseparable from the metallic sheath design; the compound does not provide the primary moisture resistance. Amine-based adhesion promoters and unvalidated semicon formulations are excluded from the triple-layer package because they can migrate into the XLPE during service and alter dielectric loss or space-charge stability under wet-heat conditions.

Submarine Interconnector Insulation Under Thermal Gradient and Near-Zero Cooling Shrinkage

For HVDC submarine interconnector insulation, the charge-transport constraints of land HVDC insulation are combined with permanent hoop stress from deep-water lay and a through-wall temperature gradient. The conductor operates above 70°C while the outside of the lead sheath remains near seawater temperature, producing a temperature difference across the XLPE wall that can routinely reach 15–25°C. Because XLPE conductivity is thermally activated, the temperature gradient shifts the point of maximum field strength away from the conductor; the design must verify that the inner semicon interface does not become field-relaxed while the outer region exceeds the intended defect tolerance. This is assessed in type testing by applying DC voltage with elevated conductor temperature and measuring space charge after thermal cycling, rather than by a room-temperature dielectric strength test alone.

Manufacturing of the submarine interconnector core uses the same dry-cure triple extrusion and degassing sequence as for land HVDC, but the formulation is subjected to stricter cleanliness screening because a longer service life with no repair access is assumed. The lead sheath is not a secondary barrier but a structural component; after lead extrusion, the core is overwrapped with armouring and outer serving in a continuous line. There is a documented operational boundary at the cooling stage: if the core is cooled too rapidly through the crystalline solidification window, internal shrinkage can produce microvoids that are not detected by standard partial discharge at factory test but can initiate DC space-charge instability under service. The cooling tube is therefore zoned to reduce the temperature gradient below 5 K/min in the outer half of the insulation wall. Published field data for this specific compound under submarine HVDC thermal gradient is limited, so qualification programs commonly require additional samples from each conductor size and insulation wall combination.

In factory-molded EHV accessory manufacture, the compound is injection-moulded into a thick annular cavity rather than extruded as a uniform tube. The WMY-4201 EHV compound, if used for such moulded bodies, must be plasticized without exceeding the peroxide decomposition threshold for a longer residence time. Mould temperatures are typically held at 115–125°C, with pressure-ramp control to avoid knit lines in the insulation body. Premature crosslinking during mould fill produces visible flow lines and decreases AC breakdown strength; the main field-observed failure mode is not short-term flashover but partial discharge inception at a knit line after installation and repeated load cycling. Moulded insulation bodies therefore require cavity pressure sensors and shot-weight tracking rather than manual visual inspection alone.

Moulded joint insulation is not produced to the same long-length cable standards but to the accessory type test requirements of IEC 60840 or IEC 62067, including partial discharge, impulse withstand, and thermal short-circuit tests. The ratio of oil or release agent is zero; all mould-release systems are eliminated because silicones and hydrocarbon greases become interfacial contaminants at the semicon boundary. The terminal enclosure or joint shell is not filled with the XLPE compound; the compound is confined to the stress-control insulation body. Operational limitations include the inability to remelt and rework crosslinked sections; any under-injection fill or sink mark requires full scrapping of the body and cannot be corrected by thermal soaking.

The finished product is a straight-through or branch joint insulation body for service in land cable circuits, usually for rated voltages of 110 kV and above. Production-scale batch records normally include shot weight, mould pressure curve, peak cavity pressure, and post-cure hot set test on a specimen cut from the same moulding run. These records serve as the only verifiable linkage between compound lot and installed accessory; published data for this specific compound in moulded EHV accessory use is limited.

What Limits Line Speed in 500 kV AC Core Extrusion?

Line speed in a 500 kV AC core is limited by the thermal cure front through the thickest insulation wall, not by extruder throughput. In a dry-cure continuous vulcanization tube, the insulation wall for a 500 kV cable is large enough that the core must remain in the tube for a period defined by the peroxide decomposition half-life at the selected cure temperature and the heat-transfer condition at the conductor interface. If line speed is increased beyond the validated cure-time window, the centre of the insulation wall remains under-cured because the peroxide decomposition front does not reach the conductor shield. The factory detects this after the cooling stage by measuring hot set elongation across the full wall; under-cured centres produce high hot set values even when the outer surface is fully crosslinked.

At production scale, operators therefore run 500 kV cores at the lower end of the EHV extrusion speed envelope, with conductor preheat and tube temperature increases used only within narrow limits. The conductor is preheated to reduce the thermal sink, but conductor surface temperature must remain below the semicon compound’s scorch limit during triple co-extrusion. The vulcanization tube is sectioned into heating and cooling zones under nitrogen pressure; the transition between them is not instantaneous, and residual heat can continue curing if the line is stopped. The highest practical line speed is therefore determined by a combination of minimum cure time, maximum allowable tube length, and the cooling rate needed to avoid shrink voids. Any attempt to raise throughput by reducing insulation thickness is rejected if the revised wall falls below the lightning impulse design margin under IEC 62067.

The end use is the insulation of the EHV AC cable core prior to on-line degassing and coiling. Specific line-speed data for WMY-4201 EHV in a defined tube length is not available in open literature; process validation is always performed by cable manufacturers on their specific continuous vulcanization line geometry, conductor size, and insulation thickness. The boundary condition is declarable not as a fixed metres-per-minute value but as the point where the cure state and void content fail the hot set and partial discharge acceptance limits.