Dry-cure continuous vulcanisation of TR-YJ-66 as the primary insulation for 6 kV to 35 kV distribution feeders places the compound under a narrow thermal corridor before peroxide decomposition begins. The granules are pre-dried at 70 °C ± 5 °C for 2 h to 4 h when open-sack storage exceeds 4 h at 25 °C and 60 % RH. Extrusion is carried out on a single-screw extruder with a barrier screw, L/D ratio 20:1 to 25:1, compression ratio 2.5:1 to 3.0:1. Barrel setpoints from feed throat to metering section are held at 105 °C, 110 °C, 115 °C, 115 °C, and 120 °C; the head and die zones are limited to 121 °C ± 3 °C. The die temperature ceiling sits below the 130 °C threshold at which premature dicumyl peroxide decomposition can generate gel particles, microvoids, and visible surface protrusions. The aluminium or copper conductor is preheated to 80–100 °C before entering the triple crosshead to reduce shrink-back. The dry-cure tube uses nitrogen at 1.0–1.5 MPa, with radiant temperature stages from 180 °C to 220 °C; steam curing is excluded above 6 kV to prevent moisture-induced microvoids that degrade partial discharge performance. Crosslinked insulation is checked by solvent extraction according to IEC 60811-2-1 or GB/T 2951.21; the accepted gel fraction for distribution-class XLPE is ≥ 80 % xylene-insoluble material. Mechanical acceptance parameters are anchored to IEC 60502-2, GB/T 12706.2, and ASTM D638-14: tensile strength before ageing ≥ 12.5 MPa, elongation at break ≥ 200 %, ageing at 135 °C for 168 h with tensile and elongation variation ± 25 %. Hot set testing at 200 °C under 0.2 MPa requires elongation under load ≤ 175 % and permanent set ≤ 15 %. Volume resistivity at 20 °C is ≥ 10¹⁵ Ω·cm. The finished insulation is used on copper- or aluminium-conductor power cables rated 6/10 kV, 8.7/15 kV, 12/20 kV, and 18/30 kV for buried, duct, and cable-tray installation in utility networks; the continuous conductor temperature is 90 °C, with a short-circuit temperature of 250 °C for 5 s.

The following acceptance anchors are used for distribution-class tree-retardant XLPE insulation according to IEC 60502-2 and related test methods.

For large ground-mounted photovoltaic collector circuits where 8.7/15 kV feeders are buried in seasonally wet soils, TR-YJ-66 is specified as the insulation layer because wet-screen and water-tree performance are procurement-critical. The cable is designed with a bonded conductor screen, the TR-YJ-66 insulation, and a strippable insulation screen. Nominal insulation thickness for 8.7/15 kV XLPE under IEC 60502-2 is 4.5 mm; for 12/20 kV collector feeders the thickness is 5.5 mm. The co-extrusion sequence at the triple head keeps the semiconductor screens within 75–95 °C at the die return channels and the XLPE melt at 121 °C ± 3 °C, which prevents screen bleed into the insulation. Water tree retardance is evaluated against ICEA S-94-649 or the utility’s comparative long-term wet ageing protocol; published data for this specific tree-retardant formulation under all field water chemistries is limited, and qualification therefore relies on the cable maker’s wet-screen performance history. The insulation must pass the hot set and ageing requirements of IEC 60811-2-1 and IEC 60811-501 after each production run. In service the cable is expected to operate at 90 °C continuous conductor temperature, 130 °C emergency, and 250 °C short-circuit. The cable is installed from string inverter stations to MV transformer stations, often in trefoil formation. This application excludes direct burial in soils contaminated with aromatic hydrocarbons unless an impermeable metallic or polymeric barrier is placed over the insulation system.

What Mechanical and Thermal Boundaries Constrain 6/10 kV Wind Turbine Tower Loop Cable Insulation?

In wind turbine tower loop applications, the cable is suspended over vertical pulls of 80–120 m and subjected to oscillating torsional loads. TR-YJ-66 insulation is extruded over flexible stranded copper conductors at 3.4 mm nominal thickness for 6/10 kV to IEC 60502-2 and HD 620 S2. The primary conflict is between crosslink density, which must be high enough to pass the 200 °C hot set test, and low-temperature flexibility, which degrades as the gel fraction rises above 80 %. Cold impact and elongation tests are referenced to IEC 60811-506; where the wind turbine cable specification calls for -40 °C cold impact, low-temperature elongation under load is typically expected to remain above 30 %. Published data for TR-YJ-66 at -40 °C is limited, so the cable design is qualified by the manufacturer’s own torsion and cold bend protocol aligned with the turbine maker’s type-test programme. The vulcanisation line normally targets a gel fraction near 82–85 % to balance hot set compliance against brittle impact at low temperature. Torsion cycling of the completed cable is performed on a vertical test rig with ± 100° rotation per 10 m of cable for thousands of cycles while the conductor remains energised or thermally conditioned. The insulation is not inherently oil-resistant; if nacelle gear oil contact is possible, an oil-resistant sheath or barrier layer is required.

When the Same Insulation Compound Is Used for 3.6/6 kV Submersible Dewatering Pump Cables

Submersible pump cables for mine dewatering and quarry groundwater extraction are constructed with TR-YJ-66 insulation at a nominal thickness of 2.5 mm to IEC 60502-2 for 3.6/6 kV XLPE. The extruded insulation is combined with a longitudinally or helically applied water-blocking tape under a heavy-duty elastomeric sheath. The insulation must retain volume resistivity above 10¹⁵ Ω·cm after submersion at 50 °C for 14 days in potable water; this is evaluated by insulation resistance measurement according to IEC 60811-201 or the manufacturer’s immersion protocol. The operational limit is the combination of mechanical flexing at the cable entry and the water pressure at depth. The insulation layer is not the primary water barrier; water ingress through the sheath must be prevented at the design stage. If the pump cable is exposed to well fluids containing aromatic hydrocarbons or hydrogen sulfide, TR-YJ-66 insulation without a metallic or polymer barrier should not be specified because polyethylene absorbs low-molecular-weight aromatic fractions, which reduces dielectric strength over time. The cable maker typically verifies insulation adhesion, hot set, and gel fraction after the first metre of each run and again after every 30 min of line time to catch drift in the dry-cure tube.

Triple Co-Extrusion Compatibility with Semiconductor Screens in Dry-Cure CV Lines

The medium-voltage manufacturing step that determines TR-YJ-66 performance is the simultaneous extrusion of conductor screen, insulation, and insulation screen through a single triple crosshead. The conductor screen melt is maintained at 110–120 °C, the insulation at 121 °C ± 3 °C, and the insulation screen at 110–120 °C so that the three melts meet without mutual contamination. The crosshead is mounted to a 90 mm or 120 mm single-screw extruder for the insulation; the two screen extruders are typically 45 mm or 60 mm. The die is designed with a flow splitter that prevents screen material recirculation into the insulation layer; any black specks in the XLPE layer indicate screen bleed and require line clearance. Because TR-YJ-66 requires dry-cure nitrogen at 1.0–1.5 MPa, the triple head sealing system must hold pressure without air ingress; oxygen ingress degrades the inner and outer screens and creates interface voids. After the CV tube, the cable passes through cooling water under pressure and is wound with a minimum bending radius of 12 × the cable outer diameter. The bonded conductor screen and strippable insulation screen are tested for interface adhesion and strippability according to IEC 60811-604 or the cable maker’s in-house method; the insulation screen must strip cleanly without leaving carbon residue on the XLPE surface. The complete insulation assembly is subjected to the hot set test and gel fraction measurement after vulcanisation; if the gel fraction falls below 80 %, the line speed is reduced by 5–10 % and the tube temperature is rebalanced. Published data for TR-YJ-66 in all triple-head configurations is limited; validation therefore requires trial batches on the specific CV line geometry.

Reference extrusion profile for a 90 mm barrier screw running tree-retardant XLPE insulation compound.