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At present, although domestic insulating materials have achieved certain results, with the increasing number of high-voltage cable projects planned in China and the continuous improvement of cable operating voltage levels, higher requirements are being placed on the overall performance of XLPE insulating materials.

• Insulation performance

High-voltage and ultra-high-voltage direct current cables have become a research hotspot due to their high transmission efficiency, low line losses, and low maintenance costs. The demands on XLPE insulation mainly include low DC conductivity and superior space charge characteristics. These properties will directly determine whether the core risk leading to cable failure—insulation breakdown—can be avoided.

Specifically, in the field of DC cables, the conductivity and space charge characteristics of XLPE respectively dominate different forms of breakdown risk.

Different breakdown modes are closely related to electric field strength and action time.

1. Conductivity is primarily associated with the risk of "electrical breakdown."

Influencing mechanism:

The electric field distribution of XLPE cables differs under AC and DC voltage. Under DC electric field, the electric field distribution of the cable insulation layer is no longer determined by the dielectric constant as it is under AC electric field, but rather depends on the conductivity of the material.

The conductivity increases exponentially with the rise in load temperature. When the cable operates under direct current voltage, the insulation layer forms a temperature gradient from the inside to the outside and from high to low. This results in the inner resistance, which is at a higher temperature and has a higher conductivity, being smaller, while the outer resistance, which is at a lower temperature and has a lower conductivity, being larger.

According to circuit principles, more voltage will be distributed in areas with high resistance, leading to "field reversal"—the electric field strength on the outside of the insulating layer is higher than on the inside, causing a risk of breakdown.

2. Space charge is mainly associated with the risk of "aging breakdown."

Influencing mechanism:

The accumulation of space charge can create a significant local electric field within the insulation. This persistent and excessive electrical stress can gradually lead to material degradation, trigger partial discharge, and ultimately result in insulation aging and breakdown after long-term accumulation.

Furthermore, in actual operation, the aforementioned two characteristics are interrelated and exacerbate each other. Their combined effects influence the actual withstand voltage strength and lifespan of the insulation system.

• Cleanliness

Regardless of the type of breakthrough, achieving a complete and closed-loop ultra-clean control process from substrate polymerization to insulation material production is the most fundamental and crucial prerequisite. It simultaneously addresses the two major issues of abnormal conductivity caused by impurities and the accumulation of space charge.

Currently, the ultra-clean route is the most mature. Borealis has achieved mass production of ultra-clean XLPE insulation materials through dedicated polymerization equipment and full-process clean control, while domestically, breakthroughs are still needed in base material synthesis and impurity detection technology.

Characteristics of Nordic Chemical's High/Ultra-High Voltage XLPE Products

• Stable extrusion process

High-voltage XLPE cable extrusion is typically carried out by co-extruding three layers of molten material (inner shielding, insulation, outer shielding) onto a metal conductor, followed by a cross-linking reaction to produce the insulated core.

Additionally, due to limitations in storage, transportation, and other aspects, there is a length limit in the manufacturing of single high-voltage submarine cables, which cannot meet the requirements for continuous extrusion. Therefore, improving the rheological properties, extrusion stability, and scorch resistance of cable insulation materials, as well as optimizing the insulation extrusion molding process, are key directions for future research and development of ultra-high-voltage insulation materials.

In 2018, domestic optical fiber cable giant Hengtong used Nordic Chemicals LS 4201 EHV XLPE insulation material to achieve a continuous extrusion of an 18-kilometer-long 500 kV submarine cable, marking the completion of China's first domestically produced 500 kV submarine cable, which is used in the Zhoushan interconnection project.

References: Company website, Chemical New Materials, Internet