I. An unavoidable honest question

In overseas projects, cross-linked polyethylene (XLPE) cables are almost standard equipment in medium and high voltage power systems. Their excellent dielectric properties, high heat resistance, large transmission capacity, light weight, and easy installation have made them a replacement for paper-insulated cables for decades.

However, they are not invincible.

According to operational statistics from several international power companies, insulation damage is the leading cause of XLPE cable failures, accounting for approximately 86.7% of all failures. Of these, failures caused by partial discharge aging, electrical aging, and water treeing account for 65.7%; failures caused by thermal aging and thermomechanical deformation account for 10.5%; and failures caused by mechanical damage and electromechanical combined aging account for the remaining 10.5%.

In other words, if your XLPE cable has a problem, it is highly likely related to insulation aging. Moreover, cable failures typically arise from three stages: defects left over from the manufacturing stage, damage introduced during installation and laying, and aging that gradually develops during operation.

Below, we will break down these most common problems for clarity.

II. Treeing – A Warning Sign Before Insulation Breakdown

First, it's essential to understand a core concept: Treeing.

In power cable engineering, "treeing" refers to the microscopic, tree-like cracks and discharge channels that form within the solid insulation medium before complete breakdown. Treeing is essentially a pre-breakdown phenomenon. Once a tree grows from one electrode to another, the insulation is breached, and the cable fails.

Based on its formation, treeing is classified into three types:

Electrical Treeing: When minute defects exist within the insulation—such as air gaps, impurities, or protrusions on the conductor surface—under long-term operating voltage, the electric field at that location undergoes severe distortion, generating partial discharge and ultimately leading to tree-like channels. Electrical trees grow rapidly and are difficult to reverse once formed, directly shortening the cable's remaining lifespan.

Under AC voltage, electrical trees can exhibit two typical forms: one is a branching structure similar to tree branches, and the other is a denser, bush-like structure. The higher the voltage, the easier it is for bush-like treeing to form.

A study on insulation failure in 33kV XLPE cables indicated that electrical treeing is a pre-breakdown phenomenon leading to premature failure of high-voltage cables, significantly negatively impacting design life. Another study based on Weibull distribution found that the higher the applied voltage, the faster the propagation rate of electrical trees, and the shorter the cable insulation life.

Water Treeing: The formation of water trees requires three conditions: moisture, an electric field, and soluble impurities.

Under the combined effects of a high electric field, a certain humidity level (relative humidity above 70%), and the presence of salt or contaminants in the water, a dense network of extremely small, water-filled voids and channels grows inside the XLPE insulation; this is water treeing. Water trees themselves are not discharge channels, but they significantly reduce the volume resistivity of the insulation material, greatly increasing dielectric losses, and causing mechanical damage at the microscopic level.

The real danger of water trees lies in their "latency." They typically grow slowly, are invisible to the naked eye, and do not immediately cause breakdown. However, over time, water trees can gradually transform into electrical trees. When sufficient moisture and impurities accumulate in the water tree area, the electric field distortion intensifies, eventually inducing the rapid growth of electrical trees and causing sudden insulation failure during operation.

It is important to note that while cable drying can temporarily alleviate the problem, water treeing is generally considered permanent and will only worsen over time.

In one real-world case, a single water tree eventually triggered an electrical tree breakdown, resulting in a direct cost exceeding $200,000 for replacing 700 meters of cable.

Electrochemical Trees
Electrochemical trees are typically associated with impurities in the insulation material. When inorganic impurities (such as salts and metal particles) remain in the insulation or semiconducting layer, they undergo electrochemical degradation under the influence of an electric field, forming tree-like structures. These chemical impurities dissociate in the electric field and, together with trace amounts of moisture, accelerate localized degradation.

Microscopic analysis revealed voids ranging from 15 to 150 micrometers in size within the semiconductive layer. These voids spatially correlated with the initiation points of electrochemical dendrites, and NaCl impurities were also present around them.

III. Initial Defects Introduced During Manufacturing
Initial defects introduced during manufacturing are inherent weaknesses of the cable. Inadequate process control during XLPE insulation extrusion and cross-linking can leave several typical problems within the insulation.

Voids
Voids are the most common type of microscopic defect. They are typically caused by the failure of decomposition products of organic peroxides (methane, acetophenone, etc.) to escape completely during cross-linking, resulting in micropores trapped within the insulation. These bubbles are typically between 15 and 150 micrometers in size.

Studies show that the size, number, area, and distribution of voids directly determine the electrical strength of the cable. Larger voids result in a more significant decrease in AC breakdown strength. Under operating voltage, partial discharge occurs within the voids, gradually eroding the surrounding insulation material.

Impurities (Contaminants)

If cross-linking byproducts are not thoroughly removed, or if external impurities (such as dust or metal particles) are mixed into the insulation material, these impurities will become "weak points" in the insulation. Under the influence of an electric field, charge will accumulate around the impurities, causing electric field concentration and thus accelerating local aging.

Conductor Protrusions and Semiconductor Layer Irregularities

If there are irregularities or protrusions in the conductor shielding layer of the cable, they will embed themselves in the insulation layer like "spikes," generating extremely high local electric field strength under operating voltage, becoming the starting point of electrical trees. Simulation analysis of spherical air gaps, pinholes, and irregularities in the internal semiconductor layer of cable terminals shows that these defects will cause significant changes in electric field strength. For example, a 2mm air gap can generate an electric field strength as high as 2.45×10⁶ V/mm near the conductor.

IV. Quality Problems Introduced During Installation and Laying

If manufacturing defects are the "inherent deficiencies" of cables, then installation quality problems are "acquired malnutrition"—and these problems are more common in the daily work of maintenance personnel. According to operational statistics from several international power grid companies, cable accessories and joints are the most frequent sources of failures.

Accessory Failures: A comprehensive analysis report published in 2025 on the causes of 35kV XLPE cable breakdowns indicated that terminal failures accounted for 58% of all breakdown incidents, main insulation failures for 32%, and external factors for 10%.

In another case from a real-world power grid operator, 24 substations operated 129 10kV and 35kV XLPE power cables. Operational data analysis showed that failures were mainly caused by electrical aging, thermal aging, and mechanical damage, a significant portion of which was directly related to the installation quality of accessories (joints and terminations).

Another set of statistics confirms this trend: In a survey report covering 6,214 cable faults (2010-2020), cable insulation faults accounted for 57% (of which XLPE cables were mainly caused by water trees), cable joint faults accounted for 23%, third-party excavation damage accounted for 11%, and secondary substation faults accounted for 9%.

Improper Joint Fabrication
Improper operation during cable joint fabrication is a significant contributing factor to faults. Common problems include: uneven cutting of the semiconductive layer, scratches or tears on the XLPE insulation surface, incomplete removal of residual foreign matter, air gaps between insulation interfaces, and improper conductor crimping. One study indicated that even tiny scratches on the insulation surface or residual semiconductive particles or metal dust can create localized electrical stress concentration on the insulation surface, thereby triggering partial discharge.

Moisture Intrusion
Moisture intrusion is another underestimated risk. In underground environments, if the cable sheath is damaged during installation or the joints are poorly sealed, groundwater can gradually seep into the insulation layer. Once moisture enters the joint, it combines with the electric field to trigger electrochemical corrosion and water tree growth.

Studies have shown that when the moisture content of XLPE insulation reaches 0.1%, its dielectric loss factor increases to more than three times that of the dry state, leading to energy waste and abnormal temperature rise.

Therefore, insulation performance testing should be performed before cable commissioning, after remaking terminations or intermediate joints, and after suspected sheath damage and water ingress. Measuring the ratio of copper shield resistance to conductor resistance is a crucial step.

Laying Quality
The on-site laying process also affects cable life. If the cable is laid beyond the specified minimum bending radius, uneven stress may be generated in the insulation, leading to mechanical damage and electromechanical combined aging over time, accelerating insulation failure. Furthermore, a certain cable length should be reserved to allow for remaking the cable termination in case of an accident.

V. Environmental Stress Encountered During Operation

Thermal Aging
During normal operation, the conductor operating temperature of XLPE cables is typically 90°C (the long-term permissible operating temperature of XLPE). However, when cables operate under overload conditions for extended periods, or in environments with poor heat dissipation (such as dense conduit installations), the insulation temperature may consistently exceed the design value.

Thermal aging induces structural changes in the insulation material, including molecular chain breakage, free radical formation, molecular weight reduction, and polymer fragmentation, leading to irreversible degradation of physical, electrical, and chemical properties. Space charge accumulation is one of the key mechanisms by which thermal aging affects insulation quality: space charge distorts the electric field distribution, exacerbating electric field inhomogeneity, thereby accelerating cable insulation aging and causing earlier breakdown.

Studies have found that at a humid aging temperature of 160°C, the space charge distribution in polymer insulation is directly related to chemical degradation. At a thermal aging temperature of 130°C, both trap level density and space charge density significantly increase, meaning that the insulation material's ability to trap charge is enhanced, further deteriorating insulation performance.

Moisture and Chemical Corrosion

In addition to the water tree mentioned earlier, moisture intrusion also brings another direct consequence: copper conductor corrosion.

In underground cables operating in coastal industrial facilities or high-humidity areas, copper conductors may blacken due to sulfate-related corrosion or oxidize to form a green oxide layer (verdigris) due to moisture intrusion.

Multi-Stress Combined Aging

In actual operation, the aging of XLPE cables is often not caused by a single factor, but by the combined effects of multiple stresses: heat, electricity, mechanical stress, and moisture.

As a semi-crystalline polymer, XLPE cables inevitably undergo gradual aging during operation. This aging stems from the combined effects of various environmental stresses, including thermal cycling, moisture exposure, continuous electric fields, and mechanical loads. Over time, these factors gradually weaken the dielectric properties, eventually leading to insulation failure.

VI. How to Detect and Assess the Health of XLPE Cables
In actual operation and maintenance, there are several methods to help determine whether XLPE cables have the above-mentioned problems.

AC Withstand Voltage Test and Partial Discharge Detection

A very important reminder: XLPE cables should not be tested using DC high voltage.

The reason is that during a DC withstand voltage test, space charge can be injected into the insulation layer through existing "trees" in the cable insulation. XLPE insulation material has extremely high resistivity, making it difficult for residual charge to dissipate after discharge. This residual charge generates an electric field that superimposes on the operating AC electric field, making the cable more susceptible to breakdown after passing the DC withstand voltage test and being put into operation.

Therefore, for XLPE power cables, it is recommended to use a combination of AC withstand voltage testing and partial discharge detection for evaluation.

Insulation Resistance Measurement: Measuring the insulation resistance of the main cable insulation is effective in detecting overall insulation moisture absorption, overall deterioration, and penetration defects. However, it should be noted that when the PVC outer sheath of a directly buried cable is subjected to long-term immersion in groundwater or external force damage but not complete failure, a decrease in insulation resistance alone cannot directly determine that the outer sheath has been damaged and infiltrated by water.

Dielectric Loss Measurement: As mentioned earlier, the dielectric loss factor of XLPE insulation material increases significantly when the moisture content reaches 0.1%. Regular dielectric loss measurement is an important means of monitoring the moisture absorption and aging status of the insulation.

VII. Why Overseas Customers Need to Pay Attention to These Issues

So far, we have discussed in detail the various problems that XLPE cables may encounter in practical applications.

Understanding these issues is not meant to make you "afraid" of using XLPE cables. On the contrary—this is engineering knowledge that an experienced cable manufacturer should provide to its customers. True professionalism is not about packaging a product as perfect, but about knowing under what conditions the product will fail, why it will fail, and helping customers avoid these problems.

For overseas customers, when purchasing a batch of XLPE cables, your concern is not "Is this cable defective?", but rather "If I install and use it according to specifications, what is its expected lifespan?", "What abnormal signals should I pay attention to during operation?", and "When should preventative testing be performed?"

A supplier who can answer these questions is worthy of long-term trust.

VIII. Summary

Common problems with XLPE cables can be summarized into several levels.

The most fundamental problem is treeing—electrical trees, water trees, and electrochemical trees are pre-breakdown signals that precede insulation breakdown; they gradually erode the integrity of the insulation material at the microscopic level.

During the manufacturing stage, initial defects such as air gaps, impurities, and irregularities in the semi-conductive layer are inherent weaknesses in cables. The source of these defects may be traced back to inadequate control over the cross-linking process or material cleanliness.

During the installation and laying stage, cable accessories and joints are the hardest hit areas for failures—terminal failures account for as much as 58%, and joint failures account for 23%. Improper joint fabrication, moisture intrusion, and excessively small cable bending radii are common contributing factors.

During the operation stage, thermal aging accelerates insulation deterioration, while moisture and chemical corrosion further damage the shielding layer and conductor. The cumulative effect of multiple stresses leads to eventual breakdown.

Throughout its lifespan, insulation damage is the core cause of XLPE cable failure—accounting for 86.7% of all failures. Thermal aging, denaturation, partial discharge, and surface creepage are the main failure modes of XLPE insulation materials.

In terms of testing, DC withstand voltage testing of XLPE cables should be avoided; instead, AC withstand voltage testing combined with partial discharge testing is more reliable.

If you are currently using or about to purchase XLPE cables, it is recommended to focus on three key aspects: the quality inspection report before delivery (especially the partial discharge test results), the quality of accessories during installation (choosing a qualified construction team), and the status monitoring methods during operation (regular inspections such as infrared thermal imaging and partial discharge detection).