DELVING INTO THE World of Cables

Cables are the most important components of power systems. There are different kinds of cable faults, which cripple the entire system. This article presents different aspects of development, fault tracing and maintenance of cables…

January 5, 2025

Most high-voltage transmission lines are underground or in the air, and various high-voltage transmission cables are progressively employed in various national grid sectors. This is due to the fast expansion of the national economy and the continual increase in the demand for power grids. Once the high-voltage cable breaks, the circuit may be disrupted, which will significantly impact both individuals and businesses. In extreme circumstances, it will cause serious incidents related to safety. For this reason, promptly detecting defects on high-voltage cables is of the utmost significance.

Components such as grounding, short circuit, and disconnection detection are the primary components of cable fault detection. Detecting cables, on the other hand, is highly challenging because, in addition to being laid above the land, cables may also be buried underground or even in the ocean. When it comes to the identification of high-voltage lines, this presents a significant obstacle. XLPE cables are susceptible to typical phenomena, including water tree ageing and metal shielding damage. The occurrence of dendritic flaws in the insulator as a result of the local concentration of the electric field is referred to as a water tree. This phenomenon occurs when there is moisture present within the cable. There is a possibility that the branches of the water tree will undergo significant oxidation in an environment with high temperatures, which will ultimately result in thermal breakdown. On the other hand, in an environment with low temperatures, the water tree branches will either be oxidised or converted into electrical branches for an extended period. The topological analysis of the power network system based on the SCADA information is the primary element referred to when discussing the intelligent grid system. This makes the kind of problem and the precise location of the fault clear. An extensive amount of logical judgments is required during the whole process of fault repair. Additionally, a complicated mathematical model must be developed to find the problem precisely and deal with it promptly. Discussing power cable fault detection technology is essential since it has emerged as one of the most advanced technologies in the current electrical grid system.

Key technological breakthroughs and research hotspots in the development of cables

In the more than a century since the invention of cables, engineers and researchers have studied higher voltage, better capacity, higher stability, and longer transmission distances. As an illustration, in 1960, many extruded cables were installed in urban underground power transmission. At that time, many believed that this type of insulation would not require replacement for a hundred years, but after a few years, a large number of failure reports were received. After further investigation, it was found that the cable insulation included many channels resembling trees. Dendritic ageing was identified as the primary issue that impacted the operational stability of extruded cables in 1968. Other researchers validated this. Because this issue was effectively resolved by utilising anti-electric tree ageing XLPE materials and a three-layer co-extrusion technique, the fast development of XLPE cables from the 1990s until the current day was significantly facilitated. XLPE cables can be gradually used in actual DC transmission projects due to the growing understanding of space charge characteristics in insulating materials. This is because using flexible DC transmission helps avoid polarity inversion, which in turn helps alleviate the problems caused by space charge.

Among the most critical research hotspots in cable development are incorporating high-performance materials, superconductivity, novel cable technology, environmental sustainability, and innovation in insulation and protection systems. The following is a comprehensive analysis of these critical areas:

High-Performance Materials

Polymeric Materials and Cross-Linked Polyethylene (XLPE): Due to its superior thermal and electrical qualities, cross-linked polyethene is a predominant insulating material for high-voltage cables. It is widely used in contemporary power transmission, particularly for underground and underwater cables, owing to its adaptability and dependability under elevated stress and temperatures. Figure 1 illustrates the Polymeric Materials and Cross-Linked Polyethylene cables.

Thermoplastic Materials: Thermoplastics have benefits like recyclability and reduced environmental impact. Research is on advancing thermoplastic materials that exhibit enhanced thermal, electrical, and mechanical characteristics.
Nano-material Enhanced Cables: Investigations into nanomaterials, including graphene and carbon nanotubes, are underway to improve cables’ thermal conductivity and mechanical strength, as illustrated in Figure 2. These advancements demonstrate significant durability, reliability, and efficiency potential, particularly in challenging conditions.

Superconducting Cables

High-temperature Superconductors (HTS): Superconducting cables, constructed from materials such as Yttrium Barium Copper Oxide (YBCO), exhibit practically zero electrical resistance, enabling very efficient power transfer with negligible energy loss, as seen in Figure 3. HTS cables function at significantly elevated temperatures compared to conventional superconductors, facilitating their applicability in practical scenarios.

Research in Cryogenic Systems: Advancements in cryogenic systems are essential because superconducting materials must be cooled to exceedingly low temperatures. Effective cryogenic cooling is crucial for the commercial viability of superconducting cables, and research is being conducted to create small, energy-efficient cooling systems.

Smart Cable Technology

Condition Monitoring and Predictive Maintenance: Smart circuit sensors check electrical properties, strain, temperature, and vibration. By enabling predictive maintenance and real-time monitoring, these technologies help minimise downtime and avoid breakdowns.
Internet of Things (IoT) and Data Analytics Integration: Smart cables provide valuable insights for utility companies by integrating IoT sensors and advanced analytics. Research focuses on developing more accurate algorithms for fault detection, life expectancy analysis, and load optimisation.
Self-Healing Cables: Self-healing cables, a cutting-edge advancement in intelligent technology, employ unique polymers to fix minor damage autonomously. Self-healing cable technology is still in its infancy but can potentially increase critical infrastructure dependability and cable longevity.

Environmental Sustainability and Recycling

Eco-Friendly Materials: Self-healing cables, a cutting-edge advancement in intelligent technology, employ unique polymers to fix minor damage autonomously. Self-healing cable technology is still in its infancy but can potentially increase critical infrastructure dependability and cable longevity.
Recyclable Cable Designs: Thermoplastic insulating material innovations facilitate recycling, lowering waste and the environmental impact. Continuous research is being done to improve recycling’s economics and efficiency, especially for intricate cable networks.
Reducing Greenhouse Gas Emissions in Manufacturing: The energy-intensive process of making cables frequently results in significant greenhouse gas emissions. By using cleaner production methods, such as cutting energy use and utilising renewable energy in industrial facilities, the research seeks to lower emissions.

Insulation and Protection Mechanisms

Improved Dielectric Insulation: Researchers continuously develop new dielectric materials with excellent resistance to mechanical stress, temperature, and electric fields since they are necessary for higher-voltage connections. Leading options include materials like improved XLPE variants and ethylene propylene rubber (EPR).
Fire-Resistant Cables: Fire-resistant cables are mandated by safety standards, particularly in high-risk environments such as buildings and industrial facilities. Research is being conducted on fire-resistant insulation materials, including halogen-free flame retardants, to evaluate their capacity to reduce toxic smoke and sustain performance in fire conditions.
Water-Blocking Techniques: Underground and submarine cables are susceptible to water ingress, resulting in potential short-circuiting and corrosion. Innovative water-blocking technologies, including hydrophobic materials and water-swellable tapes, are under development to improve cable longevity.

High-Voltage Direct Current (HVDC) and Extruded Cables

HVDC Cables: HVDC technology represents a significant research domain, offering advantages in transmitting substantial power over extended distances while minimising losses. Developing HVDC cables, primarily extruded from polymeric materials, is essential for advancing contemporary power grids.
Higher Capacity Extruded Cables: Increasing energy demands necessitate cables capable of accommodating higher capacities. Extruded HVDC cables are optimised for increased power capacity, emphasising materials and designs capable of managing higher thermal loads and enduring more significant electrical stresses.

Submarine Cables and Deep-Sea Applications

Enhanced Materials for Corrosion Resistance: Submarine cables must endure extreme underwater conditions, such as elevated pressures, salinity levels, and interactions with marine organisms. Studying corrosion-resistant materials and protective coatings is crucial for enhancing the longevity of cables.
Mechanical Reinforcement for Deep-Sea Cables: Deep-sea cables encounter distinct mechanical stresses, prompting ongoing research to enhance materials and construction techniques to bolster the mechanical strength of these cables, thereby mitigating the risk of breaks and damage under extreme pressure.
Power and Data Transmission Cables: Alongside conventional power cables, data transmission is vital in offshore wind farms, oil rigs, and submarine internet cables. Research is being conducted on combined power and fibre-optic cables to enhance the efficiency of power and data transfer through a single line.

Aging analysis of power cable lines

Standard transmission lines consist of three primary components: the insulation, conductor, and protective layers. Long-running power cables’ protective and insulation layers frequently exhibit signs of ageing or damage. Under standard conditions, the insulation paper of the oil-filled cable’s body, joints, and terminal components is typically submerged in insulation oil, which minimises the risk of ageing. Deformation of the cable, along with damage or leakage of the insulating and protective layers due to external forces, will substantially reduce the insulating effectiveness of the oil-filled cable. This condition poses a considerable risk of a serious safety incident. The charged insulating oil will interact with moisture and other airborne substances, leading to degradation and a subsequent decrease in insulating effectiveness.

Common fault conditions for XLPE cables include damage to the metal shielding layer and the ageing of the water tree. The phenomenon of dendritic ageing damage in the insulating component of a cable, which occurs when moisture is introduced, is referred to as a water tree. This damage is influenced by the local concentration of the electric field within the insulator. Exposure to a high-temperature environment over an extended period results in the gradual oxidation of the water tree. This process significantly enhances the water absorption effect and progressively increases conductivity, potentially leading to thermal breakdown. Conversely, oxidation and transformation cause the water branches to further evolve into electric tree branches in a long-term low-temperature environment. Several researchers have quantitatively analysed the impact of space charge. When subjected to a strong electric field, the cumulative effect of charge emission at the needle tip is believed to compromise the local material structure, resulting in the formation of micro-cracks and the development of electrical dendrites. The above hypothetical electrical dendritic initiation pattern, as illustrated in Figure 4, has also been summarised.

Analysing the reasons behind cable failure

Cable faults can be categorized into the following categories. Figure 5 illustrates the classification diagram for the causes of cable faults.

Mechanical Damage: The most common cause of cable accidents is mechanical damage. Even if the mechanical damage is small, the cable can still function, but with prolonged use, the damaged component will become a failure point and result in an accident.
Insulation Drops of Cable Insulation: Water in the cable insulation layer due to incorrect cable termination or subpar installation, small holes or cracks in the sheath layer during the cable manufacturing process, external force damaging the metal sheath layer, and cable protection are the primary causes of the decline in cable insulation. Damp insulation and cable damage are caused by severe sheath corrosion.
Overvoltage Faults: Overvoltage causes the power wire to malfunction by breaking down at the weak spot in the insulation.
Cable Insulation Ageing: An air gap exists within the cable insulation medium. The electric field will release the air gap, resulting in a reduction of the cable insulation. The ionisation of the insulation medium results in the generation of chemical products, including ozone and nitric acid, within the air gap. These substances can lead to corrosion of the cable insulation layer. The prolonged operation of the cable under high load conditions or inadequate ventilation can increase cable temperature. This elevated temperature can expedite the ageing process of the cable insulation. Additionally, moisture in the insulation layer may hydrolyse the insulation fibres, resulting in a deterioration of the cable insulation.

Analysis of power cable fault diagnosis technology

Diagnosing a power cable fault consists of three primary components: fault diagnosis, distance measurement, and location identification. Fault diagnosis primarily involves identifying the type of fault and assessing its severity, enabling inspectors to utilise suitable ranging and positioning technologies for subsequent operations. Assess the characteristics of the fault resistance to identify if it is classified as a closed fault or a flashover fault. Additionally, determine if it is a single-phase, two-phase, or three-phase fault. Evaluate whether the fault is high-resistance or low-resistance and ascertain if it represents a short-circuit or open-circuit fault. The distance measurement for cable fault location primarily relies on distance detection conducted by specialised equipment at one cable end.

At present, wave-ranging technology is typically employed in travel. Low-resistance and short-circuit faults are primarily identified through low-voltage pulse reflection, offering a more intuitive and straightforward approach than conventional bridge detection technology. The cable fault location technology utilises calculation results to ascertain the fault’s position, integrating the cable laying direction to estimate the fault’s specific location. This approach enables the control of the fault point within a limited interval. Additionally, methods such as the discharge acoustic method are employed to identify the precise location of the fault accurately.

Conclusions

In recent years, the magnitude of my country’s power grid system has expanded significantly, resulting in a more intricate power cable line structure and the emergence of additional issues. Conventional power cable fault detection methods are inadequate for the present substantial power system demands, making it crucial to swiftly identify problems and respond promptly using more modern diagnostic technologies.

The partial discharge detection technology for cross-linked polyethene cables underdamped oscillating wave voltage primarily relies on the damped oscillating wave broadband pulse current method for detection and localisation, employing pulse separation technology and multi-mode separation detection techniques to swiftly and precisely identify cable faults.

The detection and fault identification of the discharge sample library and intelligent, comprehensive analysis and identification software significantly enhance power grid efficiency, reduce the fault’s impact on users, and improve the efficacy of current line fault maintenance.