Challenges in Insulator Design for Overhead Transmission Lines (OHTLs)

ITG’s Group Head Application Engineer Amith Karanth explores the challenges associated with electric power transmission lines, as outlined below. 

Background

The global demand for electric power continues to rise, driven by economic development, urbanisation, and the electrification of transportation and industry. Ultra-High Voltage (UHV) transmission lines, which typically operate above 800 kV, enable power transfer over long distances, supporting the performance of transmission lines as demands continue to rise.

However, the design of insulators for these systems presents multiple challenges, stemming from the extreme electrical, mechanical, and environmental stresses. As insulators are critical to ensuring safety, reliability, and performance, their design must account for multiple complex factors, from pollution resistance to seismic resilience.

Mechanical Challenges 

Electric power transmission lines can be larger, heavier, and span over longer distances, placing enormous mechanical demands on insulators. They must support not only the conductor weight but also environmental loads such as wind, ice, and seismic forces. In addition to this, the increased span lengths often exceeding 800 meters, intensifying tensile loads on suspension insulators. These demands necessitate high mechanical strength ratings, typically ranging from 120 kN up to 530 kN in harsh environments.

Furthermore, the mass and length of insulator strings in such harsh service conditions are significantly greater, increasing the risk of dynamic oscillations and fatigue over time.

Pollution Performance 

Insulator pollution performance is a major concern, particularly in industrial, coastal, and desert areas. Accumulated contaminants such as salt, dust, and industrial pollutants reduce surface resistance, leading to leakage currents and dry-band arcing. In electric power transmission systems, higher operating voltages increase the probability of having pollution-induced flashovers.

Hydrophobicity plays a crucial role in mitigating such pollution-related issues. Composite insulators are known for their hydrophobicity, though a similar pollution performance can be achieved by application of a silicone-based RTV coating onto porcelain and glass insulators, which offer much better ageing properties than their composite counterparts.

Designers must also optimise the creepage distance, typically exceeding 50 mm/kV for extreme pollution, to minimise the risks of flashover. This often results in designs with very long insulator strings, which in turn complicate tower design and their associated maintenance.

Seismic Performance 

Regions prone to seismic activity pose additional challenges for insulators in the case of electric power transmission. Earthquakes can induce severe lateral and vertical forces on towers and insulators. The inertia of long, heavy insulator strings increases the risk of fracture and detachment during seismic events.

Seismic-prone tower designs need to incorporate dampers and flexible joints to accommodate seismic-induced motion without transmitting excessive stress to insulators, which is one way of mitigating this.

Lightning Performance 

Electric power transmission lines are vulnerable to lightning strikes due to their height and exposure. Lightning strikes can cause flashovers or puncture the insulator, particularly if energy is not dissipated effectively. Standard insulation coordination must account for both direct strikes and induced overvoltages.

Improving lightning performance involves increasing the insulation level and employing arcing horns or grading rings to manage potential gradients. Additionally, line surge arresters (LSAs) can be used alongside insulators to limit the voltage rise during transients. For UHV AC systems, careful design of shielding angles and tower grounding ensures efficient lightning performance.

Overvoltage Mitigation 

Switching operations in electric power transmission systems can cause significant over voltages due to the large inductance and capacitance in the network. These over voltages may lead to flashovers or permanent damage if not properly mitigated.

Insulator design must consider these transient stresses and incorporate features such as optimised shed profiles to handle impulse voltages. Advanced computational tools are used to simulate and mitigate electric field concentrations. Grading rings are strategically placed to redistribute electrical fields and prevent corona discharge at the ends of insulators.

Flashover Reduction 

Flashover is a critical failure mode in insulators, especially under adverse weather or pollution conditions. Avoiding flashover incidents requires an integrated approach:

• Creepage distance optimisation ensures sufficient surface path length.
• Shed geometry (alternating shed design, for instance) helps promote self-cleaning.
• Material hydrophobicity maintains performance even after surface contamination.

Flashover prediction models, using real-time pollution and weather data, are also employed to adjust operating parameters and prevent failures.

Corona and Radio Interference 

For example, at UHV levels, corona discharge becomes a significant issue. Corona leads to power loss, audible noise, electromagnetic interference, and material degradation. Insulators must be designed to minimise surface electric field intensities that lead to corona initiation.

Grading rings and corona shields are employed extensively in high-voltage insulator assemblies to smooth electric field transitions and reduce their localised intensities. The geometry of the insulators and their sheds is optimised to prevent corona initiation in both dry and wet conditions.

The impact on communication systems and health concerns due to electromagnetic radiation also necessitate strict corona control. 

Ageing and Long-Term Reliability 

Electric power transmission insulators must have service lifespans of 30–50 years, even under harsh-to-severe service conditions. Ageing factors include UV degradation, erosion, acid rain effects, and thermal cycling.

Monitoring systems using leakage current sensors, thermal imaging, and partial discharge detection are increasingly integrated into power systems to assess insulator health. Research is ongoing to develop self-healing materials or coatings that can restore hydrophobicity after degradation.

Standardisation and Testing 

Given the extreme demands, insulators for electric power transmission lines require rigorous testing. International standards such as IEC 60383, IEC 61109, and IEC 62217 provide testing protocols, but challenging applications and extreme service conditions often exceed their defined conditions.

Tests for UHV insulators include: 

• a.c. & d.c. artificial pollution tests (e.g., salt fog & solid layer)
• Mechanical load (tension & suspension) tests
• Dielectric tests
• Corona and radio interference voltage (RIV) tests
• Seismic simulations

Field validation under real operating conditions is essential to confirm laboratory results, particularly in regions with unique climate challenges.

Conclusion 

Designing insulators for electric power transmission lines is a complex, multidisciplinary challenge. It requires balancing mechanical robustness, electrical performance, environmental resilience, and long-term durability. With increasing global reliance on power systems for secure power transmission, insulator technology must continue to evolve.

Innovations in materials, smart monitoring systems, and predictive modelling are enabling more reliable and efficient insulator designs. Nonetheless, localised conditions such as pollution levels, seismic activity, and lightning frequency necessitate tailored solutions for each electric power transmission project. As grid complexity increases and climate change introduces new variables, the future of insulation design will depend on continued research, rigorous testing, and adaptive engineering.

*The information provided in this content is for informational purposes only and should not be considered professional advice. We make no warranties or guarantees, express or implied, and are not responsible for any losses or damages resulting from your use of this information.