Discussion on the Lightning Protection Level of Composite Insulators

Abstract

This article summarizes the operational status of composite insulators (110 kV and above) within the Zhongshan Power Grid. It provides a detailed analysis and discussion on the necessity of auditing whether the Lightning Full-Wave Impulse Withstand Voltage of composite insulators meets the localized lightning protection requirements. The study highlights that maintaining a sufficient effective dry arc distance is the critical factor in achieving the required lightning protection level.

Overview

In recent years, composite insulators have been widely adopted in transmission lines of 110 kV and above due to their superior anti-pollution performance, lack of “zero-value” or “low-value” units, lightweight nature, ease of installation, and minimal maintenance requirements. Since 1996, regulations in Guangdong Province have mandated the use of composite insulators for vertical strings on overhead lines in Class II and above pollution areas.

By the end of 1998, the Zhongshan Power Grid had installed 4,925 composite insulators (110 kV+), accounting for approximately 67.8% of the total towers. Overall, performance has been excellent with zero pollution flashover trips. However, 13 lightning-induced flashovers occurred, causing arc burns on grading rings and arcing horns. Analysis shows that composite insulators have both pros and cons regarding lightning protection:

• Advantage: Unlike porcelain or glass insulators, they do not suffer from unavoidable “zero-value” degradation, maintaining a consistent lightning protection level for the entire string.

• Disadvantage: Due to the smaller shed diameter, the effective dry arc distance is shorter than that of porcelain or glass insulator strings of the same height, which can lead to a reduction in the standard lightning protection level.

1. Operational Analysis

The Zhongshan region (South-Central Guangdong) experiences frequent lightning activity due to low latitude and strong solar radiation, with an average of 90 thunderstorm days per year. Lightning-related faults account for 73.6% of the total faults in the 35 kV and above transmission system.

Operational experience indicates that while the 50% impulse flashover voltage (amplitude) of standard composite insulators (540–580 kV) is 8%–13% lower than that of a 7-unit porcelain string (dry arc distance approx. 1,150 mm), the overall trip rate has decreased. This is because composite insulators are rod-shaped and do not suffer internal dielectric breakdown or “zero-value” issues.

For areas with high lightning density (4.40 times/km²·a) and high lightning current amplitude (average 38.5 kA), using standard composite insulators requires a rigorous audit of their impulse withstand voltage. Data shows that “lengthened” composite insulators (dry arc distance approx. 1,045 mm) had a significantly lower flashover rate (23% of total faults) compared to standard types.

2. Technical Countermeasures

2.1 Sufficient Effective Dry Arc Distance
A sufficient dry arc distance is the key factor. For 110 kV composite insulators, the effective dry arc distance should be selected between 1,050–1,100 mm. For 220 kV, it should be 1,950–2,000 mm. Appropriately lengthened 110 kV insulators can reach a 50% lightning impulse flashover voltage (positive polarity) of 640–680 kV, meeting design requirements.

2.2 Considerations for Sag and Wind Sway
When using lengthened insulators, technical parameters such as conductor sag and wind sway (wind deviation) must be verified. Utilizing integral injection-molded insulators with shorter metal fittings can allow for a longer dry arc distance within the same structural height.

2.3 Application of Grading Rings
Grading rings improve electric field distribution.

• 110 kV: One ring at the high-voltage end.

• 220 kV: Two rings (one at each end).

• Specs: Outer diameter 250–300 mm, tube diameter 30–40 mm, elevation distance 25–30 mm.
  Open rings with discharge electrodes are recommended for easy replacement. While a grading ring slightly reduces the impulse flashover voltage (by 1.5%–3%), it protects the sheds and core rod from severe arc burns.

2.4 Fault Inspection and Records
If a flashover occurs but the reclosing is successful, the insulator can often remain in service if no severe carbonization, erosion, or shed deformation is found. However, detailed records must be kept, including voltage level, location, fault nature (flashover vs. breakdown), and damaged components (grading rings, sheds, core rod interfaces).

2.5 Installation of Surge Arresters
As the lightning protection level of the line stabilizes with composite insulators, the probability of lightning waves invading the substation busbar increases. It is recommended to install surge arresters at substation entrances and, where possible, on line sections with historically high trip rates.

3. Conclusion

a) Standard composite insulators have both advantages and disadvantages in lightning protection; flashover issues deserve serious attention from both manufacturers and operators.
b) Auditing the lightning full-wave impulse withstand voltage to meet local design requirements is essential, and ensuring sufficient effective dry arc distance is the critical solution.