Differentiated Insulator Selection for Transmission Lines: Configuration Strategies Based on Pollution Severity and Altitude Correction

Introduction

Transmission line outages caused by pollution flashover remain one of the most persistent challenges in power system reliability. The flashover process is governed by two interacting environmental factors: the accumulation of conductive contaminants on insulator surfaces and the reduction of air density at elevated altitudes, which lowers the breakdown voltage of the surrounding air gap. A configuration strategy that addresses pollution severity alone, while neglecting altitude effects, will systematically under-dimension external insulation in mountainous or plateau regions.

The international framework for addressing this problem is the IEC TS 60815 series, which provides a structured methodology for determining the required creepage distance through the concept of Unified Specific Creepage Distance (USCD). This article outlines a practical workflow for differentiated insulator selection, combining pollution severity classification with altitude correction factors.

Step 1: Determining Site Pollution Severity

The foundation of any insulation configuration is an accurate assessment of Site Pollution Severity (SPS). IEC TS 60815-1:2025 provides the general principles for characterising pollution at a site and determining both the SPS value and the SPS class. The second edition of this standard introduced a new pollution class f (extremely heavy), applicable only when the Reference Unified Specific Creepage Distance (RUSCD) of class e cannot meet requirements.

In China, transmission line projects typically categorise pollution into five levels (a through e) based on Equivalent Salt Deposit Density (ESDD) and Non-Soluble Deposit Density (NSDD). The number of porcelain and glass insulator units required for each level is then determined from artificial contamination withstand tests. For a creepage distance of 635 mm per unit, the recommended insulator counts are 38 units for level b, 41–44 for level c, 51–55 for level d, and 60–62 for level e.

It is important to note that pollution classification alone does not determine the final configuration. Field experience shows that in ultra-severe pollution environments—where saturated ESDD exceeds 0.4 mg/cm²—flashover trips can still occur even with anti-pollution coatings, composite insulators, and insulation configured at the upper limit of the USCD method. This underscores the need for site-specific correction beyond the standard classification.

Step 2: From RUSCD to USCD — Applying Correction Factors

The core of IEC TS 60815-3:2025 is the transition from the RUSCD of a reference insulator to the USCD of a candidate insulator through a series of correction factors. The altitude correction is the most consequential for projects crossing mountainous terrain.

The physical basis for altitude correction is the power-function relationship between pollution flashover voltage and air pressure:

U₅₀% = U₀ × (P/P₀)ⁿ

where U₅₀% is the flashover voltage at pressure P, U₀ is the flashover voltage at standard reference pressure P₀ (101.3 kPa), and n is the air pressure influence characteristic index.

Experimental data from UHVDC insulator tests conducted at Beijing (45 m) and the Tibet high-altitude base (4,300 m) show that the altitude correction coefficient varies with shed profile. For standard, 2-shed, and 3-shed insulators, the coefficients were 4.24%, 3.82%, and 3.82% respectively. For AC composite insulators tested at altitudes between 2,800 m and 4,500 m, the flashover voltage decreases by approximately 4.0% to 6.4% per 1,000 m of altitude increase.

These coefficients are not interchangeable across insulator types and shed geometries. A differentiated selection approach must therefore apply profile-specific altitude corrections rather than a blanket uplift factor.

Step 3: Insulator Type Selection

With the corrected USCD established, the next decision is the choice of insulator type.

Porcelain and glass insulators rely on glaze coatings and extended creepage distance. They are well suited to light pollution areas and offer predictable, stable performance. However, their performance degrades more rapidly under combined pollution and icing conditions.

Composite (polymeric) insulators utilise hydrophobic silicone rubber sheds that cause water to bead and contaminants to shed. Their flashover performance can be improved by approximately 20% through optimisation of shed parameters. At low pressure corresponding to 4,484 m altitude, polymer insulators demonstrate a flashover voltage gradient 48.72% higher than porcelain and 72.35% higher than glass insulators. This makes composite insulators particularly advantageous for medium to severe pollution areas and high-altitude applications.

Field studies in China have confirmed that shed-type insulators are suitable for light pollution areas, while composite insulators exhibit excellent anti-pollution flashover performance for medium and more severe conditions. For UHV AC projects, a composite insulator length of 9 m is specified for areas at pollution level e or lower, based on contamination withstand experiments under weak hydrophobicity conditions.

Case Application

Consider a 500 kV AC transmission line traversing a region with SPS class d (heavy pollution) at an average altitude of 2,500 m. Using the IEC framework:

1. The RUSCD is determined from the SPS class d classification.

2. An altitude correction is applied. Using a conservative coefficient of 5% per 1,000 m for composite insulators, the correction factor at 2,500 m is approximately 1.125.

3. The corrected USCD is calculated as RUSCD × 1.125.

4. Candidate insulators are evaluated against this corrected USCD. Composite long-rod insulators are preferred for their superior performance under combined pollution and low-pressure conditions.

This differentiated approach avoids both over-design (which increases tower height and cost) and under-design (which risks flashover).

Conclusion

Differentiated insulator selection requires a systematic progression from pollution severity assessment through altitude correction to type-specific evaluation. The IEC TS 60815 series provides the methodological framework, but its effective application demands profile-specific altitude coefficients and recognition of the performance differences between porcelain, glass, and composite insulators. For projects in high-altitude polluted regions, composite insulators configured with site-specific altitude correction offer the most reliable and cost-effective solution.