Infrared Diagnostic Methods for Defects in Composite Insulator Core Rods

Introduction
Defects in composite insulator core rods are often unavoidable during the manufacturing process. How can these defects be accurately diagnosed? Today, Taporel Electrical Insulation will share some professional knowledge regarding infrared diagnostic techniques.

1. Infrared Characteristics of Internal Defects in Composite Insulators

The primary heat sources in composite insulators include polarization loss, partial discharge (PD), and leakage current. Polarization loss and PD are typically caused by excessive electric field concentration due to internal defects.

Heating caused by leakage current is mainly manifested as surface degradation. The current concentrates at these degraded areas, creating a significant temperature difference compared to the surrounding regions. Generally, due to partial discharge, the ends of air gaps and carbonization channels often become “hot spots,” while the temperature in the middle of a carbonization channel is typically lower due to a weaker electric field.

2. End-Moisture Ingress

In a specific case, several composite insulators on a 500kV line exhibited a temperature rise of 2-4 K. Laboratory temperature rise tests were conducted under both AC and DC withstand voltages, with the applied voltage amplitude equal to the maximum operating voltage.

Characteristics: Under high AC voltage, the temperature rise in high-humidity conditions reached approximately 4-5 K, which is significantly higher than in low-humidity conditions. Conversely, during DC high-voltage testing, almost no temperature rise was observed. The fluctuation in humidity causes the temperature rise to be intermittent or inconsistent.

3. Carbonization Channels

Following the fracture of one insulator in a double V-string on a 500kV tower, the broken core rod revealed multiple voids and the formation of internal corrosion channels. Damaged insulators (A and B) from the same batch were subjected to infrared thermography under operating voltage.

4. Best Practices for Infrared Testing and Diagnosis

• a) Eliminate External Interference: Composite insulators are voltage-heated devices, and their temperature field is highly susceptible to environmental factors. Strong sunlight or visible light can create a temperature gradient between the sunny and shaded sides of the insulator, potentially “masking” internal hot spots and leading to misjudgment. Therefore, it is ideal to conduct testing on cloudy days or during periods of low light.

• b) Standardize Parameter Settings: Parameters such as test distance, emissivity (set between 0.85 and 0.95), and wind speed must be configured correctly. Use rangefinders and anemometers to determine precise environmental parameters during on-site testing.

• c) Optimization of Angle and Focus: Maintain a viewing angle perpendicular to the core rod and ensure sharp focus; blurry infrared images should be discarded. Temperature rise should be determined by comparing the identified hot spot to other distant, normal sections of the core rod.

• d) Comprehensive Data Recording: Record the test time, ambient humidity, and temperature, and archive all infrared images for follow-up analysis.

• e) Software Post-Processing: If parameters are set incorrectly during the initial test, professional infrared analysis software can be used to “invert” or adjust the data. While recalculating temperature rise via software can mitigate errors, the accuracy is still inferior to correctly configured real-time testing.

• f) Long-Distance Resolution: When inspecting high-voltage towers, the resolution of infrared cameras may decrease due to the extreme distance, failing to detect subtle temperature rises. In such cases, telephoto lenses should be used to extend the effective testing distance and improve resolution.