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Polluion flashover, commonly abbreviated as pollution flashover or in technical contexts, is a surface discharge phenomenon that occurs when insulating components of an arrester become contaminated. While the metal oxide varistor (MOV) blocks inside the arrester handle the electrical stress, the external polymer or porcelain housing provides the necessary creepage distance. When this housing is covered with industrial dust, salt spray, or chemical pollutants, and subsequently exposed to humid conditions (fog, dew, or light rain), a conductive film forms on the surface.
Under normal dry conditions, the insulation resistance of the arrester housing remains high. However, when pollution combines with moisture, leakage current begins to flow across the surface. This current is non-uniform, leading to localized heating and the formation of "dry bands." The voltage stress across these dry bands becomes intense, resulting in scintillation or arcing. If the pollution severity is high enough, these arcs can bridge the insulation gap, leading to a complete flashover.
Pollution flashover does not necessarily destroy the arrester immediately, but it places immense thermal stress on the housing and can cause:
· Temporary or permanent grounding faults: Triggering circuit breakers and causing power outages.· Degradation of the housing material: Polymer housings may suffer from tracking and erosion, while porcelain housings may experience glaze damage.
· Accelerated aging: The heat generated by surface arcing can transfer to the internal MOV blocks, elevating their temperature and accelerating thermal runaway.
Unlike a sudden failure caused by a direct lightning strike, aging is a gradual degradation of the arrester’s internal metal oxide varistor blocks. Aging increases the resistive component of the leakage current, leading to increased power dissipation and eventual thermal runaway if left unchecked.
Key Indicators of Aging
Identifying aging requires a combination of visual inspection and electrical testing. Technicians should look for the following signs:
Visual and Physical Signs
· Cracking or Chalking: On polymer housings, UV radiation and electrical stress cause the silicone rubber to lose hydrophobicity (water-repellent property). Cracking allows moisture ingress.
· Rust or Discoloration: Rust stains at the ends of the arrester or on the grading rings indicate poor sealing or corona discharge.
· Swelling: In porcelain arresters, a swollen or deformed appearance often signals internal gas generation due to severe internal arcing or thermal runaway.
Electrical Signatures
· Total Leakage Current: Measuring the total leakage current at operating voltage is the primary diagnostic tool. While capacitive current is normal, an increase in the resistive leakage current (Ir) is the definitive indicator of aging. A 20-30% increase in Ir over baseline values typically warrants further investigation or replacement.
· Third Harmonic Analysis: As MOV blocks age, their non-linear characteristics change, resulting in a higher generation of third harmonic currents. Modern diagnostic tools can separate resistive current from total leakage current to provide a precise health assessment.
· Infrared Thermography: A healthy arrester operates at ambient temperature or shows a slight temperature rise. A "hot spot" or a significant temperature differential compared to identical units in the same phase indicates localized power loss due to aging or moisture ingress.
To mitigate the risks of pollution flashover and aging, a proactive maintenance strategy is required. Maintenance should be divided into routine inspections, cleaning protocols, and electrical diagnostics.
The most effective way to prevent flashover is to manage the pollution levels on the arrester surface.
· Cleaning: For porcelain arresters, live-line washing using deionized water is highly effective in high-pollution areas (coastal regions, industrial zones). This should be scheduled before the onset of foggy or rainy seasons.
· Hydrophobicity Testing: For polymer arresters, regular hydrophobicity tests (spray test) should be conducted. If the surface loses its water-repellent properties, the arrester is susceptible to moisture-induced leakage currents. While polymer housings generally require no cleaning, severe pollution may necessitate gentle washing with non-abrasive methods.
· Coating Applications: In extremely polluted environments, applying room-temperature-vulcanizing (RTV) silicone coatings to porcelain housings can significantly increase the creepage distance and maintain surface hydrophobicity.
Electrical testing is the cornerstone of identifying aging before failure occurs.
· Baseline Data: It is critical to establish baseline leakage current measurements and thermographic profiles during commissioning. Comparative analysis (comparing the same phase across three phases) is often more telling than absolute values.
· Portable Leakage Current Meters: Use portable analyzers that can separate resistive and capacitive currents. If resistive current exceeds 0.5 mA to 1.0 mA (depending on voltage class and manufacturer specifications) or shows a year-on-year increase of 30%, replacement should be scheduled.
· Surge Counter Monitoring: Many arresters are equipped with surge counters. While these do not indicate aging directly, a sudden spike in counts without corresponding lightning activity may suggest continuous arcing due to pollution or internal failure.
· Connections: Loose connections are a common point of failure. Thermal imaging can identify poor electrical contacts that generate heat. Torque checks should be performed to ensure all connections are tight.
· Seal Inspection: For porcelain arresters with gaskets, check for moisture ingress. Moisture inside the arrester leads to catastrophic failure. If the internal gap or blocks are wet, the arrester must be replaced immediately.
· Grounding: Ensure the arrester’s ground lead is intact and has low impedance. A compromised ground negates the arrester’s ability to divert surge currents, forcing all energy into the protected equipment.
Power arresters are the first line of defense against overvoltages, yet they are often neglected until a failure occurs. The dual threats of pollution flashover and internal aging represent the most common failure modes in the field.
