Synergistic Mechanism of External Insulation Flashover on Line Arresters Under Combined Action of Pollution, Moisture, and Impulse Current

1. Introduction

Line arresters, typically metal-oxide surge arresters without series gaps, are exposed to ambient conditions. Their polymeric or porcelain housings provide the necessary creepage distance. Under clean, dry conditions, the external insulation withstands nominal voltages and lightning overvoltages. However, in polluted coastal or industrial areas combined with high humidity (fog, dew, light rain), the surface condition deteriorates. When a lightning impulse arrives, the already stressed insulation may fail prematurely—not due to internal varistor failure, but via external flashover along the housing. Understanding this three-factor synergy is key to enhancing grid reliability.

2. The Three Aggravating Factors

2.1 Pollution

Pollution refers to conductive or inert deposits—salt, industrial dust, cement, or agricultural chemicals—accumulating on the arrester surface. These deposits create a saline or electrolytic layer when wetted. Equivalent Salt Deposit Density (ESDD) and Non-Soluble Deposit Density (NSDD) determine the surface conductivity. High ESDD directly increases leakage current under operating voltage.

2.2 Moisture

Moisture alone (fog, condensation, light rain) on a clean surface has relatively high resistivity. However, when moisture interacts with pollution, it dissolves soluble salts, forming a continuous conductive electrolyte film. Partial drying due to joule heating creates dry bands, where the remaining voltage concentrates, leading to local arcing. High humidity also reduces the flashover voltage by lowering the breakdown strength of air gaps along the creepage path.

2.3 Impulse Current

Impulse current from a lightning strike (typically 8/20 μs waveform) imposes a steep-fronted, high-magnitude stress. Unlike power-frequency voltage, impulse current generates intense, transient heating at the pollution layer and along dry bands. It also produces a strong electromagnetic field that can distort the potential distribution along the arrester. The rapid rise time (di/dt) can cause non-uniform voltage sharing, especially on multi-unit arresters.

3. Synergistic Flashover Mechanism

The synergistic mechanism is not a simple addition of individual effects but a cascade of coupled physical processes:

Stage 1 – Pre-Contamination:

The arrester surface accumulates pollution layers containing hygroscopic salts. Even in dry weather, some leakage current flows due to partial humidity.

Stage 2 – Wetting:

High humidity or light rain dissolves soluble pollutants. A continuous thin electrolyte film forms, dramatically reducing surface resistance from gigaohms to kiloohms or less. Leakage current under operating voltage increases significantly, initiating local heating.

Stage 3 – Dry Band Formation:

Joule heating evaporates moisture at the hottest regions (e.g., near the high-voltage terminal or between shed edges). This breaks the conductive film, creating dry bands. The remaining applied voltage now concentrates across these narrow dry gaps, generating electric fields exceeding 1 kV/mm.

Stage 4 – Impulse Current Injection:

When a lightning overvoltage occurs, the arrester conducts impulse current through its internal varistors. However, part of this impulse current also flows along the polluted, partially wet surface, seeking the path of least impedance. The high di/dt induces inductive voltage drops along the housing, shifting potential distribution. The dry bands experience an instantaneous voltage rise far above their withstand capability.

Stage 5 – Arc Initiation and Propagation:

At a critical dry band, a local spark bridges the gap. This spark transforms into a partial arc. The arc root temperature (~5000–15000 K) evaporates adjacent moisture, extending the dry zone. The arc propagates stepwise along the creepage path under the combined influence of:

· The power-frequency voltage (if the arrester remains energized)

· The trailing impulse current

· Residual charges on the pollution layer

Stage 6 – Complete Flashover:

Once the arc length exceeds a critical value (typically 60–80% of the total creepage distance), the remaining unpolluted insulation can no longer sustain the applied voltage. A full external flashover occurs, bypassing the internal varistor. The arrester fails to limit the overvoltage, potentially causing a system outage.

4. Key Influencing Parameters

Parameter Effect on Synergy

ESDD Higher ESDD → lower flashover voltage, easier dry band arcing

NSDD Thick inert layer retains moisture, prolongs wetting

Surface hydrophobicity Loss of hydrophobicity (aging) accelerates film formation

Impulse amplitude 10 kA can induce direct surface breakdown without dry bands

Moisture uniformity Non-uniform wetting (e.g., partial shadow) creates stress concentration

Creepage distance Longer creepage increases withstand but does not eliminate dry bands

5. Practical Implications & Mitigation

Understanding this synergy suggests several countermeasures:

1. Anti-pollution coatings: Room-temperature vulcanized (RTV) silicone rubber coatings restore hydrophobicity and delay conductive film formation.

2. Increased creepage distance: Use arresters with higher creepage-to-voltage ratio (≥31 mm/kV for heavy pollution).

3. Booster sheds: Special auxiliary sheds interrupt the continuous pollution layer and prevent arc propagation.

4. Regular cleaning: Hot-line washing or dry-ice blasting removes accumulated pollutants before wet seasons.

5. Diagnostic indicators: Monitor surface leakage current pulses; a sudden rise under foggy conditions predicts imminent flashover risk.

6. Conclusion

External insulation flashover on line arresters under the combined action of pollution, moisture, and impulse current is a nonlinear, time-dependent process. The presence of pollution and moisture creates a vulnerable surface state with dry bands and high local fields. Impulse current injection then acts as a trigger, rapidly converting a partial arc into a full flashover. This mechanism explains why some arresters fail during lightning storms even though internal varistors remain intact. Future work should focus on real-time surface condition monitoring and adaptive protection schemes.