Views: 0 Author: Site Editor Publish Time: 2026-06-05 Origin: Site
Metal oxide surge arresters (MOAs) have become the primary overvoltage protection devices in modern power systems due to their excellent nonlinear volt-ampere characteristics, high energy absorption capacity, and gapless design. The zinc oxide (ZnO) varistor, as the core component of MOA, determines the overall performance and reliability of the arrester. However, under real-world operating conditions—particularly in high-temperature and high-humidity (HTHH) environments prevalent in tropical coastal regions and subtropical industrial zones—the aging process of ZnO varistors is significantly accelerated, posing serious threats to the safe and stable operation of transmission lines.
According to operational statistics, moisture-related failures account for approximately 60–70% of all arrester faults, while aging induced by sustained voltage stress contributes to about 25% of failures. These figures highlight the urgent need for a comprehensive understanding of HTHH-induced varistor degradation and the development of reliable lifetime assessment methodologies.
The degradation of ZnO varistors in HTHH environments involves a complex interplay of thermal, electrical, and chemical processes. Understanding these mechanisms is essential for effective condition monitoring and lifetime prediction.
2.1 Electrical Thermal Stress
Under continuous operating voltage, a small leakage current constantly flows through MOA varistors—a phenomenon that distinguishes gapless arresters from their gapped predecessors. In high-temperature environments, the negative temperature coefficient of ZnO varistors causes leakage current to increase with rising temperature, creating a positive feedback loop that accelerates degradation.
As thermal aging proceeds, the double Schottky barrier structure at grain boundaries gradually deteriorates. Recent accelerated aging studies have revealed that modern stable ZnO varistors exhibit anomalous aging behaviors: at moderate aging temperatures (e.g., 120℃), power loss shows a continuous decreasing trend; at intermediate temperatures (150℃), it initially decreases then increases; and at elevated temperatures (180℃), sustained power loss increase occurs. The transition to irreversible aging is accompanied by the destruction of grain boundary structure, characterized by decreased relaxation time and activation energy, along with irreversible consumption of zinc interstitials.
2.2 Moisture Intrusion
Moisture is arguably the most detrimental factor for ZnO varistors in HTHH environments. Experimental investigations using damp heat chambers and multi-pulse platforms have shown that when varistors are exposed to moisture conditions, the speed at which the varistor voltage U₁mA exceeds the ±10% tolerance range is three times faster than under dry conditions. Furthermore, approximately 30% of damp samples fail after a single multi-pulse lightning strike.
The underlying mechanism involves water molecules diffusing into the inter-granular layers, where they trigger chemical reactions that deform the double Schottky barrier structure. Under multi-pulse stress conditions, the thermal conductivity of ZnO grains drops sharply, and inter-granular layers with poor thermal conductivity undergo thermal imbalance, fundamentally altering the charge distribution at grain boundaries and causing severe deterioration of voltage characteristics.
2.3 Synergistic Effects and Accelerated Failure
In real HTHH environments, thermal stress and moisture intrusion act synergistically. An electrothermal model analysis of a tropical coastal area MOA explosion accident demonstrated that high temperature and high humidity contribute to increased leakage current in long-term operation. The salt spray deposition and precipitation erosion in coastal areas accelerate the degradation of side insulation glaze on varistors. Subsequent thermal cycling damages the insulation glaze, eventually leading to internal flashover and catastrophic failure.
3.1 Critical Performance Parameters
The degradation state of ZnO varistors can be evaluated through several key electrical parameters:
· U₁mA (DC reference voltage): A voltage reduction of more than 10% below initial value is widely accepted as the failure criterion for varistors
· Leakage current: Under 0.75 times U₁mA, leakage current exceeding 50 μA indicates significant performance degradation. The resistive component of leakage current is particularly sensitive to both moisture and aging, allowing discrimination between the two degradation modes.
· Power loss and nonlinear coefficient: Changes in these parameters reflect the degree of grain boundary deterioration.
3.2 Accelerated Aging Testing Methodologies
Accelerated aging tests are indispensable for evaluating varistor longevity within realistic timeframes. Common approaches include:
· Temperature-accelerated aging: Elevated temperatures (typically 115–180℃) are applied while maintaining continuous voltage stress. The degradation rate coefficient Kt increases with applied voltage ratio (0.85U₁mA to 0.95U₁mA), with values ranging from 0.03 to 0.06 observed in additive-doped varistors.
· Moisture-accelerated testing: Controlled humid environments or direct moisture exposure methods (e.g., steam treatment followed by thermal aging) enable quantitative assessment of moisture-induced degradation.
· Multi-stress combined testing: Simultaneous application of thermal, humidity, and electrical stress best represents HTHH field conditions.
3.3 Arrhenius-Based Lifetime Modeling
The service life of ZnO varistors under continuous voltage stress follows the Arrhenius reaction rate model:
L = \exp \left( \frac{R}{T} - C \right)
where L represents lifetime, T is absolute temperature, and R and C are material-dependent constants. This relationship indicates that lifetime is inversely proportional to both temperature and voltage stress level.
For practical engineering applications, the thermal runaway condition must be integrated with degradation kinetics. The critical condition for thermal runaway can be expressed as a function of temperature rise due to surge absorption, providing a comprehensive framework for lifetime evaluation.
With proper material formulation—such as aluminum and indium doping—ZnO varistor ceramics can achieve predicted service lives exceeding 110 years under normal operating conditions, meeting the stringent requirements of power system surge arresters.
Effective lifetime management of HTHH-operated MOAs relies on timely detection of incipient degradation. Several monitoring approaches are available:
· Online monitoring systems: Real-time measurement of leakage current (full current, resistive component, and capacitive component) and power loss, combined with environmental parameter tracking (temperature, humidity, contamination).
· Infrared thermography: Abnormal temperature rise indicates localized heating caused by increased leakage current or poor contact.
· Periodic offline testing: IEC 60099-4 and GB/T 11032 standard-compliant testing, including insulation resistance measurement, DC reference voltage test (U₁mA within ±5% of nameplate value), and leakage current analysis under 0.75U₁mA.
High-temperature and high-humidity environments represent an extreme operational challenge for line MOA varistors, accelerating degradation through thermal stress, moisture intrusion, and their synergistic interaction. The degradation manifests as increased leakage current, reduced varistor voltage, and eventual grain boundary failure, with moisture involvement tripling the aging rate compared to dry conditions.
