The Physics of ZnO Arrester Degradation
Zinc oxide varistors operate on a nonlinear voltage-current characteristic that allows them to present high resistance under normal operating voltage while rapidly transitioning to low resistance during overvoltage events. This behavior is governed by the grain boundary structure of sintered ZnO ceramics doped with small quantities of Bi2O3, Sb2O3, CoO, MnO, and other metal oxides.
Under continuous AC stress, several degradation mechanisms emerge:
Thermal runaway: Increased leakage current generates Joule heating, which further increases leakage in a positive feedback loop that can lead to catastrophic failure.
Moisture ingress: Hermetic seal failure allows humidity penetration, accelerating internal partial discharges and chemical corrosion of ZnO grain boundaries.
Multiple lightning impulse aging: Repeated high-energy surges gradually erode the varistor protective grain boundary barriers.
Harmonic stress: Distorted voltage waveforms increase the resistive component of leakage current, accelerating thermal aging.
Each of these mechanisms manifests as measurable changes in the arrester electrical parameters – specifically resistive leakage current, total leakage current, and capacitive current – long before a catastrophic failure occurs.
Key Parameters for Online Arrester Monitoring
Modern online monitoring systems like the DT801 continuously track multiple parameters that serve as early warning indicators.
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