Early Moisture Diagnosis of Line Arresters Using Third Harmonic Leakage Current Analysis

Introduction

Metal oxide surge arresters (MOSAs) are critical components in power transmission and distribution networks, protecting equipment against overvoltages caused by lightning strikes and switching surges. Despite their robust nonlinear characteristics and long service life, surge arresters gradually degrade during operation due to electrical stress, environmental factors, thermal aging, and—most critically—moisture ingress.

Among various degradation mechanisms, early-stage moisture ingress is particularly challenging to detect. Conventional total leakage current measurement can identify severe deterioration but remains insensitive to early moisture-related defects. The need for online, sensitive diagnostic methods has driven substantial research into harmonic analysis of leakage current, with the third harmonic resistive component emerging as the most promising indicator for early moisture detection.

Physical Mechanism: Why Third Harmonic?

To understand the diagnostic value of third harmonic analysis, one must first examine the composition of total leakage current in a MOSA. The total leakage current I_T consists of two primary components: a capacitive component and a resistive component. Under normal operating conditions, the capacitive current dominates. However, when moisture ingress occurs, the resistive portion increases significantly due to the altered electric field distribution and accelerated degradation of ZnO varistors.

The key insight lies in the nonlinear voltage-current characteristic of ZnO varistors. When a sinusoidal voltage is applied to a nonlinear resistor, the resulting current contains not only the fundamental frequency but also odd-order harmonics. The third harmonic component of the resistive leakage current is particularly sensitive to changes in the ZnO element’s nonlinearity. As moisture penetrates the arrester housing, the resistive current increases and its third harmonic content rises proportionally—often before the total leakage current shows any detectable change.

Experimental Validation

Controlled laboratory studies have quantitatively confirmed the effectiveness of this technique. Research on six types of ultrahigh-pressure MOVs demonstrated that when continuous operating voltage changes occur, the resistive third harmonic current exhibits the most pronounced variation among all leakage current components. Temperature experiments further revealed that while resistive fundamental harmonic leakage current changes significantly with temperature, the third harmonic component remains relatively stable under varying thermal conditions—a property that enhances its reliability as a diagnostic indicator.

Field case studies provide compelling real-world evidence. A documented 110 kV lightning arrester case identified abnormal resistive current during live-line detection. Multistage tracking of resistive current and power factor angle, combined with infrared thermography and offline tests, confirmed moisture ingress. Disassembly inspection revealed that unsealed fixing holes on the top flange had allowed atmospheric moisture penetration. Notably, the third harmonic resistive current had already exhibited anomalies months before conventional diagnostics flagged any issue.

Practical Implementation and Compensation Techniques

While the theoretical foundation is sound, practical implementation requires careful compensation for two major interference sources: system voltage harmonics and ambient temperature.

Harmonic Compensation

The third harmonic content measured in leakage current comprises two components—one from the arrester’s nonlinear V-I characteristics and another from pre-existing harmonics in the system voltage. As the ABB Excount II manual explains, voltage harmonics produce capacitive harmonic currents in the arrester that may be of the same order of magnitude as the harmonics generated by the resistive current, directly interfering with diagnostic accuracy. Therefore, accurate measurement requires system harmonic compensation, typically performed in accordance with IEC 60099-5-B2

Temperature Correction

Temperature significantly affects resistive current magnitude. Advanced monitoring instruments incorporate built-in temperature sensors that correct measurements to a reference temperature (typically 20°C) per IEC 60099-5, applying correction factors for both voltage and temperature variations. Patent CN-105954632-B further describes a comprehensive method that performs temperature interference correction and harmonic interference correction on fundamental and third harmonic components of resistive current, preventing misdiagnosis caused by environmental factors.

Advanced Extraction Algorithms

Recent work has focused on improving extraction accuracy. Researchers have developed algorithms that calculate self-capacitance and stray capacitance of three-phase arresters to separate capacitive current from total leakage current, effectively reducing interphase interference. Simulation-based approaches using Prony analysis and Hilbert transform have demonstrated viability in handling both stationary and non-stationary signals with small sample sizes.

Comparative Assessment with Alternative Methods

Diagnostic Method Sensitivity to Early Moisture Suitability for Online Monitoring Key Limitation

Total current Low Yes Insensitive to early-stage defects

Capacitive current compensation Medium Limited Not suitable for three-phase synchronous measurement

Third harmonic method High Yes Requires harmonic compensation; cannot quantify aging degree alone

Fundamental wave method Medium Yes Higher accuracy but less sensitive to early moisture

Infrared thermography Medium Auxiliary Detects temperature rise after moisture has penetrated

DC reference voltage (U₁mA) Low No (offline) Requires system shutdown

The third harmonic method excels in early detection but is not without limitations. It is unsuitable when grid harmonic distortion exceeds certain thresholds and cannot provide quantitative analysis of aging degree independently. Consequently, best practice involves integrating third harmonic analysis with other diagnostic indicators—such as fundamental resistive current, infrared thermography, and DC reference voltage—for comprehensive condition assessment.

Conclusion

Third harmonic leakage current analysis represents a proven, sensitive technique for early detection of moisture ingress in line arresters. Its physical basis in ZnO varistor nonlinearity, combined with practical compensation methods for voltage harmonics and temperature effects, enables reliable online monitoring without system interruption. However, optimal diagnostic outcomes require a multi-parameter approach, integrating third harmonic analysis with complementary techniques. For utilities seeking to transition from time-based to condition-based maintenance, implementing third harmonic monitoring offers a cost-effective pathway to improved reliability and extended equipment life.