**A: Main Issues of Zinc Oxide Surge Arresters:**
1. Zinc oxide surge arresters can be affected by environmental factors such as rain, snow, dust, and other contaminants. These conditions can cause uneven potential distribution between the internal zinc oxide valve and the outer porcelain housing, leading to radial discharge and potentially damaging the entire arrester.
2. Under repeated exposure to impact voltages, the zinc oxide valve discs may age due to the energy they absorb. This aging process can reduce their effectiveness over time and increase the risk of failure.
3. Unlike traditional arresters with series gaps, modern zinc oxide arresters operate directly under grid voltage. This means a continuous current flows through the device, with the active component causing heat generation in the valve disc. This heat alters the volt-ampere characteristics of the valve, creating a positive feedback loop that accelerates aging and may eventually lead to thermal breakdown.
4. Moisture inside the arrester or poor insulation performance of the supporting structure can increase the power frequency current and power loss. In severe cases, this may result in internal discharges, further compromising the arrester’s integrity.
**II: Instrument Test Principles and Features**
1. The software is capable of identifying the voltage reference point, eliminating the need for a PT (Potential Transformer) signal. This feature can be selected within the software interface for greater flexibility.
2. The instrument uses FPGA-based hardware sampling and program-controlled amplification technology, increasing the sampling rate to 200 kHz. This allows for accurate capture of original current and voltage signals, ensuring stable and reliable test results. It also effectively filters out high-frequency harmonics and interference.
3. On-site testing is made convenient with an internal lithium-ion battery and wireless data transmission capabilities, allowing for efficient field operations without the need for external power sources.
4. The use of an embedded industrial processor ensures faster processing speeds, easier setup, and the ability to simulate various algorithms. This increases the transparency of the testing method, making the instrument more versatile as a diagnostic tool.
5. The instrument supports three-phase simultaneous testing, which helps eliminate interphase interference. This feature can be enabled via the software as needed.
6. The software includes advanced functions such as device management and database management, with a shared platform between the main computer and the instrument for seamless integration.
7. It measures both voltage and current signals, performs fast Fourier transforms, and calculates capacitive and resistive components, including fundamental and harmonic values.
**III: Theory and Practical Conclusion**
1. When the arrester degrades uniformly, the bottom capacitive current remains relatively stable. However, if the degradation is uneven, the bottom capacitive current increases significantly. When half of the arresters are deteriorated, the capacitive current at the bottom reaches its peak.
2. The fundamental component of the resistive current tends to increase substantially. If harmonic content does not rise significantly, it often indicates serious damage or moisture ingress.
3. Interphase interference may affect test readings but does not invalidate the results. Comparing current data with historical records provides a more accurate reflection of the arrester's operational condition.
4. When the harmonic content of the resistive current increases significantly while the fundamental component remains stable, it typically indicates aging of the arrester.
**IV: Why Resistive Current Testing Matters**
Resistive current testing is crucial for determining whether a zinc oxide arrester (MOA) is experiencing aging or moisture-related issues. Under normal operating conditions, the resistive current is usually a small portion of the total current—typically 10% to 20%. Therefore, detecting changes in the resistive current based solely on total current measurements is challenging.
To accurately assess the condition of the arrester, it is necessary to isolate the resistive leakage current from the total current. This allows for a clearer understanding of its behavior and provides a more reliable basis for maintenance decisions.
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