Views: 0 Author: Site Editor Publish Time: 2025-09-19 Origin: Site
Surge arresters, particularly metal-oxide varistor (MOV) type arresters, play a vital role in safeguarding power systems against lightning strikes and switching surges. While these devices are generally reliable, they are subject to aging and various environmental stresses that can compromise their performance. Traditional maintenance approaches often rely on periodic visual inspections and scheduled replacements, which may either miss developing faults or lead to premature replacements. Advanced diagnostic techniques, such as leakage current measurement and infrared thermography, offer a more scientific approach to condition-based maintenance, enabling utilities and industries to optimize their maintenance strategies.
A failed arrester can cause a short circuit, leading to equipment damage, power outages, and even safety hazards. Diagnostic testing helps in:
· Predicting Failures: Identifying arresters that are nearing the end of their service life.
· Preventing Outages: Avoiding unplanned downtime by scheduling replacements during planned maintenance.
· Reducing Costs: Minimizing the cost associated with equipment damage and emergency repairs.
· Enhancing Safety: Mitigating the risks of explosions or fires caused by arrester failures.
Metal-oxide surge arresters exhibit a small conductive current under normal operating voltage, known as leakage current. This current consists of two components:
· Resistive Current (Ir): This is in phase with the voltage and is a key indicator of the arrester's health. An increase in Ir signifies degradation of the MOV discs.
· Capacitive Current (Ic): This leads the voltage by 90 degrees and is generally stable over the arrester's life.
The total leakage current (It) is the vector sum of Ir and Ic. As an arrester ages, the resistive component typically increases due to the deterioration of the zinc oxide grains, leading to heating and potentially thermal runaway.
Leakage current can be measured using specialized instruments, often called arrester testers or leakage current analyzers. These devices typically use the following methods:
· Third Harmonic Analysis: Exploits the non-linear characteristic of the MOV to extract the resistive component from the total leakage current.
· Compensation Method: Uses a reference capacitor to cancel out the capacitive current, leaving the resistive component for analysis.
On-line vs. Off-line Measurement: On-line measurement is performed with the arrester energized and is the most common approach as it doesn't require system shutdown. Off-line testing applies a test voltage to the de-energized arrester.
The diagnostic focus is on the resistive current (Ir) and its changes over time.
· Absolute Value: Comparing the measured Ir to the manufacturer's specified limits or typical values for similar arresters (often in the range of tens to a few hundred microamperes).
· Trend Analysis: Tracking the gradual increase of Ir over successive measurements is a more reliable indicator of progressive degradation than a single absolute value. A sudden or sharp increase is a clear warning sign.
· Phase Angle: The phase difference between the fundamental frequency of the voltage and the current can also be used to assess condition.
Acceptance Criteria: While manufacturer guidelines are primary, a common rule of thumb is that an Ir value that has doubled from its initial reading or has exceeded a predefined threshold (e.g., 500 µA) warrants further investigation or replacement.
Infrared thermography detects heat generated by power losses within the arrester. A healthy arrester has very low losses and thus a minimal temperature rise above ambient. As the resistive leakage current increases due to degradation, the power loss (I²R) increases, leading to a measurable temperature increase on the arrester's housing.
1. Equipment: Use a high-quality thermal imaging camera, preferably with a temperature measurement accuracy of ±2°C or better.
2. Conditions: Perform the scan while the arrester is energized under normal load conditions. Ensure a clear line of sight to the arrester.
3. Emissivity Settings: Correctly set the emissivity value of the porcelain or polymer housing in the camera settings for accurate temperature readings.
4. Load & Ambient Factors: Consider the load current and ambient temperature, as they can influence the results. It's often best to perform scans under similar conditions for comparative analysis.
5. Reference Point: Always include a reference point (e.g., a busbar or a connection point) in the image for comparison.
· Healthy Arrester: Will show a uniform temperature distribution along its length, with a temperature rise of only a few degrees Celsius above ambient.
· Faulty/Degrading Arrester: Will exhibit a noticeable hot spot, typically in one of the modules or sheds. A significant temperature differential between the faulty section and the rest of the housing or adjacent equipment is a critical indicator.
· Severe Internal Fault: May show a very pronounced temperature rise, easily identifiable against the background.
Note: A clean surface is crucial for accurate thermography, as dirt or moisture can create misleading hot or cold spots.
The most effective strategy combines both techniques:
1. Screening with Infrared Thermography: Use IR scanning for rapid, non-contact screening of all arresters in a substation. This helps quickly identify units with apparent thermal anomalies.
2. Detailed Analysis with Leakage Current: For arresters that show thermal anomalies or are in critical applications, perform a detailed leakage current analysis to quantify the level of degradation and confirm the findings.
This combined approach provides a comprehensive assessment, leveraging the strengths of both methods to improve diagnostic confidence.
Proactive condition monitoring of surge arresters is no longer a luxury but a necessity for ensuring the reliability and safety of modern power systems. Leakage current analysis and infrared thermography are two powerful, complementary techniques that provide deep insights into the health of metal-oxide arresters. By regularly applying these diagnostics and trending the results, maintenance teams can transition from a reactive or time-based maintenance model to a predictive one, ultimately reducing costs and preventing unexpected failures.
