Interpreting VLF test data accurately involves assessing several key parameters, each revealing specific information about the condition of cable insulation:

1. Leakage Current:

• Stable, Low Leakage Current: When leakage current remains consistently low and stable throughout the test duration, it typically suggests good insulation health.
• Increasing or Fluctuating Leakage Current: If the leakage current rises over time or shows significant fluctuations, it may indicate that the insulation is degrading or has moisture intrusion. Such patterns are common in cables with deteriorating insulation quality, often leading to further investigation or additional diagnostics.

2. Insulation Resistance:

• High Insulation Resistance: High resistance generally indicates that the insulation is performing well, showing resilience against electrical stress. Cables with high resistance are less likely to fail under normal operational conditions.
• Decreasing or Low Insulation Resistance: A noticeable decrease in insulation resistance or low initial readings can be warning signs. This may be due to factors like aging, contamination, or moisture. Specific thresholds can vary by equipment and test standards, but consistently low values may necessitate repairs or replacement.

3. Breakdown Strength:

• Passing Breakdown Strength: If the cable withstands the applied voltage without failure, sudden increases in leakage current, or breakdown events, it generally indicates a pass for the insulation’s integrity. Successful testing across the voltage range shows the insulation is holding up against high-stress conditions.
• Sudden Spike or Breakdown Event: A sudden increase in leakage current, accompanied by a breakdown, signifies a fail. Such results suggest the insulation cannot withstand the stress of high voltage and may already be compromised, which often necessitates immediate action, either with further diagnostics or planning for replacement.

4. Pass/Fail Criteria Based on Standards:

• IEEE 400.2 Guidelines: Industry standards like IEEE 400.2 provide specific pass/fail criteria for VLF testing, recommending specific leakage current values and test durations. For instance, gradual leakage increase within set thresholds is sometimes permissible, while rapid or spiking leakage typically fails.
• Voltage Withstand Time: IEEE standards suggest duration benchmarks (usually 15-60 minutes) during which the insulation should remain stable. Failing within this timeframe could indicate early-stage insulation breakdown.

Interpretation should always take into account the cable’s age, environment, and historical data, as older or environmental-stressed cables may behave differently, even under normal conditions.

Nov 01,2024