This engineering document from Circuit Energy explores the frequency and causes of unexpected facility shutdowns due to external electrical disturbances. It highlights how even minor voltage fluctuations, which occur randomly, can significantly impact customer operations.

Frequency of Power Interruptions

• Global Benchmarks: Reliability performance is often measured by the System Average Interruption Frequency Index (SAIFI), which tracks the number of actual power interruptions lasting more than five minutes that a customer experiences annually.

• Developed Countries: Average SAIFI levels typically range from 0.5 to 5.0 interruptions per year.

• United States: The average SAIFI is approximately 1.3 interruptions per year.

• Influencing Factors: These rates depend on weather conditions, the age and type of infrastructure (underground vs. overhead), and system design (networked vs. radial).

   

The Impact of Voltage Sags

While prolonged outages are rare, momentary voltage sags are much more frequent and can be just as disruptive to sensitive equipment.

• Definition: Even sags lasting less than 100 ms can shut down susceptible facility operations.
• Measurement: The System Average RMS (Variation) Frequency Index (SARFI) quantifies the average number of voltage sags below a specific threshold (e.g., SARFI70 for sags below 70% of nominal voltage).

• Equipment Sensitivity:

  • In the U.S., sags below 70% occur roughly 18 times per year.

  • If equipment is sensitive to minor sags of 90%, the number of disruptive events jumps to about 50 per year.

The document emphasizes that understanding equipment sensitivity is critical for determining the true impact of these disturbances on a facility.