Voltage sags & power-quality events — IEEE 1159, ITIC, SARFI

Reference notes

Voltage sags are the single most common power-quality disturbance in industrial and commercial plants — roughly 80-90% of all PQ complaints. IEEE 1159 defines the taxonomy of PQ events; the ITIC / CBEMA curve defines the equipment ride-through envelope; IEEE 1564 defines SARFI metrics for site sag-frequency characterization.

IEEE 1159 — power-quality event categories

Voltage sag definition (IEEE 1159)

• RMS voltage 0.1 to 0.9 pu of nominal, duration 0.5 cycle to 1 minute.
• Below 0.1 pu → INTERRUPTION (not a sag).
• Above 0.9 pu → not classified as a PQ event.
• Characterized by two parameters: magnitude (remaining V in pu) and duration (cycles / ms).
• IEC term: voltage DIP (functionally identical to IEEE sag).

ITIC / CBEMA curve — equipment ride-through envelope

• Plots magnitude (% of nominal, vertical) vs duration (log scale, 1 μs — microseconds, where μ is the Greek letter mu — to 10+ s, horizontal).
• Successor of the CBEMA (Computer and Business Equipment Manufacturers Association) curve.
• Maintained by ITI (Information Technology Industry Council).
• Upper envelope (overvoltage tolerance): 200% at 1 µs → 140% at 3 ms → 120% steady.
• Lower envelope (undervoltage tolerance): 0% at 0.5 cycle → 70% at 30 cycles (500 ms) → 80% at 10 s → 90% indefinitely.
• Event INSIDE the envelope → expected to ride through. Event OUTSIDE → expected to trip.
• Tighter variants:
  • SEMI F47 — semiconductor fabs require ride-through ≤ 50% for 200 ms.
  • IEEE 1668 — adjustable-speed drive ride-through.
• Typical industrial site: 20-50 sags/year plot OUTSIDE the lower envelope.

Causes of voltage sags

• Remote transmission/distribution faults — 60-80% of events. LG fault on a transmission line up to 100 miles away depresses voltage at your facility for the fault-clearing duration:
  • Zone-1 distance protection clears in 1-3 cycles.
  • Zone-2 backup in ~30 cycles (500 ms).
  • Time-overcurrent backup in seconds.
• Motor starting — large induction motors at start draw 5-7× FLC, depressing voltage at the motor bus and nearby loads during acceleration (seconds).
• Transformer energization — inrush current 8-10× FLC for several cycles, especially severe for unloaded transformers.
• Capacitor-bank switching — oscillatory transient (not a pure sag), may cause voltage notches and harm sensitive electronics.
• Lightning — transients + may trigger faults that subsequently sag during clearing.

SARFI metrics (IEEE 1564) — site sag-frequency characterization

• SARFI-X = System Average RMS Frequency Index = count of voltage events with remaining V < X% of nominal over a defined period (typically one year).
• SARFI-90 counts all sags (broadest measure).
• SARFI-70 counts moderately severe events.
• SARFI-50 counts severe events likely to trip somewhat-tolerant equipment.
• Typical industrial site benchmark:
  • SARFI-90 ≈ 20-50 / year.
  • SARFI-70 ≈ 5-15 / year.
  • SARFI-50 ≈ 1-5 / year.
• Utility tariffs sometimes guarantee SARFI levels contractually with rebates for exceedance.
• Modern PQ monitors (Power Electronics, Schneider, Eaton, Schweitzer Engineering Laboratories) log every event and auto-compute monthly/annual SARFI indices.

Voltage swells, interruptions, transients

• Swells: 1.1-1.8 pu overvoltage, 0.5 cycle to 1 min. Common cause: LG fault on one phase elevates the unfaulted phases (especially on Yo-grounded systems). Can damage MOVs and surge arresters.
• Interruptions: < 0.1 pu, 0.5 cycle to 1 min. Caused by feeder lockout, breaker opening, brief loss of supply. Beyond 1 min → classified as outage (long-duration).
• Impulsive transients: µs duration; lightning, ESD, switching surges. Peak magnitudes 10-20 pu. Mitigated by surge arresters (MOV / gapped) and BIL coordination (see L17 insulation-coordination-bil).
• Oscillatory transients: µs-ms; capacitor-bank energization causing damped oscillation at the L-C resonance frequency. Can excite harmonic resonance (see L19 harmonics-ieee-519).

Flicker — IEEE 1453

• Voltage fluctuations at 0.5-25 Hz cause perceptible variation in incandescent lamp brightness — annoying to occupants.
• Pst (short-term flicker severity, 10-minute interval) — statistical perceptibility measure weighted by an eye-and-brain model. Pst = 1 is the threshold of irritation for a typical observer.
• Plt (long-term, 2-hour interval) — cube-root of mean cube of twelve consecutive Pst values.
• Typical utility limits at PCC: Pst ≤ 1, Plt ≤ 0.8.
• Worst flicker sources: arc furnaces (modulating kV at ~1-25 Hz — peak human-perceptibility band). Modern variable-speed drives can also contribute.
• Mitigation: SVC or STATCOM for reactive-power swing damping (see L18 facts-devices).

Mitigation hierarchy

Economics

• Single 30-cycle sag at a semiconductor fab: $100,000 - $1,000,000 in lost product / cycle time.
• DVR for one critical feeder: ~$200,000 one-time.
• Two avoided events per year typically pays back DVR investment.
• Continuous-process plants (chemical, paper, glass) target SARFI-50 < 5 per year through combination of upstream coordination + on-site mitigation.