Reliability indices — SAIDI / SAIFI / CAIDI / MAIFI / LOLE / EUE

Reference notes

Power-system reliability is measured at two levels: distribution (customer-side outage experience per IEEE Std 1366) and generation (capacity adequacy per NERC LOLE / EUE / PRM). Distribution metrics drive utility capital investment, performance-based regulation, and customer experience. Generation metrics drive resource adequacy planning, capacity markets, and renewable integration.

IEEE 1366 — Distribution Reliability Indices

Sustained vs Momentary

• IEEE 1366 defines sustained ≥ 5 minutes; shorter events are momentary.
• SAIFI / SAIDI / CAIDI count only SUSTAINED.
• MAIFI tracks MOMENTARY events separately. Most momentaries are reclosing operations after temporary faults (lightning, vegetation flashover, wildlife) — clear themselves within seconds.
• Momentaries matter for sensitive industrial loads (semiconductor fabs, paper mills, data centers) — trips processes even though supply returns.

Major Event Day (MED) Exclusion

• β-method per IEEE 1366: identifies storm days as outliers in log-space distribution of daily SAIDI.
• Compute ln(daily SAIDI) for each day in 5-year reporting period; mean μ + standard deviation σ.
• Days with ln(daily SAIDI) > μ + 2.5·σ flagged as MEDs.
• Headline SAIDI / SAIFI reported MED-EXCLUDED; MED-only metrics reported separately.
• Critical for fair comparison year-over-year and utility-to-utility — without exclusion, one hurricane dominates annual stats.
• Climate change increasing MED frequency; recent push to report BOTH MED-included and MED-excluded.

Outage causes — typical US distribution

• PSPS (Public Safety Power Shutoffs) for wildfire prevention adds 5-15% to total customer-minutes in California / Oregon / Washington in extreme years.

Generation Adequacy — NERC framework

• LOLE — Loss of Load Expectation: expected number of days per year when generation cannot meet peak load. Industry standard target: ≤ 0.1 days/year = "1 day in 10 years" reserve criterion. Used by PJM, MISO, CAISO, ERCOT, NYISO, ISO-NE.
• LOLP — Loss of Load Probability: same as LOLE expressed as probability per period. LOLP = 0.000274 per day = 0.1 days/year.
• EUE — Expected Unserved Energy: expected MWh/year of unserved load. Captures depth + frequency. Typical target < 50 MWh/year for a 50,000 MW system.
• PRM — Planning Reserve Margin: capacity above peak demand needed to achieve target LOLE. Typical 15-20%. RTOs publish PRM annually as part of capacity-market clearing.

Capacity accreditation

• ELCC — Effective Load-Carrying Capability: modern accreditation method accounting for resource correlation with load and other resources.
• Adopted by CAISO and PJM (2022-2024 transitions) as IBR penetration grew.
• FERC Order 2222 (2020) requires RTOs to allow aggregated DER market participation with appropriate capacity accreditation.

Benchmarking and performance-based regulation

• EEI — Edison Electric Institute: largest US reliability benchmarking program. Member utilities report SAIDI, SAIFI, CAIDI, MAIFI annually using consistent IEEE 1366 definitions; EEI publishes percentile rankings.
• Typical US distribution-utility benchmarks (EEI 2023): SAIDI median 120 min/customer/year; first-quartile < 80 min; fourth-quartile > 200 min. SAIFI median 1.3.
• Best-in-class underground urban: SAIDI < 30 min, SAIFI < 0.4.
• International: European utilities 30-90 min SAIDI (higher % underground + reliability investment). Japan: < 20 min, world-leading.
• State PUCs — PBR (Performance-Based Regulation): tie utility financial outcomes to measured reliability.
  • California CPUC: IOU-specific targets (PG&E, SCE, SDG&E).
  • New York PSC: REV proceeding.
  • UK Ofgem: RIIO program.
  • Australia AER: similar incentive scheme.
• Recent push to disaggregate metrics by area (urban/rural) and demographic factor (income, race) for equity reporting.

Smart-grid investment for reliability

• Distribution automation (DA) — remote-controlled switches, sectionalizers.
• FLISR — Fault Location Isolation and Service Restoration — automatic isolation of faulted section and restoration of healthy segments. Reduces SAIDI 20-50% in deployed areas.
• AMI — Advanced Metering Infrastructure — smart meters provide outage detection and verification.
• SCADA + Outage Management System (OMS) — coordinated dispatching of restoration crews.
• Microgrids — local backup for critical loads (hospitals, military bases, college campuses, community centers).
• DERMS — Distributed Energy Resource Management Systems — coordinate distributed solar, batteries, EVs, demand response.

Modern challenges

• Extreme weather — climate change increasing frequency and intensity of weather events:
  • 2021 Texas February ice storm — ERCOT load-shed 20 GW for several days.
  • 2018-19 California wildfire-driven PSPS by PG&E.
  • 2017 Hurricane Maria Puerto Rico — 11-month full restoration.
• PSPS — Public Safety Power Shutoffs: proactive shutoffs to prevent wildfire ignition during high-wind/dry conditions. Customer-minute impact enormous; CPUC tightening rules to minimize duration.
• IBR (Inverter-Based Resource) penetration: changes dynamic-stability landscape; large IBR-trip events affect downstream reliability. NERC standards (TPL-001, PRC-024, BAL-001) being updated.
• EV charging: changes load profile and peak demand patterns.
• Resilience metrics: beyond reliability — ability to recover quickly from disruptions. Customer-minutes lost during high-impact events, restoration time, percent-back-online curves. IEEE 1366 update (2024-25 cycle) addressing resilience + equity + EV/DER impacts.