Electric reliability indexes are critical metrics for assessing power system performance and customer satisfaction. These indexes quantify the frequency and duration of power interruptions experienced by consumers.
This article explores the calculation of SAIDI, SAIFI, and CAIDI based on IEEE 1366 standards. It provides formulas, tables, and real-world examples for precise reliability analysis.
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- Calculate SAIDI with 5000 customer-minutes of interruption and 1000 customers served.
- Determine SAIFI given 200 total interruptions and 5000 customers served.
- Find CAIDI using SAIDI of 120 minutes and SAIFI of 2 interruptions per customer.
- Compute all three indexes for 3000 customer-minutes, 150 interruptions, and 2500 customers.
Comprehensive Tables of Electric Reliability Indexes (SAIDI, SAIFI, CAIDI) Values
| Utility | SAIDI (minutes/year) | SAIFI (interruptions/year) | CAIDI (minutes/interruption) | Region | Year |
|---|---|---|---|---|---|
| Pacific Gas & Electric (PG&E) | 120 | 1.5 | 80 | California, USA | 2022 |
| Consolidated Edison (ConEd) | 90 | 1.2 | 75 | New York, USA | 2022 |
| National Grid | 110 | 1.4 | 78.5 | UK | 2021 |
| Eskom | 300 | 3.5 | 85.7 | South Africa | 2022 |
| Tokyo Electric Power Company (TEPCO) | 50 | 0.8 | 62.5 | Japan | 2021 |
| Hydro-Québec | 70 | 1.0 | 70 | Canada | 2022 |
| Index | Definition | Units | Typical Range | Interpretation |
|---|---|---|---|---|
| SAIDI | System Average Interruption Duration Index | Minutes per customer per year | 50 – 300 minutes/year | Average outage duration experienced by customers |
| SAIFI | System Average Interruption Frequency Index | Interruptions per customer per year | 0.5 – 3 interruptions/year | Average number of interruptions per customer |
| CAIDI | Customer Average Interruption Duration Index | Minutes per interruption | 50 – 120 minutes/interruption | Average outage duration per interruption |
Fundamental Formulas for Electric Reliability Indexes According to IEEE 1366
Understanding the precise calculation of SAIDI, SAIFI, and CAIDI is essential for power system reliability assessment. Below are the core formulas with detailed explanations of each variable.
| Index | Formula | Description |
|---|---|---|
| SAIDI | SAIDI = ∑(Ui × Ni) / Ntotal |
Average outage duration per customer per year.
|
| SAIFI | SAIFI = ∑Ni / Ntotal |
Average number of interruptions per customer per year.
|
| CAIDI | CAIDI = SAIDI / SAIFI |
Average outage duration per interruption.
|
Additional Notes on Variables and Units
- Ui (Outage Duration): Typically measured in minutes, representing the length of each outage event.
- Ni (Customers Affected): The count of customers impacted by each outage event.
- Ntotal (Total Customers): The total number of customers served by the utility or system under analysis.
These indexes are calculated annually to provide a comprehensive view of system reliability. Utilities often use these metrics to benchmark performance and identify areas for improvement.
Real-World Application Cases of Electric Reliability Indexes Calculation
Case Study 1: Urban Utility Network Reliability Assessment
A metropolitan utility serving 10,000 customers experienced three major outages during the year. The details are as follows:
| Outage Event | Duration (minutes) | Customers Affected |
|---|---|---|
| 1 | 60 | 2000 |
| 2 | 120 | 1500 |
| 3 | 30 | 1000 |
Step 1: Calculate SAIDI
SAIDI = (60×2000 + 120×1500 + 30×1000) / 10,000
SAIDI = (120,000 + 180,000 + 30,000) / 10,000 = 330,000 / 10,000 = 33 minutes/customer/year
Step 2: Calculate SAIFI
SAIFI = (2000 + 1500 + 1000) / 10,000 = 4500 / 10,000 = 0.45 interruptions/customer/year
Step 3: Calculate CAIDI
CAIDI = SAIDI / SAIFI = 33 / 0.45 ≈ 73.33 minutes/interruption
Interpretation: Customers experience on average 0.45 interruptions per year, each lasting approximately 73 minutes.
Case Study 2: Rural Distribution System Reliability Analysis
A rural utility with 5,000 customers recorded the following outage data over one year:
| Outage Event | Duration (minutes) | Customers Affected |
|---|---|---|
| 1 | 180 | 500 |
| 2 | 240 | 300 |
| 3 | 60 | 700 |
Step 1: Calculate SAIDI
SAIDI = (180×500 + 240×300 + 60×700) / 5,000
SAIDI = (90,000 + 72,000 + 42,000) / 5,000 = 204,000 / 5,000 = 40.8 minutes/customer/year
Step 2: Calculate SAIFI
SAIFI = (500 + 300 + 700) / 5,000 = 1,500 / 5,000 = 0.3 interruptions/customer/year
Step 3: Calculate CAIDI
CAIDI = SAIDI / SAIFI = 40.8 / 0.3 = 136 minutes/interruption
Interpretation: Rural customers experience fewer interruptions but longer outage durations per event.
Expanded Technical Insights on IEEE 1366 Reliability Indexes
IEEE Standard 1366-2012 provides a comprehensive framework for calculating and reporting electric power reliability indexes. It standardizes definitions, data collection methods, and calculation procedures to ensure consistency across utilities.
- Data Collection: Accurate outage data collection is critical. Utilities use SCADA systems, outage management systems (OMS), and customer reports to gather interruption duration and affected customer counts.
- Exclusion of Major Events: IEEE 1366 recommends excluding Major Event Days (MEDs) from normal reliability calculations to avoid skewing results. MEDs are identified statistically using the Beta method.
- Normalization: Indexes are normalized per customer to allow comparison across utilities of different sizes.
- Additional Indexes: IEEE 1366 also defines other indexes such as CAIFI (Customer Average Interruption Frequency Index) and ASAI (Average Service Availability Index) for more granular analysis.
Utilities leverage these indexes to:
- Benchmark performance against peers and regulatory standards.
- Identify weak points in the distribution network.
- Prioritize investments in infrastructure upgrades.
- Communicate reliability performance transparently to stakeholders.
Practical Considerations for Implementing an Electric Reliability Index Calculator
Developing a reliable and user-friendly calculator for SAIDI, SAIFI, and CAIDI requires attention to several technical aspects:
- Input Validation: Ensure inputs such as outage durations and customer counts are positive and within realistic ranges.
- Handling Large Datasets: Utilities may have thousands of outage events; efficient data processing algorithms are necessary.
- Incorporating MED Identification: Implement statistical methods to flag and optionally exclude major event days.
- Reporting and Visualization: Provide clear output tables, charts, and downloadable reports for analysis and regulatory compliance.
- Integration: Allow integration with existing outage management and customer information systems for automated data import.
Authoritative Resources and Further Reading
- IEEE Standard 1366-2012 – IEEE Guide for Electric Power Distribution Reliability Indices
- National Research Council – Electric Power Distribution Reliability
- U.S. Department of Energy – Grid Modernization and Smart Grid
- Utility Analytics Institute – SAIDI, SAIFI, CAIDI Explained
By mastering the calculation and interpretation of SAIDI, SAIFI, and CAIDI, power utilities can significantly enhance their reliability management strategies. The IEEE 1366 standard remains the authoritative guideline for these critical metrics.