SAIDI (System Average Interruption Duration Index) and SAIFI (System Average Interruption Frequency Index) are widely used reliability indices that measure the performance of power distribution systems.
SAIDI represents the total duration of interruptions for an average customer over a given time period, typically a year. It is calculated by taking the sum of all customer interruption durations and dividing it by the total number of customers served. The unit of measure for SAIDI is minutes of interruption per customer. A lower SAIDI value indicates better reliability, as it means customers experienced shorter total outage durations on average.
SAIFI, meanwhile, denotes the average number of sustained interruptions experienced by a customer over a given time period, also typically a year. It is calculated by taking the total number of customer interruptions and dividing by the total number of customers served. The unit of measure for SAIFI is power interruptions per customer. A lower SAIFI value indicates better reliability, as it means customers experienced fewer interruptions on average.
These indices allow utilities to quantify and benchmark the reliability of their distribution networks. Regulatory authorities sometimes use SAIDI and SAIFI targets to incentivize utilities to maintain and improve reliability. The indices help identify weak areas in the network that could benefit from upgrades or maintenance to reduce the frequency and duration of outages.
The U.S. Energy Information Administration (EIA) reports SAIDI and SAIFI values in its Electric Power Annual report, which is regularly released in October each year. In the most recent report, the U.S. distribution system’s average SAIDI value including all events was 335.5 minutes per customer in 2022. If major event days were excluded, which is often a worthwhile exercise to get accurate long-term trends because hurricanes and severe winter storms, for example, can skew the numbers quite dramatically in a given year, the figure dropped to 125.7 minutes per customer. Notably, this the highest SAIDI value tallied in the past decade and it continued what has effectively been a steady year-over-year decline in performance from 2013 through 2022. (2017 saw a brief improvement over 2016, but every year before and since has been worse than the previous year during the timespan covered by the report.) For comparison, in 2013, the SAIDI value was 106.1 minutes per customer.
SAIFI values do not vary as noticeably as SAIDI, but still have been worsening. In 2022, the U.S. distribution system’s average SAIFI value including all events was 1.4 power interruptions per customer. With major events excluded, SAIFI was 1.1 interruptions per customer in the U.S. While this was not substantially worse than values reported in other years over the past decade (every year from 2013 onward has been 1.0, except for 2016 when the value was also 1.1), it seems to confirm that the system hasn’t been improving.
Yet, Mike Edmonds, Chief Commercial Officer for S&C Electric Company, said several things can be done to improve the reliability and resiliency of the power distribution system. “The grid looks different depending on what state you’re in,” Edmonds said as a guest on The POWER Podcast. “We’ve got great experience with Florida Power & Light [FPL],” he said. “We’ve helped them create a resilient grid. So, that’s not only a grid that is reliable, but a grid that can actually weather the storms and all the challenges thrown at the grid.”
Notably, FPL reported in March that it had provided “the most reliable electric service in company history in 2023.” Over the past two decades, FPL said its customers have realized a remarkable 45% improvement in reliability (Figure 1). In NextEra Energy’s (the parent company of FPL) Sustainability Report 2023, the company reported FPL’s SAIDI was 47.1 and SAIFI was 0.85, confirming markedly better results than the U.S. averages noted earlier. Furthermore, FPL said this is the ninth time in the past 10 years that it achieved “its best-ever reliability rating.”
1. Florida Power & Light Company (FPL) said it provided “the most reliable electric service in company history in 2023.” Courtesy: FPL
“FPL is committed to delivering exceptionally reliable service to our customers while keeping bills as low as possible,” FPL President and CEO Armando Pimentel said in a statement issued on March 1. “Of course, we live in a state with increasingly severe weather and a year-round growing cycle, so reliability is both top of mind and always a challenge. And, that’s what makes our reliability story even more remarkable—we’re striking the balance between performance and cost. Our smart investments and ongoing efforts to strengthen the grid are making a real difference for our customers.”
“Quite often, we hear the industry talk almost exclusively around transmission—and it’s extremely important—but you can have a great transmission grid, but you’re still not going to have a resilient grid unless you’re actually taking the grid all the way to the point of consumption,” said Edmonds. “And really, that’s what Florida Power & Light has done a really excellent job of over the last 10 years.”
Among the things FPL has done to harden its energy grid are:
Placing nearly 2,000 neighborhood power lines underground since the inception of its Storm Secure Underground Program in 2018.
Installing more than 215,000 intelligent devices along the grid. This smart grid technology detects and prevents power outages and minimizes restoration times when outages do occur. The technology has helped avoid more than 13 million outages since 2011.
Inspecting power poles and replacing those that no longer meet industry-leading standards.
To better understand some of the innovative new equipment S&C Electric Company offers, Edmonds provided an example. “We have some technology that does something called ‘pulse finding,’ and what Florida Power & Light does, it just lets our equipment do what it does best. If there’s a problem, it’ll pulse to see if the problem is there or not on the grid, if it’s not, it reenergizes,” he said. “This technology is available to really change how the grid operates.”
Edmonds said S&C Electric Company invented the fuse 115 years ago, and he noted fuses have served the industry well since that time. However, today there is better technology available that doesn’t require a lineworker to respond to an outage to replace a fuse. “Let’s take fuses off the grid and have a fuseless grid, and have much more intelligent devices that can actually re-energize,” Edmonds decreed.
To hear the full interview with Edmonds, which contains more about technology available to improve power grid performance, listen to The POWER Podcast. Click on the SoundCloud player below to listen in your browser now or use the following links to reach the show page on your favorite podcast platform:
For more power podcasts, visit The POWER Podcast archives.
—Aaron Larson is POWER’s executive editor (@AaronL_Power, @POWERmagazine).
