Understanding and Managing Grid Reliability and Resiliency

By Jonathan Susser | July 25, 2018

People often fail to appreciate electricity until they lose it. But having the lights go out even for a short period of time can have enormous consequences, and the number of reported outages throughout the U.S. has been rising. In the Carolinas, most grid outages and disturbances – about two-thirds – are driven by weather. However, other causes exist as well, including animals contacting wires, auto accidents and human error. Because outages have the potential to affect thousands or even millions of people, electric utilities are constantly surveying and preparing for potential threats to their ability to deliver safe, reliable and affordable service.


Utilities often identify two main characteristics when describing the grid’s performance: reliability and resiliency. The reliability of the grid is the percent of time it is available and functional. High reliability means that when you flip a light switch, the lights turn on, and ensuring reliability requires that utilities are able to respond to common outage-causing events, such as a tree limb damaging power lines. Resiliency, on the other hand, is the grid’s ability to withstand, recover from and restore power after major events, such as hurricanes, which are less frequent but critical.

A number of metrics exist to help evaluate grid performance. Popular ones include the System Average Interruption Duration Index (SAIDI), the System Average Interruption Frequency Index (SAIFI) and Major Event Day (MED).

  • SAIDI reflects the average outage duration for each customer served over a period of time, typically a year.
  • SAIFI reflects the average number of outages a customer experiences in that period. (SAIDI and SAIFI may not capture all disturbances because briefer ones are typically addressed by other metrics.)
  • An MED occurs when SAIDI exceeds a specific threshold for a given day. It reflects outages on the longer end of the spectrum.

Certain MED events are categorized as high impact, low frequency (HILF) events. These are rare occurrences that have the potential to cause catastrophic impacts on the grid, such as cyber and physical attacks; natural disasters like earthquakes, tornadoes, tsunamis and hurricanes; and geomagnetic disturbances. Because these events are infrequent – or may have never occurred – they can be difficult to prepare for, but their potential for damage necessitates attention.

North Carolina Trends and Outcomes

For 2016, the most recent year available, North Carolina had a SAIDI of about two hours when excluding MEDs. However, it jumped to over eight hours with MEDs, an increase driven largely by Hurricane Matthew, which hit the state that fall. This SAIDI value means that North Carolina customers experienced outages averaging eight hours over the course of 2016. (Because this is an average, however, it is worth noting that many across the state did not experience disruption from Hurricane Matthew, while others, particularly those in the coastal regions, went as long as six days without power due to significant system damage and extreme flooding.)

With SAIFI, North Carolina had an index of about 1.25 without MEDs and 1.75 with MEDs (a SAIFI value between 1 and 2 is typical across the U.S.). In other words, North Carolinians, on average, experienced less than two outages throughout the year (again, this does not take into account briefer interruptions).

Outage Impacts

Outages can have a significant impact on businesses and the local economy. They can lead to decreased output or production, lost income if employees cannot commute to work, spoiled inventory that can impact sales, and just plain inconvenience. According to one estimate, Superstorm Sandy, which in 2012 struck primarily along the eastern seaboard and devastated parts of New York and New Jersey, caused $20 billion in losses from suspended business activity. The subsequent two-day shutdown of the New York Stock Exchange resulted in an estimated impact of $7 billion. Even briefer outages can be damaging, however, particularly for larger facilities.

Estimated Interruption Cost per Event by Duration and Customer Class. Source: Lawrence Berkeley National Laboratory, “Updated Value of Service Reliability Estimates for Electric Utility Customers in the United States.

As power returns from an outage, utilities work with state and local governments to prioritize the customers in most critical need of electricity. These customers typically include hospitals, water and wastewater treatment plants, gas utilities and pipelines, transportation facilities, telecommunications, data centers, the financial industry, and continuous process manufacturing.

Outage Responses

Utilities have a number of tools at their disposal to help prepare for and respond to outages. A broader category of measures is known as hardening. Hardening involves changing the physical infrastructure of the grid to make it more durable and less susceptible to damage, most often from weather-related incidents, such as wind, flooding and flying debris. For example, utilities can upgrade poles or transmission lines, use submersible equipment, elevate substations, reinforce floodwalls, or underground transmission and distribution lines. The costs of these activities vary widely, so utilities prioritize upgrades based on existing infrastructure characteristics, vulnerability factors and customer restoration needs.

Another category of resilience upgrades is self-healing and self-optimizing networks. These networks improve grid flexibility to more efficiently restore power to customers. Self-healing networks use smart sensors to detect the location of a problem and automated switches to isolate the damaged cable or equipment. The technology then identifies the best way to reroute power to restore it to as many customers as possible. Self-optimizing networks are more advanced, using additional switches to further limit the number of customers affected. For more information on these networks, view Advanced Energy’s webinar on self-optimizing grid technologies.

Microgrids are independent electric systems consisting of interconnected loads, storage and distributed energy generation that can power a defined area. Certain microgrids even have the ability to “island,” or disconnect from the main utility grid. These systems will continue to play a large role in the grid of the future, particularly to support resiliency efforts. More information can be found on the NC Smart Grid website.

Upgrading and integrating the tools used to navigate distribution management, outage management, and supervisory control and data acquisition, or SCADA, is another approach utilities take. These systems capture and analyze data from thousands of devices on the distribution system. Integrating them helps improve self-optimizing networks, provides the analytical capabilities to optimize grid performance, and enhances cybersecurity, which has become increasingly important with the additional automation and interconnection in the utility industry.

Utilities regularly pursue other activities that support resiliency as well. These include conducting preparedness planning and training, having portable generators, procuring spare equipment and having fuel for emergency vehicles.

The CR-90 is one tool that can help utilities weigh the potential costs of outages with the costs of investments in resiliency. It represents the time in hours it takes to restore power to 90 percent of customers. Steps to decrease outage time can have significant costs; therefore, each utility must balance grid enhancements with customer cost impacts to find an optimal point of investment.

Government Policies

Along with the work being done by utilities, federal, state and local governments examine policies and action planning around resilience. In January 2018, the Federal Energy Regulatory Council (FERC) asked independent system operators and regional transmission organizations, which help coordinate, control and monitor the electric system over a few states, to answer 18 questions about resiliency in their territories. The questions touched on the primary risks to resilience, the organizations’ ability to withstand an HILF event, and how challenges to resiliency affect different generation technologies. This assessment is still underway.

Every two years, the North American Electric Reliability Corporation (NERC) conducts GridEx, an exercise that simulates a cyber or physical attack on the power grid and other critical infrastructure. GridEx was last held in November 2017 and featured 5,000 representatives from electric utilities, government agencies, first responders, intelligence agencies, and supply chain stakeholders.

At the state level, regulators, agencies and industry experts partner to evaluate needs, determine vulnerabilities and develop resiliency plans. In North Carolina, the Energy Policy Council, an independent body supported by staff from the North Carolina Department of Environmental Quality, develops contingency and emergency strategies to respond to potential energy shortages. Likewise, the Department of Public Safety’s Emergency Management division works to enhance North Carolina’s resiliency by helping to manage possible disasters.

Duke Energy’s Power/Forward Carolinas Plan

For Duke Energy, grid performance and resiliency efforts are guided by its Power/Forward Carolinas plan. This plan seeks to improve day-to-day grid reliability and storm restoration, strengthen the grid against physical and cyberattacks, and enable renewable energy integration. Ultimately, Power/Forward Carolinas works to enhance overall customer service, convenience and control.

As described above, a number of approaches exist to strengthen grid resiliency and reliability, and Duke Energy is constantly evaluating the most appropriate way to support its grid and infrastructure. Prior program success, advanced data analytics, and modern grid technologies all help Duke Energy better target investments so that they are most effective and reaching territories in need.

Self-optimizing grids in particular play a big role for Duke Energy in its Power/Forward Carolinas campaign. Three primary components are needed to maximize performance of these systems: 1) multiple points of interconnectivity on the system to allow connection to alternate sources of power, 2) enough capacity on circuits to pick up new load, and 3) intelligent automated controls to optimize the grid’s performance.

One benefit of this technology is that it allows Duke Energy to think about the grid in terms of system segments rather than whole circuits. Whereas in the past, a circuit would typically serve approximately 2,000 customers, the increased connections and controls of self-optimizing grids produce segments serving roughly 400 customers. This means that fewer customers lose power in an outage, and recovery is quicker.

Targeted undergrounding is another approach Duke Energy is pursuing on other areas of the system, as those that are cost-effective to underground are generally not candidates for self-optimizing networks. While self-optimizing networks are typically applied to the larger “backbone” portions of the system that run along major roadways, for targeted undergrounding, Duke Energy often focuses on heavily treed locations in neighborhoods that experience the most interruptions and biggest consequences.

Other steps to support resiliency include investments in additional hardening measures, such as pole strengthening, animal mitigation and uplifting obsolete equipment. Improving physical and cybersecurity vulnerabilities and utilizing enhanced system intelligence for better awareness, preparedness and self-optimizing performance are also crucial steps.

Putting these efforts into perspective, Duke Energy estimates that certain Power/Forward Carolinas measures, such as targeted undergrounding, could have prevented 300 million outage minutes (more than a 30 percent reduction) in North Carolina during Hurricane Matthew in 2016. This would have improved restoration efforts, lowered restoration costs, and reduced the negative impact of the outage on the economy.

Outages can be catastrophic, and with increasing threats to electricity supply, utilities continue to make outage prevention a top priority. Fortunately, new and more sophisticated tools help combat potential issues, and utilities aren’t working alone. Preventing and navigating losses of power takes collaboration between utilities, local, state and federal agencies, and more to ensure that everyone has safe, reliable and affordable electricity when they need it. To learn even more about this topic, be sure to view our recent grid resiliency webinar.