Most Americans enjoy electric service, reliably delivered, whenever they need it. The effectiveness of the grid determines the reliability of electricity. This vast, interconnected infrastructure is highly complex and must operate under a variety of conditions.
Despite utilities’ best efforts to build the perfect grid, outages still occur. The important question to ask is this: When an outage occurs, how quickly and by what means will service be restored? The ability of the power grid to resume normal operations after a disruption is known as resiliency. In the Southeastern United States, the threat of hurricanes, ice storms and other severe storms cause the greatest disruptions for electricity customers. However, they are not the only perils. The T&D industry is filled with events that “almost occurred” but do not make headlines for anyone outside utility control rooms or the operational planning and engineering departments of electric utilities.
Microgrids can be a solution to reliability challenges. While the word “microgrid” conjures up nearly as much hype as “artificial intelligence,” actual microgrid deployments, to date, mainly have been conducted to achieve policy goals (for example, renewable integration research and development). These projects historically could not be justified economically on their own merits. However, that is starting to change. For Duke Energy Corp., microgrid deployments must be grounded in measurable benefits to customers — some of whom, because of their location, have not enjoyed the same high reliability most of the utility’s customers have come to expect.
Grid Resiliency
The term “grid resiliency” is commonly used but often not clearly defined. According to the Utility Storage Integration Program: Prioritization of Energy Storage Needs in the Southeastern U.S. — a report published in January 2018 by both the National Strategic Planning and Analysis Research Center and the Center for Advanced Vehicular Systems at Mississippi State University — here is one definition of grid resiliency: “For a power grid to be resilient, it should be robust, stable, adaptive, flexible, resourceful, agile, capable of coordination and foresight, redundant, diverse, collaborative and efficient.“
What about communities regularly at the mercy of the latest weather-caused outage? One such community is Hot Springs, North Carolina, U.S. Nestled in the beautiful mountains of western North Carolina, this town of around 600 Duke Energy customers is served by a single 10-mile (16-km) long, medium-voltage line. The line is vulnerable to “long-duration outage events due to its location in rugged mountain terrain,” said Jonathan Landy, a business development manager at Duke Energy.
The solution to this reliability challenge is a microgrid that can isolate, or island, the community when grid service is disrupted. Duke Energy began construction on the Hot Springs microgrid project in the summer of 2019. Part of the utility’s grid improvement initiative, this microgrid project was developed to improve the popular tourist town’s reliability metrics. The main source of electricity to this area is an overhead radial distribution feeder that traverses mountainous and heavily vegetated terrain.
These feeder traits not only increase the probability and frequency of faults but also make maintenance and repair operations challenging when faults do occur. Using state-of-the-art energy storage and proven inverter technologies, Duke Energy designed a microgrid solution for the town of Hot Springs in lieu of deploying a traditional wire solution.
Microgrid Design
The Hot Springs microgrid consists of alternating-current (AC) coupled solar-plus storage systems that can operate in both grid parallel and island modes. The 1.85-MW solar system uses 37 Sunny Tripower CORE1 string inverters, from SMA Solar Technology AG, connected to a 2.7-MW fixed tilt solar array. The 4.4-MW energy storage system comprises two SMA Sunny Central Storage central inverters connected to a 4.4-MWh Samsung Lithium-nickel-manganese-cobalt battery system.
When placed in service, the microgrid will isolate Hot Springs’ load from the upstream radial distribution feeder when faults occur. This will create an electrical island that maintains both the town’s load and the solar-plus storage microgrid. As a result, Duke Energy maintenance crews will be able to identify and clear faults while the microgrid maintains electrical power to the town.
In grid parallel mode, the microgrid will be controlled by Duke Energy to provide a range of ancillary services. At any point in time, the service selected will vary according to ever-changing bulk system and local feeder conditions. As such, the utility is positioning its energy storage systems to operate as fully flexible ancillary service devices to maximize their value to customers.
With islanding mode, Hot Springs will have backup power from a flexible and agile microgrid. Flexibility and agility are two other attributes of grid resiliency, as stated in the aforementioned definition. The use of renewable energy adds a third element of resiliency: resourcefulness.
When the Hot Springs microgrid is placed in service in the third quarter of 2020, it will be economically and socially important to a community fed by a single distribution feeder that is prone to being cut off from the grid. It may even help to defer new generation in the Asheville, North Carolina, region.
According to Zachary Kuznar, Duke Energy’s director of combined heat and power, microgrid and energy storage development, “We’re looking for these sorts of use cases where storage can be a more cost-effective solution or, in the case of the Hot Springs microgrid, the only real solution.”
Scaling this resiliency improvement to the larger statewide energy system is not as easily justified on economic grounds. The electricity supply in general already is highly reliable and, in many applications, the cost of deploying this technology would not be justified based on the incremental improvement to an already highly reliable section of the grid. As such, investments must be strategic and based on advanced data to identify the best locations to use microgrids.
Resiliency Evaluation
A new project funded by the State Energy Program of the U.S. Department of Energy and led by the North Carolina Department of Environmental Quality aims to help address the challenge of economically justifying investments in technology, such as microgrids. Under the leadership of North Carolina State Energy Director Sushma Masemore, the project team includes the Energy Production and Infrastructure Center (EPIC) at UNC Charlotte and the North Carolina Clean Energy Technology Center at North Carolina State University, with technical support from Duke Energy.
The project will define and test economic valuation methods to help utilities, the state and stakeholders better plan for and manage growing investments in technologies, including utility-scale and customer-sited photovoltaics, energy storage, microgrids and mini grids, and grid-management enabling technologies — within the state’s integrated resource plan as well as other relevant planning processes. The goal is to create model outputs to inform the development of new metrics (for example, economic losses experienced by customers from outages as a result hurricanes) to help the state and stakeholders better evaluate proposed utility investments within the context of a collective vision for the future grid of North Carolina and the value it needs to provide.
This work will provide new tools to evaluate grid resiliency on the value it brings to citizens who bear the brunt of severe weather events and the devastating economic losses that occur as a result of prolonged power outages. With this work, the hope is long-duration outages become increasingly a thing of the past.
For more information:
SMA | www.sma-america.com
Samsung | www.samsung.com
