Wednesday, March 31, 2021

The Network Gateway: The Missing Link for Integrating Distributed Energy Resources?

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With growing deployment of grid-connected rooftop solar photovoltaic (PV) systems, energy storage, and electric vehicles, electric utilities need to anticipate and find solutions for potential unintended side effects that may occur as a result of these distributed energy resources (DER). An example: On a sunny spring day with low customer load, a utility’s distribution grid operator notices that generation from a number of solar systems is likely to exceed load significantly, potentially causing an overload on the grid. The grid operator sends a command to an aggregation of DER to limit its output to the grid, but the command does not reach the DER due to a communications network failure, resulting in local overvoltage conditions and potentially customer outages.

This hypothetical scenario points to the need for DER to have default modes that a utility can pre-set in anticipation of network failures. In this example, if the utility had set a default mode limiting DER output during contingencies, the DER aggregation would have operated as needed even during a network failure, averting potential reliability problems.

The default mode (also known as the fail-safe mode) is one of more than a dozen new DER capabilities that utilities expect will be needed in order to manage DER effectively and reliably. This raises the question, where would these capabilities be housed and how should they be defined?

EPRI researchers and other DER integration, communications, and cybersecurity experts see a network gateway as a promising solution. A gateway is a device that connects communication networks that use different languages, translating those languages so that devices on one network can talk with devices on the other network. At a system that combines solar and energy storage, a gateway would connect the system’s local network (which may include communication among inverters, solar PV, and energy storage) with the network of the utility, vendor, or aggregator that is managing the system and other DER in the region.

“Tomorrow’s grid will be powered to a significant extent by different models and types of DER that are managed by diverse entities, including utilities and aggregators,” says Ajit Renjit, an EPRI expert on DER control systems. “Integrating all these resources over large geographical areas needs to be done in a secure, cost-effective, durable way. The gateway offers a promising solution because it’s located at the DER site, it is a low-cost device, and it doesn’t require any additional hardware to accommodate numerous DER functions and capabilities. For the gateway to work, it will need to be interoperable with DER and utility or aggregator management systems.”

A Need for Flexibility in DER Features and Functions

As DER have become more widespread, utilities and regulators have sought new tools to integrate them with grid planning and operations. An important initial step in 2018 was the publication of a revised IEEE 1547 standard, which requires that smart inverters include a communications port and various grid-support functions, such as the ability to provide voltage and frequency support. However, the standard does not specify many critical DER features and functions that are necessary for utilities and aggregators to monitor and control DER in a comprehensive way.

“The industry experts who developed the IEEE 1547 standard intentionally made the DER requirements simple,” says Renjit. “It omitted certain DER features and functions that are expected to vary by utility and region. The gateway approach provides a practical implementation path to house such features and functions.”

An EPRI-convened utility working group is identifying potential gateway applications and defining functions. Other discussion topics include determining the gateway’s physical location at a DER site and recommending gateway function refinements and commercialization strategies. The group currently has 13 participating utilities, meets every two weeks, and is open to new members.

“The utilities on the working group are already managing DER in their service territories, so they have experience in determining what capabilities are needed in the gateway,” says Renjit. “They have proposed more than 20 gateway applications so far, and EPRI is presently researching how these applications could be incorporated in a gateway.”

“Innovation and advanced grid management are key to achieving North Carolina’s electric cooperatives’ long-term vision of building a brighter future for members and local communities,” says Lee Ragsdale, senior vice president for energy delivery at North Carolina’s Electric Cooperatives. “This collaborative study identifies future applications for gateway systems and new ways to connect our centralized DERMS to remote energy resources, allowing us to more efficiently and effectively manage these resources and deliver additional value to our members.”

Fail-Safe Mode, Cybersecurity, and Universal Clock

The fail-safe mode is one of the 20 applications under investigation. IEEE 1547 did not specify a fail-safe mode for DER because settings will vary among utilities and other DER management entities. Additionally, utilities will need flexibility to configure different fail-safe triggers because of varying performance of different utility communication systems. For instance, a 10-second loss of communications may indicate a network failure for some utility systems but not for others.

A second potential gateway application is cybersecurity. IEEE 1547 does not require DER to support cybersecurity measures. Without comprehensive cybersecurity requirements and guidelines, each DER manufacturer may implement a unique or limited set of cybersecurity features in its products. This can cause DER to become less interoperable with utility control systems.

“Utilities need to securely connect their control systems with the disparate systems that make up DER sites,” says Xavier Francia, an EPRI expert on DER cybersecurity. “It would be complex and unwieldy to try to achieve this by configuring their control systems to accommodate DER sites with components made by different manufacturers and equipped with different cybersecurity features. A gateway is a more practical approach to protecting utility control systems from threats introduced through a site’s various access points, enabling more robust security.”

A third critical function envisioned for the gateway is time synchronization (also known as a universal clock). Scheduling plays an important role in how utilities manage and control DER. For DER to execute utility commands when needed, the internal clocks of DER and utility systems must be synchronized. Consider this scenario: a utility’s grid operator determines that an aggregation of DER is causing local reliability problems. The utility’s DER management system decides that these DER need to limit their output in three hours to mitigate the situation. Because significant grid events can strain communication networks, the utility system sends the commands to the DER in advance of when the output is needed. Equipped with a universal clock, the DER gateways receive the commands and execute them on time.

The gateway must handle various communication protocols with equal facility so that it can respond effectively to signals from utility, vendor, and aggregator systems. “Communication technologies and interoperability are essential for cost-effective, secure monitoring and management of gateways,” says Rish Ghatikar, an EPRI expert on information and communication technologies for DER. “Gateways can support grid reliability as long as there are robust requirements for communication protocols. This research can inform standards organizations such as IEEE in their efforts to create such requirements.”

In 2021, EPRI researchers plan to develop a gateway prototype that will be used to test the proposed applications for technical feasibility. To gauge economic viability, they will estimate manufacturing and deployment costs associated with each application. “This research will help us develop guidelines that can enable utilities to define their own gateway requirements,” said Renjit.

“It is clear that smart inverters provide value to both DER owners and grid operators,” says David Lovelady, a grid modernization solutions engineer at National Grid. “But as utilities implement more advanced smart inverter functions requiring bi-directional communications, a loss of communications can challenge their ability to manage the grid safely and reliably. The gateway being developed by EPRI and several utilities can potentially mitigate this concern through a new function that would provide local, autonomous control of a smart inverter in the event of a communications failure.”

Key EPRI Technical Experts:

Ajit Renjit, Xavier Francia, Rish Ghatikar
For more information, contact techexpert@eprijournal.com.


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