IEEE Electrification - December 2020 - 71

made the bulk power system unavailable. While isolated
microgrids have demonstrated the ability to support their
own critical end-use loads, they have not been able to
operate collaboratively to realize their full potential. While
there have been examples of limited microgrid networking, such as " nested microgrids " or " directly adjacent
microgrids, " fully networked microgrid operations must
support collaboration across a wider area. The networking
of individual microgrids is a realization of the fractal power-system operations, presented by Miller et al. in their
white paper, " Achieving a Resilient and Agile Grid. "
Miller et al. present a fractal operating concept in
which electric power systems can segment into smaller
units when there are major distributions to the bulk system. The separation can be a precautionary action for a
critical end-use load or a response to a severe system disturbance. One way to implement the concepts presented
by Miller et al. is using networked microgrids. Individual,
isolated microgrids collaboratively interconnect with one
another to increase the amount of end-use load served
and to grow the overall system resiliency. The networked
microgrids are then able to recombine with the bulk
power system during the operational-restoration phase
after the extreme event.
This article presents work being conducted by the U.S.
Department of Energy (DOE), in collaboration with electric
utilities, to support the fractal operation of power systems
using networked microgrids. Fundamental operational
issues are presented as well as practical aspects for
deployment in an operating electric utility.

and the available technologies have driven deployments
to centralized control solutions. However, emerging technologies are enabling the realization of a fractally operated
power system in the form of networked microgrids.
Miller et al. describe the ideal " agile power system, " in
which fractal operations allow for portions of the system
to separate when necessary, to support critical end-use
loads. Additionally, these segmented portions can coordinate operations and support one another, and the system
can reform as the extreme event ends and the system is
restored. According to Miller et al., ideal fractal operations
would have the following capabilities:
xx
All segments of the grid operate with the same information and control model, regardless of scale.
xx
Every segment of the grid has a decision-making
capability.
xx
The means for the exchange of peer-to-peer information are defined clearly in the standards.
xx
The rules for when to divide and when to combine are
clearly defined.
While these capabilities currently are not widely
deployed, the combination of microgrid operating principles and advances in key technologies are beginning to
enable the operations of networked microgrids and,
hence, the vision of an agile power system. Three key
technologies that enable network microgrid operations
include: 1) grid-forming inverters, which can locally regulate frequency and voltage; 2) communications networks
that support peer-to-peer communications at the application layer; and 3) distributed control architectures that
support the operations of a segmented power system. To
fully understand how the current generation of electrical
infrastructures can evolve into one with fractal operating
capabilities, it is necessary to comprehend how electric
power systems evolved and are currently operated.

Fractal Power Grid Operations
As previously discussed, power system operations are
fractal in their requirements, but the economies of scale

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Evolution of the Modern Interconnected Power System
The first commercial power stations, built during the early
1880s, were small, dc ones that were limited in their
capacities; these were effectively small, isolated
microgrids. In the United States, power was generated at
the same voltage as it was consumed, in the range of 120-
220 V, which prevented supplying distant loads using common cable sizes. This approach was expensive because
each small island of electrification required dedicated
generators, controls, and copper lines to operate. Additionally, because of the capital cost of generators, there typically were not backup units available if one were to fail.
Finally, there was not always complete service coverage
because it was not cost-effective to serve all customers
between two stations.
By 1886, ac systems, enabled by William Stanley Jr.'s
first reliable commercial transformer, proved to be more
effective for the generation and transmission of power.
However, a mix of ac and dc power still was generated
because early ac motors were not suitable for many traction

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IEEE Electrification - December 2020

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