IEEE Electrification - December 2020 - 72

and elevator loads. As a result, some regions saw power
generated and transmitted as ac but distributed and consumed as dc, with the Consolidated Edison system in New
York being the best-documented case. In New York City,
the last dc load, served by Consolidated Edison through a
solid-state rectifier, remained in service until 2007.
In the United States, the financial panic of 1893 had a
severe impact on the large number of small power systems that were financially overleveraged but rich in capital assets. The result was an extensive consolidation at the
regional level, where numerous utilities consolidated into
relatively a few. For example, by 1900, the 12 separate electric utilities and 22 streetcar operators in Seattle, Washington were consolidated into the Seattle Electric Company, a
Stone & Webster company, and the predecessor of Puget
Sound Energy. Over the next 60 years, the regional consolidation of utilities and adoption of ac systems, supported
by higher voltages and the transition from reciprocating
engines to larger turbines, allowed for the creation of large
interconnected power systems. Supplied by large centralized generators, these are the systems commonly used
today. Additionally, the limitations of controls and communications systems led toward centralized operations.
Controls were typically electromechanical with no communications between the generating stations, other than
via telephones.
Despite the efficiencies and benefits of modern, interconnected power systems, they are vulnerable to extreme
events because of the geographic separation between the

(a)

(b)

large generation units and the end-use loads. For this reason, it is not uncommon for critical end-use loads, such as
hospitals and airports, to employ local backup generation.
Typically, local generation is less efficient and more
expensive and must include the controls to maintain a
stable frequency and voltage. Because of the cost of dedicated backup generation, there typically are not redundant units available if one fails, resulting in lower
reliability, with hospitals being a notable exception.
An alternative to isolated individual backup generators
is to interconnect them into a microgrid. As defined by the
DOE, a microgrid is " a group of interconnected loads and
distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity
with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both gridconnected and islanded-mode. "
Modern microgrids have been built to address a host of
operational issues that cannot be tackled with traditional
centralized solutions or individual backup generators. For
example, microgrids leverage localized economies of
scale, where numerous resources can be combined to
support end-use loads. It has been shown that the interconnection of generators into a microgrid can increase
efficiency by 33%. Additional benefits can be realized with
the interconnection of renewable resources and leveraging redundant units.
Because electric power systems exhibit fractal characteristics regardless of scale, microgrids must accomplish
the same operational functions as a large interconnected
power system. Specifically, regardless of it being an interconnected, continent-spanning power system or a small,
isolated community system, it must maintain frequency
and voltage control, adjust generating unit outputs, and
protect the system from electrical faults. While the operational requirements are fractal, the specific implementations will vary because of fundamental differences. For
example, in a large bulk power system, the loss of a single
generator will typically not cause any loss of load. In contrast, the loss of a generator in a microgrid may cause a
complete system collapse. Despite these differences and
the impact to specific controls, the operational requirements of a power system are fractal.

Concept of Fractal Operations
(c)

(d)

Normal Utility Service

Microgrid Service

Loss of Service

Network Microgrid
Interconnection

Figure 1. (a) The electric power system with no loss of end-use load.
(b) The loss of service to portions of the electric power system. (c)
Some stand-alone microgrids supply power to portions of end-use
loads that have lost service. (d) The stand-alone microgrids network
to increase efficiency and resiliency.

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I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2020

At the core of the fractal operating concept is the idea that
centralized control is not required for all portions of electric power systems. Specifically, portions of this system can
operate independently, either in anticipation of or in
response to an event. A conceptual example of this can be
seen in Figure 1. In Figure 1(a), the power system is operating under normal conditions, with power supplied to all
end-use customers. Figure 1(a) is a conceptual diagram of
an electrical distribution system; a detailed one-line diagram of the system is not included because the fractal
nature is the focus.
IEEE Electrification - December 2020

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