IEEE Power & Energy Magazine - September/October 2019 - 78

Transactive energy platforms effectively open up the demand side
to a wide range of applications and, as a result, have the potential
to change incumbent business models in the distribution system.
and advanced energy-management solutions. In some
ways, utilities are well placed to meet these new challenges. They can undertake the role of distribution system operators to manage not just the supply of electricity
but also implement competitive retail electricity markets.
Ultimately, the coordinated control of the eIoT-enabled
grid periphery requires not only new technology but also
grid operators and planners that can ensure its physical
reliability and economic efficiency.
The eIoT is transforming the grid's architecture at multiple levels of its periphery. At the most granular level, devices
and appliances with newly integrated control functionality
are proliferating throughout homes, businesses, and industrial sites. They also have the potential to transform the grid's
architecture through microgrids and their associated energymanagement systems. Beyond their islanding capability, customer-owned microgrids can act as curtailment service providers or demand-side aggregators and participate directly
in wholesale electricity market demand-response schemes.
Such participation motivates the development of transactive
energy applications within the microgrid and implemented
as eIoT control loops. Here, transactive energy is a means
of advancing demand-response initiatives while securing the
operational needs of the grid's stakeholders. Through the use
of smart devices connected in local area networks, consumers are exposed to a larger marketplace where energy products and grid services can be exchanged.
Within transactive energy applications, consumers
and prosumers have the opportunity to exploit and benefit
from changing real-time electricity market prices. Instead
of procuring electricity directly from a utility, consumers
can implement energy arbitrage schemes within transactive
energy markets and maximize their benefits. Such arbitrage
schemes can be implemented when consumers have access
to the right eIoT devices, pricing information for wholesale
and retail electricity markets, and transactive energy platforms that can coordinate power flows in response to this
pricing information. Furthermore, the emergence of blockchain, as a distributed and virtual ledger, has advanced the
likelihood that transactive energy applications will communicate transaction information securely. Transactive energy
platforms effectively open up the demand side to a wide
range of applications and, as a result, have the potential to
change incumbent business models in the distribution system. Naturally, the needs of residential, commercial, and
industrial consumers vary, and transactive energy-management solutions have to develop accordingly. Therefore, it
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remains unclear how transactive energy applications will
take shape or how utilities will adjust to the highly activated
grid periphery.

Modeling Interdependent Infrastructure
Systems With Heterofunctional Graphs
Why Do We Need Heterofunctional Graphs?
Many things in the eIoT have a dual identity. They do not just
consume power but also provide a service to a consumer. For
example, the main purpose of a water treatment facility is to
extract and treat water and deliver it via a network of pumps
and pipes to the community. This example shows that many
energy things at the grid periphery have functions outside
the grid that drive electricity consumption patterns within
the grid. Consequently, we must differentiate between purely
electrical things and energy things with multiple functions in
multiple infrastructures.
The integration of eIoT devices at the grid periphery
introduces interdependencies between the power grid and
other infrastructure systems. Consequently, decisions in the
other systems could influence the operations of the power
grid and vice versa. What was once the conventional task
of controlling the power grid exclusively based on observations in the demand has become the much more complex
task of managing tradeoffs between multiple infrastructures. Therefore, there is a need to develop models that integrate multiple infrastructures to support decision making
and control between them. However, methodological tools
for the design of interdependent systems are required. Some
work has used graph-theoretic approaches while neglecting
the intrinsic heterogeneity in these systems. Others have
used graphical model-based systems engineering (MBSE)
techniques that do not immediately lend themselves to quantitative analysis. For example, SysML activity and block
diagrams are able to describe systems with arbitrary heterogeneity. To address the limitations of both approaches,
heterofunctional graphs have been developed over the past
decade and most recently applied to interdependent smart
city infrastructure.

What Are Heterofunctional Graphs?
Heterofunctional graphs are graphs or networks that explicitly describe the heterogeneity of function that is found in
many networked systems, including interdependent infrastructure systems. In essence, they provide a new ontological basis or vocabulary to model heterogeneous engineering
september/october 2019

IEEE Power & Energy Magazine - September/October 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2019