IEEE Systems, Man and Cybernetics Magazine - July 2021 - 15

The Virtual Microgrid as a
Cyberphysical System
The virtual microgrid is constructed with the cyberphysical
entities presented in Table 1 and communication among the
entities. For proper operations, the SDEN agents need to be
orchestrated following a protocol based on the aforementioned
five classes of messages and the prescribed functions
of each agent. Figure 4 illustrates how the proposed
virtual microgrid works as a cyberphysical system. The
dynamical behavior of the cyberphysical system can be
explained based on the agents' functions, as follows.
◆ Clients are the different flexible loads that send their
individual requests to local servers or routers, which
either locally make decisions about the requests or forward
them to the main server. The requests differ,
depending on the load and the specific kind of flexibility
that they can provide.
◆ Routers process and/or aggregate requests from the
same household before they are forwarded to the main
server. They are either smart meters or home energy
management systems. Depending on how the SDEN is
orchestrated, they can act as local servers to reallocate
the local energy packets at that specific house without
sending the request to the main server. Otherwise, they
forward the request to the main server, acting as routers.
◆ The server processes requests from different routers to
make decisions based on the energy available in the
shared pool of virtualized energy
resources. The packets can be accepted
or rejected. The accepted packets need
to be scheduled and delivered (this will
be discussed later). The rejected
requests are returned to the clients,
who are informed about the decision as
well as the procedure for resubmitting
a new request. The server is the key
agent in the proposed architecture.
◆ The source refers to the energy produced
by distributed sources such as
solar photovoltaics (PVs) and wind
generators. They are also discretized
as identifiable packets that are aggregated
as one energy source, thereby
building a virtual power plant. Energy
from other sources might be incorporated
in the source.
◆ Buffers are short-term energy storage
devices, such as batteries and heat
pumps. They are used to match supply
and demand along different operational
time scales, ranging from primary to
tertiary control. For tertiary control,
the server can allocate some packets
from the source to the buffers during
some periods, or it may allocate some
packets from the buffers to supply
Cache
Server
Clients
energy during other periods. This allocation may be
done by either a local router or the server.
◆ The cache is related to long-term (seasonal) storage,
which is not available today for households but may be
integrated in the future. This encompasses, for example,
power-to-X technologies to store energy as synthetic
fuels. In this case, extra energy production in
the summer may be transferred for use in the winter.
The proposed concept is based on the SDEN-enabled
virtual microgrid, where requests and supply are subdivided
into energy packets similar to BitTorrent Internet applications
[34], [35]. In operational terms, the resource
allocation problem to be solved by the energy server is similar
to, for example, university admission processes, where
applicants are subdivided into priority levels and the open
positions are filled accordingly. The key difference in our
approach is that it has a strong dependence on both past
allocations and predicted future availability, which is variable
because of generation fluctuation and storage.
The energy server must 1) admit new energy packets,
2) (dynamically) classify them into priority classes, 3)
define how many packets of each class can be served in
different time slots, and finally 4) allocate the time when
they will be delivered. Each of these stages has specific
challenges, as follows.
1) Admission: A reliable forecast of the energy packet generation
for a long period (e.g., 12 h) is necessary. The
Source
Source
Router
Buffer
Source
Buffer
Router
Source
Buffer
Clients
Figure 4. An example of how the agents are related and orchestrated
by the energy server to build a virtual microgrid based on the SDEN
elements and communication protocol.
July 2021 IEEE SYSTEMS, MAN, & CYBERNETICS MAGAZINE 15
IEEE Systems, Man and Cybernetics Magazine - July 2021

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