The Bridge - Issue 3, 2020 - 14

Feature

THE FUTURE OF RENEWABLE ENERGY TRANSMISSION: An Autonomous Energy Grid

this sense, it is worth emphasizing that the design
of the distributed algorithm as well as the overall
communications strategy will depend on the types of
actors participating in the real-time optimization process
(e.g., end customers, cell controllers, or aggregators).

Robustness
In the context of AEGs, robustness includes both
reliability and resilience. Reliability is the property to
be tolerant to faults, and resilience is the ability to
come back from a failure to an operational state. For
reliable operation, stability analysis can be used at
multiple timescales. Resilience to communications
drops and asynchronous operation should be
analytically established through pertinent input-tostate stability and tracking results. In other words, the
AEGs should be able to continue operating even in
the presence of these faults/errors. Mathematically,
iterative optimization algorithms have been developed
to operate with errors in their estimated parameters,
such as gradients. In fact, it can be shown that a
packet loss leads to the computation of primal or dual
gradient steps with outdated information. Thus, cells
that can switch from an islanded mode to a larger gridconnected mode may continue operating amidst faults
and/or threats to the grid. These properties can be
modeled as time-varying constraints in the underlying
optimization problem. Similarly, flexible operation-
wherein a cell (or a portion of a cell) switches to
an autonomous control setting during a prolonged
communications outage-should be enabled.

Scalability
Figure 2 illustrates an architecture wherein
communications among cells occurs when performing
distributed and/or hierarchical control. As mentioned
previously, distributed and hierarchical control
algorithms are scalable and allow for the control of
millions of devices in real time. When using distributed/
hierarchical controls, the problem is broken up into
smaller "cells," and the interactions among cells can
be reconciled using consensus to ensure consistency
constraints for electrical quantities that pertain to the
cell-to-cell connections. For example, adjacent cells
must agree on the real and reactive power exchanges
at the points of interconnection or overlap.

THE BRIDGE

Real power and reactive power set points from the
optimization are sent between levels in the hierarchy.
Intracellular communications (on the same level) can
be used to ensure that the set points of the DERs are
computed to maximize the given operational objectives
while ensuring that electrical limits are satisfied within
the cell. Communications also take place between a
cell-level control platform and individual customers;
these are necessary to optimize customer-level
objectives while respecting electrical limits within a
cell. Message passing among cells to optimize the
flow of power is based on economic and reliability
targets. These levels of hierarchy allow for scalable
distributed optimization algorithms to be designed and
implemented in AEGs.
Figure 2 shows three levels of hierarchy. The top
level, Level 3, aggregates neighborhoods to achieve
an optimization objective, such as voltage regulation
or power balancing. This level communicates to
Level 2 (e.g., a single neighborhood) about the

Figure 2. Communications architecture for distributed and real-time
optimization of AEGs. In the figure, Level 1 would be at a home or
business, Level 2 would be at a neighborhood, and Level 3 would be
multiple neighborhoods on a single distribution circuit. [2]

aggregated power designated for that neighborhood.
This information is passed from the neighborhood
level down to the homeowner (Level 1) as a power
set point to track. The homeowners might accordingly
coordinate their own distributed wind or solar,
smart home devices, and EVs to optimally balance
the grid needs and their own usage preferences.
Communications run in both directions, as indicated
in Figure 2. For example, if the homeowner is unable
to meet their power set point, information is passed

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The Bridge - Issue 3, 2020

Table of Contents for the Digital Edition of The Bridge - Issue 3, 2020