IEEE Electrification - December 2020 - 75

Individual microgrids typically range in size-from hundreds of watts to megawatts. Similar to the economies of
scale and efficiency gains of transitioning from standalone backup generation to an individual microgrid, the
networking of local microgrids can produce additional
benefits. Figure 4 shows an idealized image of a network
of microgrids, with a representative one-line diagram of
the primary distribution system that interconnects them.
The microgrids shown in in Figure 4 represent a selection of the microgrid types defined in IEEE Standard 15472011. They can range from a single customer, i.e., a house,
behind the utility revenue meter, to an entire distribution circuit. The four microgrids shown in Figure 4 would
be defined in IEEE Standard 1547-2011 as: 1) " Facility
Island (single residential meter), " 2) " Secondary Island
(multiple residential meters), " 3) " Secondary Island (multiple industrial meters), " and 4) " Lateral Island (multiple
mixed meters). "
For these microgrids to operate in a stand-alone mode
and to network and share resources, it is necessary to
accomplish operational objectives that are typically done
by large central plants and the associated control systems.
A potential method for accomplishing this is to coordinate
the operation of the microgrids using OpenFMB over a
wireless mesh network. As an example, the following discussion describes how primary frequency control can be
achieved in a network of microgrids.

OpenFMB Harness
(Publish and Subscribe)

Microgrid Controller

Recloser

Inverter

Distribution
Management System

In a traditional, centralized power system, when there
is a large change in the load or a significant change in the
generation dispatch, the frequency controls operate at
primary, secondary, and tertiary levels. The primary frequency control operates to arrest the initial decrease in
frequency and to partially restore it to nominal (60 Hz in
North America). The secondary frequency control adjusts
the generator outputs to restore the frequency to nominal. The tertiary frequency control adjusts generator outputs and possibly starts additional units to restore the
reserves that were expended during the operation of the
primary and secondary frequency controls. A three-level/
layer system can also be considered when microgrids are
operating in an islanded mode or networking for efficiency and/or resiliency.
The primary frequency-control layer is typically implemented with droop-type controllers that operate independently for each unit. In extreme cases, load shedding
can participate in primary frequency control. Generating
units can include rotating machines and grid-forming
inverters, if present; grid-following inverters do not support primary frequency control. Primary frequency control occurs at each unit to arrest the initial decrease in
frequency and to partially restore it but does not restore
it to nominal.
The secondary control layer is responsible for coordinating generating units to restore frequency and voltage to

Microgrid Controller

RTU

Volttron Node

RTU

Volttron Node

Inverter

Volttron Node

Recloser

Inverter

Building
Management System

Figure 3. A structural view of the OpenFMB harness being deployed.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 2 0

IEEE Electrification - December 2020

Table of Contents for the Digital Edition of IEEE Electrification - December 2020