IEEE Electrification - September 2020 - 96

Centralized Control Architecture

objectives. Examples of such objectives include maintaining power quality and minimizing operational costs and,
under some circumstances, providing ancillary services to
the external grid to which the microgrid is connected.
In terms of implementation of a centralized control
architecture on the Illinois C-HIL testbed, we employ the NI
cRIO 9068 as described earlier. The DERs, loads, and the
microgrid network are emulated in the Typhoon HIL device.
We use the Modbus TCP/IP protocol to set up a bidirectional
communication interface between the cRIO 9068 and the
Typhoon HIL device. The cRIO 9068 device acquires the
operating points of the assets emulated in the Typhoon HIL
device and uses that information, along with the control
objectives, to calculate modified operating points for the
controllable assets. To close the control loop, the cRIO sends
these new operating points to the controllable assets emulated in the Typhoon HIL device. The C-HIL setup for testing
the centralized control architecture is shown in Figure 7(b).
For the experiments where we use the lower-level controllers to implement the lower-level DER control schemes, a
unidirectional communication link between the cRIO and
the Typhoon HIL device, the cRIO and the lower-level controller, and the lower level controllers and the Typhoon HIL
device is sufficient for closed-loop operation.

The centralized architecture utilizes a centralized decision-making approach to control the DERs and loads in a
microgrid. This architecture requires a centrally located
computing device that maintains bidirectional communication with the controllable assets, e.g., DERs and loads, so
as to gather local information from each asset and
instruct them to operate in a specific fashion. For the
loads that are not controllable, a unidirectional communication link from the loads to the computing device is
sufﬁcient. The centralized computing device implements
different microgrid control functions, e.g., primary and
secondary frequency control, voltage control, and optimal
generation asset dispatch as well as handling transitions
from grid-connected to islanded operation and vice versa.
A layout for the centralized architecture is depicted in Figure 7(a) with a centralized computing device connected to
three DERs and three controllable loads in an islanded ac
microgrid setting. Depending on the objective, the computing device polls the relevant asset states, computes the
new operating points, and sends them back to the assets
as appropriate. The control architecture integrates and
coordinates all requisite control functions to achieve
numerous (and sometimes conﬂicting) operational

C-HIL
Testbed
Cyber Layer

Control
Nodes
Ethernet Switch

C-HIL
Testbed Rack

Lower-Level
Controllers

Typhoon
HIL
Devices
Lower-Level
Controllers

Microgrid
Network

Lower-Level
Controllers

C-HIL Testbed
Physical Layer
Zigbee

Inverter-Based DER

Communication Links

Load

Synchronous Generation

Lower-Level Controllers

Distributed Control Node

Centralized Computing Node

Figure 6. The Illinois C-HIL testbed.

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

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IEEE Electrification - September 2020

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