IEEE Power & Energy Magazine - July/August 2017 - 71

Upper-Layer
Control
Lower-Layer
Control

Secondary Control

Primary Control

Tertiary Control

Secondary Control

Droop Curves
Shifted Left (Right)

Power Transfer
Adjusted

Coordination Curve
Shifted Left (Right)

Frequency
Restored

Power
(Re-)Balanced

Power Transfer
Initiated (Updated)

Power
Balanced

Power
Deficiency

Timeline

figure 12. The time line for hierarchical control strategies applied in networked microgrids.

exchanges among networked microgrids. At the lower layer, the
hierarchical control within individual microgrids will implement the exchange flows designated at the upper layer.
Figure 12 illustrates the coordination principles for interactions between upper- and lower-layer control strategies. Once
the lower-layer primary control results in lower microgrid
frequency, the upper-layer primary control is employed to
support microgrid frequency by updating the power transfer among microgrids. Subsequently, microgrids reapply the
lower-layer primary control using the power exchanges designated at the upper level. Next, the lower-layer secondary control in Figure 12 is considered for all microgrids to restore the
respective frequencies and voltages to their initial set points.
At this time, the initial power transfers resulting from the
primary control are considered to be fixed. The upper-layer
secondary control will subsequently take action for updating
the coordination droop characteristic without modifying the
frequency adjustments in individual microgrids at the lower
layer. Next, to optimize energy management in the networked
microgrids, the upper-layer tertiary control will determine
the optimal power exchange among microgrids, followed by
the lower-layer tertiary control for the optimal energy management in individual microgrids.

Conclusions
Driven by descending costs of DERs, microgrids are becoming more appealing for managing the variability of renewable
energy. This article has explored the possibility of clustering
adjacent microgrids to form regionally networked microgrids.
Networked microgrids are operated in close coordination so
that they can improve the operational performance of individual microgrids. In case the operation of the utility grid is
affected by any disruptive events, networked microgrids utilize their inherent capability for self-organizing and self-healing of resources, which can protect local power services. In
practice, networked microgrid operations will benefit both the
utility grid and electricity customers as microgrids enhance
power system reliability, resilience, economics, security, and
july/august 2017

sustainability. The networking of microgrids is potentially
advantageous to strengthening the role of distributed systems
in smart grid implementation.

For Further Reading
L. Che, M. Shahidehpour, and A. Alabdulwahab, "Hierarchical coordination of a community microgrid with ac
and dc microgrids," IEEE Trans. Smart Grid, vol. 6, no. 6,
pp. 3042-3051, 2015.
A. Gholami, F. Aminifar, and M. Shahidehpour, "Frontlines against the darkness: Enhancing the resilience of the
electricity grid through microgrid facilities," IEEE Electrif.
Mag., vol. 4, no. 1, pp. 18-24, Mar. 2016.
A. Khodaei, S. Bahramirad, and M. Shahidehpour, "Microgrid planning under uncertainty," IEEE Trans. Power
Syst., vol. 30, no. 5, pp. 2417-2425, Sept. 2015.
W. Archibald, Z. Li, M. Shahidehpour, S. Johanns, and
T. Levitsky, "Islands in the sun: The solar power deployment
initiative at the University of the Virgin Islands," IEEE Electrif. Mag., vol. 3, no. 1, pp. 56-67, Mar. 2015.
X. Lu, S. Bahramirad, J. Wang, and C. Chen, "Bronzeville
community microgrids: A reliable, resilient and sustainable
solution for integrated energy management with distribution
systems," Electr. J., vol. 28, no. 10, pp. 29-42 2015.
M. Shahidehpour and M. E. Khodayar, "Cutting campus energy costs with hierarchical control: The economical
and reliable operation of a microgrid," IEEE Electrif. Mag.,
vol. 1, no. 1, pp. 40-56, Sept. 2013.

Biographies
Mohammad Shahidehpour is with the Illinois Institute of
Technology, Chicago.
Zhiyi Li is with the Illinois Institute of Technology, Chicago.
Shay Bahramirad is with Commonwealth Edison and
the Illinois Institute of Technology, Chicago.
Zuyi Li is with the Illinois Institute of Technology, Chicago.
Wei Tian is with Willdan Energy, Inc., Chicago.
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