IEEE Power & Energy Magazine - July/August 2017 - 68

table 2. Droop control for regulating voltages and frequency.
Control Level

Timescale

Function

Implementation Level

Primary

Milliseconds

Realize proportional sharing of real and reactive
power demand

Secondary

Seconds

Restore frequency and voltages
to set points

Tertiary

Minutes or event driven

Optimize set points for frequency and voltages

Figure 6 illustrates the hierarchical control strategy applied
to a grid-forming DER for regulating microgrid frequency. In
this case, the DER is initially operated at point a, representing
its initial set point. Once the microgrid suffers a real power
shortage (e.g., switched to the islanded mode), the real power
output of DER will be adjusted for primary control to supply the power deficit according to the droop characteristic.
Subsequently, the operating point will be shifted to point b,
representing an increase in the DER power output at a reduced
frequency. The secondary control will then take place for mitigating the regulation error introduced by the primary control.
The secondary control restores the DER operating frequency
at its set point by maintaining the adjusted real power output at point c. The tertiary control identifies point d as the
optimal DER power output without readjusting the microgrid
frequency. The secondary and tertiary controls shift the droop
characteristics up or down by applying proportional, integral,
and derivative controllers for error compensations.

Upper-Layer Control: Regulating Power
Exchanges Between Networked Microgrids

Microgrid Frequency

When the IIT-Bronzeville networked microgrids are islanded
from the utility grid, the operation of the two microgrids is
coordinated by enabling power exchanges via the IC. The MC
in the BCM is responsible for issuing supervisory commands

Primary Control
Secondary Control
Tertiary Control

Real Power Output

figure 6. Hierarchical control for microgrid frequency
regulation.
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that regulate power exchanges between the two microgrids.
When one microgrid encounters a power imbalance, the
other microgrid will provide support by exercising the power
exchange determined by the MC in the BCM. Without the
loss of generality, we assume an adequate level of reactive
power is already supplied at each microgrid level; therefore,
the IC will only consider real power transfer for adjusting
operating frequencies of both microgrids.
The coordination of the two microgrids is based on the
comparison of their steady-state frequencies. In principle,
the two microgrids can be operated at different frequency set
points, and thus the normalized values of frequencies should
be compared in those cases. The difference between the corresponding normalized values represents the coordination
error for the two microgrids. The two microgrids are normally operated at the same frequency, 60 Hz, and thus the
coordination error, which represents the deviation of the ICM
frequency from that of the BCM, ideally reaches a zero value
at steady state. The two microgrids are then coordinated as
follows. When the ICM frequency is higher than that of the
BCM (i.e., the coordination error is positive), real power is
transferred from the ICM to the BCM; when the BCM frequency is higher (i.e., the coordination error is negative), real
power flows from the BCM to the ICM.
The MC in the BCM determines the power exchange
between the two microgrids based on the coordination error,
which is shown in Figure 7 using the coordination droop
characteristic. Note that the proposed coordination droop
curve includes a vertical deadband where the power exchange
has a fixed value. Accordingly, minute coordination errors
do not constitute any power exchanges for technical or economic considerations.
A hierarchical control mechanism is developed for regulating power exchanges between networked microgrids. The
primary control applies a quick response to steady-state frequency variations in any of the networked microgrids. For
the primary control of coordinating the operation of the two
microgrids, the MC in the BCM determines the direction and
flow of real power transfer for mitigating any coordination
errors. Here we present an example to illustrate the role of
the primary control in the networked microgrids, where the
frequency regulation curve of each microgrid is obtained by
aggregating frequency-real power droop characteristics of onsite grid-forming DERs, as illustrated in Figure 8. Initially,
july/august 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2017