Table 742 droop control for regulating voltages and

Table 7.4.2 Droop control for regulating voltages and frequency
Control Level Timescale Function Implementation level
Primary Milliseconds
Realize proportional sharing of real and
reactive power demand
LC
Secondary Seconds
Restore frequency and voltages
to set‐points
LC
Tertiary Minutes or event‐driven
Optimize set‐points for frequency and
voltages
MC
Figure 7.4.5 illustrates the hierarchical control strategy applied to a grid‐forming DER for regulating
microgrid frequency. In this case, 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 active power output at point “c”. The
tertiary control identifies point “d” as the optimal DER power output without re‐adjusting the microgrid
frequency. The secondary and tertiary controls shift up/down the droop characteristics by applying
proportional, integral, and derivative (PID) controllers for error compensations.
Microgrid
Frequency
Real Power Output
a
b
c
d
Primary Control
Secondary Control
Tertiary Control
Figure 7.4.5 Hierarchical microgrid control for network frequency regulation

47
b) Upper‐Layer Control: Regulating Power Exchanges Between Networked Microgrids
When the IIT‐Bronzeville networked microgrids are islanded from the main grid, the operation of the
two microgrids is coordinated by enabling power exchanges via IC. The MC in BCM is responsible for
issuing supervisory commands 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 BCM. Without the loss of generality, we assume an adequate
level of reactive power is already supplied at each microgrid level; therefore, IC will only consider active
power transfer for adjusting operating frequencies of both microgrids.
The islanded networked microgrids coordinate the operation of the two networked microgrids by
enabling power exchanges via IC. The MC in BCM is responsible for issuing supervisory commands that
regulate inter‐microgrid power exchanges. When one microgrid encounters a power imbalance, the
other microgrid will provide support by exercising the desired power exchange for balancing the power
in each microgrid. Without the loss of generality, we assume an adequate level of reactive power is
already supplied at each microgrid level; therefore, IC will only consider the active power transfer for
adjusting the frequency.