IEEE Power & Energy Magazine - July/August 2017 - 77

Secondary Control

fo
f

V0
V1

figure 5. Droop control: frequency versus real power and
voltage versus reactive power. (Adapted with permission
from Bollman.)

Prima
ry
Respo
nse

∗
fMG

Secondary
Response

links to achieve successful control. The basic principle of
droop control is that as real power load on the system increases,
the droop control scheme will allow the system frequency to
decrease. Similarly, as the reactive power needs of the system
increase, the droop control scheme will allow the system voltage to decrease. Figure 5 shows the droop control characteristic plots.
A well-known drawback of droop schemes is the inherent
frequency deviation as it performs power sharing, which
can be much more evident during system disturbances. With
inverter-based DERs, frequency restoration is not practical due to inaccuracies in inverter frequency output. These
small frequency differences could result in increased circulating current and ultimately system instability, especially during
islanded microgrid scenarios. However, sources with rotating
machines do have frequency restoration control schemes.
In microgrid installations and as DERs are aggregated
to support system loads, frequency inaccuracies that result
from droop control schemes give reason for control designers to introduce a second layer of control that addresses
this concern.

P
P1∗

P2∗

P1max P2max

This frequency control shortcoming calls for better frequency control from a higher hierarchical layer that corfigure 6. The frequency effect from secondary and
rects the errors in frequency and voltage not corrected by
primary control. (Adapted with permission from Simpsonprimary control. Secondary control operates by setting voltPorco et al.)
age and frequency reference points for the overall system
and comparing voltages and frequencies at each DER to
these reference points. Then, commands are issued to cor- is just as important in issuing commands to other system
rect errors. Figure 6 shows secondary control in action. As assets, utilizing them as needed for frequency and voltage staprimary control's droop function attempts to meet the higher bility. These assets are system equipment such as load tap
power demand by the microgrid loads, it causes the overall changers, regulators, and capacitor banks.
When discussing secondary control, an important quesmicrogrid frequency to decrease linearly, shifting outside
the designed system frequency window. Secondary con- tion arises in relation to the type of communication that can
trol corrects these deviations by issuing new set points or be used to enable it. The industry is pretty evenly divided
directly commanding other DER assets to support the fre- into two camps: one that believes in and supports the conquency, thus bringing the system frequency back within the cept of centralized control and another that leans toward
designed window.
This approach requires a communication network that would
allow secondary control to receive
Secondary
information from all DERs and in
Control
return issue control commands to
maintain system stability. Microgrid
Communications Bus
protection and coordination depend
heavily on secondary control to toggle between relay group settings.
Primary
Primary
Primary
Primary
As the microgrid transitions from
Control
Control
Control
Control
Sensors
one mode of operation to another,
...
and
such as from islanded to grid-tied,
Telemetry
DER (3)
DER (2)
DER (1)
DER (N )
secondary control enables the corresponding protection settings that
are associated with each mode
of operations. Secondary control figure 7. Secondary control centralized architecture.
july/august 2017

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