IEEE Power & Energy Magazine - July/August 2017 - 78

the infrastructure changes and additional DERs, loads, or new configuCommunications Bus
rations are called for, it becomes
more challenging to modify the
master controller configurations and
Secondary
Secondary
Secondary
Secondary
settings. Figure 7 illustrates cenControl
Control
Control
Control
tralized control architecture for
secondary control.
Primary
Primary
Primary
Primary
Decentralized or distributed
Control
Control
Control
Control
control
uses peer-to-peer com Sensors
...
and
mu
nication
networks between the
Telemetry
DER (N )
DER (N )
DER (N )
DER (N )
DER assets. A secondary control layer is present at each DER
asset, where it collects all the
figure 8. Secondary control decentralized or distributed architecture.
measurement information from
other DERs using a communicadecentralized or distributed control. Both positions have tion network. The secondary control layer then uses the
measurement information to produce new and appropritheir pros and cons.
Centralized control is more common and suitable for iso- ate control signals to send to the primary control layer,
lated microgrids with fixed infrastructure. A central master resolving the steady-state errors in frequency and voltcontroller connects to each DER. The central controller col- age. Decentralized control with its distributed architeclects data from all DERs at the same time, runs frequency and ture offers a higher level of redundancy and improves the
voltage stability logic, and, based on the results, it then issues overall secondary control reliability. If one secondary conappropriate commands accordingly. The communication net- trol unit is interrupted, it can be excluded from the overall
work and decision-making hierarchy in this design are simple. scheme, but the rest of the microgrid system continues to
One of the major disadvantages of centralized control is that function with adequate primary and secondary controls in
if the master central controller becomes unavailable, sec- place. Due to practical limitations in computing power and
ondary control decisions are interrupted and are not made memory, distributed controllers are typically not capable of
available to the DER and microgrid assets. Another is that as hosting advanced model-based functions such as generation

Central Controller

Economic Optimization

Environmental Optimization

Environmental Optimization
Tertiary Control
Tertiary Control
Restore Voltage and Frequency Deviations
Improve Power Quality and System Reliability

Restore Voltage and Frequency Deviations
Improve Power Quality and System Reliability
Secondary Control

Secondary Control
Local Controller

Load Sharing Between DERs

Control of DERs

Control of DERs
Primary Control
Feedback

Primary Control

Feedback

Microgrids with DERs and Loads

(a)

(b)

figure 9. Control layer interoperability and typical functions. (Adapted with permission from Yang et al.)
78

ieee power & energy magazine

july/august 2017

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