IEEE Electrification Magazine - June 2016 - 52

xx
Level 3 (initial tertiary control): the control methods to
Upper-Level Operators
(Interfaces to Intentional Operation)

Tertiary Level
Economic Dispatching and Optimization
Supervisory Optimizing Generation Scheduling

Secondary Level
Power Quality Control
Voltage
Restoration

Mode Selection

Primary Level
Power Sharing Control
Droop Control

Inner Loops

Physical Level of Microgrid

Figure 7. Different levels in hierarchical control.

manage and control the power flow among the upperlayer grid and/or other microgrids.
xx
Upper levels (extended tertiary control): the control and
decision-making methods to achieve extra targets
(such as practical economic benefits).
Figure 7 shows a typical scheme of hierarchical control. At present, mature power electronic converters are
designed precisely to ensure that they remain stable and
controllable under the worst working conditions. For
this reason, hierarchical control of the microgrid is
allowed, concentrating on system-level control, references as primary, secondary, and tertiary control. Generally, the primary control performs the local control of
output voltage and current of the power electronic interfaces, following the setting points of the upper control
levels. The secondary control that appears above the primary control deals with voltage or frequency restoration
and the management of power quality. Additionally, the
secondary control is in charge of power exchange with the
external grids in the same layer (e.g., other microgrids). The
tertiary control is conventionally issued with the task of managing the power exchange between the microgrid and its
upper-layer grid. In recent studies, there is a trend to integrate the upper control levels, which are initially issued to
achieve extra targets in the tertiary control. To this end, the
tertiary control is to introduce intelligence to the microgrid
and optimize the microgrid operation based on specific interests-
normally efficiency and economics.
Figure 8 shows a typical control
architecture applying hierarchical
control in a generalized dc microgrid.
Droop control can be installed as the
primary control method for active
power sharing purposes. In recent
studies, either output power or output
current could be selected as the feedback signal in the droop control. The
droop coefficient can be regarded as a
virtual internal resistance. In this case,
the droop control consists of the
physical connection of dc sources, and
it therefore simplifies the design of
the parallel converter systems in the
dc microgrid. A small voltage deviation will be introduced
by droop-based primary control. Therefore, a secondary
control is introduced to compensate for the voltage deviation. In most cases, a straightforward proportional-integral
controller can be employed to meet the need of tracking
nominal voltage reference. However, adaptive droop control that uses adaptively changing droop coefficients
instead of fixed ones has also been introduced to some
high-requirement systems using decentralized coordination. It differs from primary and secondary control in that
the tertiary control is providing optimization functions.

For large-scale
dc microgrids,
hierarchical control
is often a preferred
choice since it
offers decoupled
behavior between
different control
layers.

operate both in grid-connected and
islanded modes. Power flows are
also expected to be managed at the
s a m e t i m e . Wi t h t h e a c t i v e
research and development in
recent years, a series of advanced
control and coordination techniques have been investigated for
dc microgrids. One of the most representative ones is the hierarchical
control scheme, which is anĀ  adaptation of the International Society
of Automation ISA-95 grid operation standard in microgrid control.
Generally, to effectively achieve
different control functions, the
hierarchical control scheme is proposed, with the following typically defined levels:
xx
Level 0 (inner control loops): the fundamental control
loops to regulate the output voltage and/or current
within each power electronic converter connected to
the microgrid.
xx
Level 1 (primary control): the control methods to emulate the physical behaviors that make the system stable and more damped power sharing.
xx
Level 2 (secondary control): the control methods to
ensure that the major variables of the system are
within the required values.

52

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2016



Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016

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http://www.nxtbook.com/nxtbooks/pes/electrification_september2019
http://www.nxtbook.com/nxtbooks/pes/electrification_june2019
http://www.nxtbook.com/nxtbooks/pes/electrification_march2019
http://www.nxtbook.com/nxtbooks/pes/electrification_december2018
http://www.nxtbook.com/nxtbooks/pes/electrification_september2018
http://www.nxtbook.com/nxtbooks/pes/electrification_june2018
http://www.nxtbook.com/nxtbooks/pes/electrification_december2017
http://www.nxtbook.com/nxtbooks/pes/electrification_september2017
http://www.nxtbook.com/nxtbooks/pes/electrification_march2018
http://www.nxtbook.com/nxtbooks/pes/electrification_june2017
http://www.nxtbook.com/nxtbooks/pes/electrification_march2017
http://www.nxtbook.com/nxtbooks/pes/electrification_june2016
http://www.nxtbook.com/nxtbooks/pes/electrification_december2016
http://www.nxtbook.com/nxtbooks/pes/electrification_september2016
http://www.nxtbook.com/nxtbooks/pes/electrification_december2015
http://www.nxtbook.com/nxtbooks/pes/electrification_march2016
http://www.nxtbook.com/nxtbooks/pes/electrification_march2015
http://www.nxtbook.com/nxtbooks/pes/electrification_june2015
http://www.nxtbook.com/nxtbooks/pes/electrification_september2015
http://www.nxtbook.com/nxtbooks/pes/electrification_march2014
http://www.nxtbook.com/nxtbooks/pes/electrification_june2014
http://www.nxtbook.com/nxtbooks/pes/electrification_september2014
http://www.nxtbook.com/nxtbooks/pes/electrification_december2014
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