08-05-2013, 12:43 PM
MICROGRID MANAGEMENT
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INTRODUCTION
THE ENVIRONMENTAL AND ECONOMICAL BENEFITS OF THE MICROGRID, AND consequently its acceptability and degree of proliferation in the utility power industry, are primarily
determined by the envisioned controller capabilities and the operational features.
Depending on the type and depth of penetration of distributed energy resource (DER) units, load
characteristics and power quality constraints, and market participation strategies, the required
control and operational strategies of a microgrid can be significantly, and even conceptually, different
than those of the conventional power systems.
Microgrid Structure and Characteristics
Figure 1 shows a microgrid schematic diagram. The microgrid encompasses a portion of an
electric power distribution system that is located downstream of the distribution substation, and
it includes a variety of DER units and different types of end users of electricity and/or heat.
DER units include both distributed generation (DG) and distributed storage (DS) units with different
capacities and characteristics. The electrical connection point of the microgrid to the utility
system, at the low-voltage bus of the substation transformer, constitutes the microgrid point of
common coupling (PCC). The microgrid serves a variety of customers, e.g., residential buildings,
commercial entities, and industrial parks.
The microgrid of Figure 1 normally operates in a grid-connected mode through the substation
transformer. However, it is also expected to provide sufficient generation capacity, controls, and
operational strategies to supply at least a portion of the load after being disconnected from the
distribution system at the PCC and remain operational as an autonomous (islanded) entity. The
existing power utility practice often does not permit accidental islanding and automatic resynchronization
of a microgrid, primarily due to the human and equipment safety concerns.
Microgrid Loads
A microgrid can serve electrical and/or thermal loads. In a
grid-connected mode, the utility distribution system often can
be considered as an electric “slack bus” and supply/absorb
any power discrepancy in the microgrid-generated power to
maintain the net power balance. Load or generation shedding
within a microgrid is also an option if the net import/export
power has hard limits based on operational strategies or contractual
obligations.
DER Controls
Control strategies for DER units within a microgrid are
selected based on the required functions and possible operational
scenarios. Controls of a DER unit are also determined
by the nature of its interactions with the system and other
DER units. The main control functions for a DER unit are
voltage and frequency control and/or active/reactive power
control. Table 2 provides a general categorization of the
major control functions of a DER unit and divides the strategies
into the grid-following and grid-forming controls.
Each category is further divided into noninteractive
and grid-interactive strategies. The grid-following
approach is employed when direct control of voltage
and/or frequency at the PC is not required. Furthermore,
if the unit output power is controlled independent of the
other units or loads (nondispatchable DER unit), it constitutes
a grid-noninteractive strategy. An example of the
grid-noninteractive strategy is the MPPT control of a
solar-PV unit. A grid-interactive control strategy is based
on specifying real/reactive power set points as input commands.
The power set points are either specified based on
a power dispatch strategy or real/reactive power compensation
of the load or the feeder.
Grid-Forming Controls
The grid-forming control strategy emulates behavior of a
“swing source” in an autonomous microgrid. A grid-forming
unit within a microgrid can be assigned to regulate the voltage
at the PCC and dominantly set the system frequency. The
unit should be adequately large and have adequate reserve
capacity to supply the power balance. If two or more DER
units actively participate in grid stabilization and voltage regulation,
then frequency-droop and voltage-droop control
strategies are used to share real and reactive power components.
In this case, the voltage and frequency of the microgrid
may deviate from the rated values, within acceptable limits,
depending on the load level and the droop characteristics.
Figure 9 shows frequency-droop (f-P) and voltage-droop
(v-Q) characteristics where each is specified by its slope (kfP
or kvQ) and a base point representing either the rated frequency
( fo, Po) or the nominal voltage (Vo,Qo), respectively. The
droop coefficients and the base-points can be controlled
through a restoration process to dynamically adjust the operating
points of the units. This is achieved by dynamically
changing the power-sharing levels to set the frequency and
voltages at new values.
Decentralized Microgrid Control
A decentralized control approach intends to provide the
maximum autonomy for the DER units and loads within a
microgrid. The autonomy of the LCs implies that they are
intelligent and can communicate with each other to form a
larger intelligent entity. In decentralized control, the main
task of each controller is not necessarily to maximize the
revenue of the corresponding unit but to improve the overall
performance of the microgrid.
Conclusions
Market acceptability of DER technologies and the gradual
and consistent increase in their depth of penetration have generated
significant interest in integration, controls, and optimal
operation of DER units in the context of microgrids. Initially,
microgrids were perceived as miniaturized versions of the
conventional power systems, and intuitively their
control/operational concepts were based on scaled-down and
simplified versions of control/operational concepts of large
power systems. This article highlighted the main differences
between microgrids and large power systems and on that
basis advocates for a fresh approach to the development of
control and operational concepts for microgrids.