13-06-2013, 04:01 PM
A Control Strategy for Islanded Microgrids With DC-Link Voltage Control
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Abstract
New opportunities for optimally integrating the increasing
number of distributed-generation (DG) units in the power
system rise with the introduction of the microgrid. Most DG units
are connected to the microgrid via a power-electronic inverter
with dc link. Therefore, new control methods for these inverters
need to be developed in order to exploit the DG units as effectively
as possible in case of an islanded microgrid. In the literature, most
control strategies are based on the conventional transmission grid
control or depend on a communication infrastructure. In this
paper, on the other hand, an alternative control strategy is proposed
based on the specific characteristics of islanded low-voltage
microgrids. The microgrid power is balanced by using a control
strategy that modifies the set value of the rms microgrid voltage
at the inverter ac side as a function of the dc-link voltage. In case
a certain voltage, which is determined by a constant-power band,
is surpassed, this control strategy is combined with -droop
control. This droop controller changes the output power of the
DG unit and its possible storage devices as a function of the grid
voltage.
INTRODUCTION
DUE TO limitations of centralized power planning and
risks of the volatile bulk power market, small-scale
generation next to the centralized generation is gaining interest
[1]. Also, the investments in distributed-generation (DG) units
are largely stimulated by the technological developments, the
increased public interest in distributed renewables, and the
reduced cost. Therefore, the number of DG units emerging
in the distribution system is largely increasing and with such
high penetration of DG units.
Droop Control
All electrical equipment in the microgrid is designed to withstand
some voltage deviation from its nominal value. Therefore,
the microgrid voltage is allowed to vary in a certain tolerated microgrid
voltage band, which is used by the -droop controller
for a stable balancing of the microgrid. However, by using
the -droop control strategy solely, voltage-limit violation
can occur. Hence, when the voltage exceeds a certain level,
which lies between the voltage limits, the power delivered to
the microgrid is changed by other means than by changing .
Therefore, the power delivered to the dc link is changed.
Changing can be done in severalways. For instance, can
be decreased by battery charge-up, by changing the generated
power , or by dumping loads.
CONCLUSION
In this paper, the -droop control principle is presented
for the control of generators in an islanded microgrid. This control
strategy focusses on the specific properties of the microgrid
since it takes the -linkage in the microgrid into account,
its lack of inertia, and the dc-link capacitor storage of the
power-electronic interface. Furthermore, to avoid voltage-limit
violation, a -droop control strategy is implemented. The
microgrid control with one power source is studied, next to
the microgrid control and power sharing with multiple power
sources, and good results are obtained. Furthermore, the simulation
results with only the -droop control are compared
with results where the -droop is turned on.