22-03-2014, 02:52 PM
Operation and Control of Hybrid Microgrid with Angle Droop Controller
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ABSTRACT:
This paper compares the performance of two
droop control schemes in a hybrid microgrid. With presence of
both converter interfaced and inertial sources, the droop control-
ler share power in a decentralized fashion. Both the droop con-
trollers facilitate reactive power sharing based on voltage droop.
However in frequency droop control, the real power sharing de-
pends on the frequency, while in angle droop control, it depends
on output voltage angle. For converter interfaced sources this
reference voltage is tracked while for inertial DG, reference pow-
er for the prime mover is calculated from the reference angle
with the proposed angle control scheme. This coordinated control
scheme shows significant improvement in system performance.
The comparison with the conventional frequency droop shows
that the angle control scheme shares power with much lower
frequency deviation. This is a significant improvement particu-
larly in a frequent load changing scenario.
INTRODUCTION
In an autonomous microgrid, all the DGs are responsible for
maintaining the system voltage and frequency while sharing
the real and reactive power. Load sharing without extensive
communication is the best option as the network can be com-
plex and can span over a large area. The most common me-
thod for this purpose is the use of droop characteristics, where
power sharing among the parallel sources is achieved by con-
trolling two independent quantities-the system frequency and
the fundamental voltage magnitude [1-6]. The sources can be
inertia less distributed generators (DG) interfaced through
voltage-source-converter (VSC) or inertial sources such as
synchronous machine. Different droop control methods have
been proposed in [4, 5, 6 and 7]. In [7], droop control method
with virtual output impedance in a microgrid is discussed,
while in [8] the implementation of conventional f/V-droops in
the respective inverters, thus down scaling the conventional
grid control concept to the low voltage grid is proposed.
Modified Droop Controller
The rates of change in power output in inertial DGs are li-
mited by their inertia constant. Therefore such DGs require a
finite time to change the power output. To avoid conflicting
power demand, the rate of change in power output of the non
inertial VSC interfaced sources must also be limited.
CONCLUSION
In this paper, the comparison of angle controller with fre-
quency controller is investigated in a microgrid that contains
both inertial and non inertial DGs. With the proposed method,
power sharing with angle controller is achieved with inertial
DG. The angle controller output is used to set the reference
power for the inertial DGs while for non inertial DGs the an-
gle controller output is used as output voltage reference angle.
It is shown that with angle droop control, it is possible to share
power with much lower frequency deviation compared to that
with frequency droop. The lower frequency deviation ensure a
better power quality and system stability in a frequent load
changing scenario.