17-05-2014, 11:00 AM
Control of Distributed Generation Systems— Part II: Load Sharing Control
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Abstract
This paper is concerned with the control strategy for
the parallel operation of distributed generation systems (DGS) in
a standalone ac power supply. The proposed control method uses
only low-bandwidth data communication signals between each
generation system in addition to the locally measurable feedback
signals. This is achieved by combining two control methods: droop
control method and average power control method. The average
power method with slow update rate is used in order to overcome
the sensitivity about voltage and current measurement errors. In
addition, a harmonic droop scheme for sharing harmonic content
of the load currents is proposed based on the voltages and currents
control algorithm presented in [28]. Experimental and simulation
studies using two parallel three-phase pulsewidth modulation
(PWM) inverters are presented to show the effectiveness of the
proposed control.
INTRODUCTION
RECENTLY, interest in distributed generation systems
(DGS) is rapidly increasing, particularly onsite genera-
tion. This interest is due to the facts that larger power plants
are economically unfeasible in many regions due to increasing
system and fuel costs, and stricter environmental regulations.
In addition, recent technological advances in small generators,
power electronics, and energy storage devices have provided
a new opportunity for distributed energy resources at the
distribution level, and especially, the incentive laws to utilize
renewable energies have also encouraged a more decentralized
approach to power delivery [1]–[16].
There exist various generation sources for DGS: conventional
technologies (diesel or natural gas engines), emerging technolo-
gies (micro turbines or fuel cells or energy storage devices)
[2], [4], [6]–[12], and renewable technologies (small wind tur-
bines or solar/photovoltaic or small hydro turbines) [3], [4],
[10]. These DGS are used for applications to a standalone [11],
[12], [16], a standby [2], [9], a grid-interconnected [12], a co-
generation [7]–[9], peak shavings [2], etc. and have many ad-
vantages such as environmental-friendly and modular electric
generation, increased reliability, high power quality, uninter-
ruptible service.
DGS CONTROL REQUIREMENTS
In general, the droop technique [16]–[22], [27] has been
widely used as a load-sharing scheme in conventional power
system with multiple generators. In this droop method, the gen-
erators share the system load by drooping the frequency of each
generator with the real power ( ) delivered by the generator.
This allows each generator to share changes in total load in a
manner determined by its frequency droop characteristic and
essentially utilizes the system frequency as a communication
link between the generator control systems [16]–[20]. Simi-
with reactive
larly, a droop in the voltage amplitude
power ( ) is used to ensure reactive power sharing.
CONCLUSION
This paper has described a new load sharing method that can
properly control the load-sharing such as the real, reactive, and
harmonic powers for parallel operation of distributed genera-
tion systems in a standalone ac power system [16]–[22]. The
proposed scheme is implemented by combined droop method,
average power control method, and harmonic sharing control
loop using low bandwidth power information exchange between
each DGS unit. In particular, the average power control method
can significantly reduce the sensitivity about voltage and current
measurement errors, and the harmonic control loop is added to
guarantee harmonic power sharing under nonlinear loads. From
the simulation and experimental results presented, it was shown
that the proposed control method is very effective in maintaining
proper load sharing under different types of loads including
linear and nonlinear loads in standalone ac power system.