ABSTRACT:
Most
common types of distributed generation (DG) systems utilize power electronic
interfaces and, in particular, three-phase
voltage source converters (VSCs) which are mainly used to regulate active and reactive power
delivered to the grid. The main drawbacks of VSCs originate from their
nonlinearities, control strategies, and lack of robustness against
uncertainties. In this paper, two time-scale separation redesign technique is proposed
to improve the overall robustness of VSCs and address the issues of
uncertainties. The proposed controller is applied to a grid-connected Solid
Oxide Fuel Cell (SOFC) distributed generation system to recover the
trajectories of the nominal system despite the presence of uncertainties.
Abrupt changes in the input dc voltage, grid-side voltage, line impedance and PWM
malfunctions are just a few uncertainties considered in our evaluations.
Simulation results based on detailed model indicate that the redesigned system
with lower filter gain (_) achieves more reliable performance in compare to the
conventional current control scheme. The results also verified that the
redesigned controller is quite successful in improving the startup and tracking
responses along with enhancing the overall robustness of the system.
KEYWORDS:
1. Power
converters
2. Solid
oxide fuel cell (SOFC)
3. Distributed
generation (DG)
4. Time-scale
separation redesign
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Schematic diagram of a grid-connected SOFC power plant with redesigned
controller.
EXPECTED SIMULATION RESULTS:
Fig.
2. Active (top) and reactive (bottom) output power in case 1 (input dc
voltage)
uncertainty using PI and redesigned controller.
Fig.
3. Output voltage (top) and current (bottom) of each SOFC array in case
1
(input dc voltage) uncertainty using PI and redesigned controller.
Fig.
4. Active (top) and reactive (bottom) output power in case 2 (grid-side
voltage)
uncertainty using PI and redesigned controller.
Fig.
5. d-axis (top) and q-axis (bottom) currents of the VSC in case 2 (gridside
voltage)
uncertainty using PI and redesigned controller.
Fig.
6. Active (top) and reactive (bottom) output power in case 3.1 (line
resistance)
uncertainty using PI and redesigned controller.
Fig.
7. Active (top) and reactive (bottom) output power in case 3.2 (line
inductance)
uncertainty using PI and redesigned controller.
Fig.
8. Additive random Gaussian noises on duty ratio of phase a (top), b
(middle),
and c (bottom) of the VSC.
Fig.
9. Active (top) and reactive (bottom) output power in case 4 (duty
ratio)
uncertainty using PI and redesigned controller.
CONCLUSION:
This
paper presents a new control technique based on two time-scale separation
redesign for the VSC of a grid connected SOFC DG system. A three-phase VSC is
used to regulate active and reactive power delivered to the grid. In addition,
variations in the input dc voltage, line impedance, grid-side voltage and duty
ratio are mathematically formulated as additive uncertainties based on the
nonlinear model of the VSC. As a result, the proposed controller is able to
address the issues of robustness and further enhance the system stability in
the presence of uncertainties. The redesigned controller also presents a fast
and accurate startup response and delivers superior decoupling performance as
compared to the conventional PI controller. Moreover, the redesigned controller
significantly reduces the maximum overshoot in the output power while the system
with a conventional controller exhibits deterioration in the output response
which leads to excessive current and voltage variations in the FC arrays.
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