ABSTRACT
In grid connected Distribution Generation
systems, Voltage Source Inverters are used for interfacing the renewable energy
source to the utility grid. DG has variety of problems during grid integration.
The power quality problems may cause problems to the industries ranging from
malfunctioning of equipment to complete plant shut down. Disturbances from the
utility grid including voltage sags, harmonics and the grid impedance will
affect the grid connected voltage source inverters connected to the grid. Hence
the control of the grid connected inverter plays an important role in feeding a
grid with high quality power. This report presents an analysis of the stability
problem of a grid connected with Voltage Source Inverter and with a LC filter.
The possible grid-impedance variations have a significant influence on the
system stability. Whenever the grid inductive impedance increases, the low
frequency gain and the bandwidth of the Proportional Integral (PI) controller
have to be decreased to maintain the system stable, thereby degrading the
tracking performance and disturbance rejection capability. To overcome this
problem an H∞ controller is proposed with an explicit
robustness in terms of grid impedance variations to incorporate the desired
tracking performance and stability margin. The proposed method is simulated by
using MATLAB/SIMULINK. The results of the proposed H∞ controller and the
conventional PI controller are compared, which validates the performance of the
proposed control scheme.
KEYWORDS:
1. Distributed Generation (DG)
2. Voltage Source Inverter (VSC)
3. LC Filter
4. H∞ Controller
5. Total Harmonic Distortion (THD).
SOFTWARE: MATLAB/SIMULINK
SIMULINK
BLOCK DIAGRAM:
Figure.1.Overall Simulink Model
EXPECTED SIMULATION RESULTS:
Figure.2.Waveform for
output voltage of PV module
Figure.3.Output current waveform of
overall system
Figure.4.THD analysis with Lg=0.3
mH and rg= 0.2Ω
Figure.5.THD analysis with Lg=0.15
mH and rg=0.2Ω
CONCLUSION
In the grid
connected VSI with LC filters, the possible wide range of grid impedance
variations can challenge the design of the controller, particularly when the
grid impedance is highly inductive. In this project, the suitability of an H∞
controller to get the desired tracking performance and stability margin is
investigated. From the software results it is seen that the grid current THD of
the H∞ controller are always lower than that of the PI controller, which
satisfy the THD requirement of IEEE Std.1547-2003 (i.e.,5%). Further simulation
work is based on demonstrating the operation of a grid in Grid connected mode
and intentional islanded mode. Through this, the system is able to determine
whether or not it is safe to remain connected to the grid. An islanding
detection algorithm is used to act as a switch between the two controllers and
this minimizes the effect of losses in the time of transition, and also to
prevent the undesirable feeding of loads during fault conditions.
REFERENCES
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