An Improved Control Scheme for Grid Connected Voltage Source Inverter

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

  • Asiminoaei, L., Teodorescu,R.,Blaabjerg,F., and Borup, U., “A New Method Of On-Line Grid Impedance Estimation for PV Inverter,” in Proc. IEEE APEC, San Diego, CA, Feb. 2004, pp. 1527–1533.
  • Bierhoff, M. H., and Fuchs,F. W., “Active Damping for Three-Phase PWM Rectifiers with High-Order Line-Side Filters,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 371–379, Feb. 2009.
  • Bin Yu and Liuchen Chang, “Improved Predictive Current Controlled PWM for Single-Phase Grid-Connected Voltage Source Inverters,” in Proc. IEEE PESC, 2005, pp. 231 – 236.
  • Blaabjerg, F., Teodorescu, R., and Liserre, M., “Overview of Control and Grid Synchronization for Distributed Power Generation Systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.

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