A Fuzzy Logic Controller for Autonomous Operation of a Voltage Source Converter-Based Distributed Generation System

ABSTRACT

This paper presents a fuzzy logic controller (FLC) for autonomous (islanded) operation of an electronically interfaced distributed generation unit and its load. In the grid connected mode, the voltage-sourced converter is operated in the active and reactive power (PQ) control mode, where a conventional control scheme is used to control the active and reactive power exchange with the grid. In the islanded mode, the proposed FLC is used to control the voltage of the islanded system despite the load variability and uncertainties. In addition, this paper also presents the use of the black-box nonlinear optimization technique to tune the parameters of the membership functions of the FLC in order to achieve optimal performance. The salient features of the proposed FLC are: 1) efficient to deal with the nonlinear systems; 2) design does not depend on the mathematical model of the system; and 3) less sensitive to the parameters variation than the conventional controllers. The frequency of the islanded system is dictated through the use of an internal oscillator. The effectiveness of the proposed FLC in controlling the voltage of the islanded system, irrespective of the load variability, is extensively validated based on simulation studies in the PSCAD/EMTDC environment. Moreover, the paper highlights the superiority of the proposed FLC over the conventional proportional-integral controllers through comparing the transient responses of the system based on both controllers.

KEYWORDS

1.     Autonomous operation

2.     Distributed generation (DG)

3.      Fuzzy logic controller (FLC)

4.      Voltage source converter

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Single line diagram of the DG system

 EXPECTED SIMULATION RESULTS

Fig. 2. Responses for transition from grid-connected to islanded mode. (a) V_d. (b) V_q. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC.

Fig. 3 Responses for the change of the load resistance R from 76 to 152 Ω(a) V_d. (b) V_q. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC.

Fig. 4. Responses for the change of the load resistance R from 76 to 304 Ω(a) V_d. (b) V_q. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC.

Fig. 5. Responses for change of the load inductance L from 111.9 to 222 mH. (a) V_d. (b) V_q. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC.

Fig. 6. Responses for connecting a nonlinear load in parallel to the load. (a) V_d. (b) V_q. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC.

Fig. 7. Responses for connecting a three-phase induction motor in parallel to the load. (a) Vd. (b) Vq. (c) Load voltage (rms). (d) Load real power. (e) Load reactive power. (f) Three-phase load voltage using FLC. (g) Three-phase converter currents using FLC. (h) Motor speed. (i) Developed torque.

CONCLUSION

This paper has presented the application of a FLC to the autonomous operation of an electronically coupled DG unit and its local load with the purpose of achieving better transient responses despite the load variability and uncertainty. A detailed dynamic model of the system under study and the control strategy are investigated. The system performance using the FLC is evaluated through the following case studies:

1) transition from the grid-connected mode to the islanded mode;

2) change of the RLC load parameters;

3) switching of a nonlinear load;

4) motor energization.

The simulation results have shown that the system performance using the proposed FLC has a better damped response and a faster transient behavior in comparison with that obtained using the conventional PI controller. It can be concluded that the FLC guarantees a robust stability and efficient performance irrespective of the load uncertainty.

REFERENCES

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