**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.

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