A Unified Control Strategy for Three-Phase Inverter in Distributed Generation

ABSTRACT:
This paper presents a unified control strategy that enables both islanded and grid-tied operations of three-phase inverter in distributed generation, with no need for switching between two corresponding controllers or critical islanding detection. The proposed control strategy composes of an inner inductor current loop, and a novel voltage loop in the synchronous reference frame. The inverter is regulated as a current source just by the inner inductor current loop in grid-tied operation, and the voltage controller is automatically activated to regulate the load voltage upon the occurrence of islanding. Furthermore, the waveforms of the grid current in the grid-tied mode and the load voltage in the islanding mode are distorted under nonlinear local load with the conventional strategy. And this issue is addressed by proposing a unified load current feedforward in this paper. Additionally, this paper presents the detailed analysis and the parameter design of the control strategy. Finally, the effectiveness of the proposed control strategy is validated by the simulation results.

KEYWORDS:
1. Distributed generation (DG)
2. Islanding
3. Load current
4. Seamless transfer
5. Three-phase inverter
6. Unified control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:
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Fig. 1. Overall block diagram of the proposed unified control strategy.

EXPECTED SIMULATION RESULTS:
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Fig. 2. Simulation waveforms of load voltage vC a , grid current iga, and inductor current iLa when DG is in the grid-tied mode under condition of the step down of the grid current reference from 9 A to 5 A with: (a) conventional voltage mode control, and (b) proposed unified control strategy.

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Fig. 3. Simulation waveforms of load voltage vC a , grid current iga, and inductor current iLa when DG is transferred from the grid-tied mode to the islanded mode with: (a) conventional hybrid voltage and current mode control, and (b) proposed unified control strategy.

CONCLUSION:
A unified control strategy was proposed for three-phase inverter in DG to operate in both islanded and grid-tied modes, with no need for switching between two different control architectures or critical islanding detection. A novel voltage controller was presented. It is inactivated in the grid-tied mode, and the DG operates as a current source with fast dynamic performance. Upon the utility outage, the voltage controller can automatically be activated to regulate the load voltage. Moreover, a novel load current feed forward was proposed, and it can improve the waveform quality of both the grid current in the grid-tied mode and the load voltage in the islanded mode. The proposed unified control strategy was verified by the simulation results.
REFERENCES:
[1] R. C. Dugan and T. E. McDermott, “Distributed generation,” IEEE Ind. Appl. Mag., vol. 8, no. 2, pp. 19–25, Mar./Apr. 2002.
[2] R. H. Lasseter, “Microgrids and distributed generation,” J. Energy Eng., vol. 133, no. 3, pp. 144–149, Sep. 2007.
[3] C. Mozina, “Impact of green power distributed generation,” IEEE Ind. Appl. Mag., vol. 16, no. 4, pp. 55–62, Jul./Aug. 2010.
[4] IEEE Recommended Practice for Utility Interface of Photovoltaic(PV) Systems, IEEE Standard 929-2000, 2000.
[5] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003.

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