A Novel Unified Controller for Grid-Connected and Islanded Operation of PV-Fed Single-Stage Inverter

 ABSTRACT:

 This paper presents a novel robust current droop controller (RCDC) using a single droop loop. This scheme is unified supporting dual mode of operation for micro-grids (MGs), including grid connected mode (GCM) and islanded mode (ISM) while ensuring seamless transition between the two modes with proportional power sharing maintained. The proposed controller is further incorporated with an improved maximum power point tracking (MPPT) technique presented for the parallel operation of single-stage inverters fed by multi-string PV array topology. In addition, an improved phase-locked-loop-less (PLL-less) method is presented supporting self-synchronization strategy of the parallel operation of PV-inverters with the main grid while maintaining the full capabilities of the unified control architecture. This obviates the usage of conventional PLLs, which are widely used with active synchronization techniques. The performance of the proposed control scheme is validated using real time simulations (RTS) developed by dSPACE MicroLabBox.

KEYWORDS:

1.      Smart Grid

2.      Distributed Generation

3.      Unified Droop Controller

4.      Self-Synchronization

5.      MPPT

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1 Typical block diagram of the MG under study. 

EXPECTED SIMULATION RESULTS:

Fig. 2 Measured PV power under varied irradiation and temperature.

Fig. 3 Regulated DC-link voltage using the proposed MPPT algorithm.

Fig. 4 AC current set point controlled by DC voltage regulator under varied irradiation and temperature.

Fig. 5 Output active power under varied insolation and temperature level.

Fig. 6 Output reactive power under varied irradiation and temperature.

Fig. 7 Output peak currents proportionally shared between DG1 and DG2.

Fig. 8 Measured direct and quadrature axes voltages at the PCC.

 

CONCLUSION:

 This paper presents an enhanced RCDC with a simplified MPPT algorithm and PLL-less SSM for the parallel operation of single-stage inverters fed by multi-string PV arrays. The modified RCDC-QPR scheme efficiently operated in both ISM and GCM without observing any resonance effect. Only one AC current sensor has been used by the simplified P&O algorithm enhancing the design simplicity, while precisely tracking the MPP under varied insolation and temperature. Besides, good dynamic response has been resulted with tenuous perturbations owing to the adopted compromise between the P&O sampling rate and incremental voltage steps. Prominently, the design has been freely built with no regards to the operating mode, accepting the last current setpoint identified by the MPPT upon islanding with no need of setpoint manipulations thanks the flexibility of the proposed unified current controller. Depending on the proposed PLL-less SSM, the active synchronization has been successfully achieved without using dedicated PLLs or communication links between inverters. This offers a simpler design obviating the complex tuning and stability issues of normal PLLs. The general guidelines for tuning the PLL-less detector have been also outlined through quantitative analysis. The simplified MPPT and proposed PLL-less detector have both enriched the control flexibility of the original RCDC-QPR controller supporting dual mode of operation with seamless transition between these two modes, while exporting the extra PV power the main grid during GCM. Design competency has been validated using real time simulations developed by dSPACE MicroLabBox.

 REFERENCES:

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[3] M. M. Hanif, “Investigation to Improve the Control and Operation of a Three-phase Photovoltaic Grid-tie Inverter,” PhD Thesis, Dublin Institute of Technology, 2011.

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