A New Transformer-less Five-level Grid-Tied Inverter for Photovoltaic Applications

 Abstract

 A new fundamental structure of a single-phase transformer-less grid connected multilevel inverter based on a switched-capacitor structure is presented in this study. By employing the series-parallel switching conversion of the integrated switched-capacitor module in a packed unit, attractive features for the proposed inverter can be obtained such as high efficiency and boosting ability within a single stage operation. Also, using a common grounding technique provides an additional advantage of reducing the leakage current. Moreover, the presented structure generates a multilevel waveform at the output voltage terminals which reduces the harmonics in the system. A peak current controller is utilized for triggering the gate of the power switches and controlling both the active and reactive powers. This results in a tightly controlled current with an appropriate quality that can be injected to the grid using a single source renewable energy resource. Operating procedures, design considerations, comparison studies and test results of a 620 W prototype are also presented to validate the accuracy and feasibility of the proposed multilevel inverter.

Index Terms

1.      Grid connected multilevel inverter

2.      Leakage current elimination

3.      Transformer-less inverter

4.      Switched-capacitor based structure

Proposed Diagram:

Fig. 1: Proposed grid connected inverter topology.

Expected Simulation Results:

Fig. 2: The Experimental results (a) output voltage waveform of inverter (200 V/div) and the injected current (4 A/div) (b) Inverter output voltage waveform (200 V/div) and the local grid voltage (200 V/div)

Fig. 3: (a) Voltage across of C1 (100 V/div) (b) Voltage across of C2 (200V/div)

Fig. 4: The Grids voltage (100 V/div) and the injected current (5A/div) waveforms under (a) Leading PF (b) Lagging PF (c) Unity PF

Conclusion

 

A new topology of the single-phase grid-tied inverter has been presented in this study. The proposed topology benefits the series-parallel switching technique of capacitors and offers both boosting ability and common ground capability. Also, low total harmonic distortion is achieved through generating multilevel waveform at output voltage terminal of the proposed inverter. The capacitors employed in the SC module of the proposed inverter are balanced well by series-parallel switching conversion and handle the single stage power boosting process in the positive and negative half-cycle of the grid frequency. Regarding the analyzed PCC technique, a tightly controlled current through a small size inductor-based filter can be injected into the grid under any demanded PF. Additionally, since the null of the grid and the negative terminal of the input source (PV panel) are commonly grounded, the problem of leakage current issues is eliminated completely. Moreover, design consideration and loss analysis of the involved switches have been developed in this study. Finally, the feasibility and advantages of the proposed topology are compared with some recently grid-tied and 5-level structures and experimental results verified the feasibility and meritorious performance of the proposed inverter.

References

 

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