High-Efficiency Two-Stage Three-LevelGrid-Connected Photovoltaic Inverter

ABSTRACT:

This paper proposes a high-efficiency two stage three-level grid-connected photovoltaic inverter. The proposed two-stage inverter comprises a three-level step up converter and a three-level inverter. The three-level step up  converter not only improves the power-conversion efficiency by lowering the voltage stress but also guarantees the balancing of the dc-link capacitor voltages using a simple control algorithm; it also enables the proposed inverter to satisfy the VDE 0126-1-1 standard of leakage current. The three-level inverter minimizes the overall power losses with zero reverse-recovery loss.

Furthermore, it reduces harmonic distortion, the voltage ratings of the semiconductor device, and the electromagnetic interference by using a three-level circuit configuration; it also enables the use of small and low cost filters. To control the grid current effectively, we have used a feed-forward nominal voltage compensator with a mode selector; this compensator improves the control environment by presetting the operating point. The proposed high-efficiency two-stage three-level grid-connected photovoltaic inverter overcomes the low  efficiency problem of conventional two-stage inverters, and it provides high power quality with maximum efficiency of 97.4%. Using a 3-kW prototype of the inverter, we have evaluated the performance of the model and proved its feasibility.

KEYWORDS:

  1. Transformerless
  2. Multilevel
  3. Dc-ac power conversion
  4. Single-phase

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Proposed high-efficiency two-stage three-level grid-connected PV inverter circuit diagram.

EXPECTED SIMULATION RESULTS:

 Fig.2. Simulation results for the leakage current of the proposed twostage

inverter.

Fig.3. Simulation results for the leakage current using a conventional three-level step-up converter of Fig. 2(b) as dc-dc power conversion stage of two-stage inverter.

CONCLUSION: 

A high-efficiency two-stage three-level grid-connected PV inverter and control system are introduced. Also, a theoretical analysis is provided along with the experimental results. By using the novel circuit configuration, the proposed two-stage inverter performs power conversion with low leakage current and high efficiency; in dc-dc power conversion stage, the connection of midpoints of capacitors enables the proposed two-stage inverter to limit the leakage current below 300mA; in dc-ac power conversion stage, the overall power losses are minimized by eliminating the reverse-recovery problems of the MOSFET body diodes. Besides, the proposed inverter with three voltage levels reduces the power losses, harmonic components, voltage ratings, and EMI; it also enables using small and low cost filters.

For the control system, the feedforward nominal voltage compensator also improves the control environment by presetting the operating point. This developed control algorithm makes the proposed inverter feasible. Thus, the proposed high-efficiency two-stage three-level grid connected PV inverter provides high power quality with high power-conversion efficiency. By using a 3-kW prototype, this experiment has verified that the proposed inverter has high efficiency, and the developed control system is suitable for the proposed inverter.

REFERENCES:

[1] B.K. Bose, “Global energy acenario and impact of power electronics in 21st century,” IEEE Transactions on Industrial Electronics, vol. 60, no. 7, pp. 2638-2651, July. 2013.

[2] Y. Zhou, D. C. Gong, B. Huang, and B. A. Peters, “The impacts of carbon tariff on green supply chain design,” IEEE Transactions on Automation Science and Engineering, July. 2015. Available: DOI: 10.1109/TASE.2015.2445316

[3] Y. Wang, X. Lin, and M. Pedram, “A near-optimal model-based control algorithm for households equipped with residential photovoltaic power generation and energy storage systems,” IEEE Transactions on Sustainable Energy, vol. 7, no. 1, pp. 77-86, Jan. 2016.

[4] Y. W. Cho, W. J. Cha, J. M. Kwon, and B. H. Kwon, “Improved  single-phase transformerless inverter with high power density and high efficiency for grid-connected photovoltaic systems,” IET Renewable Power Generation, vol. 10, no. 2, pp. 166-174, Feb. 2016.

[5] A. Shayestehfard, S. Mekhilef, and H. Mokhlis, “IZDPWMBased feedforward controller for grid-connected inverters under unbalanced and distorted conditions,” IEEE Trans. Ind. Electron., vol. 64, no. 1, pp. 14-21, Jan. 2017.

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