High-Frequency AC-Link PV Inverter

ABSTRACT:

In this paper, a high-frequency ac-link photovoltaic (PV) inverter is proposed. The proposed inverter overcomes most of the problems associated with currently available PV inverters. In this inverter, a single-stage power-conversion unit fulfills all the system requirements, i.e., inverting dc voltage to proper ac, stepping up or down the input voltage, maximum power point tracking, generating low-harmonic ac at the output, and input/output isolation. This inverter is, in fact, a partial resonant ac-link converter in which the link is formed by a parallel inductor/capacitor (LC) pair having alternating current and voltage. Among the significant merits of the proposed inverter are the zero-voltage turn-on and soft turn-off of the switches which result in negligible switching losses and minimum voltage stress on the switches. Hence, the frequency of the link can be as high as permitted by the switches and the processor. The high frequency of operation makes the proposed inverter very compact. The other significant advantage of the proposed inverter is that no bulky electrolytic capacitor exists at the link. Electrolytic capacitors are cited as the most unreliable component in PV inverters, and they are responsible for most of the inverters’ failures, particularly at high temperature. Therefore, substituting dc electrolytic capacitors with ac LC pairs will significantly increase the reliability of PV inverters. A 30-kW prototype was fabricated and tested. The principle of operation and detailed design procedure of the proposed inverter along with the simulation and experimental results are included in this paper. To evaluate the long-term performance of the proposed inverter, three of these inverters were installed at three different commercial facilities in Texas, USA, to support the PV systems. These inverters have been working for several months now.

KEYWORDS:

  1. Inverters
  2. Photovoltaic (PV) systems
  3. Zero voltage switching

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Proposed PV inverter.

EXPECTED SIMULATION RESULTS:

Fig. 2. PV current and voltage at full power.

 

Fig. 3. AC-side current and voltage at full power.

Fig. 4. Link voltage at full power.

Fig. 5. Link current at full power.

         Fig. 6. Link current and voltage at full power, using 0.1-μF link capacitance.

Fig. 7. Link current and voltage at 15 kW.

Fig. 8. AC-side current and voltage when the irradiance drops from 850 to

650 w/m2.

Fig. 9. AC-side current and voltage when the temperature changes from

25 C to 50 C.

Fig. 10. AC-side current and voltage when the AC-side voltage drops to 10% of its nominal value (at t = 0.016 s).

Fig. 11. PV current and voltage when the AC-side voltage drops to 10% of its nominal value (at t = 0.016 s).

 CONCLUSION:

In this paper, a reliable and compact PV inverter has been proposed. This inverter is a partial resonant ac-link converter in which the link is formed by an LC pair having alternating current and voltage. The proposed converter guarantees the isolation of the input and output. However, if galvanic isolation is required, the link inductance can be replaced by a singlephase high-frequency transformer. The elimination of the dc link and low-frequency transformer makes the proposed inverter more compact and reliable compared with other types of PV inverters. In this paper, the principle of operation of the proposed converter along with the detailed design procedure has been presented. The performance of the proposed converter has been evaluated through both simulation and experimental results.

 REFERENCES:

[1] S. Chakraborty, B. Kramer, and B. Kroposki, “A review of power electronics interfaces for distributed energy systems towards achieving low-cost modular design,” Renew. Sustain. Energy Rev., vol. 13, no. 9, pp. 2323–2335, Dec. 2009.

[2] Y. Huang, F. Z. Peng, J. Wang, and D. W. Yoo, “Survey of the power conditioning system for PV power generation,” in Proc. IEEE PESC, Jun. 18–22, 2006, pp. 1–6.

[3] S. Atcitty, J. E. Granata, M. A. Quinta, and C. A. Tasca, Utility-scale gridtied PV inverter reliability workshop summary report, Sandia National Labs., Albuquerque, NM, USA, SANDIA Rep. SAND2011-4778. [Online].

Available: http://energy.sandia.gov/wp/wp-content/gallery/uploads/  Inverter_Workshop_FINAL_072811.pdf

[4] Y. C. Qin, N. Mohan, R. West, and R. Bonn, Status and needs of power electronics for photovoltaic inverters, Sandia National Labs., Albuquerque, NM, USA, SANDIA Rep. SAND2002-1535. [Online]. Available: www.prod.sandia.gov/techlib/access-control.cgi/2002/021535. pdf

[5] T. Kerekes, R. Teodorescu, P. Rodríguez, G. Vázquez, and E. Aldabas, “A new high-efficiency single-phase transformerless PV inverter topology,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 184–191, Jan. 2011.

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