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.

A Switching Control Strategy for Single and Dual Inductor Current-Fed Push-Pull Converters

 

ABSTRACT:

A switching control strategy is proposed for single and dual inductor current-fed push-pull converters. The proposed switching control strategy can be used with both current-fed push-pull converters with an active voltage doubler rectifier, or active rectifier, in the secondary side of the isolation transformer. The proposed switching control strategy makes turn-on and turn-off processes of the primary side power switches zero-voltage-switching and zero-current-switching respectively. The soft-switching operation of the single and dual inductor push-pull converters, with both types of active rectifier, is explained. Simulation and experimental results are provided to validate soft switching operation of the current-fed push-pull converters with the proposed switching control strategy.

 KEYWORDS:

  1. Single and dual inductor current-fed push-pull converter
  2. Active rectifier
  3. Zero-voltage-switching
  4. Zero-current-switching.

 SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1. (a) Dual, and (b) single inductor CFPP converters with the secondary side voltage doubler rectifier.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. U1 current (A) of dual inductor CFPP converter along with its switch control signal and corresponding secondary side switch (U4) control signal.

Fig. 3. Drain to source (VDS) voltage (V) of U1 and scaled (100:1) gate control signal of dual inductor CFPP converter.

 Fig. 4. U1 current (A) of single inductor CFPP converter and control signal.

Fig. 5. Drain to source voltage (VDS) (V) and gate control signal (200:1) of U1 of single inductor CFPP converter.

 

Figure 6. U1 current (A) of dual inductor CFPP converter along with its switch control signal and corresponding secondary side switch (U4) control signal in mode II.

Fig. 7. U1 current (A) of single inductor CFPP converter along with its switch control signal in mode II.

CONCLUSION:

A switching control strategy is proposed for single and dual inductor CFPP converters with secondary side active rectifiers. The primary side power switches are turned-on and turned-off with ZVS and ZCS respectively with the help of synchronous operation of the secondary side power switches. The gain improvement of CFPP converters due to the proposed switching strategy is revealed under two modes of operation with similar switching characteristics. The performance dependency of the switching control strategy on the leakage inductance of the isolation transformer is critically analyzed. A detailed theoretical analysis is provided assuming ideal circuit conditions. Simulation and experimental results are provided to further validate the operation of the proposed switching control strategy.

REFERENCES:

[1] M. Dale and S. M. Benson, ―Energy balance of the global photovoltaic (PV) industry – is the PV industry a net electricity producer?,‖ Environ. Sci. Technol, vol. 47, no. 7, pp. 3482–3489, 2013.

[2] C. Olalla, D. Clement, M. Rodriguez, and D. Maksimovic, ―Architectures and control of submodule integrated DC-DC converters for photovoltaic applications,‖ IEEE Trans. Power Electron., vol. 8, no. 6, pp. 2980–2977, 2013.

[3] L. Bangyin, D. Shanxu, and C. Tao, ―Photovoltaic DC-building-module-based BIPV system—concept and design considerations,‖ IEEE Trans. Power Electron., vol. 26, no. 5, pp. 1418–1429, May 2011.

[4] D. D. Lu and V. G. Agelidis, ―Photovoltaic-battery-powered DC bus system for common portable electronic devices,‖ IEEE Trans. Power Electron., vol. 24, no. 3, pp. 849–855, 2009.

[5] K.-C. Tseng, J.-T. Lin, and C.-C. Huang, ―High step-up converter with three-winding coupled inductor for fuel cell energy source applications,‖ IEEE Trans. Power Electron., 2014.

Zero-Voltage Switching Galvanically Isolated Current-Fed Full-Bridge DC-DC Converter

ABSTRACT:

This paper presents a new soft-switching technique for the current-fed full-bridge DC-DC converter that enables zero voltage switching of the input side inverter switches. To achieve this, the secondary side voltage doubler rectifier has to be realized with active switches. Two control channels synchronous with the control signals of the inverter switches are added for driving those switches. Zero voltage switching achieved is assisted with the body diodes that conduct current during soft-switching transients as a result of the leakage inductance current shaping from the secondary side. Moreover, the converter does not suffer from voltage overshoots thanks to natural clamping from the secondary side. Theoretical predictions were verified with simulation.

KEYWORDS:

  1. Zero-voltage switching
  2. Current-fed DC-DC converter
  3. Full-bridge
  4. Soft-switching
  5. Switching` control method

 SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1. Galvanically isolated full-bridge current-fed DC-DC converter with controlled output rectifier stage.

EXPECTED SIMULATION RESULTS:

 Fig. 2. Simulated current and voltage waveforms along with control signals of the input and output side switches.

Fig. 3. Experimental current and voltage waveforms.

CONCLUSION:

The novel ZVS technique intended for the galvanically isolated full-bridge current-fed DC-DC converter with the controlled output rectifier stage were presented. It enables full ZVS in the input side current-fed inverter assisted with the leakage inductance and body diodes. Moreover, partial ZCS is provided in the secondary side assisted with the leakage inductance. Simulation study corroborates the theoretical predictions made. Experimental prototype operation was quite similar to the simulation model created in PSIM. Nevertheless, the prototype features oscillations caused by parasitic elements of the circuit and reverse recovery of the body diodes the input side MOSFETs. Further research will be aimed towards derivation of design guidelines that take into account reverse recovery effect and, consequently, result in high efficiency and low parasitic oscillations.

REFERENCES:

[1] Blaabjerg, F.; Zhe Chen; Kjaer, S.B., “Power electronics as efficient interface in dispersed power generation systems,” IEEE Transactions on Power Electronics, vol. 19, no. 5, pp. 1184-1194, Sept. 2004.

[2] Kouro, S.; Leon, J.I.; Vinnikov, D.; Franquelo, L.G., “Grid-Connected Photovoltaic Systems: An Overview of Recent Research and Emerging PV Converter Technology,” IEEE Industrial Electronics Magazine, vol. 9, no. 1, pp. 47-61, March 2015.

[3] Rathore, A.K.; Prasanna, U., “Comparison of soft-switching voltage-fed and current-fed bi-directional isolated Dc/Dc converters for fuel cell vehicles,” in Proc. ISIE’2012, pp. 252-257, 28-31 May 2012.

[4] Prasanna, U.R.; Rathore, A.K., “Extended Range ZVS Active-Clamped Current-Fed Full-Bridge Isolated DC/DC Converter for Fuel Cell Applications: Analysis, Design, and Experimental Results,” IEEE Transactions on Industrial Electronics, vol. 60, no. 7, pp. 2661-2672, July 2013.

[5] Iannello, C.; Shiguo Luo; Batarseh, I., “Small-signal and transient analysis of a full-bridge, zero-current-switched PWM converter using an average model,” IEEE Transactions on Power Electronics, vol.18, no.3, pp.793-801, May 2003.