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

 

[1] J. M. Shen, H. L. Jou, and J. C. Wu, “Novel transformerless grid connected power converter with negative grounding for photovoltaic generation system,” IEEE Trans. Power Electron., vol. 27, no. 4, pp. 1181–1829, Apr. 2012.

[2] M. Islam, S. Mekhilef, M. Hasan, “Single phase transformerless inverter topologies for grid-tied photovoltaic system: A review,” Renewable and Sustainable Energy Reviews, vol. 45, pp. 69-86, 2015.

[3] S. Kouro, J. I. Leon, D. Vinnikov, and L. G. Franquelo, “Grid-Connected Photovoltaic Systems IEEE Industrial Electronics Magazine,” IEEE Ind. Electron. Magazine, vol. 9, no. 1, pp. 47–61, Mar. 2015.

[4] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind. Applicat., vol. 41, no. 5, pp. 1292-1306, Sep./Oct. 2005.

[5] D. Barater, E. Lorenzani, C. Concari, G. Franceschini and G. Buticchi, “Recent advances in single-phase transformerless photovoltaic inverters,” IET Renew. Power Gener., vol. 10, no. 2, pp. 260-273, Feb. 2016.

 

A New Single-Stage Transformer less Inverter for Photovoltaic Applications

 Abstract

In this paper a new single-stage transformerless inverter for photovoltaic (PV) application is proposed. Typical PV systems have multi-stage topology in order to perform different functions. These multi-stage inverters suffer from low efficiency and high components count. The proposed inverter is a combination of boost converter and half bridge inverter which can handle voltage boosting as well as feeding ac current to the grid in one stage. The introduced converter is transformer less which increases the overall efficiency and decreases cost, size and weight of the system. This topology has several desirable features like low switching losses, low total harmonic distortion (THD) and simple control technique. In this paper, operation principle, theoretical analysis and design of the proposed topology are discussed. Also, simulation and experimental results of a l00W inverter are presented to confirm the validity of theoretical analysis.

Keywords

1.      Photovoltaic inverter

2.      Transformer less inverter

3.      Single-stage inverter

Schematic Diagram:

Fig. 1. Schematic of proposed single-stage transfonnerless inverter.

Expected Simulation Results:

Fig. 2. Simulation results of (a) current of inductors at the peak of output voltage, and (b) voltage and current of switch S ,.

Fig. 3. Simulated waveforms of (a) gate drive signals, and (b) output voltage at fullioad .

Conclusion

In this paper a new single-stage inverter which is suitable for PV application is proposed. Due to omitting galvanic isolation, the cost, size and weight of the system is reduced. Simple structure, low switching losses and easy control scheme are the main features of the presented inverter. The operation principle is disused and the inverter design is presented. Also the control scheme is presented. Simulation and experimental results demonstrates the high performance of the proposed system. Due to boosting ability and achieving low-voltage THD, this topology is suitable for PV grid connected systems.

References

[I] L. Hassaine, E. OLias, J. Quintero and V. Salas, "Overview of power inverter topologies and control structures for grid connected photovoltaic systems," Renewable and Sustainable Energy Reviews, vol. 30, pp. 796-807,2014.

 [2] S. S. Nag and S. Mishra, "A coupled inductor based high boost inverter with sub-unity turns-Ratio range," IEEE Trans. Power Electron., vol. 31, no. ll, pp. 7534-7543,2016.

[3] Y. Zhou and W. Huang, "Single-stage boost inverter with coupled inductor," IEEE Trans. Power Electron., vol. 27, no. 4, pp. 1885-1893, 2012.

[4] A. Darwish, A. M. Massoud, D. Holliday, S. Ahmed and B. W. Williams, "Single-stage three-phase diflerential-mode buck-boost inverters with continuous input current for PV applications," IEEE Trans. Power Electron., vol. 31, no. 12, pp. 8218-8236,2016.

 [5] Y. Zhou, W. Huang, P. Zhao and J. Zhao, "Coupled-inductor singlestage boost inverter for grid-connected photovoltaic system," lET Power Electron., vol. 7, no. 2, pp. 259-270, 2014

A Modular Multiport Power Electronic Transformer with Integrated Split Battery Energy Storage for Versatile Ultra-Fast EV Charging Stations

 Abstract

This paper proposes a power converter architecture for the implementation of an ultra-fast charging station for electric vehicles (EVs). The versatile converter topology is based on the concept of the Power Electronic Transformer (PET). For the direct transformerless coupling to the medium voltage grid, a Cascaded H-Bridge (CHB) converter is utilized. On the level of each submodule, integrated split battery energy storage elements play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The power interface between the stationary split storage stage and the EV batteries is performed through the use of parallel-connected dual half-bridge (DHB) DC/DC converters, shifting the isolation requirements to the medium frequency range. By choosing several different submodule configurations for the parallel-connection, a multiport output concept is achieved, implying the ability to charge several EVs simultaneously without the use of additional high-power chargers. All possible charging station operating modes among with the designed necessary control functions are analyzed. The state of charge (SoC) self-balancing mode of the delta-connected CHB converter is also introduced. Finally, the development of a down-scaled laboratory prototype is described and preliminary experimental results are provided.

Index Terms

1.      DC ultra-fast charging

2.      Electric vehicles (EVs)

3.      Cascaded H-Bridge (CHB) converter

4.      Split battery energy storage

5.      Isolated DC/DC converter

6.      Dual half-bridge (DHB)

7.      Power Electronic Transformer (PET)

8.      State of charge (SoC) balancing

9.      Multiport converter

Block Diagram:

Fig. 1. Block diagrams for (a) branch and (b) submodule balancing of battery

State of Charges.

Expected Simulation Results:

Fig. 2. Simulation results of the M2PET-based ultra-fast EV charging station

for a hypothetical power profile, exploiting all operation modes.

Fig. 3. Detailed versions of the three-phase grid voltage and current quantities

for: (a) Mode I, (b) Mode II, (c) Mode III and (d) Mode IV corresponding to Fig. 5.

 

Fig. 4. Simulation results for the active power distribution within the four charging ports.

Fig. 5. Gain scheduling control behavior of the vertical SoC balancing controller.

Conclusion

This paper has proposed a multiport power electronic transformer-based concept for the realization of multifunctional medium voltage ultra-fast EV charging stations. All system components have been tailored for the specific application and chosen accordingly. A global system control structure has been described, which is capable of handling all different power flow directions as well as capacitor voltage and battery SoC unbalances. The operating modes of the system have been presented and simulated, verifying the versatility of the introduced converter structure. An additional advantageous SoC self-balancing operating mode of the delta-connected CHBBESS unit over the star-connected one has been proposed and discussed. Preliminary results from a down-scaled developed prototype have finally supported the theoretical investigations.

References

[1] S. Haghbin, S. Lundmark, M. Alak¨ula, and O. Carlson, “Grid-Connected Integrated Battery Chargers in Vehicle Applications: Review and New Solution,” IEEE Trans. Ind. Electron., vol. 60, no. 2, pp. 459-473, Feb. 2013.

[2] J.-Y. Lee and H.-J. Chae, “6.6-kW Onboard Charger Design Using DCM PFC Converter With Harmonic Modulation Technique and Two-Stage DC/DC Converter,” IEEE Trans. Ind. Electron., vol. 61, no. 3, pp. 1243- 1252, March 2014.

[3] Z. Amjadi and S.S. Williamson, “Power-Electronics-Based Solutions for Plug-in Hybrid Electric Vehicle Energy Storage and Management Systems,” IEEE Trans. Ind. Electron., vol. 57, no. 2, pp. 608-616, Feb. 2010.

[4] M. Yilmaz and P.T. Krein, “Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles,” IEEE Trans. Power Electron., vol. 28, no. 5, pp. 2151-2169, May 2013.

[5] A. Kuperman, U. Levy, A. Zafransky, and A. Savernin, “Battery Charger for Electric Vehicle Traction Battery Switch Station,” IEEE Trans. Ind. Electron., vol. 60, no. 12, pp. 5391-5399, Dec. 2013.

A Dual-Active-Bridge-based Fully-ZVS HF-isolated Inverter With Low Decoupling Capacitance

Abstract

This paper proposes a novel dual-active bridge (DAB)-based high-frequency-isolated DC-AC converter topology suitable for PV micro inverter applications. The circuit configuration on the secondary sides of the employed three-winding transformer results in high-frequency current injection into each of the switch-nodes, thereby making zero-voltage-switching (ZVS) operation of the devices possible. The topology achieves this functionality while using the same number of devices as conventional two-stage and single-stage high-frequency-link or DAB-based solutions. Further, following a control-oriented power pulsation decoupling strategy involving dynamic variation of phase-shift, twice line frequency energy buffering can be handled on the high-voltage secondary side, thereby resulting in reduction in decoupling capacitance requirement. Working principle of the circuit and the associated control scheme is described followed by detailed ZVS analysis. Simulation studies and experimental tests on a 310 W prototype verify circuit operation and exhibit efficiency improvements compared to a similar two-stage solution.

 

Index Terms

1.      Dual-active-bridge

2.      Micro inverter

3.      Zero-voltage switching (ZVS)

4.      Power decoupling

 

Proposed Diagram:

Fig. 1. The proposed inverter topology in standalone configuration. The topological configuration on the secondary side helps in realizing ZVS.

Expected Simulation Results:

Fig. 2. Transformer voltage and current waveforms (a) over the full line cycle. (b) Near peak of output ac line cycle. Operation corresponds to mode 1b. (c) Near zero crossing of output ac line cycle. Operation corresponds to mode 2a.

Fig. 3. Illustrating occurrence of ZVS for all switches over the full 50 Hz cycle for a unity power-factor load. (a) primary-side switches (b) secondary-side switches not directly connected to the load (c) secondary-side switches directly connected to the load. The instantaneous current through the switches (light blue) and the current at the turn-on instant (dark blue) are shown. It can be observed that the turn-on currents are negative throughout the line cycle indicating ZVS turn-on. It can also be seen that for the secondary-side devices it is more difficult to achieve ZVS near the line-cycle peaks, while for the primary-side devices the same is true near the zero-crossings.

 

 

Fig. 4. Zoomed view of switch currents illustrating occurrence of ZVS under respective worst-case conditions for a unity power-factor load. (a) primary-side switches near zero crossing of vac. (b) secondary-side switches not directly connected to the load near peak of vac. (c) secondary-side switches directly connected to the load near peak of vac.

 

Conclusion

 

A novel dual-active bridge-based isolated micro inverter topology with a current-stiff interface for the PV port is proposed. The key advantage of the proposed solution is its ability to realize ZVS turn-on of all switches across the ac line cycle, unlike conventional two-stage solutions, which miss ZVS over half the ac line cycle. The topology achieves this functionality without using additional circuitry, as is required with ZVT-based solutions. Though the proposed circuit introduces additional conduction losses compared to two-stage topologies, ZVS operation can lead to better efficiency in cases where switching losses are significant, as is experimentally demonstrated. Further, by employing a control strategy involving dynamic variation of phase-shift over an ac line cycle, low-frequency power decoupling can be handled by the high voltage secondary dc bus capacitor. This results in substantial reduction in decoupling capacitance requirement, allowing a film-capacitor-based implementation, which is not possible in standard single-stage topologies.

References

 

[1] S. Chakraborty and S. Chattopadhyay, “A Multi-port, Isolated PV Micro inverter with Low Decoupling Capacitance & Integrated Battery Charger,” IEEE Energy Conversion and Congress Exposition (ECCE) 2016, Sep. 2016.

[2] N. Sukesh, M. Pahlevaninezhad, and P. K. Jain, “Analysis and implementation of a single-stage flyback pv microinverter with soft switching,” IEEE Trans. Ind. Electron., vol. 61(4), pp. 1819-1833, Apr. 2014.

[3] Y. Li and R. Oruganti, “A low cost flyback ccm inverter for ac module application,” IEEE Trans. Power Electron., vol. 27(3), pp. 1295-1303, Mar. 2012.

[4] N. Kummari, S.Chakraborty and S. Chattopadhyay, “An Isolated High- Frequency Link Micro inverter Operated with Secondary-Side Modulation for Efficiency Improvement,” IEEE Trans. Power Electron., vol. 33(3), pp. 2187-2200, Mar. 2018.

[5] S. K. Mazumder and A. K. Rathore, “Primary-Side-Converter-Assisted Soft-Switching Scheme for an AC/AC Converter in a Cyclo converter- Type High-Frequency-Link Inverter,” IEEE Trans. Ind. Electron., vol. 58(9), pp. 4161-4166, Sept. 2011.

 

A 13-levels Module (K-Type) with two DC sources for Multilevel Inverters

Abstract

This paper presents a new reconfiguration module for asymmetrical multilevel inverters in which the capacitors are used as the DC links to creates the levels for staircase waveforms. This configuration of multilevel converter makes a reduction in DC sources. On the other hand, it is possible to generate 13 levels with lower DC sources. The proposed module of multilevel inverter generates 13 levels with two unequal DC sources (2VDC and 1VDC). It also involves two chargeable capacitors and 14 semiconductor switches. The capacitors are self-charging without any extra circuit. The lower number of components makes it desirable to use in wide range of applications. The module is schematized as two back-to-back T-type inverters and some other switches around it. Also, it can be connected as cascade modular which lead to a modular topology with more voltage levels at higher voltages. The proposed module makes the inherent creation of the negative voltage levels without any additional circuit (such as H-bridge circuit). Nearest level control switching modulation (NLC) scheme is applied to achieve high quality sinusoidal output voltage. Simulations are executed in MATLAB/Simulink and a prototype is implemented in the power electronics laboratory which the simulation and experimental results show a good performance.

Index Terms

1.      Asymmetric

2.      Capacitors

3.      Multilevel inverter

4.      Power electronics

5.      Self-charging

6.      Nearest level control switching

Block Diagram:

Fig.1 The general conceptual asymmetric MLIs with capacitors

Expected Simulation Results:

Fig.2 The waveform of output Voltage (simulation) for the proposed module: (a) Waveform (b) Harmonics spectrums.

Fig.3 The waveform of output Voltage (simulation) for the first cascade topology (25 Levels): (a) Waveform (b) Harmonics spectrums.

Fig.4 The waveform of output Voltage (simulation) for the second cascade topology (169 Levels): (a) Waveform (b) Harmonics spectrums.

Conclusion

This paper introduced one module for asymmetrical multilevel inverter to produce 13 levels by two DC sources. The proposed multilevel is designed based on two back to back TType modules with some switches around them. The proposed module is named K-Type. The configuration of K-type provides two extra DC links by capacitors (as the virtual DC supply) to achieve more levels to create staircase waveform. The module needs lower components including two DC sources, two capacitors, 14 semiconductors. It can be used in power applications with unequal DC sources (with ratio 1:2). It can also be easily modularized in two strategies in cascade arrangements to form high voltage outputs with low stress on semiconductors and lowering the number of devices. This ability can be used in some special applications such as solar farm along with a lot of DC sources. DC sources can also have different voltage amplitudes. In the conventional methods, it should be considered one inverter for each DC resources and fix the output voltage the same amplitude. It increases complexity and losses from this aspect, but in asymmetrical multilevel converters, it is possible to combine some DC resources together and generate a unique AC output. It reduces the number of separated inverter, components, losses and etc. The other advantage of K-Type module is its capability to generate both positive and negative output voltage without any additional circuit. Module is tested and it shows a good performance. THD% for one module is obtained 3.87% and 4.07% in simulation and experimental results, respectively that satisfy harmonics standard (IEEE519). THD% for cascade connection (two module) is calculated 1.99% in simulation and 2.26% in experimental results.

References

[1]Essakiappan, S.; Krishnamoorthy, H.S.; Enjeti, P.; Balog, R.S.; Ahmed, S., "Multilevel Medium-Frequency Link Inverter for Utility Scale Photovoltaic Integration," in Power Electronics, IEEE Transactions on , vol.30, no.7, pp.3674-3684, July 2015

[2]A. Nami, J. Liang, F. Dijkhuizen and G. D. Demetriades, "Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities," in IEEE Transactions on Power Electronics, vol. 30, no. 1, pp. 18-36, Jan. 2015.

[3]I. A. Gowaid, G. P. Adam, A. M. Massoud, S. Ahmed and B. W. Williams, "Hybrid and Modular Multilevel Converter Designs for Isolated HVDC–DC Converters," in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 6, no. 1, pp. 188-202, March 2018.

 [4] Xibo Yuan, "A Set of Multilevel Modular Medium-Voltage High Power Converters for 10-MW Wind Turbines," in Sustainable Energy, IEEE Transactions on , vol.5, no.2, pp.524-534, April 2014

 [5] Ahmadi, D.; Jin Wang, "Online Selective Harmonic Compensation and Power Generation With Distributed Energy Resources," in Power Electronics, IEEE Transactions on , vol.29, no.7, pp.3738-3747, July 2014

A 10kW Solar-Powered Bidirectional EV Charger Compatible with Chademo and COMBO

Abstract

 Charging electric vehicles (EVs) from photovoltaic panels (PV) provides a sustainable future for transportation. This paper presents the development of a 10kW EV charger that can be powered from both a PV array and the three-phase AC grid. The goal is to realize a high power density and high-efficiency three-port power converter that integrates the EV, PV, grid and meets the Chademo and CCS/Combo EV charging standards. The EV port is designed to be isolated and bidirectional, so that both charging and vehicle-to-Grid (V2G) can be implemented. As PV and EV are both DC by nature, the converter uses a central DC-link to exchange power between the EV and PV, thereby increasing efficiency. The use of silicon carbide devices and powdered alloy core inductors enables high switching frequency and power density. The closed-loop control allows four different power flows: PVàEV, EVàgrid, gridàEV and PVàgrid. Hence the converter operates as a PV inverter, a bidirectional EV charger and a combination of both. A 10kW prototype has been successfully tested, and its experimental waveforms and measured efficiency are presented. It has three times the power density and higher partial and peak load efficiency when compared to existing solutions.

Index Terms

1.      Electric vehicle

2.      Charging

3.      powered alloy core

4.      photovoltaic systems (PV)

5.      silicon carbide (SiC)

6.      Paper produced in Windows/Microsoft Word

Block Diagram:

Fig. 1 – Block diagram of the grid connected bidirectional 10kW three–port EV-PV charger

Expected Simulation Results:

 

 

Fig. 2 – (a) Waveforms for the PV IBC for the phase shifted gate voltage VGS, Inductor current IL and MOSFET drainsource voltage Vds for (a) CCM mode (VPV=700 V, IPV=10 A); (b) DCM mode (VPV=400 V, IPV=10.75 A)

Fig. 3 – Drain-source voltage Vds and gate voltage Vgs for one phase of the IBFC for CH: (a) Quasi resonant operation LVS for Vev=250V, Iev=5A (b) Valley skipping and DCM operation at low powers for Vev=100V, Iev=1A

Conclusions

 

This paper presents the development of a 10kW, three-port, bidirectional converter for direct DC charging of EV from PV. The developed converter is compatible with CCS and Chademo EV charging standard and can operate with a PV array of wide voltage and power range. Interleaving of converters, Silicon carbide (SiC) devices, and powdered alloy core inductors are extensively used to increase the switching frequency, while keeping the converter losses within limits. This has helped to increase the power density by a factor of three when compared to conventional designs and reduce the voltage ripple at the EV, PV ports. The converter is modularly designed with three sub-converters connected on a 750V central DC-link: interleaved boost converter for PV, a three-phase inverter for the AC grid and an interleaved flyback converter for EV. While the flyback is traditionally considered suitable only for low powers, this paper shows how the use of SiC devices in a QR mode flyback converter can achieve high efficiency even at high powers. Three closed loop controls were developed and tested for the three sub-converter which enables four power flows: PVàEV, EVàgrid, gridàEV and PVàgrid. A 10kW prototype was built and tested and exhibits a peak efficiency of 95.2% for PVàEV, 95.4% for GridàEV, 96.4% for PVàGrid. The developed prototype has a much higher peak efficiency, higher partial load efficiency and three times higher power density than currently existing solutions based on AC power exchange. The charge and V2G operation at 10kW were tested with a Nissan Leaf EV with a Chademo charge controller.

References

[1] “Efficiencies and CO2 emissions from electricity production in the Netherlands, 2012 update,” Cent. Bur. Stat. - Netherlands, 2014.

[2] G. R. Chandra Mouli, P. Bauer, and M. Zeman, “System design for a solar powered electric vehicle charging station for workplaces,” Appl. Energy, vol. 168, pp. 434–443, Apr. 2016.

[3] G. R. Chandra Mouli, P. Bauer, and M. Zeman, “Comparison of system architecture and converter topology for a solar powered electric vehicle charging station,” in 2015 9th International Conference on Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015, pp. 1908–1915.

[4] D. P. Birnie, “Solar-to-vehicle (S2V) systems for powering commuters of the future,” J. Power Sources, vol. 186, no. 2, pp. 539–542, Jan. 2009.

[5] P. Denholm, M. Kuss, and R. M. Margolis, “Co-benefits of large scale plug-in hybrid electric vehicle and solar PV deployment,” J. Power Sources, vol. 236, pp. 350–356, 2013.