一种新型单相五电平无变压器光伏逆变器

中国电工技术学会成立40周年华诞

一种新型单相五电平无变压器光伏逆变器

A Novel Single-Phase Five-Level Transformer-less Photovoltaic (PV) Inverter

Author：Xiaonan Zhu; Hongliang Wang; Wenyuan Zhang; Hanzhe Wang; Xiaojun Deng; Xiumei Yue

DOI: 10.30941/CESTEMS.2020.00040

https://ieeexplore.ieee.org/document/9312762

01

Abstract

  Multilevel inverters are preferred solutions for photovoltaic (PV) applications because of lower total harmonic distortion (THD), lower switching stress and lower electromagnetic interference (EMI). In order to reduce the leakage current in the single-phase low-power PV inverters, a five-level transformer-less inverter is proposed in this paper. A total of eleven switches are required, while six of them only withstand a quarter of the dc-bus voltage, so the costs for them are low. Another four switches are turned on or off at the power line cycle, the switching losses for them are ignored. In addition, the flying-capacitors (FCs) voltages are only a quarter of the dc-bus voltage, and they are balanced at the switching frequency, which further reduces the system investment. The experimental results based on a 1 kW prototype show that the proposed modulation strategy can balance the FCs voltages at Vdc/4 very well. And the leakage current can be reduced to about 27 mA under both active and reactive operations, which satisfies the VDE 0126-1-1 standard.

02

Innovation

   In this paper, a single-phase five-level transformer-less inverter is proposed for leakage current reduction. Firstly, the RMS value of leakage current is only about 27 mA, which complies with the VDE0126-1-1 standard. Secondly, This topology combines the advantages of high voltage devices and low voltage devices. The switch with voltage stress of V_dc/4 operates at high frequency, while the device with voltage stress of V_dc operates at power line frequency, so the total loss is low.

03

Main content

  Fig. 1 shows the schematic of the proposed inverter. For the proposed inverter, five-level voltages, which are Vdc, Vdc/2, 0, −Vdc/2, and −Vdc, can be obtained. Notably, in order to achieve a constant CM voltage, the voltages of C1 and C2 are charged to a quarter of the dc-bus voltage (Vdc/4) firstly. And then they can be balanced constant by the proposed modulation strategy within one switching period. Thus, the investment cost of FCs may be lower than that of conventional FC-based topologies.

Fig. 1. Proposed single-phase five-level inverter topology.

Fig. 2. Modulation method for the proposed inverter.

   The modulation method for the proposed inverter under both active and reactive operations is shown in Fig. 2. During zone Z1, the switching sequence is (BDCD). During zone Z2, the switching sequences are (BDCD) and (ABAC). During zone Z3, the switching sequence (FEGE) is acquired. During zone Z4, the switching state rotates in the sequence of (FHGH) and (FEGE).

Fig. 3.  Waveforms of V_AN, V_BN, V_CM and i_leakage

    Fig. 3 shows the CM characteristics of the proposed inverter. From top to bottom are bridge voltage VAN, CM voltage VCM, bridge voltage VBN and the leakage current ileakage. The two bridge voltages have five voltage steps, namely, 0, 90, 180, 270, and 360 V. Moreover, VCM is almost constant within the entire period of the utility grid with little power frequency fluctuation. The RMS value of leakage current is only about 27 mA, which complies with the VDE0126-1-1 standard.

Fig. 4. Measured efficiency.

  The efficiency curve measured in the experiment is shown in Fig. 4. As we can see, the efficiency is increase with the increasement of output power. And the measured maximum efficiency is about 96.02% when the power is about 820 W.

04

Conclusion

 In this paper, a single-phase five-level transformer-less inverter and its modulation strategy for the PV systems are proposed. It adopts the symmetrical filter inductor configuration. The difference from the traditional FC-based topologies is that the FCs voltages are controlled at V_dc/4. Through the combination of dc-bus voltage and FCs voltages, the CM voltage is theoretically maintained at a constant value during the whole power frequency of unite grid, and then the leakage current is reduced. The two FCs voltages can be balanced at V_dc/4 automatically at the switching frequency through the selection of the redundant switching states. Finally, the volume and investment cost of the FCs are decreased. The theoretical analysis and experimental verifications are presented. In conclusion, the proposed topology and modulation strategy can ensure a constant CM voltage without any high-frequency components throughout the power frequency cycle. Consequently, the leakage current can be significantly reduced below 300 mA, which meets the specification in the standard VDE-0126-1-1.

引用本文

X. Zhu, H. Wang, W. Zhang, H. Wang, X. Deng and X. Yue, "A novel single-phase five-level transformer-less photovoltaic (PV) inverter," in CES Transactions on Electrical Machines and Systems, vol. 4, no. 4, pp. 329-338, Dec. 2020, doi: 10.30941/CESTEMS.2020.00040.

本文作者

Xiaonan Zhu (S’19) was born in Henan, China, 1995. He received the B.S. degree in electrical engineering from the China University of Petroleum, Qingdao, China, in 2017. He is currently working toward the Ph.D. degree in power electronics with the College of Electrical and Information Engineering, Hunan University.

His current research interests include the power converter design, analysis and modulation techniques, grid integration of renewable energy, and control algorithms in power electronics.

Hongliang Wang (M’12-SM’15) received the B.Sc. in Electrical Engineering from Anhui University of Science and Technology, Huainan, China in 2004, and received the Ph.D. degree in Electrical Engineering from Huazhong University of Science and Technology, Wuhan, China in 2011.

From 2004 to 2005, he worked as an Electrical Engineer at Zhejiang Hengdian Thermal Power Plant. From 2011 to 2013, he worked as a Senior System Engineer at Sungrow Power Supply Co., Ltd. From 2013-2018, He worked as a Post-Doctoral Fellow at Queen’s University. Since 2018, he has been with Hunan University, where he is currently a Full Professor with the College of Electrical and Information Engineering.

His current research interests include multilevel topology, High-gain topology, parallel technology and Virtual Synchronous Generator (VSG) technology for photovoltaic application and micro-grids application; resonant converters and server power supplies, and LED drivers. He has authored over 60 technical papers in the Journals and conferences.

   He is the inventor/co-inventor of 42 China issued patents, 8 US issued patents. He is currently a senior member of China Electro-Technical Society (CES), a senior member of China Power Supply Society (CPSS). He serves as a member of CPSS Technical Committee on Standardization; a member of CPSS Technical Committee on Renewable Energy Power Conversion, a China Expert Group Member of IEC Standard TC8/PT 62786, a Vice-Chair of IEEE Kingston Section, a Session Chair of ECCE 2015 and ECCE2017, a TPC member of ICEMS2012.

Wenyuan Zhang rceived the M.Sc. degree in electrical engineering from the Guangxi University, Nanning, China, in 2017. From 2017 to 2019, he worked as an electrical engineer of CRRC Dalian R & D Co., Ltd. He is currently pursuing the Ph.D. degree in electrical engineering with the College of Electrical and Information Engineering, Hunan University, Changsha, China.

His current research interests include digital control techniques, modulation strategies, topology research of multilevel converters and renewable energy systems.

Hanzhe Wang received the B.S and M.S degree in electrical engineering from China University of Mining and Technology, Xuzhou, Jiangsu, China in 2016 and 2019 respectively. He is currently working toward the Ph.D. degree in power electronics at Hunan University, Changsha, Hunan, China.

His current research interests include digital control techniques, modulation strategies, multilevel topology of inverter for photovoltaic application and renewable energy systems control.

Xiaojun Deng received the B.S. degree in electrical engineering from China University of Mining and Technology, Xuzhou, China, in 2019. He is currently working toward the Ph.D. degree in power electronics with Hunan University, Changsha, China.

His research interests include power electronics, multilevel converter in renewable energy, and control algorithms.

Xiumei Yue received the B.Sc. in Electrical Engineering from Anhui University of Science and Technology, Huainan, China in 2004, and received M.Sc. degree in Electrical Engineering from Huazhong University of Science and Technology, Wuhan, China in 2007. From 2007 to 2011, she worked at Hubei Polytechnic University. From 2011 to 2014, she worked as a Senior Intellectual Property Engineer in Sungrow Power Supply Co., Ltd. From 2014-2017, she worked in Queen’s University in Canada. Since 2018, she has been working in Hunan University. She currently researches on converter topology and control technology, DC/AC converter.

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