Abstract

۱ Introduction

۲ Proposed topology

۳ Modulation and capacitance design

۴ Comparison analysis and losses calculation

۵ Simulation and experiment analysis

۶ Conclusion

References

Abstract

     Multilevel inverters (MLIs) play an important role in research on renewable energy conversion. However, in traditional designs, the high-voltage stress of switching devices and the large number of switches limit the wide application of the inverter. To ameliorate these problems, this paper proposes a switched-capacitor multilevel inverter (SCMLI). When compared with traditional MLIs, the proposed SCMLI utilizes a switched-capacitor structure, where the capacitors can achieve voltage self-balancing without auxiliary methods. Thus, it permits changes of the positive and negative polarity of the output level without the need for an H-bridge. In addition, with the augment of the level in the expanded SCMLI structure, the maximum blocking voltage can be kept constant. To show the advantages of the proposed structure, an extensible single dc source five-level SCMLI prototype has been built. Through a comparative analysis with different topologies, this paper also presents the advantages of the proposed topology in terms of the output voltage gain, number of output levels, and voltage stress. Finally, the correctness and feasibility of the proposed inverter are validated by extensive experiments.

Introduction

     In recent years, multilevel inverters (MLIs) have become attractive converters in numerous applications such as renewable energy systems, motor drives, distributed generation systems, UPS systems, and fexible alternating current transmission system (FACTS) [1–۳]. When compared with the traditional two-level inverters, MLIs have many advantages such as lower total harmonic distortion, reduced dv/ dt stresses, lower switching frequency, etc. [4]. In general, classic MLIs include neutral-point-clamped (NPC), fying capacitor (FC), and cascaded H-bridge (CHB) inverters. However, NPC and FC inverters employ a large number of clamping diodes and capacitors along with the increase of output levels, which inevitably raises the cost of topologies and the complexity of control strategies [5, 6]. Cascaded H-bridge MLIs have problems in terms of the lack of boost capacity and the need for multiple isolated dc sources [7, 8].

     To alleviate the problems of traditional MLIs, switchedcapacitor multilevel inverters (SCMLIs) have been proposed in [9–۱۴]. SCMLIs can achieve multilevel output with a single dc source and fewer power devices. The fve-level inverter proposed in [9] uses only eight switches, but the topology contains two extra diodes and the working states for most of the switch pairs are inconsistent. In [10], a sevenlevel inverter with a single dc voltage source and 12 switches was proposed. Although the topology in [11] employs ten switches to output seven levels, there are still four extra diodes and ten gate drivers are required. The same issue can be found in [13], where the nine-level inverter can achieve twice voltage gain using many diodes and drivers. In addition, two capacitors are discharged for most of a cycle, which may bring about a large voltage ripple and the imbalance of capacitor voltage.

Conclusion

     This study presents an extensible single dc source fve-level MLI based on a switched-capacitor structure. The proposed MLI eliminates the need for H-bridge due to the inherent polarity reversal capability. In addition, the voltage of two capacitors can be self-balanced without any auxiliary circuits. Although ten switches are employed, only two switches have a voltage stress of 2Vdc, and only fve gate drivers are required, which signifcantly reduces the total voltage stress and cost of proposed inverter. In addition, the output levels and boosting factor can be raised by employing multiple switched-capacitor cells. Moreover, the maximum blocking voltage for all of the switches is kept within 2Vdc in the extended structure. A comparative study with other topologies showed that the proposed inverter has the advantages of reducing the TSV, the MBV, and the number of the power devices. Finally, the correctness and feasibility of the proposed multilevel inverter are validated through the simulation and experiment results obtained from a laboratory prototype.