A HIGHLY EFFICIENT AND RELIABLE INVERTER CONFIGURATION BASED CASCADED MULTI-LEVEL INVERTER FOR PV SYSTEMS
Abstract
This paper presents an improved Cascaded Multi-Level Inverter (CMLI) based on a highly efficient and reliable configuration for the minimization of the leakage current. Apart from a reduced switch count, the proposed scheme has additional features of low switching and conduction losses. The proposed topology with the given PWM technique reduces the high-frequency voltage transitions in the terminal and common-mode voltages. Avoiding high-frequency voltage transitions achieves the minimization of the leakage current and reduction in the size of EMI filters. Furthermore, the extension of the proposed CMLI along with the PWM technique for 2m+1 levels is also presented, where m represents the number of Photo Voltaic (PV) sources. The proposed PWM technique requires only a single carrier wave for all 2m+1 levels of operation. The Total Harmonic Distortion (THD) of the grid current for the proposed CMLI meets the requirements of IEEE 1547 standard. A comparison of the proposed CMLI with the existing PV Multi-Level Inverter (MLI) topologies is also presented in the paper. Complete details of the analysis of PV terminal and common-mode voltages of the proposed CMLI using switching function concept, simulations, and experimental results are presented in the paper.
EXISTING SYSTEM:
One elegant solution based on maintaining a constant CMV is proposed by Zhang et al. The given MLI configuration consists of eight switches for the generation of three levels in the output voltage. This topology reduces the switching losses but has the drawback of high conduction losses during both turn-ON and zero voltage states. The given MLI configuration has an asymmetric operation during each halfcycle of the fundamental component of the grid voltage. The inherent asymmetry in each half-cycle causes a DC offset in the MLI output voltage. Furthermore, the requirement of an additional number of switches for more than three-level operation limits its application. Islam et al. have proposed another interesting transformerless PV MLI topology to reduce the leakage current by maintaining CMV constant. This MLI topology uses six switches for the generation of three levels in the inverter output voltage. This circuit configuration results in high switching and conduction losses. Furthermore, this MLI topology cannot be extended to more than three levels in the output voltage. Xiao et al. have proposed another efficient three-level MLI for the minimization of leakage current by maintaining CMV constant. The given topology has low conduction and switching losses. However, this configuration suffers from the disadvantage of a high number of device count. Another interesting topology with low switching losses based on constant CMV is proposed by Ji et al. . This MLI topology consists of six switches and two diodes. Apart from resulting in high conduction losses, this topology is less amenable for an extension to a higher number of levels in the output voltage
PROPOSED SYSTEM:
This paper proposes one such solution for the minimization of leakage current in transformerless MLIs connected to PV systems. The pulse width modulation (PWM) technique for the proposed MLI is also discussed in the paper. The analysis of PV terminal and common-mode voltages using switching function is presented. This analysis leads to the development of the proposed PWM technique, which prevents high-frequency voltage transitions in the terminal voltage and CMV. Salient features of the proposed cascaded multilevel inverter (CMLI) are: 1) The topology uses eight switches for the generation of fc five levels in the output voltage. 2) During the zero voltage state only one switch and one diode conduct. 3) In the proposed topology, four switches are operated at a low switching frequency, which reduces the switching losses. 4) The dead band in PWM technique does not affect the common-mode voltage. 5) The proposed inverter can be easily cascaded to achieve more than five levels in the output. Rest of the manuscript is organized into eight sections. Section II describes the working principle and the operation for the proposed five-level grid-connected CMLI along with the generalized structure. The details of the PWM technique employed with its generalization for 2m+1 levels are explained in section III. Section IV gives the details of the Maximum Power Point Tracking (MPPT) algorithm which can be applied to the proposed five-level CMLI
CONCLUSION
In this paper, an improved five-level CMLI with low switch count for the minimization of leakage current in a transformerless PV system is proposed. The proposed CMLI minimizes the leakage current by eliminating the highfrequency transitions in the terminal and common-mode voltages. The proposed topology also has reduced conduction and switching losses which makes it possible to operate the CMLI at high switching frequency. Furthermore, the solution for generalized 2m+1 levels CMLI is also presented in the paper. The given PWM technique requires only one carrier wave for the generation of 2m+1 levels. The operation, analysis of terminal and common-mode voltages for the CMLI is also presented in the paper. The simulation and experimental results validate the analysis carried out in this paper. The MPPT algorithm is also integrated with the proposed five-level CMLI to extract the maximum power from the PV panels. The proposed CMLI is also compared with the other existing MLI topologies in Table V to show its advantages.
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