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

 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.

 KEYWORDS:

1.      Leakage current

2.      Multilevel inverter

3.      Pulse width modulation

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

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

 EXPECTED SIMULATION RESULTS:

 

                                         Fig. 2. Simulation waveforms under asymmetry filter inductance conditions. (a)

L1=1.6 mH and L2=1.44 mH. (a) L1=1.6 mH and L2=1.28 mH.

 

Fig. 3. Waveforms of output voltage VAB, Vout and current iout.

Fig. 4. Waveforms of output voltage VAB and FC voltages VC1, VC2.

Fig. 5. Waveforms of VAN, VBN, VCM and ileakage.

Fig. 6. Waveforms of VAN, VBN and VCM in the positive half cycle.

Fig. 7. Waveforms of VAN, VBN and VCM in the negative half cycle.

 

Fig. 8. Waveforms of VAB, VC1 and iout when θ is 35 degrees.

Fig. 9. Transient experiment in load.

 

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 Vdc/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 Vdc/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.

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