Transformerless Z-Source Four-Leg PV Inverter with Leakage Current Reduction

IEEE Transactions on Power Electronics, 2018 IEEE

ABSTRACT: Due to the lack of electrical isolation, the leakage current is one of the most important issues for transformerless PV systems. In this paper, a new modulation strategy is proposed to reduce the leakage current for Z-Source four-leg transformerless PV inverter. Firstly, the common mode loop model is presented. And then the common mode voltage behavior and the effect of factors on the leakage current are discussed. A new modulation strategy is proposed to achieve the step-up function and constant common mode voltage. Therefore, the leakage current can be suppressed effectively. Finally, the proposed strategy is digitally implemented and tested. The simulation results verify the effectiveness of the proposed solution.

 

KEYWORDS:

  1. Transformerless photovoltaic system
  2. Z source inverter
  3. Modulation
  4. Leakage current.

 

SOFTWARE: MATLAB/SIMULINK

 

CIRCUIT DIAGRAM:

Z-source four-leg inverter for transformerless PV systems

Fig. 1. Z-source four-leg inverter for transformerless PV systems

 

EXPECTED SIMULATION RESULTS:

(a)Common mode voltage VCM

(b) Parasitic capacitance voltage VPV

   

(c) Leakage current ICM

(d) Spectrum analysis of ICM

(e) Grid current

(f) Spectrum analysis of grid current

Fig.2 Simulation results of conventional modulation strategy

(a) Common mode voltage VCM

(b) Parasitic capacitance voltage VPV

(c) Leakage current ICM

(d) Spectrum analysis of ICM

(e) Grid current

(f) Spectrum analysis of grid current

Fig.3 Simulation results of proposed modulation strategy

(a) Conventional modulation strategy.

(b) Proposed modulation strategy

Fig. 4 Simulation results of d from 0.3 to 0.1

Fig.5 Simulation results of duty cycle and leakage current (RMS)

 

CONCLUSION:

This paper has presented the analysis and simulation verification of a new modulation strategy to reduce the leakage current of Z-source four-leg inverter for transformerless PV systems. Our finding indicates that the conventional method fails to eliminate the leakage current. Meanwhile, the leakage current will be higher as the shoot-through duty cycle increases. As for the proposed method, the effect of shoot-through duty cycle variation on the leakage current is small, and the leakage current can be effectively reduced. On the other hand, there is one drawback that the number of switching for the proposed solution is slightly more than that of the traditional one during a carrier cycle. However, compared with the conventional solution, both the leakage current and the THD of grid current can be reduced effectively with the proposed solution. Moreover, the four-leg solution can enable the zero sequence current to circulate, avoiding the dc bias in the load output currents in case of unbalanced loads. Aside from that, the power losses of semiconductor devices can be reduced significantly. Therefore, the proposed solution is attractive for transformerless PV systems.

 

REFERENCES:

  1. Guo, Y. Yang, and T. Zhu, “ESI: A novel three-phase inverter with leakage current attenuation for transformerless PV systems,” IEEE Trans. Ind. Electron., vol. 65, no. 4, pp. 2967-2974, Apr.2018.
  2. Xiao, L. Zhang, and Y. Li, “An improved zero-current-switching single-phase transformerless PV H6 inverter with switching loss-free,” IEEE Trans. Ind. Electron., vol. 64, no. 10, pp. 7896-7905, Oct. 2017.
  3. Zhang, K. Sun, Y. Li, X. Lu, and J. Zhao, “A distributed power control of series connected module-integrated inverters for PV grid-tied applications,” IEEE Trans. Power Electron., vol. 33, no. 9, pp. 7698-7707, Sept. 2018.
  4. Li, Y. Gu, H. Luo, W. Cui, X. He, and C. Xia, “Topology review and derivation methodology of single-phase transformerless photovoltaic inverters for leakage current suppression,” IEEE Trans. Ind. Electron., vol. 62, no. 7, pp. 4537–4551, Jul. 2015.
  5. Yam Siwakoti, and Frede Blaabjerg, “Common-ground-type transformerless inverters for single-phase solar photovoltaic systems,” IEEE Trans. Ind. Electron., vol. 65, no. 3, pp. 2100–2111 Mar. 2018.

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