Transformerless Z-Source
Four-Leg PV Inverter with Leakage Current Reduction
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.
CIRCUIT DIAGRAM:
Fig. 1. Z-source four-leg inverter
for transformerless PV systems
EXPECTED SIMULATION
RESULTS:
(a)Common mode voltage VCM
(b) Parasitic capacitance voltage VPV
(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
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.
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