25-08-2017, 09:32 PM
Improved Transformerless Inverter With Common-Mode Leakage Current Elimination for a Photovoltaic Grid-Connected Power System
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Abstract
To eliminate the common-mode leakage current in the
transformerless photovoltaic grid-connected system, an improved
single-phase inverter topology is presented. The improved transformerless
inverter can sustain the same low input voltage as the
full-bridge inverter and guarantee to completelymeet the condition
of eliminating common-mode leakage current. Both the unipolar
sinusoidal pulsewidth modulation (SPWM) as well as the doublefrequency
SPWM control strategy can be applied to implement
the three-level output in the presented inverter. The high efficiency
and convenient thermal design are achieved thanks to the decoupling
of two additional switches connected to the dc side. Moreover,
the higher frequency and lower current ripples are obtained
by adopting the double-frequency SPWM, and thus the total harmonic
distortion of the grid-connected current are reduced greatly.
Furthermore, the influence of the phase shift between the output
voltage and current, and the influence of the junction capacitances
of the power switches are analyzed in detail. Finally, a 1-kW prototype
has been simulated and tested to verify the theoretical analysis
of this paper.
INTRODUCTION
NOWADAYS, the gird-connected photovoltaic (PV) systems,
especially the low-power single-phase systems, call
for high efficiency, small size, light weight, and low-cost gridconnected
inverters. Most of the commercial PV inverters employ
either line-frequency or high-frequency isolation transformers.
However, line-frequency transformers are large and
heavy, making the whole system bulky and hard to install.
Topologies with high-frequency transformers commonly include
several power stages, which increases the system complexity
and reduces the system efficiency [1]–[6]. Consequently,
the transformerless configuration for PV systems is developed
to offer the advantages of high efficiency.
Unipolar SPWM Strategy
Like the full-bridge inverter with unipolar SPWM, the improved
inverter has one phase leg including S1 and S2 operating
at the grid frequency, and another phase leg including S3 and
S4 commutating at the switching frequency. Two additional
switches S5 and S6 commutate alternately at the grid frequency
and the switching frequency to achieve the dc-decoupling states.
Accordingly, four operation modes that generate the voltage
states of +Udc, 0, −Udc are shown in Fig. 4.
Fig. 5 shows the ideal waveforms of the improved inverter
with unipolar SPWM. In the positive half cycle, S1 and S6 are
always ON. S4 and S5 commutate at the switching frequency
with the same commutation orders. S2 and S3 , respectively,
commutate complementarily to S1 and S4 . Accordingly, Mode 1
and Mode 2 continuously rotate to generate+Udc and zero states
and modulate the output voltage. Likewise, in the negative half
cycle, Mode 3 and Mode 4 continuously rotate to generate −Udc
and zero states as a result of the symmetrical modulation.
CONCLUSION
This paper presented an improved grid-connected inverter
topology for transformerless PV systems. The unipolar SPWM
and double-frequency SPWM control strategies are both implemented
with three-level output in the presented inverter, which
can guarantee not to generate the common-mode leakage current
because the condition of eliminating common-mode leakage
current is met completely. Furthermore, the switching voltages
of all commutating switches are half of the input dc voltage and
the switching losses are reduced greatly. The high efficiency and
convenient thermal design are achieved thanks to the decoupling
of two additional switches S5 and S6 .Moreover, by adopting the
double-frequency SPWM, the higher frequency and lower current
ripples are achieved. Consequently, the higher quality and
lower THD of the grid-connected current are obtained, or the
smaller filter inductors are employed and the copper losses and
core losses are reduced accordingly.