Abstract
Single-phase transformerless inverters are broadly studied in literature for residential-scale PV applications due to their great advantages in reducing system weight, cost and elevating system efficiency. The design of transformerless inverters is based on the galvanic isolation method to eliminate the generation of leakage current. Unfortunately, the use of the galvanic isolation method alone cannot achieve constant common mode voltage (CMV). Therefore, a complete elimination of leakage current cannot be achieved. In addition, modulation techniques of single-phase transformerless inverters are designed for the application of the unity power factor. Indeed, next-generation PV systems are required to support reactive power to enable connectivity to the utility grid. In this paper, a proposed single-phase transformerless inverter is modified with the clamping method to achieve constant CMV during all inverter operating modes. Furthermore, the modulation technique is modified by creating a new current path in the negative power region. As a result, a bidirectional current path is created in the negative power region to achieve reactive power generation. The simulation results show that the CMV is completely clamped at half the DC link voltage and the leakage current is almost completely eliminated. Furthermore, a reactive power generation is achieved with the modified modulation techniques. Additionally, the total harmonic distortion (THD) of the grid current with the conventional and a modified modulation technique is analyzed. The efficiency of the system is enhanced by using wide-bandgap (WBG) switching devices such as SiC MOSFET. It is observed that the efficiency of the system decreased with reactive power generation due to the bidirectional current path, which leads to increasing conduction losses.
Highlights
Several transformerless inverter topologies have been presented and published [1–7]
Complete elimination of leakage current cannot be achieved with the galvanic isolation method alone because common mode voltage (CMV) during freewheeling periods cannot be identified by the switching state, which means that it is not constant
The Si MOSFET-based transformerless inverter is highly efficient in generating reactive power, but the inverter design must be optimized to address the fact that the Si MOSFET’s body diode has poor reverse recovery performance, which may lead to device failure [20,21]
Summary
Several transformerless inverter topologies have been presented and published [1–7]. One of the drawbacks of PV transformerless inverters is the generating of leakage current due to the absence of a transformer. Many transformerless inverter topologies with unipolar modulation technique have been introduced to increase system efficiency and reduce the leakage current by disconnecting the AC and DC sides during the freewheeling modes; this is known as galvanic isolation. Complete elimination of leakage current cannot be achieved with the galvanic isolation method alone because common mode voltage (CMV) during freewheeling periods cannot be identified by the switching state, which means that it is not constant. The Si MOSFET-based transformerless inverter is highly efficient in generating reactive power, but the inverter design must be optimized to address the fact that the Si MOSFET’s body diode has poor reverse recovery performance, which may lead to device failure [20,21]. This switch is known as a clamping switch and is placed between the midpoint of the freewheeling switches and the midpoint of the DC link capacitors
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