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Leakage Current Suppression with Critical-mode-based Soft-switching Modulation for Three-phase Inverters in Photovoltaic Application

Circuit schematics
Fig. 1. 8-switch topology for leakage current suppression
In photovoltaic (PV) system applications, large parasitic capacitance exists between the PV panel and earth due to the relatively large area of the PV panel.This parasitic capacitance, combined with the power converter, the grounded power grid, and the earth, forms a closed electrical loop, and therefore, there is current flowing through this parasitic capacitance, which is called leakage current in PV system applications. For safety consideration, the root mean square value of the leakage current should be lim-ited within 300mA according to related standards in leakage current for PV system applications, otherwise protection will be triggered. With the novel critical conduction mode (CRM) based soft switching modulation technique applied into a wide bandgap (WBG) semiconductor device based high-frequency three-phase bidirectional ac-dc converter system, both the high power density and high efficiency are achieved. . A 25 kW silicon carbide (SiC) based three-phase ac-dc converter prototype is built with greater than 300kHz switching frequency operation under the novel CRM based soft switching modulation. 127 W/in3 power density and approximately around 99% peak efficiency are achieved. However, the limitation of high leakage current under the above soft switching modulation prevents its use of the converer in PV system applications. Therefore, as shown in Fig. 1, the system topology is modified by adding two additional switches. The basic concept, with these two additional switches is that isolation is provided by turning off the two switches during the freewheeling period in each switching cycle and therefore, the path for the leakage current is cut off in each switching cycle. Since the voltage across the parasitic capacitance cannot be suddenly changed, the accumulation of the leakage current is avoided so the much lower leakage current is achieved. In addition, adding a three-phase common mode (CM) choke is also beneficial for the suppression of leakage current. The simulation results of the leakage current before and after adding two additional switches under the CRM based soft switching modulation is shown in Fig. 2.
Simulation results
Fig. 2. Simulation result comparison before and after topology modification
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