Abstract
This paper analyzes in detail the effect of a simple solution for ground leakage current mitigation applicable to transformerless three-phase current source inverter (CSI). The circuit modification solution is assessed for both traditional CSI topology and for CSI with an additional seventh switch, in literature named CSI7 (or H7), in particular with the splitting of the dc input inductance. In the present work, the solution is applied to grid-connected converters for string photovoltaic applications: scope of the circuit modification is to provide an internal return path from the wye connected capacitors of the output CL filter. This additional return path is able to significantly reduce the ground leakage current without adversely affecting THD. The performance of the proposed solution is assessed by the numerical simulations in case of a string of photovoltaic (PV) modules and the different behavior of CSI and CSI7 topologies is thoroughly investigated. Furthermore, the definition of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cm</sub> ZC is assessed by applying it to the common mode equivalent circuits for CSI7 with additional return path and their validation by means of a two-step simulation. The simulation results and experimental validation shows good agreement and confirm that the proposed solution is able to strongly reduce the ground leakage current.
Highlights
Current Source Inverter (CSI) topologies constitute an alternative solution to the Voltage Source Inverter (VSI) paradigm, and they have been exploited for medium voltage applications [1]
Because no electrolytic capacitor is needed for the DC link, a microinverter realized with a CSI would have a lifetime similar to the one of the PV panel [4]
Summarizing the issues of the application of the return path, the proposed solution is only viable in case of CSI7 topologies with split input inductors, as the split input inductors limit the value of iRP, as it can be seen by analyzing the common-mode circuit of Fig. 5: in case of a single input inductor the equivalent impedance related to LDC disappears
Summary
Current Source Inverter (CSI) topologies constitute an alternative solution to the Voltage Source Inverter (VSI) paradigm, and they have been exploited for medium voltage applications [1]. CSI7 TOPOLOGY WITH COMMON-MODE RETURN PATH Figure 3 shows the modified connection to the midpoint of the DC input voltage through two capacitors Cmid with the objective of reducing the ground leakage current. In this figure it is possible to see that there is a circuit formed by only one of the capacitors Cmid and DC input inductors LDC /2 (due to the presence of the two diodes) and the output CL filter connected as in Fig. 8 together with the grid phase voltages. Summarizing the issues of the application of the return path, the proposed solution is only viable in case of CSI7 topologies with split input inductors, as the split input inductors limit the value of iRP, as it can be seen by analyzing the common-mode circuit of Fig. 5: in case of a single input inductor the equivalent impedance related to LDC disappears
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