Closed-loop impedance calculation of grid-tied three-phase inverters/rectifiers in bus signaling strategy-controlled DC microgrids

The closed-loop (CL) impedances of each connected microgrid device (generator, sto rage or load) are key elements in stability studies of DC microgrids from the point of view of the DC common bus. The calculation of the CL impedance of standard DC/DC converters is widely described in the literature. The application of this method to a three-phase AC/DC converter connecting DC microgrids to the utility grid, when harmonic filters are required, is studied in this work. Four possible CL impedance types are established, depending on the operation mode of the converter (power flow direction through the interface and on the control applied to the interface at the DC-side), taking the filter into account. To calculate the four possible CL impedances of the three-phase AC/DC interface, a method used in sensitivity analysis is applied. Firstly, three voltage loops are described in the time domain. Secondly, the system is linearized around its operating point and thus the resulting dynamic system can be expressed. Finally, an existing method to close the loop gain is used to take the interface control into account and to determine the resulting CL impedance.