An Approch for the Emulation of DC Grid Admittances: Implementation an a Buck Converter

This work proposes a method to actively emulate dc grids, including their equivalent admittance, by means of a modified current control structure of a dc/dc converter. The stability of power converter units (PCUs), interconnected with electrical grids is frequently assessed by the characteristics of the equivalent admittance/impedance of the grid, at the point of connection, and the PCU. Several approaches have been proposed to shape the PCU equivalent admittance, by means of the control structure, in order to fulfil the requirements about stability, robustness and dynamic response. On top of this, the integration of dc distribution systems is steadily growing, due to its simpler implementation and the potential cost and losses reduction. The presence of PCUs, in dc grids, is inherently higher, which complicates the overall stability assessment. In order to verify an impedance/admittance stability analysis in a laboratory scaled system, the characteristics of the impedance/admittance needs to be emulated. A suitable control structure, implemented on a buck converter, to actively emulated dc grid admittance has been proposed and tested. The analytical expressions of the emulator converter admittance, with the modified control structure, are presented. The admittances of a MVDC collection network for offshore wind farms, and a MVDC drive, working with a vector control scheme, have been calculated to be used for the emulation. The experimental validation has shown the overall feasibility of the proposed emulation approach.