Nyquist stability analysis and capacitance selection for DC current flow controllers in meshed multi-terminal HVDC grids

Controllability of DC current/power flow is essential in multi-terminal HVDC (MTDC) grids, particularly for the MTDC grids in a meshed topology. In this paper, considering meshed MTDC (M
²
TDC) grids with the installation of two-line/multi-line DC current flow controllers (CFCs), a small-signal model of the DC CFCs integrated M
²
TDC grids is derived, studying the impact of the power losses of the DC CFC and their influence on the analysis of energy exchanges. The system stability analysis is analysed using the Nyquist diagram, which is more suitable for analyzing complex nonlinear systems with more compact and reliable indicators of stability in comparison with gain/phase margins shown in the Bode diagram. In addition, a selection method of the interconnected capacitor of the DC CFC is proposed under different operating conditions. The impact of the switching frequencies of the DC CFC on the control ranges of the DC current flows is analyzed. The effectiveness of the Nyquist analysis and the capacitance selection method is verified by simulation studies using PSCAD/EMTDC. The obtained control ranges of the DC CFC with different switching frequencies and capacitances would be useful for practical engineering applications.