Analysis and Optimization of Multi-DC Current Flow Controllers With Different Configuration Positions in M2TDC Grid

In multi-terminal HVDC (MTDC) grids, the voltages and currents at the converter DC terminals can be accurately regulated by the control of converter stations. For the DC line currents in meshed MTDC (M
²
TDC) grids, they are naturally distributed based on the line resistance and the voltage difference at DC ports, which greatly affects the controllability and optimal distribution of the DC power/current flow. In this paper, regarding different configurations of positioning (COP) DC CFCs in M
²
TDC grids, the dynamic behaviors and optimized configuration scheme are investigated. Firstly, topological structure of a generalized M
²
TDC is presented. Two different COP schemes for double DC CFCs in a four-terminal M
²
TDC grid are proposed according to control objectives of the DC current flows. Secondly, in order to achieve the coordinated control of DC line currents and capacitor voltage of the DC CFC, a generalized voltage-droop control strategy is applied. It can be applicable to multiple DC CFCs under different COP without the need of communication. Apart from that, considering the impact of DC CFCs under specific COP on the control characteristics of DC current flows and control margins, an auxiliary control strategy, which is capable of mitigating the confliction of multiple DC currents, is proposed. It can effectively improve the control performance of the DC CFC. Based on the time-domain simulation PSCAD/EMTDC, the M
²
TDC grids with different COP of DC CFCs are established. The merits and drawbacks of different COP, in terms of the complexity and implementation of control systems, dynamic behaviors in mitigating transient voltage spikes and current surges, control margins and robustness of the controller design, are discussed. In addition, the effectiveness of the proposed generalized voltage-droop control and auxiliary control strategies is verified with an optimized COP scheme of multiple DC CFCs proposed.