An Intelligent Implementation of Fuzzy Search Based Nonlinear Droop Control Scheme for Multi-Terminal DC Transmission Systems

Multi-Terminal DC (MTDC) transmission systems typically operate under active power and DC voltage ( $P$ - $U_{DC}$ ) droop control for active power dispatch, and its design is greatly influenced by DC network characteristics. Hence, this paper firstly analyzes $P$ - $U_{DC}$ droop characteristics when implemented on Grid-Side Voltage-Source Converters (GSVSCs) of a MTDC network, in which variations of DC cables’ resistances with temperature are taken into account. The analysis establishes that the actual droop characteristics of GSVSCs are nonlinear, different from the widely assumption of linear relationships. This finding is the fundamental for the design of the accurate power dispatch for GSVSCs. Therefore, an improved Fuzzy Search based Droop Control (FSDC) scheme for MTDC systems is proposed, which performs real-time search to capture the accurate droop curves of GSVSCs over full range of permissible DC voltage and active power. The FSDC is implemented without the assumption of the prior knowledge of DC network parameters which are subject to variations and uncertainty, and instead the accurate droop curves are periodically obtained and updated. The technical viability of the FSDC and its superiority over the conventional linear droop scheme under the variations of DC cable resistances and power flow variations is verified by a four-terminal HVDC model established in MATLAB/Simulink.