Series Current Flow Controllers for DC Grids

Large-scale grid integration of renewables and cross-country border energy exchange may be facilitated by multi-terminal high-voltage direct-current (MTDC) grids. However, as the number of terminals and dc lines increases, power flow management may become a major challenge. This paper addresses such a fundamental issue through the introduction of current flow controllers (CFCs) into the MTDC grid. A CFC is a low power rated controllable voltage source that can enhance system performance by suitably redirecting the power flow at the point of connection. This is achieved through the regulation of the dc line current by introducing a series voltage at the connection node. The characteristics, control, and operation for three configurations of series-connected CFCs are studied. These have been termed a dual-thyristor converter CFC, a cascaded voltage source converter-dc chopper-based CFC, and a dual H-bridge CFC (2B-CFC). A four-terminal MTDC grid has been modeled in Simulink/SimPowerSystems to analyze the dynamic performance of the devices. The simulation results show that all devices are capable of improving system performance. In addition, the CFCs are compared in terms of controllability and dc fault performance. For completeness, a 2B-CFC prototype has been developed to study the impact of a CFC on MTDC grid operation, with an MTDC test rig used to validate simulation results.