Control, dynamics and operation of a dual H-bridge current flow controller

In recent years, the demand for HVDC grids has greatly increased due to the integration of renewable energy, especially offshore wind energy. The current flow within the DC grid is passively determined by the resistance of the DC lines. In a complex DC grid, the current flow in each cable cannot be precisely controlled by adjusting the voltage set points of each converter and an overload may occur. In order to improve the efficiency of the branch utilization and the reliability of the grid, the current flow has to be rescheduled between DC nodes. This can be achieved by introducing a series current flow controller (CFC). This paper discusses the operation and control of a dual H-bridge CFC on a meshed connected DC grid. Two different strategies have been proposed to control the CFC, namely, master-slave control and single modulation control. A four-terminal DC grid has been modelled in Simulink/ SimPowerSystems to analyse the dynamic performance of the CFC under the proposed control strategies and a DC fault. The strategies have been compared in terms of system power losses. In addition, the operation and control of a reduced IGBT-based CFC is presented.