Level-shift Modulation and Control of a Dual H-bridge Current Flow Controller in Meshed HVDC systems

The dual H-bridge current flow controller (CFC) has been proposed as a cost-effective solution to regulate current flows in a meshed high-voltage direct-current (HVDC) system. With an interline connection, this device may increase or decrease the magnitude and even reverse the directions of transmission line currents in a meshed dc grid. However, it needs to be modulated in a different manner when its operating region changes, which makes its control more complex. This paper investigates a modulation method for the dual H-bridge CFC. The buck and boost regions of operation for the device are analyzed first. A level-shift modulation method based on pulse width modulation is proposed. This method ensures that only one bridge of the CFC is switching for each operating region, which simplifies the control of the device. In addition, the method ensures a smooth transition between the operating regions, which can enhance the reliability of the CFC. For completeness, the presented modulation and control methods are verified through time-domain simulations conducted in PSACD/EMTDC.