An Adaptive Master-Slave Technique using Converter Current Modulation in VSC-based MTDC System

The VSC-based multi terminal HVDC technology is gaining popularity in power system to integrate and efficiently operate multi-area AC systems of same or asynchronous frequency as one grid. Nevertheless, this technology has enabled to enhance controllability and stability of AC-DC integrated system. Depending upon the amount of power transfer between AC and DC systems, control strategy adopted for MTDC system varies. Master-Slave, Voltage Margin and Voltage Droop control are classical DC voltage control strategies for VSC-based MTDC systems. Although Voltage Droop technique is reliable but it suffers from drawbacks like inability to deliver constant active power to critical AC systems, inability to maintain DC voltage to a constant value, risk of irreversible switching from droop control mode to constant power control mode, etc. Voltage Margin technique is reliable than Master Slave control but suffers from power oscillations and large DC voltage overshoot when DC voltage control is transferred from main master controller to reserve controller. In this paper, an adaptive Master-Slave control technique with modulation of converter currents in Master terminal is proposed where direct-axis converter current is allowed to increase well up to maximum IGBT current carrying ability limit. Thus, augmented Master control scheme is able to compensate for more unbalanced power in DC grid than the conventional one thereby preventing severe DC overvoltage or arresting DC voltage from running down. The proposed control has enabled to improve the reliability and stability of AC-DC grid and is found to be more flexible than conventional Master-Slave control.