DC voltage and active power regulations in multi terminal high voltage direct current transmission system: A review

The multi-terminal high voltage direct current (MTDC) transmission network using voltage source converters (VSC) has emerged as a significant method for integrating renewable energy sources into the conventional alternating current (AC) system. In such a system, unequal active power allocation and fluctuation in DC voltage causes stability problems. The main technical obstacle hindering the progress of MTDC systems is the controlling of DC voltage within the permissible operational range. Herein, the paper aims to review various control methods for MTDC systems that considerably improve system performance by regulating the balanced active power and keeping the DC voltage stable. Although numerous MTDC review papers are available, they frequently focus on specific aspects related to the protection and control of system components. Conversely, this paper provides a comprehensive, up to date overview, assessment and analysis of DC voltage and active power performance in the MTDC based on the voltage and power characteristics. This study examines the technical performance of control schemes and investigates the effectiveness of these control methods under both normal conditions and significant power instabilities. Further, the performance of control techniques is summarized and compared based on various parameters, including steady state and transient conditions, the requirement of communication link and expandability, stability, and flexibility in MTDC. The findings indicate that the droop control-based methods exhibit superior performance. Finally, the future developments in MTDC are discussed.