Operating area for modular multilevel converter based high-voltage direct current systems

This study presents a ‘seek’ and ‘check’ approach to determine the operating area of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. First, the equivalent circuit for analysing the operating area of MMC-HVDC systems is derived and the operating area is represented by discrete operating points. Then, three operating constraints are proposed according to the MMC operation principles, i.e. the voltage, current and voltage stability constraints. Therefore, the determination of the operating area is transformed into checking whether the operating point meets the proposed constraints. The case studies are performed on a one-terminal MMC-HVDC system under six typical short circuit ratios. The results show that the top boundary of the operating area is limited by the voltage constraint, and the bottom boundary of the operating area is limited by the current constraint. The influence of the third harmonic injection and the ac impedance characteristic on the operating area is also studied. Lastly, the validity of the ‘seek’ and ‘check’ approach is proved by time-domain simulation results.