DC Voltage Stability Analysis and Enhancement for Grid-Forming-Based MTDC Systems

This letter first identifies a new mode of low-frequency harmonic instability when combining grid-forming control with the typical dc voltage droop control necessitated in multiterminal high voltage direct-current transmissions. This mode does not exist when grid-following control is applied and its mechanism is explicitly revealed by the dc-side input admittance modeling of the converter. Then, a phase compensator plus a virtual power system stabilizer are introduced to pacify the input admittance within the critical frequency range and derisk the related instability. Last, it is also demonstrated that the proposed control strategy achieves excellent performance in contingent active power imbalance ride-through in the dc system. All the analyses are verified by electro-magnetic-transient simulations with a modular multilevel converter model comprising 278 submodules per arm.