On implementation of primary frequency control in MMC-HVDC transmission systems

This paper investigates methodologies for the utilization of modular multilevel HVDC transmission (MMC-HVDC) for primary frequency control. In contrast to conventionally used average voltage source converter (VSC) models for frequency support system studies, advanced MMC models provide a different dynamic behaviour due to their decoupled ac and dc current control. Following the underlying converter and dynamic ac grid models, four different methods for primary frequency support by MMCs are presented in this paper. The individual control designs differ in their required input signals, controller characteristics in terms of proportional or PI structures and potential applications with respect to ac system properties. To prove the overall functionality and to highlight differences in the respective dynamic system responses as well as recovered steady-state conditions after occuring active power imbalances, a comparison using numerical simulations is carried out for a point-to-point MMC-HVDC transmission system connecting two non-synchronous areas.