Utilization of MMC-HVDC for Primary and Secondary Control in Hybrid ACDC Power Systems

This paper presents methods to implement primary and secondary control in modular multilevel converter (MMC) based hybrid acdc transmission systems. Based on an advanced MMC model and its associated control loops, novel methods to adjust the active power operating point of MMC-HVDC systems in real-time operation during grid contingencies are developed. In addition, use cases for utilization of MMC-HVDC for secondary control complete ac frequency recovering procedures for several control areas after power imbalances. Proposed concepts are analyzed regarding operational constraints, additional functional requirements like the need for telecommunication between control areas and their overall dynamic performance. Numerical simulations are carried out for a point-to-point MMC-HVDC transmission system embedded in a synchronous ac grid and further for multiterminal dc (MTDC) grids connecting ac grids in synchronous and asynchronous operation. Presented case studies evaluate the proposed control designs and highlight new opportunities for a flexible provision of ancillary services by MMC-HVDC.