Primary Frequency Control of Hybrid LCC–MMC HVDC Systems with Dual-Port Grid-Forming MMC Control for Renewable-Rich Power Systems

Frequency stability is becoming an increasingly critical issue in asynchronously interconnected (ASI) power systems, where DC interconnections inherently decouple frequency dynamics between asynchronous AC grids and conventional synchronous generators are progressively displaced by renewable energy resources with limited frequency-support capability. To address this issue, this paper proposes a primary frequency control strategy for a hybrid High Voltage Direct Current (HVDC) system composed of a line-commutated converter (LCC) rectifier and a modular multilevel converter (MMC) inverter. In the proposed scheme, the MMC adopts a dual-port grid-forming (GFM) control mode, thereby establishing voltage-source characteristics at both its AC and DC ports, while the LCC rectifier regulates the DC current. Through the interaction between the DC-voltage-forming MMC and the DC-current-regulating LCC, bilateral primary frequency support can be achieved for the two asynchronous AC systems without requiring frequency-signal communication between the converters. Furthermore, a frequency-response model is developed to characterize the frequency-coupling dynamics of the ASI system and to quantify the frequency-support capability enabled by the proposed control. Based on this model, the parameterization of the proposed frequency control is further discussed. Finally, real-time digital simulation studies are carried out on an RTDS platform to validate the effectiveness of the proposed control strategy and the accuracy of the developed analytical model.