Coordinating Grid-forming Control of MT-HVDC System to Interconnect Renewable Power Bases and Weak Grids

Using modular multilevel converter (MMC) based multi-terminal high voltage direct current (MT-HVDC) transmission to interconnect renewable power bases and asynchronous power grids is becoming a promising technology and future trend. However, conventional grid-following control faces weak grid oscillation and inertia-less problems. To solve these problems, a unified grid-forming (GFM) control strategy is proposed for all ends of the MT-HVDC system, i.e., both grid-connected and islanded MMCs. The capacitor voltage of MMC submodules is controlled to emulate the rotor speed of SGs, and a multi-target DC-side controller is designed. By distributing the control coefficient of the DC-side controller, the MMC can operate in DC-side GFM mode, AC-side GFM mode and dual-port GFM mode. Furthermore, with the coordinating of grid-forming MMCs, the MT-HVDC system can operate in different droop control modes, realizing more functions such as grid-frequency delivery and active grid frequency support. This method can not only realize the grid-forming control of each end, but also enhance the flexibility and adaptability of the MT-HVDC system. To validate the effectiveness of the proposed strategy, a 5-terminal MMCMTDC transmission system with renewable energy integration is established on the PSCAD/EMTDC simulation platform. The simulation results convincingly verify the efficacy of the control strategy presented in this paper.