Modeling and Design of Parallel LCC-VSC Interlinking Converters' Unified Controller in AC/DC Network

While both line-commutated converters (LCCs) and voltage-source converters (VSCs) have been adopted in ac/dc grids, they are mostly used individually despite some point-to-point dc transmission. The combination of both technologies as a single interlinking converter (IC) unit in an ac/dc grid can bring advantages, such as capacity expansion, bidirectional power flow, reactive power support, and harmonics compensation. For such a parallel LCC-VSC unit, a suitable control scheme and careful control parameters' design are important to ensure that the system can maintain the above advantages in different operation modes. In this article, the control scheme and parameter design for a parallel LCC-VSC unit for use in an ac/dc grid are conducted with a focus on capacity expansion scenario, where a unified control scheme that reserves most of the original LCC terminal's control while also smoothly incorporates the VSC operation is considered. The operation states of a hybrid ac/dc network are modeled. The proposed unified control scheme is considered for each state for the stability study. Then, a set of control parameters for the unified control scheme suitable for all operation states with good dynamics and stability is designed. Real-time simulations and experiments are conducted to verify the design and study.