Modular impedance modeling and stability analysis of hybrid AC/DC power systems with grid-forming and grid-following converters

The renewable energy resources integrating HVDC transmission systems, consisting of grid-following and grid-forming converters, have complex physical and control dynamic interactions. Although the impedance analysis method is advisable to study these interactions, impedance modeling is still challenging when taking various converter control schemes into account. This article proposes a modular three-port admittance modeling framework for the hybrid AC/DC power systems. This approach separates the modeling process into three levels: controller, converter, and system levels. The current and voltage feedback loops are subdivided into multiple submodules at the controller level, and the submodule connection method is proposed to obtain the small-signal model of the unified main controller, which thus can eliminate the repetitive work caused by the diversity of control schemes. Due to the modular modeling advantage, the method is also versatile and scalable, as well as using unlimited and expandable interconnecting. In addition, the modeling separates the entire system into three components, and the stability criterion of the hybrid AC/DC power systems is derived by regrouping their transfer function matrix models. Then, the proposed modeling method is applied to assess the stability of hybrid power plants integrating HVDC systems with grid-following and grid-forming sources. The conclusions can be used to plan GFM converter capacity in power System plants, the installation location of HVDC transmission systems, and other scenarios. Simulations are carried out to validate the correctness of the impedance modeling method and stability criterion. Finally, the effect of the different capacity ratios of grid-forming sources to renewable energy power plants on system stability is discussed.