Admittance Modeling and Stability Analysis of Transformerless Modular Multilevel DC-DC Converter with Internal AC Loops for all-DC System

The all-DC system is a promising solution for offshore wind power collection and transmission. The DC-DC converter, as the core equipment, affects the system's stability. However, to achieve energy balance, a necessary internal AC loop is inherently required within DC-DC converters. This AC loop induces frequency coupling characteristics. To address this, this paper investigates the frequency coupling induced by the internal AC loop in DC-DC converter and its impact on the stability of all-DC system. We establish the admittance model of the transformerless modular multilevel DC-DC converter, revealing its frequency coupling characteristics. Using this model, we analyze how DC grid impedance influences system stability through frequency coupling. Furthermore, we propose a controller to suppress this frequency coupling and improve the stability of all-DC system. Control hardware-in-the-loop and a down-scaled prototype experiments validate the controller's effectiveness, demonstrating improved stability margins and an extended power transmission distance.