Precisely Simplified Time-Varying Modeling Method Based on the Small-Signal-Waveform Assumptions for a High-Ratio MMC-Based DC Transformer

Unlike the typical MMC system, the high-ratio MMC-based DC transformer behaves as a high-speed, time-varying nonlinear system due to its greater fundamental frequency, two-stage structure, and different phase-shift control strategy. Accordingly, such a system's s-domain transfer function could not be derived, and a precisely simplified system model is also inaccessible. Thus, accurately determining the system performance and adaptively regulating the controller parameters in fluctuating working conditions are scarcely possible. This article attempts to break through this bottleneck. The delicate time-varying state equations are first developed to simultaneously certify the transformer's high-speed time-varying, phase-shift, two-stage-structured and MMC-based characteristics. A modeling method based on the small-signal-waveform assumptions (SSWA) is then provided for the first time to analytically manipulate these time-varying, nonlinear, and high-order equations. The determined model subsequently clarifies the transformer's characteristics and tackles the challenge of high-order nonlinear expression solving. It also equilibrates the model's preciseness, simplicity, and practicality. Furthermore, the study organically integrates the transformer's proposed model, the controller's quantitative design, and traditional adaptive control methods. This integration assists the transformer in automatically configuring its controller parameters, thus ensuring the transformer has sufficient stability and dynamic performance. Simulations and experiments finally demonstrate the accuracy and efficiency of the proposed modeling method and improved control strategy.