Wave Function and Multiscale Modeling of MMC-HVdc System for Wide-Frequency Transient Simulation

The detailed modeling of power electronic (PE) devices poses challenging problems to an efficient transient simulation of large-scale PE-dominated power systems. As PE paradigm, a multiscale modeling methodology of the modular multilevel converter (MMC) for simulating diverse transients from low-frequency oscillations up to high-frequency switching events in an MMC high-voltage direct current (HVdc) system is developed, implemented, and validated. The novelty lies in the creation of a wave propagation function (WPF) that describes the MMC submodule (SM) transient behavior, and then, the SM Fourier series-based shifted-frequency phasor (SFP) is developed to accelerate the computation speed of the system-level dynamics. These efforts serve as the basis for multiscale modeling of the MMC where a multiple-frequency shifting is achieved. By implementing a seamless model interface with the control systems, the multiscale modeling is applicable to an MMC-HVdc transmission system. The multiscale model is validated through case studies that cover dc fault, MMC internal fault, power oscillations, and wind power fluctuations. The computational accuracy and efficiency of the model are verified through a comparison of the results with those from the full electromagnetic transient (EMT) model.