Stable Performance of a Multi-Terminal MMC-HVDC System under Loads and DC Link Voltage Variation

In this paper, a Lyapunov control technique is proposed for AC-side Modular Multilevel Converters (MMCs) in a multi-terminal (MT) high-voltage direct current (HVDC) system while the DC-link voltage of the MMCs is changed and a related load variation occurs as well. A new dynamic model in d-q reference frame is firstly obtained in which the model consists of only the state variables of the MMC upper or lower arms. Using the dynamic model, the steady-state components of the proposed control strategy are attained supposing the zero value for the MMC arm current reference dynamics. The Lyapunov theory is employed to reach the dynamic components of proposed controller with Lyapunov coefficients that can be unlimited negative and finite positive values. The Lyapunov coefficients are assessed through three-dimension curves based on the alterations of the MMC state variable errors and the dynamic of the errors while the Lyapunov coefficient interval is extended. The results achieved from MATLAB/SIMULINK environment verify the capability of proposed control strategy for providing stable operation of MT MMC-HVDC system.