Improved comprehensive energy-based control for MMC-HVDC system

Abstract

The interconnection of two non-synchronous AC grids is one of the promising applications in the modular multilevel converter (MMC) based high-voltage direct-current transmission system (MMC-HVDC). Due to the complicated internal dynamics of the MMC, energy-based control is becoming an emerging technique to guarantee the robust stability of the MMC. However, conventional energy-based control cannot be applied simultaneously to the power-controlled MMC (PCMMC) and voltage-controlled MMC (VCMMC) interconnected by short or medium DC transmission lines. To address this problem, this paper proposes an improved energy-based control for the MMC-HVDC system aiming to achieve full control of the inner dynamics of MMCs regardless of the DC line length. A completed decoupled mathematical modeling with comprehensive state variables of MMCs is established. Then, a unified control framework for the PCMMC and the VCMMC considering AC grid conditions is developed. A distinct advantage of the scheme is that direct and fast regulation of the DC-side voltage can be achieved to improve the voltage behavior. The other is that the dynamic performance of the submodule capacitor voltages of VCMMC and PCMMC can be conspicuously improved under normal, AC system fault and arm parameter asymmetry conditions. Comparative simulations and hardware-in-loop (HIL) experimental results validate the effectiveness of the proposed methodology.