Passivity-based control of HVDC transmission system based on modular multilevel converter under unbalanced grid conditions

This paper investigates the passivity-based control of Euler-Lagrange model for modular multilevel converter based HVDC (MMC-HVDC) under unbalanced grid conditions. The topology and operation principle of MMC are firstly introduced. Then the d-q Euler-Lagrange (EL) mathematics model of MMC is derived, and the passivity-based control rule is also deduced with the consideration of desired equilibrium point, state and state error. On the basis of them, the passivity-based control scheme of MMC, for unbalanced grid conditions, is proposed including the inner decoupled positive-negative sequence passivity-based controller and the outer loop controller. An improved outer loop active power controller is integrated DC voltage control unit to reduce DC voltage fluctuation caused by grid faults. At last, the simulation model of 21-level MMC-HVDC transmission system is built in PSCAD/EMTDC. The results show that the proposed control strategy has better dynamic characteristics and stability under different fault conditions.