Advanced Control Method for VSC-HVDC Systems Connected to Weak Grids

This paper proposes a novel control method for modular multilevel converter IMMC) based high voltage direct current IHVDC) power transmission system connected to a very weak or strong AC system under balanced or unbalanced operating conditions. The control concept is developed based on no need for phase-locked loop IPLL) where the voltage source converter IVSC) is controlled directly in ABC frame. The proposed control scheme regulates the instantaneous active and reactive power independently at the Point of Common coupling IPCC). A Multi-Input-Multi-Output IMIMO) state space model has been derived in ABC frame for the MMC converter. An enhanced outer loop is presented for achieving a smooth transition between weak and strong AC network by providing reactive power support to the AC voltage at PCC and exchanging maximum amount of active power within the converter rating limits. Moreover, the controller is designed to tolerate for computational delays. The effectiveness of the proposed control is validated by simulations results.