Control strategy for voltage source converter based high voltage direct current under unbalanced conditions

Three-phase unbalance causes performance deterioration of voltage source converter based high voltage direct current (VSC-HVDC) transmission system by producing large negative sequence current in the ac side and harmonics on the dc-link voltage. Aiming at this problem, a control strategy of negative sequence voltage compensation is proposed in this paper. The solution is to generate the required negative sequence voltage at the VSC output terminal to attenuate the effect of negative sequence grid voltage by Hilbert transform and instantaneous symmetrical components. Control structure based on this method is simple and easily implemented. The digital simulation based on PSCAD/EMTDC shows that the proposed controller has good control performance under unbalanced conditions.