Model predictive control of MMC-UPFC under unbalanced grid conditions

Abstract

This paper proposes a model predictive control (MPC) method to tackle the power flow control of the modular multilevel converter based unified power flow controller (MMC-UPFC) under unbalanced grid conditions. The predictive models corresponding to the discrete-time models of MMC-UPFC are derived. For the shunt MMC, the d-axis and q-axis current references for the MPC are deduced to maintain the dc-link voltage constant according to the energy balance on the dc-link side. The series MMC utilizes a cost function with the voltage constraints to suppress the faulted current and make the power flow controllable, and thus avoid the spread of the fault. Moreover, it makes full use of the capacity of MMC by operating it on the voltage boundary defined by the maximum output voltage of the converter. The model is established using the parameters from a 500 kV MMC-UPFC practical project in Jiangsu, China. Simulation results demonstrate a favorable response of the MMC-UPFC based on the proposed MPC strategy under different unbalanced grid conditions.