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The DC-link voltage fluctuation caused by the change of working state of the load motor has been one of the key issues in the PWM rectifier-inverter system. In this study, an improved model predictive control (MPC) scheme is proposed to address this problem. The MPC is applied to both the rectifier stage and the inverter stage in the system. Direct power control is used in the rectifier stage and the direct torque control is employed in the inverter stage, with the key novelty of the active power reference values being defined by both real-time and periodic compensation power based on the system-level power balance model. Meanwhile, a MPC algorithm based on a two-step prediction is introduced to compensate for the delay of a digital controller. Comparison has been conducted between the proposed scheme and three other methods. Simulation and experimental results show that the proposed control scheme exhibits good performance in both the rectifier stage and the inverter stage with improved dynamic response and suppressed voltage fluctuation of the DC-link voltage.Index Terms-Direct power control, direct torque control, model predictive control, PWM rectifier-inverter system, system-level power balance.include nonlinear system control, advanced control theory research and its application in metallurgical industry, the rolling process automation, and electric drives. In these fields he has published more than 150 papers in international journals and conferences. He is the chief editor of Metallurgical process automation, has written 22 book chapters and he is coauthor of 4 books.Haiping Du (M'09) received his Ph.D. degree in mechanical design and theory from Shanghai

With the rapid development of modern distribution network and the access of distributed generation, the network structure is becoming increasingly complex. Frequent single-phase break faults have seriously affected equipment and personal safety and stable operation of the power system. However, with the development and application of the composite neutral grounding modes, the protection of single-phase break fault is facing new challenges. This paper proposes a protection method of single-phase break fault for distribution network considering the influence of neutral grounding modes. The characteristics of neutral voltage and sequence current are analyzed under normal operation and single-phase break fault with different grounding modes. Following this, the protection criterion based on neutral voltage and sequence current variation is constructed. The protection method of singlephase break fault for distribution network is proposed, which is applicable for various neutral grounding modes. Theoretical analysis and simulation results show that the protection method is less affected by system asymmetry, fault location and load distribution. The method has higher sensitivity, reliability and adaptability.

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