Due to the large-scale renewable energy connected to the power grid by power electronic converters, the inertia and stability of the power grid is declining. In order to improve the inertia and support the grid recovery, the three-phase converter works as a virtual synchronous generator (VSG) to respond to the frequency and voltage changes of the power grid. This paper proposes a model predictive control for the virtual synchronous generator (MPC-VSG) strategy, which can automatically control the converter output power with the grid frequency and voltage changes. Further consideration of fault-tolerant ability and reliability, the method based on improved voltage vector selection, and reconstructed current is used for MPC-VSG to ensure continuous operation for three-phase converters that have current-sensor faults, and improve the reconstruction precision. The proposed method can respond to the frequency and voltage changes of the power grid and has fault-tolerant ability, which is easy to realize without pulse width modulation (PWM) and a proportional-integral (PI) controller. The effectiveness of the proposed control strategy is verified by experiment.
Virtual vector is applied in neutral-point-clamped (NPC) three-level converter finite control set model predictive control (FCS-MPC) strategy due to its good performance of less current ripples and high power efficiency. However, in the process of adding virtual vectors, some redundant virtual vectors will be selected. Excessive virtual vector will increase the computational burden and affect grid-tied current performance. A virtual vector optimal selection strategy for model predictive control to determine the vectors is proposed. The strategy calculates the fluctuation of grid-tied current through theoretical analysis. Based on the analysis of amplitude fluctuation of grid-tied current, the graphics of coverage angle visualization can be obtained under different virtual vector. The angle of the required virtual vectors is obtained through the graphics of coverage angle visualization. The number of virtual vector can be determined when the error current angle range covers the entire vector graph. The proposed control strategy was compared with the conventional control strategies, including excessive virtual vectors or with no any virtual vector. Experimental results show that the proposed control strategy can significantly reduce THD, grid-tied current error and computational burden.
The truck with Electrically Controlled Air Suspension(ECAS) could reduce the loss of goods by setting running height when transporting, and to enhance the loading &unloading efficiency, a height modulating model was built by combining the theories of thermodynamics of variable-mass charging-discharging system and the vehicle kinetics. To solve such problems as “over-charging”, “over-discharging” and surging, a height control strategy of variable integral PID/PWM was proposed and its controller was developed base on the Freescale single chip. Finally the semi-physical rig was built for simulating the truck with the height controller, and the testing results show that the designed control system satisfies the requirement of height modulation.
The servo feed system is an important factor affecting the performance of NC (numerical control) machine tool. A high performance permanent magnet synchronous motor (PMSM) servo feed system of NC machine tool based vector control is introduced in this paper, and a double closed-loop control system with the speed and current loop is applied. The speed regulator adopts PI control, and the current regulator adopts current hysteresis band. The simulation Model of AC PMSM servo feed system is constructed in Matlab platform,according to the CNC servo system performance requirements,simulation and study for different interpolation is implemented. The reasonability and validity have been testified by simulate result,which provide a theoretic basement for optimizing performance of AC servo feed system.
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