Control Strategy Of Voltage Source Converter Based On Virtual Synchronous Generator

Nowadays, the distributed generation(DG) based on clean and renewable resources is undergoing a significant development because of environmental problem and energy crisis. As the key linkage between DG and the grid, microgrids play an important role in utilization of renewable energies, which can improve the stability of power grid and reduce the negative impact of DG. However, compared with synchronous generators(SG),the conventional inverter lack of inertia, unable to provide voltage and frequency support. Therefore, improving the dynamic characteristics and stability of DG system through proper control algorithm, is of great significance.According to traditional power system operation experience, if the inverter has the same characteristics with SGs, the performance of distributed generation system will be improved. This paper presents a virtual synchronous generator control strategy for voltage-source converters(VSCs) in microgrids based on the concept of synchronverters. The added inner control loop is in charge of the regulation of the current and voltage, which ensure high power quality injection and inherent current limitation during faults. The secondary frequency regulation is employed to eliminate the permanent frequency drop. Compared with traditional inverter control methods, the proposed virtual synchronous generator control strategy can realize seamless transition from grid-connected mode to island mode. It can increase system inertia, provide frequency and voltage support to local grid and automatically synchronize itself with the grid without a phase-locked loop(PLL).The proposed control strategy is divided into the inner control loop and the outer control loop. The inner controller is developed for fast current and voltage regulations using voltage-oriented vector control. The control parameters is designed based on the mathematic model of VSC established in d-q reference frame. The outside power controller is developed using the transient mathematic model of SG. A governor and an excitation controller are adopted for active power/frequency and reactive power/voltage regulation. In addition, the secondary frequency regulation can be achieved by modulating the scheduled power to eliminate the permanent frequency drop. The impact of main control parameters and line impedance on system stability and dynamic response is analyzed based on small-signal linearization and the frequency response techniques. The synchronous reactance is imitated by a virtual impedance. Just like SG, it needs a preliminary synchronization process to avoid overcurrent caused by the difference of voltage amplitude and phase between inverter and grid. This paper presents two methods. The one is based on virtual armature current, and the other is based on three phase phase-locked loop. The control method is verified by time simulation under various microgrids operating scenarios.