Control Strategy For Three-Phase-Four-Wire Virtual Synchronous Generator

Grid-connected inverter is the interface between the renewable energy generation system(REGS) and the power grid. Operating the grid-connected inverter to mimic a synchronous generator will make the grid-connected inverter has the ability to provid grid support by automatically changing its active and reactive power based on the frequency and amplitude of grid voltage. Thus, the grid-connected inverter is called virtual synchronous generators(VSGs). This thesis focuses on the modeling and parameter design of the power loop of the VSG, the control strategy of the independently-controlled neutral-line module and the pre-synchronization method of the VSG.This thesis firstly introduces the basic operation principle of the VSG, and then proposes the line-frequency-averaged small signal model of its power loops. Based on the model, it is further demonstrated that the coupling effects between the active power loop and the reactive power loop can be ignored under certain conditions. As a result, the control parameters of each loop can be designed independently. In order to guarantee the stability, the dynamic performance and the attenuation of the pulsating power of the system, the design method of the control parameters of the VSG is proposed in this thesis. This design method has the advantage that the control parameters can be easily determined without trial-and-error procedures.The independently-controlled neutral line module(ICNLM) is often used to provide the zero-sequence current path for unbalanced loads when the VSG operates at standalone mode. This thesis studies the control requirements from the viewpoint of the output impedance, and demonstrates that in order to reduce the voltage ripple of the neutral point, the impedance of the inductor branch should be low enough compared to the impedance of the capacitor branch at the fundamental frequency. Based on the requirements of phase margin and the impedance of the inductor branch, the design method of the control parameters of the ICNLM is proposed, with which, the control parameters can be calculated directly without try-and-error procedure.In order to reduce the inrush current at the moment when the VSG is connected to the grid,the pre-synchronization method is proposed in this thesis. With the dedicated synchronization unit, which including frequency regulator, voltage regulator and phase regulator, the output voltage of the VSG can be exactly synchronized with grid voltage, which ensures smooth transition of the VSG from standalone mode to grid-connected mode.A 10 k W three-phase-four-wire VSG prototype has been built and tested in the lab. The experimental results are given to verify the effectiveness of theoretical analysis and parameters design method presented above.