Abstract
Virtual Synchronous Generators (VSGs) can reduce the frequency and power oscillation in the grid. For parallel multi-VSGs in island microgrid, the differences of equivalent output impedance and line impedance affect the orderly sharing of reactive power and restraining of circulating current greatly. The power sharing and circulating current characteristics of VSGs are analyzed in this paper. To solve this problem, an optimization control method for parallel VSGs is proposed, which included the inner voltage and current loop and the outer reactive power loop. In the inner voltage and current loop, a virtual complex impedance including a resistive component and an inductive component is introduced. It reduces the coupling between active power and reactive power, and it reduces the disorderly distribution of power due to impedance differences. In the reactive power loop, the output voltage feedback and integrator links are adopted, which reduce the deviation of output voltage and restrain circulating current. The effects on the equivalent output impedance in the different control parameters and virtual complex impedance are analyzed, the effects on system stability in the different resistive components of virtual complex impedance are analyzed, and the proper parameters are selected. Simulation and experimental results show the correctness and validity of the proposed control method.
Highlights
By increasing the inertia and damping links in an active power control loop, the grid support capability in a weak grid of virtual synchronous generators (VSGs) is summarized in [1, 2]
When multiple VSGs are operated in parallel, there are differences in the output voltage, equivalent output impedance, and connection line impedance of the individual VSGs, which are caused by the different controller parameters, nonlinearities in the power devices, differences in filter parameters, and the distance from the grid connection point. ese differences affect the accuracy of power distribution, increase the fundamental current circulation between the VSGs, and may cause overcurrent faults [3]
From the analysis of the current circulation characteristics, it can be seen that the current circulation of the parallel VSGs is suppressed by achieving the output voltage and transmission impedance of the two VSGs is equal. e suppression of the current circulation can be realized by increasing the equivalent output impedance
Summary
By increasing the inertia and damping links in an active power control loop, the grid support capability in a weak grid of virtual synchronous generators (VSGs) is summarized in [1, 2]. In [12], a consistency-based parallel VSGs distributed control method is described This enhances the system frequency and active power characteristics, it requires communication between the parallel VSGs. is paper describes a method that improves the traditional virtual synchronous power generation technology, based on analysis of the parallel operation characteristics of multiple VSGs. e optimization control strategy includes the outer reactive power loop and the inner voltage and current loop. In the inner voltage and current loop, a virtual complex impedance including a resistive component and an inductive component is introduced. It reduces the effects on decoupling between active power and reactive power, and it reduces the effects on accurate power sharing due to the impedance difference. In the reactive power loop, the output voltage feedback and integrator links are adopted, which will reduce the deviation of output voltage and restrain circulating current
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