For virtual synchronous generator (VSG) based microgrids, it is challenging to address the adverse effect of nonlinear loads and grid background harmonics with existing harmonic suppression methods. To address this issue, a decomposed grid harmonic current suppression (DHS) method with proper feedforward point of common coupling voltage and feedback VSG output current as well as nonlinear load current is proposed for VSG-based microgrids. The proposed DHS method can separately mitigate the grid harmonic current caused by nonlinear loads and grid background harmonics, obtaining a promising harmonic suppression effect without potential system instability problem. Compared with conventional harmonic control methods, the DHS method reduces the control sensitivity to system parameters including line impedance, grid impedance, and system shunt capacitance, improving the adaptivity of harmonic control in distorted grids. The proposed method's effectiveness and reliability are verified by theoretical analysis and comparative experiments.

Generally running with frequent braking over short distances, the urban rail transit train generates great quantities of regenerative braking energy (RBE). The RBE feedback system can effectively recycle RBE and give it back to the AC grid. However, the lack of damp and inertia of generators makes conventional PWM RBE feedback system more sensitive to power fluctuations. To address this issue, a synchronverter-based RBE feedback system of urban rail transit is designed in this paper. First, the structure of the feedback system is presented. Then, the synchronverter-based control strategy with greater flexibility and higher stability is fully discussed. Furthermore, the parameter design of the system is analyzed in detail. Finally, simulation results and experimental results are provided to show the good dynamic performance of the system. Using this synchronverter-based approach, the system supplies traction power to the traction network when the train accelerates and gives the RBE back to the AC grid when the train brakes, in light of the variation of the DC bus voltage. Moreover, the system can be self-synchronized with the AC grid and make corresponding power management on the basis of changes in the voltage amplitude as well as the frequency of the grid. In this sense, the RBE feedback system becomes more flexible, effective and robust.