Research On Unbalance Suppression And Resonance Problem Of Parallel Inverter

Due to the energy dilemma, distributed generation(DG) and microgrid technology gradually become a hot topic. This dissertation focuses on microgrid islanding characteristics, unbalance suppression and the analysis of parallel resonance.Firstly, a survey of the background, development was provided. There are some constraints for existing unbalance suppression method. System parameter variation may cause sharing error of negative sequence current. Most of the parallel resonance literature was based on grid-connected inverter, while voltage-source inverter was neglected.Secondly, a model for three phase inverter and parallel inverter system was built, and related controller was designed. Double-loop control was adopted, and output voltage feed-forward was used to improve performance. The parameter of droop control was determined by maximum allowable deviation. Then the closed-loop bandwidth of power loop was verified.Thirdly, a new kind of voltage unbalanced compensation strategy was proposed. From the view of physical significance, the transformation of equivalent block diagram was deduced. Then related controller and compensation factor was designed by the system stability analysis. The performance of proposed strategy and traditional control was compared by simulation and experiment. In addition, the difference between theory, simulation and experiment performance was briefly discussed.At last, stability was analyzed in two-time scale. Power sharing controller may causes low frequency destabilization. On the other hand, impedance interaction may lead to high frequency resonance, whose frequency is normally greater than control bandwidth. A small signal model of power controller is built, and stability is analyzed by the closed-loop poles. Improper droop parameter may result in system instability. One beat delay was high order expanded, which exhibits dissimilar performance compared with inertial element. The parallel resonance can be explained by the use of impedance criterion and closed-loop poles. The result showed the gain of current controller can influence inverter output impedance, to some certain extent. The variation of output impedance may improve system stability margin. Grid-side inductance also affects system stability to a limited extent. The theory is verified by simulation and experiment.