Research And Implementation Of Dual Mode Synchronous Inverter In Microgrid

The microgrid has becomes a research hotspot of scholars from global countries due to its effective integration of distributed power generation units and flexible operation.The dual mode inverter is used as the power conversion unit and the connection and off-grid interface between microgrid and power grid,besides,its working characteristics also determines the dual mode inverter has two working modes: off-grid and grid-connected.This paper takes three phase dual mode inverter as the research object,mainly focusing on three aspects: output voltage quality control of island inverters with unbalanced and nonlinear loads,synchronous operation of parallel inverters,otherwise,the smooth switching of inverters on and off grid is also studied in this paper.In view of the output voltage’s distortion and unbalance of the off-grid inverter caused by the nonlinear and unbalanced loads,the characteristics of unbalanced and harmonic voltage under the Park transformation are analyzed,and a positive and negative double sequence voltage control is proposed based on the traditional voltage and current dual loop stracture.The unified control strategy under the negative sequence coordinate system realizes voltage imbalance control by introducing a negative sequence voltage branch and notch filters,and realizes harmonic voltage compensation in parallel with multiple resonance controllers on traditional proportional integral controller.A simulation model is established after parameters design and system stability analysis.The simulation results also show that the above control effectively eliminate the unbalance and harmonic components of the output voltage.Secondly,the synchronous operation technology of multiple inverters is studied.In order to realize the phase synchronization among multiple inverters,the synchronous phasor measurement unit PMU is proposed,the PMU device can generate 1PPS second pulse to provide the inverters with a global synchronization reference phase and realize synchronous operation.On the basis of realizing the phase synchronization,the linear adjustment of the output current amplitude is realized by adjusting the amplitude of the inverter output voltage,so as to improve the accuracy of the output current sharing and restrain the circulating current of the system.Finally,a simulation model is built in Simulink.The simulation results also verify the effectiveness of the 1PPS second pulse provided by PMU for realizing inverter synchronous operation and current sharing.In order to solve the transient shock in the process of inverter’s mode switching,the causes of voltage fluctuations and phase changes during the switching process are analyzed firstly.Aiming at the problem that the control switch and the mode switch are not synchronized,a method of voltage and current weighted control is proposed to reduce the voltage and current amplitude jump during switching time,the shock caused by the inconsistency of the phase is reduced through phase pre-synchronization and off-grid phase tracking based on PMU.The proposed smooth switching strategy was analyzed and verified by building a simulation model in MATLAB/Simulink.In order to further verify the feasibility and effectiveness of the islanding inverter’s control strategy with unbalanced and nonlinear loads,experimental verification was carried out on the relevant laboratory platform,and the hardware circuit and software program of the experimental prototype were designed.The final experimental results also further verify the effectiveness of the control strategy.