Research On Smooth Transition Control Strategy Of Inverter Between Grid-connected And Islanded Modes Under Unbalanced Voltage

With the national great support and investment in clean energy,the proportion of distributed power generation in the grid is increasing.However,distributed generation is intermittent and unstable,which will impact the stability of the grid.In order to consume large-scale distributed generation and improve the utilization of new energy,the concept of microgrid has been proposed.Microgrid has grid-connected and islanded modes,and smooth transition between this two modes is the key for the safe and stable operation of the system,which has major research potential.In this thesis,the control strategy of smooth transition between microgrid operating modes is analyzed and studied.Since microgrids are generally at the end of the power system,voltage unbalance occurs in the external grid from time to time,and often lasting for a longer period of time.Most of the existing control strategies for grid-connected transition of microgrids are based on ideal three-phase grid voltages,and the control strategy for smooth transition of grid connection under grid voltage unbalance has not yet received attention.In this thesis,the control strategy for smooth transition of grid connection under voltage unbalance is analyzed and studied.Firstly,the mathematical model of the microgrid inverter is derived based on its main circuit topology.A double closed-loop control system with capacitor voltage and inductor current as internal loop feedback variables was designed in the d-q synchronous rotation coordinate system.Droop control is selected as the external loop control strategy after analysis.Its basic principle and structure are described.The control structure framework of microgrid inverter based on droop control is given,which lays the foundation for the later research.Then,a PLL-less pre-synchronization control strategy based on virtual current is proposed,which can realize smooth transition between microgrid operating modes.The virtual impedance is put between the AC bus of the microgrid and the PCC point of the grid.Calculating the virtual current flowing on the virtual impedance and controlling it to zero can achieve the phase and amplitude synchronization of the microgrid and the grid.Compared with the traditional pre-synchronization strategy,the proposed strategy avoids the phase-locked loop and multiple PI controllers,has a simple structure,which can improves the stability of the control.The proposed strategy is combined into the droop control of the microgrid inverter which achieves smooth transition between the two modes of the microgrid.The feasibility of the PLL-less pre-synchronization control strategy based on virtual current is verified by simulation.Afterwards,an improved pre-synchronization control strategy for microgrid inverters is designed for the case of grid voltage unbalance.In the case of voltage imbalance,the virtual current will also be asymmetric,for which the positive and negative sequence separation is performed.The PLL-less pre-synchronization control strategy based on virtual current is improved by adding a negative sequence,which can control the positive and negative sequence component of the virtual current to zero.The proposed control strategy does not require a phase-locked loop which can achieve the phase and amplitude tracking of the microgrid voltage to the unbalanced grid voltage.In order to suppress the current unbalance,a negative sequence controller with the target of current balancing is designed.The effectiveness of the balanced current control and the pre-synchronization control strategy under grid voltage unbalance is verified by simulation.Finally,the software program of the inverter is designed,and the working principle and flowchart of the main program are given.A microgrid inverter hardware-in-the-loop experimental platform based on RT-LAB real-time simulator and DSP F28335 controller is built in the laboratory.The inverter software program of the PLL-less pre-synchronization control strategy is verified,followed by the pre-synchronous control strategy under voltage unbalance.