The Research Of Control Strategy Of Micro-Grid Inverter Based On LCL Filter

As the widely application of the new energy, the micro grid system is getting more attention of the world which is composed of distributed generation system. The research of control strategy of micro-gird inverter based on LCL filter is directly related to the performance of micro-grid system. There are several key issues on the micro-grid inverter LCL filter in the micro-grid power system applications:It needs add damping control strategy to suppress resonance peak due to the presence of LCL filter resonance characteristics which causes system instability; Micro-grid inverter control strategy in the unbalanced conditong has a very important practical significance; Quickly computing of grid voltage rms and phase synchronization would affect the inverter results directly. Around the key technology of micro-grid inverter, this paper carries out the theoretical and experimental research.The efficient operation of the micro-grid has a certain theoretical guiding significance and practical value, also the research results can be applied to practical engineering. The models of micro-gird inverter based on LCL filter are established respectively in three-phase stationary frame, two-phase stationary frame and two-phase rotating reference frame. The damping method is proposed after analyzing the resonant characteristics based on the models established before. The method using capacitance branch current feedback control presents a good performance to damp the resonance of LCL filter, but it needs extra voltage or current sensors in the capacitance branch which increases the hardware cost of the system, reduces the operational reliability. Therefore, no damping strategy is proposed to control micro-gird inverter based on LCL filter through avoiding extra sensors.The grid usually has phenomenon of slight imbalance or even deep imbalance caused by serious grid failure, which would lead to output voltage and current be poor stability. Based on the analysis of the impact on the control of micro-grid inverter, this paper uses the PI controller based on added resonance (R) controller to eliminate the effects of negative sequence voltage component of the micro-grid inverter operation. But adding resonant controller makes micro-grid inverter control strategy be complicated. In order to avoid increasing the complexity of the system control, voltage feed-forward control strategy is presented in this paper based on the method without R controller. And experimental verification of the control method is illustrated to prove its effectiveness. Also it is proposed a new method in this paper to get voltage effective value rapidly based on IIR low-pass filter comparing full-cycle method, derivative method and iterative method in micro-grid inverter. This algorithm only needs one sample of voltage to get voltage effective value rapidly, conveniently, and has strong anti-jamming capability comparing the three methods previously mentioned. Therefore, a novel scheme for grid fundamental positive sequence voltage detection is also proposed. By means of transferring three-phase voltage to α, β two-phase reference frame and converting with two simple mathematical transformations, it is able to obtain grid fundamental positive sequence voltage through eliminating negative sequence and harmonic component of grid voltage in α,β reference frame. Based on the grid fundamental positive sequence voltage, a software phase locked loop, which is used for idea grid, is applied to track grid voltage synchronization phase rapidly and correctly.Control Strategy based on LCL filter consists of two micro-grid inverter distributed generation system consisting of micro-grid system is proposed in this paper. The strategy analyses and uses a kind of micro-grid inverter novel droop control with factor correction algorithms, combining with virtual impedance control to achieve active and reactive power completely decoupled and improve the accuracy of the reactive power.The prototype of micro-gird inverter based on LCL filter is researched and developed to test and verify the control strategy mentioned before. The inverter can realize several functions such as reactive power compensation, black start, anti-islanding operation, strategies of charge and discharge with constant power, constant voltage or constant current etc. On the basis of the prototype of micro-grid inverter, this paper operates several experiments, and the experimental results proved that the proposed effectiveness and correctness of the inverter control strategy.