| The inverter can run as synchronous generator and obeys the regulation of power grid based on synchronous generator,then the the stability of inverter connected to the power grid can be achieved.The technology is called as virtual synchronous generator(VSG).In the paper,a three-phase four-leg inverter topology is researched and a control strategy of energy storage converter based on virtual synchronous generator is designed.The main contents are as follows.The feasibility of the inverter which simulates synchronous generator is analyzed according to the equivalent model of generator.The difference between resistance inverter and inductance inverter is analyzed considering power transmission characteristic.The inductance inverter is chosen as the research object and the torque equation,frequency modulation,voltage modulation and excitation characteristic are referenced to design the VSG power control loop,and the extemal characteristics of synchronous generator are simulated.According to the grid connected conditions of generator,a synchronous controller of frequency,phase and amplitude is designed.The mathematical model is established in stationary and rotating coordinate system.The coupling relation is analyzed and the voltage current double loop is selected after the power loop in the rotating coordinate system to improve the dynamic performance.In the case of VSG with unbalanced loads,a quasi proportional resonant controller is adopted to reduce the voltage imbalance.With the frequency fluctuation of droop control,the gain of the fixed frequency resonant controller is not enough in the resonance point.To solve this problem,an adaptive resonance control strategy is put forward and the resonance point can be changed with the variation of the voltage frequency adaptively to guarantee the effect of prohibiting imbalance.Combined with the development of microgrid,the photovoltaic module and energy storage module are introduced.The model of photovoltaic cell is established,several maximum power tracking methods are analyzed,and the effect of disturbance observation method is simulated.Based on the mathematical model of two kinds of batteries,the control strategy of bi-directional DC to DC converter for lead-acid batteries is proposed.A coordinated control strategy is to deal with the energy coordination between photovoltaic modules and energy storage modules connected to the DC side of VSG.The characteristics of VSG in island mode,synchronization modulation,power response and unbalanced load are tested by simulation and experiments.The test results verify the correctness of the control method.The photovoltaic and energy storage modules are added and the stable adjustment of energy between the two modules proves the correctness of the coordinated control strategy. |
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