Research On Stability Of DC Microgrid With Constant Power Loads

The grid-connected inverter is the key interface device between DC microgrid and large power grid,the choice of control strategy and the parameter design of line filter will directly determine the power quality of DC bus voltage.With the rapid development of DC microgrid technology and the significant increase in the permeability of constant power loads in DC microgrid,the traditional method of stability analysis of microgrid is no longer applicable to DC microgrid with a large number of constant power loads.Compared with resistive load,the negative impedance characteristic of constant power loads will reduce the stability of DC bus voltage,and the interaction between the filter device and the power electronic conversion device in front of constant power loads will cause the bus voltage resonance.Ensuring the bus voltage within the allowable range is the basis for the stable operation of DC microgrid,and reasonable parameter design and effective control method are indispensable to ensure reliable operation of DC system.This thesis intends to take the DC microgrid with constant power loads as the research object,and analyzes the influence of constant power loads on the voltage stability of DC bus in detail.Combined with the large signal analysis and the small signal analysis,the dual damping control strategy is proposed to improve the anti-interference performance of the system.The specific research contents are as follows:(1)Based on the existing literature,the topology of grid-connected converter is analyzed,and the switch function mathematical model in two-phase rotating coordinate system is established.According to the impedance characteristics,the loads connected to DC microgrid are classified,and the ideal model and linear model of constant power loads are established.(2)In view of the problem that the traditional stability analysis method cannot effectively provide the design basis for the filter parameters of the constant power loads loop and the limit value of the constant power loads connected to the DC microgrid,this thesis adopts the large signal stability analysis method to design the filter parameters of the constant power loads loop.And a damping filter circuit is added to eliminate the resonant peak caused by the interaction between the filter device and the power electronic converter,so as to ensure the stability of the system under large disturbances.(3)Aiming at the influence of the negative impedance characteristic of constant power loads on the voltage stability of DC busbar,an active compensation method based on high-pass filter is proposed to increase the damping of the system and reduce the voltage drop when the DC bus voltage is disturbed,and the correctness and effectiveness of the active compensation method are verified by simulation experiments.(4)In order to reduce the negative impedance characteristic of constant power loads and eliminate the resonance caused by the interaction between filter device and power electronic conversion,the dual damping control method,which combines the damping filter circuit and the active compensation method,is used to achieve the goal.The impedance ratio criterion is used to analyze the small signal stability of DC microgrid with damping filter circuit and active compensation.On the basis of the above,using MATLAB to simulate the DC microgrid,The simulation results show that the dual damping control strategy effectively reduce the voltage drop when the DC bus voltage is disturbed,and effectively inhibits the resonant peak caused by the interaction between the filter device and the power electronic conversion device,thus improving the power quality of the DC bus voltage.