Dynamic Stability Analysis And Control Strategy Of DC Microgrid

DC microgrid has become one of the effective networking methods to access new energy source and DC load because of its flexible power regulation ability,simplified converter link,and no complex stability problems such as power angle stability and frequency stability.However,due to the isolation effect of power electronic converter,the DC microgrid lacks inertial support of rotational kinetic energy,resulting in low inertia of DC microgrid,and the DC bus voltage will fluctuate violently during power disturbance.In addition,there are a large number of constant power loads(CPL)in the DC microgrid,whose negative damping characteristics will weaken the stability margin of the system,and the cascading effect of the converter may also cause resonance,leading to osc illation of the system,so the DC microgrid has weak damping characteristics.In this paper,low inertia and weak damping characteristics of DC microgrid are studied(1)Firstly,a four port DC microgrid model including photovoltaic power generation unit,energy storage unit and AC / DC load is established.The working principle and control mode of each port are explored Through small signal analysis of the mathematical model of each port,the corresponding small signal model of each port is obtained.Based on this,the equivalent impedance model of DC microgrid is established,which lays the foundation for the subsequent dynamic stability analysis.(2)According to the impedance ratio criterion and unit step response,the low inertia and weak damping characteristics of DC microgrid are analyzed theoretically.Then,the advantages and disadvantages of variable droop control,state feedback active damping control and traditional virtual DC machine control(VDMC)are analyzed.In addition,based on the model of DC machine,this paper proposes a VDMC strategy for energy storage converter,which enables the energy storage unit to simulate the power regulation characteristics of DC machine,provide inertia and damping support for DC microgrid,and comprehensively improve the dynamic stability of the system.(3)Because the proposed control strategy needs to add current feed-forward and variable droop control links,which makes the control structure more complex.Then,the proposed VDMC strategy is improved,and a novel VDMC is obtained,which has a more simplified control structure and better dynamic performance,The dynam ic stability of the novel VDMC is analyzed.(4)The model of DC microgrid is built in Matlab / Simulink software,and the simulation results show that the proposed control strategy can effectively improve the dynamic stability of DC microgrid,namely inertia and damping characteristics,and ensure the safe and stable operation of the system.