Stability Analysis And Control Strategies Of DC Microgrids

Due to the characteristics of low inertia and weak damping of the DC system and the negative damping of constant power load(CPL),the damping of the DC microgrid is further weakened.Hence,the stability of DC microgrids has become an important content in the research of DC microgrids.This paper mainly focuses on the stability issue of the drop-controlled DC microgrid operating in the island operation mode.The grid-forming converters(GCs)of the energy storage system parallel operate to act on the voltage stability control,and its operation mode can be divided into two kinds.In the first mode,and each GC adopts droop control.In the second mode,one GC adopts constant voltage control,and the other GC adopts constant current control.Therefore,in this paper,the transient and steady characteristics of the two operation modes are studied firstly,and then the large-signal stability analysis and stable operation control strategy design are carried out.In the first mode,the focus of the steady-state response is the current-sharing control between GCs.The current sharing is affected by the resistance between the output port of GCs and the load,making the current sharing cannot be achieved strictly proportionally to the droop coefficient.Therefore,this paper proposes a current-sharing strategy based on virtual voltage.In order to obtain the virtual voltage,according to the autonomous decentralized control,this paper proposes an improved dynamic consistency algorithm.Each GC can obtain the virtual voltage by only communicating with the adjacent GC to strictly realize the current-sharing.In addition,the virtual voltage finally converges to the average value of the output voltage of all GCs,which can reflect the bus voltage level and provide the basis for bus voltage compensation control.The transient response in the first mode focuses on how to accelerate the transient process when power fluctuation occurs.In this paper,the cause of the transient process is analyzed and the DOB(Disturbance Observer)method is first adopted.However,although this method can accelerate the transient response,the reference value transferred from the voltage loop to the current loop is forced to be zero because of the feedback of DOB to the current loop,which bypasses the droop loop and disables the virtual resistance.Therefore,the parallel GCs cannot achieve the predesigned current-sharing.An improved strategy named WDOB(Weakened Disturbance Observer)is then proposed.Through this method,the feedback is weakened and the virtual resistance is enabled again.Therefore,the transient response of the system can be finally improved without affecting the current sharing.The constant voltage strategy is adopted in the second mode,and the steady response problem is not prominent.Hence,this paper mainly concerns its transient response.In this paper,the transient process is analyzed firstly,and the reason for its limitation is given.Then,a feedforward control strategy based on voltage deviation is proposed,which can aecelerate the transient response of the current loop under the same voltage deviation.However,due to the existence of a hysteresis comparator,the overshoot of the current inner loop is easy to cause the repeated in-and-out operation of the feedforward loop in the transient process.Therefore,this paper puts forward the feedforward retainer to solve this problem,and expounds on its design method.On the basis of the pretty good steady and transient performance of DC microgrid,the large-signal modeling is carried out based on some reasonable assumptions,and the model is finally established from the aspects of power supply and equivalent loads.Because the CPLs make the system nonlinear,the stability analysis method of a linear system is no longer applicable.This paper uses the Lyapunov direct method and the mixed potential function method to analyze the large-signal stability of DC microgrids and verify each other.Besides,the stability criterion is simultaneously obtained.Then,according to the stability criterion,the multiple steady-state solutions in a mathematical sense are screened.Finally,one kind of solution is selected and its stability constraint of large-signal stability is given.The stable operation of DC microgrids needs to consider both the stability constraint and the voltage deviation constraint,thus the intersection of the two constraints is taken into consideration in the operation control strategy.It can be seen from the two constraints that the stable operation of the system is mainly affected by CPL and virtual resistance.Therefore,this paper first analyzes the power limit curve of the stable operation of the system,then analyzes the feasible region of virtual resistance design under the constraint of the power limit curve.This paper puts forward the optimization design method based on the containment algorithm to balance the requirements of both constraints.Finally,according to the power limit analysis and the optimal design of virtual resistance,a hierarchical stability control strategy of DC microgrid is proposed.