Research On Distributed Microgrid Stability And Operation Control

With the increasing pressure of environmental protection,the development of new energy resources has attracted the attention of all countries.Microgrid,as an effective application means has been proposed for nearly ten years.By integrating various distributed generation into the grid,the microgrid promotes the permeability of distributed generation,improves the reliability of power supply,and helps alleviate some environmental problems associated with burning fossil fuels.However,since the distributed generation generally adopts the inverter interface,the flexibility of the microgrid control is increased,and the characteristics of microgrid are greatly difference from the traditional grid.Further,the low inertia of the microgrid and the intermittent and random nature of the output power increase the difficulty in maintaining energy balance and stability of network.In this paper,the stability of microgrid which include the analysis of the stability boundary of the microgrid controller parameters,grid-connected stability and controller design has been studied with the support of two foundations,aiming at providing a theoretical reference for control design and micorgrid operation.The topic is divided into the following issues:(1)The modeling method.Due to the nonlinearity of distribute generation,the small signal model has been widely used in the stability analysis.However,this model essentially relies on prior equilibrium points and difficult to tackle the parameters that affect the equilibrium points.In this paper,the equilibrium points of DG system based on traditional droop control is deeply explored,and the relationship between equilibrium points and power and droop coefficient is revealed.Meanwhile,according to the operating conditions of DG unit,the dq component of equilibrium points and power and droop coefficient is further discussed and the simplified models are obtained.Based on this model,we disscuss the droop coefficient stability boundary by the eigenvalues of the system,with and without consideration the variation of equilibrium points.Besides,the stability of coefficients is also investigated when the power change.Due to the supplement of equilibrium points model,the small signal can effectively estimate the stability of system with a series of continuous equilibrium points,and provides a new approach for problem of stability boundary of parameters.(2)The method to calculate of parameters stability boundary.Due to the influents of controller on the stability,it is necessary to optimize the parameters of controller within the stability range of multi-parameters.For this reason,this paper proposes a method based on Mu for multi-parameters mixed stability range problem.By treating the parameters as interval variables and expressing as uncertain parameter,two methodologies are proposed to calculate the boundaries of interval variables.In this paper,we discuss the stability boundary of frequency droop parameter and voltage droop parameter in DG unit,the results show that the burden of exhaustive iteration is avoided when dealing with the multi-parameters,and the accuracy of results are comparable to that of eigenvalue method.(3)Stability margin and stability performance analysis of grid-connected distributed unit.This paper introduces robust stability and robust performance to evaluate grid-connected distributed unit.At present,the controller design of grid-connected is mainly based on the nominal system with an aim to achieve to dynamic performance and stability of system.However,it is inevitably affected by the perturbation of component parameters caused by the operation environment,operation conditions,and so on.Consequently,the performance of system is deviate from the theoretical mathematical expectation,and the designed controller may be not meet the operational requirements.To tackle this problem,this paper applied the robust stability to check the controller with the all possible operation conditions.For instance,the change of parasitic impedance,the uncertain of line impedance,grid impedance under weak grid and the grid voltage sag under fault.Meanwhile,the tracking performance and disturbance rejection capability are also evaluated using the Mu robust performance tools.The analysis results are very useful to the design of the controller.(4)The design of controller for an LCL-based grid-connected DG unit.The connection of a distributed generation(DG)unit to a weak power system is challenging due to stability issues resulted from the uncertainty of system impedance.This paper designs the based controller to improve the performance of grid-connected current feedback control.In order to decouple active and reactive components in control system,a multi-input multi-output model reflecting the interaction between the components of d and q is established.Under this model,the decoupling requirement,together with steady-state error and overshoot is transformed into the design problem of controller.By employing the Mu synthesis tools,the robust feedback controller is obtained.The simulations show that the controller not only guarantees the robust stability and robustness of the system under the uncertainty of the inductance of the power grid,but also realizes the decoupling of the D and Q components of the current.