Research On Islanding Control Considering The Feasibility Of Islands' SEP

The safe and steady operation of power system is a necessary prerequisite for the harmonious and stable development of society.Recently years,there have been many major blackouts worldwide,which have had a significant impact on local life and economy.Tracing back to the causes of these blackouts,we can find that when interconnected power grids encounter serious faults and the first and second lines of defense of power grid protection cannot prevent the further instability of power grids,if the splitting which is the third line of defense of power grids cannot successfully divide the unstable power grids into some stable islands which can continue to supply power for loads,it will lead to further expansion of the faults.Consequently,the blackout of power grid will occur.Based on this,in order to minimize the number of blackouts in power grids,this paper chooses the splitting,the last line of defense of the power system,as the main research content,and takes the islanding SEP(Stable Equilibrium Point,SEP)as the breakthrough point,focusing on a series of research on islanding control after the separation.This paper will mainly focus on the following aspects.The contents of this paper are discussed as follows:Firstly,this paper reviews the research status and significance of active splitting especially the current achievements in this field at home and abroad and the deficiencies need to be improved in detail.Subsequently,with the help of Lyapunov's definition of equilibrium point and system stability,the relationship between system equilibrium point and system stability is elaborated,and the viewpoint which the study of islands'stability control method can be transformed into the study of islanding SEP is put forward.In order to facilitate the analysis of the correlation between the SEP of islands and the stability of islands,this paper builds a simulation model of the whole process of active splitting based on the relevant theory of power system mathematical models.On the basis of the above theories and models.the paper then introduces the basic theory and flow charts of solving the SEP of post-fault system and islands after splitting.The effective obtaining of islanding SEP lays corresponding theoretical foundation for the analysis and study of islanding control methods.Before introducing the control methods of islands after splitting,this paper first analyses the influence of different splitting strategies and splitting time on islands' SEP,and draws the following conclusions:Firstly,the SEP of islands is different when the splitting boundary is different,that is to say,it is the topology of islands that determine whether SEP exists or not.Secondly,the splitting time has nothing to do with the island SEP itself,but the reasonable selection of splitting time can make islands run successfully on their SEP.Finally,on the basis of the above theory,model and the related knowledge of power system sensitivity analysis to be introduced later,after fully considering the feasibility of islanding's SEP,this paper proposes the following methods for voltage control and frequency control of islands:In the aspect of voltage control,this paper deduces the reactive power sensitivity formula related to islanding SEP,and puts forward the corresponding approach based on it.Considering the limitation of sensitivity analysis,voltage control method of islands based on Newton iteration is proposed.As to frequency control,in order to eliminate the imbalance of active and reactive power of islands after splitting,this paper proposes a method of simultaneous compensation of active and reactive power of islands considering the imbalance of active power.Based on this,the difference of active power distribution between the feasible islands' SEP and the original islands' SEP is compared and then proposes the corresponding generator-tripping and load-shedding methods.Finally,the effectiveness of the above methods is verified by the simulation of IEEE-118 bus system.