Study On Controlled Islanding Strategy Considering Transient Stability

The scale of modern interconnected power system is gradually expanding,and the operation state is approaching the stability limit.Local faults and continuous faults may affect the whole system through the tie-line,causing cascade outages,and even leading to catastrophic power outages of the whole system.In order to prevent blackouts,controlled islanding is regarded as the last remedial action of three lines of defense.By disconnecting transmission lines at appropriate splitting points,the system can be divided into several stable operating islands,so that each island can operate safely and steadily.How to determine and implement the islanding strategy quickly and effectively is of great significance to the safe and stable operation of power system.In order to form stable islands,the islanding strategy must satisfy multiple steady and dynamic constraints,including power balance,transmission line capacity constraints,angle stability,voltage stability and so on.Most of the existing studies are based on steady-state or quasi-steady-state information to determine the islanding strategy,while the dynamic behavior is neglected by strong assumptions.However,in extreme conditions,the system will inevitably have dynamic behavior.The state of the system will change dramatically with time,and the instability mode of the system will change along with the system state.Therefore,only based on real-time data and considering the dynamic behavior of the system,can we make a correct assessment,so as to ensure the safety and stability of the system.In view of this,this paper studies the dynamic behavior of the system in the islanding process,analyses the influence of generators grouping,islanding boundary and islanding time on the success of islanding action,studies the criteria for assessing islands' stability,and works out a reasonable and effective real-time power system islanding strategy.This paper mainly carries on the research from the following aspects:(1)Generator grouping recognition is the basis of a successful controlled islanding strategy.The islanding strategies are based on predicting generator grouping,which is traditionally done by slow coherency analysis.Slow coherency is based on the linearized model of stable equilibrium point.Therefore,slow coherency is only capable to deal with small disturbances.The analysis result shows that,the trajectory of the system before controlled islanding can be divided into three types.The first type is a small disturbance case,which is controlled by stable equilibrium point.The other two types are large disturbance cases,which depend on an unstable equilibrium point on the stability boundary of the system.Therefore,this paper presents a generator grouping algorithm taking into account the size of disturbances.According to the type of trajectory,the algorithm adopts a suitable linearized model for both stable or unstable equilibrium points.The algorithm is suitable for both small disturbances and large disturbances.The simulation results show that the influence of disturbance on generator grouping can not be neglected.Then the effectiveness of the proposed generator grouping algorithm considering disturbance size is verified.(2)In the aspect of islanding boundary,the main idea of this paper is to construct an island system with strong structure.Therefore,this paper focuses on two aspects'disturbance resistance' and 'frequency stability' to improve the structural stability of islands.Firstly,taking the two-machine equivalent system as an example,the disturbance resistance of the two-machine island is analyzed in detail,and the influence of key parameters on the disturbance resistance of the system is analyzed.For each oscillation mode of a multi-machine island,the corresponding two-machine equivalent island is established,and the disturbance resistance index of a certain oscillation mode is defined.The disturbance resistance index of a multi-machine island is then deduced,and the sensitivity relationship between the disturbance resistance index of multi-machine island and the transmission line is established.Then,the frequency stability index of islands is defined based on the power imbalance,and the sensitivity of the transmission lines to the frequency stability index is calculated.The weight graph of the system is generated by graph theory,and the grouping results obtained by the generator grouping algorithm are used as constraints.The graph is cut with the objective function of maximizing disturbance resistance and frequency stability.The effectiveness of the algorithm is verified by simulation tests.(3)Islanding time is of great important to ensure the success of controlled islanding,but the researches on this challenging "time" problem is very limited.In order to solve the islanding time problem,the concept of Feasible Islanding Time Interval(FITI)is introduced in this paper.In this time interval,the generator in an island will keep synchronization to achieve a new stable equilibrium point for a given islanding boundary.Based on the theory of nonlinear dynamic system,FITI can be achieved by BCU(Boundary of Stability Region Based Controlling UEP),which guarantees the angle stability in an island.According to the FITI obtained,an algorithm to determine the optimal time of islanding is given,considering the delay of control action and the frequency stability of islands.Finally,simulations are carried out to verify the validity of the algorithm for calculating FITI and optimal islanding time.(4)The last chapter of the paper forms a three-stage controlled islanding strategy of power system by integrating the generator grouping prediction method(Chapter 2),the algorithm for determining islanding boundary considering structural stability(Chapter 3)and the optimal islanding time algorithm based on BCU method(Chapter 4).The first stage provides coherency information for the latter two stages.The latter two stages solve the problem of "where to split" and "when to split" respectively.The islanding strategy takes into account the coherency constraints.frequency stability constraints and angle stability constraints,which enhances the third line of defense of power system.