Detection Methods Of Power Angle And Voltage Instability And Analysis Of Coupled Instability Scenarios In Power Systems

Since the 1960s,large-scale power outages have been occurring worldwide,resulting in huge economic losses and serious social impacts.In order to provide timely warning and take effective measures to avoid grid accidents,an in-depth analysis of the causes and phenomena of destabilization is needed,so the study of power system stability is of great significance.One of the basic requirements for the stable operation of a power system is that the power angle of the synchronous generator remains synchronised and that the bus voltage in the system is maintained smoothly within normal limits.Some detection methods for power angle instability and voltage instability have been summarized in traditional power systems from the perspective of engineering experience,and it is usually assumed that power angle problems and voltage problems can be handled separately.However,with the increasing proportion of power electronics in the power system,the existing detection methods can hardly meet the higher requirements for the speed and reliability of instability detection.In addition,in some cases,electrical measurements such as power angle and voltage have apparently strong coupling characteristics between them,so that control measures of one stable form can degrade the other stable form.Therefore,the purpose of this paper is to investigate online methods for fast detection of power angle/voltage destabilization to improve the speed of countermeasures and thus better prevent grid collapse.At the same time,the study of the power angle/voltage instability coupling mechanism and the identification of the coupling scenarios provide guidance for effective control measures.The research content and innovation of this paper are as follows:(1)Based on the transient energy distribution characteristics of power networks,an online fast detection method of transient power angle instability considering load potential energy and cut-set potential energy is proposed to meet the high requirements of real-time and reliability for power angle instability detection.Based on the Dirac structure characteristic of transient energy flow in power networks,the expression form of branched power at the boundary of transient stability domain is derived,revealing the theoretical limitations of existing line potential energy criterion ignoring load potential energy and relying on critical cut-set,and explaining the reasons for the slow detection speed of existing indexes.The proposed transient power-angle instability detection method extends the applicability conditions of existing transient power-angle instability detection methods based on load potential compensation and relative line potential energy,and has the advantages of fast detection speed,high reliability and easy implementation.(2)Combining the respective advantages of the continuation power flow(CPF)method and the point of collapse(PoC)method,a fast calculation method for the voltage stability critical point is proposed,which has the potential for online application.The key to the method is the proposed saddle-node bifurcation point approximation index based on the characteristics of the power system power flow equation at the saddle-node bifurcation point.By predicting the location of the saddle-node bifurcation point through the approximate linearity of the index,the hybrid calculation method of switching from the large-step CPF method to the PoC method when approaching the bifurcation point is approaching,which solves the problems of increasing the CPF method’s calculation near the saddle-node bifurcation point and the difficulty of selecting the PoC method’s initial value.In addition,the hybrid calculation method solves the problem of identifying saddlenode bifurcation and limit-induced bifurcation in large-step CPF method by adding complementary logic and overcomes the computational bottleneck in the PoC method by the implicit calculation method of Hessian matrix,which has the advantages of high numerical stability and fast computation speed.(3)In this paper,the coupling characteristics between power-angle instability and voltage instability are studied,including the power-angle dynamics of synchronous generator and the voltage dynamics of dynamic load and converter,revealing a new law of coupling between the two stability forms.On the one hand,through the stability domain analysis of synchronous generator power-angle subsystem and motor voltage subsystem,the trajectory properties near the unstable equilibrium point of the interconnected system are analyzed based on the nature of the unstable equilibrium point of the subsystem,which expands the traditional view that the unstable equilibrium point of the interconnected system determines the type of instability.On the other hand,we break the assumption that the phase-locked loop(PLL)is reduced to an algebraic equation.Based on the synchronous generator power angle-PLL dynamics equation,we also analyze the conditions that the PLL couples with the power angle of the synchronous generator when a large number of converters are connected to the grid.Using the geometric singular uptake theory,the dynamics of the transition from synchronous generator dynamics dominated to PLL dynamics dominated when the PLL loses its equilibrium point is explained from the theoretical level.By analyzing the generator speed,a new phenomenon of the interaction between the synchronous generator power angle and the PLL near the singularity point is explained,and a new understanding of the power-angle/voltage stability coupling in the new power system is formed.(4)In this study,the coupling and decoupling scenarios of power-angle instability and voltage instability are analyzed,and quantitative evaluation indexes of power-angle and voltage stability as well as coupling strength evaluation indexes are proposed.Based on the projection of the system trajectory and stability domain on the δg-P3 plane and the δgI3 plane,the power-angle/voltage coupling instability scenario classification method considering the network structure and load level is proposed based on the interaction pattern of power-angle/voltage near the singularity point.Based on the quantitative indexes of stability margin and singularity margin,a quantitative evaluation index system of powerangle/voltage stability is constructed,and a quantitative evaluation method of coupling strength is proposed based on the coupling mechanism of power-angle/voltage,which provides a new analysis method for the type of coupling instability and an analysis tool for the adaptation conditions of the control strategy.