Flexible Equivalent Of Power System And Active-reactive Power Increment Rate Evaluation Method Of Voltage Stability

With the increase of power demand and the upgrading and expansion of related infrastructure,the power system is closer to its stability limit.The power failure caused by voltage collapse will cause huge losses to the social economy.The voltage stability has been widely valued.It is of great significance to establish an accurate evaluation index of system voltage stability.The existing voltage stability evaluating index based on Thevenin equivalent principle is that the complex system is equivalent to the equivalent circuit of potential source and internal impedance in series,and the system voltage stability is quickly evaluated by comparing the relationship between Thevenin equivalent impedance and load static impedance.However,this kind of index cannot accurately distinguish the influence of internal disturbance and external disturbance on the stability of the system.In this paper,the active power and reactive power incremental rate of the injection power system node are studied,and a voltage stability evaluation method and index based on local measurement information are proposed.The significant improvement of the flexible equivalent method is that the reference phasor of the original system internal potential is converted into the reference phasor of the injection node current,so the reference phasor of the flexible equivalent measurement can change with the swing of the system equivalent potential,and the flexible equivalent parameters can reflect the dynamic change trend of the injection node power and its power factor.Because the identification of flexible equivalent impedance only needs local measurement information,it can get rid of the dependence on wide area measurement system.The identification of flexible equivalent parameters only needs to measure the voltage phasor,current phasor and their dynamic change information of each node independently,which gets rid of the dependence on wide area measurement system.When the voltage amplitude of the power supply node cannot be kept constant,the dynamic potential invariant model after the power supply node is equivalent to the dynamic impedance.The power angle stability of virtual constant potential point is used to evaluate the voltage stability of the generator terminal.Then,the dynamic equivalent method of power supply node is extended to load node,and the active-reactive increment rate index(ARPEI)of voltage stability is proposed.For the convenience of analysis,the load power time dynamic characteristic describing the external disturbance and its dynamic response and the load power voltage static characteristic describing the relationship between load power and voltage amplitude are defined.Based on the power flow equation,the calculation method of flexible equivalent parameters of power supply node is derived.In order to consider the influence of power voltage static characteristics on voltage stability evaluation,the static voltage characteristics of the system power are described as ZIP model.Because the load contains constant impedance and constant current power,which are related to the node voltage,the result of voltage derivation is no longer 0,the diagonal elements must be corrected when calculating the Jacobian matrix: The static voltage characteristic equation of system load power is derived with respect to node voltage,and the derivation results are added to the corresponding diagonal elements of Jacobian matrix.Then,the modified Jacobian matrix is obtained.Based on the modified Jacobian matrix,the calculation method of flexible equivalent parameters is improved.The 39 bus system model is used to simulate three different power disturbance modes,while the simulation results verify the rationality of proposed index.