Auxiliary Loops Of Generator Excitation System Modeling And Its Influence On Power Grid Stability

As part of the generator,excitation system plays an important role in maintaining the voltage of the machine at constant,providing reactive power support for the system and ensuring the safe and steady operation of power network.So far,voltage closed-loop control of the excitation and PSS are only taken into account in dynamic analysis of power system,but the influence of the auxiliary loops of the excitation system on safety and stability of grid isn't considered.Furthermore,most commercial power system analysis software has not established the models.Field experience shows the control function of the auxiliary loops of the excitation system has an important influence on the dynamic qualities of generators and stabilities of power grid.Therefore,in this paper,the related research is carried out from three areas: auxiliary loops modeling,parameter identification and the influence on the power grid.The main results of this paper are included below.Firstly,four widely-used auxiliary loops in real project,named Voltage Frequency Limiter(VFL),Under Excitation Limiter(UEL),Over Excitation Limiter(OEL),and Stator Current Limiter(SCL)are introduced briefly.Based on user-defined function of PSASP,the models of the four auxiliary loops are constructed.Then the accuracy and effectiveness of the models are verified through the analysis of numerical example.Secondly,a parameter identification method is proposed,which is suitable for the structure of the auxiliary loops of the excitation system.According to the characteristic that the controlling unit and the action unit of auxiliary loops apply to different stages,this method divides parameters of identified model into parameters belonged to the controlling unit and parameters belonged to the action unit.In particular,based on the generator test data in four different cases the parameter of the action unit are identified,we totally identify the parameters of the controlling unit based on wolf pack search algorithm.Numerical examples demonstrate that the proposed parameter identification method can accurately identify the parameters of the auxiliary loops.Finally,based on the extended Heffron-Phillips model with auxiliary loops,damping torque method is used to analyze the influence on the system damping after the comparing UEL and OEL put into operation,respectively.Research results show that the UEL can provide a negative damping torque for the power system,and a positive damping torque for the system is provided by the OEL.In addition,two kinds of limiter will weaken the damping torque offered by the PSS.Simulations validate the correctness of the conclusion in a single machine infinite system.