Impact Of Under Excitation Limiter On Power System Stability

Excitation system has great influence on power grid and and the generators alwaysoccupies a pivotal position in the field of power system analysis and control. With thegradual deepening of the study of the excitation system and the rapid development of manycontrol technologies, now the excitation system not only has basic control, as well asauxiliary control part and excitation limit. Under excitation limiter which is an importantpart of the excitation limit has become the standard configuration of synchronous generatorexcitation system. Units repeatedly appeared serious faults such as oscillation evendisconnection in actual operation of the power grid which were caused by the unreasonablydesigned under excitation limiter parameters and structure, so to do some in-depth study ofunder excitation limiter has the vital significance.This paper had studied the setting of limit curve of under excitation limiter,the impactof under excitation limiter on power system stability and the optimization of underexcitation limiter. According to the machine side measuring impedance criterion, we couldreceive the curve of the static stability limit impedance and the curve of The asynchronousboundary impedance in P-Q plane, and found that if the setting of allowance is reasonableaccording to the existing setting principles, the under excitation limiter and theexcitation-loss protection can coordinate very well.According to the existing theoretical knowledge of small signal analysis, aHeffron-Philips model with under excitation limiter and power system stabilizer was built.We analyzed under excitation limiter on the effect of PSS frequency band and how to set itfor having good effects on PSS in theory. We calculated the expression of the synchronoustorque coefficient and damping torque coefficient, according to the Heffron-Philips modelwhich has under excitation limiter only. It was found that the slope of the under excitationlimit curve was greater, The stability of the system was worse, according to the detailanalysis of the impact of the slope of under excitation limit curve on power system stability.By analyzing the effect of time constant on the stability of the power system, it was foundthat the different time constant can be used for under excitation limiter characteristicanalysis that will not cause the system stability problems in a certain range. Finally it wasputted forward three schemes for the optimization of under excitation limiter and analyzedthe availability of the three schemes. By carrying on the detailed discussion and analysis ofthe simulation of the most feasible solution, the simulation results found that the effect ofchanging low excitation limit curve line type is better.