Research On Coordinated Control Between Under-excitation Limiter Of Excitor And Power System Stabilizer Of Synchronous Generator

Low excitation limiter?UEL?and power system stabilizer?PSS?which play an important role in the dynamic stability of power system have become a widely configuration in the generator excitation system as the important auxiliary part of the generator.However,due to the unreasonable configuration of the UEL parameter,it degenerates positive damping that PSS provides after the UEL action.This problem leading to the serious failure of the oscillating and even dissolving has happened many times in the actual operation of the power grid.This paper first establishes the mathematical model of small interference analysis of power system elements.By linearizing at the equilibrium point,the state equations of PSS and UEL and excitation system are written.The Heffron-Phillips extension model with the addition of PSS and UEL is analyzed and the state equations are written.By the state equations of every system component,the state matrixs of the whole system are obtained.For in the below,the eigenvalue and damping ratio of the computer electric oscillation mode provide with theoretical basis for the quantitative analysis of the parameters of UEL.Moreover,the relationship between the slope,intercept and the new coefficient K₇ and K₈of the Heffron-Phillips expansion model after the addition of UEL are analyzed.The influence of the synchronous generator excitation system on the low frequency oscillation and the principle of PSS to suppress the low frequency oscillation are analyzed by the damping torque method.The additional damping torque analysis provided by UEL to the excitation system shows that UEL provides negative damping to the system.The influence of UEL on the damping torque of the excitation system after adding PSS is analyzed,and the positive damping of the PSS on the system is weakened by UEL between 0.1 and 2.5Hz,which affected the effect of PSS.Therefore,the frequency response method is used to analyze the action frequency of UEL and PSS.The simplified heffron-phillips expansion model is derived by the formula,and then draw the bird map of the excitation system with the open loop transfer of UEL and PSS.The PSS parameters can be determined by the Byrd diagram in the interaction with PSS.The effect of slope and intercept on the system are analyzed by the zero point analysis of the open loop transfer function of the excitation system.In the end,this paper takes the output conflict of UEL and PSS of a certain power plant of yunnan power grid as an example.The equivalent model of an infinite system of single machine is set up under PSASP software.The equivalent model of the single machine infinite system is set up under PSASP software,and the state matrix of the whole system is written.Both the simulation analysis and the eigenvalue analysis show that the parameters of UEL with the frequency response method can coordinate the PSS and UEL.The larger the slope K_u and the intercepting|C|are,the worse the stability of the system is.The setting of the slope K_u and intercept|C|should be smaller according to the actual situation.