Research On Nonlinear Characteristic And Control In Automatic Generation Control System Of Thermal Power Unit

ABSTRACT:Automatic Generation Control(AGC)has always been a research focus,considering its prominent position in the power grid’s frequency regulation and the units’ scheduling and control.With AGC technology,units power and tie-line power can be regulated so that the balance between generation power and electrical load is regained and frequency stabilization can be maintained,while a load disturbance occurred in power system.Nevertheless,as the structure of power system changed and the operation mode diversified,the influences and dynamic interaction characteristics between the power system and the units has become prominent.The nonlinear dynamic characteristics make traditional AGC technology fail to sustain the power quality.Thermal power units(TPU)play an important role in AGC regulation.The power response abilities of TPU affect the power grid’s frequency control performance.At this stage,the "Two Rules" put forward strict requirements of frequency modulation for TPU.With power grid’s enlargement and the new energy generation grid-connected,the problems of the power grid’s uncertainty and random load disturbance are becoming serious.A new AGC control strategy must be studied to inhibit load disturbance and ensure its dynamic performance.Therefore,the researches on dynamic interaction characteristics between TPU and the power grid,the researches on AGC control strategies are important for the power system’ safety.This thesis focuses on TPU’s coordinated control system(CCS)optimization,interaction characteristics analysis between the power grid and TPU and AGC control strategy.To improve TPU’s AGC performance,a new computation method of AGC performance indexes from the power plants is designed based on the historical data.Combining the new computed AGC performance indexes with the main steam pressure’s ITAE indexes,a new optimization object function is applied to optimize CCS control parameters with genetic algorithm(GA).It is demonstrated that the proposed object function can maintain stable operation and improve TPU’s AGC performance.Finally,the CCS optimization with the proposed object function is adapted in AGC model.TPU’s AGC performance is improved and the maximum frequency deviation is reduced after the load disturbance.To lucubrate the nonlinear characteristics of the power system,an Adomian modified analytic-numeric integration algorithm(AMANIA)is proposed.Differential transform theory is used to simplify the analytical expression and a matrix form of the analytical solution is obtained.With the defined recursive error,automatic adjustment of step size and truncation order is realized,which improves the precision of global simulation and increase the iteration step.The effect of different truncation criterion on accuracy for coupling nonlinear system is analyzed.Nonlinear analysis of single machine infinite bus(SMIB)system based on AMANIA reveals that the increasing disturbance amplitude causes bifurcation and finally lead to chaos.Considering the oscillations caused by active power disturbances,automatic voltage regulation(AVR)circuit and AGC circuit are combined.A coupling model with two saturation is constructed based on Phillips-Heffron model of SMIB.Generation rate constraint(GRC)and excitation saturation constraint(ESC)are analyzed.ADM analytical solution based on AMANIA reveals that improper PI parameters will cause oscillation or divergence of both frequency and active power,while reasonable GRC and ESC limit this unwelcome response.When the resonance occurs under the conditions of periodic steam turbine regulating valve opening disturbance or periodic load disturbance,the resonance frequency increases as the proportional coefficient of PI increases in reasonable range and resonance amplitude increases as the integral coefficient of PI increases in reasonable range.Based on saturation characteristics analysis under the periodic disturbance,it is demonstrated that GRC and ESC parameter value affect the resonance amplitude of the coupling system.For the problems of GRC and time delay in AGC system,Hopf bifurcation Routh criterion and the time delay stability criteria are separately used to compute the saturation PI stability domain and the delay PI stability domain.By AMANIA,the two kinds of AGC systems’ ADM analytical solutions are obtained.The sequence and phase diagrams are provided to verify the correctness of the PI stability domain.AGC system is equivalent to cascade control system made up of load frequency control loop and unit control loop.Using PI stability domain,PI control parameters are optimized based on GA to ensure the good dynamic performance of the inner and outer control loops.Considering the problem of uncertainty and random disturbance in AGC system,the relationship between pole assignment and noise suppression is analyzed.The optimal performance indexes integrated steady state variance and the control energy is constructed.A robust variance control(RVC)strategy with optimal performance indexes is proposed.The optimal performance indexe is transformed into linear matrix inequality with constraint conditions,hence the problem has changed into an optimization problem with robust variance constraints.By GA,the pole coordinates and positive scalar are optimized for the optimal control parameters.Simulation and comparison in two-area AGC system have demonstrated that the proposed control strategy can reduce noise and has better robustness and dynamic performance.