Primary Frequency Control Ability Modeling And Optimization Of Large-scale Thermal Power Units

Frequency is an important indicator in the electric power production,reflecting the balance of power between supply and demand.Before the advent of large-scale energy storage technologies,the primary frequency control(PFC)is still needed as a principal measure to maintain frequency stability.In this paper,the digital electric-hydraulic control system(DEH)and the coordinated control system(CCS)are researched and modeled.On the basis of it,the assessment method of primary frequency control ability(PFCA)is studied.The PFC of thermal power unit is optimized based on PFCA.The accuracy of DEH servo system model directly affects the accuracy of valve simulation,which affects power response simulation of the primary frequency control in high-power units.The servo system in steam turbine DEH is affected by nonlinear factors when it is working.In order to accurately simulate dynamic characteristics of the DEH,a new nonlinear servo system is proposed,which has limit,dead zone and correction coefficient caused by unknown factors.The model parameters are divided into linear parameters and nonlinear parameters to be identified respectively.Neural networks are used to identify linear parameters.The nonlinear parameters should be identified according to flow characteristic curve.In order to verify the validity of the proposed model and parameter identification method,the Actual data of primary frequency control from a 1000 MW Ultra Supercritical Unit is adopted.Meanwhile,the linear model with no nonlinear factors is used for comparison.Where the fitting degree of valve opening is 98.445% and power is 96.986%,the output of nonlinear model coincides with actual output well.Where the relative error of stable result is 5% of valve opening and 1.58% of power,the error of linear model is larger.Improving the effectiveness and reliability of the PFC plays a significant role in ensuring the stability of electrical power systems.The PFC power response is influenced by the valve opening of turbine and the boiler energy supply.Therefore,it is necessary to improve the accuracy of DEH model and conduct modeling in combination with the boiler and the turbine.In order to deeply simulate and study the PFC,an innovative method combining the black box modeling and mechanism modeling is proposed to develop the CCS model,including pulverizing system,boiler,pressure loss of pipe from superheater outlet to turbine valve and turbine model.Transfer function and differential equation model are presented for pulverizing system,pressure loss of pipe and turbine link,respectively with model parameters identified by genetic algorithm.In addition,neural network model is put forward for boiler link due to complicated process,high inertia and delay characteristic.Furthermore,every link is simulated to validate the correction of models.Results show almost all the errors are within the acceptable range.In addition,the integral difference equation model of CCS is used to acquire the simulation curve based on given coal instructions,feedwater and valve opening.The simulation curve can be in accordance with actual curve and all the curve fitting degrees are more than 90%,which verified the proposed CCS model.Finally,the PFC dynamic response is simulated by integral CCS model.The simulation value of outlet pressure,main steam pressure and power is also well in accordance with actual value,which verifies this CCS model can simulate the PFC well.The study and evaluation of the PFC capability is helpful to master the PFC capability of the regional power system,which is of great significance for preventing the low-frequency risk of the power grid.Based on the above DEH and CCS modeling research,the PFC capability evaluation method is proposed.The coupling model of DEH and CCS is created to analyze function of frequency modulation.The simulation results show effect of frequency modulation by the DEH and CCS together is best.The primary frequency control ability is discussed on sliding pressure operation mode.Several operation mode are introduced to solve the lack of primary frequency control ability on sliding pressure operation.Increasing sliding pressure,small feed water bypass and mixed frequency modulation are tested by primary frequency control experiment.The results show that it is the most effective method for increasing sliding pressure to improve the primary frequency control ability.The initial response is slow for small feed water bypass and the characteristic is better on the high load.It can be the effective complement to adjust the frequency for regulating valve.The regulating effect is similar with increasing sliding pressure.To improve the reliability of the PFC,an evaluation method of PFCA is proposed.First,based on the coupling model of CCS and DEH,principle and control mode of the PFC are introduced in detail.The simulation results show that the PFC of the CCS and DEH is the most effective control mode.Then,the analysis of the CCS model and variable condition reveals the internal relationship among main steam pressure,valve opening and power.In term of this,the radial basis function(RBF)neural network is established to estimate the PFCA.Because the simulation curves fit well with the actual curves,the accuracy of the coupling model is verified.On this basis,simulation data is produced by coupling model to verify the proposed evaluation method.The low predication error of main steam pressure,power and the PFCA indicate that the method is effective.In addition,the actual data obtained from historical operation data is used to estimate the PFCA accurately,which is the strongest evidence for this method.It is of great significance to study flexible operation of thermal power unit for grid reliability.High pressure heater(HPH)can become an effectively auxiliary way to improve the primary frequency control ability.In order to study HPH feedwater bypass,the EBSILON model of 1000 MW unit is created.Small,mixed and big feedwater bypass are simulated,respectively.The results show that only the first HPH bypassed can power increase.Based on it,the relation between power increment and bypass flow is acquired under different load rate.Heat rate curves are fitted with load rate.Furthermore,by comparing power increment,heat rate and economy of three bypass,we found the mixed HPH feedwater bypass is the best.Finally,the heat rate of mixed bypass is compared with throttling valve.The results show HPH feedwater can effectively improve operation economy of units on the basis of ensuring enough primary frequency control ability.Parameters of high power units are key factors to influence dynamic stability of units frequency modulation and keep grid frequency stable.The grid standard of primary frequency control and necessary conditions for the stability of the power grid and units are set as the constraint,where coal consumption and pollution emissions minimum are set as the objective function of optimization model.Twelve units are used to validate the algorithm based on the actual operation data.Results show units can finish normally and quickly primary frequency control tasks with the best economy.For the deeply peak shaving of units,the rate of speed inequality should be set without constraint of the grid standard.To keep the rapidity and economy of adjusting frequency,this work can give an important reference value to set the actual unit rate of speed inequality.It exists a contradictory relationship between ability of peak shaving and load rate.In order to improve the economy of electric power system operation and ability of peak shaving simultaneously,optimal load distribution model considering the PFCA is proposed.Furthermore,sine cosine algorithm(SCA)is introduced to solve it.Taking four units as simulation example to verify the model.The results show the SCA can achieve better performance than genetic algorithm(GA)in calculating optimal value.Compared with the automatic generation control(AGC),the new method is more economical and meet the requirements of the PFCA.In addition,the relationship between optimal economic cost and primary frequency control capacity under different load rate is summarized by simulation.It has great guiding significance for load optimal distribution.Finally,load optimal distribution in low load rate is studied.The results show the units with better economy are selected preferentially to participate in deep peak-regulating.In this paper,the PFC of high-power units is studied longitudinally.Firstly,the accuracy of DEH and CCS modeling is improved to ensure the accuracy of PFC model.Then,the maximum PFC capability evaluation method based on neural network and EBSILON modeling are proposed.The PFC capability of the unit can be obtained simply and efficiently.Finally,a new optimization strategy is proposed,which includes the primary frequency modulation capability into the constraint condition of the optimization.It can realize the joint optimization of speed inequality and load distribution of different units on the basis of ensuring the grid’s sufficient PFC capability.The research content has an important reference value to enhance the capacity of power grid to absorb new energy and increase the flexibility of operation of high-power units.