Research On Dynamic Frequency Measurement Algorithm And Low Frequency Load Shedding Scheme Of Power System

Low frequency load shedding is an effective measure to ensure the safe and stable operation of power system and prevent system frequency collapse.Being able to quickly and accurately measure the system frequency and frequency change rate is a necessary condition to ensure the rapid and correct action of low-frequency load shedding.When the frequency drops slowly due to a small active power shortage in the system,the accuracy of the existing low-frequency load shedding scheme and system frequency measurement algorithm is relatively high,while the measurement accuracy decreases significantly under the condition of dynamic change of power system frequency.In order to improve the measurement accuracy of low-frequency load shedding scheme and frequency measurement algorithm under the condition of dynamic change of system frequency,this thesis makes an in-depth study.Firstly,the static characteristics of power system frequency and the dynamic characteristics under the condition of dynamic change of system frequency are analyzed.The simulation results show that the static characteristics of power system have the ability to suppress the change of system frequency,and the dynamic characteristics have the characteristics of space-time distribution,which are inseparable from the structure of power network and the operation mode of system.Secondly,after studying the characteristics of the dynamic change of system frequency,the change rate of system frequency is introduced into the system signal model,and the mathematical model of voltage signal under the condition of dynamic change of frequency is established.Based on this mathematical model,the mathematical relationship between instantaneous frequency,frequency change rate and period at each time is deduced;Newton iterative method,Qin Jiushao algorithm,Newton interpolation algorithm and whale algorithm are used to accurately solve the zero crossing time of interpolation polynomial function,and then the accurate signal period value is calculated;Using the above calculation and derivation results,the frequency and frequency change rate under dynamic conditions are calculated,and the measurement accuracy and response speed of frequency and frequency change rate are improved.The simulation results show that the algorithm has high measurement accuracy of frequency and frequency change rate under various working conditions,such as standard sinusoidal signal,high-order harmonic and noise in the signal,linear change of frequency,nonlinear change of frequency and so on;It has the advantages of small amount of calculation,fast response speed and high measurement accuracy.It can be applied to the device application in the case of large frequency offset,rapid frequency change and nonlinear fluctuation of power system.Finally,after analyzing and discussing the influence of voltage fluctuation and frequency oscillation on low-frequency load shedding,a low-frequency load shedding strategy based on frequency curve integral is proposed.This strategy can automatically adjust the first round action frequency and load shedding of each round of low-frequency load shedding according to the integral value of frequency curve,and can also eliminate the influence of voltage fluctuation and frequency oscillation on load shedding strategy in multi machine complex system.In view of how to reasonably distribute the load capacity to each node to be cut off,a load cut-off algorithm based on active power deficiency sensitivity is proposed,which can truly reflect the recovery of frequency after the load to be cut off of the node to be cut off.After calculating the sensitivity,sort according to the absolute value from large to small,and then cut off the corresponding amount of load in this order.After cutting off the load,calculate the sensitivity of each node again.Although the value of sensitivity has changed,the arrangement order has not changed,so only one sorting can be carried out.The effectiveness of the low-frequency load shedding strategy and load shedding algorithm is verified by simulation.