Research On Frequency Measurement Algorithm For Load Fast Response

Frequency is not only an important indicator of power quality,but also an important control parameter of relay protection devices in power system.Frequency measurement runs through the whole process of power generation,power transmission,power distribution and power consumption.With the development and application of demand response technology,the quick response resources in the load side actively participate in two-way interaction with power grid.Meanwhile,frequency and rate of change of frequency(ROCOF)have gradually become important control parameters for the quick response of load,which poses a higher demand for the real-time requirements and accuracy of the frequency measurement algorithm.In this paper,the basic principle of traditional frequency measurement algorithms is studied,two frequency measurement algorithms for different conditions are put forward and a load control strategy based on frequency and ROCOF is proposed.The main research results obtained are summarized as follows.(1)This paper introduces the basic requirements of frequency measurement and common models of signal in power system.Traditional frequency measurement algorithms are classified according to matched models of signal in power system.Meanwhile,these algorithms are analyzed and simulated from the aspects of principle,application situation,measurement accuracy and response speed.(2)Considering the influence of harmonic pollution,noise disturbance and impulse load from load side,a frequency measurement algorithm based on dynamic model is proposed.Firstly,the dynamic model is expanded with Tayloy expansion to extract frequency deviation,which is used to establish the system state model of Kalman filter.Secondly,the Kalman filter algorithm is utilized to iterate the state model to obtain the optimal estimate of the frequency deviation,and the frequency tracking is realized.The algorithm is not sensitive to noise,harmonics and decaying direct-current component,and features high accuracy and real-time measurement.Simulation results show that in the case of frequency fluctuation and steps in phase,the proposed algorithm can track frequency after half a cycle of delay.(3)In order to realize the direct measurement of ROCOF,a frequency measurement algorithm considering ROCOF is proposed.Firstly,the algorithm is derived using the second-order Taylor expansion formula,then parameters of frequency and ROCOF are extracted.The appropriate time window is selected for data sampling,thus linear equations of system state model are established.Finally,the least squares method is applied to calculate the linear equations,and the deviation of frequency and ROCOF are obtained.This algorithm is very suitable for dynamic model integrated with linear changes in frequency,which has the good features of high accuracy and real-time measurement.Simulation results show even in the case of frequency fluctuation and steps in amplitude,the algorithm can continue to track deviation of frequency and ROCOF after less than one cycle of not-applicable area.(4)A load control strategy based on frequency and ROCOF is proposed.Compared with adaptive under frequency load shedding schemes(UFLS),the control strategy estimates load shortage by ROCOF to determine response capacity of each round.By making full use of the load response resources and responding to changes in frequency and ROCOF,the control strategy can particiate in the frequency regulation of the power system.Simulation results show that,the proposed strategy can restrict the fault-caused frequency drop in a more effective way,which is beneficial to enhance the frequency recovery ability of the power grid.