Research On Dynamic Frequency And Phasor Measurement Algorithm And Its Application In Power System

Measuring the frequency,phasor and related electrical quantities quickly and accurately is a prerequisite for embedded automatic device to work correctly and reliably.Existing frequency and phasor algorithm under the condition of the system frequency constantly and offset the smaller has high precision,and the dynamic change of the system frequency and frequency offset is larger,the calculation accuracy will be reduced significantly.Therefore,it is of great significance to improve the measurement precision and real time of the embedded device to the frequency and phase of the embedded device under the condition of the dynamic change of the system frequency and the large deviation of the frequency.This dissertation focus on the following problem of measurement accuracy of the frequency and phasor is not high under the condition of the dynamic variation of system frequency and frequency offset.The following innovative research progress is mainly made in the dissertation through the in-depth study of the related measurement methods.(1)The algorithm research of Instantaneous Frequency,rate of change of frequency(ROCOF)and Signal Period under the condition of Frequency Dynamic Variation.First of all,the ROCOF is introduced into the system signal model,and a voltage signal model under dynamic conditions is proposed.The exact expression of the instantaneous frequency,the frequency change rate and the period are derived at any time.Secondly,the mathematics tools such as Newton interpolation method,Qin Jiushao algorithm and Newton iterative method are used to solve the zero-crossing time of the interpolation polynomial function,and then the accurate signal period value is calculated.Finally,using the foregoing calculation and derivation,the frequency and frequency change rate on the dynamic condition are calculated,and the measurement precision and response speed of the frequency and frequency change rate are improved.The simulation results show that the algorithm has a high measurement accuracy of frequency and frequency change rate under various working conditions,such as pure sine signal,signal contains higher harmonic and noise,frequency linear,nonlinear frequency changes,The algorithm has the advantages of small calculation,fast response,and high measurement accuracy.The algorithm can be applied to the rapid change of frequency and the nonlinear fluctuation in a wide range of 30Hz~70Hz.(2)Research on Discrete Fourier Transform(DFT)Correction Algorithm for Frequency OffsetWhen the frequency offset of the system is large,non-synchronous sampling will increase the error of the DFT algorithm,and it is difficult to meet the measurement and calculation accuracy requirements.Through analysising the Fourier transform formula,using the Fourier transform of a identity(real and imaginary part of phasor amplitude and frequency and electric parameters satisfy a certain relationship),A correction algorithm of DFT phasor and other electric parameters based on frequency measurement values is proposed.The algorithm is modified on the basis of the existing DFT algorithm,and adopts a fixed sampling period.The algorithm does not need to adjust the sampling frequency,iterate operation and increase data window,so the algorithm is small in computation and fast in response.The simulation results show that the algorithm is applicable to the DFT algorithm correction in the frequency range of 45Hz~55Hz,which has a higher measurement precision of the phase and electric quantity.Meanwhile,the algorithm is not sensitive to the measurement error of frequency,and only requires low sampling frequency.(3)Design of digital filterAiming at the problem that the algorithm error increases in the case of harmonic and noise seriously,two digital filters are designed to supplement the algorithm to ensure that the automatic device can operate stably and reliably under severe disturbances.A three-phase integrated filter with positive sequence components is designed based on the condition of three-phase voltage input to suppress high-order harmonics,noise and inherent DC components and to solve the problem that single phase voltage measurement is susceptible to PT disconnection and asymmetric fault.A synthetic digital filter is designed by combining the Butterworth filter with the Difference filter in the case of single-phase voltage input to filter out the DC component and suppress the high-order harmonics.Simulation experiments and device verification show that both of digital filters have better filtering effect and meet measurement requirements of the device measurement.(4)Development and application of automatic device.Based on the theoretical research and the implementation of the algorithm,the research results have been applied to the automatic safety disconnection device and the automatic quasi-synchronization device successfully,and a synchronous closing control strategy for the grid-connected system of hydropower station is proposed.Aimed at the present situation of medium and small hydropower plant configuration of the safety automatic existence isolated net column device detection criterion is not comprehensive,accurate,and device has such problems as refusing action and maloperation.To solve the problem that uncomprehensive criterion,detection blind area,protection misoperation caused by the problem of incomplete detection in the system disturbance,a more comprehensive criterion is designed through frequency,frequency variation,voltage,impedance and and other electrical quantities.The correct action rate of the device is improved by using the high precision frequency measurement and phase measurement algorithm as well as the digital filter designed.On this basis,the hardware and software development of the device was completed,and the prototype was tested.The results show that the device has high precision,stable and reliable operation,and can identify the operation state of the isolated network in time,and ensure the reliable operation of the hydropower station system.Based on the research of the precise algorithm of frequency and phasor and the digital filter,the software and hardware development of the automatic quasi-synchronizing device was completed,the prototype was tried and the prototype was tested.A system control strategy for the synchronous switching of the generator is proposed by analyzing the various influence factors of the synchronous closing of the generator,It solved the problem of coordination effectively among automatic quasi-synchronization device and generator excitation system,speed control system,dispatching command system and other related parts.It can achieve better impact and smooth grid-connection control.On the basis of the above research,the system control strategy for the synchronous switching of the generator is combined with the developed automatic synchronization device,and it is successfully applied to the hydropower station.The results of field application show that the automatic quasi-synchronous device designed is in good cooperation with each control system of the generator,grid connected with power system is stable,fast and accurate and no impact.The operation of the device is stable and reliable.