Study On Fast Frequency Shifting And Filtering Algorithm And Its Applications In Micro PMU

With Global Position System(GPS)timing technology,the Wide Area Measurement System(WAMS)achieves synchronized phasor estimation with accurate time tag through Phasor Measurement Unit(PMU),which improves the efficiency of monitoring and control for the power system.As the increasing of the number of non-linear devices as well as the connections of the large-scale distributed power sources,the safe and stable operation of power systems,especially distribution networks encounter challenges.However,the disadvantages of traditional PMU,e.g.,bulky and costly,limit the wide application of PMU in distribution networks.Due to the advantages of small size,low cost and easy installation,the Micro PMU(μPMU)provide a new idea for solving the dynamic monitoring of the distribution network.As the key of synchronous phasor measurement technology,the synchronous phasor estimator affects the performance of power system stability control,fault analysis and relay protection directly.Therefore,it is of great significance to study a synchronous phasor estimator with high accuracy and low delay under the measurement condition of the distribution network to ensure the safe and stable operation of the power system.DFT-based synchronous phasor estimator is widely used in WAMS due to its high computational efficiency and accuracy.However,under asynchronous sampling,DFT is affected by the spectrum leakage and fence effect,and these defects reduce its phasor measurement accuracy significantly,which makes the DFT-based estimator difficult to meet the phasor measurement requirements.Moreover,the low cost and miniaturization requirements of μPMU place higher demands on the synchronous phasor estimator.In order to fulfill the requirements,this thesis proposes a synchronous phasor estimator based on fast frequency shifting and filtering algorithm(FSF).Firstly,traditional phasor estimators are reviewed.Aiming to overcome the limitations of traditional methods,a FSF-based phasor estimator is proposed and established.The basic steps of FSF are: 1)shifting the target component to zero frequency by a reference signal;2)filtering out the interference components by an iterative filtering process based on averaging filter;3)obtaining the phasor estimation by the related equations.To lower the computational burden of filtering process,this thesis proposes the fast FSF based on the equivalent weighted filter.The simulation results show that the fast algorithm can effectively reduce the computational burden and provide satisfying the phasor estimation as the original one,which can meet the requirements of synchronous phasor measurement for distribution networks.Secondly,the systematic error of the FSF-based phasor estimator is analyzed.Due to the non-ideal amplitude-frequency response of the averaging filter,the interference components cannot be filtered out completely and this produces the systematic error of FSF.The systematic error is modeled and analyzed under pure sinusoidal and harmonics conditions.According to the characteristics of systematic error,an improved FSF called Smoothing FSF(SFSF)is proposed by systematic error compensation.The simulation results show that the system error compensation can effectively improve the phasor measurement accuracy under the premise of slightly increasing the calculation amount,also it is verified that the SFSF can enhance the performance of μPMU in practical measurement.Subsequently,random error of the FSF-based phasor estimation is analyzed.It is known that the phasor estimation will be inevitably affected by the background noise and the noise introduced by the signal acquisition system in practical,which produces the random error.Fortunately,these noise can be approximated as white noise.Therefore,the model of FSF-based phasor estimator is established using the additive white Gaussian noise.The frequency,amplitude and phase variance expressions are derived theoretically.Moreover,the variances of FSF-based phasor estimations under white noise are compared with the Cramer-Rao Lower Bound.The correctness of the derived variance expressions is verified through simulations,which provide a basis for improving the anti-noise level of μPMU in synchronous phasor measurement.Finally,a platform of the designed μPMU is introduced to test the performance of the FSF-based phasor estimator.The FSF is improved according to the requirements of μPMU and the specific measurement flow is given.The error source of the μPMU is analyzed based on the procedure of the improved FSF.Besides,the error correction methods are given to lower the error produced by the measurement system.The experimental results show that the accuracy of FSF-based phasor estimator in different measuring conditions can meet the requirements of μPMU for distribution network monitoring.