Study On The Hign-accuracy Three-point Interpolated DFT Method And Its Applications In The Measurement Of Unbalanced Three-phase Power System

Nowadays,with the growing demand of electricity,the renewable energy sources(RESs)like wind or solar energy are used extensively to generate power.Due to varying weather conditions of RESs and the different nonlinear loads such as rectifiers in power distribution systems,there is a variety of undesirable phenomena in microgrid,especially the unbalanced supply voltage.The impacts of such unbalance are profound such as generating harmonics,increasing distribution loss and so on.These impacts demonstrate that severe imbalance can deteriorate the power quality.The advent of the microgrid is a promising solution to make more intelligent and reliable distribution grids.It is highly significant to mitigate these potential impacts of imbalance to ensure the power quality.Considerable effort has been devoted to control technologies to mitigate the unbalance by operating the power system effectively.As the crucial basis of these control technologies,accurate and fast parameter estimation of the unbalance must be included.Therefore,it is of great importance to investigate parameter estimation in power system.Due to varying weather conditions and the non-ideal characteristics of the power electronics,signals of power system are characterized with poor power quality.Among the available estimators,it is a challenge to find the estimator for the parameter estimation of the unbalance which have a predictable accuracy and low computational burden.Therefore,based on the introduction of the available parameter estimation methods and estimation theory of unbalanced three-phase power system,this dissertation proposes the novel three-point interpolated DFT(Ip DFT)method to achieve the parameter estimation,which includes:(1)the parameter estimation method based on the proposed three-point Ip DFT;(2)the systematic errors of parameter estimation based on the proposed method;(3)the random errors of parameter estimation based on the proposed method;(4)the proposed method is speed-up by using the sliding transform scenario for its application in the unbalanced three-phase power system.Firstly,the novel three-point Ip DFT method based on the Maximum Sidelobe Decay windows(MDWs)is proposed after the available window function and their characteristics are presented.Among the available windows,the expressions of parameter estimation based on the MDWs will be more explicit and simpler to implement since the complicate calculations involved in the determination of the coefficients of MDWs can be avoided.As for other window functions,the expressions of parameter estimation are also provided by the polynomial approximations based on Least Square Method(LSM)at the cost of a little more calculation.The simulation results show the proposed method can achieve high-accuracy parameter estimation.Especially for the three-point Ip DFT method based on MDWs,it can achieve parameter estimation by analytical formulas,which are expected to be applicable in the measurement of unbalanced three-phase power system.Subsequently,it theoretically derives the systematic errors of the proposed method caused by the spectral leakage from the image components or harmonics.Although the spectral leakage can be reduced by weighting the analyzed signal with the MDWs,the systematic errors of parameter estimation cannot be neglected when the number of the acquired signal is small or the signal is distorted by large harmonic components.Thus,when the DFT samples that the image,harmonic components are taken into consideration is determined on the basis of the Taylor’s formula,the systematic errors of the proposed method are quantitatively investigated.An improved three-point Ip DFT-based procedure capable to compensate all the significant effects of harmonics on the parameter estimation accuracy is proposed.The simulation results validate the effectiveness of derived theoretical expressions and the proposed procedure,which can provide some suggestions to improve the accuracy of parameter estimation.Then,the random errors of the proposed method are derived theoretically when the wideband noise is considered.The analytical expressions of the parameter estimation variances are derived according to the law of uncertainty propagation.The factors affected the estimation variances are determined from the derived expressions,which can suggest an efficient way to improve the accuracy of parameter estimation in unbalanced three-phase power system.Last but not least,this dissertation presents the speed-up parameter estimation by three-point Ip DFT with the sliding transform scenario,namely mHSDFT.The mHSDFT method fits well in three-point Ip DFT.It can be expected that the presented method would not compromise the accuracy of the three-point Ip DFT method but only reduce the computational cost.Besides,the application of this method in the parameter estimation of unbalanced three-phase power system is presented.The estimation system can calculate the voltage unbalance factor based on the parameter estimation of unbalanced three-phase signals,which includes the signal conditioning module+ADC+STM32.And the estimation errors and related calibration of this system are discussed,which can be viewed as a basis for further practical application in unbalanced three-phase power system.In conclusion,the proposed method can have a tradeoff between the response time and accuracy under the complicate working conditions of power system.Under the harmonic conditions specified in IEEE C37.118.1-2011,the maximum TVE values of three-point Ip DFT method is 0.0354%,which is below threshold of 1% required by M-class PMU.And the response time of transient conditions is not larger than 32 ms,which meets the requirement of the P-class PMU in this standard.Besides,it can be concluded that the proposed method exhibits the accurate and fast characteristics enabling the parameter estimations for unbalanced three-phase power system.