Here, a novel method for estimating utility harmonic impedance (UHI) based on probabilistic evaluation is proposed. By analysing harmonic data, it is determined that UHI tends to obey a particular distribution, that varies with the amplitude of the background harmonics. Therefore, calculating the probability distribution of the UHI is considered to estimate its value. The UHI can be estimated by calculating the conditional probability density function of the background harmonics and prior distribution of UHI. The estimation results are not affected by the correlation of harmonic impedances between the two sides of the point of common coupling. The validity and accuracy of the proposed method were further verified through simulations and the analysis of field testing data.

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Due to the limitation of frequency resolution and the spectrum leakage caused by signal windowing, the spectrums of harmonic and interharmonic components with close frequencies overlap each other. When the dense interharmonic (DI) components are close to the harmonic spectrum peaks, the harmonic phasor estimation accuracy is seriously reduced. To address this problem, a harmonic phasor estimation method considering DI interference is proposed in this paper. Firstly, based on the spectral characteristics of the dense frequency signal, the phase and amplitude characteristics are used to determine whether DI interference exists in the signal. Secondly, an autoregressive model is established by using the autocorrelation of the signal. Data extrapolation is performed on the basis of the sampling sequence to improve the frequency resolution and eliminate the interharmonic interference. Finally, the estimated values of harmonic phasor, frequency and rate of change of frequency are obtained. The simulation and some experimental results demonstrate that the proposed method can accurately estimate the parameters of harmonic phasors when DIs exist in the signal, and has a certain anti-noise capability and dynamic performance.

This paper presents a novel harmonic state estimation (HSE) methodology based on graph signal processing. The proposed approach can perform HSE without calculating harmonic impedance. The proposed methodology involves graph signal theory for constructing a graph Laplacian matrix and an unconstrained optimization function, the adaptive K-means algorithm for building a prior information model of the target bus, as well as data fuzzification and the domain averaging method for state estimation. Simulations are carried out using a 30-bus power distribution system, and results are presented and discussed to prove the efficiency of the proposed method. The practical application further verifies the availability of the proposed method.

Accurate estimation of the utility harmonic impedance is significant for harmonic control and responsibility division. Traditional methods of estimating the utility harmonic impedance are valid on the premise that the harmonic impedance on the wind-farm side is significantly greater than that on the utility side. However, with the installation of filters and reactive power compensation devices in wind farms, traditional estimation methods are no longer applicable. The accuracy of most existing estimation methods depends only on the accuracy of the model. This can cause large errors in the estimation of the utility harmonic impedance on a wind farm. To address this challenge, this paper proposes a method for estimating the utility harmonic impedance for a direct-drive permanent magnet synchronous generator wind farm based on a modified model with measured data. The proposed method uses the measured data to modify a model to obtain a more accurate estimation of the utility harmonic impedance. The results of the simulation and case study indicated that the traditional estimation methods have large errors when the background harmonic fluctuates significantly or the wind-farm-side harmonic impedance is smaller than the utility-side harmonic impedance. In contrast, the proposed method has a higher accuracy for the above scenarios.INDEX TERMS D-PMSG wind farm, power quality, utility-side harmonic impedance， wind-farm-side harmonic impedance.