The existing methods estimating utility harmonic impedance usually require some assumptions such as the utility side is the dominant harmonic source and the harmonic impedance of customer side is much larger than that of the utility side, etc. However, these assumptions may not always hold in modern power grid. This paper proposes a novel method without any assumption, the estimation process only uses the correlation of the harmonic data measured at a point of common coupling (PCC). Through analysis of various field harmonic data, we find that measured data have short correlation characteristics, and there always exists a common weak correlation interval for the self-correlation and cross-correlation of the measured data. According to Norton equivalent circuit modal, it is easy to deduce that the utility harmonic current source is weak self-correlated in this interval. By selecting the self-correlation of the utility harmonic current source less than a small threshold, the utility harmonic impedance can be estimated accurately, whether the traditional assumptions are valid or not. In addition, the proposed method only requires a small sample size. Performance and effectiveness of the proposed method are verified through the simulation and field cases.

This article proposes a novel method for estimating multiharmonic sources in the underdetermined measurement system. In the proposed methodology, supposing that the number of harmonic measurements (HMs) is less than that of unknown state variables, the HM system can be mathematically described as underdetermined matrix equations, which has infinitely many solutions corresponding to all possibilities of the locations and the current emissions of the actual harmonic sources. Because the harmonic current emissions from nonharmonic source buses are approximately zero, by fitting the harmonic current emission of every possible nonharmonic source bus combinations and selecting the minimum fitting residual of all combination cases, thus, the true number, locations, and current emissions of harmonic sources are determined. Compared with the existing methods, the proposed method has less dependence on the placement of HMs and requires less prior information of harmonic sources. The effectiveness of the proposed method is verified by simulation and field case, with respect to a high correct location rate of more than 90%.

The existing noninvasive methods estimating utility harmonic impedance are usually based on the assumption that the impedance is constant during a time interval of concern. In fact, the utility harmonic impedance is variable due to various reasons such as possible short-circuit faults, grid operation changes, and capacitor switching. Thus, this paper proposes a novel method to estimate the variable utility harmonic impedance by minimizing the sum of the weighted impedance variations and the weighted variance of background harmonic voltage variations. The proposed method can track the continuous and abrupt changes of utility harmonic impedance. The effectiveness of the proposed method has been evaluated and verified through simulation and field cases.

The existing online cable insulation condition (CIC) evaluation methods are mainly based on the insulation defects signals inside the cable, such as DC signals, or the transient disturbances from outside, such as ground faults. However, the former are usually weak and susceptible to environments, and the latter are not frequently obtainable. On the contrary, the steady-state disturbances, such as harmonics, which carry CIC information, have been widely measured and are readily available. Therefore, it is very convenient if we can use these data to evaluate CIC. In fact, based on the harmonic measurements at one end of a cable and the cable parameters corresponding to any CIC, the theoretical harmonic amplification factor (HAF) can be obtained, and the HAF is the ratio of harmonic values at both ends. In other words, if there is a deviation between the measured and theoretical HAF, it means that CIC has changed. Given this, we propose a novel method to evaluate CIC by minimizing the deviation, which includes three steps: 1) derive the theoretical HAF by using the cable parameters and harmonic measurements at one end of a cable, 2) construct the objective function to minimize the deviation and 3) solve the function to evaluate CIC by employing the Gauss-Newton iteration algorithm. The performance and effectiveness of the proposed method are verified through the simulation and field cases.

The existing methods for estimating harmonic impedance usually assume that harmonic impedance is constant in a measurement period. However, for a variety of practical reasons, such as the load change of power grid, utility harmonic impedance may be changed. So a reasonable hypothesis is that utility harmonic impedance is time-varying, but the variation of the impedance is not large at adjacent sampling time. Thus, this paper proposes a novel method to estimate utility harmonic impedance by taking the criterion of minimum norm of impedance differences at adjacent sampling times with a measurement period. According to the proposed criterion, an objective function is built with the norm of the harmonic impedance differences and utility harmonic voltage differences of adjacent moments. And then, the utility harmonic impedance can be obtained by minimizing the objective function. The proposed method has high accuracy even while background harmonic emission level is high and harmonic impedance of customer side is not much larger than that of utility side. The effectiveness is verified by simulation and the field cases, and the corresponding estimation errors for most cases are within 10% of the nominal value.

The existing methods for evaluating harmonic impacts of multiple harmonic-producing loads (HLs) usually assume that all the network impedances are constant in the concerned whole-time interval. While considering the actual situations in power grids, the network impedances may have continuous or sudden variations due to grid operation changes, such as power line maintenance, filter switching, and capacitor switching, which causes the unreliable estimation of harmonic impacts by the existing methods. In view of this shortcoming, a novel multi-point model considering the variations of multiple network impedances is proposed. Furthermore, two weight matrices are formulated to fit the abrupt changes in network impedances. Based on the model and the matrices, the continuous and sudden variations of multiple network impedances can be tracked simultaneously according to the proposed minimum weighted variation constraint. Finally, more realistic and accurate harmonic impacts can be obtained. The performed simulation and experiment have demonstrated the validity of this method.