Wenhao Hou

To tackle the aging problem of metal oxide arresters (MOA) in online monitoring, a novel genetic algorithm (GA)-based method is proposed for monitoring MOA degradation. Using the operating voltage and measured leakage current of MOA, the GA's optimization capability is employed to determine the parameters k and c in the MOA equivalent model, which vary with aging. This approach ultimately enables the monitoring of MOA degradation. Additionally, simulations using Matlab are conducted to analyze the effects of voltage harmonics, frequency fluctuations, and voltage fluctuations in the power grid on the algorithm. The study demonstrates that the new algorithm can adapt the calculated leakage current to match the measured leakage current of MOA (with a standard deviation of 1.498%) and accurately calculate the parameters k and c . Furthermore, Matlab simulation results show that the maximum errors of k and c are only 0.08% and 0.05% respectively, even under voltage harmonics, voltage fluctuations, and frequency fluctuations. This demonstrates the strong anti-interference robustness of the proposed algorithm. By relying on only two simple parameters, the method also simplifies the monitoring process, improving both computational efficiency and practical applicability for online MOA aging assessment.

Optimizing control parameters is one of the measures to suppress oscillation in new power system. Firstly, a three-port impedance model with input characteristics is established for the grid connected inverter, and the optimal stability region of inner current control parameters considering AC side stability is given. Then the parameters of the control loop are optimized to form the optimal stability region, which is verified by simulation and experiment.

The impulse charge moment change (iCMC) is an important electrical property of cloud-to-ground (CG) lightning. In this paper, a new method of measuring the iCMC at distances of several hundred kilometers is proposed. The method is based on the vertical electric field below 1 kHz measured by the widely used fast electric field antenna with low frequency/very low frequency (LF/VLF) band. The impulse response of Earth-ionosphere waveguide (EIWG) is modeled using a finite difference time domain (FDTD) method considering an anisotropic ionosphere. By comparing the observed waveform with the simulated impulse response, the lightning discharge is classified into the impulsive discharge and the non-impulsive discharge. For the impulsive discharge, its iCMC is obtained directly by comparing the measured ELF waveform to the modeled impulse response at the same distance. For the non-impulsive discharge, its current moment waveform is assumed to be a sum of two Heidler’s functions, and the genetic algorithm is used to search the unknown parameters in the functions. The good agreement between the measured ELF waveform and the simulated waveform implies that the extracted current moments are reasonable. This method can be used to continuously monitor the lightning iCMC in a given time and space.

The effects of real terrain on the lightning magnetic fields and location accuracy are studied by using two-dimensional finite difference time-domain method. Two estimation methods of wave arrival time (using the time corresponding to the peak field and 10% threshold of the peak field) are applied to the arrival time delay, and then using the ToA technique with three approaches of arrival time differences to evaluate the location accuracy. It is found that the effect of real terrain on the lightning magnetic fields is due to the enhancement of reflection wave and propagation attenuation of terrain topography. The arrival time of peak value method is delayed from 0.15 to 2.3 μ s, while that of 10% threshold method varied from −0.11 to 0.92 μ s. The lightning location accuracy using the 10% threshold method is least, about tens of meters. Furthermore, considering the specific frequency bandwidth of detection sensors, three-order 10- to 500 kHz Butterworth filter was applied. After filtering, the effect of real terrain on the lightning magnetic fields becomes more obvious. The 10% threshold method is still the best way for lightning location, and the frequency bandwidth of different sensors on the effect of the location accuracy cannot be ignored.

In this article, we studied the propagation effect of finite ground conductivity and earth's curvature on the far vertical electric fields at ground level radiated by narrow bipolar events (NBEs), within several hundreds of kilometers from the discharge channel. Two different NBE current moments with risetimes of 5 and 8 μs are adopted, and two discharge heights of 5 and 15 km are considered, respectively. Simulation results show that the effect of earth's curvature should be considered when the propagation distance is larger than 200 km, and the propagation effect is closely related the NBE discharge current waveform, the discharge height, the ground conductivity, and the observation distance. With the decrease in ground conductivity, the field attenuation obviously increases. Besides, the attenuation is more when the source contains more high frequency component. It is also found that the propagation effect is more obvious when the discharge height is lower, especially for ground conductivity lower than 0.01 S/m. The reduction of NBE's far field peak caused by the propagation effect can reach about 42%, 60%, and 70% at the propagation distance of 100, 200, and 300 km, respectively. Furthermore, the current moment of NBE will be underestimated by about 5–35% from the observed far field without considering the propagation effect, within 300 km from NBE channel. However, the propagation effect on the estimation of NBE charge moment can be approximately ignored.

The polarity reversal of the lightning‐generated first sky wave as a function of the observation distance is studied using a novel approach combining the finite‐difference time domain (FDTD) method and the superposition principle of electromagnetic waves. In this method, the sky wave is generated by radiation from the induced current produced by the motion of charged particles driven by the lightning‐radiated electromagnetic waves in the ionosphere. The horizontal and vertical components of the induced current density under the daytime and nighttime ionospheric conditions are evaluated. Their different contributions to the sky wave at different observation distances are analyzed in detail. Furthermore, a physical explanation for the polarity reversal in the time domain is proposed. It is found that, for relatively short observation distances (within ~200 km), the first sky wave is dominated by the component generated by the horizontal equivalent current in the Fresnel zone, while for longer observation distances (larger than ~300 km), the first sky wave is dominated by the component generated by the vertical equivalent current in the Fresnel zone. Since the polarities of the sky wave components generated by the vertical current source and horizontal current source are opposite, the polarity of the sky wave will reverse when increasing the observation distance.

In this article, the effect of a mountain located on the propagation path of lightning-radiated electromagnetic fields was systematically studied by using the two-dimensional finite-difference time-domain method (2-D FDTD), considering the influence of the return stroke (RS) type, the ground conductivity, the mountain height and the width at the base, its position, and the observation distance. The field peak reduction and the wave arrival time delay, compared with those for the perfectly conducting flat ground, are analyzed in detail. The results show that, with an increase in the mountain height, the field peak reduces and the wave arrival time delay increases. The wave arrival time is insensitive to the change of mountain widths. The terrain envelope method can be used to estimate the additional time delay introduced by the mountain. For a mountain lower than 1 km or so, the additional attenuation effect of the mountain on the field peak can be ignored for most practical purposes. Furthermore, the propagation process of the lightning-radiated electromagnetic wave over the mountain was studied by analyzing the spatiotemporal evolution characteristics of the field. The analysis shows that the electromagnetic wave propagates to the back of the mountain through the diffraction process at the mountaintop.

In this paper, we analyzed the performance of the moving computational domain technique (MCDT) for the calculation of lightning-generated electromagnetic fields in the Earth-ionosphere waveguide (EIWG) using the finite-difference time-domain (FDTD) method. In the FDTD method with MCDT, a spatial window moving along with the wavefront is defined and, at each time step, the field components are updated only in that window. The FDTD efficiency can be improved by at least a factor of two with respect to the conventional FDTD, while the accuracy remains unchanged. The improvement in computational efficiency is dependent on the size of the simulation domain, the cell size and the time window of interest.

We present an improved finite-difference time-domain (FDTD) method based on the moving computational domain technique for the calculation of lightning-generated fields over large-scale irregular terrain. In the improved method, a spatial window moving along with the wavefront at the speed of light is defined and, at each time step, the field components are updated only in that window. The time delay of the wavefront arrival caused by the propagation effect and the irregular terrain is considered when determining the width of the moving window. The efficiency and accuracy of the improved model are validated by comparing its results with those calculated by conventional FDTD, considering an example of lightning-radiated electromagnetic field propagation over complex mountainous terrain. The gain in the CPU time can be as high as a factor of 4 to 5, while the accuracy remains unchanged with respect to the conventional FDTD.