Electromagnetic Time Reversal Research Articles

The Effect of the Ring Mains Units for On-Line Partial Discharge Location With Time Reversal in Medium Voltage Networks

The performance of a new on-line partial discharge (PD) location method based on the Electromagnetic Time Reversal (EMTR) theory and the Transmission Line Matrix (TLM) method are investigated for characterization of Medium-Voltage (MV) networks. The distortion of the PD signal during its propagation along a cable connection including network components modelled based on experimental data is reproduced in simulation and the effectiveness of the EMTR-based method to localize the PD source is analyzed. In particular, the effects of the ring main units (RMUs), that behave as a complex impedance, the variation with frequency of the MV cable impedance and the reflection patterns, due to impedance mismatches, are considered and investigated. Simulation results are given showing the performance of the EMTR method in two different networks configurations: the former one with a RMU at the end of a MV cable and the latter one with a second MV cable connected to the RMU of the first configuration having a distribution transformer at its far end. The results show that the EMTR method is able, with only a single observation point, to localize PDs also in the presence of RMU with a relative error, with respect to the line length, of approximately 1%.

Transmission Line Fault Location Based on Improved Algorithm of Electromagnetic Time Reversal

For most traditional fault location methods, the fault type needs to be known in advance or there may be dead zones using the decoupled modulus. To solve the problems, an improved algorithm based on electromagnetic time reversal is proposed. The improved algorithm can realize fault location by using the coupling signal in the non-fault phase of the transmission line, so fault location can be realized effectively even when the fault phase is unknown. The feasibility of using non-fault phase coupled signals to locate faults is theoretically verified. A 750kV overhead transmission line model is established by ATP-EMTP to obtain the fault signal at the observation point. The fault signal is time reversed and then connected to the observation point as an injection source. Each hypothetical fault location is traversed and its current is calculated by the BLT super-matrix equation. The 2-norm based on current energy is proposed as the criterion, and the position with the largest norm is the fault location. The simulation results show that the algorithm is not affected by the fault types, transition resistance and the initial phase angles of the power sources on both sides, and has high accuracy.

Analysis of time reversal cavity characteristics based on multipath channel model

The electromagnetic wave time reversal technology using time reversal cavity (TRC) has potential applications in many areas, such as pulse compression, power synthesis, perturbation detection, beamforming, etc. Microwave chaotic cavity with multi-path transmission characteristics is usually used in TRC. Based on diffraction theory, it can prove that this kind of cavity has spatiotemporal focusing characteristics and can be used for compressing pulses, but it cannot be used to analyze the reversal performance of the cavity. In order to obtain a suitable analysis method and guide the design of TRC, in this work, the scattering, diffusion and attenuation characteristics of electromagnetic wave propagation are analyzed and a multipath channel model of TRC is built by the channel theory. Moreover, the crosstalk characteristics between paths are studied, and the generation mechanism of time sidelobe, time sidelobe shift and interference are also investigated. In addition, under the assumption of random plane wave, the distribution characteristics of spatial focal spot are analyzed, which is consistent with the diffraction theory. Moreover, the actual focal spot size is not only limited by the diffraction limit, but also related to the initial focal spot size. The theoretical analysis results are basically consistent with the experimental and numerical simulation results.

Distribution Line Fault Location With Unknown Fault Impedance Based on Electromagnetic Time Reversal

Rapid and accurate location of power distribution line faults is essential for power system reliability improvement. Based on the electromagnetic time reversal (EMTR), an improved expression of fault current considering the influence of both sides of the transverse branch in the reversed time was derived in this research. In the reversed time, the transient voltage signals were injected into both ends of the line and multiplied one signal by −1, as suggested in An <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> 's work, and an assumed impedance of the transverse branch was set to 1000 Ω. A new criterion of the mirrored minimum fault current signal energy was proposed to determine the fault location. In the direct time, by setting different ground fault impedances, the distribution of the fault current signal energy in the frequency domain was calculated. Furthermore, the time-domain simulation verification was performed in the power systems computer-aided design software. The results indicate that when the actual fault impedance was unknown, the accurate fault location of the distribution line can be achieved by the proposed EMTR-based method, and it is less affected by time synchronization.

A New Hybrid Algorithm to Image Lightning Channels Combining the Time Difference of Arrival Technique and Electromagnetic Time Reversal Technique

Very-high-frequency (VHF) electromagnetic signals have been well used to image lightning channels with high temporal and spatial resolution due to their capability to penetrate clouds. A lightning broadband VHF interferometer with three VHF antennas configured in a scalene-triangle shape has been installed in Lhasa since 2019, to detect the lightning VHF signals. Using the signals from the VHF interferometer, a new hybrid algorithm, called the TDOA-EMTR technique, combining the time difference of arrival (TDOA) and the electromagnetic time reversal (EMTR) technique is introduced to image the two-dimensional lightning channels. The TDOA technique is firstly applied to calculate the initial solutions for the whole lightning flash. According to the results by the TDOA method, the domain used for the EMTR technique is predetermined, and then the EMTR technique is operated to obtain the final positioning result. Unlike the original EMTR technique, the low-power frequency points for each time window are removed based on the FFT spectrum. Metrics used to filter noise events are adjusted. Detailed imaging results of a negative cloud-to-ground (CG) lightning flash and an intra-cloud (IC) lightning flash by the TDOA method and the TDOA-EMTR are presented. Compared with the original EMTR method, the positioning efficiency can be improved by more than a factor of 3 to 4, depending on the scope of the pre-determined domain. Results show that the new algorithm can obtain much weaker radiation sources and simultaneously occurring sources, compared with the TDOA method.

A Correlation-Based Electromagnetic Time Reversal Technique to Locate Indoor Transient Radiation Sources

To overcome the multipath interference in locating transient electromagnetic (EM) radiation sources in an indoor environment, we propose a criterion that calculates the correlation between back-propagated signals from observation points, to be used in EM time reversal (EMTR) algorithms. The method introduced in this article has three main advantages with respect to classical methods that use full-wave simulations and other criteria, such as maximum field strength. First, compared with full-wave techniques in the back-propagation phase, the proposed correlation-based method utilizes approximated transfer functions from the ray-tracing technique, which can improve the computation efficiency. Second, an inverted-loss model is used for the back-propagation, which could reduce the localization error caused by multipath effects due to signal attenuation and time delay. Third, the proposed correlation criterion has weak correlation with the source characteristics, which makes it applicable to the localization in indoor reflective environments with only two observation points. Several numerical simulations are carried out to assess the performance of the proposed method. The results indicate that the proposed correlation-based EMTR technique is able to locate radiation sources accurately and efficiently in indoor reflective environments.

A 3D controllable diffraction‐limited spot array generated by means of a spaced‐dipole array

The creation of an array of three-dimensional (3D) multifocal spots in the focal region has attracted interest due to potential applications in parallel or simultaneous process areas. Based on the theory of pattern synthesis of antenna array and the electromagnetic time reversal technique, an optimisation-free approach is reported to construct a 3D controllable diffraction-limited spot array in the focal volume of a 4pi focussing system formed by two high-numerical-aperture (NA) objectives. The proposed method can be implemented readily by inversely focussing the field radiated from a virtual spaced-dipole antenna array mounted at the focus of the 4pi configuration. By solving the inverse problem, the required illumination in the pupil plane for producing the 3D spot array can be found. It is demonstrated that the 3D diffraction-limited focal spot array owns the properties of controllable polarisation, scheduled number, tunable location, and adjustable interval. This array may find applications in 3D simultaneous optical manipulation and trapping, 3D parallel fabrication, 3D optical data storage, and so on.

Electromagnetic Time Reversal Method to Locate Partial Discharges in Power Networks Using 1D TLM Modelling

This letter sets out to describe the first results of the design process that will lead to a new on-line partial discharge location method based on Electromagnetic Time Reversal theory and using the Transmission Line-Matrix method. A description of the basic steps of the method under design is given together with the modeling procedure used to describe time inverted signal propagation. Finally, the ability of the method to locate partial discharges on power cables both using two observation points and a single observation point is proved in simulation.

Fault detection using electromagnetic time reversal: The problem of losses

Fault detection using electromagnetic time reversal: The problem of losses

Time domain sources identification in the near field: comparison between electromagnetic time reversal and genetic algorithms-based methods

This study presents a comparative study between two electromagnetic inverse methods based on electromagnetic time reversal (EMTR) technique and genetic algorithms (GAs) for radiating sources characterisation in the near field (NF). Indeed, frequency-inverse methods reveal insufficiencies in transient radiations study. For instance, it leads to a repetitive process for multiple distinct frequencies. To overcome the frequency method limitations, EM inverse method in time domain (TD) successively based on GA and EMTR technique are implemented using analytical equations governing equivalent dipoles radiations. The performance of the two methods is compared in terms of complexity, accuracy, and calculation time. A TD measurement test bench is given using the NF scanning technique. Experimental results confirm that both methods are efficient and provide adequate equivalent models. Nevertheless, GA-based method depends on simplifying assumptions and thus the convergence is not always guaranteed. Therefore, EMTR-based method is more suitable, particularly for high switching frequencies in electronics systems.

Electromagnetic Time Reversal in the Near Field: Characterization of Transient Disturbances in Power Electronics

In the article, the electromagnetic time reversal technique (EMTR) is studied in the electromagnetic compatibility context. The main issue addressed by this approach is to deal with electromagnetic (EM) radiating source identification. This article is intended to provide a time-domain (TD) study of EMTR in the near field (NF) and prove its efficiency in characterizing transient disturbances in power electronics. The reconstruction of EM emissions is based on two stages. First, the modeling process relies on the use of TD analytical expressions. Second, signal processing is referring to the time reversal technique, which defines an inverse problem resolution. The method proposes to identify a set of equivalent dipoles. The validation is carried out experimentally using a TD measurement bench. The main purposes of this method is to extract the dipoles locations, moments, and orientation. The magnetic field maps calculated at each time step using obtained equivalent sources are compared with measured distributions. Adequate agreement is achieved, which confirms the efficiency of the proposed method. In addition, for validation purposes, the equivalent model is compared with that obtained by a frequency-domain (FD) method based on the standard genetic algorithms. Unlike FD, TD investigations allow characterizing transient disturbances in power electronic systems that emit strong EM interferences, by finding a sufficient equivalent model valid on a wide frequency band at once.

An Effective EMTR-Based High-Impedance Fault Location Method for Transmission Lines

This article summarizes the electromagnetic time-reversal (EMTR) technique for fault location, and further numerically validates its effectiveness when the fault impedance is negligible. In addition, a specific EMTR model considering the fault impedance is derived, and the correctness of the model derivation is verified by various calculation methods. Based on this, we found that when the fault impedance is large, the existing EMTR methods might fail to accurately locate the fault. We propose an EMTR method that improves the location effect of high-impedance faults by injecting double-ended signals simultaneously. Theoretical calculations show that this method can achieve an accurate location for high-impedance faults. To further illustrate the effectiveness, the proposed method is compared with the existing EMTR methods and the most commonly used traveling wave-based method using wavelet transform. The simulation results show that the proposed double-ended EMTR method can effectively locate high-impedance faults, and it is more robust against synchronization errors compared with the traveling wave method. In addition, the proposed method does not require the knowledge or a priori guess of the unknown fault impedance.

An Effective Method Based on Time Reversal and Optimization Techniques for Locating Faults on Power Grids

Electromagnetic time reversal (EMTR) has recently emerged as a promising technique applied for locating faults in power networks. It directly transposes the idea of focusing energy back to its source introduced in original time-reversal (TR) methods. Accordingly, we present in this paper, FasTR, a method based on the tenets of TR, that estimates the fault location by employing optimization based algorithms for fetching the highest peak amplitude with maximum coherence in space and time . However, it uses an alternative approach for executing the cumbersome TR post-processing, thanks to a simplified analytical model capable of evaluating the voltage (or current) at any position and any instant of the tested network resulting from the back-injection of the recorded time-revered signals. FasTR is shown to accurately locate a fault in a complex network with just a basic knowledge of its topology in no more than a couple tens of seconds. More importantly is its ability to locate multiple faults in non-homogeneous networks. The performance of the proposed method is validated by numerical simulations as well as an experimental setup by making reference to a reduced-scale coaxial cable network where real faults are hardware-emulated.

Locating Transient Directional Sources in Free Space Based on the Electromagnetic Time Reversal Technique

Traditional location methods such as time of arrival and direction of arrival have been widely used to locate transient radiation sources. However, these approaches neglect the directional radiation pattern of the sources. Locating directional sources is a complicated task because the waveform and the amplitude of radiated signals in the time domain vary for different directions. In this article, the ability of the electromagnetic time reversal (EMTR) technique to locate directional sources is investigated. As a typical directional source, the impulse radiation antenna (IRA) is used to validate the method. A radiation model is first established, and the time reversal procedure is carried out in the frequency domain. The amplitude of the signals is kept constant in the reverse phase in order to eliminate the distance attenuation. After analyzing the characteristics of directionality, we choose the energy criterion to estimate the position of the source since the time reversed signals would add up in phase only at the position of the source. It is shown that the point with the maximum energy of the synthesized signal corresponds to the position of the source. Numerical validations are presented and the relationship between the spatial resolution and the influencing factors such as the central frequency and the bandwidth of the excitation, the background noise level, the baseline length, and the number of sensors, are discussed in this article. Experimental results in an anechoic chamber with an impulse radiation antenna as the radiator and multiple horn antennas as the receivers are presented which confirm the ability of the EMTR-based method to locate directional radiation sources. Furthermore, we show that the location accuracy obtained with the energy criterion is better than that obtained with the ∞-norm criterion in the considered scenarios.

Partial Discharge Localization Using Electromagnetic Time Reversal: A Performance Analysis

In this study, first, a comparison on the application of electromagnetic time reversal (EMTR) and time difference of arrival (TDoA) in partial discharge localization in power transformers is presented. A two-dimensional finite-difference time-domain simulation is used to calculate the signal recorded by the sensors. Results show that, in a transformer tank excluding its windings, both methods yield similar results in terms of location accuracy, although the EMTR method only needs one sensor to localize the partial discharge (PD) source while the TDoA method needs at least three sensors in the 2D localization problem. However, the presence of transformer windings leads to a degradation of the performance of the TDoA method if the line of sight from the source to the sensor is blocked by any of the winding blocks. On the other hand, the presence of the transformer windings has an effect on the localization of PD sources that occur between two adjacent phase windings when the distance between the outer winding distances is shorter than the minimum wavelength, λ min . The degradation is directly caused by the diffraction limit. It is shown that, if the distance between two adjacent phase windings is greater than λ min , the EMTR process can locate PD sources occurring between two adjacent phase windings with acceptable accuracy. A case of occurrence of PDs in close proximity (less than λ min /2) to a single metallic object is analyzed both numerically and experimentally. The analysis reveals that although a degradation in the accuracy of the localization is observed compared to the case of longer distances between the PD source and the metallic object, a reasonable localization error of 10 mm (corresponding to λ min /10) is obtained.

Numerical and Experimental Validation of Electromagnetic Time Reversal for Geolocation of Lightning Strikes

We implement an electromagnetic time reversal technique (EMTR) to locate lightning return strokes. The two-dimensional finite difference time domain is employed to simulate the EMTR process in both, the forward-time and the backward-time phases. Scatterers are included in the computational domain to emulate the presence of objects. Three possible criteria to find the optimum time slice of the EMTR process that includes the maximum peak field, maximum peak energy, and last local minimum of entropy are tested and it is found that only the entropy criterion can successfully locate the lightning discharge. Our analysis shows that the EMTR process in both, using an unchanged and a simplified medium for the backward time works reasonably well even with only two sensors. Furthermore, we validated the proposed method via experimental results using waveforms recorded at two sensors at distances of 14.7 and 380 km from the Säntis Tower. The results demonstrate that the EMTR back-propagation process leads to a refocusing of the radiated energy at the location of the Säntis Tower. The ambiguity in the obtained location when only two sensors are used can be resolved either by using an additional sensor or through a more accurate modeling of the terrain.

Sensitivity enhancement of super resolution breast tumour imaging with far-field time reversal mirror integrating with multi-layered sub-wavelength patch scatterers

We present numerical simulation study of super-resolution imaging of closely spaced sub-wavelength size breast tumours with low cost microwave MIMO bowtie antenna array integrating with near-field planar multi-layered sub-wavelength patch (MLSWP) scatterers by using a monopole as the excitation source. The MLSWP scatterers, serving as transforming structures for the propagation of evanescent waves for sub-wavelength breast tumours imaging with far-field time reversal technique incorporating with bowtie antenna time reversal mirror, are investigated in this paper. The use of the MLSWP scatterers has enhanced the directivity and gain of the traditional monopole resulting in the enhanced scattering amplitudes of the tumour inclusions in the breast. The scattering intensity of the tumours has been enhanced as compared to imaging without the MLSWP scatterers. We propose a tumour imaging scheme using a bowtie antenna array with far-field electromagnetic time reversal for which tumours of sub-wavelength dimensions of 2 mm, 4 mm and 6 mm at sub-wavelength inter tumour distances have been imaged from our simple breast model with super-resolution operating at 2.45 GHz. This will greatly enhance the early imaging of breast tumours in the fight against breast cancer whiles attaining a cost effective biomedical imaging system since the antennas and MLSWP scatterers are low cost microwave components.

Single-Sensor Source Localization Using Electromagnetic Time Reversal and Deep Transfer Learning: Application to Lightning

Electromagnetic Time Reversal (EMTR) has been used to locate different types of electromagnetic sources. We propose a novel technique based on the combination of EMTR and Machine Learning (ML) for source localization. We show for the first time that ML techniques can be used in conjunction with EMTR to reduce the required number of sensors to only one for the localization of electromagnetic sources in the presence of scatterers. In the EMTR part, we use 2D-FDTD method to generate 2D profiles of the vertical electric field as RGB images. Next, in the ML part, we take advantage of transfer learning techniques by using the pretrained VGG-19 Convolutional Neural Network (CNN) as the feature extractor tool. To the best of our knowledge, this is the first time that the knowledge of pretrained CNNs is applied to simulation-generated images. We demonstrate the skill of the developed methodology in localizing two kinds of electromagnetic sources, namely RF sources with a bandwidth of 0.1–10 MHz and lightning impulses. For the localization of lightning, based on the experimental recordings in the Säntis region, the new approach enables accurate 2D lightning localization using only one sensor, as opposed to current lightning location systems that need at least two sensors to operate.

Electromagnetic Time Reversal Similarity Characteristics and Its Application to Locating Faults in Power Networks

The existing Electromagnetic Time Reversal (EMTR)-based fault location methods generally leverage the features (e.g., energy) of the signals simulated in the EMTR reversed-time stage (relative to the direct-time stage wherein a fault occurs and its originated transient signals are measured). This paper presents for the first time an integrated time-frequency analysis that identifies the similarity between the signals respectively present in the direct time and in the reversed time. It is demonstrated that the fault current observed at the true fault location exclusively behaves as a quasi-scaled copy of the time-reversed back-injected transient current, such that, the fault location can be identified through a similarity assessment. In this respect, the paper proposes calculating a cross-correlation metric to quantitatively represent the level of similarity between the back-injected transient signal and the fault current signal simulated at the guessed fault locations. The similarity characteristic and the fault location performance of the proposed cross-correlation metric are validated first through simulation case studies based on the IEEE 34-bus test distribution feeder, and then using a pilot test wherein a live medium-voltage radial distribution network was subject to realistic fault occurrences. It is demonstrated that the proposed similarity metric is applicable to locating various types of faults and is robust against uncertainties like fault impedance and fault inception angle.

On the Spatial Resolution of Fault-Location Techniques Based on Full-Fault Transients

This article discusses the mechanisms enabling spatial resolution in fault location methods based on full transient signals, as opposed to those only using their early-time portion. This idea is found in recent travelling-wave methods (TWM) and those based on electromagnetic time reversal (EMTR). Their spatial resolution is discussed in terms of the sensitivity of a system resonances to change in the fault position and their coherence bandwidth. It is proven that using the entire transient signal it is possible to bypass the Fourier transform uncertainty principle, which limits the spatial resolution of time-domain reflectometry and standard early-time TWM. Super-resolved fault location is shown to be possible only for resonating systems, enabling high spatial resolution without relying on wide-band data. A detailed theoretical analysis for laterals and numerical results for networks and a three-phase line show that significant differences can be observed for the spatial resolution associated to each resonance, most often resulting in a loss of spatial resolution. The interaction between separate resonant structures, such as laterals in networks and coupled conductors in three-phase lines are shown to be the main cause of resolution loss.