Hybrid Electromagnetic Model Research Articles

Assessing the Impact of DC Bipole Configuration on the Lightning Performance of a HVDC Transmission Line in terms of Backflashover

This work presents a discussion on the influence of both the polarity and position of the phase conductors (DC poles) of a double circuit 500 kV HVDC transmission line (TL) on lightning overvoltages developed across their line insulator strings and the corresponding lightning performance in terms of backflashover, considering computational simulations with the Hybrid Electromagnetic Model (HEM) and the Leader Progression Model (LPM). Several polarity arrangements of the DC poles were considered and their influence on the probability of backflashover occurrence due to negative downward lightning was assessed for tower-footing grounding impedances varying from 10 to 100 Ω. The study indicates the worst lightning performances for configurations B, C1 and D2, with critical current and backflashover probability varying from 96 to 49 kA and from 4% to 26%, respectively. These results show the importance of both the position and polarity of the phase conductors and tower-footing grounding impedance to define the HVDC TL performance, indicating the TL configurations with the lower phases with positive polarity as the one with worst lightning performance in terms of backflashover.

Hybrid Electromagnetic Modeling and NVH Co-Design of Switched Reluctance Motors

Hybrid Electromagnetic Modeling and NVH Co-Design of Switched Reluctance Motors

New Approaches to Represent the Frequency Dependence of Soil Parameters on the Time-Domain Hybrid Electromagnetic Model (HEM-TD)

New Approaches to Represent the Frequency Dependence of Soil Parameters on the Time-Domain Hybrid Electromagnetic Model (HEM-TD)

Modeling guyed towers of transmission lines in the assessment of backflashover occurrence

This paper evaluates the modeling of guyed towers of transmission lines in EMT-type programs to assess the backflashover occurrence of transmission lines and proposes a new modeling approach to be applied for this kind of towers. Two approaches to model the guyed towers by the revised Jordan model were evaluated: (i) the approach of CIGRE Brochure 63 that suggests representing the tower surge impedance as the parallel of the surge impedances of the mast and the guyed wires, ignoring the mutual coupling among them, and (ii) a new approach assuming the surge impedance of all the conductors in the tower section, guyed and mast conductors, including their mutual coupling effect. The evaluations considered the results provided by the Hybrid Electromagnetic Model (HEM) as reference for assessing the quality of the approaches. The approach (i) provided results of lightning overvoltage significantly lower than the reference, HEM model, leading to an underestimation of the probability of backflashover of the line. On the other hand, the approach (ii) was responsible for the results closest to those provided by HEM, indicating its quality, and the recommendation of applying this proposed approach for modeling guyed towers in the evaluations of backflashover occurrence.

Assessing the impact of the Corona effect on the lightning response of transmission lines using the Time Domain Hybrid Electromagnetic Model (HEM-TD)

Assessing the impact of the Corona effect on the lightning response of transmission lines using the Time Domain Hybrid Electromagnetic Model (HEM-TD)

Assessing the influence of peak current distributions of first return strokes on the lightning performance of transmission lines: Instrumented tower distributions versus standard distributions

• The effect of peak current distributions on backflashover is shown. • Instrumented tower and standard distributions are adopted. • IEEE and CIGRE distributions overestimate backflashover for high voltage level TLs. • MCS distribution leads to largest BFOR probability for low insulation level TL. • A new formulation to reproduce high current region of MCS distribution is proposed. The influence of peak current distributions of first return strokes of instrumented towers and international standards on the calculated lightning performance of 138 kV and 500 kV transmission lines is evaluated. This performance, expressed by the backflashover probability, is determined by computational simulations using the Hybrid Electromagnetic model (HEM) and the Disruptive Effect method (DE). The performance is calculated considering the cumulative peak-current distributions obtained from direct measurements at the instrumented towers of Morro do Cachimbo (MCS) and Monte San Salvatore (MSS) stations and those provided by IEEE and CIGRE standards. According to the results, using the standard distributions yields larger backflashover probabilities of transmission lines of higher insulation level, in relation to those calculated using instrumented-tower distributions. Considering transmission lines of lower insulation level, the performances calculated using standard distributions are close to those obtained using MSS distribution, but yields lower backflashover probabilities in relation to those obtained using MCS distribution. The latter leads to backflashover probabilities about 40% and 47% larger than those obtained by using IEEE and CIGRE distributions, respectively, in the tower-footing impedance range of 20 Ω – 40 Ω.

Machine Learning Assisted Hybrid Electromagnetic Modeling Framework and Its Applications to UWB MIMO Antennas

Machine learning (ML) has gained recognition as an efficient and robust technique to realize the solution of electromagnetic forward and inverse problems. This article introduces a hybrid ML framework that simultaneously acts as a forward and inverse model based on a mode input. Multivariate relevance vector regression (MVRVR) is adopted for implementing the hybrid ML model. MVRVR models for forward and inverse modeling are also presented. In addition, three hybrid ML models based on support vector regression (SVR), Gaussian process regression (GPR), and artificial neural network (ANN) are also implemented and a thorough comparative analysis between these ML models with the proposed MVRVR model is investigated to verify its accuracy. The proposed hybrid framework can be used to replace the requirements of the two separate models for solving forward and inverse problems. Two examples of ultra-wideband (UWB) MIMO antennas are employed to validate the effectiveness of the proposed modeling framework.

HEM-TD: New Time-Domain Electromagnetic Model for Calculating the Lightning Response of Electric Systems and Their Components

A new electromagnetic time-domain model dedicated to calculating the lightning response of electric systems and their components is presented. It is formulated taking as reference the main features of the frequency-domain Hybrid Electromagnetic Model. A differential aspect of this model is the capability to efficiently address problems involving nonlinear effects, such as soil ionization around grounding electrodes, the surge-arresters operation and the Corona Effect in applications related to the lightning performance of transmission lines. In addition, it is able to consider frequency-dependent effects, for instance that of soil resistivity, with an accuracy compatible with engineering applications. The basic formulation of the model is shown along with the validation of the results it yields by comparison with reliable references, including experimental results.

Influence of tower modeling on the assessment of backflashover occurrence on transmission lines due to first negative lightning strokes

This paper assesses the influence of tower modeling on the calculation of voltages across insulator strings, critical currents and percentage of backflashover occurrence for typical 138-, 230- and 500-kV towers. Three different model types are considered: geometric, multistory and multiconductor models. The results are compared to those obtained with the Hybrid Electromagnetic Model (HEM), which is taken as reference. The extended Jordan model belonging to the multiconductor model type is responsible for the results closest to those provided by HEM. The geometric and multistory models also lead to consistent results but with some limitations. The former provides good results for tower-footing impedance above 20 Ω, for the 138-kV tower, and above 40 Ω for the 230- and 500-kV towers. The latter is more appropriate to represent towers with geometry close to the one adopted on the development of its formulation, such as double circuit transmission towers with heights above 60 m.

Closed-Form Approximation for Grounding Grids Transient Analysis

Traditionally, wideband modeling of grounding system demands the usage of the method of moments (MoM) which implies a large number of segments to represent the conductors involved and the solution of a double integral for each and every one of these segments. Approaches presented in the literature as PEEC (partial element equivalent circuit) and HEM (hybrid electromagnetic model) are based on these assumptions. In either of these approaches, a heavy computational burden occurs, leading to a very time-consuming simulation. This paper proposes to use a two-term series expansion to allow a closed-form solution to the double integral found in HEM and thus is named as sHEM. The proposed approach, sHEM, is compared with HEM with respect to the harmonic impedance and the time responses associated with impulse currents. The results indicate only a small loss of accuracy with respect to the peak value of the overvoltage with a significant gain in the computational time. Finally, a comparison with actual measurement results is provided to evaluate the adequacy of using sHEM in the analysis of grounding grids.

A Computational Improvement in Grounding Systems Transient Analysis

This study investigates the adequacy of a modification in the evaluation of transversal and longitudinal impedance matrices for wideband modeling of grounding electrodes without a significant loss of accuracy. It is based on an approximation of the so-called hybrid electromagnetic model (HEM) and is called modified-HEM (MHEM). It exploits the average distance between segmented electrodes, allowing to pre-calculate the integral prior to the evaluation of the frequency loop improving considerably its numerical performance. Apart from some discrepancies for frequencies above a few MHz, the results for the harmonic impedance presented a very good accuracy when compared with the ones obtained using HEM. A comparison with an experimental test is carried out to enforce the adequacy of the investigated formulation within suitable parameters.

Closed-Form Approximation for Horizontal Grounding Electrodes Transient Analysis

The method of moments (MoM) is widely used in several areas of knowledge to solve integral equations. In the specific case of solving electrical grounding transient behavior problems, the computational time is usually high. This paper presents new alternative solutions for solving the double integrals in the well-known hybrid electromagnetic model (HEM) applied to horizontal grounding electrodes. These new alternative solutions correspond to using an approximation to the exponential term appearing in the finite integral needed to obtain the parameters. Two approaches are considered either using Maclaurin series or Pade approximation. The corresponding results are compared with those obtained by using HEM with MoM. It is shown that the proposed solutions allow reducing the computational time, without jeopardizing accuracy.

Constraints on the use of surge arresters for improving the backflashover rate of transmission lines

The effectiveness of using transmission line surge arresters (TLSA) under different conditions for improving the lightning performance of lines is assessed by means of computational simulation. These conditions comprise using TLSA only in critical towers and/or using devices only in certain phases, leaving one or two-phases unprotected. In addition to assess the effect on stricken towers, the transfer of overvoltage to unprotected adjacent towers is also addressed, considering different values of tower-footing impedance. In the analyses, lightning overvoltages across insulator of typical single-circuit 138-kV and double-circuit 230-kV transmission lines were calculated by using the Hybrid Electromagnetic Model (HEM) and flashover occurrence was assessed by using the Disruptive Effect method (DE), in each simulated condition. The results indicate that the operation of TLSA protects the own insulator and diminishes the backflashover risk at unprotected insulators. However, this risk remains high, depending on tower-footing impedance. Also, the risk of flashover at the unprotected adjacent towers due to the operation of surge arresters at the stricken tower was found to be significant in the case of all phases of the stricken tower protected, notably when the tower-footing impedance of the protected tower is high and that of the adjacent tower is low.

Using circuit elements to represent the distributed parameters of a grounding system under lightning strokes

This work presents a proposal to represent electrical grounding systems directly in the time domain. The advantage in using this type of representation is that voltage and current are better understood in this domain, and the model can easily be inserted into conventional software such as the Alternative Transient Program (ATP). The proposed method is applied to horizontal electrodes, vertical rods and grounding grids. The grounding is initially modelled in the frequency domain to obtain harmonic impedance, which is then represented by means of an approximation in the time domain, by passive elements (resistors, inductors and capacitors) associated in series and/or parallel. Validation was achieved by comparing the results with those obtained using the TLM (transmission line model), the HEM (hybrid electromagnetic model) and technical literature. The TLM and HEM were used to obtain the harmonic impedance of the grounding systems, but any other model could have been used, since this process has no effect on the proposed method, making the proposed method very versatile and attractive.

A Contribution to the Study of Grounding Systems Based on Circuit Synthesis

This work presents an unprecedented technique capable of representing grounding grids directly in the time domain using an association of passive and discrete circuit elements (RLC circuits: resistors, inductors and capacitors). It is based on the frequency response of RLC circuits and ensures that all elements will be positive (physically implementable), solving situations that other techniques, such as vector fitting, generally presented in the literature are not able to solve using only passive circuits (feasible). The proposed technique is applied from the harmonic impedance previously provided, and therefore this impedance can be obtained using any model. It is accurate; it presents excellent results and was validated by comparing the harmonic admittance and transient voltage curves to those obtained with the hybrid electromagnetic model. The advantage of this technique is that it allows that several components in power systems, such as grounding systems and transmission towers, to be represented by a feasible circuit. Once this circuit is implemented in the laboratory, analysis and comparisons with conventional software (e.g. ATP) can be made and accurate transient electromagnetic responses can be obtained.

Lightning performance of transmission lines: Assessing the quality of traditional methodologies to determine backflashover rate of transmission lines taking as reference results provided by an advanced approach

This paper presents a critical review of CIGRE and IEEE traditional methodologies for calculating the backflashover outage rate (BFOR) of transmission lines, considering their adopted simplifications. These methodologies, along with an advanced computational approach named HEM-DE, were applied to a real 138-kV line to determine its backflashover rate. The quality of the results produced by the methodology at each step of their calculation procedure was assessed and discussed, taking as reference the accurate results provided by the advanced approach, based on the application of the hybrid electromagnetic model (HEM) and disruptive effect model (DE). According to this assessment, in the 40-to-10-Ω range of tower footing grounding resistance, the CIGRE methodology overestimates outage rate of the line in relation to the reference values (98–21% larger in the 40-to-10-Ω range of tower footing grounding resistance). The IEEE methodologies underestimates the BFOR, yielding rates 47–23% lower in the same resistance range.

• 139. HIGH FREQUENCY ENHANCEMENT OF THE HYBRID ELECTROMAGNETIC MODEL BY IMPLEMENTING COMPLEX IMA

Abstract A b s t r a c t: The well know hybrid electromagnetic model (HEM) is widely used for analysis of grounding systems. However, it has been proven that results obtained by HEM diverge from referent results in the high frequency (HF) domain. This paper presents implementation of complex image theory to take into account the earth-air interface instead of the quasi-static images used up-to-date with HEM. Parametric analysis of the achieved improvement in precision of the HEM method on high frequencies is visualized in multiple figures by comparison to referent results calculated by the renowned numerical electromagnetic code NEC-4. It may be concluded from the research presented in this paper that usage of complex images with HEM extends notably the applicability domain of the method. Key words: hybrid electromagnetic model; complex images

A hybrid electromagnetic model for acoustic optimization of claw-pole alternators

A hybrid electromagnetic model for acoustic optimization of claw-pole alternators

Lightning-Induced Voltages Over Lossy Ground: The Effect of Frequency Dependence of Electrical Parameters of Soil

The impact of the frequency dependence of soil resistivity and permittivity on lightning overvoltages induced on overhead lines over lossy ground is investigated. The Visacro-Alipio expressions were implemented on the hybrid electromagnetic model to take this effect into account. Systematic simulations were performed considering representative current waveforms of first and subsequent strokes and different stroke locations near the simulated overhead line. In general, the results indicated that this effect is responsible for a reduction of the induced overvoltage. This reduction increases with increasing soil resistivity and becomes relevant for soil resistivity above 1000 Ω·m.

A Broadband Focal Plane Array Camera for Real-time THz Imaging Applications

We present a large-format, sub-millimeter-resolution, focal plane array sensor for THz imaging. Each pixel in the sensor array consists of broadband THz antennas monolithically integrated with ultra-fast heterostructure backward diodes for THz sensing. With the aid of in-house hybrid electromagnetic modeling tools, the focal plane array is optimized for diffraction limited image resolution and conjugate impedance matching for highest THz sensitivity. The camera is designed to operate in the 0.6–1.2 THz band with 5 frames-per-second image acquisition speeds, making it ideal for THz imaging applications, such as security screening, non-destructive evaluation and chemical, pharmaceutical, and medical imaging. The simulation results are validated by measurements to demonstrate sub-millimeter resolution with a pixel optical responsivity of 600 V/W at 0.7 THz.