Behzad Kordi

Current and electromagnetic field associated with lightning-return strokes to tall towers

An analysis of electric and magnetic fields radiated by lightning first and subsequent return strokes to tall towers is presented. The contributions of the various components of the fields, namely, static, induction, and radiation for the electric field, and induction and radiation for the magnetic field are illustrated and discussed. It is shown in particular that the presence of a tower tends, in general, to increase substantially the electric and magnetic field peaks and their derivatives. This increase is mainly caused by the presence of two oppositely propagating current wavefronts originating from the tower top and by the very high propagation velocity of current pulses within the tower, and depends essentially on the wavefront steepness of the channel-base current. Because of the last factor, the increase of the field magnitudes is found to be significantly higher for subsequent return strokes, which are characterized by much faster risetimes compared to first return strokes. The presented results are consistent with experimental observations of current in lightning strokes to the Toronto CN Tower and of the associated electric and magnetic fields measured 2 km away. These findings partially explain the fact that subsequent return strokes characterized by lower current peaks but higher front steepnesses and return stroke speeds may result in higher field peaks. The results obtained have important implications in electromagnetic (EM) compatibility. It is found that lightning strokes to tall metallic objects lead to increased EM field disturbances. Also, subsequent return strokes are to be considered an even more important source of EM interference than first return strokes. Indeed, EM fields from subsequent strokes are characterized by faster fronts and additionally, they may reach greater peaks than first strokes. Lastly, findings of this study emphasize the difficulty of extracting reliable lightning return stroke current information from remote EM field measurements using oversimplified formulae.

A new lightning return stroke model based on antenna theory

A new approach based on antenna theory is presented to describe the lightning returnstroke process. The lightning channel is approximated by a straight and vertical monopole antenna with distributed resistance (a so-called lossy antenna) above a perfectly conducting ground. The antenna is fed at its lower end by a voltage source such that the antenna input current, which represents the lightning return-stroke current at the lightning channel base, can be specified. An electric field integral equation (EFIE) in the time domain is employed to describe the electromagnetic behavior of this lossy monopole antenna. The numerical solution of EFIE by the method of moments (MOM) provides the time-space distribution of the current and line charge density along the antenna. This new antenna-theory (or electromagnetic) model with specified current at the channel base requires only two adjustable parameters: the return-stroke propagation speed for a nonresistive channel and the channel resistance per unit length, each assumed to be constant (independent of time and height). The new model is compared to four of the most commonly used “engineering” return-stroke models in terms of the temporal-spatial distribution of channel current, the line charge density distribution, and the predicted electromagnetic fields at different distances. A reasonably good agreement is found with the modified transmission line model with linear current decay with height (MTLL) and with the Diendorfer-Uman (DU) model.

Time-Domain Distortion Analysis of Wideband Electromagnetic-Field Sensors Using Hermite–Gauss Orthogonal Functions

A time-domain distortion characterization of electromagnetic-field sensors is performed using Hermite-Gauss functions. For every electromagnetic-field sensor, a transformation matrix is calculated in the spanned Hermite-Gauss signal subspace. The calculated matrix is then compared to a reference transformation matrix to give a measure of distortion. Distortion characteristics for different frequency ranges are calculated to obtain a spectral interpretation. Unique properties of Hermite-Gauss functions under the Fourier transform are used to calculate the distortion characteristics from a single set of time-domain measurements. Distortion characteristics of an asymptotic conical dipole are compared to three wire monopoles of different lengths. A time-domain simulation based on solving an electric-field integral equation using the method of moments is performed. Distortion characteristics resulted from the simulation and those obtained using measurement data are compared to each other. Finally, the sensitivity of the sensors is taken into account in the distortion analysis.

On-line monitoring of partial discharges in a HVDC station environment

This paper deals with the on-line measurement of partial discharges (PD) in a high voltage direct current (HVDC) converter station. The HVDC station is a particularly challenging environment for measurement due to elevated interference levels caused from the switching of thyristor-controlled converters. In this work, online PD measurements were performed on two specimens; one is an HVDC converter transformer and the other an HVDC converter wall bushing. The measurements were performed in the high-frequency range from 400 kHz to 30 MHz with modern wideband PD measurement instrumentation. Results demonstrate that on-line measurement of PD in an HVDC station environment is possible and that a combination of input filtering and modern signal processing methods for feature extraction can be used to mitigate the converter interference. The feature extraction method used plots partial discharge data on a time-frequency classification map. The map enabled isolation of individual PD phenomena, and demonstrated that trending to monitor insulation degradation is possible with online partial discharge measurement. Furthermore, specific strategies for PD measurement and analysis are developed for each of the transformer and bushing specimens.

Passive Wireless Sensor for Measuring AC Electric Field in the Vicinity of High-Voltage Apparatus

A passive wireless sensor for the measurement of ac electric fields emanating from high-voltage (HV) apparatus using radio-frequency (RF) resonant cavities is presented. The sensor is composed of a coaxial transmission line resonator, where the resonant frequency is perturbed by capacitively coupled varactors. HV apparatus induces a bias voltage on the varactors through capacitively coupled electric fields. A printed circuit board on the top of the cavity provides coupling between the cavity and varactors and also between the varactors and the external electric field. The sensor is designed with a resonant frequency in the range of 2.4-2.5 GHz in the industrial, scientific, and medical band. Using capacitive coupling to an HV transmission line with 60 Hz at 1.7 kV, a resonant frequency shift of 7.2 (kHz)/12.5 (V/m) was obtained. The resonance frequency is determined remotely by sending pulses of RF signal to the sensor and recording the ring back of the resonator. RF pulses, with a frequency of 2455 MHz, are used to remotely interrogate the sensor at a repetition rate of 2.5 MHz and a pulsewidth of 100 ns. Downconverting the received signal, a ring back signal is recorded whose analysis determines the resonant frequency.

Application of time-domain antenna techniques in electromagnetic field sensors characterization

Time-domain techniques are advantageous over frequency-domain methods in the transient analysis of wideband systems. A major class of passive electromagnetic field sensors are treated as antennas in the receiving mode and hence can be characterized using antennas time-domain parameters such as effective gain and fidelity. In this paper, these parameters have been calculated for a monopole antenna and an Asymptotic Conical Dipole (ACD). The antennas are modeled as wire structures. By solving a time-domain Electric Field Integral Equation (EFIE) using the method of moments (MoM), the induced current in the wires due to the incident electromagnetic field is determined. Knowing the induced time-domain current and the incident electric field, antenna effective height is calculated as a key parameter to evaluate time-domain characteristics of the sensors.

Impulse Generator Optimum Setup for Transient Testing of Transformers Using Frequency-Response Analysis and Genetic Algorithm

A novel optimization-enabled electromagnetic transient simulation approach is proposed for calculating the resistor setting of a multistage impulse generator for insulation impulse testing of high-voltage transformers. This approach enables test engineers to overcome most of the major challenges attributed to the use of conventional trial-and-error methods for determining resistor settings, including the excessive time consumption and the potential damage to the test transformer due to a greater number of trial tests. The proposed method uses the frequency response of the test transformer to synthesize a circuit model for it. The test setup, including the test transformer and the impulse generator, is simulated using an electromagnetic-transients-type simulator. A genetic-algorithm-based approach is used to optimize the setting of the impulse generator. Optimized resistor values are then used in impulse testing of a three-phase power transformer. Different cases of impulse generator resistor arrangements are studied in this paper and simulated waveforms are compared with those obtained from measurements.

Transient Electromagnetic Fields associated with a power transmission line above a lossy ground

This paper provides a step by step procedure for the computation of electromagnetic (EM) fields in the vicinity of power transmission lines. By decomposing the transmission line into a number of Hertzian dipoles, the electric and magnetic fields at any point above the ground can be found by summing up the contribution of each segment. Two analytical frequency-domain approaches for the calculation of radiation from a horizontal electric dipole (HED) located above a finitely conducting ground are implemented and compared. Numerical electromagnetic code (NEC-4) is used in order to verify the results.

Buried cable parameter extraction using a full-space unbounded conformal mapping technique

In this paper a full-space conformal mapping technique is used to extract the parameters of buried multiconductor cables. The quasi-electric and quasi-magnetic approximations of Helmholtz equation are employed and necessary scaling factors are applied to the governing equations and post processing energy relations. The modified equations are numerically solved using a Finite-Element-based method. The calculated results for typical buried circular cable geometries are compared with those obtained from bounded-space cases and with analytic formulation. Comparison shows very a good agreement between analytical and the mapping technique results in a wide frequency range.

Classification of simultaneous multiple partial discharge sources based on probabilistic interpretation using a two-step logistic regression algorithm

In online condition assessment monitoring of high voltage (HV) insulators, it is often required to identify multiple, simultaneously activated partial discharge (PD) sources that happen in the insulation of the HV apparatus. Phased resolved partial discharge (PRPD) patterns are commonly used to identify PD sources. However, multiple, concurrent PD sources sometimes result in partially overlapped patterns, which make them hard to be identified. In this paper, we develop an accurate, reliable algorithm by constructing a novel two-step logistic regression (LR) model to conduct probabilistic identification of multi-source PDs. To this end, principal component analysis is applied on a database to construct a low dimensional space associated with single-source PDs. Samples of multi-source PDs are then projected onto this space and one-class kernel support vector machine is adopted to distinguish multi-source PDs from single-source ones. Finally, classification is performed by estimating the probability (degree of membership) of each PRPD pattern arising from different multi-source PDs following two rounds of LR modeling. To evaluate the performance of our proposed method, we study a number of multi-source PD models to simulate common defects of Gas-Insulated Switchgear (GIS) in small-scale laboratory test cells with realistic SF6 gas condition. Observations are obtained using fingerprints generated by a novel approach from recorded PRPD patterns. Comprehensive performance evaluation of the proposed algorithm and its advantages are conducted and the development of analytical equations is presented. The results of this paper can be used to design a solid basis for an automated multi-source classification system, which facilitates multi-source PD identification in early stages and safe operation of HV apparatus.