Francisco Roman

Corona from floating electrodes

Abstract It is not unusual to have insulated conducting objects located close to the conductors of a Lightning Protection System. However, the separation of these objects from the Lightning Protection System could vary from a few millimetres to some centimetres. When the system is exposed to thunderstorm electric fields, discharge could be initiated between the Lightning Protection System and the floating conductive body. We have performed the study reported in this paper to gain knowledge concerning this discharge process. The critical field for the initiation of corona currents from a floating electrode, when a negative DC voltage is applied to the main gap electrodes, is detected by the discharge of the floating electrode to the earth electrode. The electric field intensity at the high curvature (sharp) points on the floating electrode surface was calculated numerically to determine this critical electric field. The discharge repetition frequency was studied as a function of the applied background electric field and a well-defined relationship was observed between these two parameters. The corona currents were calculated and good agreement was found between calculated and measured values.

On the Electromagnetic Susceptibility of Hot Wire-Based Electroexplosive Devices to RF Sources

We present an analysis of the thermal response of a hot-wire electroexplosive device (EED) excited with different transient signals. First-order and second-order analytical models to calculate the thermal response of an EED are assessed taking as reference numerical simulations obtained using ANSYS. For the early-time response, when the time is much smaller than the thermal constant of the EED, the best approach corresponds to a first-order differential model in which the thermal capacitance is calculated with short-pulse excitations. A linear simplification to calculate the maximum temperature due to short excitations is also shown to be adequate. On the other hand, the most appropriate model for the late-time response is a second-order model. The models are used to assess the electromagnetic susceptibility of a wired EED for different electromagnetic pulsed environments. Radiated signals produced by a mesoband radiator, two types of radars, and a hyperband radiator are considered. The radar signal proved to be the most disturbing source because of its highest duty cycle and its flat spectral response around a specific frequency. Even the temperature firing threshold can be exceeded with the radiated field produced by a radar of 200 kW of output power located at a distance of 5 m.

Influence of the wind for positive corona in a single-needle-electrode on thunderstorm-like conditions

The effect of the wind velocity on a corona-point electrode in a point-to-plane arrangement is presented in this paper. An important relationship between the wind velocity, the electric field and the magnitude of the corona current was observed. The experiment is performed in a shielded room at the atmospheric pressure and temperature of Bogotá, Colombia (2.600 m over sea level). The wind velocity was controlled.

Corona current in a cactus-like electrode under natural-like electric field: Comparison with a single-needle electrode

In this paper a working hypothesis was proposed: If an air stream can remove the corona space charge close to a corona source, then the corona current is modified in a different manner in different types of corona electrodes. This phenomenon must be polarity dependent. To test this hypothesis two types of corona electrodes were tested: a cactus-like and a single-needle electrode under a natural-like electric field of both polarities. Both corona current sources were tested inside of a shielded room to avoid high frequency interference produced by external HF sources and were immersed in a constant DC electric field of both polarities, while the corona current and the background electric field were measured. In a plate-to-plate electrode arrangement a DC voltage was applied to the tested electrodes during 3 subsequent 30 s time intervals. In the second time interval a 7,06 m/s air stream was applied to the measured electrode, while in the first and third ones the air was calm. Results show that the measured corona current is larger in the presence of a current stream and in some specific cases of air flow velocity and background electric field, the measured current is larger in the single-needle electrode. Those results are polarity dependent.

Influence of thunderstorm-like wind velocities on point discharge corona currents

To study the influence of the wind speed on the generation of corona current in two different needles, a wind tunnel was designed and constructed. Needles were tested in a point-to-plane arrangement under laminar air flow. A constant background voltage is applied to the electrode arrangement, while air velocity varied from 0 m/s up to 16 m/s, a wind velocity near to thunderstorm conditions. Results show that the corona onset voltage of the needles is independent of the air velocity; however, the corona current magnitude increases with an increase of the air velocity.

Switched Oscillator: Parameter Effects on the Generated Signals

This paper describes a methodology to design a switched oscillator (SWO) together with its experimental validation. The dependence of the SWO resonance frequency, bandwidth, and quality factor upon spark-gap pressure, interelectrode distance, and transmission-line (TL) length is evaluated. The SWO simulation results obtained with both the TL model developed in the computer program EMTP-alternative transient program (ATP) and the electromagnetic (EM) model developed in the computer program Computer Simulation Technology (CST) Microwave Studio are compared. An SWO based on simulation results, connected to an impedance of 100 Ω, was constructed and tested in the laboratory. Simulations and laboratory test results fit well. The proposed methodology, which combines TL and EM simulations, was experimentally validated, and it can be used to design SWO systems for different applications.

Analysis of wavelet based denoising methods applied to measured lightning electric fields

This paper describes the results of a wavelet based noise reduction procedure applied to three lightning electric field signals recorded in Bogotá-Colombia, Colombo-Sri Lanka and Toronto-Canada. In general, the evaluated signals were affected by three different noise sources. These are the analog-to-digital converter (ADC), the electronic circuit and the environmental noise.

Nonfatal lightning injuries in Colombia: Case reports

Lightning accidents involving several fatalities and injuries are reported every year in the world. Colombia, a country with a high keraunic level, reports frequently lightning fatalities involving several injury mechanisms. However, there are also some fortunate cases where the victims have survived without disabling traumas. Two nonfatal events in which lightning strikes persons are reported. In both cases it is observed that some metal objects such as necklaces or buckle belts saved the life and integrity of the victims. Moreover, both the relative localization and the comprehensive description of the cases scenarios involving visual consequences in the victims skin are presented. The analysis of such accidents could be of interest to protect persons against indirect lightning currents.

Characterization, Modeling, and Statistical Analysis of the Electromagnetic Response of Inert Improvised Explosive Devices

The experimental characterization of the electromagnetic (EM) response of a set of inert improvised explosive devices (IEDs) is presented in this paper. The characterization is based on measurements of gain and input impedance of the firing circuits of inert IEDs in the range of 200 MHz to 2 GHz. First, a theoretical model to predict the average gain is developed. A comparison between the measurement results and theoretical predictions is presented. Then, the power delivered to a hot wire-based electro-explosive device connected to the inert IED is calculated for each measured case and it is statistically analyzed. The effects on the IED response of different characteristics of the coupling, such as the IEDs elements, the incident field polarization, and the impedance mismatch, are also studied. In addition, the statistical performance of the induced power and current are determined and compared with the results of a numerical model based on Monte Carlo simulations. It is demonstrated that both approaches, a theoretical model based on an equivalent antenna and Monte Carlo simulations, can be used to describe the average EM response of the firing circuit of IEDs.

Experimental Study of a Submillimeter Spark-Gap

This paper describes an experimental study on a submillimeter spark-gap and the effect of the gas type, pressure and interelectrode distance on the gas discharge. A coaxial transmission-line test switch and stainless steel electrodes have been used for the laboratory tests. The results presented in this paper are obtained using air, SF6, and argon, at gas pressures between 174 and 774 kPa and interelectrode distances from 330 to 450 μm. Results show that there is a linear change in breakdown voltage as the pd-relation increases. Important differences are observed when the time of the resistive phase is calculated and compared with the one obtained with some empirical models. It is also found that the arc-resistance depends on the gas type, and it is almost proportional to pd. The Kushner arc-resistance model fits well with the laboratory test results. From these results, a new constant of proportionality depending on electric field, pressure, and interelectrode distance is proposed. Additionally, for the studied pd-range, the gas with the best-fitting results was SF6. Finally, a good fit is observed between the laboratory test results and the obtained models programed in the computer program electromagnetic transient program.