Tower models for power systems transients. A Review

Fast-front transients play an important role in the insulation design of any power system. When a stroke hits the shield wire or the tower of high-voltage overhead power lines, flashover may occur either along the span or across tower insulators, depending on the relevant voltages and insulation strength. As a result, backflashover may take place from the tower structure to the phase conductor whenever a huge impulse current flows along the tower towards considerably high footing impedances. For these reasons, tower modeling for transients studies is an important step in the insulation design, and also for lower voltage applications, where indirect lightning effects may play a predominant role. However, after decades of research on tower modeling, starting from the 1930s with the first model proposed by Jordan, no consensus has been reached neither on a widely accepted tower model nor on the quantitative effect of the tower models on insulation design. Moreover, the fundamental mechanisms at the base of the transient response of towers and the definition of some fundamental parameters have not been totally clarified yet. The aim of this review is to present the available tower models developed through the years in the power community, focussing mainly on lumped/distributed circuit models, and to help the reader to obtain a deeper understanding of them

Electromagnetic pulse technology : biological and terahertz applications

Since the mid-1970s, the field of Electromagnetic Pulse (EMP) technology has extended to include High-Power Electromagnetic (HPE) sources/antennas. Two such EMP/HPE antennas, designed to address unique applications, are presented in this dissertation. The first is the Prolate-Spheroidal Impulse-Radiating Antenna (PSIRA). Such an antenna uses a prolate-spheroidal reflector and has two foci. A fast (\u3c= 100 ps), high-voltage (\u3e 100 kV) pulse launched from the first focal point is focused into a target located at the second focal point (near-field). It has been found that these pulses are useful for a variety of biological applications, such as accelerated wound healing and skin cancer (melanoma) treatment. Two lens designs for the PSIRA are explored. The first lens, called the focusing lens, is used at the second focal point of the PSIRA to better match the focused pulses into the (biological) target medium. Analytical calculations, numerical simulations and experimental results on a five-layer, hemispherical, dielectric focusing lens are detailed. The second lens, called the launching lens, is used at the first focal point of the PSIRA. For input voltages of 100 kV or more, a switch system, i.e., switch cones, pressure vessel, hydrogen chamber and launching lens, are required to effectively launch a spherical TEM wave from the first focal point. Various switch configurations are explored. It is shown that the pressure vessel can also serve as the launching lens, which considerably simplifies the design of the switch system. Spherical and cylindrical pressure vessel designs are investigated. The second is the Switched Oscillator (SwO) antenna. A SwO is essentially an electrical, shock-excited resonant structure. The SwO is adopted as a high-power antenna to radiate high-energy pulses in the terahertz frequency range. The primary focus is to use these pulses for secure communications. Analytical calculations for the SwO are detailed. Numerical simulations are used to optimize and more thoroughly study the antenna. Various characteristic relations obtained are used to provide a deeper insight into the working of the SwO radiator

Do Wind Turbines Amplify the Effects of Lightning Strikes A Full-Maxwell Modelling Approach

Wind turbines (WTs) can be seriously damaged by lightning strikes and they can be struck by a significant number of flashes. This should be taken into account when the WT lightning protection system is designed. Moreover, WTs represent a path for the lightning current that can modify the well-known effects of the lightning discharge in terms of radiated electromagnetic fields, which are a source of damage and interference for nearby structures and systems. In this paper, a WT struck by a lightning discharge is analyzed with a full-wave modelling approach, taking into account the details of the WT and its interactions with the lightning channel. The effects of first and subsequent return strokes are analyzed as well as that of the rotation angle of the struck blade. Results show that the lightning current along the WT is mainly affected by the ground reflection and by the reflection between the struck blade and the channel. The computed electromagnetic fields show that, for subsequent return strokes, the presence of a WT almost doubles their magnitude with respect to a lightning striking the ground. Such enhancement is emphasized when the inclined struck blade is considere

A Dual Resonant Transformer and a Dielectric Antenna for Picosecond Pulse Radiation

This thesis discusses the development of a pulsed power system for high power picosecond pulse radiation. In the system, a charging transformer, which generates a high voltage pulse of ~100 kV, can be used for charging a transmission line in less than 100 ns. Such a short pulse could cause a peak gap switch to break down and generate a picosecond pulse transient for radiation. A dielectric antenna, if fed with the high voltage picosecond pulses, can radiate them to targets made of high dielectric materials. Biological tissues, for instance, can be targeted for electrostimulation. The transformer was designed considering the needs to deliver a high gain and fast output. We showed that a transformer in the dual resonant mode, in which the resonance of the primary and the second is equal, can produce a voltage gain of approximately 6. The output voltage of the transformer is more than 100kV with an input of 15kV. This shows the average gain of the transformer is 7. The fast output requires the voltage at the secondary winding needs to be less than 100 ns in order for achieving a picosecond transient in the oil peak switch. This was done by low-inductance windings with an air core. Two winding configurations were explored: a cylindrical winding and a toroidal winding. The cylindrical winding appears to be a better option in terms of the gain. Experimental results show that for a capacitive load (30pF), the voltage can be charged up to 33 kV in 20 ns. A conical dielectric antenna was investigated through simulation and experiments. The antenna is made of a V-shape transmission line on a ceramic conical body with dielectric constant of 28. This antenna was immersed in transformer oil for high voltage insulation, which allowed for the feed voltage to be as high as 50 kV. The antenna was characterized by an electric field sensor immersed in water. We found that the emitted field increases as the voltage increases, but it reaches a saturation for 40 kV. The highest electric field is 1.5 kV/cm even for the input voltage 50 kV. This is 6 times less than simulation. We speculate that the discrepancy is caused by the dielectric tangent loss, which was not taken into account in the simulation. Future work towards a complete system includes a choice of a linear dielectric material which is capable of sustaining its dielectric constant for a high electric field and the study of an oil peak switch, which is a critical component between the transformer and the antenna

Non-Invasive Picosecond Pulse System for Electrostimulation

Picosecond pulsed electric fields have been shown to have stimulatory effects, such as calcium influx, activation of action potential, and membrane depolarization, on biological cells. Because the pulse duration is so short, it has been hypothesized that the pulses permeate a cell and can directly affect intracellular cell structures by bypassing the shielding of the membrane. This provides an opportunity for studying new biophysics. Furthermore, radiating picosecond pulses can be efficiently done by a compact antenna because the antenna size is comparable to the pulse width. However, all of the previous bioelectric studies regarding picosecond pulses have been conducted in vitro, using electrodes. There is not yet a device which can non-invasively deliver picosecond-pulsed electric fields to neurological tissue for therapeutic applications. It is unclear whether a radiated electric field at a given penetration depth can reach the threshold to cause biological effects. In this dissertation, a picosecond- pulsed electric field system designed for the electrosimulation of neural cells is presented. This begins with the design of an ultra-wideband biconical dielectric rod antenna. It consists of a dielectrically loaded V-conical launcher which feeds a cylindrical waveguide. The waveguide then transitions into a taper, which acts like a lens to focus the energy in the tissue target. To describe the antenna delivery of picosecond pulses to tissues, the initial performance was simulated using a 3-layer tissue model and then a human head model. The final model was shown to effectively deliver pulses of 11.5 V/m to the brain for a 1 V input. The spot size of the stimulation is on the order of 1 cm. The electric field was able to penetrate to a depth of 2 cm, which is equal to the pulse width of a 200 ps pulse. The antenna was constructed and characterized in free space in time domain and in frequency domain. The experimental results have a good agreement with the simulation. The ultimate biological application relies on adequate electric field. To reach a threshold electric field for effective stimulation, the antenna should be driven by a high voltage, picosecond-pulsed power supply, which, in our case, consists of a nanosecond charging transformer, a parallel-plate transmission line, and a picosecond discharging switch. This transformer was used to charge a parallel-plate transmission line, with the antenna as the load. To generate pulses with a rise time of hundreds of picoseconds, an oil switch with a millimeter gap was used. For the charging, a dual resonance pulse transformer was designed and constructed. The novel aspect of this transformer is has a fast charge time. It was shown to be capable of producing over 100 kV voltages in less than 100 ns. After the closing of the peaking switch and the picosecond rise time generation, the antenna was able to create an electric field of 600 V/cm in the air at a distance of 3 cm. This field was comparable to the simulation. Higher voltage operation was met with dielectric breakdown across the insulation layer that separates the high voltage side and the ground side. Before the designed antenna is used in vivo, it is critical to determine the biological effect of picosecond pulses. This is especially important if we focus on stimulatory effects, which require that the electric field intensity be close to the range that the antenna system can deliver. Toward that end, neural stem cells were chosen to study for the proliferation, metabolism, and gene expression. Instead of using the antenna, the electrodes were used to deliver the pulses to the cells. In order to treat enough cells for downstream analyses, the electrodes were mounted on a 3-D printer head, which could be moved freely and could be controlled accurately by programming. The results show that pulses on the order of 20 kV/cm affect the proliferation, metabolism, and gene expression of both neural and mesenchymal stem cells, without reducing viability. In general, we came to the conclusion that picosecond pulses can be a useful stimulus for a variety of applications, but the possibility of using antennas to directly stimulate tissue functions relies on the development of a pulsed power system, high voltage insulation, and antenna material

Aeronautical engineering: A special bibliography with indexes, supplement 82, April 1977

This bibliography lists 311 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1977

Non-conventional sensors for measuring partial discharge under DC electrical stress

Partial discharge (PD) is a micro discharge that occurs in defected regions within the insulating media. As these discharges are the main culprits that cause dielectric material aging, PD measurements have been used for assessing insulating materials, including solids, liquids, and gases for power applications. There are various methods and sensors available for measuring PD sensitive to specific characteristics and operable over a wide range of frequencies. Most PD measurement techniques provide patterns that enable PD interpretation more comfortable for users. For example, in AC applications, the phase-resolved partial discharge (PRPD) technique provides identifiable patterns for distinguishing various types of PDs. However, the establishment of meaningful patterns to multiple types of PD in DC systems requires more sensitive and accurate measurements of individual PD pulses with noise rejection functionality due to the lack of phase-resolved information. Investigating of the transient phenomena such as individual PD pulses requires well-designed circuits with sufficiently large bandwidths. Waveshapes can be easily disturbed by background noise and deformed by the frequency response of measuring circuits and data acquisition systems (DAQ). Noises are unwanted disturbances that could be suppressed by suitable filters or mathematical methods. Measurement circuits and DAQ systems consist of transmission lines, sensors, cables, connectors, DAQ hardware, and oscilloscopes. Therefore, matching the impedance of all components guarantees a reflectionree path for traveling signals and addresses most of the challenges relevant to transient measurements. In this dissertation, we proposed and designed an appropriate testbed equipped with high bandwidth transmission line and electromagnetic field sensors suitable for investigating PD under DC electrical stresses. We comprehensively used finite element analysis simulations through the COMSOL Multiphysics software to design the dimensions and evaluate the frequency response of the testbed, transmission line, and electromagnetic sensors. Furthermore, based on the new testbed, DC PD measurements were performed using conventional and non-conventional sensors. Finally, various types of DC PD were statistically classified based on the proposed testbed

Aeronautical engineering: A continuing bibliography with indexes (supplement 256)

This bibliography lists 426 reports, articles, and other documents introduced into the NASA scientific and technical information system in August 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

An Ultrawideband Dual-Linear Polarization Feed for Solar Microwave Observation

The study of Solar Microwave Bursts (SMB\u27s) emanating from the sun is important from several perspectives. SMB\u27s are well correlated to Coronal Mass Ejections (CME\u27s) and therefore can provide insight into the physics of the sun. SMB\u27s and CME\u27s can interfere with microwave communication systems such as cell phones, satellites, and radar, and can adversely affect the accuracy of the Global Positioning System. Furthermore, CME\u27s can be hazardous to individuals and equipment in earth orbit as well as causing power grid blackouts. The rapid detection of SMB\u27s and their subsequent effect on space weather is a key element of responsibility of the United States Air Force (USAF). Identification of the source eliminates the possibility of intentional jamming or a systems failure. When such a determination is made, warnings can be issued so that measures can be taken such as using different communication frequencies or modes, or to place satellites in a safe mode. Currently the USAF operates the Radio Solar Telescope Network (RSTN) consisting of three parabolic dish antennas each at four locations to continuously observe the sun in the microwave spectrum. The 2.4m RSTN dishes have feeds that are single polarization at four discrete frequencies between 1.4 and 8.8GHz. Expanding the capability of these existing dishes with a single ultrawideband feed to cover 1-10GHz would improve observations, while adding a dual polarization capability could facilitate improved monitoring should there be future developments in spectrum usage. A feed with folded diamond-shaped elements in a damped cavity has been designed and constructed, funded under contract FA9453-09-C-0309 from the USAF Solar Disturbance Prediction Program, with a simulated bandwidth of 0.9 - 12 GHz. Subsequent characterization from 2 - 8 GHz showed good correlation between simulation and measurement, and that the feed meets virtually all performance specifications that were tested

LASER PULSE DRIVEN TERAHERTZ (THz) GENERATION IN INHOMOGENEOUS PLASMAS

Intense, short laser pulses propagating through inhomogeneous plasmas can ponderomotively drive terahertz (THz) radiation. Theoretical analysis and full format PIC simulations are conducted to investigate two mechanisms of laser pulse driven terahertz generation: (i) a resonant transition radiation (RTR) mechanism occurring as a pulse crosses a plasma boundary and (ii) a slow wave phase matching mechanism (SWPM) that occurs in corrugated plasma channels. These studies are the first to investigate ponderomotively driven THz self-consistently in the interesting situations in which the interaction occurs over a scale many wavelengths long. For the resonant transition radiation mechanism, both theory and simulation results show the conical THz emission originates in regions of varying density and covers a broad spectrum with maximum frequency close to the maximum plasma frequency. In the case of a sharp vacuum-plasma boundary, the radiation is generated symmetrically at the plasma entrance and exit, and its properties are independent of plasma density when the density exceeds a characteristic value determined by the product of the plasma frequency and the laser pulse duration. For a diffuse vacuum-plasma boundary, the emission from the plasma entrance and exit is asymmetric: increasing and decreasing density ramps enhance and diminish the radiated energy, respectively. Enhancements by a factor of 50 are found, and simulations show that a 1.66 J, 50 fs driver pulse can generate ~400 μJ of THz radiation in a 1.2 mm increasing density ramp. We present a model that attributes this effect to a plasma resonance process in the density ramp. The results from the model match those of the simulations for ramp lengths less than 600 μm. For longer ramps for which simulations are too time consuming, the model shows that the amount of radiation reaches a maximum at a ramp length determined by collisional absorption. For the slow wave phase matching mechanism, excitation of terahertz radiation by the interaction of an ultra-short laser pulse and the fields of a miniature, corrugated plasma waveguide is considered. Plasma structures of this type have been realized experimentally, and they can support electromagnetic (EM) channel modes with properties that allow for radiation generation. In particular, the modes have subluminal field components, thus allowing phase matching between the generated THz modes and the ponderomotive potential of the laser pulse. Theoretical analysis and full format PIC simulations are conducted. We find THz generated by this slow wave phase matching mechanism is characterized by lateral emission and a coherent, narrow band, tunable spectrum with relatively high power and conversion efficiency. We investigated two different types of channels, and a range of realistic laser pulses and plasma profile parameters were considered with the goal of increasing the conversion of optical energy to THz radiation. We find high laser intensities strongly modify the THz spectrum by exciting higher order channel modes. Enhancement of a specific channel mode can be realized by using an optimum pulse duration and plasma density. As an example, simulation results show a fixed driver pulse (0.55 J) with spot size of 15 μm and pulse duration of 15 fs excites approximately 37.8 mJ of THz radiation in a 1.5 cm corrugated plasma waveguide with on axis average density of 1.4 × 10^18 cm^−3, conversion efficiency exceeding 8% can be achieved in this case