High-risk region of bird streamer flashover in high-voltage transmission lines

Bird-caused outages are more common and widespread than generally believed, and greatly threaten the safety of transmission lines. However, few reports are concerned with laboratory research of bird-caused faulting, especially in the case of long air gap insulators. This paper carried out experimental and simulation work on bird streamer flashover of 220 kV insulators. It was found that the radial distance from the insulator axis shows great influence on the bird streamer flashover. Significant is that we also found a high-risk region of bird streamer flashover which can be restricted in an elliptical region located at the center of 220 kV insulator string, the major and minor axes being 500 mm and 250 mm. The processes of flashover due to bird streamer are discussed in detail. Simulation work further demonstrates the existence of the elliptical region of high flashover risk, which is of significance for transmission line protection.     

Evaluation of probe size in STEM imaging at 30 and 60 kV

In order to estimate the probe size on the specimen surface in a newly developed low-acceleration-voltage (30–60 kV) atomic-resolution scanning transmission electron microscopy (STEM), we compared the intensity profiles of experimentally obtained annular dark field (ADF)-STEM images of Si–Si dumbbells and those of images simulated using a multislice method which takes chromatic aberration into account. However, the simulated ADF images at 30 and 60 kV were found not to match the corresponding experimental images. Subsequently, the simulated images were convolved with probe functions (normal distributions) of different widths until a good match was obtained between the images. This allowed the probe shapes corresponding to the experimental conditions to be determined. ADF-STEM images with chromatic aberration could then be calculated by an incoherent superposition of these probe functions over a range of energies. The full widths at half maximum for the probe functions were estimated to be 99.2 pm for 30 kV and 92.8 pm for 60 kV. The D59 diameters were calculated to be 154.0 pm for 30 kV and 127.8 pm for 60 kV. This means that the 30-kV probe has a larger tail than the 60-kV probe.     

The lightning striking distance—Revisited

First return stroke current waveforms measured by Berger [Methods and results of lightning records at Monte San Salvatore from 1963–1971 (in German), Bull. Schweiz. Elektrotech. ver. 63 (1972) 21403—21422] and Berger and Vogelsanger [Measurement and results of lightning records at Monte San Salvatore from 1955–1963 (in German), Bull. Schweiz. Elektrotech. ver. 56 (1965) 2–22] are used to estimate the charge stored in the lightning stepped leader channel. As opposed to previous charge estimates based on the entire current waveform, only the initial portion of measured current waveforms (100 μs in duration) was used in order to avoid the inclusion of any charges not involved in the effective neutralization of charges originally stored on the leader channel. The charge brought to ground by the return stroke within the first 100 μs, Q_f,100 μs (in C) is related to the first return stroke peak current, I_pf (in kA), as Q_f,100 μs=0.61 I_pf. From this equation the charge distribution of the stepped leader as a function of the corresponding peak return stroke current is estimated. This distribution (along with the assumed average electric field of 500 kV/m in the final gap) is used to estimate the lightning striking distance S (in meters) to a flat ground as a function of the prospective return stroke peak current I (in kA): S=1.9 I_pf^0.90. For the median first stroke peak current of 30 kA one obtains S=41 m, while the traditional equation, S=10 I_pf^0.65, gives S=91 m. In our view, the new equation for striking distance provides a more physically realistic basis for the electro-geometric approach widely used in estimating lightning incidence to power lines and other structures.     

‘The mesh method’ in lightning protection standards – Revisited

At present the design of the Lightning protection systems (LPS) for structures as stipulated in standards is based on the electro – geometrical method, which was initially used to protect power lines from lightning. A derivative of the electro-geometrical method is the rolling sphere method. This method together, with the protection angle method and mesh method are used almost in all lightning standards as the measure in installing the lightning protection systems of grounded structures. In the mesh method, the dimension of the cell size in different levels of protection is determined using the rolling sphere method. Since the rolling sphere method does not take into account the physics of the lightning attachment process there is a need to evaluate the validity of the stipulated value in standards of the minimum lightning current that can penetrate through the mesh in different levels of protection. In this paper, meshes of different sizes as stipulated in the lightning protection standards were tested for their ability to intercept lightning flashes using a lightning attachment model that takes into account the physics of connecting leaders on. The results are in reasonable agreement with the specifications given in the lightning protection standards.     

Estimation of charge neutralized by negative cloud-to-ground flashes in Catalonia thunderstorms

Charges neutralized by lightning flashes have been usually located and inferred from the quasi-static changes in the vertical component of electric field (ΔE) obtained from multiple synchronized measurement stations. In this paper, the charges neutralized by negative cloud-to-ground (?CG) lightning flashes are located and inferred using single station electric field measurements in combination with total lightning (cloud-to-ground and intra-cloud) data from lightning detection networks in the north-eastern region of Spain. The altitude of the negative charge region in 8 thunderstorms examined here was in the range of 5.8–7.2 km as inferred from temperature soundings. Charge locations are assumed to be in the region where the VHF (Very High Frequency) sources associated with the discharge are located. The quantity of charge neutralized is calculated using a point charge model and the measured vertical component of electric field (ΔE) associated with each flash. The results are compared with those previously obtained from measurements carried out in the NASA Kennedy Space Center (KSC) in Florida. The analysis of 260 ?CG flashes from eight thunderstorms reveals a median charge value of ?9.7 C with 95% of the values ranging between ?4.5 C and ?45 C.     

Performance analysis of optical network based on optical add drop multiplexers with different MZI techniques

This paper presents an investigation on the performance of an optical network in terms of crosstalk based on optical add drop multiplexers with Mach–Zehnder interferometer (MZI), MZI-semiconductor optical amplifier (SOA) and MZI-fiber Braggs gratings (FBG) techniques obtained at 8 × 10 Gbps with 0.1 nm channel spacing wavelength division multiplexing (WDM) transmission with optical add drop multiplexer (OADM) placed at the 20 km point of a 40 km link. It is found that the signal can be transmitted with least BER and better Q-factor with MZI-FBG based OADM and the worst case is found with the MZI-SOA based OADM.     

Novel ultracompact wavelength division demultiplexer based on photonic band gap

We propose ultra-compact wavelength division demultiplexer based on photonic band gap in two dimensional photonic crystals. The structure consists of two different types of lattice, which can separate two communication wavelengths, 1310 and 1490 nm. In addition, it also can be used to separate the wavelengths of 1310 and 1550 nm if we change the structural parameters. The total size of the present structure is only 12.5 μm × 12.5 μm. The transmission efficiency is above 97%, and the good performance is verified with plane wave expansion and finite-difference time-domain simulation.     

Nanoscale device modeling: the Green’s function method

The non-equilibrium Green’s function (NEGF) formalism provides a sound conceptual basis for the devlopment of atomic-level quantum mechanical simulators that will be needed for nanoscale devices of the future. However, this formalism is based on concepts that are unfamiliar to most device physicists and chemists and as such remains relatively obscure. In this paper we try to achieve two objectives: (1) explain the central concepts that define the ‘language’ of quantum transport, and (2) illustrate the NEGF formalism with simple examples that interested readers can easily duplicate on their PCs. These examples all involve a short n ^+  + – n ⁺ – n ^+  + resistor whose physics is easily understood. However, the basic formulation is quite general and can even be applied to something as different as a nanotube or a molecular wire, once a suitable Hamiltonian has been identified. These examples also underscore the importance of performing self-consistent calculations that include the Poisson equation. The I–V characteristics of nanoscale structures is determined by an interesting interplay between twentieth century physics (quantum transport) and nineteenth century physics (electrostatics) and there is a tendency to emphasize one or the other depending on one’s background. However, it is important to do justice to both aspects in order to derive real insights.     

Investigation on different ESD protection strategies devoted to 3.3 V RF applications (2 GHz) in a 0.18 μm CMOS process

ESD protection for radio frequency (RF) applications must deal with good ESD performance, minimum capacitance, zero series resistance and good capacitance linearity. In order to fulfill these requirements, different ESD protection strategies for RF applications have been investigated in a 0.18 μm CMOS process. This paper compares different ESD protection devices and shows that a suitable ESD performance target for RF applications (200 fF max, 2 kV HBM) can be reached with a diode network scheme. The optimization of the diodes is then a key point which is detailed. A trade-off has to be found between the ESD performance, the voltage drop during ESD and the parasitic capacitance. Poly as well as shallow trench isolation (STI) bounded diodes have been studied and it appears clearly that a solution based on poly bounded diodes is the best choice.     

A computational study of the HyShot II combustor performance

Experimental and flight data for hypersonic air-breathing engines are both difficult and extremely expensive to obtain, motivating the use of computational models to enhance the understanding of the complex physics involved. Here, a comprehensive numerical study has been carried out for the HyShot II scramjet combustor. This study makes use of Reynolds Average Navier Stokes (RANS) based models and Large Eddy Simulation (LES) based models with semi-detailed reaction kinetics. In this investigation we focus on the underlying flow-mixing-combustion physics at different operating conditions tested in the High Enthalpy Shock Tunnel Göttingen (HEG). To account for the complex flow in the HEG facility a zonal approach is employed in which RANS is used to simulate the flow in the HEG nozzle and test-section, providing the necessary inflow boundary conditions for the combustor RANS and LES, being the focus of this analysis. Specifically, we here combine results from RANS and LES computations with data from the HEG experiments and the target HyShot II flight-tests at two different flight-altitudes (28 and 33 km). The LES model is observed to capture the experimental wall-pressure and heat-flux data very well for both the 33 and 28 km altitude cases, whereas the RANS model is only able to predict the wall-pressure and heat flux data for the 28 km altitude case. Based on the LES results, the flow at both altitudes is found to be unsteady, but with unsteady transitional flow features dominating the 33 km case. Moreover, these results show that the equivalence ratio is of key importance to the resulting flow, mixing and combustion physics, with richer mixtures being prone to transitional flow features. The LES results are also used to describe the flow physics in detail for both altitudes, and the key processes responsible for the transition between the two combustion modes observed.     

Efficient high voltage pulser for piezoelectric air coupled transducer

The design of high voltage pulser for air coupled ultrasound imaging is presented. It is dedicated for air-coupled ultrasound applications when piezoelectric transducer design is used. Two identical N-channel MOSFETs are used together with 1200 V high and low side driver IC. Simple driving pulses’ delay and skew circuit is used to reduce the cross-conduction. Analysis of switch peak current and channel resistance relation to maximum operation frequency and load capacitance is given. PSPICE simulation was used to analyze the gate driver resistance, gate pulse skew, pulse amplitude influence on energy consumption when loaded by capacitive load. Experimental investigation was verified against simulation and theoretical predictions. For 500 pF capacitance, which is most common for piezoelectric air coupled transducers, pulser consumes 650 μJ at 1 kV pulse and 4 μJ at 50 V. Pulser is capable to produce up to 1 MHz pulse trains with positive 50 V–1 kV pulses with up to 10 A peak output current. When loaded by 200 kHz transducer at 1 kV pulse amplitude rise time is 40 ns and fall time is 32 ns which fully satisfies desired 1 MHz bandwidth.     

Development of a liquid stub tuner for the KSTAR ICRF system

A liquid stub tuner was developed to transmit a MW level of RF power continuously in the KSTAR (Korean Superconducting Tokamak Advanced Research) ICRF (ion cyclotron range of frequencies) heating and current drive systems. It is made of a 9–3/16″ coaxial transmission line, and the property of the RF wavelength difference in a liquid and in a gas due to their different dielectric constants is utilized. Silicon oil with a relative dielectric constant of 2.74 was used as the liquid, because of its low vapor pressure and low dielectric loss. The liquid stub tuner components were equipped with electrostatic probes, thermocouples, and a humidity sensor to measure the RF voltages, the liquid temperature, and the humidity in the coaxial transmission line, respectively. The level of the liquid for the liquid stub tuner can be changed from 0 to 2.8 m, which corresponds to 2.0 m mechanical length variation at 30 MHz. The RF power test of the liquid stub tuner was performed with various conditioned silicon oils at 30 MHz to compare the high voltage characteristics among various conditioned silicon oils. The stand-off voltage of 43.82 kV (average) was achieved for the liquid stub tuner during the 300 s operation.     

Study of all-fiber asymmetric interleaver based on two-stage cascaded Mach–Zehnder Interferometer

In order to improve the transmission efficiency of optical-fiber communication system with 10 Gb/s + 40 Gb/s, an all-fiber interleaver with unequal passband is proposed and discussed, which is based on a two-stage cascaded Mach-Zehnder interferometer (MZI). The optimum value of structural parameters, such as splitting ratios of the couplers and the physical length differences of the interferometer arms, were chosen. One set of optimized data is validated in the experimental result. The experimental results and the theoretic analysis indicate that an all-fiber optical interleaver with 3 dB passband width in odd channels and even channels could be obtained, which having more than 60 GHz passband and 30 GHz passband, for transmission speed of 40 Gb/s and 10 Gb/s, respectively. By assigning different portions of spectrum to the 10 Gb/s and the 40 Gb/s channels, the bandwidth efficiency requirement of the 40 Gb/s channel is relieved, and therefore longer transmission distance can be achieved.     

Realization of periodic transmission filter based on a pair of cascaded long-period fibre gratings of different strength/wavelength position

Using high-intensity (560–650 GW/cm²) 264 nm 220 femtosecond laser pulses, we inscribed a periodic (comb) transmission filter in a photosensitive Ge/B-codoped fibre, based on a pair of long-period gratings of different strength/wavelength position. The irradiation conditions and grating parameters for the successful realization of the 24–28-nm-wide transmission filter in the region 1480–1580 nm with the fringe period of 1.7–3.1 nm and the fringe bandwidth of 0.8–1.3 nm were established.     

Design and VNA-measurement of coplanar waveguide (CPW) on benzocyclobutene (BCB) at THz frequencies

The low permittivity and the low loss tangent of the benzocyclobutene polymer (BCB) offers to coplanar waveguides (CPW) a low dispersive propagation properties at THz frequency. These transmission lines have been designed, modeled with a three dimensional (3D) solver of Maxwell equations based on finite element method (FEM) from 20 to 1000 GHz at various characteristic impedances (Z_c). Their dispersion and losses (radiation, conduction and dielectric) have been investigated separately versus the waveguide size, the nature of the substrate (dielectric or semiconductor) to optimize the THz signal propagation. Monomode CPW on BCB numerically designed for various Z_c were realized and measured with vector network analyzer (VNA). S-parameters of CPW are de-embedded by optimization of the accesses’ model. A good agreement is found between experimental and numerical results with low attenuation constants of 2.7 dB/mm and 3.5 dB/mm at 400 GHz and 500 GHz, respectively.     

Towards a 1 MW, 170 GHz gyrotron design for fusion application

The electrical design of different components of 1 MW, 170 GHz gyrotron such as, magnetron injection gun, cylindrical interaction cavity and collector and RF window is presented in this article. Recently, a new project related to the development of 170 GHz, 1 MW gyrotron has been started for the Indian Tokamak. TE_34,10 mode is selected as the operating mode after studied the problem of mode competition. The triode type geometry is selected for the design of magnetron injection gun (MIG) to achieve the required beam parameters. The maximum transverse velocity spread of 3.28% at the velocity ratio of 1.34 is obtained in simulations for a 40 A, 80 kV electron beam. The RF output power of more than 1 MW with 36.5% interaction efficiency without depressed collector is predicted by simulation in single-mode operation at 170 GHz frequency. The simulated single-stage depressed collector of the gyrotron predicted the overall device efficiencies >55%. Due to the very good thermal conductivity and very weak dependency of the dielectric parameters on temperature, PACVD diamond is selected for window design for the transmission of RF power. The in-house developed code MIGSYN and GCOMS are used for initial geometry design of MIG and mode selection respectively. Commercially available simulation tools MAGIC and ANSYS are used for beam–wave interaction and mechanical analysis respectively.     

Multifrequency super-thin cloaks

Bandwidth and thickness have become the most troublesome problems for EM cloaks. In this paper, we propose to solve the two problems using connected patches based on the microwave network model. By covering an obstacle with combined connected patches, cloaking effect can be achieved at multiple frequencies so as to expand the operating band. As an explicit example, a dual-band super-thin cloak using two different connected patch unit cells is demonstrated. Cloaking effect can be achieved at 3.50 GHz and 4.14 GHz simultaneously with an 8 dB transmission enhancement. The cloak design method provides a new route to broadening the bandwidth of thin EM cloaks.     

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5 THz and 1.1 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1.325 THz to 1.38 THz. These results will be applied to THz modulation and tunable filtering.     

Normal incidence sound transmission loss in impedance tube – Measurement and prediction methods using perforated plates

For the determination of the transmission loss of samples in an impedance tube, two different approaches is found in the literature, one based on determining the full transfer matrix (TM method) of the acoustic element, the other based on the wavefield decomposition theory (WD method). In this paper both methods are implemented and measured results are compared using samples which includes different types of perforated plates, also combined with porous material. Measurements are conducted in a tube of square cross section with dimensions 200 × 200 mm, thereby limiting the workable frequency range upwards to approximately 850 Hz. The main purpose of the paper is, however, to compare measured results with predictions using the transfer matrix method. For a bare plate with cylindrical apertures two models are compared as well; a “classical” one and another based on modeling the perforated plate as a porous material having a rigid frame. As for these transmission loss measurements, the two measurement approaches turn out to give identical results within the numerical accuracy. The fit between measured and predicted results are reasonably good with a maximum deviation mostly within 2 dB.