Overvoltage Values Research Articles

Numerical simulation of narrow bipolar electromagnetic pulses generated by thunderstorm discharges

Using the concept of avalanche relativistic runaway electrons (REs), we perform numerical simulations of compact intracloud discharge (CID) as a generator of powerful natural electromagnetic pulses (EMPs) in the HF-VHF range, called narrow bipolar pulses (NBPs). For several values of the field overvoltage and altitude at which the discharge develops, the numbers of seed electrons initiating the avalanche are evaluated, with which the calculated EMP characteristics are consistent with the measured NBP parameters. We note shortcomings in the hypothesis assuming participation of cosmic ray air showers in avalanche initiation. The discharge capable of generating NBPs produces REs in numbers close to those in the source of terrestrial γ-ray flashes (TGFs), which can be an argument in favor of a unified NBP and TGF source.

Energy profile of hydrogen evolution on metals with consideration of their surface energy

It has been established that there is a quantitative relationship between the experimental value of hydrogen overvoltage, \(a = \eta _{H_2 }\), in the Tafel equation and the surface energy, ΔUS, of metal with fcc and hcp structure, which is calculated theoretically in metal surface physics. An equation of slow discharge of hydrogen ions on metal is deduced, in which the a value is a linear function of the value where ΔUS is the minimal surface energy of the specified metal, α = 0.5 is the transition coefficient, and F is the Faraday number. A prevailing tendency toward a decrease in hydrogen overvoltage on polycrystalline surfaces of metals with fcc, hcp, and bcc lattices with an increase in the minimal surface energy related to the appropriate (111), (0001), and (110) facets of these lattices has been observed. Results of analysis of the relationships between and the surface energy of metal are interpreted within the theory of active centers for heterogeneous catalysis with involvement of a thermodynamic model of the metal surface enriched with atom vacancies.

Co-Mo and Co-Mo-C Alloys Deposited in a Magnetic Field of High Intensity and their Electrocatalytic Properties

Co-Mo and Co-Mo-C Alloys Deposited in a Magnetic Field of High Intensity and their Electrocatalytic PropertiesThe article presents results of tests on electrodeposition in a magnetic field of Co-Mo and Co-Mo-C alloys characterised by low overvoltage of hydrogen evolution. Addition of molybdenum and carbon was to lower the values of overvoltage of hydrogen evolution on cobalt. The influence of a magnetic field of high intensity (1 - 12 T) on the deposits composition, structure and morphology was determined. The deposition was conducted in a magnetic field of parallel orientation of the magnetic field forces lines in relation to the working electrode. Electrocatalytic properties of the obtained alloy coatings within the range of hydrogen evolution were tested in a concentrated solution of NaOH. The obtained alloys were analysed with the XRD method, their composition was tested with the WDXRF technique and they underwent observation applying scanning electron microscopy (SEM). The comparison of electrocatalytic properties of Co-Mo alloys with properties of Co-Mo-C ones enabled determining the influence of carbon presence in cathodic deposits on the overvoltage value of hydrogen evolution on them.

Delta-Bar-Delta and directed random search algorithms to study capacitor banks switching overvoltages

This paper introduces an approach to analyse transient overvoltages during capacitor banks switching based on artificial neural networks (ANN). Three learning algorithms, delta-bar-delta (DBD), extended delta-bar-delta (EDBD) and directed random search (DRS) were used to train the ANNs. The ANN training is based on equivalent parameters of the network and therefore, a trained ANN is applicable to every studied system. The developed ANN is trained with extensive simulated results and tested for typical cases. The new algorithms are presented and demonstrated for a partial 39-bus New England test system. The simulated results show the proposed technique can accurately estimate the peak values of switching overvoltages.

Development of high frequency circuit model for oil-immersed power transformers and its application for lightning surge analysis

The lightning impulse withstand voltage for an oil-immersed power transformer is determined by the value of the lightning surge overvoltage generated at the transformer terminal. This overvoltage value has been conventionally obtained through lightning surge analysis using the electromagnetic transients program (EMTP), where the transformer is often simulated by a single lumped capacitance. However, since high frequency surge overvoltages ranging from several kHz to several MHz are generated in an actual system, a transformer circuit model capable of simulating the range up to this high frequency must be developed for further accurate analysis. In this paper, a high frequency circuit model for an oil-immersed transformer was developed and its validity was verified through comparison with the measurement results on the model winding actually produced. Consequently, it emerged that a high frequency model with three serially connected LC parallel circuits could adequately simulate the impedance characteristics of the winding up to a high frequency range of several MHz. Following lightning surge analysis for a 500 kV substation using this high frequency model, the peak value of the waveform was evaluated as lower than that simulated by conventional lumped capacitance even though the front rising was steeper. This phenomenon can be explained by the charging process of the capacitance circuit inside the transformer. Furthermore, the waveform analyzed by each model was converted into an equivalent standard lightning impulse waveform and the respective peak values were compared. As a result, the peak value obtained by the lumped capacitance simulation was evaluated as relatively higher under the present analysis conditions.

Influence of Substation's Elements on Electromagnetic Transient Occurrences Caused by Disconnector Switching

This article presents the results of investigating switching overvoltages caused by the operation of disconnectors in air-insulated substations rated at 220 kV. Measurements of switching overvoltages were performed in air-insulated substations at the Hydro Power Plant Grabovica on the River Neretva and at the Thermo Power Plant Kakanj. These power plants are of great importance to the operation of the electric power system in Bosnia and Herzegovina. Operation of air disconnectors was investigated in order to determine the switching overvoltages' shapes and levels in primary and secondary circuits, which could cause malfunctioning of signaling devices or burning of protection relays' supply units. Results of computer simulations using an electromagnetic transients program are presented. Comparison of measured and calculated overvoltage values confirms their concordance. These comparisons proved computer calculation to be useful for investigating transient overvoltages due to disconnector switching. Furthermore, experimental and computing investigations on the 220-kV switchyard of the Hydro Power Plant Grabovica demonstrate an influence of substation's elements on the waveform of switching overvoltages.

Power system reactive compensation: evaluation of expansion plans taking into account electromagnetic transient restrictions

In this paper, the authors present developments regarding Power System Capacitive Shunt Reactive Compensation and Voltage Control, with respect to Expansion Activities, in an Electromagnetic Transient context. The definition of the amount and location of shunt reactive compensation to be installed in power systems considers several aspects, as reactive power optimization, short circuit calculations, voltage stability and harmonic analyses, among others. Different alternatives must be evaluated, regarding their impact (positive and negative) to system behavior, in order to select the most adequate compensation. Expansion decision process usually does not include studies about the transient impact of reactive shunt compensation caused by switching operation. Traditionally, this is done only after expansion definition, when the detailed project is elaborated, mainly to prepare equipment specifications. In this paper, it is proposed an innovative procedure to include a technical evaluation of different shunt compensation based on a preview of the overvoltage and overcurrent values caused by the corresponding capacitors bank switching. The new methodology is designed to (i) indicate the critical system buses that must be carefully investigated, considering transient behavior, and (ii) promote the adequate shunt compensation adjust. The proposed procedure is detailed and results of its application in real systems are presented.

Power Network Asymmetrical Faults Analysis Using Instantaneous Symmetrical Components

Although the application of Symmetrical Components to time-dependent variables was introduced by Lyon in 1954, for many years its application was essentially restricted to electric machines. Recently, thanks to its advantages, the Lyon transformation is also applied to power network calculation. In this paper, time-dependent symmetrical components are used to study the dynamic analysis of asymmetrical faults in a power system. The Lyon approach allows the calculation of the maximum values of overvoltages and overcurrents under transient conditions and to study network under non-sinusoidal conditions. Finally, some examples with longitudinal asymmetrical faults are illustrated.

Optimization of Surge Arrester's Location on EHV and UHV Power Networks Using Simulation Optimization Method

Switching surges are of primary importance in insulation co-ordination of EHV and UHV networks, as well as in designing insulation of apparatuses. The magnitude and shape of the switching overvoltages vary with the system parameters, network configuration and the point-on-wave where the switching operation takes place. Although some studeis were carried out for design of the networks, but power systems are gradually developed and their configuration is changed. It is important for the utilities to ensure that the peak overvoltage and global risk of the network resulting from the switching operations are well within safe limits. Optimization problems of power networks are very difficult to solve because the power networks are complex and widely distributed. This paper presents a simulation optimization approach in order to find the peak value of switching overvoltages and optimum location of arresters, as protective device, in EHV and UHV networks. In the proposed method, the Artificial Neural Network (ANN) as meta model estimates the peak value of overvoltages and global risk of failure generated by switching transients. An optimization method based on genetic algorithm (GA) determines the best positions of surge arresters on power networks so as to minimize the global risk of the network.

Initial stages of nucleation of molybdenum and tungsten carbide phases in tungstate–molybdate–carbonate melts

The electrochemical nucleation of Mo2C and W2C crystals for tungstate–molybdate–carbonate melts of specific compositions on Ag, Au, Cu, Pt, and Ni substrates was studied by the electrodeposition method. The influence of electrocrystallization conditions (temperature, deposition time, initial current pulse, and current density) was investigated. Experimental measurements indicate that crystallization overvoltage is associated with three-dimensional nucleation. Carbide deposition onto the substrates prepared from the same solid materials was not associated with crystallization overvoltage. The maximum value of the initial overvoltage, ηmax, which includes crystallization and electrolysis overvoltages, is proportional to the electrode surface area. Under these conditions, surface diffusion limits the electrode process rate. An increase in carbide deposition rate leads to an increase in the number of crystallization centers. This reduces the duration of surface diffusion. A rise in the melt temperature complicates the crystallization process by alloy-formation phenomena. Overvoltage maximum height for metals, which do not form alloys, is proportional to the reciprocal of their formation time. Melt temperature increase promotes interdiffusion of refractory metal, carbon, and substrate, and also intensifies their chemical interaction. Structural mismatch is observed during molybdenum and tungsten carbide electrodeposition onto different single-crystal substrates.

The effect of H 2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H 2S). Trace amount of CO and H 2S can cause considerable cell performance losses. A comparison between the effect of CO and that of H 2S on PEMFC performance was made in this paper. Under the same conditions, the H 2S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H 2S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H 2S and can be only partially recovered by reintroducing neat H 2. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.

A study on Medium Voltage Shunt Reactor Switching in Montevideo’s Trasmission Network

The paper describes first briefly the power installation under study and the relevant aspects of the well known theory of shunt reactor switching. A preliminary theoretical approach to the expected values of the switching overvoltages is presented as well as the relevant data acquired during the type tests performed on the circuit breakers. The detailed digital models used during the simulations, and the main results obtained from the electromagnetic studies performed using ATP are shown. The corrective measures proposed in order to match the insulation design criteria of the shunt reactors are also described.

Influence of Selected Parameters of Overvoltages on Hazard of Insulating Systems in MV Power Lines

Chosen problems relevant to overvoltage hazard of MV overhead power lines and power cables with polymeric insulation are discussed. Computer simulation results regarding the hazard due to lightning discharges occurring within MV power network systems are presented. These prove that 20 kV power network systems can be threatened by overvoltages during thunderstorms. Maximum values of total overvoltages induced within such conductors in dependence on distance between lightning stroke channel and a line have been estimated. Ill. 5, bibl. 6 (in English; summaries in English, Russian and Lithuanian).

Estimation of switching transient peak overvoltages during transmission line energization using artificial neural network

Overvoltages are one of the most frequently encountered problems during line energization. At the time of restoration transmission line switching is also one of the major causes, which creates overvoltage. The magnitude and shape of the switching overvoltages vary with the system parameters and network configuration and the point-on-wave where the switching operation takes place. Though detailed electromagnetic transient studies carried out for the design of transmission systems, such studies are not common in a day-to-day operation of power system. However it is important for the operator to ensure that peak overvoltages resulting from the switching operations are well within safe limits. This paper presents an Artificial Neural Network (ANN)-based approach to estimate the peak overvoltage generated by switching transients during line energization. In proposed methodology Levenberg–Marquardt method is used to train the multilayer perceptron. The developed ANN is trained with the extensive simulated results, and tested for typical cases. The simulated results presented clearly show that the proposed technique can estimate the peak values of switching overvoltages with good accuracy.

Towards the mechanism of electrochemical hydrogen storage in nanostructured carbon materials

The mechanism of electrochemical hydrogen storage in a nanostructured carbon electrode using the electrodecomposition of KOH and H2SO4 aqueous solutions has been investigated by means of galvanostatic and voltammetry techniques. The role of charging the electrical double layer is carefully considered during the process of hydrogen insertion and deinsertion into carbon, i.e. electroreduction and electrooxidation, respectively. Once the electrode potential becomes lower than the equilibrium potential, hydrogen in the zero oxidation state is formed by the reduction of water in alkaline solution or the reduction of hydronium ions H3O+ in acidic medium. In the next step, hydrogen is physically adsorbed (Had) onto the carbon surface and diffuses into the bulk of the carbon material with an efficiency which depends on the type of electrolyte. A higher amount of hydrogen is stored using the KOH medium, and the galvanostatic oxidation shows a well-defined plateau around -0.5 V vs. Normal Hydrogen Electrode (NHE). Due to the high overvoltage value in KOH (η=0.55 V), the recombination steps of Had leading to molecular hydrogen evolution through the chemical (Tafel) or electrochemical (Heyrovsky) reactions are less favoured than in an H2SO4 medium (η=0.32 V). Hence, a meaningful sorption of hydrogen is observed only in the basic electrolyte which shows a reversible capacity of 350 mA h/g (i.e. 1.3 wt. %) with a good electrical efficiency. Such performance demonstrates that nanostructured activated carbons might be a promising alternative to metallic alloys for electrochemical hydrogen storage.

Reliability of surge protective devices stressed by lightning

Surge protective devices (SPDs) are used to limit the maximum overvoltage value on protected circuits. The great part of such overvoltages are originated by direct and nearby lightning flashes to structures and to lines, it is then important to evaluate the reliability of the protective device. Actual SPDs are affected by ageing, due essentially to number and amplitude of stresses, but also to other factors such as overheating, pollution and humidity. A method, based on probabilistic arguments, to evaluate the ageing process of SPDs, when stressed by lightning pulses, is proposed and analysed. Its relevance stands in the fact that National and International standards do not give complete indications to decide when a SPD must be changed before its failure occurs (reliability estimation), instead the estimated expected life, obtained with the method proposed, can be directly used in the design of logistic/maintenance procedures.

Texture formation of electrodeposited fcc metals

The influence of the thickness of fcc metals electrodeposited on an indifferent substrate on the formation of a preferred orientation of the crystalline grains has been investigated. Following the atomistic theory of nucleation, the work of formation of both mono- and poly-layered critical nuclei oriented according to the most important symmetry axes of the fcc lattice was evaluated. It resulted that the initial texture axis of fcc metals is always the 〈1 1 1〉 axis which does not depend on the value of the crystallization overvoltage and of the energy of interaction between the nucleus and the substrate. The theoretical results on the initial texture are in agreement with the experimental findings on Cu and Ni electrodeposits. Moreover, by increasing the thickness of the latter electrodeposits, they presented a typical growth texture with 〈1 1 0〉 and 〈1 0 0〉 texture axis, respectively.

Electrochemical hydrogen evolution at a single-crystal zirconium face in sulfuric acid solutions

The electrochemical behavior of a zirconium electrode in 1 N H2SO4 is studied at various values of overvoltage, temperature, pH, and concentration of a surface-active substance. A mechanism of hydrogen evolution on the partly reduced surface of a zirconium single crystal is put forth. The assumption that zirconium readily forms zirconium hydride, which considerably alters the surface electrochemical properties, is confirmed. The hydrogen absorption on the single crystal results in the emergence of cracks on the metal surface in a certain crystallographic orientation. X-ray diffraction analysis is used to determine the index of the working surface of the zirconium single crystal and calculate the crystal lattice parameters.

Gas-Generating Porous Electrodes: Large Ohmic Resistances

It is shown how it becomes possible, in gas-generating porous electrodes (GGPE), to construct polarization curves (PC) that encompass also the region of high ohmic resistances up to an ultimate possible overvoltage that is reached at practically complete expulsion of the electrolyte out of the electrode pores near the front surface of the electrode, without restricting oneself, while taking into account ohmic limitations, by not-too-high values of overvoltages. As the true appearance of the dependence of the magnitude of an effective specific ionic electroconductivity of GGPE on the specific features of the structure of its porous space and the degree of occupation of its porous space by the electrolyte and gas is unknown, an attempt is undertaken to evaluate possible values of limiting ohmic limitations, specifically, to take into account the presence of both maximum and minimum ohmic losses. Calculations of PC and some other characteristics of GGPE are conducted using the constants that are close to the constants that take place for the process of formation of chlorine in dimensionally stable anodes. It is discovered that, if one selects a nonoptimum structure of the porous space of the electrode (this means that, with the emergence of gas pores in the electrode, ohmic losses would rapidly increase), then one fails to realize the values of the electrochemical activity of the electrode in excess of a few A cm–2.

The inhibition of the corrosion of Armco iron in HCl solutions in the presence of surfactants of the type of N-alkyl quaternary ammonium salts

The polarization behaviour of Armco iron in aqueous solutions of HCl with and without inhibitors has been studied by the potentiostatic method. At lower overvoltage values, the dissolution process is controlled by activation, while at higher overvoltage values, the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using four surfactants. Under the critical micelle concentration (CMC), the inhibition of these four surfactants is negligible. At a concentration higher than the CMC, the inhibiting action of tetradecyl trimethyl ammonium iodide (TTAI), tetradecyl trimethyl ammonium bromide (TTAB), hexadecyl trimethyl ammonium bromide (HTAB) and dodecyl trimethyl ammonium bromide (DTAB) increases rapidly. The process of inhibition was attributed to the formation of adsorbed film on the metal surface, that protects the metal against corrosive agents. Protection efficiency improved when the inhibitor concentration and the length of the alkyl chain were increased. The four quaternary ammonium salts tested were adsorbed on the Armco iron surface according to a Langmuir isotherm.