Voltage Oscillograms Research Articles

Time and frequency domain analyses based expert system for impulse fault diagnosis in transformers

The presence of insulation failure in the transformer winding is detected using the voltage and current oscillograms recorded during the impulse test. Fault diagnosis in transformers has several parameters such as the severity of fault, the kind of fault and the location of the fault. Detection of major faults involving a large section of the coils have never been a big issue and several visual and computational methods have already been proposed by several researchers. The present paper describes an expert system based on re-confirmative method for the diagnosis of minor insulation failures involving small number of turns in transformers during impulse tests. The proposed expert system imitates the performance of an experienced testing personnel. To identify and locate a fault, an inference engine is developed to perform deductive reasoning based on the rules in the knowledge base and different statistical techniques. The expert system includes both the time-domain and frequency-domain analyses for fault diagnosis. The basic aim of the expert system is to provide a non-expert with the necessary information and interaction in order to make fault diagnosis in a friendly windowed environment. The rules for fault diagnosis have been so designed that these are valid for the range of power transformers used in practice up to a voltage level of 33 kV. The fault diagnosis algorithm has been tested using experimental results obtained for a 3 MVA transformer and simulation results obtained for 5 and 7 MVA transformers.

Molding loose metal particles into briquettes with the use of short pulses of high density current

Theoretical estimates and experimental investigations were carried out on the process of molding a mixture of metal particles and a dielectric (air) into briquettes with the use of short pulses of high density current. Four groups of specimens were studied consisting of metal fragments and the dielectric in various proportions. In the experiments and the current and voltage oscillograms, the electrical resistance and the temperature of the specimens before and after passing the current were measured and their ultimate strength was determined. It has been found that the processes affecting the distribution of the current over the specimen cross section and the specimen inductance during the current pulse play an important role in molding briquettes of sufficient strength for transportation and processing.

Multi-turn railguns: concept analysis and experimental results

The multi-rail concept of railguns, the so-called multi-turn railguns, has been considered and dependences of their main parameters such as accelerating force, current density distribution in rails and breech voltage upon bore sizes, number of turns and the launcher's scale are presented. The experimental designs of multi-turn railguns built in the TRINITI in 1995-1999 are described: a three-turn RBM-3-1 and MTR-3-1.3 railgun with a 5.6 /spl mu/H/m inductance gradient and two five-turn RBM-5-05 and RBM-5-2 railguns with 0.5 m and 2 m bore length, correspondingly and a 16.5 /spl mu/H/m inductance gradient both. The experimental design of the armature for these railguns is described as well. The results of tests of launching armatures with a 0.14-1.1 kg mass up to velocities of 800-1300 m/s and accelerations up to 8/spl middot/10/sup 5/ m/s/sup 2/ are given. Representative oscillograms of current and muzzle voltage of each rail pair are shown. Some methods of providing electrical strength for rail insulation in the bore and armature plates at metal contact fault are considered. Strengths and weaknesses of multi-turn launchers in comparison with conventional ones are discussed and an advantage of the former railguns at launching bodies with large volume and mass is shown.

Characteristics of transverse volume discharge in Cf2Cl2 and in Ar/Cf2Cl2 mixture

The conditions of ignition and the characteristics are given for a nanosecond transverse volume discharge in Freon-12 (CF2C12) and in an Ar/CF2C12 mixture. It is shown that, in the ultraviolet and visible ultraviolet spectral region, a plasma based on CF2C12 molecules is a source of radiation of excited carbon atoms and Cl2 molecules. The volumetric stage of the discharge exists atU charge ≤ 12 kV and [CF2C12] = 0.1–3 kPa, and the current and voltage oscillograms are similar to the respective characteristics of a transverse discharge with prior-to-prebreakdown electron multiplication in He(Ne)/SF6(F2) mixtures. A plasma of an Ar/CF2C12 mixture is a source of radiation of the λ = 258 nm bands for Cl 2 * , 193 nm for ArF, and 175 nm for ArCl. Such sources may be used in short-wave pulse photometry, microelectronics, and photo- and biochemistry.

Time-domain study of a shielding cable system with a nonlinear load

An impulse current of several kiloamperes was injected to the shield of a shielded cable, which was terminated by a varistor. The induced voltage on the inner conductor caused by this impulse current reaches an amplitude in excess of the varistor's threshold level. The clamped voltage across the varistor and the injected current have been studied for different termination conditions of the shielding cable. Furthermore, this paper also presents the use of a lumped circuit to simulate the transfer impedance of an "electrically short" shielded cable in the time domain. In combination with the varistor circuit model, the shielded cable with the nonlinear load, a varistor, was also simulated straightforwardly in the time domain. Good agreement was found between the measured voltage and current oscillograms and the calculated waveforms. It is thereby demonstrated the validity of the developed lumped circuit model for the transfer impedance of a shielded cable.

Reply To "Comments On "The Time Evolution Of The Resistances And Inductances Of The Discharges In A Pulsed Gas Laser Through Its Current Waveforms'"

The physics of a pulsed gas discharge introduces adequate restrictions into the mathematical problem for the definition of the time evolution of the resistance and inductance. These restrictions permit the determination of the above-mentioned parameters uniquely. In a previous paper, 1 we had published a method for finding the time evolution of the resistance and inductance of a pulsed discharge. According to this method, the elaboration of the current waveforms through the integrodifferential equations of the system leads to the revelation of the time history of the resistance and inductance. However, two objections in our method have arisen in a paper by Friedman. According to the first objection, the definition of the time evolution of the resistance and inductance through the oscillograms of the current and voltage is not one-valued, but it has a set of solutions. As a verification, the author of the above Comments gave an example (an - - circuit) of various temporal dependencies of inductance and resistance corresponding to the identical current and voltage oscillograms. The set of solutions and consisted of the fundamental solutions plus oscillatory terms with frequencies multiples of the fundamental frequency of the system. Two restrictions were established by the author of the above Comments for the choice of the functions and : the physical restriction, that is, and , should be positive, and the mathematical restriction, that is, when , the inductance is

Comments on "The time evolution of the resistances and inductances of the discharges in a pulsed gas laser through its current waveforms" [with reply

The opportunity of definition of the time evolution of the resistance and inductance through the oscillograms of a current and voltage is discussed. It is shown that this problem is not single-valued, and it has a set of solutions. The restrictions are established, which should be in accordance with these solutions. An example of various temporary dependencies of inductance and resistance is Indicated, which corresponds to the identical current and voltage oscillograms.

Electrical characteristics of various contact contaminations

Contact reliability may be endangered by various types of contact contaminations as particles, corrosion products, polymers or carbon deposits. Often information about the type of contamination may be gained by easily applicable measurements of current and contact voltage only: i.e., contact resistance measurements after each operation of a routine life test (employing increasing voltage and current), and voltage oscillograms across opening contacts.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

RL-multivibrator and retrieving the coil magnetization curve

A method for retrieving the magnetization curve of a nonlinear coil is described. The parameters of a piecewise-linear or a third-order polynomial approximation can be obtained. A simple bridge RL-multivibrator, which includes an operational amplifier and this coil, is used. The coil parameters are calculated observing two oscillograms of the voltage at one of the bridge branches and measuring the oscillation period. >

Control System of Cycloconverter Drive

This paper describes control system of cycloconverter fed synchronous motor drive. For the purpose of assurance of the drive system operation with high performance static and dynamic characteristics over a full speed range from zero to maximum speed the field-oriented control method is applied. The motor phase and excitation current references calculator is described in detail because it is the kernel of the whole control system of this cycloconverter drive. First part of this paper presents the requirements that should be satisfied by control system to allow the most efficiency synchronous motor work. In the second part the structure of phase and excitation current references calculator is shown . The voltage and current method of synchronous motor flux calculation is described. The proper calculation of motor flux is the most important and difficult problem to solve because the accuracy of the flux calculation impacts fundamentally on motor control quality. The functions and the principles of operation of each parts of current references calculator are also presented here. The control system described here was manufactured for 900 kW hoisting machine drive using 3-phase cycloconverter and ordinary 32-pole low-speed synchronous motor. At the end of the paper the oscillograms of motor voltages, currents and speed waveforms at various states of drive operation are shown and discussed.

Steady state analysis of an inverter-fed induction motor employing natural commutation

A steady-state analysis of a three-phase induction motor fed by a DC link inverter commutated by machine-induced EMF (electromotive force) is presented. The active power requirement of the motor is met by the DC link, and the necessary reactive power is supplied by a capacitor bank connected at the motor terminals. A generalized analytical model is developed for no-load as well as loaded conditions of the motor. The steady-state performance of the motor is numerically evaluated. The computed results are compared with corresponding experimental results for a 4 HP squirrel cage induction motor. The relevant oscillograms of voltage and current waveforms of the inverter and motor are presented and discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quasicontinuous excitation regime of electric-discharge exciplex lasers

An investigation is made of the feasibility of establishing a quasicontinuous discharge stage in exciplex lasers. An analysis is made of a discharge model based on allowance for multistage ionization and attachment processes. It is shown that stabilization of the discharge may be achieved by selecting the parameters of the electric circuit whereby the electron density and the metastable concentration of the rare gas atoms should correspond to certain steady-state values. A quasicontinuous discharge stage of up to 1 ..mu..sec duration was achieved experimentally. The proposed model was used to make numerical calculations of oscillograms of the plasma voltage and discharge current. The calculations give good qualitative agreement with the experiment. Laser radiation was achieved at the quasicontinuous stage of a discharge in XeCl* of 250-nsec duration. It is shown that the quasicontinuous excitation regime can substantially increase the radiation energy of electric-discharge lasers.

Vacuum breakdown in electric fields in the range (5?7)�106 V/cm

1) After electron and ion processing of electrodes, with monitoring of their surface by measurement of exoelectronic (post-discharge) emission, gaps are produced which have very high electrical strength. Thus complete breakdown of a gap of about 0.1 mm occurs in an electric field of (5–7)·106 V/cm. 2) Oscillograms of current and voltage reveal that in a majority of cases the breakdown which develops is of very brief duration with no stable are developed, and this phenomenon is most probably produced by explosion of a microscopic projection heated by current during the thermoelectronic emission stage preceding breakdown. Upon breakdown a current pulse of electrons from the microscopic plasma produced in the explosion appears.

Cavity resonance effect in large HV laboratories equipped with electromagnetic shield

The electromagnetic screening of High Voltage laboratories against external interference is required in order to facilitate measurements inside the laboratory of partial discharges and radio interference voltages during dielectric testing. Such laboratories have been found to behave as high quality cavity resonators when excited from inside by the rapid discharges of impulse generators. Due to the large physical dimensions of high voltage halls, some resonant frequencies may be as low as a few megahertz. The oscillatory electromagnetic fields produced by the currents which circulate in the cavity induce stray signals in the impulse voltage measuring circuits. Such circuits usually have pass-band widths sufficiently large to reproduce these oscillations. This interference is then superimposed on the impulse oscillograms and, in certain circumstances may hamper the interpretation of the test voltage oscillograms. The interference can be suppressed by coating the inner side of the shield with a special resistive paint, which reduces the Q factor of the resonant cavity. Although such paint is not yet commercially available the method has been experimentally verified on a reduced scale model.

Theoretical description of ac arcs in mercury and argon

The energy balance and circuit equations, together with Ohm’s law, are solved numerically to obtain temperature profiles, current, and voltage as functions of time for wall−stabilized ac arcs. Experimental measurements of current and voltage oscillograms and also temperature profiles have been made for 1.5− and 7.5−A rms arcs in mercury vapor. Derived net emission coefficients of radiation as a function of temperature for the 1.5− and 7.5−A arcs are consistent with one another; furthermore, the experimental temperature profiles for both arcs are in good agreement with our theoretical predictions. Theoretical current and voltage waveforms and central temperatures for a 20−A rms arc in argon are in good agreement with the experimental results of Detloff and Uhlenbusch. Radial convection has little influence on these results. An approximate criterion for arc extinction or reignition at current zero is given in terms of the steady−state V−I characteristic and the arc time constant. By relating the present analysis to methods using the Cassie−Francis and the Mayr equations, expressions are derived for the undetermined constants used in these equations for arcs dominated by thermal conduction losses.

Lightning Surges in Open Wire, Coaxial, and Paired Cables

During the 1968 and 1969 lightning seasons, member companies of the Telephone Association of Canada conducted a field investigation of lightning voltages induced in outside plant toll facilities at ten selected sites across Canada. Oscillograms of longitudinal surge voltages occurring in open wire, paired, and coaxial cable were continuously photographed with an automatic camera system especially developed for the investigation during each season. On completion, approximately 10 000 useful surge photographs were obtained and analyzed. The results indicate that a standard test wave, with 1000-V peak and 10 \times 1000-\mu s waveshape, simulates 99.8 percent of the lightning surges encountered in paired and coaxial cables. For open wire circuits, a more suitable test wave with 2000-V peak amplitude and 4 \times 200-\mu s waveshape is required to simulate 99.8 percent of the lightning surges. The number of surges per storm was found to be far higher than previously assumed. The highest recorded incidence during a storm was 484 surges greater than 10 V. Surge repetition intervals were short in all three types of facility; about 15 percent were less than 2 s. The surge information presented provides fundamental data on lightning surge effects in Canadian telephone plants which were previously unavailable. The assembled data will be of value to the protection engineer when designing future protection systems, and also to the transmission engineer concerned with noise problems.

Ignition Criteria for Resistive Circuits

The criterion of no ignition is stated in general. The relation between the electrical power output and the discharge voltage, and the power output and time are discussed for resistive circuits with source voltages ranging from 24-60 V, tested by the German breakflash. Shadow photographs of the electrode area with the incipient kernel and the relevant oscillograms of the discharge voltage are recorded. From the oscillograms a general discharge voltage pattern emerges. The theoretical considerations as well as the experimental results suggest that the critical section when ignition is likely to occur under minimum igniting conditions coincides with the peak of the power transfer function. Suggestions are made about the practical application of these results to monitor the sensitivity of the breakflash or forecast the probable value of minimum igniting current or power for a given circuit.

Electrical explosion of tungsten wires in a vacuum

The present work demonstrates the disintegration of thin electrically exploding tungsten wires in a vacuum. The drawn tungsten wires split into tiny fibers in a time close to that at which the wire reaches its melting point. Similar results were obtained for wires exploded at low pressures and at atmospheric pressure. The results show, within the limits of the experimental accuracy, that, when the wire reaches its melting point, dips are observed on the current derivative and the voltage oscillograms and x rays are produced. The results support the suggestion that the production of hard x rays from drawn thin exploding tungsten wires is due to the splitting of the wire, which has a fiberlike crystalline structure. Moreover the oscillograms of the first current pulse show that for thin wires the electrical conduction up to melting point is due only to the solid wire material and that the influence of the ambient pressure on the crest value of the first current pulse for wires exploded at the same condenser-bank voltage is almost negligible.

The Behavior of Input-Power Density in Exploding-Wire Restrikes

The input power per unit volume of the restrike channel of thin exploding wires reaches its maximum slightly before the electrical conductivity has reached its maximum. In its turn, the electrical conductivity reaches its maximum before the current and the total power input, ui, have reached their maxima but slightly after the ledge observed on the voltage oscillograms. The reason why the input power per unit volume and the electrical conductivity reach their maxima much earlier than the current and the total power input reach theirs seems to be closely connected with the magnetogasdynamics of the restrike channel, which have not yet been satisfactorily treated because of the complexity of the problem.

Recording arc discharges at the cathode in models of MHD devices

The analysis of current-voltage characteristics for channel models of MHD devices gives reason to believe that, in specific cases, the uniformly distributed discharge at the cathode is "stripped" and that an eIectric arc appears [1]. It has also been surmised [2] that a "unidirectional" are exists, which penetrates the boundary layer and extends on one side into the cathode glow and on the other side into the uniform current in the body of the plasma stream. These phenomena have been observed both on "cold" [2], and hot electrodes made of silicized graphite [1]. In order to verify these assumptions, a series of experiments was performed on a channel model to detect arc discharges. The work was carried out in a stream of air (T G ~ 3000 ~ K) with an applied electric field (UEF F = 80 V, f = 50 Hz); the temperature range of the operating surface of the electrode T w was from 1200 to 2000 ~ C, and the additive content varied from 0 to 1.2% by weight of the flow rate. The channel mode1 and the measurement circuit are described in [1]. Together withthe usual oscillograms for the current-voltage characteristics (electrode-electrode and electrode-probe types), the experiments also gave the corresponding values of current and voltage displayed on a time scale. Fig. 1 gives a typical current-voltage electrode-probe characteristic for an electrode made of silicized graphite; Fig. 2 gives an oscillogram of the current and voltage displayed on a time scale. The section OD of the electrode-probe oscillogram (Fig. 1) (the potential drop in a gas layer of thickness ~5 mm as a function of the current flowing in the electrode circuit) corresponds to an operating regime in which the electrode under investigation acts as an anode, and the segment OABC corresponds to an operating regime where this electrode acts as a cathode. We see from Figs. 1 and 2, that the arcs are struck at a specific breakdown voltage U~', corresponding to the point B, after which the voltage decreases and the current increases. While the arc is burning, the voltage remains practically constant during one-half period or decreases a little (point C). After the voltage reaches a value corresponding to the point A, the conditions necessary for maintaining an arc regime disappear and the electrode again begins to operate in a distributed discharge regime. Here, as in the initial sector OB, a sinusoidal variation of current and voltage in the electrode circuit is observed. The oscillograms obtained for the variation of voltage and current in the electrode circuit of a channel model for a MHD device correspond in shape to the voltage and current oscillograms of the stabilized arc between the electrodes of a variable current plasmatron. 9 Fig. I