Showing papers in "Transactions of The American Institute of Electrical Engineers in 1914"

Effect of Altitude on the Spark-Over Voltages of Bushings, Leads and Insulators

Abstract: The dielectric strength of air decreases with decreasing pressure and increasing temperature; that is, with the density. Therefore, at high altitudes where the barometric pressure is low, brush discharge starts and spark-over takes place at lower voltages than at sea level. The effect of air density on corona, and spark-over between spheres, etc., has already been given. In the following investigation the effect of altitude and temperature on the surface spark-over of leads and insulators was studied by placing them in a large wooden cask and gradually exhausting the air. Correction factors are given for various standard types. The spark-over voltage decreases almost directly with the air density.

Submarine Signaling

Abstract: Submarine signaling has been greatly advanced by the introduction of a powerful sound transmitter and receiver called the ``Fessenden telegraph oscillator.'' By means of this, telegraph messages can be sent and received through the water by moving ships and for short distances speech can be transmitted, icebergs can be located, and soundings taken instantaneously. The apparatus consists of an oscillating electric motor-generator which has a strong electromagnet surrounding a central core on which is an alternating-current winding. Between the core and the magnet is a copper tube which acts as a closed secondary to the core winding. This copper tube is attached to a large diaphragm. When the alternating current passes through the core winding it induces a current in the copper tube, which, being free to move, vibrates back and forth, thus setting the diaphragm in vibration. This apparatus is installed in a ship so that the face of the diaphragm is in contact with the water and its vibrations set up sound waves in the water. Signals have been sent a distance of 31 miles. The oscillator can also be used as a receiver. Sound waves striking against the diaphragm cause the copper tube to vibrate, thereby generating a current in itself which is induced in the core winding. A telephone receiver in the armature circuit enables the observer to hear the sound.

The Sphere Gap as a Means of Measuring High Voltage

Abstract: A gap method of measuring high voltage is desirable in certain tests because a gap measures the maximum point of the voltage wave. The needle gap is unreliable, and is subject to large corrections due to humidity, etc. The sphere gap is free from complicated corrections and the curve is subject to calculation. Wherever voltage is measured by a gap, correction must be made for air density?that is, altitude or barometric pressure, and temperature. The laws for this correction have been deduced for the sphere gap, from tests, over an air density range equivalent to that from sea level to 18,000 ft. elevation. An equation is given for calculating sphere spark-over curves for various spacings, radii, air density, etc. Standard measured curves for convenient sizes of spheres at sea level are given for practical use, as well as a table for applying these curves to any altitude. The altitude correction may be made very accurately to the standard curve, either by applying the calculated correction constant or by applying the correction factor given in the tables. The complete standard curves should not be calculated when reliable measured curves are available. The effects of high frequency and impulse voltages are discussed. In making measurements certain precautions are necessary for accuracy, as well as for protection of apparatus. These precautions are discussed and test results are given.

Influence of Transformer Connections on Operation

Abstract: In this paper the relative advantages and disadvantages in operation of the more important three-phase transformer connections are discussed. Three conditions of operation are given: First, normal; second, operation of a bank with one phase disabled; third, effect of line grounds on operation. The paper is not complete, particularly in that high-frequency or switching phenomena are not discussed. Its major purpose is to give a fairly adequate presentation of insulation stresses at relatively low frequencies to which transformers are subject in either normal or abnormal conditions of operation. These frequencies include the fundamental or generated frequency and its harmonics and the natural frequency of the system. The behavior of three-phase auto-transformers under the various conditions of operation given above is also analyzed.

Harmonic Voltages and Currents in Y- and Delta-Connected Transformers

Abstract: The paper reviews the conditions under which triple harmonic voltages and currents are produced in Y- and delta-connected transformers. These voltages are produced by hysteresis in the core. In a single-phase transformer, increase of series resistance tends to suppress the current harmonic and produce the voltage harmonic. In three-phase transformers, a Y connection suppresses the current harmonic and allows the full flux and voltage harmonics to appear. Delta connection provides a closed path for the current harmonic, and suppresses the triple voltage. A case is cited where a Y-connected auto-transformer was used to step up from 6600 to 12,000 volts at a substation. The neutral was not grounded, and trouble resulted due to partial resonance at triple frequency between line capacity and transformer reactance. The paper shows that, although not generally recognized, a triple component can exist in the line-to-line e.m.f. wave of a three-phase system. This is possible in a case where a two-to-three-phase transformation is used, and when the e.m.f. wave of the two-phase generator contains a triple harmonic. Vector diagrams and curves are given illustrating this possible effect.

The Electric Strength of Air-V

Abstract: The paper presents the account of an investigation of the influence of frequency on the corona, between 60 and 3000 cycles per second. The wire and coaxial cylinder method was used. A simple method for measuring the maximum value of an alternating voltage wave has been developed and used for obtaining this maximum value at the critical corona voltage. The gold-leaf electroscope was used to detect the first appearance of corona. It has been found that for frequencies above 275 cycles per second the corona voltage (maximum value) is not so uniform and constant as it is at frequencies in the neighborhood of 60 cycles. The indications of the investigation are that at 2000 cycles the corona voltage is lower than it is at 60 cycles by about 3 or 4 per cent. The experiments reveal several interesting instances of resonance phenomena in the high-tension transformer circuits.

The Effect of Delta and Star Connections upon Transformer Wave Forms

Abstract: The purpose of the paper is to show the distortions resulting from different symmetrical three-phase connections of generator and transformers without transmission line, as dependent upon the hysteresis cycle and the admittance of the transformers at no-load. Tests were made with the oscillograph to show the no-load exciting current and voltage waves of three single-phase step-up transformers when the windings of the generator and both sides of the transformers were connected in all possible symmetrical delta and star relations. Tests are divided into four groups, according to the connection of the generator. In all cases normal low-tension line voltage was held, but voltage and current measurements were considered less important than the recording of wave forms. Oscillograms are given in each case and the relations between the flux, voltage, exciting current, and the hysteresis cycle are shown in two instances. The author points out that the best voltage wave forms will, in general, be obtained with a star-connected generator and delta-star or star-delta connected transformers.

Some Investigations on Lightning Protection for Buildings

Abstract: A description is given of investigations conducted for a large manufacturer of explosives to determine upon a suitable system of lightning protection for buildings containing explosives. The work is divided into the following sections: (1) An analysis by oscillograph of the secondary currents induced by actual lightning discharges in vertical earthed conductors, the results seeming to indicate that such currents are non-oscillatory and aperiodic, though occurring in either a negative or positive direction. An attempt is made to explain the phenomena generally attributed to high, frequency oscillations by the existence of unidirectional waves of almost vertical front. (2) An investigation of the primary effects of a 20-in. spark in air, having the same essential characteristics as those attributed to lightning, when applied to a model protective system consisting of isolated vertical conductors surrounding a small building. (3) An investigation of the secondary effects produced under the above conditions, which are considered to be static inductive rather than dynamic inductive. A brief description is given of a general protective system recommended for explosives buildings.

Theory of the Corona

Abstract: The theory of the corona developed in this paper explains the coronal discharge by the application of the known laws of the discharge of electricity through gases. The experiments of Dr. Whitehead and Mr. F. W. Peek and others have shown that the surface gradient Xs at the surface of a smooth wire necessary to start the corona increases very greatly as the radius of the wire decreases. As an illustration of this increase it is found that the coronal gradient Xs at the surface of a wire of 0.5 cm. radius is 46,000 volts per cm., while a gradient of 97,000 volts per cm. is required for a wire of 0.02 cm. radius. The phenomenon is due to ionization by impact. The ions no move in toward the wire from the place at which they cross a region where the gradient is X0 = 26,600 volts per cm. up to the wire. They produce other ions, and n ions arrive at the wire. The condition for the corona is that n shall be a constant. If ? = the number of ions that one ion produces in a cm., then fb ? dx n = no e a The ? is a function of X at every point. This function is expressed in terms of x. It is introduced into the above equation, and on integration the equation for the corona is obtained. The equation also contains a term for the density of the air.

Magnetic and Other Properties of Electrolytic Iron Melted in Vacuo

Abstract: Electrolytic iron of a purity of 99.97 to 99.98 per cent was melted in fused magnesia crucibles in a modified Arsem furnace, forged into rods, machined, and annealed in an electric furnace according to various heat cycles. The magnetic properties were obtained by the Burrows double bar method. Determinations were also made of the electrical resistance, chemical composition, and physical properties, including microstructure and critical temperatures as well as tensile tests. A few commercial iron and steel samples were tested for comparison. The following valuable results were obtained: 1. Pure iron melted in an atmosphere of carbon monoxide under atmospheric pressure will absorb both carbon and oxygen with the result that the iron thus produced is of an inferior magnetic quality. 2. Low-carbon iron melted in vacuo will lose 50 to 90 per cent of its original carbon content. 3. The magnetic quality of electrolytic iron melted in vacuo is decidedly superior to any grade of iron thus far produced, the maximum permeability obtained being 19,000 at a flux density of 9500 gausses. The average hysteresis loss obtained is less than 50 per cent of that found in the best grade of commercial transformer steel, due to the fact that the coercive force is very much lower than for silicon steel, although the retentivity is higher. 4. The specific electrical resistance of pure iron melted in vacuo is 9.96 microhms per centimeter cube. 5.

A Milliampere Current Transformer

Abstract: (1) It is pointed out that by the use of a current transformer having a primary to secondary current ratio of the order of 1 to 100, oscillograms can be obtained of the charging current of a single high-tension insulator or of a few feet of high-tension transmission line; that is, oscillograms of currents of the order of 0.1 to 0.5 milliampere may be obtained. (2) The drawings and specification are given for a transformer for this purpose. (3) The transformer relations are discussed; the methods of determining the transformer constants are outlined, and the performance of transformers constructed in accordance with the specifications is determined. (4) A series of oscillograms is given to illustrate some of the applications of the transformer, such as to the study of corona, high-tension insulators, and leakage currents in evacuated lamps.

Toll Telephone Traffic

Abstract: Experiments are described to determine the relationship between telephone circuit loads and the corresponding delay to traffic The operating methods employed and the number of circuits available determine in general the number of messages per day which can be handled over a single toll circuit The average delay to traffic obviously depends upon the number of messages per circuit per day, or the circuit loads With a given load factor, increase in the circuit loads will increase the average delay to traffic At the same time the revenue per circuit mile will correspondingly increase The practical limit, however, is approached when the delays to traffic reach a point where the service is unsatisfactory The results of the experiments described illustrate the fact that increasing circuit loads increase the delay to traffic, and vice versa The revenue per circuit mile is directly proportional to the product of the circuit load and the toll rate per minute-mile; consequently the relationship between the quality of service and the toll rate is generally obvious, assuming a certain rate of return on the plant investment

The Electrically Driven Gyroscope in Marine Work

Abstract: The gyroscope when continuously driven by electric power may be used for various purposes, many of its applications having especial value in marine work. Small gyroscopes are used to establish base lines for the measurement or control of angular motion. The gyro-compass makes use of the earth's daily rotation and consequent changing direction of gravity to secure a directive force a hundredfold greater and more accurate than that of the magnetic compass. A general description is given of the gyro-compass as adopted by the United States Navy for use on all the battleships and submarine vessels. Many electrical and mechanical devices have been developed to perform the various functions whereby an instrument of great precision has been secured. Large gyroscopes are made which are capable of counteracting enormous wave forces and completely stabilizing any ship against rolling in the heaviest seas.

Insulator Depreciation and Effect on Operation

Abstract: Investigation shows that insulator trouble increasing with time is not due to fatigue in the material under applied working loads, but rather to depreciation caused by the absorption of water by porous material or by the cracking of the dielectric from high internal mechanical stress set up by uneven temperature in the dielectric, or by greater expansion of cement or metal, or stress from a combination of these. The shape of the dielectric may cause high maximum stresses under comparatively mild conditions, necessitating the working of material with a lower factor of safety than that permissible even in steel work. The high maximum internal stress under which insulators operate will cause considerable depreciation in some types through cracking, necessitating a careful study of the effect of depreciation upon the operation of the system. Trouble comes largely through the matching up of faulty parts so that the remainder of the insulator will be destroyed by a comparatively mild surge. Applying the theory of probability, it is then possible to obtain a relative operating hazard for the insulator under the same conditions or for varying degrees of depreciation. An equation for the operating hazard may be developed which gives a good idea of the relative economic importance of the number of sections in the insulator, the magnitude of the switching surge and the rate of depreciation as affecting the reliability of the system.

150,000-Volt Transmission System

Abstract: The paper describes operating conditions on the 150,000-volt transmission line of the Pacific Light and Power Corporation which delivers power from the Big Creek hydroelectric development to Los Angeles, Cal., 240 miles away. In daily operation 60,000 kw. are generated, utilizing a total hydraulic head of 4000 ft., in two steps. Plans for the future contemplate the building of two more power houses, operating under somewhat lower heads. Of particular interest is the complete success of the constant potential system, i. e., operation at the same voltage at the generating and receiving stations, by means of synchronous condensers at the receiving end, in conjunction with automatic voltage regulators, one for each condenser as well as for the generators at each of the power houses. The line has been operated with unusual freedom from short circuits. Appendixes describe the development of the system, and give data relating to the equipment of the Big Creek transmission line.

Protective Reactors for Feeder Circuits of Large City Power Systems

Abstract: This paper outlines the use and limitations of protective reactance coils in feeder circuits. When no feeder reactors are used, doubling up the station capacity increases the number and severity of short circuits. The insertion of feeder reactors cuts down the severity of a short circuit and practically renders the effect local, so that beyond a certain point additional generator capacity does not appreciably increase the severity. Curves are given showing what the effects of feeder reactors are, with and without bus reactors, for generators of various reactances. The advantages that might be gained by parallel operation of feeders are discussed and the difficulties to be encountered are pointed out.

Some Simple Examples of Transmission Line Surges

Abstract: The paper treats of the true wave phenomena which take place on a transmission line when switches are opened and closed or when a circuit breaker operates. These phenomena are practically the same in alternating- and direct-current systems for transmission lines up to say 150 or 200 mi. (240 or 320 km.) in length, and therefore the surges which are described in the paper are the surges which take place when a steady voltage is connected to the line or when the line is disconnected with a steady current flowing in it. The details of line surging which takes place under such conditions are most easily described in terms of what may be called the ``ribbon wave'', and the first part of the paper is devoted to a discussion of the ribbon wave. Then a number of practical examples are described in detail.

The Cost of Electricity at the Source

Abstract: The Stott-Gorsuch method is applied to the determination of the cost of manufacturing electricity in a 60-cycle station of 100,000 kilowatts installed capacity. The results correspond to the assumption that such a station is capable of delivering from 350,000,000 to 700,000,000 kelvins per annum for load factors ranging from 0.50 to 1.00. For unity load factor the cost of three-phase electricity at the outgoing cables ranges from 0.65 cent per kelvin with coal at $5.00 per ton, down to a matter of 0.20 cent per kelvin for fuel of negligible cost. A method is indicated for tracing through the increase in the cost of the electricity at later stages of its journey from the source to the consumer.

Use of Reactance with Synchronous Converters

Abstract: The paper presents the results of overload and short-circuit tests made about two years ago upon some synchronous converters in circuit with auxiliary reactors. Two entirely separate sets of tests upon two synchronous converters of widely different operating characteristics are described. In the one case the reactor is in the a-c. circuit and in the other in the d-c. circuit; yet in each it may be called, and in fact is, a protective reactor. In presenting this original information the author, with a view to indicating its commercial application and hoping to provoke discussion, has endeavored to divide synchronous converter installations in a general way into classes with respect to the need or desirability of employing protective reactance, and also with respect to the general design or type of the reactor to be employed. In order to do this, synchronous converter installations are divided into a few general classes with respect to the character and exactions of the service conditions under which they are required to operate. The paper was written shortly after the tests were made, but although much has been learned or written about protective reactors since that date, the author believes nothing has transpired to affect the value of these tests or to make it necessary to change the form in which it was originally intended to present them.

A Study of Some Three-Phase Systems

Abstract: The star-star, delta-delta, delta-star and star-delta connections are taken up in order and their individual peculiarities and characteristics, precautions that must be taken in operation to avoid trouble, and where and when the system may be grounded with best results, are discussed. The question of delta-delta vs. delta-star is considered: (1) For moderate voltages the delta-delta system has the advantage on account of its greater flexibility. (2) For high voltages the delta-star connection is preferable, chiefly on account of its greater strength and lower cost. There is a probability that a delta-star bank of transformers will cause less disturbance in switching than one that is delta-delta connected. (3) There are much greater possibilities of development in transformers designed for delta-star connection than for any other. (4) Past and present practise upholds the use of the delta-star connection in preference to the delta-delta for high-voltage transmission.