Showing papers in "Transactions of The American Institute of Electrical Engineers in 1930"

Loading Transformers By Temperature

Abstract: It is pointed out that safe loading of transformers by temperature requires not only an accurate knowledge of the thermal laws but also a knowledge of what is a safe temperature limit to be maintained continuously which condition seldom, if ever, happens with the present method of limiting the load to nameplate rating. In view of our present knowledge and experience the author questions the advisability of loading transformers continuously up to the present A. I. E. E. limit of 105 deg. cent. hot spot and argues for the establishment of a differential of 10 deg. cent. between the limit to be maintained continuously by means of overloads and the limit to be reached occasionally with rated load. In other words, for continuously maintained maximum temperatures the hot spot should not exceed 95 deg. cent. It is shown by the use of the thermal laws that without increasing the maximum or hot spot temperature, transformers can be overloaded 1 per cent for each degree centigrade by which the ambient is below 30 deg. cent. (air) for self-cooled transformers, 25 deg. (water) for water-cooled transformers. The results of laboratory aging tests conducted on class A insulations in air and in oil are given. These tests show that the rate of aging is roughly double for each 8 deg. cent. increase in temperature.

Generalized Theory of Electrical Machinery

Abstract: In the following pages electrical machinery is analyzed from a new point of view. Analytical quantities like magnetizing current, armature reaction, leakage flux, transient reactance are not introduced; only such quantities are used as actually exist in the machine at one particular load. Thereby the theory of electrical machinery is expressed in terms of the minimum possible number of quantities. No hypothetical currents or fluxes are used and no actual physical quantity is left out. The concept of ``free energy,'' used in thermo dynamics, is introduced and generalized. The criterion of good design of all electrical machines is expressed by a constant, the ``thermodynamic efficiency'' which gives a measure of the effective utilization of iron and space for the transformation of energy. This constant plays a most fundamental role in the steady and transient behavior of the machine. A method is given by which the direction of flow of energy between different parts of any complicated machine can be easily read off the diagrams. The theory of constant-potential and constant-current electromagnets is used as a stepping stone to show that the theory of the polyphase alternator is identical with the theory of the constant-potential polyphase transformer if flux linkages and magnetomotive forces are interchanged. The circle diagrams of the transformer and the alternator are developed as well as the elliptical locus diagram of the alternator with salient poles. Problems in the sudden short circuit and the sudden load variation of the polyphase alternator are also solved.

Starting Performance of Salient-Pole Synchronous Motors

Abstract: This paper presents an analysis of the sub-synchro-nous speed phenomena of a salient pole synchronous motor in which formulas and equivalent circuits are developed, making it possible to calculate the torque and current for a broad range of Conditions. Equations in terms of the impedances of the direct and quadrature axes are first derived from the revolving field point of view. The equivalent circuits for the two axes are then set up and expressions for the separate impedances of the circuit elements established. Adequate means of taking into account the combined effects of salient poles, incomplete amortisseur windings, and open or closed field windings are presented. The results obtained are illustrated by a comparison of the predicted performance and test data for a large 60-pole motor. The calculated distribution of current between the amortisseur bars of the same motor is shown by curves. Other curves show the calculated performance of the motor with and without continuous end rings between the poles and with the field winding open and closed. A final example shows the theoretical effects of changing the positions of the bars in the poles of a large waterwheel generator.

Voltage oscillations in armature windings under lightning impulses-I

Abstract: Experience has shown that rotating machinery connected directly to overhead lines is more vulnerable to surges than many other types of apparatus. This fact together with a desire on the part of some to connect important units to the line in this manner, has necessitated a study of the protection problem. Such a study is here described, showing oscillograms taken when steep voltage surges were applied to machine windings, measuring internal voltages to ground which are 200 per cent of the voltage allowed by the terminal lightning arrester. A simple traveling wave analysis of these oscillations is developed, which has successfully explained the peculiarities of over 400 oscillograms taken under various terminal conditions. Practical methods of eliminating th. e oscillations with neutral impedance are outlined in the light of the theory developed, and oscillographic evidence supporting their reliability is given. A generalized theory of neutral protection is proposed. The importance of wave-front, surge impedance of incoming line, arrester resistance, and other factors is discussed. Methods for protection of the turn insulation and the insulation to ground of such machines are suggested, showing that the lightning arrester only limits the potential of waves entering the machine and cannot control oscillations which may take place within the machine. The advantages of thyrite as a neutral resistor are pointed out in connection with the short-circuit protection, telephone interference, and lighting protection problems of such machines.

Double Windings for Turbine Alternators

Abstract: Large turbine alternators with two similar independent armature windings have recently been proposed, to permit the electrical segregation of bus sections in large stations without loss of synchronizing power. Several such generators have been installed, or are under construction, and it now seems that they will become of paramount importance in very large future stations. This paper explains the theory, and design limitations of these double winding generators, and describes their application in systems having different types of bus connections.

Extinction of a Long A-C. Arc

Abstract: The extinction of an a-c. arc is analyzed as depending on two factors, the rate of recovery of dielectric strength of the arc space after current zero, and the rate at which voltage tending to reignite the arc is applied by the external circuit. In the short arc, most of the recovered dielectric strength resides in a deionized layer next to the cathode but in the long arc the rest of the arc space contributes largely to the dielectric strength. The breakdown gradient of the still ionized arc space is defined, and using a thermal ionization theory, a formula for growth of breakdown gradient is derived. The extinction of long a-c. arcs in the open is greatly influenced by the sectional area which the arc stream has at current zero. By confining arcs to slots and holes, the rate of deionization at current zero is greatly increased, and so large voltages per cm. of arc can be interrupted. A gas blast passing turbulently through an arc stream greatly accelerates deionization at current zero and so is effective in increasing the capacity of the a-c. arc to interrupt high-voltage circuits. The expulsion fuse is an example of a gas blast circuit interrupter, the gas blast resulting from the decomposition of the fiber fuse case. The oil circuit breaker is also a gas blast circuit interrupter, the blast arising from the gases produced by the decomposition of the oil.

Three-phase short circuit synchronous machines-V

Abstract: This paper is the fifth of a series which the authors have presented on the subject of synchronous machines. Part I was an extension of the fundamental theory; Part II, a treatment of torque-angle characteristics under steady-state conditions; Part III, a study of torque-angle characteristics under transient conditions; and Part IV, an analysis of single-phase short circuits. In the present paper, equations for the currents in the armature phases and field under three-phase short circuit are developed. The analysis is divided into three parts. The first covers the case of short circuit at no-load when the resistance is negligible in determining the magnitude of the current although it must naturally be considered in finding the decrement factors; the second considers the same case where the resistance does affect the magnitude of the current; the third covers the case of short circuit under load when resistance is negligible as in the first case. The effect of the nature of the load, i. e., whether the power factor is lagging or leading, is studied and some interesting results are brought out. It is shown that short circuits under load may give less current than at no-load, and it is further developed that under certain practical operating conditions it is even possible to obtain sub-stantially no fundamental current. The paper is illustrated with comparisons of calculated curves and oscillograms showing the actual currents obtained in test.

Controlling Power Flow with Phase Shifting Equipment

Abstract: In the operation of an electric power distributing system involving various kinds of networks, a fundamental necessity is that each transforming and transmitting unit carry a reasonable share of the total load. In the case of a single system that has been developed in a coordinated manner the problem of load division has not in general been troublesome and has been solved by judicious selection of parallel circuits, use of reactors, proper system set-up, etc. The advent of inter-company connections has brought together systems of varying characteristics, changed the relation between the basic elements involved, and in some cases has made difficult the control of power flow over tie lines small in comparison to the systems which they unite. It is the purpose of this paper to discuss the case of load control where closed rings or parallel path circuits are involved. After briefly reviewing the principles of voltage phase angle and power flow, a description is given of some actual tests performed on a very large interconnected system with a 250-mi. transmission loop, involving five power companies and a total generator capacity well over 1,000,000 kw. The results of these measurements are analyzed for the purpose of checking theoretical against actual values and forming a basis for the solution of problems of a similar nature.

Stability of Synchronous Machines Effect of Armature Circuit Resistance

Abstract: The theory of synchronous machines as developed by Doherty and Nickle1 has been extended to include a determination of the effect of armature circuit resistance on damping torque. Equations are developed for the damping torque of synchronous machines in general, i.e., both the salient-pole and round rotor types. These equations assume an exciting winding in the direct axis and an amortisseur winding in the quadrature axis, and further assume that all damping is due to currents induced in these two windings. The effect of an amortisseur winding in the direct axis is not considered because its damping action at the low frequency of hunting is small compared to that of the exciting winding. It is shown that the damping torque of any synchronous machine can become negative, giving instability, if the armature resistance is increased beyond a critical limiting value. This fact has been known,2 but an actual determination of the value of the critical resistance in terms of constants of the machine has not, to the authors' knowledge, been available. This value, for a salient-pole generator with normal excitation and no amortisseur winding, is r = x q tan ?? where r is armature circuit resistance?, x q is quadrature synchronous reactance, and ?? is the steady-state displacement angle. If r is less than the critical limiting value, the damping torque is positive; if greater, negative. The damping of a generator increases in the positive direction with increase in load.

Progress in the Study of System Stability

Abstract: In the second part of this paper and in the appendices attention is given to simplified methods of treating the problem of system stability. Methods are recorded which have been found useful in making many system studies. These methods have had considerable verification by tests both on a model system and on large operating systems and have been simplified to such an extent that many operating companies are now finding it to their advantage to undertake the work of making careful studies of their own systems, just as they now make short circuit studies which formerly were thought too difficult and too highly theoretical. The first part of the paper gives some comments and conclusions of the authors and their colleagues as a result of many such system studies as well as studies involving more detailed methods, and also as the result of practical experience through contact with various operating companies. Some of these comments are based directly on an example which is given in detail in Appendix I.

Dancing Conductors

Abstract: The phenomenon of dancing cables is discussed; the merits of different theories are considered, reference being made to the theory that vibrations of small amplitude, but relatively high frequency, travel along the line to points where the constants of the line change as at dead-ending insulators and there reflect and combine into slower waves of great amplitude. The suggestion is made that much experimental work should be done with regard to these theories before drawing any definite conclusions. Reference is made to Magnus effect and to the ``lift'' of ice-coated cables in wind storms. This lift, perpendicular to the direction of the wind, is carefully considered, and diagrams show experimental values of this lift on models similar to the ice-coated cables. The diagrams indicate variations and reversals of this thrust due to small changes of angle of the specimen to the direction of the wind. The suggestion is made that these alternate lifting and depressing effects should be considered as the cause of some, if not a large percentage, of the phenomenal movements of ice-coated conductors in relatively light winds. As in most earlier reports and discussions there does not seem to be any remedy as a result of this study, other than that of heating the conductor electrically throughout sleet-forming periods to such a temperature that the sleet cannot form on the wires.

Effect of Armature Resistance Upon Hunting of Synchronous Machines

Abstract: The ability of systems to withstand shocks, such as faults or switching operations, is much greater for small operating angles. The present paper has to do with system stability for small oscillations as contrasted with the more usual concept of stability as being its ability to withstand shocks. The analysis shows that synchronous machines without amortisseur windings and with no resistance in the armature are inherently stable for small oscillations in prime mover input or load output. With resistance in the armature, stability for such conditions is dependent upon load. A new proof is given for the relation previously derived by Nickle and Pierce which states that the limiting angle at which a machine becomes stable is tan ?1 r/x q . Arguments are presented which show that any standard type of damper which may have been installed in generators for other incidental reasons, possesses the property of preventing spontaneous hunting except for the most abnormal conditions met in practise.

Quiet Induction Motors

Abstract: Magnetic noise in induction motors is caused by the vibration of magnetic parts produced by the varying forces associated with the changing flux density in the adjacent air parts of the magnetic circuit. Torsional vibration of the motor is caused by unbalanced windings or applied voltages. Vibration of the rotor simulating a mechanically unbalanced rotor is caused by two fields differing by two poles and not of the same frequency or direction of rotation. Vibration of the stator by distortion from a circular shape results from the presence of other simple combinations of fields. Teeth tend to vibrate axially at double applied frequency. The magnetic noise produced by a motor can be predicted from a qualitative analysis of the air-gap field supplemented by an approximate quantitative analysis and consideration of the stiffness of the parts.

Traveling Waves on Transmission Lines with Artificial Lightning Surges

Abstract: The paper describes tests on a transmission line of the Consumers Power Company using a portable million-volt impulse generator. Data were secured on attenuation with one, two, and three conductors in parallel, with and without ground wires. A study was made of the influence of ground wires in reducing voltages induced by overhead discharging clouds and also the effect on surges of entering or leaving a section of line having ground wires. A brief study was made comparing steel and copper for ground wires. The effect of ground resistance in tower footings was considered and a method given for measuring surge impedance of transmission lines.

Effects of Lightning Voltages on Rotating Machines and Methods of Protecting Against Them

Abstract: Rotating machines directly connected to overhead lines are subject to damage from lightning surges. Many methods of protection against such damage are possible, several of which are discussed in this paper. Laboratory experiments have been made to show the normal distribution under various steepnesses of waves, and the improvement of unsatisfactory distributions by means of condensers or lightning arresters connected to various parts of the windings has been studied.

Resistance Welding

Abstract: This paper is directed to engineers interested in the application of resistance welding to the industries. The discussion foreshadows some of the latest tendencies of development. It deals with some fundamentals of design though it is not written as a complete treatise on the subject, nor especially for those engaged in the designing of resistance welding machinery. Differentiation is made between the older form of resistance butt welding and the later method of flash butt welding. The probable action taking place in the flashing zone is discussed, along with the effect that increased potential across the weld has on this action. The main object in flash butt welder designing should be to combine the maximum degree of acceleration of the metals into the flashing zone with the proper and necessary e. m. f. A study of the power division of a typical butt welder shows that the efficiency may be as low as 7 per cent or lower, but that nevertheless it competes with other methods of welding when applied to problems of mass production. The effect of external resistance in a flash butt welding machine is to offset variations in die contact and variations in stock resistance, thus stabilizing the flow of current in the flashing zone. The power factor of welding machines is not regarded by the designers as a major element, since the time of application of the current is at most but a few seconds.

The M. I. T. Network Analyzer Design and Application to Power System Problems

Abstract: Previous network-computing devices with their uses and limitations are reviewed briefly, following which is a statement of the requisites for an effective a-c. calculating table. The M. I. T. Network Analyzer, a static miniature a-c. power system, designed and constructed jointly by the Massachusetts Institute of Technology and the General Electric Company for the computation of actual power networks, is described in detail and its operation outlined. The fields of its application are enumerated and include the study of normal operating conditions, stability, and short circuits. An example illustrating its application to the solution of a normal-operation problem is given.

A Cathode Ray Oscillograph with Norinder Relay Its Design and Application

Abstract: The paper describes the Norinder cathode ray oscillograph as used in lightning investigations on transmission lines in 1928. It then describes a new model of a cathode ray oscillograph developed for the expansion of field and laboratory tests in 1929. One of the novel features of this modified instrument is the steel housing. A circuit is shown for obtaining a unidirectional time axis on the oscillograms, in which the timing movement is started by an impulse from the surge to be measured. In the appendix a simple and concise formula for the sensitivity of a cathode ray oscillograph is developed. This has been derived by Mr. Lewis R. Smith and takes into account the change of the mass of the electron at speeds approaching that of light. At the frequently used cathode voltage of 60 kv., this phenomenon already causes a deviation of 5 per cent from the value which would be expected otherwise.

Effect of Electric Shock

Abstract: This paper describes an investigation of the effects of electric shock on the central nervous system. A total of 286 rats was employed in the investigation. These were shocked at 110, 220, 500, and 1000 volts on both alternating and continuous current circuits for varying lengths of time. In each series the duration of the shock was increased until it was found impossible to resuscitate the rats. When possible the rats were resuscitated by means of artificial respiration, and kept alive for about a week. Then they were killed by an overdose of ether and an autopsy was immediately made. The rats reacted in entirely different manner on the two types of circuit. At the lower voltages the alternating current was deadlier than the continuous, but at the high voltage the opposite was found to be the case. In many instances the rats were paralyzed by the application of the current and in the majority of these animals gross hemorrhages were found in the spinal cord at autopsy. All of the deaths that occurred as a result of the shock were caused by respiratory failure, which could usually be traced to an injury of the central nervous system. In some cases the electric current produced peculiar effects upon the rats.

Thyrite A New Material for Lightning Arresters

Abstract: A new material has been developed which is peculiarly adapted for use in lightning arresters. It is physically similar to dry process porcelain and it can be made in any shape that can be successfully moulded. The manufacturing processes have been perfected so that the electrical and mechanical characteristics can be duplicated or varied as desired within practical limits. This material, which is called thyrite, does not follow Ohm's law, for each time the voltage is doubled the current increases 12.6 times. The ratio of change of current to change in voltage is constant over ranges of current of 10,000,000 to one. This means the resistance decreases as the current and voltage increase. The resistance at any particular value of voltage or current is fixed by the physical dimensions of the piece. It is slightly lowered at high temperatures but returns to normal when cooled. The resistance does not depend upon the rapidity of current or voltage change and it is unaffected by long service. Because of these definite characteristics of thyrite the action of lightning arresters can be accurately calculated for the first time. Thyrite is now being used experimentally in three types of lightning arresters. The station type is made in 11.5-kv. sections, each one containing its own gap unit so that no series arrester gap is required. These gap units have the desirable property of sparking over at practically the same voltage irrespective of wave front. The 11.5-kv.

Impulse Insulation Characteristics of Wood Pole Lines

Abstract: This paper gives the results of a rather comprehensive series of tests on the impulse insulation characteristics of wood and on combinations of insulators and wood as used in wood pole transmission line construction; and suggested methods of protecting wood from damage due to lightning discharges: also a brief discussion on the application of the data.

Transient Torque-Angle Characteristics of Synchronous Machines

Abstract: Mechanical oscillations of synchronous motors following the application of abrupt shaft loads have not been hereto-fore easily calculated for cases of large angular swings, taking into account the damping currents in the rotor, except by tedious point-by-point methods. The chief difficulty has been due to the form of the basic differential equation upon which the solution of hunting problems depends. Within the lastfew years Dr. V. Bush and others at the Massachusetts Institute of Technology have devised the integraph which is capable of solving the equations of the synchronous motor. Into the integraph are put curves representing the non-linear differential equations and the boundary conditions. The desired results come from the integraph as curves. In this paper the problem of sudden load on the non-salient pole alternator is solved by the integraph for enough different conditions so that the performance of practically any machine of this type may be easily predicted from the compiled results. Knowing the moments of inertia of the machine and its load, the characteristics of the machine running as a synchronous motor and as an induction motor, the maximum amount of sudden load for which the machine will remain in synchronism may be determined for different values of initial load. Other curves give the maximum angle of the first swing of the rotor and the time interval for the rotor to change from its initial position to this maximum angle. The simplest type of equation representing an ideal synchronous machine is solved in this paper.

Three Regions of Dielectric Breakdown

Abstract: In a previous paper, the writers published results on the dielectric strength of various glasses and concluded that for ordinary thicknesses there were three distinct types of breakdown. The present paper emphasizes the fact that this is true not only for glass but also for fused quartz and for celluloid. By the elimination of edge-effect the following breakdown gradients were obtained: For celluloid, 2,500,000 volts/cm.; for glass, 3,100,000 to 5,000,000 volts/cm.; and for mica, 10,600,000 volts/cm. The various methods of eliminating edge-effect are discussed and compared and an attempt is made to correlate the electrical properties of the glasses with their chemical composition.

Surge Characteristics of Insulators and Gaps

Abstract: This paper presents the surge characteristic, in the form of time lag curves, of transmission line and substation insulation. The forms of insulation discussed are: I. Line insulation: suspension type with and without arcing rings, pin type, and wood alone and in combination with porcelain. II. Post type insulators with and without arcing rings. III. Safety gaps with either needle or spherical electrodes. IV. Oil gaps.

Arcing Grounds and Effect of Neutral Grounding Impedance

Abstract: This paper was written to review and extend the theory of overvoltages due to arcing grounds because of the increasing tendency to use impedances between the neutral point and the ground, thereby losing the advantage of the solidly grounded neutral. The ``third class conductor'' theory of Steinmetz is touched upon very briefly and is considered as not applying to transmission line conditions. The theory when the phenomenon is controlled by normal frequency arc extinction, as presented by Peters and Slepian, is reviewed and the maximum voltage for this analysis is found to be 3 ? E, where E is the normal line to neutral voltage. The theory when the phenomenon is controlled by oscillatory frequency arc extinction as originated by Dr. Petersen is given in detail but in a modified and extended form. The maximum voltage for a single-phase circuit when no damping is considered is found to be 6 E. The analysis for the three-phase circuit is newly developed for the case in which there is an impedance between the neutral and ground and the maximum voltage is found to be 7 ? E when the effect of the damping factors and capacitance between lines is neglected. The method of determining the various reductions or damping factors is outlined. The effect of a neutral grounding resistor is discussed and it is pointed out that a suprisingly high value of resistance can be used without incurring the possibility of dangerous overvoltages.

A General Switching Plan for Telephone Toll Service

Abstract: This paper outlines a comprehensive plan for improved switching of long haul toll telephone traffic in the United States and eastern Canada. A brief discussion is given of the methods of designing the toll plant to give adequate transmission efficiency for all connections established in accordance with this plan, including a new method of providing amplification at intermediate switching points replacing the cord circuit repeater method.

Lightning Laboratory at Stillwater, New Jersey

Abstract: This paper outlines the purpose of the cooperative installation of a lightning laboratory, the organization employed, and the various tests which have been made during the summer of 1929. In general, the paper presents the preliminary analysis of some of the most outstanding of a rather long series of tests and forms the background for a more complete paper giving a more detailed analysis which will be presented in the near future.

Synchronous Motor Effects in Induction Machines

Abstract: Many induction motors do not have the smooth speed torque curves which are to be expected from elementary theory. Many motors at no load show a tendency to run at some speed way below that normally expected. This is sometimes called ``sub-synchronous'' speed, or the motor is said to be ``crawling.'' In the present paper one cause of this phenomenon is explained. It is found that certain combinations of phases, rotor slots, and poles cause the motor to run as a synchronous motor at some intermediate speed between zero and normal no-load induction motor speed. This effect has been christened the ``synchronous motor effect'' in induction machines and is shown to be due to the locking of harmonic fields, generated by the stator winding with similar harmonic fields, generated by the rotor windings. The method of avoiding this trouble by the proper selection of the number of rotor slots is given.

Lightning Investigation on 220-K. System of the Pennsylvania Power & Light Company (1928 and 1929)

Abstract: The purpose of this paper is to describe the results of a lightning investigation conducted during 1928 and 1929 on 114 circuit miles of a 220-kv. system located in a territory where severe lightning storms are frequent. During the 1929 investigation, which was a continuation and expansion of previous years' work, a number of devices was successfully used, such as surge voltage recorders, cathode ray oscillographs, electric field intensity recorders, and lightning stroke recorders. Some of these devices are new. Many valuable data on magnitude and wave shape of actual lightning surges were obtained, proof of the existence of both single- and multiple-phase faults due to lightning; some data on the shielding effect of overhead ground wires and qualitative data on the nature and time of discharge of lightning strokes, together with the atmospheric gradients resulting therefrom, were also obtained.