R. D. Evans

Series Capacitors for Transmission Circuits

This paper presents the results of an analytical and experimental investigation of the use of series capacitors to increase the permissible loadings of long high-voltage a-c transmission lines. Because of limitations imposed by synchronous stability and reactive kilovolt-ampere requirements, conventional lines, when extended to the higher voltages and greater lengths, cannot be loaded to values of power sufficiently high to develop maximum over-all economy. It is shown that, when suitably applied, series capacitors will make possible the desired optimum loadings and will very materially reduce transmission costs. The theoretical possibilities of using series capacitors to compensate the excessive inductive reactance of long lines have received consideration for many years. Practical series capacitors require protective equipment, but shunting of the capacitors in the earlier schemes resulted in a decrease instead of an increase in the transient-stability limits. The authors propose series capacitors of the limited-voltage type in combination with auxiliary equipment which not only protects the insulation but quickly restores the capacitors to the circuit after the faulted conductors are isolated, thus preventing a decrease in system power limit at the time of need. The application requirements of necessary apparatus are also briefly outlined. Results of analytical studies and miniature-system tests in connection with a typical application of series capacitors to one of the major lines of a typical transmission system are included. This work deals with relative transmission costs, transient stability, spontaneous hunting, and the sub-synchronous operation in the induction starting of machines. The conclusion is reached that the proposed series-capacitor scheme appears practicable for long transmission lines.

Static Stability Limits and the Intermediate Condenser Station

The principal features of this paper may be summarized as follows: For static stability calculations, power systems may be represented by a network with constant impedance and admittance branches with as many terminals as the number of synchronous machines requiring individual consideration. This method is justified because, as developed in the paper, loads may be represented by the equivalent constant admittances and synchronous machines may be replaced by their equivalent impedances. A criterion for static stability of systems is presented together with formulas for the calculation of the two-, three- and four-machine cases. It is shown that the maximum angle between machines for which synchronism can be maintained is, in general, somewhat dependent upon the inertia of the synchronous machines. Under some conditions synchronism can be maintained between two machines operating at angles greater than π/2. The delivered power corresponding to definite excitation of machines will, in general, be a maximum for a smaller angle than the maximum stable angle. While the investigations have shown that synchronism can be maintained in regions heretofore considered inherently unstable, operation in such regions is held to be inadvisable. There are a number of generalizations which may be made as to the limiting stable condition.

The Petersen Earth Coil

Accidental arcing grounds on transmission lines constitute the foremost problem to be solved in the transmission of electrical energy over great distances. There has come into use to a limited extent, arcing ground suppressors. This device consists in principle, of a switch in the station which is automatically closed in parallel with the accidental arc at any point out on the system. The parallel path through the switch shunts the current from the arc and thereby extinguishes the arc. This development is not yet completed. This paper gives the results of some experiments on an entirely different device for suppressing accidental grounds — a device that was first advocated by Prof. W. Petersen of Darmstadt, Germany. The essential part of this new apparatus is a suitable reactor connected between the neutral of the circuit and ground. This reactance is chosen of such a value as to neutralize the capacitance of the circuits when an accidental ground of one phase takes place. Under this accidental condition the reactor is electrically in parallel with the active capacitances and, by the well-known fundamental law, the only current that flows to the combination of the inductance and capacitance in parallel is the current necessary to supply the energy loss in the combination. The simplified equivalent conditions are shown in Fig. 4. This energy current can be made very small and it is this relatively small current that passes through the accidental arc to ground. If the ground is of the arcing type, the arc will, under favorable conditions be extinguished, as the energy flowing through the fault is only that necessary to supply the losses in the resonant circuit. If the losses are low, the energy flowing through the fault will be insufficient to support an arc and the voltage of the resonant system is gradually reduced to zero, while the voltage between the former faulty wire and ground gradually rises to normal value. In a comparison of the various methods of grounding and their effects on the operation of a power system, the solid and the low-resistance grounds assume first and second place in the order of desirability. The distinction however, between these two is slight and choice will be determined by local conditions. Either the Petersen earth coil or the critical-resistance ground will assume third place in the order of desirability as the relative advantages and disadvantages of these two are about equal. The advantages of the Petersen earth coil system are: first, the suppression of arcing grounds under favorable conditions; second, the reduction of insulator trouble; and third, small earth current when a fault occurs to ground. The disadvantages are: first high potentials between line and ground due to series resonance; second, maintenance of a series of arcs under unfavorable conditions, that is, resonance and high loss, or large dissonance and either high or low loss; third, difficulty in obtaining selection of the faulty line by means of relay protection; fourth, reduced lightning protection due to the necessity of high settings on arresters; and fifth, increased system insulation due to the shifting of the neutral with abnormals or transients.

Power Limitations of Transmission Systems

Review of the Subject — Several independent studies have been made recently to determine the economies of a large, uniform power system. The two studies of more general interest were those conducted by the Department of the Interior, under the direction of W. S. Murray, for the Superpower Zone, and by F. G. Baum for the United States. Both of these investigations are available in published form. During the progress of the Superpower Survey, one of the longest transmission lines proposed was that extending 350 miles from the Niagara Falls Development to New York City. Under emergency conditions on this line, the power limit for the maximum amount of power was approached by two twin-circuit lower lines with three circuits carrying the emergency load. The maximum power limit would have been exceeded if two single-circuit tower lines had been employed, even though the transmission voltages and the total copper cross-section were the same as with the two twin-circuit lower lines. Similarly, several long, high-voltage lines will be required in a nation-wide system, especially through the middle western region as shown by Mr. Baums report. The tendency to extensive transmission systems has emphasized the necessity of considering the factors which will limit the amount of power that can be transmitted any distance with the highest practical transmission voltage. On account of the transmission line characteristics, the power limits will be greater when the system is regulated by synchronous apparatus than when those such apparatus is used so that two power limits will be considered in this paper; first, for an unregulated system; and second, for a regulated system. However, while we are primarily interested in high-voltage systems in this paper, it should be kept in mind that these same methods of calculation may be applied to lower voltages in determining the power limitations of station tie lines. It is commonly accepted that different types of networks have certain power limitations. For example, a very simple case quite generally known is that of a simple resistance circuit in which the power delivered is a maximum when the resistance of the load is equal to that of the line. Another familiar case is that of the electric arc furnace where the maximum power occurs when the resistance of the furnace arc is equal to the reactance of the electric furnace leads. The general phenomenon of maximum power limit in circuits of fixed reactance and variable resistance or load has been recognized and Us theory worked out for numerous cases, such as short transmission lines, rotating machines, and transformers. A power transmission system may be regarded as a special type of network. Ordinarily it consists of long, high-tension transmission lines and apparatus connecting generating stations with distant load centers which may be either at the terminus or at intermediate points on the high-tension lines. In large systems, the high-tension lines may form a network similar to an ordinary local distribution system. Where synchronous condensers are not installed, the problem of the maximum amount of power which may be delivered through the system is similar to the simple resistance and reactance cases cited above in that additional load or shunt impedance simply alters the load and voltage in accordance with the relative impedances of the system. The employment of synchronous condensers at the load centers or along the transmission lines to alter the power factor and maintain the voltage at the load materially increases the maximum amount of power that may be desired over a given transmission network. The theoretical maximum amount of power however, cannoi be obtained under operating conditions because the synchronous equipment at the receiver drops out of step with the supply. Also, fluctuations in load will produce unstable conditions, which may accumulate sufficiently to cause the momentary swings in load to exceed the power limit, resulting in the receiver falling out of step with the supply at a lower load than it would under steady conditions. This is usually characterized as “hunting out of step.” In order to investigate the power limitations of a transmission system, it was necessary to rearrange and extend the present methods of transmission line calculations to make them more convenient for the study of the practical limit of maximum power. The method which has been found best adapted for this purpose is a development of the power circle diagram combined with the characteristic curves of the synchronous machines used to regulate the system. This power diagram has been mode applicable to all the types of transmission systems by including the transmission line, step-up and step-down transformers, series and parallel circuits, so that the most complex transmission system may be represented by a single equivalent set of transmission constants. A general discussion of the methods of calculation, the maximum power limits, the practical operating limit and illustrated examples is given in Part I of the paper, and the analytical development upon which the discussion is based is given in Part II

Recovery-voltage characteristics of typical transmission systems and relation to protector-tube application

THE SUCCESSFUL operation of an are-interrupting device requires that the insulation recovery-voltage characteristics of the device be higher than the recovery-voltage characteristics of the system. A broad understanding of the recovery-voltage problem from the application point of view, requires a knowledge of the effects caused by the range of systems, by the different types of faults and the different fault conditions encountered in practice. With this object in mind an investigation was undertaken to provide a general picture of the recovery-voltage characteristics of transmission systems.

Inductive Co-ordination Aspects of RectiFier Installations

Discussion of paper 46-105, an AlEE committee report, presented at the AlEE summer convention, Detroit, Mich., June 24-28, 1946, and published in AIEE TRANSACTIONS, 1946, July section, pages 417-36.

Harmonics in the A-C Circuits of Grid-Controlled Rectifiers and Inverters

This paper summarizes the results of an investigation of the harmonic currents and voltages in the a-c circuits of grid-controlled rectifiers and inverters. The principal part of the paper presents the development of a theoretical method for predetermining the magnitude of the harmonics in terms of the d-c load current, the commutating reactance, the rectifier transformer secondary voltage, and the amount of grid control. Harmonic voltages in the supply circuit may then be calculated from the harmonic currents and the supply-circuit reactances at the various harmonic frequencies.

Harmonics and Load Balance of Multiphase Rectifiers Considerations in the Selection of the Number of Rectifier Phases

This paper analyzes the operating characteristics of multiphase power rectifiers from the standpoint of harmonics, load balance, and their interrelation, and their effects on the apparatus and circuits to which they may be connected. These factors are considered in connection with selection of the number of rectifier phases, which varies from six (four for very small sizes) to combinations of six-phase units to form a much larger number, such as 36, 72, or even 108 phases. More specifically, the problems of harmonics, load balance, and number of phases are considered from the standpoint of the effects on 1. Apparatus, particularly turbine generators and capacitors. 2. The wave-shape distortion on circuits that may be important in inductive co-ordination. 3. The possibility of resonance being encountered in the supply system to amplify one or more of the rectifier harmonics. These problems have increased greatly because of the great number and large size of rectifier installations made in connection with the war.

Arc-Backs in Rectifier Circuits Artificial Arc-Back Tests

THE enormous development in the use of rectifiers for the electrolytic reduction of light metals has been an outstanding development of World War II. One important problem in this rectifier development has been the performance at the time of rectifier arc-back. Currents of between 50,000 to 100,000 amperes maybe encountered on an individual rectifier and associated transformer. Still higher currents would obtain if back feed from other units on the same bus were not eliminated by prompt isolation from the circuit or d-c bus.

Abridgment of static stability limits and the intermediate condenser station

THE development in power transmission has been in the direction of delivering increased amounts of power per circuit over greater distances. This trend raises two fundamentally important questions: 1. What constitute the output limitations of the alternating current system of power transmission, and 2. How close to these limits is it feasible to operate transmission systems?