S. B. Crary

Overvoltages During Power-System Faults

OVERVOLTAGES may be produced by lightning, switching surges, faults, both solid and arcing, and the overspeeding of machines due to loss of load. The effect of circuit and machine characteristics on the duration and magnitude of these overvoltages has received considerable attention.1–10 Furthermore, field tests have been made in order to determine the effectiveness of different methods of grounding, and the accuracy of the methods used to calculate the magnitude of overvoltage during system faults.11–13

The Doubly Fed Machine

SYNCHRONOUS machines, operating with a-c excitation on both stator and rotor are used in many applications, for example, as induction frequency converters, as power and instrument Selsyn drives, and as variable speed power drives. Reference 1 has mentioned particularly the variable speed fan drive, and presented equations for the small oscillations of one such doubly fed machine. Reference 2 has also previously given the equations of hunting of the doubly fed machine (part XIV, section IV) in connection with the general study of oscillations of rotating machines. However, since the present authors have been using in their own work equations which seem to them to be more convenient and simpler in form for calculations, and since it now seems desirable to present not only general equations but also some of the more fundamental and significant performance characteristics of these machines, it is thought that this paper may now be appropriate. The form of the equations developed possesses the additional novelty of facilitating the setting up of equivalent circuits for hunting on the a-c network analyzer, and allowing the quick determination of the damping and synchronizing torques directly by wattmeter readings.

Pull-In Characteristics of Synchronous Motors

The differential analyzer has proved its worth in solving various types of difficult problems, of which the determination of the pull-in characteristics of salient pole synchronous motors is typical; but even with the aid of this device it has been necessary in solving this problem to make several simplifying assumptions. The differential analyzer recently completed at the University of Pennsylvania, however, is sufficiently comprehensive in design so that few assumptions are necessary, and the major electrical transients and mechanical forces may be taken into account. Typical solutions obtained by means of this analyzer are presented herewith, together with practical pulling-into-step criteria.

Stability Limitations of Long-Distance A-C Power-Transmission Systems

Conventional methods of transmission are analyzed and discussed both for existing transmission distances up to about 300 miles and for greater distances up to a full wave length; also eight different methods that have been proposed for increasing the loading per circuit or distance to which a-c power can be transmitted are discussed. Of these methods, series compensation of the line reactance appears to be the most favorable for straightaway distances up to about 600 to 700 miles at 60 cycles. The problem is analyzed mathematically and with the aid of an a-c network analyzer. Curves are included which give the power limits and reactive kilovolt-ampere requirements for a large number of parameters, including the effect of distance, voltage, conductor size, series and shunt compensation, terminal impedances, and stability margin. This analysis indicates that the limitation of the transmission of a-c power greater straightaway distances than has been accomplished heretofore is primarily that of the cost of the line, the stability limitation can be overcome by a comparatively small increase in cost.

Long-Distance Power Transmission as Influenced by Excitation Systems

THE renewed attention now being given to transmission of power from hydroelectric long distances from their load areas requires a re-evaluation of some of the factors affecting power system stability.

Torque-Angle Characteristics of Synchronous Machines Following System Disturbances

The DETERMINATION of the behavior of synchronous machines during transient conditions subsequent to a system disturbance is becoming increasingly interesting and important. In determining this behavior it is essential to know the torque-angle characteristic of the given machine under conditions resulting from disturbances of various sorts. The most important of such disturbances are variations in mechanical torque, excitation voltage, amount of external reactance, and system voltage. System voltage may vary in either magnitude or angular position.

Supplementary Control of Prime-Mover Speed Governors

THE development of interconnected systems has required improvements in the manual and automatic control of load, frequency, and time. The speed governors of the prime movers have always provided the medium for supplementary control of frequency, load, and time. Experience with various types of control devices and the factors influencing their functioning have been discussed frequently in the technical literature. Recently the problem has been given renewed attention1–5 with an effort to correlate experience with theory and to provide a basis for determining the proper characteristics of governors2 and their supplementary control.

Synchronous Starting of Generator and Motor

A SYNCHRONOUS motor, generator, or condenser can be started from rest, by means of shaft torque, induction-motor (amortisseur winding) torque, or synchronizing torque. This paper presents data on the last method of starting. The system studied consists of a prime mover driving a synchronous generator, and a synchronous machine driven electrically by the generator. The whole system is started from rest by means of torque applied to the generator shaft, the motor being accelerated to normal speed by the synchronous torque developed. The performance of such a system was determined both by tests on several machines and by calculations with the aid of the differential analyzer at the University of Pennsylvania, and limiting conditions and criteria for successful starting were found in terms of the various pertinent system parameters. In a few cases a system consisting of a generator and two motors was studied.

Analysis of the Application of High-Speed Reclosing Breakers to Transmission Systems

IN recent years many new or improved kinds of apparatus have been made available for increasing the reliability of transmission circuits. These include high-speed relays, high interrupting speed breakers, ground-fault neutralizers, protector tubes, and high-speed reclosing breakers. Though they may be used in combination, ground-fault neutralizers, protector tubes, and high-speed reclosing breakers offer alternate methods for reducing the outages on the transmission circuit. The reliable loading of a transmission circuit may also be increased by introducing intermediate switching stations and by increasing the interrupting speeds of existing breakers.

Long-Distance Power Transmission

MANY STUDIES have been made recently to determine the performance characteristics of long-distance transmission systems. This particular analysis considered straightaway transmission systems of 600 miles in length, a distance which is representative of transmission now under study and in actual development.1,3 The system characteristics and requirements for economic transmission outlined here may be used to guide the future development of long-distance as well as moderate-distance transmission systems.