A large number of transmission lines have been erected without adequate consideration having been given to the ability of the line to stay in circuit electrically during the occurrence of lightning or other disturbance conditions. These come, however, within the range of the transmission engineer's field of action. The authors' experience with a 55-mi., 132-kv. transmission line since 1917 to date is cited, the particular line being one which has been very successful from a standpoint of continuity of service. Further experience with other lines built with variations on the original design and placed in service in 1924 and 1925, is given. A detailed analysis is given of eighty-eight cases of lightning trouble on one of these lines during 1925, the various steps taken to reduce the frequency of this trouble, the effect of increased clearances and separations, of additional insulators, of flux control units, and of arcing horns and shields on the amount of trouble and on the damage to insulator and conductor. These are discussed both from the standpoint of actual experience and from the laboratory analyses and investigations made. A general discussion of the ground wire and its effect on lightning voltages is given and the arguments for and against it are discussed. The importance of paying attention to the mechanical side of the ground wire and the effect of such attention on continuity of service is pointed out.

Lightning and Other Experience with 132-Kv. Steel Tower Transmission Lines, and its Bearing on Tower-Line Design from the Continuity of Service Standpoint

Overhead transmission lines are usually considered to be a class of equipment subject to certain types of troubles which cannot be avoided by means accepted as ordinary practise. It is the purpose of this paper: (1) to describe improvements in design of wood pole lines of the 33,000-volt class, which improvements have been developed and applied to the system with which the authors have been identified and which, it is believed, will greatly reduce the characteristic troubles, (2) to present an explanation of flashovers on wood structures, (3) to give the facts found in field investigations which form the basis for the improvements adopted, and (4) to describe the method used for field investigations and analysis of troubles. The studies and investigations referred to deal primarily with the matter of insulation. Considerable information relative to the insulation values of wood in a structure as developed by laboratory tests has been published. Experience is cited in this paper to show the value of wood insulation in the structure developed from field experience on 33,000-volt lines, and there is presented a practical economical design of wood pole structures using wood braces, in place of the usual metal braces, which, it is believed, will be practically immune to lightning troubles. Data from laboratory tests are given to substantiate the conclusions developed in the field investigations. The performance of insulators with respect to mechanical and electrical breakage is shown and the use of insulators of more sturdy designs is discussed.

Transmission Research and Design with the Field as a Laboratory

Records of the transients actually occurring on transmission lines of widely varying characteristics have been obtained recently with the klydonograph. 1. These records substantiate many of the theories of transients on lines. 2. They indicate the incorrectness or incompleteness of some of these theories. 3. They suggested modifications or extensions of these latter theories. This paper is a coordination of those theories which agree with test data obtained up to the present time. Since the results of the klydonograph surge investigations are presented in a companion paper, only the data required to make this paper complete in itself are included.

Transmission Line Voltage Surges

K. B. McEachrom The comparison made by Dr Slepian between the energy to be handled by the valve-type arrester and the arc-resistance type for equal protection is important. With the arc type, if the flow of system energy following the impulse is not to be excessive; it is necessary to employ considerable series resistance with the result that the arrester's ability to discharge the impulse is seriously impaired. To prevent system current from following the discharge of an impulse through an arrester, it is necessary that the voltage across the arrester, as the impulse ceases, be equal to or greater than the system crest voltage. The modern valve-type arresters are designed to operate on this principle.

Theory of the autovalve arrester

Overhead grounded wires have been in extensive use since the construction of the earliest transmission circuits. The fundamental theory of their protective value is based on Faraday's ice-pail experiment. As the resulting law goes, there is no electrostatic field emanating from the inner surface of a charged hollow conductor. The parallel grounded wires do not surround the power wires. Consequently the protection of these grounded wires against induced electric charges by thunderclouds, is only partial?usually of the order of 25 to 40 per cent. It might be erroneously inferred that several decades of use of the overhead grounded wire had established by practise its value. The several factors involved in its use do not lend themselves easily to experimental observations. For example, power lines extend over hundreds of mile while any particular induced charge is localized at some point in these vast distances. Taking into account the broef period of a lightning stroke. the unwilling observer stands a small chance of being near the point of discharge. Furthermore, thunder-clouds differ from one another. Still further, at the instant the lightning bolt takes place the distance from the thunder-cloud to the power wires varies quite indefinitely. In fact, there is a long list of difficulties involved in experimental observation of the effect of cloud lightning on power wires. As a result, except for a few small-scale experiments performed in the laboratory, knowledge of the subject is confined almost entirely to theoretical analyses.

Questions on the Economic Value of the Overhead Grounded Wire

This paper is an endeavor to answer questions of practise and criticism of arresters brought out by an investigation conducted by the Protective Devices Committee. For the most part practise in lightning arresters is standardized. In fundamental principles there have been no changes for many years. Improvements in details, especially of construction, are still being made. A new arrester, the oxide film arrester, gets rid of the oil and electrolyte and avoids the necessity of daily charging, but fundamentally it is designed along the same principles as the aluminum arrester. The important principle is the electric valve action?there are but a few milliamperes of discharge rate at normal line voltage, but at abnormal line voltages the discharge current rise to hundreds of amperes. In answer to criticisms made by a few prominent engineers, it is maintained as fundamental that a large discharge rate for an arrester is an absolute essential. The burden of proof falls on those engineers who use arresters of low discharge rate. These arresters cannot discharge the dangerous lightning. surges on overhead lines. Since there are lightning arresters of low discharge rate in apparently satisfactory use, an explanation for this anomaly is found in the use of insulators of low arc-over voltage. Either the lightning potential is relieved locally at the insulator or the resultant traveling wave is of too low voltage when it reaches the transformer greatly to endanger the insulation. Poor line insulation is not a solution of the problem of continuity of service.

E. E. F. Creighton

Papers

Questions on the Economic Value of the Overhead Grounded Wire

On Deviations From Standard Practise in Lightning Arresters