G. Camilli

Transformer magnetizing inrush currents and influence on system operation

When a transformer is energized, a transient current, known as magnetizing inrush current, generally flows for a short period of time until normal flux conditions are established. Under most practical system conditions, this current transient is of little consequence. However, in very rare cases a combination of circumstances may be obtained which results in this inrush being of such consequence as to impair momentarily the proper operation of the system. Because of the numerous faetors bearing upon this general problem, an investigation has been made to determine the effects of transformer inrush currents under a wide variety of system conditions. It is the purpose of this paper to discuss the mechanism by which inrush currents are produced, the results of tests and calculations, and studies made with the miniature-system analyzer. Factors that determine the significance, of inrush current from the standpoint of system operation and methods for reducing the inrush current or mitigating its effects are also discussed.

Gaseous Insulation for High-Voltage Apparatus

A group of halogenated gaseous compounds have been investigated to determine their impulse (1½x40-microsecond wave) and 60-cycle strength in uniform and nonuniform fields at pressures of 1,2, and 3 atmospheres. The scope of this investigation was confined to bare electrodes, and additional testing will be required to determine the behavior of these gases with various types of solid insulation. It has been found that sulphur hexafluoride (SF6) possesses superior insulating properties for high-voltage apparatus, even at these relatively low pressures. Data obtained in an approximately uniform field indicate that at 30 pounds gauge. pressure this gas approaches the impulse strength of oil. Therefore, certain classes of high-voltage apparatus advantageously may use this gas for their insulation, if other properties are adequate.

A Proposed Method for the Determination of Current-Transformer Errors

CURRENT TRANSFORMERS for relay service must be able to function with reasonable accuracy under very high overload conditions, and it is therefore necessary that their accuracy for such high currents be verified. However, in performing the usual accuracy test at very high currents, certain difficulties are encountered — the principal ones being:

A Flux Voltmeter for Magnetic Tests

A voltmeter is described for a-c. circuits the voltage indications of which are directly proportional to the maximum flux density, regardless of the wave shape of the voltage. While the instrument is suitable for many varieties of magnetic tests, its most important application is to the reduction of transformer coreloss measurements to sine-wave basis. Test data indicate excellent accuracy for the meter in this application in comparison with other schemes or outfits used at the present time for that purpose. Losses determined by this new meter are, in general, appreciably larger than those determined by the older methods. The new meter makes it unnecessary to use, for reliable results, large generators to reduce the wave distortion caused by transformer excitation loads, and permits the use of any generator that will carry the load thermally.

New Developments in Potential-Transformer Design

This paper describes certain design ideas applied to potential transformers to accomplish substantial reductions in volume and weight, with improved reliability and with better accuracy than the best requirements of the American Standards. The size of these units is such that they appear mostly as bushings. Perhaps the most striking feature of construction is the arrangement and the insulation of the high-voltage winding. Liquid-impregnated porous paper has been substituted for the combination liquid and solid used in the conventional designs. Another notable improvement incorporated in these transformers is the elimination of all gaskets. The amount of liquid insulation required is extremely small, and therefore premium insulating liquids (such as the Askarels) can be used without much additional expense.

Overvoltage Protection of Current-Transformer Secondary Windings and Associated Circuits

It has long been recognized that excessive potentials may be developed in current-transformer secondary windings under unusual conditions, such as open circuits. Recent experience discloses that dangerous overvoltages (several thousand volts) may be produced as a result of normal switching operations on circuits containing lumped capacitance. A simple procedure for circuit analysis and evaluation of approximate voltage magnitude for the switching transient case is reported. For easy reference, there are included tables of calculated secondary voltage magnitudes covering a broad range of application. Under certain conditions, overvoltage protection is desirable and important. Aside from the potential hazard to life, current-transformer circuit insulation may be damaged, yet not be evident immediately. Performance at normal rated current may not be noticeably impaired, yet serious failure may occur in the presence of fault-current flow, thus nullifying the action of current-actuated protective relays. The characteristics of a new overvoltage protector expressly designed for current-transformer protection is presented. With this device current-transformer secondary voltages are limited to moderate values. The protector is small and compact,. and easily applied to existing as well as new current-transformer installations. The characteristics are permanent, not affected by repetitive operation, and result in negligible ratio error in the normal operating current range.

A Survey of Bushing-Type Current Transformers for Metering Purposes

The great simplicity, reliability, and low cost of bushing-type current transformer has led inventors to devise various schemes to improve its accuracy to metering standards. This paper reviews these schemes, culminating in the orthomagnetic device described in detail in a paper by J. W. Farr.160

Reduction of Transformer Exciting Current to Sine-Wave Basis

As a sequel to an earlier investigation and development of a method for the reduction of core-loss measurement to sine-wave basis, this paper describes two methods developed for the reduction of exciting current to sine-wave basis. The first method consists of making two measurements at wave shapes as widely different as possible, setting the voltage in each case by means of the flux voltmeter. The current corresponding to sine-wave voltage is obtained, by extrapolation from the observed values of currents and form factors. Although the method might be considered to some extent empirical, it is found to yield an accuracy within one per cent even under extremely unfavorable conditions. The second method utilizes as before the flux voltmeter for setting the voltage but uses a “crest ammeter” (developed for this purpose) for reading the instantaneous maximum values of the corresponding currents. Measurements are made at 100 per cent, 86.6 per cent and 50 per cent voltages. These data determine the fundamental, third and fifth harmonics of the exciting current corresponding to sine-wave voltage and hence the exciting current itself, because these harmonics are the only important components in determining the effective value of the exciting current. Theory of the crest ammeter is given, and its applicability (in conjunction with the flux voltmeter) to the determination of d-c. saturation curves by means of a-c. tests in magnetic investigations is indicated.