Phenomena accompanying transmission with some types of star transformer connections — II

Abstract

1. Hysteresis, as well as variation of permeability, gives rise to harmonics of considerable magnitude in the exciting current of a transformer; the components from the two sources are in quadrature for each harmonic. 2. There is a fallacy in the idea that the harmonics in the exciting current of a transformer are entirely wattless. 3. For transmission lines of modern lengths, the effect of the line is to reduce the third harmonic voltage between neutral and line terminals of a star-star connected bank of transformers having grounded neutral on the line side only. 4. In transmission with star-star transformer banks having grounded neutral on the line side only, the interrelation of line susceptance from conductor to ground in series with the varying susceptance of the individual transformers may cause the total m.m.f. of one of the transformers to pass through zero at the same instant as the impressed e.m.f.; and thus, give opportunity for a reversal of this transformer due to inertia, thereby producing a periodically reversing leg and abnormal voltages. 5. In a similar system with high quality line insulation, the effect of atmospheric charges on the line conductors may be to cause the transformer of the bank to assume relations with one leg permanently reversed when energized from the station side; thus, imposing unbalanced voltage conditions on the bank and producing, in two of the units, fundamental frequency components of voltage 264 per cent of normal. 6. Combinations and accentuations of the above causes may, through natural requirements, give rise to a double frequency pulsation of the potential of the hypothetical neutral of the line conductors; thus, producing, in the leg voltages, second harmonic components of extremely large dimensions which are limitless so far as is yet known. 7. The above applies also in case a group of star-connected ironclad reactors, such as auto-transformers without tertiary windings, are employed to ground the neutral of an otherwise isolated neutral transmission. 8. Results, obtained in the laboratory with a three-phase star-star connected bank of transformers feeding condensers between line conductors and line side neutral, though not yet sufficiently extensive to confirm all of the above statements, show that the voltages from line terminals to neutral are in considerable measure independent of the primary impressed delta voltages so far as concerns wave shape and frequency.