Instability in transformer banks

This paper considers the instability which sometimes occurs in banks of transformers supplying a capacity load when certain harmonics in the primary current are suppressed, either by the type of transformer connections or by a resonant circuit in series with the primary of the transformer, and the similarity between the several unstable circuits is pointed out. Curves showing the triple-frequency voltage distortion as a function of the capacity load have been included for two of the unstable circuits. For one case, oscillograms taken during the instability are shown. An explanation, substantiated by actual analyses, has been brought forward for the simplest unstable circuit, consisting of three branches connected in Y across a three-phase line with balanced, sinusoidal line voltages, with the neutral unconnected, each branch of the Y consisting of an iron-cored reactance in parallel with a capacity. This explanation is extended to the other cases, two with experimental evidence as justification, and the third by analogy only.