Ferroresonance During Single-Phase Switching of 3-Phase Distribution Transformer Banks

It is possible to obtain ferroresonant overvoltages during single-phase switching of distribution transformer banks. When energizing the bank with single-pole devices at higher distribution voltage, the internal winding capacitances of ungrounded wye-delta banks can react with the bank saturation characteristics producing high sustained overvoltages. Transient network analyzer (TNA) studies have determined the necessary conditions for the occurrence of the phenomena and have investigated remedies. One promising solution is the substitution of wye-wye grounded banks for the ungrounded wye-delta banks on 19.9/34.5-kV distribution circuits.