Ferroresonant Overvoltage Caused by Dispersed-Generation Islanding on Power Distribution Line

Ferroresonant overvoltages can occur on an isolated feeder connected to Distribution System Generators (DSGs). This paper analyzed the overvoltages and the following results are obtained. (1) The overvoltages occur on a three-phase four-line wye-grounded system such as 400V class distribution lines but do not occur on a three-phase three-line ungrounded system such as 6.6kV distribution lines. (2) The overvoltages can occur when DSGs are separated from the utility source not at the primary side but at the secondary side of a delta-wye grounded transformer connected to the feeder. (3) The saturated reactances of transformer excitation circuit and the phase-to-neutral capacitances on the isolated feeder eatablish the ferroresonance circuit. (4) The ferroresonance generates the phase-to-neutral voltages of 2.6 pu. The voltages are suppressed to 1.8 pu with installation of surge-absorbers but continue. The DSGs have to be disconnected from the islanding system by overfrequency or overvoltage relays to drop the continuing over-voltages.