Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Electrical Engineering


As far as electric utilities are concerned, ferroresonance is undesirable on distribution or transmission systems because it may result in either transient or sustained over voltages under certain conditions. Lightning arrestor flashover, excessive transformer hum, transformer damage, cable damage, and phase reversals have been attributed to ferroresonance. The probability of ferroresonant conditions resulting in damage to equipment has only relatively recently risen to an appreciable level. The difference lies in the fact that the farroresonant circuit is nonlinear due to the inductive component. This component is a saturable core reactance, which has its impedance value dictated by the saturation characteristic of the iron core. This nonlinear element accounts for the fact that three modes of irregular voltage may be possible on the same circuit. One mode is an unstable, transient overvoltage that quickly disappears. A second mode is a sustained voltage of nearly normal value. The third mode is a sustained overvoltage that can reach destructive values under certain conditions, and this high mode represents a ferroresonant overvoltage condition. If a ferroresonant circuit appears within a power system, the inductive component will in most oases be the winding of an unloaded transformer. An early report of 60 years ago attributes over voltages on a test circuit to ferroresonance. The subject was apparently not of great concern until after World War II as evidenced by the amount of attention it has received within the past 25 to 30 years. Several tests have been conducted on ferroresonance, including network analyzer studies, actual field tests on existing equipment, and extensive laboratory tests. Although the subject has been covered in quantity, there are at least two good reasons for researching ferroresonance. The first reason is supported by the complexity of the subject. Ferroresonance is affected by several factors, many of which are not readily apparent in most of the available literature. One goa1 of this thesis will be to present a discussion of ferroresonance that is more clear and complete. The second reason lies in the fact that the prediction of ferroresonance, using system parameters, does not appear to be entirely satisfactory. The second goal of this thesis will be to present a fresh approach to this problem via a computer solution. The computer program will be an attempt to solve for ferroresonant over voltages given information about the primary cable and transformer involved. A solution will be presented for one situation only. This will consist of an unloaded, ungrounded wye-connected transformer bank supplied by a length of underground cable with only one phase energized. If the solution is accepted for this case, it could be applied to other situations.

Library of Congress Subject Headings

Transients (Electricity)
Electric transformers



Number of Pages



South Dakota State University