Scottishpower´s experiences of power system ferroresonance

Ferroresonance is a generic term used to describe resonant conditions which may be experienced on either distribution or transmission systems and involves capacitive coupling in series with iron cored inductance. Its occurrence on the system may be random and unpredictable. Whilst there may be many combinations of circumstances where ferroresonance may occur it is more commonly experienced in three specific areas. The first is on distribution systems and is usually associated with live line working where one or more phases may be disconnected and may incorporate sections of overhead line, with cable, terminated with a transformer. The second area is where, on a transmission system, a transformer is teed-off the end of an overhead line with an adjacent live circuit on the same line towers. The third area which may be encountered is where an electromagnetic voltage transformer may be connected via an open circuit-breaker to a short isolated section of busbar. The paper discusses the second condition where an actual ferroresonant condition gave rise to problems. This led to a series of controlled systems tests being undertaken. The paper then discusses the results of these tests and highlights both the need for methods of predicting when the conditions will occur and for the development of economic techniques for switching such conditions