Predetermination of lateral length to prevent overvoltage problems due to open conductors in three-phase systems

The opening of one or two conductors in three-phase circuits supplying an ungrounded transformer produces a steady-state overvoltage due to ferroresonance between the lateral capacitance and the transformer nonlinear inductance. Available methods of determining the conditions under which such an overvoltage may occur are mostly based on experimental studies, and hence are limited in application to systems whose design parameters are similar to those used in the tests. A simple and elegant approach, presented here, reveals that an incorrect choice of overvoltage limit leads to the selection of an unsafe lateral length, resulting in a potentially dangerous high voltage across the connected apparatus and protective equipment. Generalised mathematical equations are developed to determine the safe lateral length as a function of transformer size for the prevention of the overvoltage problem due to open conductors in three-phase systems. The computed results are in good agreement with experimental observations.