Overvoltage Protection of Current-Transformer Secondary Windings and Associated Circuits

It has long been recognized that excessive potentials may be developed in current-transformer secondary windings under unusual conditions, such as open circuits. Recent experience discloses that dangerous overvoltages (several thousand volts) may be produced as a result of normal switching operations on circuits containing lumped capacitance. A simple procedure for circuit analysis and evaluation of approximate voltage magnitude for the switching transient case is reported. For easy reference, there are included tables of calculated secondary voltage magnitudes covering a broad range of application. Under certain conditions, overvoltage protection is desirable and important. Aside from the potential hazard to life, current-transformer circuit insulation may be damaged, yet not be evident immediately. Performance at normal rated current may not be noticeably impaired, yet serious failure may occur in the presence of fault-current flow, thus nullifying the action of current-actuated protective relays. The characteristics of a new overvoltage protector expressly designed for current-transformer protection is presented. With this device current-transformer secondary voltages are limited to moderate values. The protector is small and compact, and easily applied to existing as well as new current-transformer installations The characteristics are permanent, not affected by repetitive operation, and result in negligible ratio error in the normal operating current range.