Guide to Describe the Occurrence and Mitigation of Switching Transients Induced by Transformers, Switching Device, and System Interaction
|Publication Date:||9 December 2010|
This guide addresses the application of transformers in the presence of oscillatory switching transients. These oscillatory transients are typically produced by the interaction of the switching device, transformer, load, and system. This guide defines operating conditions that may produce switching voltages damaging to the transformer insulation system. It discusses the electrical characteristics of the system source, switching device, transformer, and load and the nature of their transient interaction. It outlines several mitigation methods. Two examples are included.
This guide recognizes that many devices and/or system operations can produce oscillatory transient waveforms. The focus of this guide is on the interaction between a transformer, the system, and a switching device as a result of several reports of transformer internal winding failures.
This guide focuses only on mechanical switching devices and does not cover semiconductor switching devices.
Transformers that are inductively loaded (reactors, or stalled or accelerating motors) are not covered by this document but are addressed by several other referenced publications
When a transformer is switched into or out of a system, the transient voltage produced at the terminals of the transformer may contain several high frequency oscillatory components. When this oscillatory transformer terminal voltage has a frequency component near one of the natural frequencies of the transformer and is of sufficient magnitude and duration, permanent damage to the transformer internal insulation structure may result.
The purpose of this guide is to provide aid in the recognition of conditions and applications where transformers are subjected to terminal oscillatory switching transients that may produce internal winding voltages damaging to their internal insulation structure. This is accomplished by developing necessary expressions that characterize this interaction. This guide also characterizes the transformer, switching device, system, and load during this type of event. Many of the parameters that influence this system are presented, to assist the user in developing an understanding when a potential concern exists. Finally, several mitigation methods are outlined.