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Description / Abstract:
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
Purpose
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.