scope:

This guide addresses the application of transformers in the presence of oscillatory switching transients. In addition, there is some limited information on shunt reactor switching. 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. Six 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. The guide applies to both medium voltage and high voltage systems and also includes some shunt reactor switching information.

NOTE - The initial publication of this guide did not include high voltage (HV) and extra high voltage (EHV) interaction examples. In reviewing the principles and theory within the guide, the content of this guide applies to both medium voltage (MV) and higher voltage (HV and EHV) systems. For this reason, the guide has been revised to include example material for HV and EHV interactions. In addition, some limited material has been added to give users information about system interactions during shunt reactor switching. These system interactions can produce very fast rising voltages which can cause damage to the transformers primarily near the line terminals. Some information has also been included on the damage that may occur with these very fast voltage transients.

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. In addition to oscillatory transients, transformers damage can also occur from very fast rising voltage transients produced by voltage breakdowns. The voltage distribution during these transients can be significantly different than those during the high frequency oscillatory transients. The transient voltage distribution across the windings and insulation will typically not be linear and will many times be concentrated near the line terminals. Some information on the steep voltage transients has also been included.

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.