Development of New Winding Design Techniques for Superior Surge Performance of High Voltage Transformers

Abstract

Transformers operating in the high and extra-high voltage power systems are frequently subjected to over-voltage produced by natural lightning phenomenon and switching operations, both of which may produce transient over-voltage in high voltage transformers. Such over voltages, although transient or short-term in nature, may subject the transformer windings to high voltage stresses, which may even lead to their dielectric failure. Windings of a transformer are conventionally protected by external protective devices like surge arrester installed at the transformer s terminal. Also, the windings of a transformer need to be insulated adequately to withstand such occasional over-voltages. However, the external protective devices are not always reliable and effective, and often the residual voltage of such protective devices can be high enough to cause breakdown of internal insulation in transformers. On the other hand, enhancement of the winding insulation invariably increases the cost of the transformer, apart from worsening heat-dissipation by the conductors. Therefore, efficient design of the winding is considered to be the most perceptible solution against internal over-voltage and dielectric failure in high voltage transformers. It is imperative, that adoption of special winding techniques is the key to control and mitigate the voltage stresses in transformer winding insulation to warrant fail-safe operation under plausible external disturbances. newline Interleaving is one common winding technique, which is widely used in the transformer industry for voltage stress control in high voltage transformers. Shielded winding is another alternative approach, which has also been proposed by several researchers in varied forms, to safe-guard transformer winding from transient over-voltages. However, the present research work highlighted that conventional interleaved winding , or the shielded winding may offer limited benefits under oscillatory transient over-voltages. Even worse, some of these methods may cause reductio