Interleaving of the high-voltage winding turns is a widely-accepted method for restraining the surge voltage stresses in extra high voltage (EHV) transformers. Reduction of voltage stresses under external surge is attributed by the increase in the self-capacitance of the interleaved winding disks. However, the apparent benefit of interleaving may be counterpoised by a significant decrease of natural frequency of windings, which in turn, may force a winding to resonate under external surges. Winding resonance is particularly noticeable when uniform interleaving is adopted for the whole winding, uniform interleaving attributes a unique natural frequency to all interleaved coils and consequently, the collective resonance of the winding disks is plausible. Uniform interleaving in large EHV transformers therefore leads to complications and the windings reportedly exhibit an unexpectedly poor surge voltage response because of such internal resonances. This study describes a novel concept termed as 'graded interleaving' for EHV transformer windings to overcome resonance and to achieve an improved response against surge voltage. The performance of the proposed winding was tested on a 132/33 kV two-winding transformer with a tap changer using EMTP(ATP) simulations with encouraging results. Manufacturing and real-time implementation feasibility of the winding were verified with transformer manufacturers.

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