Interleaved winding and suppression of natural frequencies

Abstract

Ideally, it is desirable to design and manufacture a transformer winding that can render all its internal resonances nonexcitable. This study examines the effectiveness of an interleaved winding in achieving this goal. While investigating its effectiveness, it led to the establishment of a much desired theoretical basis that reinforces the reasons put forward in the literature to explain internal insulation failures observed in interleaved windings used in extra high voltage (EHV) transformers. Numerical calculations along with experimental verification on actual transformer windings are presented. This study reveals that most of the natural frequencies that are normally non-excitable in the line and neutral current responses of an interleaved winding have been rendered excitable in the disk-to-disk voltages, thus, providing favourable conditions for insulation overstress because of resonant overvoltages. Prevalence of such a condition is an inherent characteristic of interleaved windings.

Enhancing physical resolution of constructed ladder network for an interleaved winding

“…Here, a 7section model was constructed compared to a 4-section model. Unlike the DPA, the high frequency poles in the voltage ratio spectrum are not closely associated with their neighboring zeros which would cancel the corresponding resonance peaks [5] and hence, three additional peaks are observable in Fig. 3.…”

“…In an earlier work [5], it was found that the normally suppressed natural frequencies in an interleaved-disk winding when seen through its line or neutral current, were rendered excitable when seen in its disk-to-disk voltages. This was a unique feature of the interleaved winding's behaviour.…”

“…(Note: Information about 'zeros' of ADPA corresponding to the appended 'poles' is unavailable from the voltage ratio spectrum. Given the fact that these natural frequencies are rendered non-excitable in the DPA function, it is appropriate to initially assign a value to 'zero' which is numerically very close to the immediate neighboring 'pole' [5]. Furthermore, while doing so, the alternating arrangement of pole and zero must be strictly adhered to.…”

“…Typically, it is desirable to construct a ladder network wherein at least one or two pair of disks would correspond or map onto one section of the ladder network. Although, in case of the continuous-disk windings this resolution achieved is higher (as relatively a larger number of natural frequencies are observable in its frequency response plot), the same is certainly not the case for interleaved-disk windings, wherein a maximum of 2-3 natural frequencies are observable [4,5]. So, the physical resolution achievable, especially for interleaved windings, by this method of approach is very poor, thus, defeating the very purpose of this exercise.…”

Modelling of foil‐type transformer windings for computation of terminal impedance and internal voltage propagation

“…The accurate modelling of transformer windings at high frequencies was performed by many researchers [1]. Equivalent electrical circuit representations of either distributed [2][3][4][5][6][7] or lumped R, G, L and C parameters [8][9][10][11][12][13][14][15][16][17][18] have been successfully developed. These models require detailed winding data and core geometry.…”