engleski

Dimensioning Stabilizing Windings for Y-Y Transformers

Stabilizing/equalizer winding is a delta-connected tertiary, not foreseen for loading purposes, often added to the Y-Y connected transformers or autotransformers in order to stabilize the neutral point of fundamental frequency voltages. This is mainly done to protect the transformer and the system from excessive third-harmonic voltages and currents, and to prevent the interference with telecommunication circuits due to third-harmonic currents in the lines and earth return path [1]. In the last few decades, the number of the transformers without stabilizing winding is increasing. Therefore, it is of major importance to discuss when are they necessary and how to dimension them. The necessity of the stabilizing winding depends on the zero-sequence and third harmonic characteristics of the transformer and its compatibility with the network configuration in which it will be installed. Moreover, the events such as non-symmetrical transformer loading, short circuits and various transients have to be taken into account when it comes to dimensioning the stabilizing tertiary winding [2]. Nowadays, the interference with the telecommunication circuit is not a major problem as most of the telecommunication lines are equipped with fiber optics or various filters. Omission of the stabilizing windings can also cause overheating of metal parts, therefore a calculation of the zero-sequence flux and the associated stray flux losses in metal part must be calculated to prevent overheating. Common practice is to specify the rating for the stabilizing for 1/3 of the rated power of main windings. This rating is sufficient to sustain a case in which “a Yyn + d transformer (a transformer provided with an equalizer winding) may have its secondary neutral terminal loaded with current up to the rated current, provided that the delta-connected winding has a power rating equal to at least one-third of the power rating of the secondary winding“ [3], or in simpler wording, a single phase loading with one neutral terminal solidly grounded and the other isolated. This is not necessarily the realistic scenario. To serve its purpose, the stabilizing winding must be dimensioned by two criteria – short-circuit withstand and thermal rating for realistic loading cases of the transformer, for which the allowable asymmetry between phases should be known. Methods for calculation of forces, stabilizing winding temperature rises and metal parts temperature rise in the case of the transformer with and without delta stabilizing winding will be presented in the article.

stabilizing winding, thermal, YY transformers

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