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Reply To: معماي ترانسفورماتور شماره 6- مدار توالی صفر در ترانسفورماتورهای قدرت

#1343
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    For a three-phase three-limb core, since fluxes in three limbs are in the same direction, they have to return through a path outside the core. In this case, tank acts as an equivalent delta winding, and the magnetizing zero-sequence reactance is the reactance between the tank and excited winding. Since the gap/area between the excited winding and tank is much larger than the gap/area between the windings, the magnetizing zero-sequence reactance is considerably higher than the positive-sequence leakage reactance. The tank influences the zero-sequence reactance in the following way. It provides a comparatively lower reluctance return path (as compared to air) to the zero-sequence flux, which has the effect of increasing the reactance; on the other hand the tank enclosing three phases acts as a short-circuited winding reducing the reactance. In a three-phase five-limb core, the zero-sequence flux has a return path through the end yokes and end limbs and hence the zero-sequence magnetizing reactance has the same value as the corresponding positive-sequence magnetizing reactance (quite a high value) unless the voltage applied is such that yokes and end limbs saturate. For an applied value of zero-sequence voltage, close to the rated voltage, yokes and end limbs will get completely saturated (area of yokes and end limbs are too less to carry the zero-sequence flux of all the three phases at the rated voltage) giving a lower value of magnetizing zero-sequence reactance close to that of a transformer with three-phase three-limb core. For single-phase three-limb cores, since the zero-sequence flux has a return path through end limbs, the zero-sequence magnetizing reactance is equal to the positive-sequence magnetizing reactance (~infinite value). Thus, for a three-phase bank of single-phase transformers, the zero-sequence reactance for any type connection is usually equal to the positive-sequence reactance