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معماي ترانسفورماتور شماره 6- مدار توالی صفر در ترانسفورماتورهای قدرت
نویسنده : شهبازی - از: ایران
 
#1
ساعت: 17:58 - تاریخ: 12 تیر 1386
ترانسفورمر "شل" دارای مسیر برگشت آزاد فلوی مغناطیسی است ولی این مسیر برای ترانسفورمر "کور" هوا ، روغن و ورق بدنه ترانسفورمر که دارای رلوکتانس بالایی هستند مي باشد . در ترانسفورمر "شل" حتی به ازاء  مقدار کمی بار نامتعادل یا بار تکفاز نقطه نیوترال شدیداً جابجاشده ولتاژ فاز بار شده را کوچک می کند و ولتاژ دو فاز دیگر را بزرگ می کند و همچنین باعث نوسان نقطه نیوترال می شود . در ترانسفورمر "کور " این بار تکفاز تا حدی قابل تحمل است و نقطه نیوترال را خیلی زیاد جابجا نمی کند . هر چند باعث گرم شدن غیر عادی ترانسفورمر می شود ولی تا حد 10%توان نامی قابل تحمل است . 
#2
ساعت: 16:51 - تاریخ: 13 تیر 1386
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
 
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