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معمای ترانسفورماتور شماره 18 - بارگذاری ترانسفورماتور
در صورت وجود اختلاف بین دمای محیط و ماکزیمم دمای طراحی ترانس، ظرفیت باردهی آن چگونه محاسبه می شود؟ نحوه صحیح دریافت اضافه بار از ترانسفورماتورها چگونه است؟
نویسنده : بهنام - از: فسا
 
#1
ساعت: 13:24 - تاریخ: 24 خرداد 1389
بر اساس آخرین ویرایش IEEE ماکزیمم ظرفیت بار دهی یک ترانس می تواند تا 200 درصد ظرفیت قید شده Name plate آن افزایش یابد.  IEEE C57.91-1995 معادلات و جداول مرتبط با چگونگی این تغییرات را مشخص می نماید. با این حال به عنوان یک تقریب خوب می توان یک افزایش یا کاهش 1 درصدی در ظرفیت بار دهی ترانس به ازای 1 درجه سلسیوس افزایش یا کاهش دمای محیط در نظر گرفت. برای مثال میزان باردهی ترانسی که برای دمای محیطی 30 درجه طراحی شده در دمای 40 درجه حدود 10 درصد کاهش می یابد.
در صورتی که hottest-spot دمای هادیهای ترانس از 110 درجه تجاوز نکند یک رابطه لگاریتمی بین طول عمر ترانس و تغییرات دمای کارکرد آن وجود دارد. بطور سر انگشتی به ازای هر 8 درجه کاهش دمای کارکرد ترانس طول عمر آن دو برابر می شود.
در مورد اضافه بار ترانس ،بطور کلی اپراتور مجاز به افزایش بار ترانس تا 200 درصد ظرفیت نامی برای مدت زمان محدود و hottest-spot حدود 180 درجه می باشد. جدول زیر نیز نحوه اضافه بارگذاری توصیه شده در ترانسفورماترهای قدرت را نشان می دهد.



The loading recommendations in above table assume that the load is summer and winter peaking, that the fall and spring peak is approximately 60% of summer peak, that the average ambient temperature on a normal summer day is 30˚C, that the daily load profile is similar to the cyclical load profile described earlier, that the winter peak is less than 120% of the summer peak, and that noncyclical loading is constant except for downtime.
1. The normal summer loading accounts for periods when temperatures are abnormally high. These might occur every three to five years. For every degree that the normal average ambient temperature during the hottest month of the year exceeds 30˚C, derate the transformer 1% (i.e., 129% loading for 31˚C average ambient).
2. The percent load is given on the basis of the current rating. For MVA loading, multiply by the per unit output voltage. If the output voltage is 0.92 per unit, the recommended normal summer
MVA loading is 120%.
3. Exercise caution if the load power factor is less than 0.95 lagging. If the power factor is less than
0.92 lagging, then lower the recommended loading by 10% (i.e., 130% to 120%).
4. Verify that cooling fans and pumps are in good working order and oil levels are correct.
5. Verify that the oil condition is good: moisture is less than 1.5% (1.0% preferred) by dry weight, oxygen is less than 2.0%, acidity is less than 0.5, and CO gas increases after heavy load seasons are not excessive.
6. Verify that the gauges are reading correctly when transformer loads are heavy. If correct field measurements differ from manufacturer’s test report data, then investigate further before loading past nameplate criteria.
7. During heavy load periods, verify with infrared camera or RTD that the LTC top-oil temperature relative to the main tank top-oil temperature is correct. For normal LTC operation, the LTC top oil is cooler than the main tank top oil. A significant and prolonged deviation from this indicates LTC abnormalities.
8. If the load current exceeds the bushing rating, do not exceed 110˚C top-oil temperature (IEEE,
1995). If bushing size is not known, perform an infrared scan of the bushing terminal during heavy load periods. Investigate further if the temperature of the top terminal cap is excessive.
9. Use winding power-factor tests as a measure to confirm the integrity of a transformer’s insulation system. This gives an indication of moisture and other contaminants in the system. High-BIL
(basic insulation level) transformers require low winding power factors (<0.5%), while low-BIL transformers can tolerate higher winding power factors (<1.5%).
10. If the transformer is extremely dry (less than 0.5% by dry weight) and the load power factor is extremely good (0.99 lag to 0.99 lead), then add 10% to the above recommendations.


 
نویسنده : حمید - از: فیروزآباد فارس
 
#2
ساعت: 11:29 - تاریخ: 31 خرداد 1389
The rated overload of a transformer depends on the transformer’s previous load or
the corresponding oil temperature at the beginning of the overloading. The permissible
duration and the respective levels of the acceptable overload of commercial
oil-immersed distribution transformers are shown in Table below (Schneider
Electric AE 2002). For example, if the transformer is loaded to 50% of its rated
power continuously, then the transformer can be overloaded to 150% of its rated
power for 15 minutes or to 120% of its rated power for 90 minutes, as shown in
Table.
It should be noted that the oil temperature is not a safe measure for the winding
temperature, since the time constant of the oil is 2 to 4 hours, while the time constant
of the winding is 2 to 6 minutes (Schneider Electric AE 2002). Therefore, the
determination of the permissible duration of the overload must be done very carefully,
since there is a danger that the winding temperature exceeds the critical
temperature of 105°C, without this being indicated by the oil temperature.


 
نویسنده : حمید - از: فیروزآباد فارس
 
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