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معمای فشار قوی شماره 7 - طول عمر عایق در تجهیزات برق
من دانشجو برق-قدرت هستم.می خواستم اطلاعاتی در باره طول عمر عایقها در تجهیزات برق بهم بدید.این 1تکلیف برای دانشگاه است.لطفا کمک کنید
نویسنده : مسعود
 
#1
ساعت: 11:53 - تاریخ: 22 آبان 1390
گذر زمان بویژه در مورد عایقهایی که تحت تنش میادین الکتریکی واقع می شوند با تغییر تدریجی در ساختمان شیمیایی عایقها بر خواص مکانیکی و الکتریکی آنها اثر می گذارد.  روشهای متعددی برای مونیتورینگ شرایط عایقی عایقهای تحت سرویس وجود دارد که در نشریات تخصصی مرتبط با IEEE یا CIGRE بدانها پرداخته شده است. برای مثال آنالیز گازهای غیر محلول روغن ترانسها،آزمایش تلفات عایقی و خازنی روغن در فرکانس نامی، اندازه گیری مقاومت عایقی روغن و شاخص پلاریزاسیون آن از آزمایشهای معمول برای ارزیابی کیفیت عایقی روغن ترانس با گذر زمان و تأثیر عمر عایق بر کیفیتش می باشد.  امروزه روشهای جدیدی نیز برای ارزیابی تأثیر عمر عایق بر عملکرد آن معمول است که در زیر مختصراً به آنها اشاره می شود.

New methods for transformer in-service monitoring include: Furan Analysis and HPLC (High Performance Liquid Chromatography) to quantify chemical ageing products and the Dielectric Response Analysis (DRA), the fundamentals of which have been treated above. The methods related to DRA are briefly explained below.

Dielectric response analysis (DRA)
This method is based on the measurements of polarization as well as depolarization currents and is thus sometimes described as the ‘PDC method’. It should be noted that the dielectric response function f(t), if quantified for times larger than several milliseconds, also provides access to the frequency-dependent dielectric parameters starting from ultra-low frequencies up to power frequencies.
Recovery voltage
The measurement of recovery or return voltages is another method to quantify the dielectric response of materials. The principle of the measurement can be traced back to the last century. With reference to Figure below, it can be explained as follows: a constant voltage Uc charges the test object for 0 < t < t1; after a relatively short period between t1 < t < t2 during which the sample is shortcircuited, the test object is left in open-circuit condition. Then for times t > t2 a recovery voltage ur(t) caused by residual polarization is built up across the test object, resulting in repolarization. If the voltmeter recording ur (t) has an extremely high input impedance, the test object remains charged until it discharges through its internal resistance. If the dielectric response function of the test object is known, the time dependence of recovery voltage for t > t2 can be derived from equation below.



The advantage of this method is that it includes a self-calibration with respect to the capacitance of the test object, but for numerical evaluations the response function has to be measured as shown previously.

The ‘polarization spectrum’
The so-called ‘polarization spectrum’ is a quantity derived from a special measurement procedure of recovery voltages. With reference to above Figure, it is determined from the peak values (at time instants tr) of many recovery voltages, which vary depending on charging duration Tc. These recovery voltage maxima are then presented as a function of increasing charging duration.
Normally the charging time Tc is chosen to be twice as long as the discharge duration, Td. It can be shown that the peak of the ‘polarization spectrum’ will be reached for a very dominant time constant t of a Debye process. This measurement method, for which a commercial equipment exists, has often been applied in practice during recent years with the goal of identifying the moisture content of the pressboard within power transformers. The interpretation of the results, however, is in general very difficult. Misleading results can be obtained if the moisture content is quantified in accordance with instructions provided by the manufacturer.

 
نویسنده : حمید - از: فیروزآباد فارس
 
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