ماهو فرق بين قياس مادة العزل ميجا اوم AC , DC

[محمدالسعودية]

السلام عليكم :عندي سوال ارجو منكم مساعدتي


عندي ستيم بويلر اريد قياس مادة العزل MHOMS .... سوال هل في فرق بين قياس ميجا اوم ب AC,DC

[محمد سعادة]

السلام عليكم
قبل الاجابة على السؤال
لماذا تريد قياس مقاومة العزل لمعدة غير كهربية ؟ وبالتالى خصائص مادة العزل مختلفة فهي غير مصممة لعزل اجزاء كهربية حية رجاء التوضيح

[محمد سعادة]

[LEFT][CENTER][LEFT]

الاجابة من كتاب ELECTRICAL INSULATION
FOR ROTATING MACHINES
Design, Evaluation, Aging,
Testing, and Repair
The major differences between the DC and AC tests are the test voltage applied and the
voltage distribution across the groundwall insulation. Both are linked. With DC voltage, the
voltage dropped across insulation components within the groundwall and in the end-winding
depends on the resistances (resistivity) of the components. Components with a lower resistance
will have less voltage dropped across them. In contrast, the AC voltage dropped across
each component in the groundwall or in the end-winding depends on the capacitance (dielectric
constant) of each component. Thus, there tends to be a completely different electric stress
distribution across the groundwall between AC and DC tests. In older insulation systems,
particularly asphaltic mica systems, the differences between the AC and DC stress distributions
were less pronounced because of the finite resistivity in older groundwalls due to the
absorption of moisture. However, with modern epoxy–mica insulations, the resistivity is essentially
infinite; thus, the DC voltage may all be dropped across a very thin layer of insulation.
Consequently, significant flaws that might cause puncture may escape detection with a
DC test but would be easily detected with an AC test because of the more uniform voltage
distribution with AC stress.
For modern windings, the AC hipot test yields an electric stress distribution that is the
same as that which occurs during normal operation. Consequently, the AC hipot test is more
likely to find flaws that could result in an in-service stator failure if a phase-to-ground fault
occurs in the power system, causing an overvoltage in the unfaulted phases. For this reason,
the AC hipot is considered superior to the DC hipot, especially with modern thermoset insulation
systems.
In the 1950s there was considerable research on the relationship between AC and DC
hipot tests, specifically, the ratio of the DC to AC hipot voltages . Eventually, a
consensus was reached that, under most conditions, the DC breakdown voltage is about 1.7
times higher than the AC rms breakdown voltage. This relationship has been standardized in
IEEE 95. This research was based on older insulation systems and, unfortunately, is largely
irrelevant in modern insulation systems, since, as described above, the voltage distribution is
completely different under AC and DC. There have, however, been a few studies of the relationship
between AC and DC breakdown in modern groundwall insulation systems. One of
the largest of these studies pointed out that the ratio of DC to AC breakdown voltage on average
was 4.3 in epoxy–mica insulation . The 1.7 factor, then, no longer seems to be
valid, but since the variability is so large, no replacement ratio has been proposed. Thus, we
are left with 1.7.[]

[محمد سعادة]

السلام عليكم
ملحوظة هامة
المشاركة السابقة بتتحدث اساسا عن الفرق بين اختبارات ال high pot tests والفرق بين استخدام جهود ac و dc فيها وهى اختبارات commissioning فى الاساس وتختلف طبعا هم اختبارات قياس مقاومة العزل الي بيستخدم فيها جهد ال dc فقط لان في النهاية بيتم قياس مقاومة اومية ولكن الشرح السابق بيدي فكرة عامة عن الفرق بين استخدام الجهدين
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