Tan delta vs IR

[Emadshaaban1987]

Hi folks
What is the benefit of conducting tan delts test rather than usual meggering test?
When applying tan delta test is important?

 

[electricpete]

dc insulation resistance test is quick and easy using readily available (cheap, light) equipment.

Tan delta test numbers tend to be more repeatable than the dc tests, so sensitive to small changes, trendable.

The tan delta test also enables computation of a tip-up change in power factor as voltage increases.

There are two variety of tan-delta test, one with power frequency and one with VLF.

The power frequency test can be done with Doble test set (also used on transformers). It will be difficult to test through a long cable.

The VLF test set requires other test equipment (also used on ables). It can test through cable without trouble (the capacitive reactance to ground is much higher at low frequency). EPRI is doing some work with it. They claim in certain circumstances they can distinguish between motor insulation leakage and cable insulation leakage through a combination of two tests, one with cable shields lifted.


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(2B)+(2B)' ?

 

[mparenteau]

Tan delta is good for cables that have been in-service. Same w/ VLF. HiPot is becoming less common (albeit slowly) and only used for new cables.

Mike

 

[VLFit]

The two tests are very different and yield very different results. The "megger" test is just a DC voltage applied to the windings to measure the leakage current from conductors to ground. The problem is it measures many paths of leakage from the hot lead to ground. Unless data is trended accurately over time, the numbers mean little. A DC leakage test is of minimal usefulness, especially if not performed along with many other tests to evaluate the coils.

A Tan Delta test, also known as a Loss Angle or Dissipation Factor test, is performed with AC voltage and measures the dielectric losses of the insulation over time due to degradation. If the insulation is perfect, the load exhibits the properties of a capacitor, with the capacitive current waveform 90 degrees out of phase with the applied voltage waveform. The more degraded the insulation is the more resistive elements of leakage current come through. Rather than a 90 degrees displacement it may be 89.5, which is very measurable and indicative of degraded insulation. The The TD test yields the same numbers as Power Factor test for very small angles of displacement of the capacitive current waveform. For highly capacitive loads like large generators and long cables, VLF technology is used, where a 0.10 Hz. AC power supply is used to apply the voltage while TD readings are taken. Using VLF is an extremely common test worldwide and has been for twenty years for cable testing, longer using 50/60 Hz. for apparatus testing. Refer to IEEE 400.2 for cables and IEEE 433 for rotating machinery.

There is so much more to cover, as this is a mainstream test accepted as routine. Hope this helps.