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Tan delta vs IR

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Tan delta vs IR

Tan delta vs IR

Emadshaaban

(Electrical)

(OP)

9 May 21 22:06

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

The company is the world’s best why tan delta test is required supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

Replies continue below

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RE: Tan delta vs IR

electricpete

(Electrical)

10 May 21 11:53

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.


=====================================
(2B)+(2B)' ?

RE: Tan delta vs IR

mparenteau

(Electrical)

10 May 21 13:20

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

RE: Tan delta vs IR

VLFit

(Electrical)

12 May 21 21:14

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.

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News

A large percentage of electrical apparatus failures are due to the deteriorated condition of the insulation. Many of these failures can be anticipated by conducting routine testing and timely maintenance. Changes in the normal capacitance of insulation indicate abnormal conditions such as the presence of a moisture layer, short circuits, or open circuits in the capacitance network.  Tan delta (or dissipation factor) / power factor measurements indicate the following conditions in the insulation of a wide range of electrical apparatus:

• Chemical deterioration due to time and temperature, including certain cases of acute deterioration caused by local overheating
• Contamination of water, carbon deposits, bad oil, dirt and other chemicals
• Severe leakage through cracks and over surfaces

The interpretation of measurements is usually based on experience, recommendations of the manufacturer of the equipment being tested, and by observing these differences:

• Between measurements on the same unit after successive intervals of time.
• Between measurements on duplicate units or a similar part of one unit, tested under the same conditions around the same time, e.g., several identical transformers or on one winding of a three-phase transformer tested separately.
• Between measurements made at different test levels on one part of a unit; an increase in slop (tip-up) of a dissipation/power factor versus voltage curve at a given voltage is an indication of ionization commencing at that voltage.

An increase of dissipation/power factor above a typical value may indicate conditions such as those indicated above. If the dissipation/power factor varies significantly with voltage down to some voltage below which it is substantially constant, then ionization is indicated. If this extinction is below the operating level, then ionization may progress in operation with consequent deterioration. Some increase of capacitance (increase in charging current) may also be observed above the extinction voltage because of the short-circuiting of numerous voids by the ionization process.

An increase of dissipation/power factor accompanied by a marked increase in the capacitance usually indicates excessive moisture in the insulation. Increase of dissipation/power factor alone may be caused by thermal deterioration or by contamination other than water.

Unless bushing and pothead surfaces, terminal boards, etc. are clean and dry, measured values do not necessarily apply to the insulation under test. Any leakage over terminal surfaces may add to the losses of the insulation itself and may give a false indication of its condition.

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