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:

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• 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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Tan Delta test kit rating

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Tan Delta test kit rating

Tan Delta test kit rating

power

(Electrical)

(OP)

20 Jun 06 09:31

1. Any one pl. suggest us that how to calculate the current rating of Tan-Delta test- kit for testing of 16.5kV, 260MVA Generator / 400/16.5kV 260MVA transformer (for both HT and LT).

2. For transformer bushing tan-delta testing, why should not inject rated voltage of the busing?

Advanced thanks,

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RE: Tan Delta test kit rating

electricpete

(Electrical)

20 Jun 06 10:02

2 &#;
2A - For one thing you are limited by the capability of your test set, generally 10kvac.
2B &#; For another thing you want to compare your results to other tests at standard voltage, generally 10kv or less.
2C - It's been awhile since I looked at the setup of the test taps. I remember there were two different varieties. If you did have the capability to put full line-to-ground voltage, you want to doublecheck to make sure you are not overstresing portions of the insulation system.  Maybe someone else can refresh my memory on this.

1 - As I rememer, the Doble test set was limited to 300 milliAmps at 10kv test voltage. (3kva). If the device you are testing draws more current than the capability of the test set, then you have to reduce your test voltage to remain within the capability of the test set.2 &#;2A - For one thing you are limited by the capability of your test set, generally 10kvac.2B &#; For another thing you want to compare your results to other tests at standard voltage, generally 10kv or less.2C - It's been awhile since I looked at the setup of the test taps. I remember there were two different varieties. If you did have the capability to put full line-to-ground voltage, you want to doublecheck to make sure you are not overstresing portions of the insulation system. Maybe someone else can refresh my memory on this.

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RE: Tan Delta test kit rating

edison123

(Electrical)

21 Jun 06 05:42
I use 12 KV inductors in parallel with the gen/motor windings get my 100 mA TD kit up to 3 Amps.

I use 12 KV inductors in parallel with the gen/motor windings get my 100 mA TD kit up to 3 Amps.

* Basically, I would like a full-time job on part-time basis *

RE: Tan Delta test kit rating

power

(Electrical)

(OP)

21 Jun 06 08:03

1st queiry is clear.
Pl discuss regarding 2nd queiry.
Thanks...

RE: Tan Delta test kit rating

prc

(Electrical)

22 Jun 06 02:47

No problem .You can inject full rated voltage to bushing and measure tan delta at  that voltage.But then you require a standard capacitor of that voltage to measure tandelta.Doble kit normally is with a 10 kV capacitor and hence measuremet at that voltage.In fact bushing manufacturer will be measuring tan delta not only at rated voltage, but also at full test voltage.

RE: Tan Delta test kit rating

power

(Electrical)

(OP)

23 Jun 06 09:09

Thanks Mr. PRC, I have seen in many sites that Doble test of HV bushings is carried with 2.5kV however Doble kit rating is upto 10kV. Any specific reason?

RE: Tan Delta test kit rating

prc

(Electrical)

24 Jun 06 01:01

Doble kit is provided with a 12 kV power source and hence all site testsare done at voltages below this.In case you want at a higher voltage, you require a voltage source, a standard capacitor and an additional instrument accessory from Doble.
Reason of selecting 2.5 kv may be availability of earlier bench mark values at that voltage.Some utilities have standardised at 10 kV.

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