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Power Transformer testing - tan delta test

01/31/2009 6:04 PM

Recently, we did an acceptance test of our newly delivered brand new mobile substation our company bought. One of the insulation tests we performed on the power transformer is the tan delta test. The test showed 0.457% at 60 hz.

Now, NETA and DOBLE standards recommends that for new transformers the dissipation factor or insulation power factor should be 0.5% or below.

Additionally, our existing power transformers aged 10+ years have lower power factors in the range of 0.25% - 0.35%.

Based on the NETA and DOBLE standards it is a passing mark. We had some reservations on the test results since it is almost 0.5%. Our old transformers faired better.

Should we accept the mobile sustation? What do you think?

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Anonymous Poster
#1

Re: Power Transformer testing - tan delta test

01/31/2009 8:26 PM

What you are actually testing is the moisture content of the dielectric or insulation. The reason that your older transformers tested at a lower percentage is because thet had been in use and the heat generated caused more moisture to be driven from the insulation thereby power factor to be lower. When the transformers are built, especially the large ones, they have a vaccuum of under 10 microns pulled on them and then hot oil is circulated on them to remove as much moistrue from them as possible.

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#4
In reply to #1

Re: Power Transformer testing - tan delta test

02/01/2009 2:26 AM

Tnx a lot!

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Engineering Fields - Power Engineering - Siswanto

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#2

Re: Power Transformer testing - tan delta test

01/31/2009 8:51 PM

Dear myn1996

If you have concern to Tan Delta Test only , its very difficult to made a decision, because its still in acceptable limit value even its in worst limits.

May you must concern also to the insulation acceptance test parameters if the other winding parameter test (TTR, Winding resistance, Polarity, Vector group, etc) are good, the insulation test require, are:

- Voltage breakdown

- Insulation resistance / PI

- Induced test

- Capacitance test

- SFRA

- DGA test even for new oil and new winding its concern to water absorption content

Be careful and good luck…..

Rgds

Siswanto

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Engineering Fields - Power Engineering - Siswanto

Join Date: Mar 2008
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#3

Re: Power Transformer testing - tan delta test

01/31/2009 10:48 PM

Dear myn1996

Following explanation may can help you to made a decision , its taken from literatures...

1. Oil condition assessment

Transformer oil degradation comes largely from decomposition, contamination and oxidation.

For oil condition assessment, some test items, their limit values and testing methods are listed in

Table 1. Other tests are also being used in industry but not listed here. The principle of using

these tests can be found in various publications.

Table 1 Power Transformer Oil Condition Assessment Tests

a) Dielectric breakdown voltage indicates the presence of electrically conductive contaminants in oil.

b) Interfacial tension and acid number (sometimes called neutralization number or acidity) are affected by

oxidation and contamination

c) Water content is temperature dependent..

d) Power factor is also temperature dependent..

e) PFVO stands for Power Factor Valued Oxidation. SFL stands for Sludge-Free Life. PFVO/SFL is a Doble test unit.

In this study, IFT, KOH, H2O and PF are used to implement an AI based oil condition assessment, because these tests are relatively easier to perform, and their results often correlate closely with each other. Actually, IFT test is an excellent means of detecting oil-soluble polar contaminants and oxidation products, KOH is a measure of the acidic by-products of the oxidation, H2O is a major contaminant, and PF is an overall indication of the oil deterioration and contamination. Although they may yield an oil condition assessment individually, their proper combination could lead to a more precise conclusion.

A fuzzy model is developed to do the job. This is a fuzzy inference system aims to combine all the aforementioned information.

First, a fuzzy membership function was defined for each test. The input of the function is the outcome of the test, and the output of the function is an index representing the condition of the oil. The indices are a number in the range of 0 to 1, where 1 means good oil. These functions are shown in Figure 1 and they are monotonic fuzzy logic transfer functions.

Then, an unconditional fuzzy proposition based on Equation 1 was used to combine these indices, yielding an overall oil condition assessment index.

Eq. 1

When Idx_oil_condition is 1, the oil is in good condition. When Idx_oil_condition is changing towards 0, the quality of oil becomes poor.

It must be emphasized that in Figure 1 (c) and (d) the H2O and PF inputs need be the values at 25°C, otherwise the conclusion could be wrong.

Water has higher solubility in transformer oil at higher temperature [Gri881]. Unless water is saturated in oil and the oil temperature is dropped, there is no danger of free-water forming, which is a major concern because free-water can increase the risk of electrical breakdown significantly. Therefore, high water content at high temperature is allowed as long as it is not saturated.

Measured PF at a temperature other than 25°C must be adjusted using a correction factor before being used, because PF of transformer oil is highly temperature dependent [Gri87].

A sample correction factor vs. temperature curve is given in Figure 2.

Figure 1 Transformer Oil Condition Assessment

Figure 2 Transformer Oil Power Factor Correction Factor

2. Solid Insulation Condition Assessment

Solid insulation of transformers refers to paper insulation of windings and leads, and pressboard mechanical supporting parts. The requirement for solid insulation is high electrical and mechanical integrity. New transformers usually meet the requirement.

Aged transformers may or may not, due to deteriorations such as decomposition, developing of voids/gaps, and being burnt away. Tests to assess the insulation condition are listed in Table 2. Principles behind these tests are complex and some of them are still under investigation

Table 2 Tests for Power Transformer Solid Insulation Assessment

Induced voltage tests are live tests, which means that the transformer needs to be energized.

Among them, RIV is the most preferred method, while PD and AE can be on-line applications. The problem with on-line PD and AE tests is interferences. Therefore, in this study only the result of RIV test was used.

Insulation resistance tests are basically off-line DC test. Test results are influenced by environmental factors such as temperature and humidity, and also by external leakage paths such as contaminated insulators and bushings. Nevertheless, polarization index was used in the study.

Power factors of solid insulation are often tested to detect inside water buildup [Griffin89]. The 0.5% limit is for new transformers and was considered to be valid for aged transformers as well in this study.

Furfural test of transformer oil and degree of polymerization (DP) test of paper samples drew great attention in the last decade

Furfural test is relatively convenient because the oil sample can be obtained while the transformer is in operation.

DP test needs a paper sample obtained directly from the winding insulation, which may damage the integrity of the transformer insulation. It is therefore not preferred in most cases. A correlation curve between the furfural concentration in oil and DP was found by Burton [Burton85] and verified by Corvo [Anto91], which may partly solve this problem. The relationship can be written as:

( eq. – 2 )

DP 187.5log (FUR) 487.5

Where FUR denotes the ppm concentration of 2-furfuraldehyde, or simply 2-furfural, the most common and abundant furan derivative present in oil samples.

Some researchers used the amount of CO + CO2 in oil to predict the solid insulation life of transformers [Kgoto90, Kawa91], but this is not a definitive technique because these gases can present simply because the oxidation of transformer oil or paper involved discharging activity.

The same as oil condition assessment, a set of indices can be defined for solid insulation condition assessment. These are also fuzzy membership functions, as shown in Figure 3. It should be noted that the index function for PI is based on [C57.125], and the index function for 2-furfural is based on all the experiences obtained from the literature. The transition part of the latter is described as:

( eq - 3 )

The same as oil condition assessment, an unconditional fuzzy proposition based on Equation 4, was used to combine the indices, yielding an overall solid insulation condition assessment index.

( eq - 4 ).

Idx _ paper _ condition = min(Idx_RIV,Idx_PI, Idx_DP,Idx_FUR)

When Idx_paper_condition = 1, the solid insulation is healthy. When Idx_paper_condition is close to 0, the solid insulation is weak.

Figure 3 Transformer Paper Insulation Condition Assessment

Good Luck

Rgds

Sis

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#5
In reply to #3

Re: Power Transformer testing - tan delta test

02/01/2009 2:36 AM

Dear Siswanto:

We tested the power txfr for TTR, Winding Resistance, SFRA, Excitation Current, Tan delta and oil dielectric breakdown voltage. All tests registered passing marks. This is our first time to do acceptance testing on a mobile substation so we're wary about making mistakes.

Thank you very much for the quite lengthy and detailed answer. It's really helpful. You really took time to explain it. I really appreciate it.

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#11
In reply to #3

Re: Power Transformer testing - tan delta test

04/19/2011 1:39 AM

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#6

Re: Power Transformer testing - tan delta test

02/02/2009 9:31 AM

If the tan delta recorded has been converted at 20 degree centi. then it is a matter of concern.It only shows that the unit has not been processed /handeled after processing properly.

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Anonymous Poster
#7

Re: Power Transformer testing - tan delta test

02/17/2009 5:04 PM

HI,

I have just had a very similar experience & almost the same results. Upon investigation the source of the problem was a high moisture content in the LTC barrier board. It was decided that the transformer was not acceptable at these levels. We insisted on a replacement LTC and when the 2nd unit arrived we did some basic "dry" testing & found it to be very much dryer than the 1st unit.

We have not finished the replacement yet but expect that the end result will be a good Tan delta result.

I strongly sugget you do not accept the result - it will cause problems over the long term. You should ask the supplier ti fully investigate the source and provide you with an action plan for rectification. Mobile substations are often used in an emergency for when another transformer has failed. The last thing you need is to put in a unit that is suspect as you could end up with a doubling of your problem.

Good Luck - KRW43

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#8
In reply to #7

Re: Power Transformer testing - tan delta test

02/17/2009 5:16 PM

Thanks a lot for all your help. I appreciate it a lot.

Anyway, during the SFRA test the txfr failed due to severe deformation in the x0-x3 sec winding plus the impact recorder showed a 10G vertical acceleration.

The manufacturer decided to remove the txfr and bring it back to the factory to be disassembled, checked and tested.

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#9
In reply to #8

Re: Power Transformer testing - tan delta test

02/19/2009 2:36 AM

That's the very reason why SFRA should be carried out as a acceptance test after transportation of the unit to the site.

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Anonymous Poster
#10
In reply to #8

Re: Power Transformer testing - tan delta test

03/07/2009 5:56 PM

Good move the manufacturer insisted to have it checked. The unit might have been mishandled on its way from the factory to your facility.

Mobile substations are designed to be towed into service in emergency situations. Its very nature of use requires that it is stable enough for field transport and should withstand more mechanical abuse than non-mobile units.

If this unit exhibited sever deformation from SFRA after shipment then the internal structure is not secure enough for acceptance from a mechanical point of view.

Insulation power factor may probably be taken from a different light, specially your transformer is a brand new unit. Try to take water in oil tests (ASTM D1533B) to check if the oil was properly processed into the transformer. Make sure the oil is sampled via a gas tight DGA syringe and rinsed properly. If the moisture content of the oil is good for a newly-built transformer, then we might infer back that the solid insulation is probably be dry.

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