sampling transformer oils-part1.pdf

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Winter 2002-2003 1

The results obtained from the analysis of an insulating oil sample

can provide unparalleled information concerning the condition of

the insulating materials within electrical apparatus, life assessment,

and the operating condition of the device. However, a sample that is not

representative of the bulk oil insulation can provide erroneous informa-

tion which can easily mislead maintenance personnel to incorrectly as-

sess the condition of the oil or the electrical apparatus. In one case,

samples taken from two transformers showed very high concentrations

of hydrogen and no other gases, prompting maintenance personnel to

give these units priority for diagnostic surveys. It was later found that

the cause of the high hydrogen was a galvanic reaction occurring in the

drain valves in which water was converted to hydrogen because of the

interaction of a galvanized fitting with a dissimilar metal.

Why Sample?For in-service oil-filled electric apparatus, sampling of the oil provides

a method to determine the condition of the solid and oil insulation as

well as the operating condition of the apparatus without opening or de-energizing the apparatus. This is especially important in the present utilityand industrial climates, as equipment outages for out-of-service testinghave become very limited. Sampling provides a means to check the con-dition of oil in storage, whether it be new or used, and to determine if itcomplies with specifications such as TOPS, ASTM D 3487, IEC 60296,IEEE C57.106, or company specifications. Sampling can also help to de-termine:

1) If accidental mixing of different dielectric oils has taken place;

2) If the method of transportation contaminated the oil;

Chemist’s Perspective

Sampling Transformer Oils

Part One – How and Why to Take a Good Sample

by Lance R. Lewand

Doble Engineering Company

3) If the handling equipment totransfer the oil contaminated theproduct.

What is a Good Sample?Simply put, a good sample is

one that is representative of thecontent of the bulk oil insulation.Since samples are usually retrievedfrom a drain valve or the attachedsampling cock, preparation of thatarea is important to obtain a good

sample. Cleaning the drain valveand the sampling cock inside andout is the first step in avoidingsample contamination. Cleaningthe outside of the drain valve is justas important as cleaning the inside.The dirt and debris falling off theoutside of the valve into thesample container during the sam-pling process can contaminatemany samples.



2 NETA WORLD

Most of the contamination in the apparatus con-sists of water and particles (paper fibers, metal par-ticles, etc.), which over time will settle out on the bottom of the apparatus near the drain valve. Thismaterial needs to be flushed out of the system toget to the bulk oil insulation. It is necessary to re-move at least one to two liters of oil from the drainvalve, cap the drain valve, and then flush out thesampling cock before proceeding with sampling.

On occasion, two liters will not be sufficient, espe-cially when sampling a nonenergized transformeror certain OCBs and LTCs. Specific sampling tech-niques and precautions, especially those dealingwith low volume electric equipment, are detailedin the Doble Reference Book on Insulating Oils andGases and ASTM Practices D 923 and D 3613.

Good Samples Versus Bad SamplesIt is sometimes very clear to the laboratory perform-

ing the analysis on the oil that the sample was takenimproperly. For example, the presence of free water

or foreign objects such as insects, pipe sealing tape orputty are strong indicators that the drain valve wasnot adequately flushed out prior to sampling. Onceanalysis has begun and it is determined that there is ahigh or free water content coupled with a low dielec-tric strength, with all the other test results being ac-ceptable, then it strongly indicates that the proper sam-pling technique was not adhered to. It may even im-ply that some chemical reactions were taking place inthe drain valve that were not representative of the bulkoil insulation.

Lab Tests Most Easily AffectedAs indicated previously, the analytical tests most

easily affected by sampling are dielectric strength andwater content. This is due to the fact that drain valvesare usually at very low points in the tanks where debris and water accumulation occurs. Water can also be present as a result of condensation that occurs inthe drain valve, which is due to the position of thedrain valve on the tank. In most cases the drain valveprotrudes 15 to 30 centimeters (six to 12 inches) awayfrom the main tank. From experiments performed atDoble Engineering, it was found that the oil in manyof these valves varies in temperature from eight de-

grees Celsius to 15 degrees Celsius cooler than the bulkoil insulation. When oil or air has an elevated relativesaturation or humidity and there is a significant cool-ing, condensation of water will occur. This is exactlywhat happens in a drain valve. Other analytical testseasily affected by sampling are dissolved metals, par-ticulate metals, particle counts, dissolved gases-in-oil,and power factor.

The concentration of metals, whether dissolved orin a particulate state, are especially impacted by theamount of cleaning performed on the drain valve and

the amount of flushing that is performed. Debris thatsettles to the bottom of the apparatus and subse-quently into the drain valve can consist of metal par-ticles. In addition, just the simple fact of removing thedrain-valve plug or opening the sampling cock willcreate particulate metals. This is due to the grindingof the surfaces between the valve body and the drainplug or sampling cock. In fact, it is becoming moreapparent that that these types of samples should only

be retrieved after a minimum of two (and sometimesthree to four) liters of oil have been passed throughthe drain valve.

The same is true of retrieving a sample for particlecount where valve debris, whether inside or outside,can severely skew the results. The debris, soot, andgrime that exist on the outside of the drain valve areof serious consequence, especially in industrial loca-tions. This debris can be easily transferred to thesample bottle while the sampling process is takingplace. This validates the importance of cleaning theoutside of the valve prior to taking the actual sample.

Dissolved gas-in-oil analysis is another test im-pacted by sampling, drain valve components, andsampling materials. When galvanic fittings (zinccoated) are used in the drain valve assembly — suchas the drain plug — a galvanic reaction with watercan cause very high levels of hydrogen to be produced.If this residue is not flushed out adequately then itwill be transferred to the sample and included in theanalysis, causing a level of concern that is not war-ranted. Galvanic plumbing fittings such as nipples canalso have the same effect. Brass, bronze, stainless steelor black iron should be the only materials used. Inaddition, drain valve assemblies should not be com-

posed of dissimilar metals as corrosion can result,which may end up in the sample. Debris, water andother ionic contaminants also affect the power factortest. These materials increase dielectric loss, whichincreases the power factor. Incompatible inorganic andorganic materials from the drain-valve stem packingor drain-plug sealants can also have the same effecton the power factor.

Costs Associated with a Bad SampleIn the case of a single sample, the costs for routine

oil quality analysis and DGA testing are just a verysmall fraction of the total costs associated with takingand analyzing a sample. Some of the items and costsassociated with sampling and analysis are:

ITEM COST ($)

Labor to take sample 275

Materials to take sample 15

Packaging and shipping cost 8

Analysis cost 70

Engineering evaluation of the data (10-15 min) 35

TOTAL 403



Winter 2002-2003 3

Every situation is different, but in many cases theanalysis cost is only about 17 percent of the entire sam-pling and data review process. In a situation wherethe sample has been determined to be nonrepresenta-tive of the bulk oil insulation, the following additionalcosts may also be incurred:

ITEM COST($)

Labor to take original sample 275

Materials to take original sample 15Packaging and shipping cost for original sample 8

Analysis of original sample 70

Engineering evaluation of the data (10-15 min)of original sample 35

Additional engineering time to confirmsample was nonrepresentative 35

Labor to take 2nd sample 275

Materials to take 2nd sample 15

Packaging and shipping cost for 2nd sample 8

Analysis cost of 2nd sample 70

Engineering evaluation of the data

(10-15 min) of 2nd

sample 50

TOTAL 856

The cost of taking a bad or nonrepresentativesample has more than doubled from the original to-tal. This is in part due to the fact that review of datafrom the second sampling takes longer as there is amore critical and thorough review.

If the original sample was not recognized as bad,the costs associated with that sample can be stagger-ing. For example, a bad sample could cause a customerto try a remedial effort in an attempt to improve the

condition of the insulating oil — such as processingthe oil through clay or vacuum-processing a trans-former to remove moisture — then associated costsmay skyrocket to between $10,000 and $30,000. Thisis one reason why Doble always recommends takinga second sample to confirm the results of the first be-fore any remedial activities begin. Other factors, suchas accidental sample switching or misidentification,can also be the source of an erroneous assessment.

Part two of this series will cover the sampling prac-tices to follow and the science of sampling.

References“Items of Interest” in The Doble Exchange, The Doble

Engineering Company, Watertown, MA, USA, Vol-ume 11, Number 3, September 1993, Page 4.

Transformer Oil Purchase Specification (TOPS), edited by the Doble Oil Committee, Rev. TOPS-884, DobleEngineering Company, Watertown, MA.

“ASTM D 3487: Standard Specification for MineralInsulating Oil Used in Electrical Apparatus” in Elec-trical Insulating Oils and Gases; Electrical ProtectiveEquipment, Annual Book of ASTM Standards, Vol.10.03, ASTM, West Conshohocken, PA, 2001.

“IEC 60296: Specification for Unused Mineral Insu-lating Oils for Transformers and Switchgear” In-ternational Electrotechnical Commission, 3, rue deVarembe, Geneva, Switzerland, 1982.

“IEEE C57.106-1991: IEEE Guide for Acceptance and

Maintenance of Insulating Oil in Equipment”, IEEE,345 East 47th Street, New York, NY, 1992

Reference Book on Insulating Oils and Gases, edited bythe Doble Client Committee on Oil Insulation, 1993,Doble Engineering Company, Watertown, MA.

“ASTM D 923: Standard Practice for Sampling Elec-trical Insulating Oils” in Electrical Insulating Oils andGases; Electrical Protective Equipment, Annual Book of ASTM Standards, Vol. 10.03, ASTM, WestConshohocken, PA, 2001.

“ASTM D 3613: Standard Practice for Sampling Elec-trical Insulating Oils for Gas Analysis and Deter-mination of Water Content” in Electrical InsulatingOils and Gases; Electrical Protective Equipment, An-nual Book of ASTM Standards, Vol. 10.03, ASTM, WestConshohocken, PA, 2001.

Griffin, P. J. “Water in Transformers – So What!” Na-tional Grid Condition Monitoring Conference, May1996.

Lance Lewand received his BS degree at St. Mary’s College of Maryland in 1980. He has been employed by the Doble Engineer-ing Company since 1992 and is currently Project Manager of Re-search in the materials laboratory and Product Manager for theDOMINOTM product line. Prior to his present position at Doble,he was the Manager of the Transformer Fluid Test Laboratory andPCB and Oil Services at MET Electrical Testing in Baltimore, MD.Mr. Lewand is a member of ASTM committee D 27.

sampling transformer oils-part1.pdf

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