Choosing a Sample Container for Transformer Oil Analysis.pdf

7/27/2019 Choosing a Sample Container for Transformer Oil Analysis.pdf

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Fall 2002 1

The last several articles discussed understanding water in transformer

systems and which tests to choose and when. The results obtained

from the analysis of a sample can provide unparalleled informa-

tion concerning the condition of the insulating materials within electri-

cal 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 information that can easily mislead personnel to

incorrectly assess the condition of the oil or the electrical apparatus. The

art of sampling is very important and will be covered in a future article.

One aspect of the sampling function that is often overlooked is the sample

container itself. Unfortunately, there are many instances in which the

sample container plays a major role in the quality of the sample taken.

Purpose of a Sample Container Sample containers are used to retrieve and store samples. It is impor-

tant for them to prevent contamination and to provide the best samplefor a specific test. ASTM Practices D 923 and D 3613, the Doble ReferenceBook on Insulating Liquids and Gases, or IEC Methods and Guides 60475and 60567 are to be consulted when choosing a sample container. Sample

containers should have the following characteristics. They must:

• Be large enough to hold the volume of liquid necessary for analysis.

• Not impart any contamination (chemicals or particles) to the samplefrom the container itself.

• Seal the sample from external contamination.

• Shield the sample from direct sunlight to prevent photodegradation.This can be done either by having a dark container or by having acovering for that container.

• Prevent the loss or gain of gasesor water when testing for theseproperties.

The volume of the sample is of the utmost importance, as variousanalytical tests require very differ-ent sample volumes. For example,a test for inhibitor content (ASTMD 2668) requires only a few millili-ters of oil whereas the test for im-pulse breakdown (ASTM D 3300)may require as much as two to four

liters. In general, it is good prac-tice to provide the sample vol-ume required for each test plusten percent. If unsure of thesample volume required for spe-cific test or tests, laboratoriessuch as the Doble MaterialsLaboratory have reference liststhat detail such information.

Chemist’s Perspective

by Lance R. LewandDoble Engineering Company

Choosing a Sample Container

for Transformer Oil Analysis



2 NETA WORLD

Which Sample Container to UseFor general oil quality tests, glass bottles — either

amber or clear — function well. Amber bottles pro-vide protection against photodegradation while clear bottles enable visual inspection of the sample. To pre-vent photodegradation in clear bottles, shielding fromdirect sunlight by storing them in cardboard or someother type of container works well. Bottle caps must be constructed from a compatible material that will

not contaminate the sample. For example, bottle capswith liners composed of paper or having glue that issoluble in the oil are not appropriate. Liners made outof foil, Teflon® or polyethylene are usually safe to use.If sealed tightly, the glass bottle is an appropriate con-tainer from which to draw a sample for water analy-sis. However, problems have been encountered withcaps working loose over time. If not properly pro-tected, glass bottles are more apt to break than othercontainers, and they are not suitable containers fordissolved gas-in-oil analysis, as gases such as hydro-gen and carbon monoxide will be lost.

Metal cans have become popular because they aremore resistant to breakage than glass bottles. Canswith soldered seams prepared with some fluxes willcontaminate the sample; therefore, welded seams arepreferred. Cans made of tin, aluminum, and stainlesssteel have been used and are especially good contain-ers to hold samples for oil quality tests. Metal bulbsconstructed out of stainless steel are also used andcan be invaluable when sampling in areas that have alot of airborne environmental contamination such ascoastal areas where salt spray is common.

The use of plastic bottles has grown significantlyin the past decade. Like cans, they resist breakage and

shield the sample from sunlight when dark plasticsare used. They are appropriate containers for samplesin which oil quality tests are to be performed. Samples being tested for water should not be stored in plastic bottles as water ingress or egress can occur in just afew hours. Not all plastics are compatible with oil soselection of the construction material is important, andcontainers made of substances such as polypropylene,polyvinyl chloride (PVC), and especially polystyreneare not appropriate. Tests such as power factor will be affected by incompatible plastic bottles because of dissolved components transferred to the sample thatwill cause increases in dielectric loss. Bottles made of

high density polyethylene have been found suitableand are one of the preferred sample containers whenelectrostatic charging tendency or furanic compoundanalysis is to be performed. In the case of furanic com-pounds, high-density polyethylene does not have thesilanol groups found in glass that attracts the

semipolar furanic compounds to the glass walls andthus removes them from the sample. Samples with low concentration of furanic compounds are not aaffected as samples with larger amounts of furanicompounds.

The most appropriate container for takinsamples for dissolved gas-in-oil and water contenanalyses, and the easiest to manipulate, is thground glass syringe whose barrel and plunge

have extremely tight tolerances. This type of syringhas been found to satisfactorily prevent the ingresof gases and water into the sample, and in turn prevent the egress of water and dissolved gases fromthe sample over a period of time. Care should btaken with glass syringes to ensure the stopcock itight and in the closed position once the sample itaken. Also, the samples collected in syringes mus be quickly protected from photodegradation (degradation by light) by immediately placing them inthe dark or in their shielding containers. Stainlessteel cylinders may also be used but can be mordifficult to manipulate or to determine when all th

air has been removed from the cylinder. Metal cylinders will also add significant shipping costs because of weight but are definitely more durable intransit.

Sample identification is an extremely vital aspecof the sampling process, and the identification is usually attached to the container itself to avoid confusionMany laboratories receive samples that can not be related to a specific device. Even if the sample was takencorrectly, the lack of proper identification makes thsample useless, and is therefore a bad sample. Whensampling personnel retrieve samples from electricaapparatus, it should be done in a prescribed sequenc

so nothing is forgotten.

Sample Storage and TransportSamples should not be stored longer then a few

days before shipping to the laboratory for analysisThe properties of the dielectric liquid tend to decaover time, and gases for dissolved gas analysis (DGAcan be lost or gained if the sample is held too long, athere is a finite time in which the sample containecan maintain the sample integrity. Remember that thsample is supposed to represent the bulk liquid insulation and storing it too long will change those prop

erties. Store samples in the dark to prevenphotodegradation, and keep them away from temperature extremes and humid environments.



Fall 2002 3

Package the sample containers to protect from leak-age and breakage, especially if the samples are inglass. The United States Environmental ProtectionAgency Resource Conservation and Recovery Act(RCRA) listed in 40 CFR Part 261.4(d) requires thatall samples shipped must be packaged so they do notleak, spill, or vaporize from their packaging. Othercountries may have similar requirements as well. Usemethods to prevent sample container breakage or

leakage such as the use of cardboard, Styrofoam, andother protective materials. Package the samples insuch a way as to avoid container-to-container con-tact. Also, some shipments become too heavy and un-wieldy, and the shipping company may not be ableto handle it safely. In this case, it is advisable to sepa-rate the shipment into several packages.

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 for the past seven years and is currently ProjectManager of Research in the materials laboratory and ProductManager for the DOMINOTM product line. Prior to his presentposition at Doble, he was the Manager of the Transformer FluidTest Laboratory and PCB and Oil Services at MET Electrical Test-ing in Baltimore, MD. Mr. Lewand is a member of ASTM commit-tee D 27.

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Choosing a Sample Container for Transformer Oil Analysis.pdf