luk sich paper 2004

8/13/2019 Luk Sich Paper 2004

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Evaluating New and In-Service Vegetable Oil Dielectric Fluids

John LuksichCooper Power Systems

Waukesha, WI

Abstract

Natural ester (vegetable oil) dielectric fluid is finding growing acceptance and application inelectrical equipment using liquid insulation. Its fire safety, interaction with insulation, and

environmental characteristics make it an excellent choice for many applications. This paper

examines natural ester and mineral oil specifications, inherent differences in properties, and

variations in in-service aging. The application of dissolved gas analysis as a diagnostictechnique is discussed.

Introduction

Vegetable oil dielectric fluids are a recent addition to the category of less-flammable fluids.Less-flammable fluids, sometimes called high fire point fluids, were introduced in the mid 1970s

as alternatives to Askarels for installations requiring a high degree of fire safety. The National

Electrical Code and National Electrical Safety Code recognize their use as an optional firesafeguard [1,2]. Factory Mutual Global considers them an equivalent safeguard to space

separation, fire barriers, and fire suppression systems for transformers containing up to 10,000

gallons of fluid [3].

Less-flammable fluids come in a variety of chemical types. First were the high molecular weight

hydrocarbons (HMWH) and silicones, followed in the 1980s by synthetic fluids such aspolyalphaolefins (PAO) and polyolesters (POE). Of these, the POEs easily have the most

attractive environmental properties. Their high cost limits them to niche applications and

spurred the search for more affordable alternatives.

Enter the vegetable oils. These natural esters are chemically similar to the POEs and share

many of their properties. The natural esters have excellent environmental properties, a high

degree of fire safety, and are shown to retard the degradation of paper insulation [4-6]. Availablecommercially since the late 1990s in distribution transformers, they are now beginning to see use

in power transformers.

Properties

A combination of electrical, chemical, and physical properties are used to evaluate new and in-

service insulating fluids [7-14]. Periodic fluid testing of in-service transformers monitors thecondition of the oil and establishes baseline values and trends over time. Changes over time are

often more telling than a single set of measurements.

Natural ester fluids are inherently different in their chemistry from mineral oils. We mightexpect the chemical, physical, and electrical properties to differ as well, requiring the results of

testing to be interpreted differently. Values for dissipation factor, water content, and acid

number are inherently higher in natural esters than for mineral oil; resistivity and interfacial


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tension are lower. Table 1 compares some of the properties of mineral oil and natural ester fluids

in existing standards.

Water content of natural ester fluid can be dramatically different from mineral oil. At room

temperature, the water saturation of mineral oil is about 60 mg/kg. Natural esters have room

temperature saturations in the neighborhood of 1000 mg/kg. The dielectric strength of aninsulating fluid starts to decrease when the relative saturation increases to 40-50%. Using

percent saturation instead of absolute water content allows direct comparisons between natural

ester fluids and mineral oil to be made.

In many cases, test methods routinely used to characterize new mineral oil and evaluate in-

service mineral oil can be directly applied to natural ester fluids. Some methods require minor

modifications in technique or apparatus [15]. For example, ASTM method D1816 for dielectricbreakdown strength of mineral oil is measured 3-5 minutes after filling the test cell. Natural

ester fluids require 10-15 minutes in order to obtain reliable results.

Table 1. Comparison of some specification values for new as-received mineral oil and natural ester fluid.

ASTM Mineral Oil Natural EsterProperty Method ASTM D3487 ASTM D6871

ELECTRICAL

Dielectric strength (kV) D1816 (2mm gap) 35 35

Dissipation (%) 25 C D924 0.05 0.20100 C 0.30 4.0

CHEMICAL

Water content (mg/kg) D1533 35 200

Acid number (mg KOH/g) D974 0.03 0.6

PHYSICAL

Color D1500 0.5 1.0Visual examination D1524 clear & bright clear & bright

Relative density D1298 0.91 0.96

Flash point ( C) D92 145 275Fire point ( C) D92 - 300Viscosity (cSt) 100 C D445 3.0 15

40 C 12.0 50Pour point ( C) D97 40 0Interfacial tension (dyne/cm) D971 40 -


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Table 2. Limits for continued use of service-aged insulating fluid. Mineral oil values are from existing IEEE Guide.Natural ester values, loosely based on laboratory testing and limited in-service history, are offered for discussion.

Note that water content is given in percent saturation.

Mineral Oil Natural EsterProperty IEEE C57.106 (IEEE guide not yet available)

ELECTRICAL

Dielectric strength, D1816, 2mm gap (kV)Voltage Class: 69 kV 40 40

Voltage Class: 69 < x < 230 kV 47 47

Voltage Class: 230 x < 345 kV 50 50

Voltage Class: 345 kV 50 50

Dissipation, D924 (%) 25 C 0.5 1.0100 C 5.0 15

CHEMICALWater content, D1533 (% saturation)





Acid number, D974 (mg KOH/g)Voltage Class: 69 kV 0.20 2.0

Voltage Class: 69 < x < 230 kV 0.15 1.5

Voltage Class: 230 x < 345 kV 0.10 1.0

Voltage Class: 345 kV 0.10 1.0

PHYSICAL

Interfacial tension, D971 (dyne/cm)





Changes Over Time

First installed in 1996, long term in-service data from natural ester transformers are not available.Laboratory accelerated aging tests let us take an educated guess at the changes taking place over

the lifetime of a transformer. Aging vessels containing transformer construction materials in

typical proportions were aged at 130, 150, and 170 C for 500, 1000, 2000, and 4000 hours. The

times at temperature were converted to the equivalent number of IEEE normal lifetimes of20.54 years. [16].

Table 2 shows the in-service limits suggested by IEEE for mineral oil [8] and offers fordiscussion limits based on both IEC standards for synthetic esters [13, 14] and accelerated aging

results. It should be noted that IEEE has an active working group developing a maintenance

guide for natural esters currently designated as C57.147xxx.

Electrical Changes

The dielectric breakdown strength (ASTM D1816, 2mm gap) of mineral oil and natural esters

change in the same way over time. Both started around 60 kV and decreased slightly to 50 kVafter 3.8 normal lifetimes. The dissipation factor (Figure 1) of both fluids followed similar

curves with the natural ester reaching values similar to those seen in aged Askarels.


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Figure 1. Dissipation factor at 25 C during accelerated aging (triangles = natural ester, circles= mineral oil). Each normal lifetime represents 20.54 years.

Chemical ChangesThe absolute water content of the natural ester fluid during aging reached higher values during

aging than did the mineral oil systems (Figure 2a), but is actually drier in terms of percentsaturation (Figure 2b). The curve shapes are again similar to each other. Changes in acid

number follow the same trend of the same curve shape and higher values for the natural ester

(Figure 3). Each fluid produces a different type of acid: mineral oils generate short chain organic

acids, and natural esters produce long chain fatty acids. Although the test method determines theamount of acid present, it does not reveal anything about the reactivity of the acids. Fatty acids

are considered to be less reactive than short chain organic acids.

Physical Changes

Relative density, flash and fire points, viscosity, and pour point remained unchanged for bothfluids for the duration of the test. Color and appearance degraded over time similarly for bothfluids. Interfacial tension (Figure 4) has the same curve shape for both fluids with the natural

ester starting lower and remaining lower.

Equivalent "Normal" Lifetimes

0 1 2 3 4

DissipationFactor(%)

0.001

0.01

0.1

1

10


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a bFigure 2. Water content of natural ester and mineral oil during accelerated aging (triangles = natural ester, circles =mineral oil). Figure a shows the absolute water content; b shows the same data as percent of room temperature

saturation. Each normal lifetime represents 20.54 years.

Figure 3. Acid number of natural ester and mineral oilduring accelerated aging (triangles = natural ester, circles= mineral oil). Each normal lifetime represents 20.54

years.

Figure 4. Interfacial tension of natural ester and mineral oilduring accelerated aging (triangles = natural ester, circles =mineral oil). Each normal lifetime represents 20.54 years.

Dissolved GasesThe gases dissolved in an insulating fluid can give evidence of abnormal conditions inside a

transformer. Several methods are available to aid interpretation and fault diagnosis using

dissolved gas data [17-19]. The combustible gases generated by faults in natural ester fluids are

similar to those in mineral oil. High levels of hydrogen indicate that partial discharge may betaking place. Carbon oxides in certain ratios suggest overheated paper. Hydrocarbon gases

could result from a thermal fault in oil. The presence of acetylene points to arcing. The changesover time of amounts of gases and their rates of generation are more important than thesnapshot given by a single sample.

Because transformers using natural ester fluid are a recent development, the opportunities toevaluate actual faulted transformers are slow in coming. The few available to us have been very

useful to begin validating the application of dissolved gas analysis to natural ester fluid.


0 1 2 3 4

Water

Content(mg/kg)

0

20

40

60

80

100


0 1 2 3 4

WaterContent(%saturation)

0

20

40

60

80

100


0 1 2 3 4

AcidNumber(mgKOH/g)

0.001

0.01

0.1

1

10


0 1 2 3 4

InterfacialTension(dyne/cm)

0

10

20

30

40

50


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Case 1

A new mineral oil-filled transformer showed abnormal gas levels soon after installation.

Methane, ethane, and ethylene levels were high and increasing. The customer retrofilled thetransformer with natural ester fluid. The initially clean fluid developed the same hydrocarbon

gas signature and high levels seen in the mineral oil. An autopsy of the transformer discovered a

7.5 inch piece of metal banding steel inside the coil window of the B phase coil. This caused ahole to burn through the 90-mil window insulation.

Case 2

A 28-year-old mineral oil transformer was retrofilled with natural ester. No dissolved gas

history was available for the unit. During the retrofill process, it was noted that the tap changer

contacts showed significant coking. After a year in service, high levels of acetylene were found.

After verifying the acetylene level, an outage was scheduled to examine the transformer. The tapchanger contacts were heavily coked. Replacing the switch returned the gas levels to normal.

They remain stable.

Typical Operation

The opportunity to measure the dissolved gases in a variety of new and retrofilled natural ester

transformers with no known problems do not show abnormal gas levels. The natural estertransformers seem to have slightly higher baseline levels of hydrogen and carbon dioxide thanfound in mineral oil transformers. The Doerenburg and Rogers ratio methods have so far not

been reliable indicators of the condition of natural ester transformers. We do not yet have the

decades of dissolved gas history and fault correlation for thousands of transformers needed forconsistent dissolved gas analyses of natural esters.

However, laboratory determinations of the types and amounts of gases generated in natural estersas well as their absorption characteristics confirm that interim use of a combination of the IEEE

key gases and condition methods presented in the mineral oil dissolved gas guide [17] are

valid for natural esters.

Discussion

Natural ester fluid properties and dissolved gas content for condition assessment can be usedmuch in the same way as is currently done for mineral oil transformers. The inherent differences

in fluid properties, both starting values and changes over time, must be taken into account in

order to get the most benefit of such measurements.

The primary functional property of dielectric strength is interpreted just as for mineral oil. Water

content, if in percent saturation, also translates across fluid type. As with mineral oil, viscosity,pour point, flash point, and fire point should be expected to be stable. Other properties useful in

diagnostics change in ways similar to mineral oil but differ in magnitude. As in mineral oil, the

acid number, dissipation factor, and resistivity increase over time. In natural esters, they begin at

higher initial values and show greater increases than are seen in mineral oil. Interfacial tensionstarts at lower values than does mineral oil and decrease to lower values.


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References

[1] Less-Flammable Liquid-Insulated Transformers, Article 450.23, National ElectricalCode, NFPA 70, National Fire Protection Association

[2] Location and Arrangement of Power Transformers and Regulators, Section 152, National

Electrical Safety Code, Institute of Electrical and Electronics Engineers

[3] 5-4 Transformers, Section 2.3.1.1.1 Separation Distance, Property Loss Prevention DataSheets, Factory Mutual Global, May 2003

[4] C.P. McShane, K.J. Rapp, J.L. Corkran, G.A. Gauger, J. Luksich, Aging of PaperInsulation in Natural Ester Dielectric Fluid, 2001 IEEE/PES Transmission & Distribution

Conference, October 28 - November 2, 2001, Atlanta, GA USA

[5] C.P. McShane, K.J. Rapp, J.L. Corkran, G.A. Gauger, J. Luksich, "Aging of Plain Kraft

Paper in Natural Ester Dielectric Fluid", IEEE/DEIS 14th International Conference on

Dielectric Fluids, July 7-12, 2002, Graz, Austria

[6] C.P. McShane, K.J. Rapp, J.L. Corkran, J. Luksich, "Aging of Cotton/Kraft BlendInsulation Paper in Natural Ester Dielectric Fluid", TechCon 2003 Asia-Pacific, May 7-9,

2003, Sidney, Australia

[7] Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus, ASTMD3487-00, ASTM International

[8] IEEE Guide For Acceptance and Maintenance of Insulating Oil in Equipment, IEEE

C57.106-2002, Institute of Electrical and Electronics Engineers

[9] "Specification for unused mineral insulating oils for transformers and switchgear",Amendment No. 1, March 1986, IEC Standard 60296, International Electrotechnical

Commission

[10] Standard Specification for High Fire-Point Mineral Electrical Insulating Oils, ASTM

D5222-00, ASTM International

[11] "IEEE Guide for Acceptance and Maintenance of Less Flammable Hydrocarbon Fluid in

Transformers", C57.121-1998, Institute of Electrical and Electronics Engineers

[12] "Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical

Apparatus", ASTM D6871-03, ASTM International

[13] "Specification for unused synthetic organic esters for electrical purposes", IEC Standard

61099, International Electrotechnical Commission

[14] "Synthetic organic esters for electrical purposes - Guide for maintenance of transformer

esters in equipment", IEC Standard 61203, International Electrotechnical Commission

[15] Envirotemp FR3 Fluid Testing Guide, Section R900-20-12, Cooper Power Systems, July

2004

[16] IEEE Guide for Loading Mineral-Oil-Immersed Transformers, IEEE Std. C57.91-1995,

Institute of Electrical and Electronics Engineers

[17] IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers,

IEEE Std. C57.104-1991, Institute of Electrical and Electronics Engineers

[18] Mineral oil-impregnated electrical equipment in service Guide to the interpretation of

dissolved and free gases analysis, IEC Standard 60599, International ElectrotechnicalCommission

[19] Transformer Maintenance, Facilities Instructions, Standards, and Techniques, Vol. 3-30,

pp. 39-53, Hydroelectric Research and Technical Services Group, Bureau of Reclamation,

U.S. Dept. of Interior, Denver, CO, October 2000

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