ANALYTICAL SERVICES INCORPORATED

TJ|H2b – Ledicia Salgado

Alfonso de Pablo

Significance of Low Degree of Polymerization of Papers in

Transformers

Transformer 17 – Torun (Poland) – 9‐11 May 2017)

SUMMARY

INTRODUCTIONINTRODUCTION

DEGRADATION MODELSDEGRADATION MODELS

SHORTCOMINGSSHORTCOMINGS

UNCERTAINTIES IN DP MEASUREMENTS

UNCERTEINTIES IN DETERMINING DP0

ACTUAL AMOUNT OF DEGRADED PAPER

TRANSFORMER THERMAL PROFILE

EFFECT OF OIL TREATMENTS

STRUCTURE OF CELLULOSE

STRUCTURE OF CELLULOSE

Supramolecular fibers:•High tensile strength due to extensive hydrogen bonds•600 – 2.000 µm long and about 20 µm in diameter

Individual fibers:•About 1.200 units of monomer (glucopyranose)

End‐of‐life:•Degree of polymerization < 200 units•Tensile strength < 40% original value

PUBLISHED DATADP Company Reference

150‐220 NTPC (IN) Chakraborty

87 Alliant (US) Baldyga

150 EDF (FR) Bellesort

140 FKH (CH) Knab

100 TJH2b (US) Hanson

<200 VP Diagnose (SE) Ardvidsson

100‐150 Hydro One (CA) Figuerola

100 NL Salverada

75 Trafo Service (FR) François

<100 Ameren (US) Bizzoff

DP Company Reference

150 AU/NZ Wright

200 EIMV (SI) Gradnik

150 NZ Bhumiwat

170 HQ (CA) Duval

173 EDF (FR) Alzieu

145 ELMA (IT) Amirouche

110 EPSA (CO) Jaimes

210 Polimeri Europa De Pablo

50‐200 Israel Electric (IL) Grisaru

Furans dissolved in the oil Furans dissolved in the oil of overheated of overheated transformers were first transformers were first reported by Peter Burton reported by Peter Burton (CEGB) in 1982.(CEGB) in 1982.

DEGRADATION MODELS

DEGRADATION MODELS

Burton log[2FAL] = 2,5 – 0,005 DP

Vuarchex log[2FAL] = 2,6 – 0,0049 DP

Chendong log[2FAL] = 1,51 – 0,0035 DP

Stebbins log[2FAL] = 1,5655 – 0,0035 DP

DEGRADATION MODELS

CIGRE TF.15.01.03

DEGRADATION MODELS

CIGRE TF.15.01.03

Cellulose Furans + by‐products y = 30%

DEGRADATION MODELS

OO

O O

O

H

HOH

H

CH OH2

CH OH2

H

HO

H H

HOH

HH

OHH

O CHO

E A

1/T

At relatively low temperatures (<140/150 °C), activation energy is relatively constant and reaction yield is close to 30%

(A. de Pablo and B. Pahlavanpour. Electra 175, 8‐32, 1997)

OO

O O

O

H

HOH

H

CH OH2

CH OH2

H

HO

H H

HOH

HH

OHH

O CHO

E A

1/T

At higher temperatures (>140/150°C), activation energy varies and reaction yield is not constant (from 40 % to 85 %)

(B. Pahlavanpour, M. A. Martins and A. de Pablo. IEEE ISEI 2002)

S. D. Myers experts published a survey on furans formation in transformers having TU paper. They reported furans concentrations in ppb rather than ppm. This has wrongly induced to think that TU paper forms lower amounts of furans than normal Kraft paper.

Normal Kraft vs Thermally up‐graded paper

DEGRADATION MODELS

Influence of type of paper

The results of this study supported the theory that there is a relationship between the concentration of 2‐Furaldehyde and the DP of paper. There was no difference in this relationship between thermally upgraded and non‐thermally upgraded paper,

Normal Kraft vs Thermally up‐graded paper

DEGRADATION MODELS

Normal Kraft vs Thermally up‐graded paper

Lower levels of furans are formed in transformers using TU paper simply because TU paper degrades more slowly than regular Kraft paper does and therefore reaches low DPs more slowly.

DEGRADATION MODELS

SHORTCOMINGSUncertainties in DP measurements

Lab DP units Remarks

# 1 1550 As received

# 2 1500 As received

# 3 1150 Vacuum dried at rt

# 4 1050 Vacuum dried at 80°C

# 5 944 Vacuum dried at 70°C

SHORTCOMINGSUncertainties in DP measurements

IEC 450:1974:“5.1 Apparatus for solutionRound‐bottomed 50 ml flask, preferably with a short narrow neck, or a narrow‐necked 50 ml Erlenmeyer flask.Rubber stopper, fitting the neck of the flask, through which passes a capillary tube fitted with a small‐bore glass cock.Glass balls 4 mm to 6 mm diameter, which shall not be able to enter the bore of the cock.…7.1.2 SolutionPut a few balls in the flask.Add the weighed test sample.Carefully shake or stir by hand, to ensure…Add 22,5 ml of distilled water.Again shake or…Leave for half an hour to allow thorough impregnation with water.Add 22,5 ml 1 M solution of cupriethylene‐diamine.Add a number of glass balls so that, when the stopper with capillary tube, etc., is fitted, the liquid will entirely fill the flask and reach the bore of the cock.…”

SHORTCOMINGSUncertainties in DP measurements

LabNew paper Aged paper

Glass balls N2 purged Glass balls N2 purged

# 1 1206 1206 664 621

# 2 1125 1163 620 603

# 3 1214 1281 657 669

# 4 1303 1303 633 647

# 5 918 1084 494 557

# 6 1260 1215 725 665

# 7 596 1174 152 606

# 8 1197 854

# 9 1291 1387 640 658

SHORTCOMINGSUncertainties in determining DP0

SHORTCOMINGSUncertainties in determining DP0

“Finally, as mentioned earlier, use of a linear relationship between life remaining or life used and DP, with a “beginning life” value for DP of 1200 is not completely appropriate. New paper before being installed in a transformer typically has a degree of polymerization of 1200 to 1400. After factory drying and assembly, our experience is that a new paper in a new transformer has a DP of 800 to 1200, and it is usually more appropriate to figure life assessment based on changes from the lower limit of this range”

R. Stebbins et al., Proc. 7th Int. Conf. on Properties and Applications of Dielectr. Materials, Nagoya 2003

SHORTCOMINGSIgnorance of the actual amount of paper degraded at a given time

Two unknowns: Amount of degraded paper at any timeRatio paper/oil in the transformer

Amount of degraded paper at any time: 2% ‐ 10% (V. Sokolov)

SHORTCOMINGSIgnorance of the actual amount of paper degraded at a given time

Italy 1970‐1990

kV MVA Duty R

420 370 Step‐up 20

240 180 Step‐up 24

420 250 Transmission

26

220 100 Transmission

46

130 30‐40 Distribution 38‐45

Spain nowadays (distribution)

kV MVA R kV MVA R

<66 10 6 <66 65 8,6

66 10 2,6 66 65 6,3

132 10 2,5 132 65 5,23

<66 40 4,6 <66 75 8,75

66 40 4,83 66 75 6,4

132 40 5,24 132 75 5,56

SHORTCOMINGSIgnorance of the actual amount of paper degraded at a given time

661

561

384

SHORTCOMINGSIgnorance of transformer´s thermal profile

110/20 kV, 31.5 MVA substation transformer 1958

SHORTCOMINGSIgnorance of transformer´s thermal profile

SHORTCOMINGSEffect of oil treatments

• Degassing the oil partially removes furfural because furfural is relatively volatile.

• Oil reclamation with Fuller´s earths almost completely removes furfural because its polarity.

• After oil replacement may soon appear some furfural because part of the old furfural is absorbed on paper and dissolved again by the new oil

END OF LIFEStatistics

CIGRE Berlin 1993

[2FAL] ppm # transformers %

< 0,10 2.700 54,0

0,11 – 0,50 1.347 26,9

0,51 – 1,00 415 8,3

1,01 – 5,00 466 9,3

5,01 – 10,00 57 1,1

> 10,00 20 0,4

Electr. Insul. Magazine 2015

Age (years)

# transformers

% [2FAL] ppm

90 % 95 % 99 %

0 – 10 4,622 25,28 0,2 0,4 0,9

11 – 20 4.528 24,77 0,7 1,7 3,7

21 – 30 4.517 24,71 1,1 2 4,3

31 ‐ 40 3.540 19,37 1,8 3,4 6,1

> 40 1.006 5,50 2,8 6,9 26,5

CONCLUSIONS

Relationship between DP and furfural content is not evident because:

•Uncertainties in DP measurements,•Uncertainties in defining value of DP0,•Ignorance of the actual amount of paper degraded at any time,•Ignorance of transformer thermal profile•Effect of oil treatments

Low degree of polymerization observed in many transformers working correctly may be true or may be due to these uncertainties.

CONCLUSIONS

However, analysis of furans, mainly furfuraldehyde, if considered with due regard and proper engineering

judgement, constitute a very important tool to estimate transformer cellulosic paper condition

CONCLUSIONS

The End of Life (EOL) criteria, now considered to be at DP of 200 units, needs to be revised.

Lower limit does not represent a serious risk for the transformer and may assist in saving asset investments.

THANK YOU VERY MUCH FOR YOUR ATTENTION