Changes of Oil-immersed transformers

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Dissolved gas analysis (DGA) for oil-filled transformers and testing of insulating oil properties
Among equipment in substations, the percentage of aging and deteriorated devices continues to increase. Noticing deterioration conditions of aging devices and managing failure spot became important for maintaining and also for improving a reliability of power supply.
Internal faults and the degree of aging deterioration of oil-filled transformers can be determined by analyzing their insulating oil.

1.Dissolved gas analysis

Inside an oil-filled transformer, localized overheating and discharge causes thermal breakdown of the insulating oil and insulation materials, and the decomposed gas that is generated gets dissolved into the insulating oil. Dissolved gas analysis is a method of diagnosing the presence or absence of internal faults in the transformer by sampling and analyzing the concentrations of the gases dissolved in the insulating oil.

Dissolved gas analysis
Dissolved gas volume and criteria for determining deterioration
Caution I
level
Total
combustible
gas
TCG
Hydrogen
H2
Methane
CH4
Ethane
C2H6
Ethylene
C2H4
Acetylene
C2H2
Carbon
monoxide
CO
500 400 100 150 10 0.5 300
Caution II
level
1. C2H2≧0.5
2. C2H4≧10, TCG≧500
Abnormal
level
1. C2H2≧5
2. C2H4≧100, TCG≧700
3.  C2H4≧100, TCG rate of increase≧70/per month

Diagnosis flowchart of maintenance management using dissolved gas analysis

Examples of report details
1. Trends in dissolved gas
Sampling
date (Year,
month, day)
Oxygen
(O2)
Nitrogen
(N2)
Hydrogen

(H2)
Carbon
monoxide

(CO)
Carbon
dioxide
(CO2)
Methane

(CH4)
Ethane
×
(C2H6)
Ethylene

(C2H4)
Acetylene

(C2H2)
Total
combustible
gas
(TCG)
TCG rate of increase per month
(ppm)
1994.6.5 1321 8801 14 88 144 5 7 n nd 114
1996.6.16 2163 14540 6 78 145 4 14 t nd 102 0
2000.6.4 1709 66031 6 92 239 43 83 4 n 228 3
2000.6.4 1053 62953 -3 79 128 53 136 4 n 275 0
2004.5.30 1120 76332 31 102 412 89 245 8 n 475 4
2007.5.17 712 74592 8 102 469 110 348 9 n 577 3
2008.5.31 484 77131 5 66 625 84 310 9 n 474 -8
2008.10.1 1322 87451 5 76 521 104 345 11 n 541 17
2009.2.16 1659 87212 6 77 345 107 356 10 n 556 3
2009.8.25 911 74437 -4 53 405 83 324 9 n 473 -13
2010.2.22 1259 82253 5 53 331 83 320 9 n 470 0
2011.2.21 921 88712 5 53 444 85 346 10 n 499 2
2012.2.24 665 77104 7 55 428 88 370 11 n 531 3
2012.2.24 665 77104 7 55 428 88 370 11 n 531 3

Determination criteria

CautionⅠ
level
400 300 100 150 10 0.5 500
CautionⅡ
level
1. C2H2≧0.5ppm
2. C2H4≧10ppm and TCG≧500ppm
Abnormal
level
1. C2H2≧5ppm
2.  C2H4≧100ppm and TCG≧700ppm
3.  C2H4≧100ppm and TCG rate of increase≧70ppm per month

2. Gas pattern

3. Abnormality diagnosis chart (Composition ratio)

4. Specific gas

IEC-60599 (1999) CO2/CO<3,cellulose is degraded.
IEEE Std.C57.104-1991 CO2/CO>7, normal.
International Council on Large Electric Systems (CIGRE) materials 15/21/33-19 (TF15.01.01) (1996) CO2/CO<10, cellulose is overheated.
CO2/CO<3, cellulose is degraded by electrical abnormalities.

2.Insulating oil property testing services

This is a method for diagnosing the health of insulation properties achieved by managing the moisture content, insulation breakdown voltage, total acid number, and volume resistivity in order to determine the deterioration level of insulating oil during operation.

Insulating oil property testing
Examples of report details
Steps of insulating oil diagnosis for oil-filled transformers
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